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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 CoCrNi MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Co x (CrNi) (100¹x) (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/m 2 ), while, in addition to them, deformation-induced martensitic transformation to HCP ¾-martensite was observed in Co40 having lower SFE (SFE = 10 mJ/m 2 ), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of ¾-martensite greatly increased in Co60 (SFE = 0 mJ/m 2 ) 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.

In this study we report the deformation microstructures and strength of medium entropy alloys (MEAs) and high entropy alloys (HEAs), which are defined as alloys composed of four or less, and five or more principal elements, respectively, with (near-) equi-atomic concentrations. The friction stress (fundamental resistance to dislocation motion in the crystal lattice) and Hall-Petch relationship of various MEAs (CoCrFeNi, CoCrNi, etc.), taken as subsystems of the equi-atomic CoCrFeMnNi HEA, were precisely measured at room temperature. Experimental values of the friction stresses were found to fit with a theoretical model proposed by Toda-Caraballo et al. very well, which indicates that the strength of the alloys is closely related to a heterogeneously distorted crystal lattice. At the same time, values of the average lattice distortion in the alloys were found to be comparable to those in some dilute alloys, contradicting the belief that “severe” lattice distortion is a reason for the higher strength than in dilute systems. Finally, a strengthening mechanism due to element-element interactions was proposed as an additional mechanism in FCC HEAs and MEAs.

This study revealed characteristics of the deformation behavior in high/medium entropy alloys (HEAs/MEAs) with face-centered cubic (FCC) structure. A Co60Ni40 alloy and a Co20Cr40Ni40 MEA having low and high friction stresses (fundamental resistance to dislocation glide in solid solutions), respectively, but similar in other properties, including their stacking fault energy and grain sizes, were compared. The MEA exhibited a higher yield strength and work-hardening ability than those in the Co60Ni40 alloy at room temperature. Deformation microstructures of the Co60Ni40 alloy were composed of coarse dislocation cells (DCs) in most grains, and a few deformation twins (DTs) formed in grains with tensile axis (TA) nearly parallel to <111>. In the MEA, three microstructure types were found depending on the grain orientations: (1) fine DCs developed in TA~//<100>-oriented grains; (2) planar dislocation structures (PDSs) formed in grains with other orientations; and (3) dense DTs adding to the PDSs developed in TA~//<111>-oriented grains. The results imply difficulty in cross-slip of screw dislocations and dynamic recovery in the MEA, leading to an increase in the dislocation density and work-hardening rate. Our results suggest that FCC high-alloy systems with high friction stress inherently develop characteristic deformation microstructures advantageous for achieving high strength and large ductility.