Current Status of Research on the Oxidation Behavior of Refractory High Entropy Alloys

Refractory high entropy alloys (RHEA) are considered as novel promising high-temperature materials for structural applications at ultrahigh temperatures primarily due to their attractive mechanical properties. By contrast, the oxidation behavior of RHEA has raised concern owing to pest oxidation, significant weight changes, scale spallation, or even complete oxidation at elevated temperatures. Herein, the currently available literature on high-temperature oxidation behavior of RHEA is reviewed with respect to alloy composition, mass changes, corrosion products, and scale constitution. While many RHEA indeed suffer from poor oxidation resistance similar to that of pure refractory metals, some RHEA exhibit very good protectiveness, which is attributed to the formation of either well-known protective scales such as alpha-Al2O3 or rarely encountered complex oxides such as CrTaO4. Thermodynamic and kinetic aspects of oxide formation and growth are discussed to understand the oxidation mechanisms typical of RHEA. Further research directions with respect to additional in-depth studies elucidating the oxidation mechanisms as well as the further consequent improvement of the oxidation resistance of RHEA are developed from the current intermediate stage of research in the field.