Table of contents

The 15^th International Conference on the Strength of Materials (ICSMA 15) took place in Dresden, Germany, August 16–21, 2009. It belongs to the triennial series of ICSMA meetings with a long tradition, starting in 1967 - Tokyo, 1970 - Asilomar, 1973 - Cambridge, 1976 - Nancy, 1979 - Aachen, 1982 - Melbourne, 1985 - Montreal, 1988 - Tampere, 1991 - Haifa, 1994 - Sendai, 1997 - Prague, 2000 - Asilomar, 2003 - Budapest, 2006 - Xian. ICSMA 15 was hosted by the Dresden University of Technology, Institute of Structural Physics.

Following the tradition of this conference series, it was the main focus of ICSMA 15 to promote and strengthen the fundamental understanding of the basic processes that govern the strength of materials. Nonetheless, it was the aim to forge links between basic research on model materials and applied research on engineering materials of technical importance. Thus, ICSMA 15 provided a forum for the presentation and discussion of research on the mechanical properties of all materials which are of interest to materials scientists and engineers from many different areas. The topics covered by ICSMA 15 were:

1.Atomistic and microstructural aspects of plastic deformation 2.Atomistic and microstructural aspects of fracture 3.Adhesion and interfacial strength 4.Cyclic deformation and fatigue 5.High temperature deformation and creep 6.Mechanical properties related to phase transformations 7.Large and severe plastic deformation 8.Nano- and microscale phenomena in plasticity and fracture 9.Strength issues in biological systems and biomaterials 10.Mechanical behaviour of glasses and non-crystalline solids 11.Multiscale modelling and experimental validation 12.Insight through new experimental methods 13.Other new developments related to the field

While there was large interest in the new topics 7 and 8, contributions to topic 9 were much less than expected.

ICSMA 15 attracted 352 scientists from 30 countries with one fourth of the participants being students. This is a very good ratio showing that we could attract the young generation. There have been 272 oral and 135 poster presentations. It is our pleasure to thank the members of the International ICSMA Committee for their valuable help, especially for proposing and choosing the 18 plenary speakers. 187 papers were submitted for publication in the proceedings, 167 were accepted after reviewing. We would like to express our thanks to all referees for their efficient and prompt efforts. We acknowledge particularly support from the German Research Society (DFG), the Saxon Ministry for Science and Art and the City of Dresden. We are also grateful for industrial support from PLANSEE Metall GmbH, Goodfellow GmbH, MTS Systems GMB, Nagoya University and IOP Publishing. Finally we thank all members of the Local Organizing Committee, Intercom Dresden and Conwerk / Laboratory Ten for the excellent organization of ICSMA 15 and the very pleasant collaboration.

During the conference the International ICSMA Committee decided to convene the next conference in Bangalore, India, in 2012. We wish the organizers of ICSMA 16 great success and look forward to meeting you in Bangalore.

Werner Skrotzki (Technische Universität Dresden) Carl-Georg Oertel (Technische Universität Dresden) Horst Biermann (Bergakademie Technische Universität Freiberg) Martin Heilmaier (Technische Universität Darmstadt) Guest Editors Dresden, June 23, 2009 (* Corresponding author; e-mail address: werner.skrotzki@physik.tu-dresden.de)

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The deformation mechanisms involving ordinary dislocations are studied at various temperatures, in single-phased and lamellar TiAl alloys. In situ straining and heating experiments are performed in the transmission electron microscope (TEM) as well as postmortem studies of deformed samples. It is shown that the dislocations can move by glide or climb depending on the temperature range and that some dynamic strain aging occurs at intermediates temperatures. In lamellar alloys, the crossing of interfaces is found to play a determining role.

In this work, we carry out a detailed study, by in-situ Transmission Electron Microscopy (in-situ TEM), focused on two single-crystals of Cu-Al-Ni shape memory alloys with different transformation temperatures. The first single crystal is in beta phase at Room Temperature (RT) and has been cycled under stress, by super-elastic effect, inside the TEM. Two different mechanisms for the nucleation of β'₃ and γ'₃ martensite phases were observed: a) Martensite can nucleate on dislocations during super-elastic tests and when withdrawing the stress, the reverse transformation takes place by the disappearance of the martensite plate on the dislocation. b) During mechanical cycling martensite plates nucleate in other plates. The second single crystal is in martensite phase at RT, and when the stress is applied different mechanisms are observed: a) Reorientation and interface motion of the plates under the external applied stress, b) nucleation of mobile dislocations inside the martensite. A quantitative analysis of the experimental results, having into account the images and the diffraction patterns, has been realized and different mechanisms have been proposed to explain the experimental results.

Large densities of crystalline defects are created in severely deformed metallic alloys, pushing systems far from the thermodynamic equilibrium. Thus, strain induced phase transformations may occur and affect the mechanical properties like in the well known white etching layer on the surface of rail tracks. Here we briefly report about different phenomena reported in the literature such as solid state amorphization, super saturated solid solutions, precipitate dissolution and disordering. Phase separation and precipitation in nanostructured metallic alloys by severe plastic deformation prior to aging are also discussed.

The mechanical behaviour of NiTi shape memory alloys superficially resembles that of certain biomaterials, such as bones or tissues: By virtue of a reversible martensitic phase transformation, NiTi alloys can recover relatively large strains; uniaxial stress-strain curves exhibit constant stress-plateaus (at several hundreds of MPa, depending on alloy composition and testing temperature) associated with the phase transition. These novel functional properties, in combination with high mechanical strength in ultra-fine grained NiTi and good biocompatibility, are utilized in various implants and medical devices. Yet – and quite similar to hierarchically structured biomaterials – the deformation behaviour of NiTi is intricately linked to distinct deformation processes on several length scales, and there remain significant gaps in our understanding of the microstructure-property relations. In the present paper, recent experimental and theoretical results from first-principles calculations, micromechanical modelling and nanoindentation are discussed with a focus on the role of inelastic deformation processes, twin boundaries and the interaction of plastic deformation and stress-induced phase transformations. These novel findings challenge our understanding of the fundamental mechanical properties of NiTi. They highlight the importance of inelastic deformation mechanisms for the overall mechanical properties and strength of NiTi.

The deformation behaviour of the aluminium alloy AA6016 at low temperatures was investigated in the initial state and after 4 and 8 cycles of accumulative roll bonding (ARB). Tensile tests at 25 K, 77 K, 180 K and 296 K were performed at constant strain rate 10⁻⁴ s⁻¹. Stress relaxation experiments performed during the tensile tests were used to determine the experimental rate sensitivity λ as a function of the flow stress σ. In all cases λ (σ) is found to be linear revealing that the Cottrell-Stokes law holds. The effect of grain size on σ can be adequately described through an additive athermal stress contribution σ_d, which is the higher the higher the degree of pre-deformation is. Moreover, the temperature dependence of the strain rate sensitivity m(T) indicates that the rate controlling mechanism in the initial state is local single slip. The ARB states deviate from the single slip behaviour already at 25 K. The reason probably is the occurrence of additional thermally activated slip processes in the ARB states.

The three dimensional features of the intermetallic microstructure that develop across the thickness of a 1 mm thick casting has been studied using SEM and dual beam FIB. The intermetallics form a closely interconnected spatial network which resembles a scaffold or micro-truss structure near the casting surface. The degree of interconnection decreases, and the overall scale of the microstructure is coarser at the core of the casting. The contribution of this pseudo micro-truss structure to the overall strength of the casting is discussed.

Deformation bands (DMB) are crystal slabs that develop slip on systems different from neighbours; they rotate in different directions developing intervening transition boundaries (TB) that rise rapidly in misorientation. In polycrystals, grains divide into DMB, slipping on 2 or 3 systems to provide the 5 required components with minimum energy, as confirmed through OIM boundary misorientations and band rotation poles. TEM exposed the subgrains but seldom TB, except in single crystals as layers of cells. As the TB extend and align into layer bands, the microtextures are similar in cold and hot working. In hot working, TB are quite narrow and able to migrate; at large strains, lengthening and rotating TB (like grain boundaries) are responsible for rapid accumulation of high angle facets of many subgrains. Nevertheless sub-boundaries persist defining steady-state cellular dimensions and flow stress.

Due to a lower symmetry of the tetragonal C11_b structure when compared with the cubic BCC lattice, the 1/2<331] dislocation cores are split asymmetrically contrary to the 1/2<111> BCC dislocations. This has essential impact on their behaviour and, consequently, on mechanical properties. Various types of dislocation dissociations are analyzed in the frame of anisotropic elasticity with the help of the data from ab initio calculations of γ-surfaces for generalized stacking faults.

An analysis of moving dissociated dislocations subjected to both a quasi-static viscous friction force and a periodic lattice resistance is performed: Metastable configurations are expected, the range of the dissociation distances being asymmetrically distributed with respect to the equilibrium dissociation distance. The data of earlier investigations on the influence of high external stress on the dissociation of dislocations in silicon and germanium, as determined by transmission electron microscopy, are re-examined. It is shown that in most cases the range of observed dissociation widths is consistent with the predictions of the model

Statistics of acoustic emission accompanying plastic deformation and of stress serrations caused by the Portevin-Le Chatelier effect are studied during tension of an Al3%Mg alloy at room temperature. Power-law distributions of acoustic emission reflecting self-organization of dislocations and intermittency of plastic flow are found, irrespective of the strain rate, both before and after the critical strain for the onset of the serrated flow. In contrast, several regimes including both power-law and peaked distributions are observed at the macroscopic scale of stress serrations, depending on the applied strain rate.

Large-scale molecular dynamics simulations have been widely used to investigate the mechanical behaviour of materials. But complex datasets, involving the positions of millions of atoms, generated during the simulations make quantitative data analysis quite a challenge. This paper presents a novel method to determine not only dislocations in the crystal, but also to quantify their Burgers vectors. This is achieved by combining geometrical methods to determine the atoms lying in the dislocations cores, like for example the common neighbour analysis or the bond angle analysis, with the slip vector analysis. The first methods are used to filter out the atoms lying in undisturbed regions of the crystal; the latter method yields the relative slip of the remaining atoms and thus indicates the Burgers vector of those atoms lying in the dislocation cores. The validity of the method is demonstrated here on a single edge dislocation in a relatively small sample. Furthermore a way will be sketched how this analysis can be used to determine densities of statistically stored and geometrically necessary dislocations, respectively. Hence, this method can be expected to provide valuable input for strain gradient plasticity models.

In situ TEM straining experiments were carried out in pure Fe, to investigate the origin of the discontinuity observed at 250K in the temperature variation of the deformation activation parameters. The results show that the motion of screw dislocations is steady at 300K, in agreement with a kink-pair mechanism, but jerky at 110K. This change has been attributed to a transition from a kink-pair mechanism to a locking-unlocking mechanism, similar to that observed previously in Ti.

A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have high Peierls barriers has been developed. It is justified that the impurity kinetics during atmosphere formation includes two stages. The initial stage leads to a negative strain rate and positive temperature dependence of the yield stress of the material in some range.

Al-matrix material composites were produced using hot isostatic pressing technique, starting with pure Al and icosahedral (i) Al-Cu-Fe powders. Depending on the processing temperature, the final reinforcement particles are either still of the initial i-phase or transformed into the tetragonal ω-Al0_0.70Cu_0.20Fe_0.10 crystalline phase. Compression tests performed in the temperature range 293K − 823K on the two types of composite, i.e. Al/i and Al/ω, indicate that the flow stress of both composites is strongly temperature dependent and exhibit distinct regimes with increasing temperature. Differences exist between the two composites, in particul ar in yield stress values. In the low temperatureregime (T ≤ 570K), the yield stress of the Al/ω composite is nearly 75% higher than that of the Al/i composite, while for T > 570K both composites exhibit similar yield stress values. The results are interpreted in terms of load transfer contribution between the matrix and the reinforcement particles and elementary dislocation mechanisms in the Al matrix.

The temperature dependences of the critical resolved shear stress (CRSS) governed by the Peierls mechanism in pure NaCl type crystals, those in pure bcc transition metals, those by dissociated dislocations in covalent crystals of the diamond and the zinc blende structures and those by perfect dislocations at low temperatures in zinc blende crystals have been demonstrated to be roughly scalable with respect to the non-dimensional normalization of the CRSS by the shear modulus G and the temperature by Gb³/k_B, where b is the strength of the Burgers vector and k_B the Boltzmann constant. Furthermore, CRSS vs. T relations have been shown to be scaled universally by normalizing respectively the CRSS by the estimated Peierls stress τ_p and the temperature by the kink-pair energy parameter of (τ_p/G)^1/2(bd)^3/2G/k_B, where d is the period of the Peierls potential.

Ultrafine grained (UFG) metals have remarkably high strength in comparison with that of coarse grained metals. However, the UFG metals often exhibit low tensile ductility. It has been reported that the UFG metals having bimodal grain-size distributions consisting of coarse grains and fine grains perform large elongation keeping high strength. However, the exact deformation mechanism of the bimodal UFG metals has not yet been clarified. Deformation behaviors of a bimodal UFG metal were studied in the present paper. A sheet of Cu-30%Zn was highly deformed by accumulative roll-bonding (ARB) process and then annealed. The annealed specimen showed a fully-recrystallized UFG structure with a bimodal grain size distribution. To clarify deformation behaviors of the bimodal metal, an identical region was observed by the electron backscattered diffraction (EBSD) method at various tensile strains. The grain average misorientation (GAM) increased with increasing the tensile strain. The increase in GAM of the coarse grains was larger than that of the fine grains, suggesting the difference in local deformation and strain-hardening behaviors between different grain sizes. The enhanced uniform elongation of the bimodal UFG specimen was considered to be associated with the difference in strain-hardening between the coarse grains and fine grains.

The present work focuses on the transformation of high-purity Ni powder blends of controlled volume fractions (40 and 60 %) of nanometre-sized (100 nm) and micrometre-sized (544 nm) particles into bulk samples as part of a strategy for producing ultrafine-grained materials usefully exhibiting both strength and ductility. The process involved cold isostatic pressing at 1.5 GPa and sintering. The resulting bulk samples had relative densities near 95 %, were texture-free, and exhibited two different grain size distributions with an average value of 600 ± 30 nm. The mechanical properties were investigated by compression and microhardness tests, both at room temperature, and compared to the behaviour of a sample processed from micrometre-sized powder only. Samples prepared from the blends exhibited high yield stresses of 440 and 550 MPa after compression, and they did sustain work hardening. Tests conducted before and after compression up to 50 % deformation showed the same relative amount of hardness increase around 20 %, which was three times lower than that of the monolithic sample for which a decrease of the average grain size close to 26 % was measured.

Compression testing of micropillars was used to investigate the gain boundary effect on the strength of metals which is especially interesting in ultra fine grained and nanocrystalline metals. Single and bicrystal micropillars of different sizes and crystallographic orientations were fabricated using a focused ion beam system and the compression test was performed with a nanoindenter. A reduction of the pillar size as well as the introduction of a grain boundary results in an increase in the yield strength. The results show that the size and the orientation of different adjoining crystals in bicrystalline pillars have an obvious effect on dislocation nucleation and multiplication.

