Diffusive-to-ballistic transition in grain boundary motion studied by atomistic simulations

An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature (T) and applied driving force (P). At low P and T, grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high P and T, grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both P and T lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. The recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).