Direct observation of the atomic scale mechanism of grain rotation

A polycrystalline material is an aggregate of crystallites with different lattice orientations. The crystallites, i.e., grains, can undergo nearly-rigid-body rotation. Such rotations have been widely observed in polycrystalline materials during grain growth, plastic deformation and recrystallization. Grain rotation plays a central role in polycrystal dynamics (e.g., changing grain growth kinetics) and constitutes a primary deformation mechanism in nanocrystalline solids and may dominate texture evolution. The mechanism of grain rotation remains elusive, despite decades of investigation, largely because of the difficulty in observing the evolution of internal interfaces. Here, we report direct, atomic-scale evidence that grain rotation takes place via the propagation of disconnections (i.e., GB line defects with step and/or dislocation character) along GBs in polycrystals, as observed using in situ and four-dimensional scanning transmission electron microscopy  with atomic resolution. The propagation of disconnections creates a jump in the parallel displacement of the crystal lattices (shear displacement) across the GB and is correlated with GB migration and grain rotation. Our results demonstrate the critical role of disconnections in determining the behavior of polycrystals akin to dislocations in crystals. These findings provide new, quantitative, insight into polycrystal dynamics.