S. J. Zhou

Nonlinear dynamics and the problem of slip at material interfaces

Abstract The problem of dry friction between two metallic interfaces is discussed from the perspective of large scale molecular dynamics (MD) simulations. For flat interfaces between identical metals, two-dimensional MD simulations using embedded-atom-method potentials for copper have shown a variety of phenomena associated with a velocity weakening of the tangential force at high relative velocities (approaching significant fractions of the transverse sound speed). These include dislocation generation, dislocation motion both parallel and normal to the sliding interface, large plastic deformation, nucleation of microstructure, diffusive coarsening of microstructure, and material mixing. The early time behavior of a flat sliding interface is dominated by dislocation motion parallel to the interface. For this early stage, lower-dimensional models are useful in interpreting some of the simulation data. A two-chain forced Frenkel-Kontorova model reproduces some of the behavior of the larger scale simulations when the phenomenological damping is taken to be consistent with the MD simulations. This model exhibits four velocity regimes of steady state flow which will be discussed. Some of the implications for the nucleation of microstructure will be addressed.

Fracture simulations via large-scale nonequilibrium molecular dynamics

We describe the historical development of large-scale nonequilibrium molecular-dynamics (NEMD) computer simulations of fracture at Los Alamos. We have found that dynamic crack propagation leads to energy buildup that seeds instabilities, such as dislocation emission and branching. Recent three-dimensional simulations have shed light on ductile fracture mechanisms, including sequences of dislocation emission in various modes that are strikingly different from anything previously conjectured. The future for constitutive modeling, based on observations of dynamical features in such NEMD simulations is quite bright, and one such example is discussed.

Studies of sliding friction in compressed copper

We present the results of simulations for sliding copper interfaces for pressures in the kilobar range. The velocity dependence and density dependence are discussed and the evolution of plastic damage is described. Density dependent embedded atom model (EAM) potentials are used to describe the atomic interactions. Simulations were typically for N=65,000 atoms and were carried out using molecular dynamics on massively parallel CM200 and CM5 platforms. A transition to a low friction state at high velocities is discussed.

Modeling of failure in metallic thin films induced by stress and electromigration: A multiscale computational analysis

A common failure mechanism in metallic thin-film interconnects is void propagation driven by electric fields and thermomechanical stresses. In this paper, a multiscale computational analysis is presented for predictive modeling of transgranular void dynamics. The modeling approach is hierarchical and involves atomistic simulations for property database development, molecular-dynamics simulations based on boundary=element methods and techniques for moving boundary propagation. An extremely rich void dynamical behavior is predicted, which includes faceting, facet selection, propagation of slits from the void surface, as well as formation of fine-scale crack-like features on the void surface, in agreement with recent experimental data.

Short-range dislocation interactions

Abstract Short-range dislocation interactions are inherently atomistic and can only be investigated at present by means of atomistic simulations. Large-scale molecular-dynamics (MD) simulations can provide a great deal of quantitative information about the generation, motion, and interaction of dislocations, which are processes not easily accessible at present to experiments. In this paper, we will show the necessity of atomistic simulations in investigating short-range dislocation interactions by analyzing junction formation and jog formation in the process of dislocation intersection.

Large-scale molecular dynamics simulations of fracture and deformation

SummaryWe have discussed the prospects of applying massively parallel molecular dynamics simulation to investigate brittle versus ductile fracture behaviors and dislocation intersection. This idea is illustrated by simulating dislocation emission from a three-dimensional crack. Unprecedentedly, the dislocation loops emitted from the crack fronts have been observed. It is found that dislocation-emission modes, jogging or blunting, are very sensitive to boundary conditions and interatomic potentials. These 3D phenomena can be effectively visualized and analyzed by a new technique, namely, plotting only those atoms within the certain ranges of local potential energies.

Atomistic studies of jogged screw dislocations in {gamma}-TiAl alloys

The behavior of jogged screw dislocations in {gamma}-TiAl alloys has been investigated with large-scale molecular dynamics (MD) simulations. The authors find a new mechanism for formation of pinning points in jogged screw dislocations. They also find that the critical height for the jogs in the {+-}[{bar 1}10] directions on the (001) plane to move nonconservatively is between 3r{sub 0} and 4r{sub 0}, where r{sub 0} is the nearest neighbor distance of aluminum atoms. Interstitials and vacancies are created during the nonconservative motions of the jogs. In addition, the formation of dislocation dipole and loops around the jogs is also observed.

Molecular dynamics of very large systems

Abstract We briefly present recent results obtained with our parallel molecular dynamics (MD) code, SPaSM, performing large-scale multi-million atom simulations to study crack blunting and dislocation generation in copper. We also discuss some recent work addressing the problems associated with analyzing and visualizing the data generated from multi-million particle MD simulations.

A nonlinear-discrete model of dynamic fracture instability

Abstract Molecular-dynamics simulations of dynamic fracture show that local shear deformation at the crack tip increases, and local potential energy builds up, eventually triggering dislocation emission — the instability that leads to crack branching. Neither the build-up nor the instability can be predicted by linear-continuum theories. We propose a simple nonlinear-discrete model, which demonstrates that the excitation of nonlinear modes generates this instability.