Oxidation Competition Between FCC and B2 Phases in Al-Cr-Fe-Ni Eutectic High-Entropy Alloys at 850 °C

Al-Cr-Fe-Ni eutectic high-entropy alloys (EHEAs) with an FCC + B2 dual-phase microstructure are promising candidates for high-temperature structural applications, yet the competitive oxidation behavior between the two phases at elevated temperatures remains unclear. In this work, four AlxCrFeNi3−x (x = 0.75, 0.8, 0.85, 0.9) EHEAs are prepared by vacuum arc melting, and their oxidation resistance at 850 °C is evaluated for up to 150 h. All alloys possess an FCC + B2 eutectic microstructure. With increasing Al content, the B2 phase fraction rises from 20.5% to 66.4%, and the microstructure evolves from an FCC matrix with dispersed B2 islands, via a lamellar eutectic transition, to a B2 matrix with isolated FCC islands. The oxidation kinetics of all alloys follow a parabolic rate law, and the Al0.85 and Al0.9 alloys exhibit the best oxidation resistance with a mass gain of only 0.49 mg/cm2 after 150 h of oxidation. The Al-rich B2 phase preferentially oxidizes to form Al2O3-rich oxide, while the Fe/Cr/Ni-rich FCC phase forms Cr2O3-rich oxide and spinel oxides. The oxide scale evolves into a bilayer structure with a dense continuous Al2O3-rich oxide inner layer and a Cr/Fe-rich oxide outer layer. An Al-depleted layer accompanied by B2-to-FCC decomposition forms in the subsurface, and its thickness decreases with rising Al content. The superior oxidation resistance of high-Al EHEAs stems from the synergistic effect of the B2 phase acting as an Al reservoir and eutectic interfaces acting as a possible short-circuit diffusion pathway, which jointly promote the formation of a continuous protective Al2O3-rich oxide scale.