A study of strain localization under compression of <111> Hadfield steel single crystals at room temperature was done by light and transmission electron microscopy. At <1%, macro shear bands (MSB) form that have non-crystallographic and complex non-linear habit planes and are the results of the interaction of dislocation slip on conjugate slip planes. Mechanical twinning was experimentally found inside the MSB. After the stage of MSBs formation, deformation develops with high strain hardening coefficient and corresponds to interaction of slip and twinning inside as well as outside the MSBs.

Stress relaxation tests along stress-strain curves of two magnesium alloys AJ51 and AE42 are used for determining the activation volume at different test temperatures. The applied stress has been separated into its effective stress and internal stress components. The variations of the activation volumes with the effective stress are examined.

Tensile tests on a Fe22Mn0.6C steel at room temperature and different strain rates show serrations on the curves similar to Portevin-Le Chatelier (PLC) serrations of type A, associated with negative strain rate sensitivity. Propagation of deformation bands have been observed by high-rate extensometry over more than two orders of magnitude of the applied strain rate. This constitutes a remarkable difference with the PLC effect which shows a transition to static bands (type B or C) when the applied strain rate decreases. In this steel, bands moving as slow as a few tenth of mm/s are observed instead of static bands, which is two orders of magnitude lower than what is reported for type A PLC bands. This emphasises a strong correlation between plastic events, also confirmed by multifractal analysis of the tensile curves. Twinning which is responsible of the high strain hardening rate of this steel at room temperature is discussed as one of mechanisms of correlation between instabilities.

Plastic deformation behavior of pure titanium has been investigated using a tensile test at a various range of strain rates. Flow stress curves reveal that the work hardening behavior depends on the strain rates. The higher strain rates, the more work hardening. Dislocation cell structure has been developed after the deformation at a low strain rate. At a high strain rate, in addition to that, microstructure of twinning is well evolved. Much occurrence of twinning at the high strain rate leads the pronounced work hardening.

Different morphologies of α+β microstructures were obtained in a commercial Ti-6Al-4V alloy by cooling at different rates from the single β-phase region into the two phase region. The effect of such morphologies on mechanical properties was studied using hot compression tests in a Gleeble thermomechanical simulator. A variety of complex morphologies could be obtained since the cooling rate has a significant influence on the β to α phase transformation and the resulting morphological development. While most of the β phase transformed to colonies of α at high cooling rates, it was possible to obtain a complex mixture of a colonies, grain boundary a and lamellar structure by decreasing the cooling rate. These complex morphologies each exhibited distinctive mechanical properties and characteristic dynamic phase transformation behaviour during deformation as a function of strain rate.

A fine precipitation of spherical vanadium carbides is obtained in a Fe22Mn0.6C base steel during the final recrystallisation heat treatment. Precipitates formed in recrystallised grains have a cube-cube orientation relation with the matrix, confirmed by Moiré patterns observed in TEM. The theoretical size for loss of coherency is below the nm, much lower than the precipitates' size. Deformation contrasts were observed around the precipitates and their residual coherency was measured. It was shown to decrease when the carbides' size increases, to vanish above 30 nm. The net increase of the yield stress was estimated to be 140 MPa. Precipitation hardening by vanadium carbides do not alter the strain hardening rate by TWIP effect, as they do not seem to act as obstacles for the propagation of microtwins.

The texture and local orientation after compression of a NiMnGa polycrystal and a bicrystal with 5M modulated structure were measured with synchrotron radiation and electron backscatter diffraction (EBSD), respectively. Compression of both samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. EBSD directly confirms the motion of twin boundaries within individual grains.

Ge single crystals are deformed in compression at 850K and the same strain rate to various extents of strains. In each sample, the internal stress is measured through stress reduction tests and the dislocation densities by X-ray measurements. Data about these two parameters follow fairly well the Taylor-Saada relation, provided a correction term is added. It probably corresponds to dislocations which are seen by X-rays, though they do not contribute to crystal hardening.

Hard oriented NiAl single crystals (<100> deformation axis) have been compressed at temperatures between 296K and 963K. At about 600K the yield stress changes from a plateau to a steep fall. Slip line investigations show that this coincides with a transition from {112}<111> to {110}<110> slip. At low temperatures slip of <111> dislocations is determined by the Peierls mechanism while in the plateau region the deformation mechanism is still not understood. For the deformation by {110}<110> slip the activation enthalpy measured supports the deformation mechanism proposed by Mills et al. [1] based on the diffusion-assisted motion of macro-kinks in <110> edge dislocations which are found to be decomposed into <100> edge dislocations. The slip transition temperature is discussed with regard to the brittle-to-ductile transition temperature of polycrystalline NiAl.

The ultrafine grained aluminium alloy AA6016 produced by accumulative roll bonding (ARB) up to eight cycles was deformed in tension at 25K, 77K, 180K and 296K at a constant strain rate. The stress-strain curves were analyzed with regard to the evolution of yield stress σ_p, ultimate stress σ_m and fracture strain ef as a function of temperature for zero, four and eight ARB cycles processed aluminium plates. For all materials σ_p, σ_m, σ_m−σ_p and _f decrease with increasing temperature. With increasing number of ARB cycles, σ_p and σ_m increase, while σ_m-σ_p and _f were found to decrease. Based on the experimental results the influence of temperature and number of ARB cycles is discussed.

Single crystals of Ni₃Al, intrinsic Ge, Cu, Cu with 5at%Al and 7.5at%Al are submitted to constant strain rate compression tests and stress reduction experiments. Curves of the initial creep rate as a function of the amount of stress reduction are analysed in terms of a hyperbolic sine relation based on the thermal activation of the dislocation velocity. It is shown that this relation satisfactorily fits the curves of the two first crystals. However, this is not observed for the three last ones. Reasons for this behaviour are proposed as well as a method for the effective stress evaluation.

New steel with fine lamellar structure consisting of austenite and ferrite was developed. 15mass%Mn-3%Al-3%Si steel sheet was used in this study. First of all, the effect of the cooling rate on the microstructure was examined. The cooling at the slower speed of 100 deg/hour created the dual phase structure consisting of both austenite and ferrite. The additional rolling developed the fine lamellar duplex structure. Improvement of both the tensile strength and elongation was achieved by rolling. The strength increases furthermore by the rolling up to larger reduction. The 90% rolled sheet shows high tensile strength around 1000MPa with large elongation (15%–20%). These results indicate that the multi-phased structure with controlled lamellar morphology is beneficial for the management of both high strength and large ductility.

The modified split-cantilever beam is a specimen type applied in fracture mechanics to measure the mode-III fracture properties of composites. In this paper an improved beam model is developed, which is based on the superposition of four different effects. The analytical model is validated by numerical calculations using the finite element software ANSYS and it is demonstrated that the agreement is very good between the analytical and the numerical models. Experimental measurements are also performed on unidirectional glass/polyester composite specimens, and a very good agreement is obtained between the results of the analytical model and the experiments. Apart from the excellent accuracy of the beam model it is shown that the modified split-cantilever beam has an important role in fracture mechanics, because it applies the same specimen geometry as the standard mode-I and mode-II tests and it is suitable to investigate the mode-III fracture properties in a quite extended crack length range.

Dynamic dislocation-defect analysis is derived from precise determination of the activation volume ν and distance d. Upon precipitation of nano-particles of Al₆Fe in nominally pure aluminum, the Haasen plots can reveal their existence and a new activation distance plot can determine their magnitudes for specific obstacles from which particle size can be assessed.

The high-temperature plastic deformation of BaCe_0.95Y_0.05O_3−δ polycrystals with average grain size of 0.50 μm has been studied in compression between 1000 and 1250°C in air at different initial strain rates. The stress-strain curves display yield drop at strains close to 5%, followed by steady state or strain-softening stages. Large ductilities were achieved at the higher temperatures, without appreciable changes in grain shape and size. Mechanical data and microstructural observations are consistent with a flow mechanism by grain boundary sliding.

It is well known that aluminum/gallium couple causes liquid metal embrittlement. Gallium atoms penetrate the grain boundaries of polycrystalline aluminum and degrade it. Polycrystalline aluminum specimens were contacted with a small droplet of gallium for 24 h. After gallium was removed from the surface of the specimens, tensile tests were performed between 77 K and 313 K. The specimens are ductile below 230 K and brittle above 303 K, the melting temperature of gallium. Between 280 K and 300 K, the maximum stress is larger in the specimens heated from 77 K than in those cooled from 313 K. This thermal history dependence of the maximum stress is considered to be attributed to the solidification of supercooled gallium in the grain boundaries.

ISAS/JAXA is now planning to adopt a thruster made of monolithic silicon nitride (SN282 manufactured by Kyocera Co.) onto a Venus exploration probe, PLANET-C, in replacement of conventional niobium heat-resistant alloy. Silicon nitride is still brittle and requires precise analysis on multiaxial thermal stresses induced during firing, though it has high toughness among other structural ceramics. This study evaluated quasi-static fracture characteristics of SN282 considering the surface conditions through compression-torsion biaxial fracture tests as well as the conventional four-point-bending tests. The samples were applied to the mechanical tests either as-ground or after annealing at 1300°C in air for 1 h, which formed an oxidation layer of more than 250nm on the specimen surface. Symmetry four-point-bending tests showed that annealing improves flexure strength and reduce the difference caused by grinding directions. Biaxial stress fracture tests showed the high compressive stress makes the influence of facial crack insensitive.

We studied the process of damages accumulation at tension of low carbon steel specimens with lateral notch. The fractal dimension of microcrack patterns in a plastic zone and angular parameter of microcracks distribution have been estimated. It is established that these parameters change in opposite phase. The analogy to the similar dependences estimated according to seismic activity is drawn.

This study is concerned with crack propagation in a soft steel sheet during drawing. The drawability is considered in relation with the structural anisotropy. Most existing studies on crack propagation are based on the global mechanical properties. However, microstructural inhomogeneity can lead to micro-crack formation. Micro-texture can affect crack propagation and stops in soft steel during drawing. The EBSD technique is used to show that the adjustment of the grain orientation from the initial recrystallization component {111}<112> towards the deformation orientation {111}<110> incites a trans-granular crack inside a {111}<112> grain in a globally ductile material.

In this investigation the fatigue properties of specimens manufactured with different turning parameters were investigated in stress-controlled constant amplitude tests at ambient temperature. The change of feed rate and depth of cut lead to a change in the near surface microstructure. Hence the fatigue properties were influenced significantly due to different surface roughness and surface residual stress resulting from the unequal turning processes. The cyclic deformation behaviour of AMC225xe is characterised by pronounced initial cyclic hardening. Continuous load increase tests allow a reliable estimation of the endurance limit of AMC225xe with one single specimen on the basis of cyclic deformation, temperature and electrical resistance data.

The cyclic stress-strain behaviour of metals and alloys in cyclic saturation can reasonably be described by means of simple multi-component models, such as the model based on a parallel arrangement of elastic-perfectly plastic elements, which was originally proposed by Masing already in 1923. This model concept was applied to thermomechanical fatigue loading of two metallic engineering materials which were found to be rather oppositional with respect to cyclic plastic deformation. One material is an austenitic stainless steel of type AISI304L which shows dynamic strain aging (DSA) and serves as an example for a rather ductile alloy. A dislocation arrangement was found after TMF testing deviating characteristically from the corresponding isothermal microstructures. The second material is a third-generation near-gamma TiAl alloy which is characterized by a very pronounced ductile-to-brittle transition (DBT) within the temperature range of TMF cycling. Isothermal fatigue testing at temperatures below the DBT temperature leads to cyclic hardening, while cyclic softening was found to occur above DBT. The combined effect under TMF leads to a continuously developing mean stress. The experimental observations regarding isothermal and non-isothermal stress-strain behaviour and the correlation to the underlying microstructural processes was used to further develop the TMF multi-composite model in order to accurately predict the TMF stress-strain response by taking the alloy-specific features into account.

Minor addition of B to the Ti-6Al-4V alloy reduces the prior β grain size by more than an order of magnitude. TiB formed in-situ in the process has been noted to decorate the grain boundaries. This microstructural modification influences the mechanical behavior of the Ti-6Al-4V alloy significantly. In this paper, an overview of our current research on tensile properties, fracture toughness as well as notched and un-notched fatigue properties of Ti-6Al-4V-xB with x varying between 0.0 to 0.55 wt.% is presented. A quantitative relationship between the microstructural length scales and the various mechanical properties have been developed. Moreover, the effect of the presence of hard and brittle TiB has also been studied.

Engineering materials often undergo a plastic deformation during manufacturing, hence the effect of a predeformation on the subsequent fatigue behaviour has to be considered. The effect of a prestrain on the microstructure is strongly influenced by the strengthening mechanism. Different mechanisms are relevant in the materials applied in this study: a solid-solution hardened and a precipitation-hardened nickel-base alloy and a martensite-forming metastable austenitic steel. Prehistory effects become very important, when fatigue failure at very high number of cycles (N > 10⁷) is considered, since damage mechanisms occur different to those observed in the range of conventional fatigue limit. With the global strain amplitude being well below the static elastic limit, only inhomogeneously distributed local plastic deformation takes place in the very high cycle fatigue (VHCF) region. The dislocation motion during cyclic loading thus depends on the effective flow stress, which is defined by the global cyclic stress-strain relation and the local stress distribution as a consequence of the interaction between dislocations and precipitates, grain boundaries, martensite phases and micro-notches. As a consequence, no significant prehistory effect was observed for the VHCF behaviour of the solid-solution hardening alloy, while the precipitation-hardening alloy shows a perceptible prehistory dependence. In the case of the austenitic steel, strain-hardening and the volume fraction of the deformation-induced martensite dominate the fatigue behaviour.

The evolution of dislocation structures was investigated by means of TEM in Fe-Si alloys with 0, 0.5 and 1.0 mass% Si during a cyclic bending test in conjunction with fatigue crack behavior. The addition of Si increased the fatigue strength. In steel without Si the cell structure develops, whereas in steel with 1%Si the vein structure evolves, which is considered to lead to the increased fatigue strength. The cell structure in 0%Si steel is postulated to be caused by the easy cross slip of dislocations, whereas the vein structure in the steels with Si is inferred to be caused by the difficulty in cross slip presumably due to the decrease in stacking fault energy. Furthermore, the steel containing Si shows a dislocation free zone (DFZ) along grain boundaries. A transgranular fracture takes place in 0%Si steel, while in 1%Si steel many intergranular cracks were observed just beneath the top surface, which was thought to be caused by the fact that a) strains are dispersed within grains owing to the vein structure and b) micro cracks are initiated and propagated along a DFZ.

The low-cycle fatigue behaviour of copper and a-brass CuZn30 was investigated in uniaxial and biaxial tests. Planar biaxial fatigue tests were carried out using cruciform samples with proportional stain paths with and without phase shift between the two axes. Microcharacterisation was performed by electron microscopy as well as by high-resolution X-ray line profile analysis. The biaxial cyclic stress-strain curves show good agreement with the uniaxial ones using the von Mises equivalent strain hypothesis. The dislocation densities and microhardness values of the biaxial case, however, show significantly lower values compared to the uniaxial case at equivalent von Mises stresses.

