Unexpected strength–ductility response in an annealed, metastable, high-entropy alloy

Abstract The design of non-equiatomic high-entropy alloys (HEAs) opens huge compositional space to develop new materials with exceptional properties. Among them, HEAs with flexible microstructures showed an adaptive phase stability that enhanced the work hardening (WH) ability of the material drastically. With the same motive, here we present a new friction stir processed Fe39Mn20Co20Cr15Si5Al1 HEA that demonstrated an unexpected strength–ductility response just upon annealing. The inter-competing precipitation and grain/twin formation events during low-temperature annealing resulted in an unexpected f.c.c. (γ) → h.c.p. (ɛ) transformation. This unusual phase evolution triggered development of refined, ɛ-dominated microstructure coupled with a uniformly dispersed fine γ phase. The controlled 〈c+a〉 slip and twinning in the ɛ phase along with the transformation of a refined γ matrix resulted in higher elongation of 52% with enhanced ultimate tensile strength of 1.12 GPa during deformation. Thus, the metastability-assisted design of ɛ-martensite-dominant HEAs by annealing opens a new path to obtain strong yet ductile alloys, which is otherwise not feasible in conventional steel/HEA designs.