Regulation of cryogenic mechanical behaviors of C-added non-equiatomic CoCrFeNiMo ferrous medium-entropy alloy via control of initial microstructure

This study demonstrated the potential for customizing the desired properties of the Co18.5Cr12Fe55Ni9Mo3.5C2 (at.%) ferrous medium-entropy alloy by manipulating the deformation-induced martensite transformation (DIMT) behavior at liquid nitrogen temperature. This was achieved by modifying various initial microstructures through annealing at temperatures ranging from 900 to 1200 °C. The variations in DIMT kinetics were analyzed based on two main factors. (1) Inducing carbide precipitation by annealing at 900 and 1000 °C results in changes in the composition within the matrix, which may affect the stability of the face-centered cubic phase. Samples with a higher volume fraction of the carbide precipitates exhibit lower ΔGFCC→BCC and faster DIMT kinetics. (2) The onset and kinetics of DIMT are also affected by the use of martensite nucleation sites, which may vary depending on the presence of non-recrystallized regions or the grain size. In fine-grained structures, martensite primarily nucleated in the non-recrystallized regions and grain boundaries. However, in coarse-grained microstructures, martensite mainly nucleated along the in-grain shear bands and their intersections. This precise control of the microstructure results in superior properties. The samples annealed at 900 and 1000 °C with carbide precipitates and fine grains exhibit ultrahigh ultimate tensile strength, which may reach elevated values up to ∼1.8 GPa, while those annealed at 1100 and 1200 °C with larger grains and no precipitates exhibit a uniform elongation that exceeds 100%.