Strain-driven grain boundary motion in nanocrystalline materials

We report fully three-dimensional atomistic molecular dynamics studies of strain-induced grain boundary mobility in nanocrystalline Ni at room temperature. The position of a statistically significant number of grain boundaries was monitored as a function of the strain level for a strain rate of 3.3 × 108 s−1 for two different interatomic potentials. The results show the grain boundaries migrating with velocities of 2–3 m/s, depending on the interatomic potential used. Detailed analysis of the process shows that grain boundary migration is accompanied by grain rotation and in many cases dislocation emission. The results suggest that grain rotation, grain boundary sliding, and grain boundary migration occur simultaneously in nanocrystalline metals as part of the intergranular plasticity mechanism. The effects of free surfaces present in the sample on these related mechanisms of plasticity were investigated in detail and it was found that the presence of free surfaces lowers the flow stress observed for the samples and increases the amount of grain boundary sliding, while actually decreasing the average velocity of grain boundary migration parallel to itself. Finally, we report observations of grain coalescence in the samples with a free surface. The results are discussed in terms of the coupling of grain boundary sliding and migration.