High-temperature oxidation behaviour of low-entropy alloy to medium- and high-entropy alloys

The high-temperature oxidation behaviour of CoCrNi, CoCrNiMn, and CoCrNiMnFe equimolar alloys was investigated. All three alloys have a single-phase face-centred cubic structure. Thermogravimetric analyses (TGA) were conducted at temperatures ranging from 800 to 1000 °C for 24 h in dry air. The kinetic curves of the oxidation were measured by TGA, and the microstructure and chemical element distribution in different regions of the specimens were analysed. The oxidation kinetics of the three alloys followed the two-stage parabolic rate law, with rate constants generally increasing with increasing temperature. CoCrNi displayed the highest resistance to oxidation, followed by CoCrNiMnFe and CoCrNiMn exhibiting the least resistance to oxidation. The addition of Mn to CoCrNi increased the oxidation rate. The oxidation resistance of CoCrNiMn was enhanced by the addition of Fe. Less Mn Content and the formation of more Cr₂O₃ were responsible for the reduction in the oxidation rates of CoCrNiMnFe. The calculated activation energies of CoCrNiMn and CoCrNiMnFe at 800, 850 and 900 °C were 108 and 137 kJ mol^?1, respectively, and are comparable to that of Mn diffusion in Mn oxides. The diffusion of Mn through the oxides at 800–900 °C is considered to be the rate-limiting process. The intense diffusion of Cr at 1000 °C contributed to the formation of CrMn_1.5O₄ spinel with Mn in the outer layer of CoCrNiMn and Cr₂O₃ in the outer layer of CoCrNiMn.