Compositional stability in medium and high-entropy alloys of CoCrFeMnNi system under ion irradiation

Abstract

The equiatomic high-entropy alloy (HEA) CoCrFeMnNi not only has excellent mechanical properties but also good irradiation resistance. However, the mechanical properties of some equiatomic medium-entropy alloys (MEAs) are superior to those of CoCrFeMnNi HEA. In this study, the irradiation resistance and changes in composition due to irradiation in CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA are investigated. Thin film samples of the MEAs and HEA and Ni used for comparison were irradiated with up to 1.7 × 1019 ions/m2 of 2.4 MeV Cu ions at 673 and 873 K. The average damage in the observed area was 1 displacement per atom (dpa). No voids were observed in any of the MEA and HEA samples even after irradiation at 873 K; however, large voids were formed in Ni irradiated at 873 K. This indicates that the irradiation resistance of CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA was better than that of Ni. In addition, the formation of stacking fault tetrahedra (SFTs), a type of vacancy cluster, at 873 K was much more pronounced in CoCrNi and CoCrFeNi MEAs than in CoCrFeMnNi HEA. Therefore, the irradiation resistance of CoCrNi and CoCrFeNi MEAs is lower than that of CoCrFeMnNi HEA. Moreover, significant Cr segregation occurred in the CoCrNi and CoCrFeNi MEA samples irradiated at 873 K. In contrast, no segregation occurred in CoCrFeMnNi HEA. First-principles calculation results show that the formation rate of Cr-dumbbells is higher in CoCrNi and CoCrFeNi MEAs than in CoCrFeMnNi HEA, and that Cr interstitials are more stable in the MEAs. Therefore, Cr segregation is more likely to occur in the MEAs. Element segregation may affect the irradiation resistance of the alloys.