| Materials properties depend on processes that take place on a variety of time scales. These range from atomic vibrations or dislocation-mediated slip processes, which have typical time scales of hundreds of femtoseconds (fs) to hundreds of picoseconds (ps), to diffusion, which may take place on the order of seconds or longer. This disparity in time scales leads to difficulties when trying to model slower processes where individual atomic motions may be important, such as diffusion controlled boundary migration and dislocation climb. A straightforward molecular dynamics (MD) approach, with a typical time step of 1 fs, would require an enormous computation time to adequately capture these processes. This work presents a novel method, called Diffusive Molecular Dynamics (DMD), which can capture the diffusion time scale while retaining the atomic spatial resolution by coarse-graining over atomic vibrations and evolving a site-probability representation of atomic density clouds. DMD solves master equation on a moving atomic grid. It combines long-range elastic effects and short-range atomic interactions simultaneously with gradient thermodynamics.;DMD has been applied to nanoindentation, hot isostatic pressing of nanoparticles, climb of edge dislocation and diffusional void growth. In nanoindentation, the simulations demonstrate that displacive plasticity depends sensitively on the remnant debris of prior diffusional plasticity. This is evident from dislocation structure, reduction in yield load and stiffness due to surface step formed by surface diffusion at low indentation rates and/or at high temperatures. In hot isostatic pressing, DMD captures the evolution of multiple nanoparticle compact to theoretical density revealing significance of rigid-body motion, and diffusional and displacive processes in obtaining the final microstructure. Dislocation nucleation triggered by diffusive void growth is also a coupled diffusive-displacive phenomenon captured by DMD. This plays an important role in ductile failure, where near a crack tip, the intensive dislocation interactions produce vacancies well in excess of their equilibrium concentration facilitating condensation and void growth. Finally, a possible mechanism for climb of edge dislocation in FCC crystals and energetics associated with it is presented. It is shown that, when simultaneous displacive and diffusive events are allowed, there exists a coupled diffusive-displacive pathway along which the activation energy is substantially lower than the previous theoretical predictions and on par with the experimental observations. |