A simple mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack-tip opening displacement (ΔCTOD) and the crack growth increment (da/dN). The objective of this work is to compare analytical estimates of ΔCTOD with results of numerical calculations under large scale yielding conditions and to verify the physical basis of the model by comparing the predicted and the measured evolution of the crack length in a 10%-chromium-steel. The material is described by a rate independent cyclic plasticity model with power-law hardening and Masing behavior. During the tension-going part of the cycle, nodes at the crack-tip are released such that the crack growth increment corresponds approximately to the crack-tip opening. The finite element analysis performed in ABAQUS is continued for so many cycles until a stabilized value of ΔCTOD is reached. The analytical model contains an interpolation formula for the J-integral, which is generalized to account for cyclic loading and crack closure. Both simulated and estimated ΔCTOD are reasonably consistent. The predicted crack length evolution is found to be in good agreement with the behavior of microcracks observed in a 10%-chromium steel.

At the Institute of Materials Science and Engineering at the University of Kaiserslautern an ultrasonic testing system for the fatigue assessment of metallic materials in the very high cycle fatigue (VHCF) regime was developed. The ultrasonic testing system allows to control the test and to measure detailed fatigue data. The achieved results can be used to describe the cyclic deformation behaviour of wheel steels at ultrasonic frequencies. In load increase tests (LIT), the critical stress amplitude can be determined, which leads to a defined change of process parameters like generator power, dissipated energy and specimen temperature. With SEM investigations it was proved that the change of the process parameters correlates with irreversible changes in the microstructure. It can be shown that the stress amplitude, leading to first irreversible changes in the microstructure, strongly depends on the depth position within the original wheel rim. New and basic results on the fatigue mechanisms of high strength steels in the VHCF-regime can be achieved.

Austenitic-ferritic stainless steel has been cycled in symmetrical push-pull regime with constant strain rate and different strain amplitudes. Hysteresis loops were recorded and analysed. The plot of the second derivative of the half-loop vs. relative strain shows one peak for low amplitude straining corresponding to the cyclic deformation of softer phase, austenite. With increasing strain amplitude second peak appears and corresponds to the cyclic deformation of the harder phase, ferrite. Transmission electron microscopy study revealed parallel bands in austenite and initial virgin dislocation structure in ferrite at low amplitude while cell structure in austenite and secondary wall structure in ferrite at high amplitude.

The cyclic deformation and fatigue behavior of the γ-TiAl alloy TNB-V5 is studied under thermo-mechanical load for the three technically important microstructures Fully-Lamellar (FL), Near-Gamma (NG) and Duplex (DP), respectively. Thus, thermo-mechanical fatigue (TMF) tests were carried out with different temperature-strain cycles, different temperature ranges from 400°C to 800°C and with two different strain ranges. Cyclic deformation curves, stress-strain hysteresis loops and fatigue lives are presented. The type of microstructure shows a surprisingly small influence on the cyclic deformation and fatigue behavior under TMF conditions. For a general life prediction the damage parameter of Smith, Watson and Topper P_SWT is well suitable, if the testing and the application temperature ranges, respectively, include temperatures above the ductile-brittle transition temperature (approx. 750°C). If the maximum temperature is below that temperature, the brittle materials' behavior yields a high scatter of fatigue lives and a low slope of the fatigue life curve and therefore the damage parameter P_SWT cannot be applied for the live prediction.

The present paper deals with the in-situ observation of local fatigue damage processes by using a conventional servohydraulic testing machine and a new developed piezo-driven miniature testing system in the scanning electron microscope in combination with electron backscatter diffraction to correlate crack initiation and propagation with local microstructural features. In the case of metastable austenitic 304L steel it is shown and supported by finite element calculations that fatigue-induced strain localization causes the formation of α' martensite. Martensite nuclei lying parallel to the slip planes in the austenite grains and twin boundaries can be considered as crack-initiation sites. Fatigue crack propagation causes an increasing martensite transformation rate ahead of the crack tip leading to the occurrence of crack closure effects. As shown in an earlier study, crack closure in the case of short cracks follows a transient regime, i.e. immediately after crack initiation the crack stays open almost during the complete fatigue cycles. The development of a short crack model based on the boundary element method aims to the prediction of crack initiation and short fatigue crack propagation rates.

Cyclic deformation characteristics of a recently developed AM30 Mg extrusion alloy in the transverse direction were evaluated under strain-controlled tests at different strain amplitudes. The alloy exhibited strong cyclic hardening especially at higher strain amplitudes. While the initial tensile Young's modulus was essentially the same in both transverse and longitudinal directions, the hysteresis loops were asymmetric in the longitudinal direction, but nearly symmetric in the transverse direction. This tension-compression asymmetry was associated with the presence of strong texture in the extruded Mg alloy. With increasing strain amplitude the mid-life hysteresis loops showed a clockwise rotation which was related to nonlinear or pseudoelastic deformation behavior. Fatigue crack initiation occurred at the specimen surface, and multiple initiation sites were observed at higher strain amplitudes. Crack propagation was basically characterized by the formation of characteristic fatigue striations.

Al-Mg-Sc alloy polycrystals bearing Al₃Sc particles with different sizes, i.e. 4, 6 and 11 nm in diameter, have been cyclically deformed at 423 K under constant plastic-strain amplitudes, and the microstructural evolution has been investigated in relation to the stress-strain response. Cyclic softening after initial hardening is found in specimens with small particles of 4 and 6 nm, but no cyclic softening takes place in specimens with larger particles of 11 nm. These features of cyclic deformation behavior are similar to the results previously obtained at room temperature. Transmission electron microscopy observations reveal that dislocations are uniformly distributed under all applied strain amplitudes in the specimens containing large particles of 11 nm, whereas slip bands are formed in the cyclically softened specimens bearing smaller particles. The cyclic softening is explained by a loss of particle strength through particle shearing within strongly strained slip bands. The 6 and 11 nm Al₃Sc particles have a stronger retardation effect on the formation of fatigue-induced stable dislocation structure than 4 nm particles at 423 K.

It is known that hydrogen embrittlement occurs in austenitic stainless steels like type 304 because of the strain induced martensitic transformation. Rotary bending fatigue tests of type 310S stainless steel with hydrogen cathodic charging were carried out to investigate the effect of hydrogen charging on the austenitic phase. High dissolved hydrogen decreases the fatigue strength. It also enhances the plasticity and induces _H martensitic transformation. Hydrogen may promote crack initiation in slip bands.

Surface state plays a major role in the crack nucleation process of pure metals in the High-Cycle-Fatigue (HCF) as well as in the Ultra-High-Cycle-Fatigue (UHCF) regime. Therefore, in studies dealing with HCF or UHCF, special attention is paid to the evolution of surface degradation during fatigue life. The accelerating structures of the future Compact Linear Collider (CLIC) under study at CERN will be submitted to a high number of thermal-mechanical fatigue cycles, arising from Radio Frequency (RF) induced eddy currents, causing local superficial cyclic heating. The number of cycles during the foreseen lifetime of CLIC reaches 2×10¹¹. Fatigue may limit the lifetime of CLIC structures. In order to assess the effects of superficial fatigue, specific tests are defined and performed on polycrystalline Oxygen Free Electronic (OFE) grade Copper, a candidate material for the structures.

Surface degradation depends on the orientation of near-surface grains. Copper samples thermally fatigued in two different fatigue experiments, pulsed laser and pulsed RF-heating, underwent postmortem Electron Backscattered Diffraction measurements. Samples fatigued by pulsed laser show the same trend in the orientation-fatigue damage behavior as samples fatigued by pulsed RF-heating. It is clearly observed that surface grains, oriented [1 1 1] with respect to the surface, show significantly more damage than surface grains oriented [1 0 0].

Results arising from a third fatigue experiment, the ultrasound (US) swinger, are compared to the results of the mentioned experiments. The US swinger is an uniaxial mechanical fatigue test enabling to apply within several days a total number of cycles representative of the life of the CLIC structures, thanks to a high repetition rate of 24 kHz. For comparison, laser fatigue experiments have much lower repetition rates. The dependence of surface degradation on grain orientation of samples tested by the US swinger was monitored during the fatigue life. Results are presented and compared to the ones arising from the two other test methods.

To analyse interactions between single steps of process chains, variations in material properties, especially the microstructure and the resulting mechanical properties, specimens with tension screw geometry were manufactured with five process chains. The different process chains as well as their parameters influence the near surface condition and consequently the fatigue behaviour in a characteristic manner. The cyclic deformation behaviour of these specimens can be benchmarked equivalently with conventional strain measurements as well as with high-precision temperature and electrical resistance measurements. The development of temperature-values provides substantial information on cyclic load dependent changes in the microstructure.

The morphology of specimen surface after fatigue fracture was evaluated in connection with grain orientation distribution and grain boundary microstructure to reveal a mechanism of fatigue fracture in nanocrystalline materials. The electrodeposited and sharply {001} textured Ni -2.0 mass% P alloy with the average grain size of ca. 45 nm and high fractions of low-angle and Σ3 boundaries showed 2 times higher fatigue limit than electrodeposited microcrystalline Ni polycrystal. The surface features of fatigued specimen were classified into two different types of morphologies characterized as brittle fracture at the central area and as ductile fracture at the surrounding area.

Surface relief within persistent slip markings (PSMs) was studied using confocal scanning laser microscopy (CSLM) and atomic force microscopy (AFM) in cast nickel base superalloy Inconel 738LC cyclically strained in strain control at room temperature. Extrusion and intrusion topography and kinetics are documented. The dependence of extrusion height on the number of cycles is obtained. Two regimes of extrusion growth are identified. Average intrusion growth rate is assessed.

Fatigue properties and fatigue crack growth rate were examined in an Al-Mg-Li-Sc-Zr allow subjected to equal channel angular extrusion (ECAE) with rectangular shape of channels up to a total strain of ~4 at a temperature of 325°C followed by solution treatment with subsequent oil quenching with aging. After this processing the fraction recrystallized was ~80pct; the deformed microstructure remains essentially unchanged under solution treatment due to high density of Al₃Sc coherent dispersoids playing a role of effective pinning agents. It was shown that the fatigue limit of this material attained a value of ~185 MPa. Thermomechanical processing provided a decrease in fatigue crack propagation growth rate and an increase in the stress intensity factor, K_1c, in comparison with extruded bar. However, characteristics of crack propagation resistance did not attain values suitable for application of this alloy for critical aircraft components.

Grain size seems to have only a minor influence on the cyclic strain strain curves (CSSCs) of metallic polycrystals of medium to high stacking fault energy (SFE). Many authors therefore tried to deduce the macroscopic CSSCs curves from the single crystals ones. Either crystals oriented for single slip or multiple slip were considered. In addition, a scale transition law should be used (from the grain scale to the macroscopic scale). The Sachs rule (homogeneous stress, single slip) or the Taylor one (homogeneous plastic strain, multiple slip) were usually used. But the predicted macroscopic CSSCs do not generally agree with the experimental data for metals and alloys, presenting various SFE values. In order to avoid the choice of a particular scale transition rule, many finite element (FE) computations are carried out using meshes of polycrystals including more than one hundred grains without texture. This allows the study of the influence of the crystalline constitutive laws on the macroscopic CSSCs. Activation of a secondary slip system in grains oriented for single slip is either allowed or hindered (slip planarity), which affects strongly the macroscopic CSSCs. The more planar the slip, the higher the predicted macroscopic stress amplitudes. If grains oriented for single slip obey slip planarity and two crystalline CSSCs are used (one for single slip grains and one for multiple slip grains), then the predicted macroscopic CSSCs agree well with experimental data provided the SFE is not too low (austenitic steel 316L, copper, nickel, aluminium).

Fatigue properties of the new generation of TiAl alloys with high Nb content are studied. Comparison with the previous alloy with 2at% Nb shows that the new alloy resists better to cyclic deformation at high temperatures. The microstructural observation proved that at 750 °C, the easiest deformation mode is the glide of ordinary dislocations followed by superdislocation glide and twinning. Nevertheless, all three modes seem to be active.

Focused ion beam (FIB) technique together with other advanced microscopic techniques were applied to study early microstructural changes leading to crack initiation in fatigued polycrystals. Dislocation structures of persistent slip bands (PSBs) and surrounding matrix were revealed in the bulk of surface grains by electron channelling contrast imaging (ECCI) technique on the FIB cross-sections. True shape of extrusions, intrusions and the path of initiated fatigue cracks were assessed in three dimensions by serial FIB cross-sectioning (FIB tomography). Advantageous potential of FIB technique and its other possible utilization in fatigue crack initiation studies in polycrystals are highlighted.

High-strength steels typically fail from inclusions. Therefore, to increase the fatigue limit of high-strength steels it is necessary to modify the inclusions and/or the surrounding matrix. The goal must be a higher threshold for crack initiation and/or crack propagation. One possibility to reach this goal seems to be a deep cryogenic treatment which is reported to completely transform the retained austenite as well as to facilitate the formation of fine carbides. Therefore, specimens were annealed before or after deep cryogenic treatment, which was carried out with different cooling and heating rates as well as different soaking times at −196° C. Hardness and retained austenite measurements and fatigue experiments were used to evaluate the different sequences of treatments mentioned above. The fatigue limit increases only after some of the sequences. The results show that the soaking times are not relevant for the fatigue limit but it is very important to temper the specimens before the deep cryogenic treatment. Also, repeated deep cryogenic treatments had a positive influence on the fatigue limit.

Investigations of constant and variable amplitude loading, including a variety of overloads, were carried out experimentally as well as numerically in order to characterize the fatigue crack growth behaviour of nodular cast iron with a ferritic matrix. Under constant cyclic loading the crack growth rate, which depends on the graphite size can be described well by the NASGRO equation. The mean distance between the graphite particles and the shape factor also influence the fracture behaviour. The experimental investigations show that overloads yield to acceleration effects in fatigue crack growth. The calculation of the a-N curves based on different usual life prediction models yields non-conservative results. Therefore, a modified strip yield model is presented which allows the prediction of the crack growth acceleration after overloads in nodular cast iron.

Fatigue experiments were conducted up to N=10⁸ cycles on a duplex stainless steel. The investigations revealed important information: (i) most of the slip markings on {111}-slip planes form during the early stage of fatigue (~10⁴), although many grains stay active in developing more and/or longer slip bands (investigations with SEM/EBSD). It was found that the damage in these grains often has its origin at grain or phase boundaries, growing into the interior of the grains; (ii) in most of the recrystallisation twins slip markings are present in all parts of the grain leading to the assumption that just tilted grain boundaries are not very effective as microstructural barriers. In cases where the twin grain boundary is favorably oriented, it even can promote microcrack initiation; (iii) phase boundaries are most effective in restricting dislocation movement to the softer austenite, since no evidence of growing fatigue damage was found in ferritic grains. These findings lead to the conclusion that for the material studied an endurance limit exists despite occurring plastic deformation. This behaviour is a consequence of the barrier effect induced by phase boundaries against short crack propagation and holds true as long as no inclusions are present.

