Metastability engineering of partially recrystallized C-doped non-equiatomic CoCrFeNiMo medium-entropy alloy

Ferrous medium-entropy alloys (FeMEAs) are coming into attention these days for their excellent mechanical properties. Most of the FeMEAs developed so far form metastable face-centered cubic (FCC) matrix, and “metastability engineering” that utilizes deformation-induced martensitic transformation (DIMT) from FCC to body-centered cubic (BCC) as a method to enhance work hardenability has been the key to the exceptional mechanical behaviors. However, the FeMEAs have a significant weakness: low yield strength compared with high tensile strength and ductility. In this study, partial recrystallization is presented as a solution to the current drawback of the FeMEAs. A Co18.5Cr12Fe55Ni9Mo3.5C2 (at. %) FeMEA was annealed at 800 °C for 10 and 30 min and partially recrystallized microstructures with relatively coarse non-recrystallized grains that contain profuse mechanical twins and ultrafine recrystallized grains were attained. In addition, nanosized Cr-rich M23C6-type and Mo-rich M6C-type carbides were precipitated during the annealing. The partially recrystallized FeMEA showed a yield strength of ∼1.07 GPa, significantly enhanced from ∼600 MPa of the recrystallized counterpart. Dislocation strengthening, precipitation strengthening, grain boundary strengthening, and twin boundary strengthening led to the improved strength of the partially recrystallized FeMEA. Back stress hardening owing to the heterogeneity also contributed to the high strength and work hardenability. Moreover, the transformation-induced plasticity effect from the FCC-to-BCC DIMT activated by BCC nucleation at defects within the non-recrystallized grains effectively enhanced the work hardenability, leading to ∼1.34 GPa of tensile strength and ∼30% of elongation. This study provides an insight to optimize the microstructure and corresponding mechanical properties of metastable metallic materials.