Martensitic transformation and mechanical behavior of a medium-entropy alloy

Diffusionless martensitic transformation (MT) exerts one of the most significant influences on the mechanical properties of alloys. However, the application of martensitic transformation to improve mechanical performance was seldom involved in the manufacture of high-entropy alloys (HEAs) and medium-entropy (MEAs) alloys. In this work, an innovative non-equiatomic MEA, Fe42Co42Cr16, was proposed with incorporation of martensitic transformation during water quenching and plastic deformation. Water quenching for the alloy in the high-temperature single-phase region produced a partial MT; i.e., transformation of gamma-FCC austenite phase into an epsilon-HCP martensite phase, responsible for the coexistence of gamma and epsilon phases in the dual-phase (DP) alloy. Another triple-phase (TP) alloy, including gamma-FCC austenite, epsilon-HCP martensite and B2-BCC precipitates, was obtained by quenching the alloy in gamma + B2 phase region. Owing to the low intrinsic stacking fault energy (gamma(I)), both DP (gamma(I) = 10.9 mJ/m(2)) and TP (gamma(I) = 12.2 mJ/m(2)) alloys involved the complete polymorphic MT process during plastic deformation; i.e., the transformation of the gamma-FCC austenite phase into the alpha-BCT martensite phase with an intermediate e-HCP martensite phase. Due to the transformation-induced plasticity effect and precipitation strengthening, the produced TP alloy exhibited a yield strength above 1 GPa with a total elongation of as high as 25%.