Self-diffusion in high-angle fcc metal grain boundaries by molecular dynamics simulation

Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature s tructure which, at lower temperatures, undergoes a reversible structural an d dynamical transition from a confined liquid to a solid; low-energy bounda ries, by contrast, were found to remain solid all the way up to the melting point. Here we demonstrate for the case of palladium that fcc metal GBs be have in much the same manner. Remarkably, at high temperatures the few repr esentative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotro pic liquid-like diffusion mechanism that is independent of the boundary mis orientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simula tions demonstrate that the decrease in the activation energy at elevated te mperatures is caused by a structural transition, from a solid boundary stru cture at low temperatures to a liquid-like structure at high temperatures. Consistent with the experiments, the transition temperature decreases with increasing GB energy, that is with increasing degree of short-range GB stru ctural disorder. By contrast, the degree of long-range structural disorder in the zero-temperature GB appears to play no role in whether or not the GB undergoes such a transition;It elevated temperatures.