Oxidation and Hot Gas Corrosion of Al-Cr-Fe-Ni-Based High-Entropy Alloys with Addition of Co and Mo

Multicomponent, high-entropy alloys (HEAs) are promising candidates for replacing conventional alloys in high-temperature applications. Herein, the high-temperature corrosion of AlCrFeNiX0.5 (X = Co, Mo) is investigated. The samples are tested for their oxidation resistance at temperatures up to 1200 °C for 120 h and their behavior in NaCl/Na2SO4 at 900 °C for 96 h. They are benchmarked against commercial alloys such as FeCrAl. Despite the same contents of Al and Cr, the HEAs form different oxide layers showing very different oxidation resistance. The type of oxide is related to the multiphase microstructure. The samples exhibit different amounts of ordered and unordered body-centered cubic (bcc) phase. The Co-containing specimen shows an oxidation resistance that performs similarly well as FeCrAl. Its behavior is ascribed to the formation of an Al2O3 layer, which is very stable at high temperatures. The sample with X = Mo exhibits an additional Mo-rich sigma phase, thus posing the risk of catastrophic oxidation. However, the Mo-containing HEA is more resistant in the environment of molten salt. Preoxidation treatment at a lower oxygen partial pressure proves to prolong life span of the Mo-containing HEA in hot air. Furthermore, a positive impact on oxidation resistance by addition of Y is affirmed.