• Using in-situ atomic resolution HAADF-STEM imaging, the study provides direct experimental evidence that grain rotation in nanocrystalline materials is primarily driven by disconnection-mediated shear-coupled grain boundary (GB) migration, confirming a long-hypothesized but previously unproven mechanism.

• Large-area in situ 4D-STEM mapping of approximately 2000 grains statistically revealed that grain rotation is tightly coupled with grain growth and shrinkage, highlighting the dominance of shear-coupled GB migration during microstructure evolution.

• The study, supported by molecular dynamics simulations, demonstrates that disconnection-mediated shear-coupled migration and associated grain rotation occur across a wide range of grain boundary (GB) types, including both symmetric and asymmetric boundaries.

Y. Tian, X. Gong, M. Xu, C. Qiu, Y. Han, Y. Bi, L. Estrada, E. Boltynjuk, H. Hahn, J. Han, D.J. Srolovitz, and X. Pan. Science 386,49-54 (2024).