Vasily V. Bulatov

Automated identification and indexing of dislocations in crystal interfaces

We present a computational method for identifying partial and interfacial dislocations in atomistic models of crystals with defects. Our automated algorithm is based on a discrete Burgers circuit integral over the elastic displacement field and is not limited to specific lattices or dislocation types. Dislocations in grain boundaries and other interfaces are identified by mapping atomic bonds from the dislocated interface to an ideal template configuration of the coherent interface to reveal incompatible displacements induced by dislocations and to determine their Burgers vectors. In addition, the algorithm generates a continuous line representation of each dislocation segment in the crystal and also identifies dislocation junctions.

INTERATOMIC POTENTIAL FOR SILICON DEFECTS AND DISORDERED PHASES

We develop an empirical potential for silicon which represents a considerable improvement over existing models in describing local bonding for bulk defects and disordered phases. The model consists of two- and three-body interactions with theoretically motivated functional forms that capture chemical and physical trends as explained in a companion paper. The numerical parameters in the functional form are obtained by fitting to a set of ab initio results from quantum-mechanical calculations based on density-functional theory in the local-density approximation, which include various bulk phases and defect structures. We test the potential by applying it to the relaxation of point defects, core properties of partial dislocations and the structure of disordered phases, none of which are included in the fitting procedure. For dislocations, our model makes predictions in excellent agreement with ab initio and tight-binding calculations. It is the only potential known to describe both the 30°- and 90°-partial dislocations in the glide set

Enabling strain hardening simulations with dislocation dynamics

111%. The structural and thermodynamic properties of the liquid and amorphous phases are also in good agreement with experimental and ab initio results. Our potential is capable of simulating a quench directly from the liquid to the amorphous phase, and the resulting amorphous structure is more realistic than with existing empirical preparation methods. These advances in transferability come with no extra computational cost, since force evaluation with our model is faster than with the popular potential of Stillinger-Weber, thus allowing reliable atomistic simulations of very large atomic systems. @S0163-1829~98!04026-0#

Generalized-stacking-fault energy surface and dislocation properties of aluminum

Numerical algorithms for discrete dislocation dynamics simulations are investigated for the purpose of enabling strain hardening simulations of single crystals on massively parallel computers. The algorithms investigated include the /(N) calculation of forces, the equations of motion, time integration, adaptive mesh refinement, the treatment of dislocation core reactions, and the dynamic distribution of work on parallel computers. A simulation integrating all of these algorithmic elements using the Parallel Dislocation Simulator (ParaDiS) code is performed to understand their behavior in concert, and evaluate the overall numerical performance of dislocation dynamics simulations and their ability to accumulate percents of plastic strain.

Dislocation multi-junctions and strain hardening

We have employed the semidiscrete variational generalized Peierls-Nabarro model to study the dislocation properties of aluminum. The generalized-stacking-fault (GSF) energy surface entering the model is calculated by using first-principles density functional theory (DFT) and the embedded-atom method (EAM). Various core properties, including the core width, dissociation behavior, energetics, and Peierls stress for different dislocations have been investigated. The correlation between the core energetics and the Peierls stress with the dislocation character has been explored. Our results reveal a simple relationship between the Peierls stress and the ratio between the core width and the atomic spacing. The dependence of the core properties on the two methods for calculating the GSF energy (DFT vs EAM) has been examined. Although the EAM gives the general trend for various dislocation properties, it fails to predict the correct finer core structure, which in turn can affect the Peierls stress significantly (about one order of magnitude). (c) 2000 The American Physical Society.

Atomic modes of dislocation mobility in silicon

At the microscopic scale, the strength of a crystal derives from the motion, multiplication and interaction of distinctive line defects called dislocations. First proposed theoretically in 1934 (refs 1–3) to explain low magnitudes of crystal strength observed experimentally, the existence of dislocations was confirmed two decades later. Much of the research in dislocation physics has since focused on dislocation interactions and their role in strain hardening, a common phenomenon in which continued deformation increases a crystals strength. The existing theory relates strain hardening to pair-wise dislocation reactions in which two intersecting dislocations form junctions that tie the dislocations together. Here we report that interactions among three dislocations result in the formation of unusual elements of dislocation network topology, termed ‘multi-junctions’. We first predict the existence of multi-junctions using dislocation dynamics and atomistic simulations and then confirm their existence by transmission electron microscopy experiments in single-crystal molybdenum. In large-scale dislocation dynamics simulations, multi-junctions present very strong, nearly indestructible, obstacles to dislocation motion and furnish new sources for dislocation multiplication, thereby playing an essential role in the evolution of dislocation microstructure and strength of deforming crystals. Simulation analyses conclude that multi-junctions are responsible for the strong orientation dependence of strain hardening in body-centred cubic crystals.

Dislocation-Stacking Fault Tetrahedron Interactions in Cu

Abstract Mechanisms of partial dislocation mobility in the {111} glide system of silicon have been studied in full atomistic detail by applying novel effective relaxation and sampling algorithms in conjunction with the Stillinger-Weber empirical interatomic potential and simulation models of up to 90000 atoms. Low-energy pathways are determined for the generation, annihilation and motion of in-core defects of the 30°-partial dislocation, specifically, the individual left and right components of a double-kink, an antiphase defect (APD), and various kink-APD complexes. It is shown that the underlying mechanisms in these defect reactions fall into three distinct categories, characterized by the processes of bond-breaking, bond switching, and bond exchange, respectively. The quantitative results reveal a strong left-right asymmetry in the kinetics of kink propagation and a strong APD-kink binding; these have not been recognized previously and therefore hold implications for further experiments. The present wo...

Chapter 64 – Dislocation Core Effects on Mobility

In copper and other face centered cubic metals, high-energy particle irradiation produces hardening and shear localization. Post-irradiation microstructural examination in Cu reveals that irradiation has produced a high number density of nanometer sized stacking fault tetrahedra. The resultant irradiation hardening and shear localization is commonly attributed to the interaction between stacking fault tetrahedra and mobile dislocations, although the mechanism of this interaction is unknown. In this work, we present results from a molecular dynamics simulation study to characterize the motion and velocity of edge dislocations at high strain rate and the interaction and fate of the moving edge dislocation with slacking fault tetrahedra in Cu using an EAM interatomic potential. The results show that a perfect SFT acts as a hard obstacle for dislocation motion and although the SFT is sheared by the dislocation passage, it remains largely intact. How-ever, our simulations show that an overlapping, truncated SFT is absorbed by the passage of an edge dislocation, resulting in dislocation climb and the formation of a pair of less mobile super-jogs on the dislocation.

Highly optimized empirical potential model of silicon

This chapter discusses the dislocation core effects on mobility. Its purpose is to discuss the atomic structure and interactions in the dislocation core and their effects on dislocation mobility from the standpoint of theoretical concepts, physical models and simulation studies, with due consideration of relevant experimental results. The major emphasis is placed on physical ideas and observations. In fact, technical details of modeling and experiments are only briefly mentioned and only where required for clarification of physics issues or for interpretation of results. The existence of glide and shuffle sets of dislocation positions in diamond cubic semiconductors introduces still more complexity with regard to the role of core mechanisms in dislocation motion. Plasticity behavior of BCC metals is controlled, to a large extent, by the motion of screw dislocations.

A stochastic model for continuum elasto-plastic behavior. I. Numerical approach and strain localization

We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects.