Achieving superior strength-ductility synergy in a CrCoNi medium-entropy alloy via grain refinement and precipitation strengthening

Abstract Although the CrCoNi medium-entropy alloy (MEA) possesses outstanding ductility and fracture resistance, its strength at room temperature often falls short for demanding structural uses. To address this limitation, this study investigates the synergistic effect of adding 3 at.% Mo and 0.5 at.% C on enhancing its mechanical performance. The recrystallized (CrCoNi) 96.5 Mo 3 C 0.5 MEA developed a unique microstructure consisting of ultrafine grains (∽1.2 μm) and a uniform dispersion of nanoscale M 23 C 6 carbides, a significant refinement compared to the single-phase CrCoNi. This engineered microstructure leads to a dramatic improvement in mechanical properties: yield strength rises from 400 MPa to 840 MPa, while ductility remains considerable at ∽40% elongation. Furthermore, the alloy demonstrates respectable retention of strength and an interesting transition to enhanced ductility (∽57% elongation) at 600°C, showcasing its potential for intermediate-temperature applications. Quantitative analysis revealed that the room temperature strength increment was primarily governed by precipitation strengthening, with significant contributions from grain refinement and a minor solid solution effect. Analysis of the post-deformation microstructure indicated a change in the dominant deformation mode from mechanical twinning in the base alloy to the development of dislocation cells and stacking faults in the (CrCoNi) 96.5 Mo 3 C 0.5 MEA, explaining its modified work hardening characteristics. This work demonstrates a potent microstructural engineering strategy for developing high-strength, ductile MEAs, successfully breaking the strength-ductility trade-off at room temperature while maintaining promising performance at elevated temperatures.