2020 年 61 巻 4 号 p. 587-595
Non-equiatomic high entropy alloys (HEAs) and medium entropy alloys (MEAs) are expected to have the potential to exhibit good mechanical properties due to abundant composition designs compared to equiatomic alloys. It has been reported that an equiatomic CoCrNi MEA shows better strength-ductility balance than CoCrFeMnNi HEA, and there is a possibility that the mechanical properties can be further improved by changing chemical composition. Among the constituent elements, cobalt (Co) has the effect of decreasing stacking fault energy (SFE). In this study, we clarified the effect of Co-content on mechanical properties of non-equiatomic Co–Cr–Ni MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Cox(CrNi)(100−x) (x = 20 (Co20), 40 (Co40), 60 (Co60) at%) MEAs were processed to very high plastic strains by high-pressure torsion (HPT) and subsequently annealed under proper conditions to obtain FCC single-phase and uniform fine grain sizes. Mechanical properties of the specimens with fully recrystallized microstructures were characterized by tensile tests at room temperature. Their deformed microstructures at different tensile strain levels were observed by electron microscopy. The result of the tensile tests showed that the work-hardening rate was enhanced with increasing the Co-content although early fracture before reaching plastic instability condition occurred in Co60. Planar slip of dislocations and deformation twinning were observed in Co20 (SFE = 30 mJ/m2), while, in addition to them, deformation-induced martensitic transformation to HCP ε-martensite was observed in Co40 having lower SFE (SFE = 10 mJ/m2), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of ε-martensite greatly increased in Co60 (SFE = 0 mJ/m2) compared with Co40, and early fracture occurred due to stress concentration at intersects between martensite and grain boundaries. The present results suggested that the mechanical properties of the present materials could be effectively designed by controlling the SFE.