Abstract: Nanoscale twins and a 9R structure (a periodically faulted rhombohedral configuration) were directly observed under microscopy in a medium/high stacking-fault energy face-centered cubic high-entropy alloy (HEA) subjected to high strain rate shear deformation, and demonstrated to be related to a special activity of partial dislocations, namely, their cooperative activation, which is manifested by detwinning when the alloy is subjected to conditions that involve a high local stress at a high strain rate. These mechanisms significantly enhance work-hardening behavior, providing a new pathway for designing high-toughness HEAs. Furthermore, the significant grain refinement associated with dynamic recrystallization increases the likelihood of partial-dislocation-related defects. This promotes the direct formation of the 9R phase through the sequential activation of partial dislocations from grain boundaries, leading to continuous strain hardening following the onset of shear localization. The corresponding formation mechanisms are elucidated through molecular dynamics simulations.

Key words: High-entropy alloy, High strain-rate performance, Nanocrystalline structure, 9R phase, Shear localization