The creep behaviour of a high Ta containing γ-TiAl alloy was investigated in the temperature range of 700–850°C with applied stresses in order to have rupture times up to 3000 h. Decelerating primary and accelerating tertiary regimes dominated the experimental curves, whilst the secondary regime with constant minimum creep rates was absent. Reporting the experimental data in plot log vs. the tertiary data lie on lines with similar slopes at all temperature and load conditions, indicating that a damage mechanism, depending only on the accumulated creep strain causes the accelerating tertiary regime. Creep tests with step like changes in load and/or temperature changes were run and microstructure investigations were performed through X ray diffraction, scanning and transmission electron microscopy to have an insight on the nature of the damage mechanisms that control the accelerating tertiary regime.

Directionally solidified (DS) alloys of the eutectic systems NiAl-10Mo and NiAl-34Cr (at.%) are potential candidates for high-temperature structural applications. Here, these alloys were first arc-melted and drop-cast. Thereafter, they were directionally solidified (DS) at growth rates of 20 and 80 mm/h while rotating at a fixed rotation speed of 60 revolutions per minute. Specimens of the DS alloys were tested in three-point-bending and uniaxial compression to obtain mechanical properties, including the ductile to brittle transition temperature (DBTT). For the NiAl-Cr system DBTT was found to be around 300 °C. Microstructural observations revealed that in the section perpendicular to the growth direction a uniform distribution of fibres was observed. The expected decrease of the fibre diameter with increasing growth rate was not observed. Instead, the fibre diameter slightly increased with increasing crystal growth rates. First compression tests were performed to get insights into the creep behaviour of these fibre-reinforced microstructures.

A constant-indentation creep rate test (CICRT) has been carried out for an Al-Mg solid-solution alloy using a microindenter in the temperature range of 636–773 K. When a conical indenter is pressed into the specimen surface under a load condition of F=F₀ exp(γt) (F the indentation load, F₀ the initial load, γ the loading rate parameter, t the loading time), the indentation pressure and indentation creep rate approach constant values of p_s and _in(s), respectively. The representative points of the deformation in the underlying material are defined on a contour line of the equivalent stress σ_r = C₁p_s, where C₁ is the so-called constraint coefficient of 1/3 reported by Tabor. The finite element simulation of a power-law material subjected to the CICRT shows that the relationship between the equivalent plastic strain rate _r at these points and _in(s) is _r=C_2in(s) and that C₂ ≊ 1/3.6 in the case of a creep stress exponent of 3.0. The constitutive equation of _r versus σ_r obtained from experimental data and the computed value of C₂ is in good agreement with that evaluated from conventional uniaxial creep tests.

Thermal ageing experiments at 850 and 950°C for 1000 h with and without an applied stress have been applied to INCONEL 617 alloy. The consequences of these treatments characterized in terms of microstructure evolution and the tensile behavior have been studied at room and elevated temperatures. It is shown that thermal ageing strongly reduces the ductility of the alloy at room temperature, whereas the saturation stress measured at elevated temperature is only slightly affected. Applying a small stress of 7MPa during thermal ageing has negligible effects on the tensile behavior.

Cobalt-Rhenium base alloys are being developed for applications at temperatures beyond Ni-base superalloys. The high melting refractory element Re readily dissolves in Co and thereby changes the character of a Co-based alloy to a high melting point material. So far Co-Re system has not been investigated from the point of view of strengthening and oxidation and therefore different possibilities of strengthening mechanisms have been explored for our alloy development. Cr, a common element for oxidation resistance in many systems, is added along with Si in Co-Re alloys to improve oxidation behaviour at high temperatures.

Thermal barrier coatings are used to protect turbine blades from the high temperature of the process gas inside a turbine. They consist of a metallic bond coat and of a ceramic top coat with low thermal conductivity. During service, an additional oxide layer forms between bond coat and top coat that eventually causes failure. Finite element simulations show that the roughness of the interface between top and bond coat is crucial for determining the stress state. Lifetime models have been inferred that assume that cracks form in the peak positions at small oxide thickness and propagate when the oxide layer grows and the stress field shifts. A two-dimensional finite element model of crack propagation in the TBC layer is presented. Since the cracks propagate near a material interface and since plasticity may occur in the bond coat, standard tools of fracture mechanics for predicting the crack propagation direction are difficult to apply. This problem is circumvented in a very simple way by propagating short "test cracks" in different directions and optimising to find the crack direction with the maximum energy release rate. It is shown that the energy release rate and the crack propagation direction are sensitive to the details of the stress state and especially to the creep properties of the materials. Implications for failure models are discussed.

The low density of magnesium alloys makes them attractive for lightweight constructions. However, creep remains an important limitation of Mg alloys. To gain a more detailed understanding of the correlation between microstructure and creep properties in Mg alloys, creep tests have been performed on MRI 230D samples featuring various microstructures. For this purpose, the MRI 230D Mg alloy has been thixomolded into a plate with four steps of different height, which gives different microstructures in each step due to different cooling rates. With an increase in cooling rate (e.g., a decrease in step height) the interconnectivity of the eutectic phase increases at virtually constant volume fraction. The creep strength is found to decrease with decreasing interconnectivity of the eutectic phase. This implies that a eutectic phase morphology, which is highly interconnected, benefits the creep properties and should therefore be one goal in further developments for creep resistant Mg alloys.

In order to study the behavior of the complex failure mechanisms in thermal barrier coatings on turbine blades, a simplified model system is used to reduce the number of system parameters. The artificial system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956) as the substrate with a Y₂O₃ partially stabilized plasma sprayed zircon oxide TBC on top and a TGO between the two layers. A 2-dimensional FEM simulation was developed to calculate the growth stress inside the simplified coating system. The simulation permits the study of failure mechanisms by identifying compression and tension areas which are established by the growth of the oxide layer. This provides an insight into the possible crack paths in the coating and it allows to draw conclusions for optimizing real thermal barrier coating systems.

Structural changes in 9%Cr martensitic steel during creep were examined. The grip section of the crept specimen was characterised by a lath martensite structure, which hardly changed during the test. In contrast, quite different microstructure developed in the necking portion of the specimen. The structural changes were characterized by the evolution of relatively large equiaxed subgrains with remarkably lowered density of interior dislocations at places of initial martensite laths. The development of the well-defined subgrains in the necking portion was accompanied with a coarsening of second phase precipitations. The structural mechanism responsible for the microstructure evolution during creep is considered as a dynamic polygonization.

Tensile properties of the 18Cr-9Ni-W-Nb-V-N austenitic stainless steel were studied at strain rates ranging from 6.7×10⁻⁶ to 1.3×10⁻² s⁻¹ in the temperature interval 20−740°C. It was found that this steel exhibits jerky flow at temperatures ranging from 530 to 680°C and an initial strain rate of 1.3×10⁻³ s⁻¹. This phenomenon was interpreted in terms of Portevin-Le Chatelier (PLC) effect occurring due to dynamic strain aging (DSA). PLC yields significant increase in high temperature strength of this steel due to extending of plateau on temperature dependence of yield strength (YS) and ultimate tensile strength (UTS) to higher temperatures. As a result, YS and UTS remain virtually unchanged with increasing temperature from 350 to 740°C. Role of additives of tungsten and vanadium in DSA and high temperatures strength of the austenitic stainless steel is discussed.

Only hexagonal close-packed (h.c.p.) materials show creep behaviour significantly at ambient temperature or less even below their 0.2% proof stresses with their stress exponents of 3.0 and their apparent activation energies of 20 kJ/mol. Transmission electron microscopy revealed dislocation arrays as a planar slip without any tangled dislocations inside each grain. Atomic force microscopy and electron backscatter diffraction pattern analyses brought about the occurrence of grain boundary sliding. The grain-size exponent was evaluated as 1.0, which means grain boundaries work as the barrier of the dislocation motion. Ambient-temperature creep of h.c.p. materials is schematically illustrated as that lattice dislocations move inside each grain without any obstacles and then pile up at grain boundaries. To continue the creep deformation, these dislocations are absorbed by grain boundaries to accommodate the internal stress and lead to grain boundary sliding.

The deformation behaviour of high-purity aluminium at low temperatures was investigated in order to re-examine Ashby-type deformation mechanism map. All specimens with different purities showed significant creep below room temperature. Under the same stress and temperature, the steady-state creep rate increased with increasing purity of the material. They showed stress exponents around 5.0 and apparent activation energies around 20 kJ/mol at temperatures below about 400 K, and 4.0 and 70–80 kJ/mol at temperatures above that temperature. The grain size had no effect in the low temperature region. From the microstructural observation, secondary slip system was observed. These features imply that pure aluminium deforms in the different mode from the ambient temperature creep of h.c.p. metals which has similar activation energy.

Certification tests on turboshaft engines for helicopters can expose components as high pressure turbine blades to very high temperature during short time periods. To simulate these complex temperature and mechanical stress loadings and to study dimensional and microstructural stability under severe testing conditions, an experimental set-up has been recently developed. In this paper, we first present this new device and describe its performances. Then, the device is used to study the effect of heating procedure on creep results at 1200°C and rafting during primary creep on the single crystal nickel-based superalloy MC2.

The influence of carbon in solid solution on the stress-strain curves of α-iron was investigated using model alloys prepared from high purity iron. Uniaxial compression tests were carried out within the ferritic domain at temperatures between 700 and 880 °C. Oscillating stress-strain curves observed at high temperatures and low strain rates indicate that discontinuous dynamic recrystallization takes place. The macroscopic strain rate sensitivities m and apparent activation energies Q associated with the flow stress are not significantly modified by carbon additions. By contrast, the "mesoscopic" parameters h and r associated with strain hardening and dynamic recovery, respectively, are strongly dependent on the carbon content. Finally, an estimation of the grain boundary mobilities during dynamic recrystallization was carried out from the above rheological data.

The region around the tip of a creep crack obtained using an interrupted test on a C(T) specimen of the 9 wt.% Cr power plant alloy P92 has been examined using electron backscatter diffraction to study the relationship between the crack morphology and the martensitic substructure. Nucleation of voids was found to occur primarily on prior austenite grain and packet boundaries. This agrees well with previous results in which minimum creep rate decreased with increasing prior austenite grain size for small prior austenite grains. Growth and coalescence of voids to form a continuous crack appears to occur by recovery, recrystallisation and eventual rupture of the bridging regions between the voids.

As a simplified model system for thermal barrier coatings (TBCs) in gas turbines, Y₂O₃ partially stabilized ZrO₂ TBCs were applied on FeCrAlY substrates. The creep strength of the substrate was varied by using an oxide-dispersion-strengthened (ODS) alloy and a non-ODS alloy with similar chemical composition. Defined interface profiles were produced before coating. Creep properties of the oxide layer between substrate and TBC were varied by either coating the test pieces with nanocrystalline PVD-alumina or with coarser grained naturally grown Al₂O₃ before plasma spraying the TBC. During thermal cycling (T_max=1050°C, T_min=60°C, dwell of 2 hours at T_max) periodic 2-d interfaces resulted in very low lifetime independent from substrate strength and interface oxide type. With stochastic 3-d interfaces lifetimes up to 900 cycles were reached, especially for the substrate with low creep strength combined with a coarse grained alumina interlayer.

The dislocation structure forming in the single crystal Ni₃Ge alloy during the deformation is studied in this paper. It is established that plastic deformation leads to the uniform strain stability loss in the Ni₃Ge alloy at increasing the temperature. The homogeneous dislocation structure loses its stability in local places, which are situated close to the zone of localization. The internal structure of the localization band of deformation consists of cleaned from dislocations fragments in size about 1μm that are disorientated from each other to 10–60°. Then the polycrystalline local band is formed, in which the plastic deformation reaches hundreds of percents.

The molybdenum alloy TZM (Mo-0.5wt%Ti-0.08wt%Zr) is a commonly used constructional material for high-temperature applications. It is well known that molybdenum and its alloys develop a distinct subgrain structure and texture during hot deformation. These microstructural aspects have a significant effect on strength at elevated temperatures. It was observed that with proceeding primary recrystallization and therefore with disappearance of subgrains the yield strength drops almost to the level of pure molybdenum. The aim of the present work was to investigate and describe the strain hardening of hot deformed TZM on a microstructural basis. For this purpose sintered and prerolled TZM rods were recrystallized and each of them deformed to a specific degree of deformation afterwards. Especially the evolution of disorientation distributions was analyzed by electron backscattering diffraction (EBSD) and used to describe the work hardening effect. The yield strength was determined by tensile tests between room temperature and 1473 K. By analyzing disorientation profiles the formation and evolution of geometrically necessary and incidental dislocation boundaries could be observed. A model developed by Pantleon was used to describe the work hardening of TZM.

During in situ investigation of the distribution of lattice parameters within a superalloy with a rafted microstructure by Three Crystal Diffractometry, both the γ and γ' peak positions and shapes are shown to change. While the peak positions can be used to measure the average stresses and strains within both phases, the changes in shape are related to these in the dislocation distribution within the material. It is shown that the density and order of the dislocation array at the γ/γ' interface and within the γ' phase give different contributions to the peak width, and that the widening of both peaks under high stresses is due to an increase of the dislocation density within the rafts.

The mechanical behavior of a coarse-grained (100 μm) nickel-base alloy nichrome (Ni-20%Cr) was studied in compression at temperatures ranging from 150 to 1000°C. It was shown that in the temperature interval of 300-600°C this alloy demonstrates the following features of mechanical behavior: i) positive temperature dependence of yield stress; ii) jerky flow associated with the Portevin-Le Chatelier (PLC) effect; 3) very high value (115 MPa) of "threshold" stress at 650°C. These features of mechanical behavior can be related to short-range ordering (SRO). It was shown by differential scanning calorimetry that SRO takes place in this temperature range, causing PLC effect and positive temperature dependence of yield stress. In addition, SRO has persistency effect on yield stress and creep resistance.

The work hardening behaviors of 4 steels deformed under dynamic recrystallization (DRX) conditions were analysed. The h and r parameters necessary to describe the work hardening or dynamic recovery (DRV) curves in the absence of DRX were determined from the experimental flow curves. The fractional softening X attributable to DRX, defined as the difference between the calculated DRV and experimental DRX curves, was used to derive the Avrami kinetics of DRX. Some conclusions are drawn about the effects of deformation temperature, strain rate and steel composition on the kinetics of DRX.

This study describes the tensile test results of AISI type 304 deformed at room and elevated temperatures. It has been known that the normal distribution fits well with the strength of typical structural materials. The Weibull distribution has many characteristics in the reliability design. In this paper, we showed that the Weibull distribution can be used to describe the scattering of strength data as well as normal distribution by testing goodness of fit. It is found that the parameters of Weibull distribution have decreasing tendency for temperature rise and the coefficient of variation has increasing tendency for temperature rise.

