Graphene has received a lot of attention in the past few years for its superior electronic, optical, and thermal properties. Many of these properties depend on the geometrical structure of the flat crystalline membrane, and can be altered by deformation. Deformations can be caused by various factors, including thermal fluctuations, chemical functionalization as well as defects that arise during growth or processing. While there is a large body of modelling work on the electronic properties, the effect of defects and functional groups on morphology and the formation and structure of defects, ripples and wrinkles has not been as well studied or understood in comparison.

This focus section invites papers that describe recent progress in the area of growth modelling, defect formation mechanisms, mechanics of rippling and wrinkling and the role of functional groups on morphology in graphene, the successes and also the fundamental challenges that yet remain. It is the intent o...

We compare behaviour of quasi-two-dimensional aluminium crack tips undergoing mode I loading using orbital-free density functional theory (OFDFT) and the classical embedded atom method (EAM). Low-index crack orientations are compared in the context of the Griffith, Rice and Tadmor–Hai continuum criteria, using values from Kohn–Sham DFT (KSDFT). All orientations are predicted to be ductile, and twinning is expected to occur only in certain orientations of low-dimensional or low-temperature Al. OFDFT and the EAM predict similar values to KSDFT for the relevant properties. In simulations of two crack orientations, the critical stress intensity factor in EAM simulations is close to continuum predictions while crack tips modelled by OFDFT do not exhibit plasticity until loaded at least 13% over the continuum prediction. The EAM and OFDFT give qualitatively similar results for a crack orientation that emits edge dislocations. For a twinning orientation, OFDFT simulations emit partial d...

The application of discrete dislocation (DD) dynamics methods to study materials with realistic yield stresses and realistic cohesive strengths requires new algorithms. Here, limitations of the standard algorithms are discussed, and then new algorithms to overcome these limitations are presented and their successes demonstrated by example. With these new methods, the stability, accuracy and robustness of the DD methodology for the study of deformation and fracture is significantly improved.

In this work, we examine the kink-nucleation process in BCC screw dislocations using atomistic simulation and transition pathway analysis, with a particular focus on the compact core structure. We observe the existence of a threshold stress, which results in an abrupt change in the minimum energy path of the kink-nucleation process, and hence, a discontinuity in the activation energy versus stress for the process. The magnitude of the discontinuity is found to be related to the degree of metastability of an intermediate split-core structure. This feature appears to be a direct consequence of the so-called 'camel-hump' nature of the Peierls potential, which manifests itself in the existence of a metastable, intermediate split-core structure. The effect is observed in a number of empirical EAM potentials, including Fe, Ta, V, Nb and Mo, suggesting a generality to the observations.

Interfaces continue to provide an endless source of stimulating problems. With support from the DOE/BES supported Computational Materials Science Network (CMSN, www.phys.washington.edu/users/cmsn), a group of us studied various aspects of both solid–liquid and solid–solid interfaces.

The papers in this focus section represent a variety of approaches and scales. Since understanding the sensitivity of properties to the choice of interatomic potential is an essential precursor to obtaining useful results with atomistic simulations, Becker and Kramer compared the properties of liquid aluminum predicted by molecular dynamics simulations with experimental measurements (including x-ray diffraction), demonstrating significant differences based only on the choice of interatomic potential. Mendele...

A three-dimensional finite element method is developed to predict the critical conditions necessary to nucleate electromigration or stress induced voids in interconnects. Our approach is to model interfaces using a modified cohesive zone, which permits the interfaces to separate at a critical stress, and also accounts for mass transport along the interfaces. A simple boundary value problem is solved to verify the numerical procedure. The method is then used to simulate the nucleation and initial growth of voids in a realistic interconnect test structure. Predicted void nucleation sites are in good agreement with experimental observations. In addition, modeling analysis is also consistent with qualitative effects of geometric and material parameters such as the line length, cap layer and passivation material on critical times to nucleate voids.

Aluminium–lithium alloys provide a lower density and higher stiffness alternative to other high strength aluminium alloys. However, many Al–Li alloys exhibit a non-traditional failure mechanism called delamination, which refers to the failure of the elongated grain boundary interface. In this investigation, delaminations were observed after cyclic deformation of both uniaxial and torsion experiments. A cyclically stable rate-independent crystal plasticity framework with kinematic hardening was developed to address many experimental trends of stabilized cyclic plasticity. Utilizing this framework, meso-scale grain boundary interface stresses were estimated with uniform deformation and bi-crystal models. These models are computationally amenable to investigate both orientation dependence and the statistical nature of the grain boundary stresses for a given bulk texture and nominal loading. A coupled shear-normal Findley-based damage parameter was formulated to quantitatively charac...

The objective of this paper is to develop further a framework for computationally modeling microstructurally small fatigue crack growth in AA 7075-T651 (Bozek et al 2008 Modelling Simul. Mater. Sci. 16 065007). The focus is on the nucleation event, when a crack extends from within a second-phase particle into a surrounding grain, since this has been observed to be an initiating mechanism for fatigue crack growth in this alloy. It is hypothesized that nucleation can be predicted by computing a non-local nucleation metric near the crack front. The hypothesis is tested by employing a combination of experimentation and finite element modeling in which various slip-based and energy-based nucleation metrics are tested for validity, where each metric is derived from a continuum crystal plasticity formulation. To investigate each metric, a non-local procedure is developed for the calculation of nucleation metrics in the neighborhood of a c...

Computer simulations of tribological phenomena, such as they occur when two solid bodies are slid past one another, or squeezed into each other, have become increasingly more realistic over the last decade. This progress is important because many different scenarios lead to similar behaviour. For example, the number of processes that can lead to stick slip motion is incredibly large. If we want to suppress this irregular motion in a given system, instabilities leading to undesired dynamics must be revealed. These processes, however, cannot be identified with toy models. Of course, realistic simulations demand a heavy computational overhead, which can be alleviated with the help of new methods, such as improved model potentials, mapping schemes from the atomistic scale to continuum descriptions, or even concurrent multi-scale techniques.

This special issue consists of papers dealing with material-specific simulations of friction and contact mechanics. It includes contribut...

Serial sectioning methods continue to produce an abundant amount of image data for quantifying the three-dimensional nature of material microstructures. Here, we discuss a methodology to automate detecting and characterizing eutectic particles taken from serial images of a production turbine blade made of a heat-treated single crystal Ni-based superalloy (PWA 1484). This method includes two important steps for unassisted eutectic particle characterization: automatically identifying a seed point within each particle and segmenting the particle using a region growing algorithm with an automated stop point. Once detected, the segmented eutectic particles are used to calculate microstructural statistics for characterizing and reconstructing statistically representative synthetic microstructures for single crystal Ni-based superalloys. The significance of this work is its ability to automate characterization for analysing the 3D nature of eutectic particles.