It was recalled that molecular dynamics simulations of high-energy high-angle twist grain boundaries had revealed a universal liquid-like high-temperature structure which, at lower temperatures, underwent a reversible structural and dynamical transition from a confined liquid to a solid. On the other hand, low-energy boundaries were found to remain solid up to the melting point. It was demonstrated here that face-centered cubic metal grain boundaries behaved in a similar manner. At high temperatures, representative high-energy high-angle (tilt or twist) boundaries exhibited the same rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that was independent of the boundary misorientation. The observations were in qualitative agreement with recent grain-boundary self-diffusion and impurity diffusion data. The present simulations demonstrated that the decrease in activation energy at high temperatures was caused by a structural transition from a solid boundary structure at low temperatures, to a liquid-like structure at high temperatures. The transition temperature decreased with increasing grain boundary energy; that is, with increasing degree of short-range grain-boundary structural disorder. On the other hand, the degree of long-range structural disorder in a grain boundary at zero-temperature appeared to play no role in whether or not the grain boundary underwent such a transition at high temperatures.

Self-Diffusion in High-Angle FCC Metal Grain Boundaries by Molecular Dynamics Simulation. P.Keblinski, D.Wolf, S.R.Phillpot, H.Gleiter: Philosophical Magazine A, 1999, 79[11], 2735-61