Mechanisms of Grain Boundary Dynamics and Grain Rotation in Nanocrystalline Platinum: Insights from in situ 4D-STEM

Near-rigid-body grain rotation is frequently observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials, yet its underlying mechanisms remain unresolved. This study provides direct evidence that grain rotation occurs via the motion of disconnections—line defects with step and dislocation character—along grain boundaries (GBs) in platinum thin films. Using advanced in situ four-dimensional scanning transmission electron microscopy (4D-STEM), we uncovered a statistical correlation between grain rotation and GB migration.

High-resolution 4D-STEM datasets, acquired during elevated-temperature experiments, captured the temporal evolution of grain orientations and GB dynamics. Grain segmentation and inter-frame association analyses revealed the pervasive nature of grain rotation and its strong coupling with GB migration. Complementary atomic-resolution high-angle annular dark-field STEM (HAADF-STEM) observations demonstrated that disconnection propagation drives shear-coupled GB migration, leading to localized shear strain accumulation.

These findings establish shear-coupled GB migration, mediated by disconnection motion, as the dominant mechanism underpinning grain rotation. By bridging atomic-scale observations with statistical analyses, this work provides a comprehensive understanding of GB dynamics, paving the way for improved prediction and engineering of macroscopic properties—such as mechanical strength and thermal stability—in polycrystalline materials.