Compared to Ti-rich γ-TiAl-based alloys Al-rich Ti-Al alloys offer an additional reduction of in density and a better oxidation resistance which are both due to the increased Al content. Polycrystalline material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical, scanning and transmission electron microscopy and XRD analyses. The high temperature creep of two binary alloys, namely Al₆₀Ti₄₀ and Al₆₂Ti₃₈ was comparatively assessed with compression tests at constant true stress in a temperature range between 1173 and 1323 K in air. The alloys were tested in the cast condition (containing various amounts of the metastable phases Al₅Ti₃ and h-Al₂Ti) and after annealing at 1223 K for 200 h which produced (thermodynamically stable) lamellar γ-TiAl + r-Al₂Ti microstructures. In general, already the as-cast alloys exhibit a reasonable creep resistance at 1173 K. Compared with Al₆₀Ti₄₀, both, the as-cast and the annealed Al₆₂Ti₃₈ alloy exhibit better creep resistance up to 1323 K which can be rationalized by the reduced lamella spacing. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded apparent activation energies for creep of Q = 430 kJ/mol for the annealed Al₆₀Ti₄₀ alloy and of Q = 383 kJ/mol for the annealed Al₆₂Ti₃₈ material. The latter coincides well with that of Al diffusion in γ-TiAl, whereas the former can be rationalized by the instability of the microstructure containing metastable phases.

The effect of Cr content on the thermal stability of tempered laths (or elongated subgrains) and precipitates has been studied during long-term aging at 650 °C in three P122 grade steels with increasing Cr content from 9 to 10.5 and 12%. Addition of Cr accelerates the coarsening of subgrains during long-term aging. The number fraction of MX precipitates does change up to 10⁴ h aging in 9% and 10.5% Cr steels, whereas it decreases significantly in 12% Cr steel due to the formation of Z phase. The coarsening rate of M₂₃C₆ precipitates, mostly located on the subgrain boundaries, increases from 9 to 12% Cr and steel containing 9% Cr has the highest number density of M₂₃C₆ after 10⁴ h aging. The addition of Cr from 9 to 12% accelerates the coarsening rate of Laves phase particles during aging. As a result, 9% Cr steel shows the most stable tempered lath martesitic structure during long-term aging.

Initial transient stage in low-stress creep experiments was observed in all such experiments. Recently, evidences were presented that this stage cannot be considered as a normal creep primary stage, though the shape of the creep curve is similar. The strain reached during this so called pre-primary stage is fully recoverable upon unloading; the internal stresses must play important role in the effect. Model of standard linear anelastic solid was modified by introduction of creeping body instead of viscous dashpot. Both power law and hyperbolic sine creep law were used to fit observed creep curves of model and structural materials. Mainly the model using hyeprbolic sine creep law provides good fit to individual creep curves and sets of creep curves at different stresses.

Mo-Si-B materials consisting of a Mo(Si) solid solution and the intermetallic phases Mo₃Si and Mo₅SiB₂ (T2) were prepared by mechanical alloying (MA) as the crucial step of a powdermetallurgical process. After consolidation via an industrial processing route (cold isostatic pressing, sintering, hot isostatic pressing) the resulting microstructures of Mo-Si-B alloys up to 45% of intermetallic phases reveal a continuous α-Mo matrix with embedded, homogeneously distributed intermetallic particles. Clearly, increasing the amount of Mo solid solution reduces the BDTT (demonstrated by three point bending tests between room temperature and 1200°C), however, values below 900°C could not be obtained due to grain boundary embrittlement caused by Si segregation. Alloying with Zr was proven by Auger analysis in Mo-Si solid solutions to reduce this segregation. Therefore, in a second trial Zr as a (micro-) alloying element was added. The influence of microalloying on ductility and strength is comparatively discussed with reference compositions Mo-6Si-5B and Mo-9Si-8B.

The effect of niobium additions up to 2.36 wt% on the creep behavior of a series of seven extra low carbon 18Cr-12Ni austenitic stainless steels at 700°C has been investigated. Grain size and hardness measurements, hot tensile tests and constant stress creep tests from 90 to 180 MPa were carried out for each alloy, in the solution treated condition at 1050, 1200 and 1300°C followed by quench in water. The mechanical behavior at high temperature was related to the amount of NbC precipitation occurring during the tests. Solid solution and intermetallic compound effects were also considered. Creep data analysis was done to determine the parameters of the creep power-law equation = A.σⁿ and the Monkman-Grant relation .t^m_R = K. Niobium-carbide precipitation in these steels reduces the secondary stage dependence of strain rate with applied stress, resulting in n-values which indicate the possibility of operation of various creep mechanisms. The creep strength during the secondary stage is primarily controlled by the amount of NbC available for precipitation. However, the rupture times increase progressively with niobium content, as the amount of undissolved carbide particles in grain boundaries and the Laves phase precipitation increase.

A method of creep life prediction by means of Strain-Acceleration-Parameter (SAP), α, is presented. The authors show that the shape of creep curve can be characterized by SAP that reflects magnitude of strain-rate change in secondary creep. The SAP-values, α are evaluated on magnesium-aluminium solution hardened alloys. Reconstruction of creep curves by combinations of SAP and minimum-creep rates are successfully performed, and the curves reasonably agree with experiments. The advantage of the proposed method is that the required parameters evaluated from individual creep curves are directly connected with the minimum creep rate. The predicted times-to-failure agree well with that obtained by experiments, and possibility of precise life time prediction by SAP is pronounced.

Nanocrystalline Fe-12Cr-2W (wt.%) and oxide dispersion strengthened (ODS) Fe-12Cr-2W-0.25 Y₂O₃ (wt. %) alloy powders were produced through mechanical alloying in a high energy planetary ball mill. Microhardness studies on individual milled powder particles revealed a clear increase of hardness for the Yttria containing alloy, thus, proving the ODS effect to be operative in the powders. Consolidation was carried out in a uniaxial hot press at 900°C with a pressure of 200 MPa. TEM and XRD analyses consistently revealed the nanocrystalline grain size before (12 to 30 nm) and after consolidation (120 nm), respectively. Compression creep studies in a temperature range between 800 and 1000°C revealed that diffusional creep is obviously suppressed in these materials. However, the creep resistance of the ODS ferritic steel is only marginally higher than its base alloy and both alloys are by far less creep resistant than that of a ferritic steel of comparable baseline composition strengthened by complex and exceptionally thermally stable Ti-Y-O nanoclusters [1]. Both differences can be rationalized by the instability of the microstructure leading to significant particle and grain growth after creep testing.

An evolution of microstructure has been investigated with increasing compression creep strain in intermetallic alloys Ti-48Al-2Cr-2Nb-1B (alloy Nb2) and Ti-46Al-7Nb-0.6Cr-0.2Ni-0.1Si (alloy Nb7) loaded to 350 MPa at 973 K. Scanning electron microscopy and transmission electron microscopy analysis focused on individual deformation modes and on the phase stability of the alloys. Dislocation densities and deformation twinning characteristics were systematically evaluated for strains up to 0.38. Results of these quantitative studies showed that the evolution of total and non-zero <c>-component dislocation densities with strain was similar for all the investigated materials in spite of relevant differences in the initial γ/α₂ microstructures. However, the spacing between deformation twins seemed to scale with the creep strength of the two alloys. These results are discussed in terms of the fundamental deformation modes and their contributions to the strain accumulation kinetics during high temperature creep in TiAl-base intermetallics with a different Nb content.

Single crystals of Magnesium have been deformed in plane strain compression from room temperature to 450 °C. The deformed crystals have been analysed by EBSD to identify the operative twin variants and parent rotations for comparison with the expected systems. In parallel with these experiments, we have developed a crystal plasticity model based on Taylor principles and the Schmid law, to compute the activated systems together with the twin reorientations and the lattice rotations for large strains. The CRSS values are determined by an iterative procedure that best correlates the experimental data with the numerical simulations for each orientation. From the measured stress/strain data the CRSS for twinning and some slip systems are given over this wide temperature range.

Compression tests of an undercooled aluminium alloy Al-0.6Mg-0.7Si have been performed in a quenching and deformation dilatometer. Samples have been solution annealed and quenched in the dilatometer with varying quenching rates (0.1 K/min to 1000 K/min) and varying quenching temperatures. Immediately after quenching, compression tests on quenching temperature have been performed in the dilatometer. The results have been correlated with the precipitation behaviour of the undercooled aluminium alloy Al-0.6Mg-0.7Si. Stress-strain-curves of quenching rates higher than the critical cooling rate for precipitation differ from those of quenching rates lower than the critical cooling rate. Further, stress-strain-curves of temperatures above and during precipitation differ from those of temperatures below precipitation.

In this study, the contributions of the various strengthening components following the application of cold work and precipitation in AA6111 has been evaluated and correlated by means of tensile testing and transmission electron microscopy (TEM). The results show a considerable improvement in yield and tensile strength with increasing level of cold work. The component of strength developed from cold work and precipitation respectively increases with increasing level of cold work. The recovery strength (softening) also increases with increasing level of cold work. TEM showed a strong interaction of strengthening precipitates with dislocations. The density of dislocation tangles is shown to increase with increasing degree of cold work.

The addition of Si and Ti to low carbon steel allows to obtain a nanometric precipitation of an ordered intermetallic phase, Fe₂SiTi, with a volume fraction of the order of 6%; which is much higher than usual for microalloyed steels and more comparable to the situation encountered in aluminum alloys. The resulting precipitation hardening leads to extremely high mechanical properties, depending on the optimization of metallurgical route, which makes these materials very promising for the development of innovative, weight saving automotive solutions. In this contribution the precipitation sequence and kinetics are studied in details by a combination of experimental techniques including small angle neutron scattering and scanning or transmission electron microscopy. Some properties (yield stress, strain hardening, strain to fracture) are discussed in view of the measured precipitate characteristics, notably their size and volume fraction. The fracture mechanism, and particularly the occurrence of brittle fracture, is discussed in view of the state of precipitation.

3rd generation Al-Li-Cu alloys such as AA2198 are known to present extremely high strength levels because of the precipitation state of the material. The final precipitation state is reached via a complex sequence that can involve notably solute clusters, GP zones, θ', T₁ and δ' phases. In this work, we have studied the evolution of mechanical properties during heat treatment. Microhardness and tensile tests have shown a direct link with the evolution of the size of intra-granular T1 precipitates. The evolution of the fracture mode in relation with the precipitation state has been followed by the so-called short bar test that allows to load relatively thin plates in the short transverse direction, which is particularly sensitive to the inter-granular fracture mode. Along the heat treatment, from the naturally aged T351 state and along ageing at 155°C, we show a clear transition from a ductile transgranular fracture mode to a ductile inter-granular fracture mode, controlled by the precipitation of the T₁ phase at the grain boundaries.

The development of stresses and distortions in solidifying metallic components during casting processes is not only influenced by the temperature field created across the mould but can also be strongly influenced by the transformation kinetics occurring in the solidifying metal. It is challenging to apply numerical methods which are strictly based on a coupled thermo-mechanical model because the models do not account for the inhomogeneous distribution of alloying elements, particularly carbon. Therefore a viable model has to take into account some preselected difference by heated states using short-time austenitizing, which includes a carbon gradient in order to understand the influence of carbon on thermomechanical properties. The aim of the present work is to identify high temperature parameters for a lamellar cast iron GJL-350 in order to include them into a material model. The transformation kinetics for non-isothermal transformations are determined from measured isothermal TTT-diagrams at differently heated states. Doing so, it was also possible to determine the isothermal TTT-diagrams as a function of the carbon content in the matrix.

Metastable austenitic steels show excellent mechanical properties, such as high strength combined with excellent ductility and toughness due to martensitic transformation under mechanical loading (transformation induced plasticity effect). A good energy consumption, and, in the case of high-alloyed metastable austenitic steels, a high corrosion resistance, increase the potential of these materials for diverse applications, also in regard of safety requirements. Up to now, numerous wrought alloys were investigated concerning mechanical behaviour, TRIP-effect, martensitic transformation behaviour and modelling of transformation kinetics or stress-strain behaviour. New high alloyed cast CrMnNi-steels, developed at Technical University Bergakademie Freiberg, provide the chance to reduce processing steps, production time and costs. In order to understand the influence of temperature on the martensitic phase transformation behaviour and therefore on mechanical properties and failure, the mechanical response under tensile loading in a temperature range between -70°C and 200°C was investigated. The mechanical behaviour under compressive loading was also examined in a wide range of strain rates between 10⁻⁴ s⁻¹ and 10³ s⁻¹ to obtain information about the strain rate effect on stress-strain behaviour and microstructural changes.

A novel steel-based composite material, composed of metastable austenitic stainless steel as matrix and up to 15 % zirconia as reinforcement, is processed by two powder metallurgy routes. The matrix exhibits the so-called TRIP-effect (TRIP: TRansformation-Induced Plasticity) and shows a deformation-induced formation of martensite. Compression tests of rod samples processed by cold isostatic pressing show increased strength compared to the non-reinforced steel matrix up to 20 % strain. Three-point bending tests show, however, reduced ductility for high zirconia contents. Filigree honeycomb structures were produced by a novel extrusion technique with extraordinary high values of specific energy absorption.

The effect of tempering temperature on mechanical properties of an E911+3%Co creep resistant steel was investigated. The mechanical tensile tests were carried out at temperatures from 298 to 1073 K and at strain rates varying from 2.1 × 10⁻⁵ s⁻¹ to 2.1 × 10⁻¹s⁻¹. The Portevin-Le Chatelier (PLC) effect was found in the temperature range of 473 to 623 K. Various attributes of dynamic strain aging (DSA) like serrated flow with an acoustic emission were observed. With increasing temperature the ultimate tensile strength (UTS) and the yield strength (YS) increased while the ductility decreased. The dependences of the critical plastic strain on strain rate and temperature exhibited "inverse" behavior that was associated with concentrated solid solution in the DSA regime.

Stress-strain behavior of lamellar Ti-38Al-3Zr and -3Nb alloys with average lamellar thickness ranging from 10 to 1000 nm were studied at room temperature. Their yield stresses decrease from a high value of coherent lamellar structures to a low value after introducing misfit dislocations onto lamellar boundaries. Their strain hardening rates increase with decreasing lamellar thickness, and then drop to a low level when the misfit dislocations become absent. There is critical thickness of γ lamellae for introduction of misfit dislocations. The thickness decreases with increasing lattice misfit between the constituent phases.

The effects of addition of 0.17wt%Zr, 0.1wt%Mg and 0.1wt%Co on the mechanical properties of a Cu-1.2wt%Ni-0.2wt%Be alloy have been investigated. Adding Zr, Co or Mg to the Cu-Ni-Be alloy brings about the improvement in strength and stress relaxation property. The Zr, Co or Mg addition decreases the inter-precipitate spacing of γ" precipitates, resulting in the increase in strength. The higher resistance to the stress relaxation of the Zr- or Co-added alloy is attributed to the lower density of mobile dislocations. The improvement of stress relaxation property by the Mg addition is explained by the viscous glide motion of dislocations dragging Mg atoms, in addition to the lower density of mobile dislocations.

