Engineering hierarchical γ/ε/γ-twin laminate via cyclic tension–compression for strength–ductility synergy in a metastable high-entropy alloy

Metastable high-entropy alloys (HEAs) offer a practical strategy for achieving a good balance between strength and ductility through both deformation-induced (martensitic) phase transformations and solid solution strengthening effects. However, HEAs often have a low yield strength (YS) and a tendency toward deformation-induced martensite coarsening, which impedes the formation of refined microstructures that would improve the mechanical properties. Here, we demonstrate that in a prototype metastable Fe₆₀Mn₁₂Cr₁₂Ni₈Si₈ HEA, a hierarchical γ/ε/γ-twin laminate structure can be achieved simply by cyclic tension–compression (CTC) processing. Compared to the as-received material, the engineered microstructure contains a higher density of coherent interfaces and exhibits more pronounced microstructural evolution during tensile deformation, resulting in a 130% increase in YS while maintaining comparable ductility. Atomic-scale characterization integrated with density functional theory calculations reveals that during successive CTC cycles, partial dislocations nucleate within the ε martensite and propagate along the 111_γ/0001_ε planes, leading to retransformation back into γ or nano γ twins, thereby effectively reducing the interspacing. Crucially, in this metastable HEA, transformation-mediated twinning (TMT) exhibits a lower energy barrier than the classical layer-by-layer mechanism. Significant dislocation accumulation at the γ/ε interfaces generates intense local stresses, supplying the critical energy required to activate TMT. Our work offers a valuable insight into the twinning mechanism and highlights a practical new way of developing very fine hierarchical γ/ε/γ-twin laminate microstructures with improved strength.