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

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Abstract
Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature structure which, at lower temperatures, undergoes a reversible structural and dynamical transition from a confined liquid to a solid; low-energy boundaries, 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 behave in much the same manner. Remarkably, at high temperatures the few representative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that is independent of the boundary misorientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simulations demonstrate that the decrease in the activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure 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 structural 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 at elevated temperatures.