Activating metastability engineering via control of initial microstructure in C-added CoCrFeNiMo medium-entropy alloy: in situ X-ray diffraction study

Deformation-induced martensitic transformation (DIMT) from face-centered cubic to body-centered cubic or body-centered tetragonal martensite in metastable ferrous medium-entropy alloys (FeMEAs) can be effectively tuned by modifying initial microstructures to achieve the desired mechanical response. For example, partial recrystallization can promote a faster rate of DIMT by providing numerous nucleation sites for the martensite embryos, such as profuse shear bands and nanotwins. Precipitation-driven changes in matrix concentrations can also influence the phase stability and the DIMT kinetics. In this work, two Co_18.5Cr₁₂Fe₅₅Ni₉Mo_3.5C₂ (at%) FeMEA samples with different fractions of recrystallized regions and precipitate volume fractions were fabricated, and the difference in their DIMT behaviors and work hardening responses was investigated with in situ high energy X-ray diffraction analysis during tensile testing. The sample with a higher fraction of non-recrystallized regions and carbide precipitates showed low FCC phase stability mediated by the profuse nucleation sites in non-recrystallized regions and precipitation-driven metastability, and rapid DIMT to α′ martensite from the beginning of plastic deformation. Meanwhile, the other sample showed a delay in DIMT until the final stage of deformation. The lattice strain evolution demonstrated dynamic stress partitioning onto the newly born α′ martensite. Furthermore, the phase stress calculation showed that the stress partitioning creates a meaningful contribution to the enhancement of the work hardening and flow stress of the sample with the faster DIMT kinetics, validating our strategy to regulate DIMT via initial microstructure in search of improved mechanical performances.