The microstructure-mechanical properties relationship in ultrafine-grained Ti-Fe-Sn alloys with high strength and large plasticity was investigated. The alloys are mainly composed of a hypereutectic microstructure with micrometer-sized primary dendrites embedded in an ultrafine-grained eutectic matrix. The bimodal composites exhibit a fracture strength higher than 2350 MPa and an enhanced plasticity larger than 7%. The excellent mechanical properties are critically related to the microstructure features of the phase constituents in the alloys.

Boron Nitride (BN) films were synthesized onto silicone wafer by depositing B metal vapour under simultaneous irradiation of N ions. Here, film thickness, ion beam energy and transport ratio (B/N) were selected as a preparation parameter and they were controlled in the range of 0.2-1μm, 0.2~2keV and 1~5, respectively. The BN films prepared were characterized using several analytical techniques and their internal stresses were estimated using Stoney's equation. From Fourier transform infrared spectroscopy, it was found that use of low energy N ions is effective for the formation of cubic BN (cBN) phase using ion mixing and vapour deposition (IVD) technique. At this condition, high compressive stress is measured and strong correlations were found among crystal structure, internal stress and Knoop hardness of BN films.

In many ferrous austenitic alloys a strain induced martensitic transformation is important for determining bulk mechanical response. In this work molecular dynamic simulations have been performed in an attempt to further elucidate the mechanisms by which bcc-α' martensite may form at deformation induced hcp- martensite bands. The question of critical nucleus size and fault band arrangement are discussed in relation to the Olson-Cohen model for nucleation at fault band intersections as is the resulting orientation relationship.

Severe plastic deformation (SPD) processes were used in mixing and consolidating aluminum based nanocomposites containing various amounts of C nanoparticles. A planetary ball milling machine was used to mix the powders, and back pressure equal channel angular pressing (BP-ECAP) for consolidation. For comparison, Al-Al₂O₃ nanocomposites were processed by the same processing method. TEM observations showed that the combination of the two SPD processes was effective in dispersing the C nanoparticles in the Al matrix, and better dispersion was achieved by increasing the number of ECAP passes. Especially in Al-5 wt% C after ECAP consolidation for 24 passes, the compressive plastic strain was significantly improved. The Al-Al₂O₃ materials appeared to have better particle distribution and ductility compared to the Al-C materials although their strengths were lower.

Magnesium and their alloys have been extensively studied in recent years, not only because of their potential applications as light-weight engineering materials, but also owing to their biodegradability. Due to their hexagonal close-packed crystallographic structure, cold plastic processing of magnesium alloys is difficult. The preliminary researches carried out by the authors have indicated that the application of the KOBO method, based on the effect of cyclic strain path change, for the deformation of magnesium alloys, provides the possibility of obtaining a fine-grained structure material to be used for further cold plastic processing with large total deformation.

The main purpose of this work is to present research findings concerning a detailed analysis of mechanical properties and changes occurring in the structure of AZ31 alloy wire during the multistage cold drawing process. The appropriate selection of drawing parameters and the application of multistep heat treatment operations enable the deformation of the AZ31 alloy in the cold drawing process with a total draft of about 90%.

In this study Ti-foils were roll bonded together with commercially pure aluminium AA1050. The laminates were produced by using two 1 mm thick AA1050 sheets at the outer side of the stack combined with 100 μm thick Ti-foils as an intermediate layer for each accumulative roll bonding process. The samples were rolled up to 4 ARB cycles. Subsequently the sheets were post-process heat treated at 180°C, 400°C or 600°C, respectively, for 24 hours. The local mechanical behaviour of the Al/Ti intermetallic interfaces have been investigated using nanoindentation experiments. A strong dependence between annealing-temperature, – time and deformation grade is detected. While a heat treatment at 180°C only leads to a weak bonding between Al and Ti with a preservation of the UFG structure, temperatures up to 600°C are causing a complete recrystallisation of the microstructure and formation of diffusion layers with different Al and Ti concentrations.

Cube {100} <001> oriented single crystals of Al 1% Mn were compressed in channel-die. Their lateral faces were covered with transferable carbon grids with a step of 100mm . At a deformation of about 0.3, the vertical bars of the grids show undulations whose characteristic length is of the order of the millimetre and which become sharper and smaller as the deformation proceeds. Fiducial golden grids with a step of 20 mm remain largely unaffected. This shows that the investigated heterogeneity is typical of the mesoscopic scale and has no directly related patterns at the macroscopic and microscopic level. Microfocussed X-rays were used to measure the crystallographic rotations during the process. The investigated spot was a few 0.1 mm². At a deformation of 0.6, the lateral faces of the crystal undergo a split into two Cube orientations each rotated of about 15° around the transverse axis. This is put in relation with the undulations of the bars. At 0.9 an additional rotation around the longitudinal axis appears. The local material rotation and the lattice spin at the mesoscopic scale are interpreted in accordance with previous analyses of the evolution of the Cube texture based on EBSD and the observation of the traces of slip systems.

Interstitial-free steel and OFHC copper were subjected to 8 passes, route B_C room temperature ECAE followed by cold-rolling up to 97.5% thickness reduction. Uniaxial tensile tests and Electron Back-Scattering Diffraction were used to characterise the evolution in mechanical properties, microstructure refinement and micro-texture. IF-steel showed continuous increase in strength whereas Cu returned reduced strength and a small gain in ductility at 97.5% reduction. In both metals substructure refinement was accompanied by an increase in high-angle boundary fraction, average misorientation and a slight increase in Σ3 boundaries. An evolution of crystallographic orientations from negative shear to predominantly cold-rolled textures after 95% and 97.5% reduction was observed in both metals.

A commercial purity aluminum was highly deformed by the accumulative roll-bonding (ARB) process and subsequently annealed. The specimens having various grain size distributions were obtained. In case of the specimen ARB-processed with lubrication, the specimens with mean grain size larger than 3μm showed continuous yielding. On the other hand, in case of the specimen ARB-processed without lubrication, the specimens with mean grain size larger than 3μm showed discontinuous yielding. It suggests that appearance of the yield-drop phenomena can not be decided by the mean grain size. In order to consider effect of grain size distribution, the volume fraction of grains was summed from coarser grains, and the grain size when the summed volume fraction reached 70%, d_70% was estimated from the grain size distribution. it was found that d_70% of specimens which showed continuous yielding were larger than 8 μm while the specimens which showed discontinuous yielding were smaller than 6 μm, regardless of the lubrication condition in the ARB process. The result suggests that the appearance of the yield-drop phenomena depend on d_70%.

The mechanical properties of Cu–7m.%Ag–0.05m.%Zr-alloys have been improved by optimised thermo-mechanical treatments. After processing the conductor material shows an ultimate tensile strength of more than 1.1 GPa, a yield strength of about 1 GPa, a plastic strain of 0.7%, and a conductivity of 60 %IACS (at room temperature). The yield strength of a cold deformed Cu – 7m.%Ag – 0.05m.%Zr-alloy has been assessed on the basis of different hardening mechanisms: solid solution, grain boundary, precipitation and dislocation hardening. The critical shear stress which is determined from the experimentally observed critical shear strength by Schmid's law is between a linear and a quadratic superposition of its individual contributions and hence is in good agreement with the theoretical prediction.

In pure Ti, the influence of shear deformation on the α to ω transformation and the development of texture in the ω-phase under high-pressure torsion (HPT) straining were investigated by means of X-ray and neutron diffractions. The fraction of ω-phase increased with strain in the ω-phase state. Bulk submicrocrystalline ω-Ti was fabricated by HPT-straining under the compressive pressure P = 5 GPa with the equivalent strain _eq > 110 at the rotation speed of 3.3 × 10⁻³ rev. per sec. (0.2 rev. per min.) at room temperature. The texture of ω-phase evolved by HPT-straining with the prismatic planes parallel to the shear direction of HPT-straining and the basal planes perpendicular to it.

Deformation behavior of a commercial purity aluminium (99% purity) highly deformed by the accumulative roll-bonding (ARB) process was investigated by analyzing the change in the specimen shape during tensile test precisely. True stress obtained by the specimen shape analysis of the conventionally coarse grained material was found to increase continuously even after necking, which indicated that the necking region was work-hardened. On the other hand, true stress of the ARB-processed Al which showed lamellar boundary structure having the mean lamellar spacing of 220 nm decreased after necking, which was quite different from the result of the coarse grained material. This result suggests that the necking region is not hardened but softened by deformation. The softening is considered to be caused by dynamic recovery at grain boundaries.

An interstitial free ferritic stainless steel was cold worked to a total strain of 4.6. The largely strained steel is characterized by a submicrocrystalline structure consisting of elongated grains/subgrains with the transverse size of about 210 nm; and the fraction of high-angle grain boundaries is about 0.6. Following a rapid rise at an early processing stage, the dislocation density in (sub)grain interiors unusually decreased after total strains of above 2. Nevertheless, the samples are characterized by high residual stresses that result in complex elastic distortions of the crystal lattice within the elongated crystallites. Such internal stresses are shown to be originated from deformation grain boundaries including low-angle subboundaries.

A high purity gold composed of coarse grains (d = 1.7 mm) was repeatedly thermo-mechanically processed by multi directional forging (MDF) at room temperature and annealing at warm temperature range. The effects of small addition of Ca (180 at. ppm) to Au on the static recrystallization (SRX) behavior and ultrafine grain evolution were investigated. Extensive SRX was observed in the MDFed Au sample to cumulative strain of ΣΔ = 1.2 even at 453 K. After 2 cycles of the repeated TMP, fine grains of about d = 10 μm was obtained in the Au sample. The Au-Ca alloy MDFed to ΣΔ = 1.2, however, was hardly SRXed even by annealing at 543 K. Subsequently, the fine-grained Au and Au-Ca alloy were further MDFed to ΣΔ = 8.0 at maximum at room temperature. By the prolonged MDF, average (sub)grain size of about 200 nm was achieved in the Au sample. The hardness increased with increasing cumulative strain. Although the tensile strength was also raised with cumulative strain, large loss of ductility did not appear.

Fe-Ni-Mn martensitic steels are one of the major groups of ultra-high strength steels that have good mechanical properties and ductility in as annealed condition but they suffer from severe inter-granular embitterment after aging. In this paper, the effect of heavy shaped cold rolling and wire drawing on the mechanical properties of Fe-Ni-Mn steel was investigated. This process could provide a large strain deformation in this alloy. The total strain was ~7. Aging behavior and tensile properties of Fe-10Ni-7Mn were studied after aging at 753 K. The results showed that the ultimate tensile strength and ductility after cold rolling, wire drawing and aging increased up to 2540 MPa and 7.1 %, respectively, while the conventional steels show a premature fracture stress of 830 MPa with about zero ductility after aging.

This paper will describe the investigations of a nanostructured (NS) state of nickel based INCONEL^® alloy 718. This structure was generated in bulk semiproducts by severe plastic deformation (SPD) via multiple isothermal forging (MIF) of a coarse-grained alloy. The initial structure consisted of γ-phase grains with disperse precipitations of γ"-phase in the forms of discs, 50-75 nm in diameter and 20 nm in thickness. The MIF generated structures possess a large quantity of non-coherent plates and rounded precipitations of δ-phase, primarily along grain boundaries. In the duplex (γ+δ) structure the grains have high dislocation density and a large number of nonequilibrium boundaries. Investigations to determine mechanical properties of the alloy in a nanostructured state were carried out. Nanocrystalline Inconel 718 (80 nm) possesses a very high room-temperature strength after SPD. Microcrystalline (MC) and NS states of the alloy were subjected to strengthening thermal treatment, and the obtained results were compared in order to determine their mechanical properties at room and elevated temperatures.

The additional cold rolling and the aging process were applied to Cu-0.85Cr-0.07Zr alloy sheets processed by ARB, and mechanical properties and structural information were investigated for the purpose of further improvement of the mechanical properties and the electrical conductivity. From the results of the tensile test and the measurement of electrical conductivity, ARB/aged/CR was most appropriate processing in order to achieve technical advantages. The high tensile strength of 745 MPa and the high electrical conductivity of 68 %IACS were obtained simultaneously. In addition, the improvement of incomplete boundaries generated during ARB processing was possible by thermo-mechanical treatment.

An Al-5%Mg–0.18%Mn–0.2%Sc-0.08%Zr-0.002%Be was subjected to equal-channel angular extrusion up to true strains of ~3 and ~8, that resulted in the formation of partially recrystallized and fully recrystallized structure, respectively. It was shown that the alloy with partially recrystallized structure exhibits highest strength and ductility. The material with fully recrystallized structure showed lowest fatigue crack growth rate and highest value of fracture toughness. Reasons of this unusual effect of microstructure on crack propagation resistance under fatigue are discussed.

The problem of crystal structure refinement under severe plastic deformation has been examined in the framework of the multiscale approach used in strength physics and nonequilibrium thermodynamics. This approach assumes that fragmentation of a solid in the entire hierarchy of the structural-scale levels (nano-, micro-, meso- and macroscale levels) plays a crucial role in the plastic deformation and fracture of materials. Formation of submicrocrystalline or nanocrystalline structure in solids subjected to severe plastic deformation is associated with fragmentation of the strain-induced band structures in the bending internal stress field. Flowing defect boundaries at the lower scale accomodate fragmentation of any curvature in a highly nonequilibrium material under SPD.

Among technological problems related to manufacturing materials with controlled and localized properties, the effect of a microstructure notch is still unsatisfactorily recognized. Laser modification of selected areas of material structures is one of the most promising technologies in the field. The work presents selected variants of generating the intended microstructure changes in the near-surface areas of steel plates by means of a laser beam. A field of residual stress was generated in the examined steel as a result of phase transformations accompanying the laser treatment. Both the microstructure changes and configuration of residual stresses were identified by means of the optical microscopy, and X-ray diffraction technique. The results, additionally related to crystallographic texture, allowed to characterize the microstructure modifications and to interpret a potential change of mechanical properties. It was found, that the level of the stresses and its range can vary essentially with parameters of the laser treatment, which gives one a chance to control the behaviour of a material in the conditions of mechanical loading, by exploitation of a structure element.

Since fully-dense ultrafine or nanocrystalline bulk materials can be processed, there has been an increasing scientific interest in several plastic deformation (SPD) procedures, particularly in the last decade. Especially the equal-channel angular pressing (ECAP) has widely been investigated due to its ability of producing billets sufficiently large for industrial applications in functional or structural components. The significant strength increase based on grain refinement is typically accompanied by a significant decrease in ductility and toughness. Within this work, a new methodology was applied for combining ECAP with a subsequent high-temperature short-time aging for the 6063 aluminium alloy. An increase in strength, ductility as well as impact toughness regarding its coarse grained counterparts was reached. More precisely, ultimate tensile strength, elongation to failure and impact toughness were increased by 46%, 21% and 40% respectively. This was observed after only one run of ECAP at room temperature in a solid-solution treated condition and an aging at 170° C for 18 minutes. The regular aging time for maximum strength at 170° C is around 6 hours. Longer exposure times lead to recrystallisation and, as for regular aging, it leads to overaging, both causing a decrease of properties. The work demonstrates a strategy for an efficient processing of commercial Al-Mg-Si alloys with outstanding mechanical properties.

