Atomistic twinning process with ultra-high shear in hexagonal close-packed crystals.

Twinning is a critical deformation mechanism that can significantly enhance the mechanical performance of materials. Predicting active twinning modes has long been a fundamental challenge. It is generally believed that active twinning modes should exhibit low to moderate shear strains to minimize the associated strain energy. Here, using in situ atomic-scale straining experiments, we identify an unconventional twinning mode in hexagonal close-packed (HCP) rhenium nanocrystals, characterized by an extraordinarily large shear strain of ~1.2, the highest reported for HCP crystals to date. Remarkably, this twinning process proceeds via pure lattice shear without the atomic shuffling typically required for conventional HCP twinning. Our results indicate that high stress states attainable in nanocrystals, together with favorable energetics, enable activation of this mode. This ultra-high-shear twinning mechanism holds significant potential for enhancing the formability and ductility of HCP metals.