On the atomistic mechanisms of grain boundary migration in [0 0 1] twist boundaries: Molecular dynamics simulations

Molecular dynamics simulations were performed to characterize the atomic motions that govern grain boundary migration in a series of twist boundaries. In particular, migration of θ = 36.87° Σ5, θ = 22.63° Σ13, and θ = 40.23° general high angle [0 0 1] twist boundaries driven by stored elastic energy in fcc nickel were investigated. Atomic motions during migration were identified as single-atom jumps and multiple-atom collective motions (including general string-like cooperative motions and special four-atom shuffles) using quantitative string measurement, the self-part of the van Hove correlation function, and an angular distribution function. The simulation results confirmed that collective four-atom shuffle motion was the rate controlling atomic motion during migration of the Σ5 twist boundary. Furthermore, simulations showed no correlation between individual four-atom shuffles, suggesting migration was controlled by random shuffles rather than a propagation of kinks along ledges. As grain boundary local symmetry decreased (i.e., from a low Σ boundary to a high Σ boundary), string-like cooperative atomic motions (not collective shuffle motions) became increasingly important. Both random single-atom jumps and general string-like cooperative motions were dominant during migration of a general non-Σ twist boundary. Simulations also showed that the activation energy for grain boundary migration was well correlated with the average string length occurring within the boundary. This implies that grain boundary drifting velocity is sensitive to factors (e.g., external stresses, impurities) that can alter string motions within a boundary.