FeMnCoCr-based complex concentrated alloys (CCAs) dominated by austenite usually exhibit low yield and ultimate tensile strength, which restricts their practical applications. Traditional strengthening strategies often achieve high strength at the expense of ductility. Here, we report a novel CCA with the nominal composition of Fe 69.4 Mn 10 Co 10 Cr 10 N 0.6 (at.%), processed via a two-step heat treatment consisting of austenitization at 700 °C followed by tempering in the range of 200∼400 °C. This yields a microstructure dominated by α′-martensite, with concurrent austenite reversion and martensitic transformation during tempering and water quenching, generating high densities of geometrically necessary dislocations (GNDs) within both α′-martensite matrix and the retained austenite (RA). Uniaxial tensile tests reveal that yielding of these materials is governed by the stress-induced martensite transformation of RA, while the elevated GND density within RA enhances its stability, thereby delaying the onset of phase transformation and increasing yield strength without sacrificing uniform elongation. Furthermore, plastic deformation of the α′-martensite contributes significantly to strain hardening. Consequently, yield strength increases continuously from 512 MPa for the annealed material to 764 MPa after tempering at 400 °C, while ultimate tensile strength rises from 1310 MPa to 1388 MPa, with uniform and total elongation maintained at ∼17% and ∼25%, respectively. This study successfully breaks the strength-ductility dilemma of FeMnCoCr-based alloys, offering a novel microstructural engineering paradigm for advanced alloys.
Zhang et al. (Fri,) studied this question.