Nano-scale characterisation, deformation and failure mechanisms on enhanced-performance modern steels

This research project aims to experimentally study the nature and fundamental characteristics of microstructure, deformation and failure mechanisms, focused on various and complex nanoscale phases, such as precipitations and paraequilibrium phases, not yet completely explained in modern nanophase steels by state-of-the-art literature. Moreover, this work focuses on the investigation of correlation of the microstructure with its impact on the mechanical properties in laboratory-developed novel nanophase steels, which mainly consist of a ductile ferrite matrix, and are strongly affected by appropriate alloy design and thermomechanical treatment differentiations. The microstructural investigation will be accomplished by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), in conjunction with Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). Mechanical properties evaluation is assessed by hardness tests. The experimental approach has been designed based on the -nano to macro- examination of the materials under investigation to overall understand and explain their nature and properties. Finally, the outcome endeavours to interpret the nanophases’ influence as well as the contribution of alloying elements and thermomechanical and heat treatment routes on the microstructure-properties relationship of these novel nanophase steels.


Introduction
Modern microalloyed steels are designed and developed based on nano-scale configuration to meet their demands.However, the essential explanation of their microstructural phenomena and therefore their impact on deformation and failure mechanisms are limited.Their microstructures may consist of ferrite, perlite, bainite, martensite, retained austenite and their common coexistence, producing various types of modern steels, such as Complex Phases steels (CP steels), High Strength Low Alloyed Steels (HSLA steels) and Transformation Induced Plasticity steels (TRIP steels) [1][2][3].The majority of them also consist of carbides and/or carbonitrides precipitations in micro or nano scale with variable morphology, composition and distribution, and/or multiphase microstructure composed of micro to nano dispersions of bainitic ferrite, carbon-controlled martensite morphologies as well as metastability-related retained austenite.[3][4][5][6][7].This complexity in their microstructures derives from strictly controllable alloy design, as well as complex thermomechanical treatments of multiple paths, which results to advanced and tailored enhanced mechanical properties [1,3,7].The consideration of overall enhanced mechanical properties and failure resistance is the highest and most challenging target of metallurgists and it endeavours to be reached by the suitable formation and understanding the nature of nanoscale precipitations, metastable and paraequilibrium phases in steels, in recent years [5][6][7][8][9][10].On this aspect, NANOMODS Project try to characterise, interpret and explain phenomena of nucleation and growth, phase transformation and volume fraction, size and spatial distribution, and their impact on mechanical behaviour in a great extent, focused on nano-scale.There are two kind of nano-scale microstructural consideration; nanoprecipitation of complex nitrides/carbides/carbonitrides and non-equilibrium phases, such as retained austenite, martensite and bainite.

Experimental procedure
The studied materials are divided to two main categories as abovementioned.The first is the singlephase nanoprecipitated steels, which present essentially soft ferritic matrix strengthening by complex, fine nanoprecipitates and grain size refinement.Two sub-categories are based on the presence or absence of Molybdenum addition in V/Nb-bearing microalloyed steels, that critically influence the precipitation sequences phenomena and grades mechanical response.Both sub-categories were subjected to 80% cold rolling reduction approximately and sub-critical annealing treatments at 650, 675 and 700 o C for various time of isothermal holding.The second is the secondary nanophase-based Si-Mn steels with low carbon, which present ferritic matrix accompanied with secondary para-equilibrium phases in nanoscale, such as metastable retained austenite films and islands, bainitic ferrite and martensite laths produced from different isothermal bainitic temperatures, which altogether influence the metastability, volume fraction and spatial distribution of nanoscale retained austenite and finally the ability to exhibit a trip effect upon deformation.Appropriate metallographic preparation (cutting/grinding/polishing) was accomplished for all studied samples.Hardness evaluation samples were in as-polished condition.TEM thin foils were subjected to thinning and fine polishing by a Gatan model 691 Precision Ion Polishing System (PIPS).XRD samples were chemically polished by aqueous solution of HF 10% vol.mixed with hydrogen peroxide 30% vol.LOM investigation samples were chemically etched by aqueous solution of sodium metabisulphite 10% vol.SEM study samples were chemically etched by Marshall's and Nital 2% solution for the studied categories respectively.Advanced electron microscopy and microanalysis characterisation through SEM (JEOL JSM 6380-LV) and TEM (JEM 2100 HR) coupled with EDS detectors (EDS-INCA X-Sight and X-Max N 100 TLE Silicon Drift Detectors accordingly) was conducted to the obtained microstructures in order to study the influence of different nanophases volume fraction, spatial distribution, morphology and metastability to mechanical properties.Light optical microscopy and X-ray diffraction (Bruker D8focus, CuKa, used at 40 mA/40 kV) employed for initial microstructural analysis.11 hardness Vickers measurements were accomplished via a laboratory hardness tester (HV-50Z) on the surface parallel to the rolling direction of each studied sample and category in order to evaluate the mechanical response changes of the studied samples in conjunction with their microstructural evolution; the selected parameters for the studied categories is 98 N and 196 N for 15 seconds, accordingly.

