High temperature oxidation of Al₀.₇CoCrFeNi high-entropy alloys doped with Y, Hf, and Zr in dry and humid atmospheres

The high-temperature oxidation behaviour of Al 0.7 CoCrFeNi high-entropy alloys (HEAs) doped with trace reactive elements (Yttrium (Y), Hafnium (Hf), and Zirconium (Zr)) was investigated at 1000 °C in Ar-20%O 2 , Ar-20%H 2 O, and Ar-20%O 2 -20%H 2 O atmospheres. All alloys underwent a phase transformation from the as-cast A1 + A2 + B2 structure to a stable A1 + B2 dual-phase configuration upon oxidation. Oxidation may proceed via phase-selective pathways, wherein Al-rich B2 domains promoted the formation of alumina, while Cr-rich FCC regions favoured spinel-type oxides. Water vapour accelerated oxidation kinetics but simultaneously enhanced oxide scale adherence. In contrast to the undoped alloy, which exhibited severe scale spallation, all reactive element (RE)-doped alloys exhibited continuous and adherent oxide layers under all test conditions. At a doping level of 0.04 at.%, Y, Hf, and Zr all significantly improved scale adhesion, with interfacial peg formation and a reduction in interfacial voids. However, Hf and Zr formed readily observable oxide clusters (HfO₂, ZrO₂) at the metal/oxide interface, whereas Y did not produce similarly distinct oxide features in the same region. When the Y content was increased to 0.42 at.%, the alloy, which already contained coarse Y-rich precipitates in the as-cast state, developed pronounced Y-rich oxide intrusions along the oxide/substrate interface after prolonged oxidation. Moreover, raising the Hf content to 0.42 at.% promoted the development of Hf-rich oxides both within the scale and along the interface. These findings elucidate the distinct roles of RE dopants and oxidation atmosphere in governing scale evolution and provide insights into the design of oxidation-resistant HEAs for high-temperature applications in steam-containing environments. • Water vapor accelerates oxidation but suppresses scale spallation in undoped Al₀.₇CoCrFeNi HEA. • Trace Y, Hf, and Zr doping enables adherent Al₂O₃ scale formation in dry and wet O₂ at 1000 °C. • Y doping at 0.04 at.% develops interfacial pegs without detectable oxides, whereas 0.42 at.% forms Y-rich intrusions. • Hf and Zr doping at 0.04 at.% form interfacial pegs composed of Hf- and Zr-rich oxide clusters.