Revealing the origin of the exceptional elevated-temperature oxidation resistance of an L12-strengthened NiCoCrFe-based high-entropy alloy

Nanoparticle-strengthened high-entropy alloys (HEAs) are currently served as potential high-temperature materials for modern industrial applications. In this work, the oxidation behavior of an L1₂-strengthened HEA with superior mechanical properties in elevated temperatures was systematically investigated by isothermal oxidation tests at 700, 800 and 900 °C in air. The results reveal that this alloy exhibits exceptional oxidation resistance at 700–900 °C, compared to typical nickel-based and high-entropy alloys. At 700 ℃, continuous Cr₂O₃ is the key to the oxidation resistance. More complex hierarchical oxide scales formed at 800 and 900 °C, primarily consisted of the TiO₂, spinel, Cr₂O₃ and Al₂O₃. The synergy of Cr₂O₃ and Al₂O₃ oxide scale at 800 °C ensure the oxidation resistance, but the spinel oxide scales formed lead to the spallation of the outermost oxide film and the internal oxidation. After oxidation at 900 ℃, the formation of spinel and TiO₂ oxide layer can effectively reduce the evaporation of Cr₂O₃ oxide layer. Furthermore, the blocking effect of TiTaO₄ layer on Al ions and the high diffusion rate of Al element resulting in the formation of a continuous and dense Al₂O₃ oxide layer, playing a key role in the oxidation resistance at 900 ℃.