High-Temperature Oxidation Behavior of FeCoCrNi+(Cu/Al)-Based High-Entropy Alloys in Humid Air

Crystals 14 (2024),  60, DOI: 10.3390/cryst14010060

Previous studies showed some transition metal high-entropy alloy (HEA) compositions can have good oxidation resistance in air up to 800 °C. Four equiatomic HEAs have been developed based on FeCoCrNi with additions of Mn, Cu, Al or Al+Cu. The oxidation behavior of these HEAs was compared in humid (10 vol.% H₂O) air at 800 °C for 100–500 h to investigate the influence of water vapor on the oxidation mechanisms. The Cu- and Al-containing alloys exhibited improved oxidation resistance over the Mn composition. For the Cu-containing alloy, a local attack of the Cu-rich phase was observed, which formed an Fe/Ni/Co/Cr spinel that was surrounded by Cr₂O₃. This oxide was thicker for the humid air atmosphere when compared to dry air, and the transition of the Cu oxide to the spinel was accelerated. The Al-containing HEA formed a thin Al₂O₃ scale with humidity suppressing AlN formation and forming a smoother oxide layer. The Al+Cu composition had the highest overall oxidation resistance (minimal local attack, no nitridation) and also showed a smooth oxide scale topography under humid air oxidation as opposed to a plate-like, rougher scale under dry air.