Effects of Mo and Nb on the microstructure and high temperature oxidation behaviors of CoCrFeN based high entropy alloys

A series of CoCrFeNi(Mo, Nb) high entropy alloys (HEAs) were prepared using vacuum arc melting. The effect of Nb and Mo on the microstructure and high temperature anti oxidation properties of HEAs was investigated. The addition of only Mo was found to not cause formation of new phase, but did increase the lattice distortion. After the addition of Nb, as-prepared CoCrFeNiMo0.2Nb0.1 and CoCrFeNiMo0.2Nb0.2 alloys are characteristic of duplex phase structures of FCC and Laves, in which the Laves phase is semi-coherent with FCC matrix. Our studies also revealed that the oxidation kinetics of the alloys followed parabolic rate law and the corresponding oxidation rate constants were calculated. Due to the synergistic effects of Mo and Nb, the anti-oxidation properties of HEAs improved significantly. It was also discovered that the new Laves phase containing Mo and Nb at the inner oxidation layer hindered the outward diffusion of active metal cations, ergo improving the oxidation resistance of HEAs. With increase of Nb content, it was further recognized that the layered structure of Laves phase was thinner and denser. Therefore, the oxygen ion diffusion process in the CoCrFeNiMo0.2Nb0.2 alloy can be inhibited effectively by the new formation of duplex phase structures with undulated Laves phase and FCC matrix.(c) 2023 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

The addition of Nb and Mo to CoCrFeNi high entropy alloys improves their oxidation resistance due to the formation of a new Laves phase that hinders the diffusion of metal cations. The duplex phase structure of FCC and Laves contributes to the inhibition of oxygen ion diffusion, leading to enhanced anti-oxidation properties.