Diffusion mechanisms in Cu grain boundaries

Abstract

We investigate atomic mechanisms of grain boundary (GB) diffusion by combining molecular dynamics (MD), molecular statics, the harmonic approximation to atomic vibrations, and kinetic Monte Carlo (KMC) simulations. The most important aspects of this approach are the basin-constrained implementation of MD and an automated location of transition states using the nudged elastic band method. We study two $\ensuremath{\Sigma}=5$ [001] symmetric tilt GB's in Cu, with atomic interactions described by an embedded-atom potential. Our simulations demonstrate that GB's support both vacancies and interstitials, and that vacancies can show interesting effects such as delocalization and instability at certain GB sites. Besides simple vacancy-atom exchanges, vacancies move by ``long jumps'' involving a concerted motion of two atoms. Interstitials move through concerted displacements of two or more atoms. More complex mechanisms (such as ring processes) involving larger groups of atoms have also been found. The obtained point defect formation energies and entropies, as well as their migration rate constants calculated within harmonic transition state theory, are used as input to KMC simulations of GB diffusion. The simulations show that GB diffusion can be dominated by either vacancy or interstitial-related mechanisms depending on the GB structure. The KMC simulations also reveal interesting effects such as temperature-dependent correlation factors and the trapping effect. Using the same simulation approach we study mechanisms of point defect generation in GB's and show that such mechanisms also involve collective transitions.