Annealing-induced hardening in single-phase FCC, equiatomic CrCoNi and non-equiatomic CrCoNiMo medium-entropy alloys

Abstract

A two-step processing route including cold working (primary hardening) followed by annealing-induced (secondary) hardening is proposed to strengthen single-phase face-centered cubic CrCoNi-based medium-entropy alloys (MEAs). Here, we investigate two compositions (in at.%): Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6. The former has received considerable interest and is now regarded as a model MEA. The latter was designed to investigate how replacing 6 at.% Cr with Mo affects mechanical properties and annealing-induced hardening. It is worth mentioning that this latter composition is similar to that of the industrial MP35 N alloy (Cr22·9Co35·4Ni35·5Mo6.2), which is well-known to exhibit a strong annealing-induced hardening, but Cr27·3Co33·3Ni33·3Mo6 is outside its specification. In MP35 N, it has long been thought that Mo segregation at planar defects is responsible for annealing-induced hardening. In the present work, we show that the strength of both Cr33·3Co33·3Ni33.3 and Cr27·3Co33·3Ni33·3Mo6 can be increased by ∼25% after post-deformation annealing, thus demonstrating that Mo is not essential to cause annealing-induced hardening. Either another element, which could be Cr as revealed by atom probe tomography, is able to segregate at planar defects and induce annealing-induced hardening, or different mechanisms produce a similar strengthening effect. Based on these results, we discuss different models that can be used to quantify annealing-induced hardening in CrCoNi-based MEAs.