The evolution of subsequent yield loci after large plastic strains has been studied for pure iron and steel with ferrite-pearlite structure. Large strains have been applied by torsion of thin walled tubes with different strain paths: monotonic and cyclic incremental torsion. Points on the yield loci have been probed by biaxial torsion-tension or torsion-compression load using a single-probe method. The results show strong influence of the microstructure towards anisotropic hardening. Microstrain values from XRD profile analysis are added and confirm the findings for hardening behaviour.

Our work deals with accumulative roll bonding (ARB) of pure Mg sheet (0.9mm thickness) and of Al5052 sheet (0.5mm). A stacking of Al-Mg-Al was firstly rolled to 50% reduction at 400°C and secondly ARB has been processed up to 3 cycles. In such multilayers as well as highly mixed composites of two-phased system texture development, phase reactions and strain accumulation are of basic interest, which needs a combination of different experimental methods for characterization. The present paper deals with the global texture evolution measured by thermal neutrons to average always over the whole sample thickness, SEM and optical microscopy indicates the macroscopic development of Mg and Al layers. The initial materials show typical and strong basal plane texture of hexagonal Mg (17.9mrd) and a recrystallization texture of cubic Al (8.5mrd). Co-deformation of Al/Mg/Al leads to strong decrease of both textures, whereas Mg has always a much stronger texture than Al5052. ARB processing produces only weak Al-textures. After sandwich-rolling and 1 cycle ARB rotated cube is observed in Al5052, which does not exist after 2 and 3 cycles of ARB.

The method of severe plastic deformation (SPD) is often used to produce bulk ultra-fine grained materials. SPD is a procedure, whereby the structure of the material changes from the initial coarse grained state to ultra-fine grained structure. For these techniques shear deformation as well as the so-called non-monotonic deformation are characteristic. The aim of this paper is to compare different metal-forming techniques with respect to non-montonity of deformation. A quantity and its calculation for the characterization of non-monotonity are introduced. Study of different metal-forming processes (simple shear, ECAP, upsetting) concerning non-monotonity and the comparison of these results are presented.

Upon the ECA pressing at 400°C of the AISI 1045 steel with 0.45% C, the processes of dynamic recovery are developed with the formation of subgrains of ~320 nm in size and isolated grains of submicron size with high-angle boundaries. The fragmentation and partial spheroidization of the cementite lamellae is observed within the pearlite colonies. Deformation in pearlite grains is not sufficient, either to break down the cementite lamellae structure or to produce the refinement of ferrite grains. Annealing of the deformed steel with 0.45%C after ECA pressing causes the perfection of the recovered structure in ferrite and further processes of fragmentation, spheroidization, and coagulation of carbide phase. Annealing at T = 550°C for 5 h leads to almost complete spheroidization of carbides with an average size of ~280 nm and increases to some extent the average ferrite grain (subgrain) size to ~410 nm. The submicron grain-subgrain structure of the steel with 0.45%C causes a significant strengthening (YS = 960 MPa) at a retention of satisfactory elongation (EL = 8%). Subsequent annealing at T = 550°C for 5 h increases ductility to EL = 14% and decreases the yield strength to YS = 745 MPa.

The aluminium alloy AA5754 is used for many technical applications. In this work, the accumulative roll bonding process is applied to this alloy in order to investigate the potential of an ultrafine-grained structure on the mechanical properties of this Al-Mg alloy. Sheets from AA5754 (AlMg3) were successfully processed by accumulative roll bonding in order to obtain an ultrafine-grained microstructure. The ARB process was performed at 230 °C or 250 °C up to 7 or 8 cycles respectively. Thus the grain size decreased from 10 μm (initial state) to approximately 80 nm (ultrafine-grained state, normal direction). The microstructural evolution and the mechanical properties have been investigated by means of scanning electron microscopy, hardness measurements and tensile testing. After one ARB cycle the samples showed an increase in hardness by a factor of almost 2 in comparison to the as-received material. Further processing causes a linear increase of hardness with each additional cycle. Yield strength and tensile strength of the roll bonded specimens are highly increased in comparison to the as-received samples whereas the ductility declined. A considerable increase in ductility is obtained by heat treatment of the ARB specimens at 250 °C, but on the expense of a moderate decreased strength. The deformation behaviour is also influenced by the ultrafine-grained structure. The occurrence of the Portevin-Le Chatelier effect is manifested by serrated stress-strain curves. The amplitude of serrations increases with increasing number of ARB cycles but can be reduced by the appliance of a higher strain rate. Lüders strain only occurs at the as-received, i.e. not strained, samples.

A change of dominant slip system (CDSS) in tensile FCC single crystals, as illustrated by properties of Cu-11at.%Al alloy single crystals, is quantitatively analyzed. The analysis shows that before and after the changeover cumulative shear of the dominant slip system is of the order of a magnitude larger than that of any other active secondary system. It was also found that at the critical condition for the CDSS to occur the rate of work hardening of the previously dominant slip system becomes equal to that one which is taking over.

Titanium and F-138 stainless steel are employed in bone replacement and repair. The former material was ECAP-deformed at room temperature and at 300°C, followed in some cases by cold rolling. The steel was ECAP-deformed at room temperature only. Work-hardening behavior was studied by making use of the Kocks-mecking plots and microstructural evolution was followed by TEM. Conclusions show that for Ti, ECAP combined with cold rolling gives the best strength-ductility combination, whilst room temperature ECAP increases the tensile strength of the steel but caused substantial ductility loss.

High pressure torsion (HPT) deformation method at increased temperature of 400°C with varying strain was applied to refine microstructure of the medium carbon steel (AISI 1045). To investigate the deformation behaviour of the ferrite-pearlite two phase structure the different shear deformation and constant high hydrostatic pressure of 7 GPa were applied. The shear stress evolution during deformation and measurement of the torque were recorded and related to structure development and mechanical strength. In order to characterize microstructure development the transmission electron microscopy (TEM) was accomplished. By execution of the first turn the grain refinement was observed at deformed disc periphery. In the centre of the disc, no matter what strain was introduced, the ferrite grain structure showed moderately deformed features and pearlite colonies were preserved. With further equivalent strain increase, equilibrium between the refinement of coarse phases and new grains restoration processes led to saturation of the grain refinement process. Upon tensile properties testing, the yield strength and ultimate strength increased with increasing equivalent strain (_eq) and only short region of strain hardening period prior failure appeared, regardless the strain applied. A small drop in the hardness across the disc was measured after execution of N= 4 and 6 turns, which may be related to formation of fine grain structure and structure recovery in disc centre.

Recently severe plastic deformation processes have been the essence of metal forming researches to produce ultrafine-grained materials. Twist Extrusion (TE) is one of the most unprecedented methods developed in recent years, but remained in laboratory scales. The main reason for this matter refers to some deficiencies like microstructure heterogeneity occurring after TE. However, employing conventional forming techniques could make TE industrial and, moreover, reduce the bulk structure grain size. Using a conventional forming process such as rolling after twist extrusion has been suggested by this paper. T.E process of Al 8112 samples was carried out using a twisted die with 60° die angle and the samples were processed through rolling subsequently. The results demonstrated that implementation of rolling not only reduced heterogeneity but also decreased the grain size and, consequently, enhanced the bulk strength.

Mg of purity 99.8 wt% was deformed by High Pressure-Torsion at hydrostatic pressures 1 to 4 GPa and RT, up to plastic shear strains of 120. X-ray texture analysis showed up deviations from expected shear texture, which increased with increasing shear strain and hydrostatic pressure. According to TEM and SEM investigations these deviations can be understood in terms of recrystallization. The current paper aimed at the differences of the recrystallization processes which occur during HPT deformation and unloading (dynamic recrystallization, DRX), and those after deformation (static recrystallization, SRX). For this purpose, two sorts of samples were investigated: (i) such being stored at RT immediately after HPT deformation, and (ii) such being stored at 77 K immediately after HPT deformation, and stored at RT for a minimum and constant time needed for preparation. The results show that SRX brings the texture closer to the ideal shear texture and to higher strength values, but to smaller ductilities than DRX does. The mechanical properties can be attributed to changes of texture rather than to those of grain size.

Nanoporous Ni-based superalloy membranes are a new material class. They are fabricated from the two phase γ/γ' base material by thermomechanical processing, followed by selective phase extraction. Compared to other metallic membrane materials, they stand out due to an extremely regular and fine channel-like porosity on the nanoscale. This allows for particularly interesting applications in areas such as particle filtration, catalysis of chemical reactions or heat exchange. However, fundamental understanding of the mechanical behaviour is a prerequisite in all these cases. Thus, the microstructure property correlation of these novel materials is analyzed here, examining a γ'-membrane (where the γ-phase is leached out) by tensile testing. It will be demonstrated that nanoporous superalloy membranes are remarkably strong materials, provided the processing parameters are properly selected.

Although a number of nanoscale metallic materials exhibit interesting mechanical properties the fabrication paths are often complex and difficult to apply to bulk structural materials. However a number of steels which exhibit combinations of plasticity and phase transitions can be deformed to produce ultra high strength levels in the range 1 to 3 GPa. The resultant high stored energy and complex microstructures allow new nanoscale structures to be produced by combinations of recovery and recrystallisation.

The resultant structures exhibit totally new combinations of strength and ductility to be achieved. In specific cases this also enables both the nature of the grain boundary structure and the spatial variation in structure to be controlled. In this presentation both the detailed microstructural features and their relation to the strength, work-hardening capacity and ductility will be discussed for a number of martensitic and austenitic steels.

The deformation resistance of ultrafine-grained (UFG) materials is modelled on the basis of the evolution of the average dislocation density with strain in the course of glide and recovery of dislocations. In contrast to materials with conventional grain size (CG), dislocations are stored and annihilated solely at the high-angle boundaries, where screw dislocations glide and edge dislocations climb towards annihilation sites. The high-angle boundaries enhance both the rates at which dislocations are stored and recovered. Depending on the spacing of high-angle boundaries, temperature and strain rate, UFG materials are softer or harder than their CG counterparts.

An in situ transmission electron microscopy study has been performed on ultrafine-grained (UFG) aluminium during tensile loading and unloading in the microyield regime. The goal was to assess the reasons for the unusually large inelastic backflow that had been observed earlier during unloading on UFG material, as compared to that of conventional grain size material. It was noted that in particular edge dislocations emitted by sources within the grains during loading run back into the dislocation sources and disappear during unloading, explaining at least semi-quantitatively the rather large inelastic backflow in UFG material.

Present paper represents an examination of strain rate response of nanocrystalline (NC) Pd with the aim to shed a light at deformation mechanisms operating in this material with extremely small grain size of 14 nm. Specimens were prepared by inert gas condensation method, and studied using conventional and in situ in SEM compression tests, and instrumented high pressure torsion (HPT). NC Pd demonstrated very high compression strength and good ductility, furthermore, strain rate jump tests revealed high strain rate sensitivity of 0.05, and corresponding activation volume was only 5b³. Microstructure investigation in the gauge sections revealed signatures of various deformation mechanisms: dislocation slip, twinning, cooperative grain boundary sliding and shear banding. These results raise a question about the applicability of kinetic analysis for nanocrystalline materials.

Hadfield steel single crystals have been deformed by high pressure torsion at room temperature (P=5GPa) for 1, 2, 3 revolutions. The resulting microstructure has been studied by means of transmission electron microscopy (TEM) and X-ray analysis. The size of fragments decreases with increasing number of revolutions due to interaction of slip dislocations, microbands and thin twins. As a result of severe plastic deformation, the microhardness of the Hadfield steel has been increased, and a portion of , α' martensite has been found.

Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α₂-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures.

The brittle-to-ductile transition (BDT) in boron, antimony and arsenic doped Cz silicon crystals has been experimentally studied, respectively. The BDT temperatures in antimony and arsenic doped silicon wafers are lower than that in a non-doped wafer while the BDT temperature in a boron doped wafer is almost the same as that in the non-doped wafer. The activation energy was obtained from the strain rate dependence of the BDT temperature. It was found that the values of the activation energy in the antimony and arsenic doped wafers are lower than that in the non-doped and boron doped wafers, indicating that the dislocation velocity in the antimony and arsenic doped silicon is faster than that in the non-doped while the dislocation velocity in the boron doped is the same as that in the non-doped. The effect of increasing in dislocation velocity on the BDT temperature was calculated by two-dimensional discrete dislocation dynamics simulations, indicating that the increasing in dislocation velocity decreases the BDT temperature in silicon single crystals.

3D observations of dislocations at a crack tip were attempted by transmission electron microscopy and computed tomography in order to reveal the 3D structure of dislocations emitted around a crack tip. {011} cracks were introduced into a (001) silicon single crystal wafer by using an indentation method at room temperature. The specimens indented were heated and kept at high temperatures to introduce dislocations from the crack tip. The specimen holder was tilted ±31° by 2° step and dislocation images were taken at every step. The diffraction vector was kept nearly 220 during the tilting operation. The Burgers vectors of the dislocation segments were determined, which included the signs of Burgers vectors. The dislocations observed here were those which accommodate mode II stress intensity around the crack tip. 3D observations using electron tomography reveal these complex crucial processes around the crack tip, which should contribute to understanding the dislocation process improving fracture toughness of crystalline materials.

In the present paper the influence of pre-strain direction on energy balance during deformation of austenitic steel was investigated and the analysis of microscopic phenomena responsible for this influence was performed. The specimens with different pre-strain directions were prepared and the ratio of the stored energy increment to plastic work increment, called energy storage rate, as a function of plastic strain was experimentally determined. At the initial stage of plastic deformation of annealed materials this quantity vs. plastic strain has a maximum. It has been shown that for specimens strained in the same direction as pre-strain the energy storage rate decreases monotonically with deformation while for specimens where strain path was changed, the maximum of the energy storage rate is observed (as in case of annealed material). The study of slip and microstructure evolution at meso- and micro-scales have shown that the change in pre-strain direction leads to the redistribution of internal stresses generated by incompatible slip in neighbouring grains of different orientation. Just after change in strain direction the accommodation of these stresses takes place not only by generation of geometrically necessary dislocations but also by micro-shear banding.

The subject of the present paper is decomposition of energy storage rate into terms related to different mode of deformation. The stored energy is the change in internal energy due to plastic deformation after specimen unloading. Hence, this energy describes the state of the cold-worked material. Whereas, the ratio of the stored energy increment to the appropriate increment of plastic work is the measure of energy conversion process. This ratio is called the energy storage rate. Experimental results show that the energy storage rate is dependent on plastic strain. This dependence is influenced by different microscopic deformation mechanisms.

It has been shown that the energy storage rate can be presented as a sum of particular components. Each of them is related to the separate internal microscopic mechanism. Two of the components are identified. One of them is the storage rate of statistically stored dislocation energy related to uniform deformation. Another one is connected with non-uniform deformation at the grain level. It is the storage rate of the long range stresses energy and geometrically necessary dislocation energy. The maximum of energy storage rate, that appeared at initial stage of plastic deformation is discussed in terms of internal micro-stresses.

