Shear-assisted Grain Coarsening in Colloidal Polycrystals.

Significance Grain growth and grain-boundary motions during shear play key roles in polycrystal annealing. Notably, complex interactions between neighboring grains give rise to nontrivial grain-boundary kinetics that are not well understood, in part because electron microscopy cannot dynamically track atoms. Here, we observe grain-growth behaviors and elucidate kinetics from experiments with colloidal polycrystals that resolve single-particle motions; the observations reveal shear-coupled grain-boundary migration, grain rotation via emission of dislocations across the grain, melting-point shifts for different lattice orientations, and melting–recrystallization phenomena, which provide a mechanism for unexplained dynamic abnormal grain growth. These fundamental findings are critical results for understanding and processing polycrystalline materials. Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting–recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting–recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation R×θ=constant (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials.