The oxidation behavior of the quinary FeCoNiCrSix high-entropy alloys

The oxidation of three quinary FeCoNiCrSix high-entropy alloys (x = 0.25, 0.5 and 1.0) was investigated at 700-900 °C in dry air and at 900 °C in various O2-containing atmospheres (FeCoNiCrSi0.5 only). The two lower Si-content alloys contained an FCC single-phase structure, while the equimolar alloy had a BCC plus FCC dual-phase structure. The oxidation kinetics of all the alloys followed the parabolic rate law, with their oxidation rate constants increased with increasing temperature and decreasing Si content. In addition, the oxidation rate constants of the FeCoNiCrSi0.5 alloy steadily increased with increasing oxygen partial pressure, indicative of a typical oxide scale exhibiting a p-type semiconductivity. The scales formed on all the alloys consisted mostly of Cr2O3 and minor amounts of SiO2 (α-quartz). The oxidation mechanism of the HEAs is due primarily to outward diffusion of cations.