Results & Discussion
Regarding the single-phase nanoprecipitated steels, it is observed that the increase of temperature and/or time of isothermal holding during a sub-critical annealing process on cold-rolled strips, essentially influence the nanoprecipitation and recrystallisation phenomena (Figs. 1 and 2), and therefore the mechanical properties of these microstructures (Fig. 3).Ultra-fine precipitates mainly detected by TEM are related to precipitation strengthening via dislocation movement impediment, whereas fine and larger nanoprecipitates mainly monitored via SEM contribute to grain size control and refinement via Zener pinning effect, and essentially improves the grain boundary strengthening mechanism of the studied alloys.The phenomena of recrystallization and precipitation are co-dependent on this kind of steels.The selection of annealing temperature determines the kinetics of nucleation, growth and possible coarsening of the precipitates, as well as may delay the recrystallization procedure of the matrix phase.The effect of time of isothermal holding principally determine the quantity of completion of the abovementioned microstructural phenomena.According the temperature ranges of nucleation and growth of precipitates, microstructure may present fine and ultra-fine or combined fine and coarsened nanoprecipitates sequences.However, these ranges and the micro-to-nano phenomena are related to alloying addition elements; Figure 3 present the effect of Mo addition of the abovementioned phenomena.The comparative lower kinetics of Mo-including precipitates delay the completion of precipitation and recrystallisation phenomena and enhance the mechanical and deformation response.Regarding the secondary nanophase-based steels, it is observed that the change of temperature and/or time of isothermal bainitic holding, essentially influence the nanoretention phenomena (Fig. 4-5) and the mechanical properties of these microstructures confirm that (Fig. 6).This effect is based on the tailored selection of bainitic temperature and time, because this define if nanoretention of austenite though carbon partition is sufficient or its will be transformed to martensite without stimulate a trip effect.The described mechanism of this retention phenomenon is supported by the combined interpretation of morphological characteristics of obtained microstructure (nanoscale and film-like austenite), increasing austenite volume fractions alongside with hardness's drop and stabilization, after peak ageing, as a function of isothermal bainitic time.The presence of austenitic populations with different morphologies, distribution and metastability are confirmed through SEM and TEM microstructural analysis, a fact that is mainly governed by the bainitic transformation temperature and secondly by the intercritical annealing parameters.The finest microstructures are obtained through low temperature prolonged isothermal bainitic treatments, for partial completion of the bainitic ferrite transformation, and best spatial distribution and metastability is achieved in intermediate time and temperatures, in which, the nucleation sites are enough to accomplish detailed final spatial distribution of nanoscale austenite, and the carbon diffusion keep up with the bainitic ferrite growth in film-like morphology and hence stabilize a significant volume fraction of high carbon film-like nanoscale retained austenite.

Conclusions
Overall, the influence of nano-scale constituents can be considered significant for the mechanical response of the studied grades.On the one hand, the evolution of precipitation phenomena (finish of precipitation or start of particle growth) play vital role on properties, fact that is detected on hardness evaluation.On the other hand, the growth kinetics of bainitic ferrite laths through a displacive mechanism and diffusional partition of carbon to adjacent nanoscale metastable austenite films to stabilize them or be transformed to high carbon martensite play a vital role on mechanical properties also, fact that the hardness evolution confirms.Both steel categories studied in this research present a soft ferritic matrix and secondary nanophases through alloy design considerations and complex thermomechanical and heat treatments processes.

Figure 1 .
Figure 1.SEM-SEI and BES, as well as BF TEM micrographs of sub-critical annealed samples as a function of Larson-Miller f(T*log(t)) increase, from top to bottom.Different groups of precipitation phenomena are detected.

Figure 2 .
Figure 2. BES-SEM micrograph and the related EDS microanalyses of annealed sample at 675 o C for 21 hours.Characteristics of the nanoprecipitation phenomena and their effect on grain size control are observed.

Figure 3 .
Figure 3.Comparison of hardness evolution of sub-critical annealed samples for 16 hours and 650, 675, as well as 700 o C isothermal holding for Mo added/non-added grades.The comparative lower kinetics of Mo-including precipitates delay the completion of precipitation phenomena and improve mechanical properties.

Figure 4 .
Figure 4. LOM micrographs and XRD patterns of isothermal bainitic transformation samples as a function of Larson-Miller f(T*log(t)) increase, from top to bottom.

Figure 5 .Figure 6 .
Figure 5. SEM-SEI, as well as BF TEM micrographs of isothermal bainitic transformation samples as a function of Larson-Miller f(T*log(t)) increase, from top to bottom.