Kinetics of grain boundary migration in nanosized Al polycrystals

Classical theories of grain growth assume that grain boundary curvature and velocity are linearly correlated. However, recent experimental observations seem to imply that the relationship is more complex in polycrystalline materials. Here, we determined the velocity and curvature of approximately 12 000 grain boundaries from molecular dynamics simulations of the annealing of randomly textured nanosized polycrystalline Al. The grain boundary kinetics was studied at different scales and compared with theoretical predictions. At the grain level, that is, attributes averaged over all grain boundaries of a grain, the kinetics behavior showed a linear correlation of curvature and velocity. When studying grain-boundary-level behavior (i.e., attributes of individual grain boundaries are considered), we found a clear correlation between the sign of the grain boundary curvature and the direction of migration. Nevertheless, considering all the grain boundaries, the correlation coefficient between the magnitude of velocity and curvature was found to be low (0.34) although a strong linear correlation was observed for a subset of high angle, long-lived, initially large grain boundaries with uniform curvature distribution and similar 5 crystallographic degrees of freedom. Furthermore, some grain boundaries that are close in crystallographic space showed different kinetics. This elucidates the complexity of grain boundary migration in the nanocrystalline ensemble as a result of complex dependence of grain boundary mobility and energy on grain boundary character, and the existence of other driving forces that affect the behavior of grain boundary migration. Among others, defect distributions and variation in stress distribution between neighboring grains are found to influence grain boundary migration.