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

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. (gamma) > c.p. (epsilon) transformation. This unusual phase evolution triggered development of refined, epsilon-minated microstructure coupled with a uniformly dispersed fine gamma hase. The controlled < c+a > slip and twinning in the epsilon phase along with the transformation of a refined gamma matrix resulted in higher elongation of 52% with enhanced ultimate tensile strength of 1.12 GPa during deformation. Thus, the metastability assisted design of epsilon-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. (C) 2018 Elsevier Ltd. All rights reserved.