The atomic mechanisms of grain-boundary diffusion were investigated by combining molecular dynamics, molecular statics, harmonic approximations to atomic vibrations and kinetic Monte Carlo simulations. The most important aspect of the approach was a basin-constrained implementation of molecular dynamics, and the location of transition states using the nudged elastic band method. Two Σ = 5 [001] symmetrical tilt grain boundaries were studied, with the atomic interactions being described by an embedded-atom potential. The simulation results demonstrated that grain boundaries supported both vacancies and interstitials, and that the vacancies could exhibit interesting effects such as delocalization and instability at certain grain boundary sites. As well as simple vacancy-atom exchanges, the vacancies moved via so-called long jumps which involved the concerted motion of 2 atoms. Interstitials moved via the concerted displacement of 2 or more atoms. More complex mechanisms, such as ring processes which involved larger groups of atoms, were also found. The deduced point-defect formation energies and entropies, as well as their migration-rate constants – as calculated within harmonic transition state theory - were used as input data for kinetic Monte Carlo simulations of grain boundary diffusion. The simulations showed that grain boundary diffusion could be dominated by vacancy-related or interstitial-related mechanisms; depending upon the grain boundary structure. The kinetic Monte Carlo simulations also revealed interesting effects; such as temperature-dependent correlation factors and trapping effects. By using the same simulation techniques, a study was made of the mechanisms of point-defect generation in grain boundaries and it was shown that such mechanisms also involved collective transitions.

Diffusion Mechanisms in Cu Grain Boundaries. M.R.Sørensen, Y.Mishin, A.F.Voter: Physical Review B, 2000, 62[6], 3658-74