Atomic-level study of twinning behaviors in metastable hexagonal high-entropy alloys

High-entropy alloy (HEAs) with a hexagonal close-packed (hcp) structure can be generated from the high-entropy face-centered cubic (fcc) matrix phase through martensitic transformation (MT) as deformed at low temperatures. {10 $\bar{1}$ 1} deformation twinning (DT) was widely observed in these deformation-induced hcp HEAs. Corresponding to local heating by plastic work, the deformation-induced hcp phase is generally metastable during plastic deformation. The metastability of the hcp phase facilitates the formation of high-density basal stacking faults (BSFs) and two types of fcc nano-bands with a {111} twinning relationship, significantly influencing {10$\bar{1}$ 1} twin propagation and thickening. Using high-resolution transmission electron microscopy (HRTEM) and molecular dynamics (MD) simulations, we systematically investigated the behaviors of {10$\bar{1}$ 1} DT and its interactions with BSFs and fcc nano-bands associated with reversible martensitic transformation (RMT) in the deformation-induced hcp phase. The dynamically coupled deformation mechanisms of RMT (fcc ↔ hcp) and {111} DT generate complex structural evolutions within {10 $\bar{1}$ 1} twins, where kinematic paths of RMT are influenced by twin boundaries. Interactions between {10$\bar{1}$ 1} twin and fcc nano-bands are categorized into “non-crossing” and “apparent crossing” mechanisms, depending on their crystallographic orientations. HRTEM characterizations and MD simulations reveal that {10 $\bar{1}$ 1} twin transmission through fcc nano-bands with low misorientation angles is facilitated by indirect slip transmission via re-nucleation of {10$\bar{1}$ 1} twinning dislocations from the hcp/fcc phase boundaries. These findings provide an in-depth understanding of the deformation mechanisms in metastable hcp HEAs, highlighting the role of dynamically coupled DT and RMT mechanisms in governing microstructural evolutions during plastic deformation.