The oxidation behavior of the quinary FeCoNiCrSi_x high-entropy alloys

The oxidation of three quinary FeCoNiCrSi_x 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 (FeCoNiCrSi_0.5 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 FeCoNiCrSi_0.5 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.