Roles of Yttrium and Zirconium Additions in High-Temperature Oxidation Resistance of Al0.7CoCrFeNi High-Entropy Alloys

This study systematically investigates the high-temperature oxidation behavior of undoped, Y- and Zr-doped Al_0.7CoCrFeNi high-entropy alloys (HEAs). After 100 h at 1000 °C, the undoped alloy consisted of the outer Cr₂O₃/spinel layer and an inner region exhibiting either an internal oxidation zone or a continuous Al₂O₃ layer, together with AlN underneath. Significant scale spallation was observed throughout. In contrast, both Y- and Zr-doped alloys exhibited thinner, denser, and more adherent oxide scales without spallation, indicating a significant improvement in scale stability. The Y-doped alloy formed a discontinuous spinel layer, a continuous α-Al₂O₃ layer, and AlN precipitates underneath. In contrast, the Zr-doped alloy developed a similar α-Al₂O₃-based structure but no clear AlN precipitates. There were Zr-rich oxide intrusions at the interface. Under the present experimental conditions at 1000 °C, Y appeared to provide more effective oxidation resistance than Zr. In addition, the oxidation behavior of the Zr-doped alloy was further examined at 900 °C and 1100 °C in comparison with that of the Y-doped alloy reported previously. The Zr-doped alloy formed a mixed scale of spinel, α-Al₂O₃, and AlN at 900 °C, and developed pronounced oxide intrusions with Zr-rich cores encapsulated by Al₂O₃ shells at 1100 °C. Overall, Y and Zr doping effectively enhances oxidation resistance by stabilizing the α-Al₂O₃ scale and suppressing spallation.