Oxidation Behavior of Al x (CoCrFeNi) 100- x High-Entropy Alloys Under Thermal-Cycling Conditions

The oxidation behavior of five different high-entropy alloys from the Al–Co–Cr–Fe–Ni metallic system, namely Al x (CoCrFeNi) 100- x ( x = 0; 3; 6; 9; 12), was studied under thermal-cycling conditions, at 1273 K in air atmosphere. The choice of selected compositions allowed for covering the chromia-to-alumina former transition, as well as the transition from the FCC single-phase solid solution structure to multiphase alloys with Al-enriched B2-structured constituent. The measurements were taken for 500 cycles (1 cycle - 1 h of heating, 20 min of cooling). The results indicate a profound influence of the thermal-cycling conditions on the oxidation products, with extremely complex scale structures and extensive internal oxidation and nitridation zones, as well as severe spallation of the oxide scale in most cases, showing the limited usefulness of these alloys for high-temperature applications at the current stage of their development.