Tensile failure in superplastic alumina-base materials shows a close similarity to ductile failure in metallic materials containing voids or hard inclusions. The occurrence of microcracks, which is followed by macroscopic cracking towards final failure, cannot be explained from the brittle propagation of preexistent defects, but from the coalescence of intergranular cavities formed and grown by superplastic deformation. The cavity coalescence is shown to occur as the cavity separation distance decreases to a certain level owing to an increase in the number and size of the cavities.

The evolution of the microstructure during compressive deformation of the biodegradable polymer poly(3-hydroxybutyrate) (P3HB) was investigated in-situ via X-ray diffraction using synchrotron radiation. Flow curves were measured in-situ together with X-ray profiles for several degrees of deformation. The profiles were analysed using Multi-Reflection X-ray Line Profile Analysis (MXPA) adapted by the authors for semicrystalline polymers providing lamella thickness, crystallinity, and the presence and density of dislocations as a function of the deformation. In contrast to previous investigations in α crystallised isotactic polypropylene (α-iPP), P3HB does not exhibit a deformation induced increase of the dislocation density which suggests mechanisms other than dislocations to be involved in plastic deformation of P3HB.

The mechanical properties of bulk ultrafine-grained materials produced by severe plastic deformation can be modified (sometimes enhanced) by a mild annealing treatment which leads, in some cases, to a bimodal grain size distribution, characterized by a good combination of strength and ductility. Bimodal grain size distributions can also evolve during cyclic deformation at rather low homologous temperature. Here, the conditions under which bimodal grain size distributions evolve and how they affect the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed.

In the present study, an electrodeposited nanocrystalline (nc) Ni sample with high strength and superior ductility relative to many other electrodeposited nc-Ni was prepared. The superior ductility in the present nc-Ni sample free of defects was ascribed to mixed grains, the size of which spanned nano- and sub-micro scales at its as-deposited state with a grain size distribution ranged from 5 to 120nm. Obvious dislocation motion happening in coarse-grained polycrystalline was observed in large grains of nc-Ni matrix resulting in a remarkable enhanced ductility without a decrease in the strength. The present nc-Ni with an average grain size of 27.2nm prepared by direct current electrodeposition shows the average ultimate tensile strength of 1200MPa and the average elongation to failure of 10.4%.

Nickel produced by additive-free pulsed electro-deposition with grain size of about 150 nm was deformed in tension at 320 K and 4 K. At both temperatures the specimens exhibited strong parabolic hardening and good ductility. However, at 4 K a sudden transition from homogeneous deformation to "repeated catastrophic shear" took place. Microstructural characterization of the sheared region showed deformed grains elongated in between tensile axis and growth direction. The texture information from the sheared region was obtained using conical dark field measurements in the transmission electron microscope. The pole figures of the deformed 4 K specimen show a typical shear texture which clearly differs from the starting texture.

In ceramics, dopants offer the possibility of higher creep rates by enhancing diffusion. The present study examines the potential for high strain rate superplasticity in a TiO₂ doped zirconia, by conducting creep experiments together with microstructural characterization. It is shown that both pure and doped zirconia exhibit transitions in creep behaviour from Coble diffusion creep with n~1 to an interface controlled process with n~2. Doping with TiO₂ enhances the creep rate by over an order of magnitude. There is evidence of substantial grain boundary sliding, consistent with diffusion creep.

The porous polymer-based biomaterial has been synthesized from PLGA, dioxane and tricalcium phosphate (TCP) by low-temperature deposition process. The deformation behaviours and fracture mechanism of polymer-based biomaterials were investigated using the compression test and the finite element (FE) simulation. The results show that the stress-strain curve of compression process includes linear elastic stage I, platform stage II and densification stage III, and the fracture mechanism can be considered as brittle fracture.

Cu-Zr-Ti metallic glass was subjected to high pressure torsion applying different revolution times (180s, 120s, 60s). Both deformation and deformation rate dependent microstructural and thermal properties were characterized by scanning electron microscopy, X-ray diffraction and calorimetry, respectively. In order to estimate the temperature rise in the metallic glass during high pressure torsion, quasi three-dimensional heat conduction equation with a source term was considered. Solutions indicate that the saturation temperature strongly depends on the revolution time, i. e. on the deformation rate.

Al-based metal matrix composites containing different volume fractions of nanocrystalline Al₇₀Ti₂₀Ni₁₀ reinforcing particles have been produced by powder metallurgy and the effect of the volume fraction of reinforcement on the mechanical properties of the composites has been studied. Room temperature compression tests reveal a considerable improvement of the mechanical properties as compared to pure Aluminum. The compressive strength increases from 155 MPa for pure Al to about 200 and 240 MPa for the samples with 20 and 40 vol.% of reinforcement, respectively, while retaining appreciable plastic deformation with a fracture strain ranging between 43 and 28 %.

Highly dense bulk samples were produced by spark plasma sintering (SPS) through combined devitrification and consolidation of partially amorphous Al₈₅Y₈Ni₅Co₂ gas atomized powders. The microstructure of the consolidated samples shows a mixed structure containing crystalline, ultrafine-grained and amorphous/nanocrystalline particles. The sintered sample exhibits a remarkable high strength of about 1050 MPa combined with 3.7 % fracture strain.

We have fabricated micro-/nano- pillars of a Zr-based metallic glass, Zr₅₀Ti_16.5Cu₁₅Ni_18.5, with pillar tip diameters ranging from ~750 nm to ~110 nm. These pillars were mechanically tested quantitatively in-situ in a Transmission Electron Microscope (TEM). Due to a slight tapering of ~3°, the deformation accommodated with shear bands is driven downwards from the top. However, the real time monitoring of shear bands evolution in-situ in TEM makes it possible to measure the effective load-bearing diameter, i.e., the diameter ahead of the most forefront shear band, which enables the precise measurement of yield strength of these pillars. Consequently, it turns out that the yield strength is essentially size independent and close to the bulk value. Statistical physics description of strength of small size metallic glasses is discussed, and it is concluded that Weibull Statistics is inappropriate in describing strength of metallic glasses at micro and nano length scales.

The influence of initial grain orientation and geometrical restrictions (grain boundaries, sample geometry) on the microstructural evolution was investigated at ambient temperature. Single- and polycrystalline austenitic steel samples, strained under uniaxial tension, show pronounced orientation changes which are strongly dependent on initial orientation. The numerical simulation results from a crystal plasticity FEM model match the experimental ones.

In the article a method of numerical verification of experimental results for magnetorheological elastomer samples (MRE) is presented. The samples were shaped into cylinders with diameter of 8 mm and height of 20 mm with various carbonyl iron volume shares (1,5%, 11,5% and 33%). The diameter of soft ferromagnetic substance particles ranged from 6 to 9 μm. During the experiment, initially bended samples were exposed to the magnetic field with intensity levels at 0,1T, 0,3T, 0,5T, 0,7 and 1T. The reaction of the sample to the field action was measured as a displacement of a specimen. Numerical calculation was carried out with the MSC Patran/Marc computer code. For the purpose of numerical analysis the orthotropic material model with the material properties of magnetorheological elastomer along the iron chains, and of the pure elastomer along other directions, was applied. The material properties were obtained from the experimental tests. During the numerical analysis, the initial mechanical load resulting from cylinder deflection was set. Then, the equivalent external force, that was set on the basis of analytical calculations of intermolecular reaction within iron chains in the specific magnetic field, was put on the bended sample. Correspondence of such numerical model with results of the experiment was verified. Similar results of the experiments and both theoretical and FEM analysis indicates that macroscopic modeling of magnetorheological elastomer mechanical properties as orthotropic material delivers accurate enough description of the material's behavior.

Various distributions of particle size are considered in the present work to study the effect of pinning by a bimodal distribution of particles on grain growth simulation in comparison with that of the real distribution of MnS (30–50 nm) and AlN (5–8 nm) in a Fe3%Si alloy grade HiB. A dividing site technique is used to make easy consideration of different size of particles together without increasing matrix size for calculation. The dissolution of precipitates introduced after the stagnation stage allows grain growth to start again.The dividing sites technique allows the consideration of particle dissolution by gradual vanishing of peripheral layers of particles, in agreement with the experimental evolution.

Two copper samples, pre-deformed in tension to 5% plastic strain, are subjected to an in situ tensile deformation of 1% plastic strain while X-ray peak profiles from individual bulk grains are obtained. One sample is oriented with the in situ tensile axis parallel to the pre-deformation axis, and peak profiles are obtained with the scattering vector parallel to this direction. The profiles show the expected asymmetry explained by the composite model as caused by intra-grain stresses. The other sample is oriented with the in situ tensile axis perpendicular to the pre-deformation axis, and peak profiles are obtained with the scattering vector parallel to the in situ tensile axis. In this case the profiles initially show an opposite asymmetry, but during the in situ deformation the asymmetry reverses sign as the deformation under new loading conditions leads to changes in the intra-grain stresses.

Polycrystalline copper is severely plastically deformed by equal channel angular extrusion to an equivalent strain of 10. Cylindrical ASTM standard tensile specimens machined from the extruded material are deformed in tension until failure. The forming neck develops an anisotropic cross section with its long axis along the original transversal direction. Microstructure and texture are investigated by electron backscatter diffraction in the as-deformed condition and after additional tensile deformation. The texture is dominated by two components close to <110>{112} directly related to the shear deformation in the intersection plane of the channels, weakens with tensile deformation and is finally replaced by a restricted <111>+<100> fibre texture characteristic for tensile deformation in the neck region.

Beside the presentation of interesting parts of the experimental equipment the thermal shock behaviour of two ceramics will be discussed. Specimens of silicon carbide and zirconia were investigated in air and vacuum. The influence of the testing conditions on the strength will be addressed.

A bulge test device has been built, with the aim to perform mechanical tests on membranes with a thickness in the 100 nm to 10 μm ranges, between room temperature and 900 °C.Our set up a differential atmospheric pressure is applied across a membrane, while the deflection is measured using a laser interferometer. We show first results obtained from gold membranes with different thicknesses.

To understand in more detail the behaviour of deformed metallic materials, the knowledge of defects and defect distributions at an atomistic level is important. To investigate the plastic zone in front of a crack tip, the positron annihilation lifetime spectroscopy with a pulsed monoenergetic positron beam of variable energy allows the detection of vacancies, dislocations, vacancy clusters and micro voids in the crack surface near region. Moreover, for a given defect type it is possible to determine its concentration. The positron lifetime measurements in samples of different materials (aluminium, copper) showed different defect profiles for crack surfaces produced by monotonic and cyclic deformation. In addition, this technique was able to characterize the kind of damage (monotonic or cyclic) by analysing the different positron lifetimes.

Nanostructured WC-17Co coatings have been prepared by means of High Velocity Oxy-fuel (HVOF) technique. The wear resistance at the elevated temperature (500°C) of nanostructured coatings was compared using GCr15 steel as counterpart in sliding wear tests. The results show that when at the temperature of 500°C, the wear failure mechanism turns from plastic deformation to fracture resulted from crack propagation and adhesive wear. With the wear going, abrasive wear dominate in the coating, then turns into adhesive wear with changes of microscope.

A relation between the residual strength and the dispersed damage accumulated in a short fiber reinforced polyoximethylene (GFR/POM) samples under tension is found. For that purpose dependencies of damage and residual strength on loading percentage are used. Damage as a function of loading percentage is known for the material under study. To find the dependency of residual strength on loading percentage a subsidiary function is introduced and a method is proposed for determination of the parameters in the dependency on the basis of the experimental data. Both damage and residual strength are measured after unloading samples that have been loaded applying different loading percentages. Damage is the accumulation of new internal surfaces that arise under mechanical loading in the whole volume of the material. They are registered by a new original method of X-ray refraction. The analytical relation between the residual strength and damage accumulated is compared to the experimental results found for the residual strength under different damage degrees.

The effects of an applied tensile stress on the growth rate and morphology of discontinuous precipitation (DP) product have been studied for a Cu-5wt%Ag alloy aged at 300 °C. The DP cell consists of lamellae of the rod-shaped Ag precipitates and solute-depleted Cu matrix. The tensile stress accelerates the growth of DP cells along both the loading direction (LD) and transverse direction (TD) but the cell growth rate along the TD is faster than that along the LD. Transmission electron microscopy has revealed that the tensile stress is apt to produce the Ag precipitates elongated in a <110> direction nearly perpendicular to the LD in a cell, irrespective of the cell growth direction. The observed morphology of the Ag precipitates and the promotion of the cell growth, namely precipitate growth under tension can be understood through the interaction energy between the external stress and the misfit strains of precipitate. The growth of Ag precipitates toward the direction perpendicular to the LD explains the faster cell growth along the TD.

Six types of metal matrix syntactic foams (MMSFs) were produced by pressure infiltration technique. The foams were investigated by upsetting tests at increased (220°C) and at room (25°C) temperature. The parameters were the constituents of the composites and the aspect ratio (height-diameter ratio, H/D) of the specimens. The characteristic properties were: the compressive strength, the fracture strain, the structural stiffness of the foams and the absorbed energy. The strength, the strain and the energy were decreased while the stiffness was increased by increasing the H/D. Increased temperature caused ~25 % drop in the strength and in the stiffness. Macrohardness, depth sensitive and dynamic hardness tests were also performed on MMSF blocks: macrohardness is a structural property and independent from the matrix material. The depth sensitive hardness is sensitive to the deformation capability of the matrix and to a possible change reaction. The dynamic hardnesses of the MMSFs were higher than the hardness of the matrices and this is a microballoon related property.

The environmental and pollution materials emission standards in Europe are going to be more and more strict. In order to keep the standards, a large European automotive manufacturer makes a laser treatment on the cast iron cylinder bores of the V-engine blocks. Samples of laser treated cast iron cylinder bore with lamellar graphite were investigated. Four samples were treated with Nd-YAG laser and Yb-fiber laser sources in three different configurations. Microhardness measurements were made to evaluate the hardness profile of the treated layer. In order to evaluate the microstructure and grain size of the laser treated layer, scanning electron microscopic (SEM) images were taken in cross section with a SEM/focused ion beam (FIB) dual beam electron microscope. The opened graphite area percent were also determined by image analysis method on the surface after laser treatment with a SEM in backscattered electron (BSE) mode, because the outburned graphite holes are the oil reservoirs for lubrication during operating conditions of the engine.

In this study effects of small amount of yttrium addition on grain growth of C.P titanium (single α), Ti-14Mo-3Nb-1.5Zr (single β) and a newly developed Ti-4.5Al-6Nb-2Mo-2Fe (α+β) titanium alloy are investigated. By adding yttrium nano-sized Y₂O₃ particles are formed in 'in-situ' mode. These particles lead to significant suppression of grain growth in single α and β alloys rather than in the α+β alloy. The results are discussed with regard to conventional grain growth and grain boundary pinning models. It is concluded that yttrium is a potential micro alloying element in titanium alloys and could be used as a grain refining agent.