Dan Mordehai

Introducing dislocation climb by bulk diffusion in discrete dislocation dynamics

We report a method to incorporate dislocation climb controlled by bulk diffusion in a three-dimensional discrete dislocation dynamics (DDD) simulation for fcc metals. In this model we couple the vacancy diffusion theory to the DDD in order to obtain the climb rate of the dislocation segments. The capability of the model to reproduce the motion of climbing dislocations is examined by calculating several test-cases of pure climb-related phenomena and comparing the results with existing analytical predictions and experimental observations. As test-cases, the DDD is used to study the activation of Bardeen–Herring sources upon the application of an external stress or under vacancy supersaturation. Loop shrinkage and expansion due to vacancy emission or absorption is shown to be well described by our model. In particular, the model naturally describes the coarsening of a population of loops having different sizes.

Cross-Split of Dislocations: An Athermal and Rapid Plasticity Mechanism

The pathways by which dislocations, line defects within the lattice structure, overcome microstructural obstacles represent a key aspect in understanding the main mechanisms that control mechanical properties of ductile crystalline materials. While edge dislocations were believed to change their glide plane only by a slow, non-conservative, thermally activated motion, we suggest the existence of a rapid conservative athermal mechanism, by which the arrested edge dislocations split into two other edge dislocations that glide on two different crystallographic planes. This discovered mechanism, for which we coined a term “cross-split of edge dislocations”, is a unique and collective phenomenon, which is triggered by an interaction with another same-sign pre-existing edge dislocation. This mechanism is demonstrated for faceted α-Fe nanoparticles under compression, in which we propose that cross-split of arrested edge dislocations is resulting in a strain burst. The cross-split mechanism provides an efficient pathway for edge dislocations to overcome planar obstacles.

Signature of dislocations and stacking faults of face-centred cubic nanocrystals in coherent X-ray diffraction patterns: a numerical study.

Crystal defects can be identified by their fingerprint in coherent X-ray diffraction patterns. Realistic defects in face-centred cubic nanocrystals are studied numerically, revealing various signatures in diffraction patterns depending on the Miller indices and providing an identification method.

Annealing of Dislocation Loops in Dislocation Dynamics Simulations

We report of 3-dimensional discrete dislocation dynamics (DDD) simulations of dislocation loops coarsening by vacancy bulk difiusion. The calculation is based upon a model which couples the difiusion theory of vacancies to the DDD in order to obtain the climb rate of the dislocation segments. Calculation of isolated loops agrees with experimental observations, i.e. loops shrink or expand, depending on their type and vacancy supersaturation. When an array of dislocation loops of various sizes is considered, and the total number of vacancies in the simulation is maintained constant, the largest dislocations are found to increase in size at the expense of small ones, which disappear in a process known as Ostwald ripening.

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.

A Percolative Deformation Process Between Nanograins Promotes Dynamic Shear Localization

Shear localization is an important failure mechanism in crystalline solid materials. However, while nanograins (NGs) are universally observed in shear bands, their connection with the formation of the bands is not clear yet. Based on molecular dynamics (MD) simulations, we propose that the shear localization evolves as a percolative process of deformation zones between NGs. The presence of NGs is a necessary condition to relieve some of the elastic energy by dislocation emission and absorption. Moreover, the density and spatial arrangement of those grains are of prime importance to accelerate or delay the formation of a strong localization path.

Cross-slip in face-centered cubic metals: a general Escaig stress-dependent activation energy line tension model

Abstract Cross-slip is a dislocation mechanism by which screw dislocations can change their glide plane. This thermally activated mechanism is an important mechanism in plasticity and understanding the energy barrier for cross-slip is essential to construct reliable cross-slip rules in dislocation models. In this work, we employ a line tension model for cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier under Escaig stresses. The analysis shows that the activation energy is proportional to the stacking fault energy, the unstressed dissociation width and a typical length for cross-slip along the dislocation line. Linearisation of the interaction forces between the partial dislocations yields that this typical length is related to the dislocation length that bows towards constriction during cross-slip. We show that the application of Escaig stresses on both the primary and the cross-slip planes varies the typical length for cross-slip and we propose a stress-dependent closed form expression for the activation energy for cross-slip in a large range of stresses. This analysis results in a stress-dependent activation volume, corresponding to the typical volume surrounding the stressed dislocation at constriction. The expression proposed here is shown to be in agreement with previous models, and to capture qualitatively the essentials found in atomistic simulations. The activation energy function can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality.

Shear relaxation behind the shock front in 110 molybdenum – From the atomic scale to continuous dislocation fields

Abstract In this work we study shock-induced plasticity in Mo single crystals, impacted along the 〈1 1 0〉 crystal orientation. In particular, the shear relaxation behind the shock front is quantitatively inspected. Molecular dynamics (MD) simulations are employed to simulate the deformation during shock, followed by post-processing to identify and quantify the dislocation lines nucleated behind the shock front. The information on the dislocation lines is ensemble averaged inside slabs of the simulation box and over different realizations of the MD simulations, from which continuous dislocation fields are extracted using the Discrete-to-Continuous method. The continuous dislocation fields are correlated with the stress and strain fields obtained from the MD simulations. Based on this analysis, we show that the elastic precursor overshoots the shear stress, after which dislocations on a specific group of slip planes are nucleated, slightly lagging behind the elastic front. Consequently, the resolved shear stress is relaxed, but the principal lateral stress increases. The latter leads to an increase in the resolved shear stress on a plane parallel to the shock wave, resulting in an additional retarded front of dislocation nucleation on planes parallel to the shock front. Finally, the two-stage process of plasticity results in an isotropic stress state in the plane parallel to the shock wave. The MD simulation results are employed to calculate the dislocation densities on specific slip planes and the plastic deformation behind the shock, bridging the gap between the information on the atomic scale and the continuum level.

Nucleation‐Controlled Plasticity of Metallic Nanowires and Nanoparticles

Nanowires and nanoparticles are envisioned as important elements of future technology and devices, owing to their unique mechanical properties. Metallic nanowires and nanoparticles demonstrate outstanding size-dependent strength since their deformation is dislocation nucleation-controlled. In this context, the recent experimental and computational studies of nucleation-controlled plasticity are reviewed. The underlying microstructural mechanisms that govern the strength of nanowires and the origin of their stochastic nature are also discussed. Nanoparticles, in which the stress state under compression is nonuniform, exhibit a shape-dependent strength. Perspectives on improved methods to study nucleation-controlled plasticity are discussed, as well the insights gained for microstructural-based design of mechanical properties at the nanoscale.

Cross Slip in Cu - A Molecular Dynamics Study

The annihilations of screw dislocation dipoles via cross-slip in Cu were simulated using constant-temperature constant-stress molecular dynamics. The cross-slip mechanism and annihilation process of flexible dislocations in a large dipole configuration was identified as a dynamic variant of the Friedel-Escaig mechanism. The cross-slip rate was found to exhibit exponential dependence on the temperature, from which the activation enthalpy for the cross-slip process was calculated by the Arrhenius relation.