Oxidation Behavior of Al_xHfNbTiVY_0.05 Refractory High-Entropy Alloys at 700–900 °C

Refractory high-entropy alloys (RHEA) are considered as potential candidates for new-generation energy-related high-temperature applications. However, the poor high-temperature oxidation resistance of RHEAs, resulting in phenomena such as significant weight gain, scale spallation, pesting, and even complete oxidation, limits their applications. In this study, the oxidation behavior of Al_xHfNbTiVY_0.05 (x = 0.75; 1; 1.25) high-entropy alloys was investigated at 700–900 °C. The isothermal oxidation tests showed that the oxidation resistance of Al_xHfNbTiVY_0.05 RHEA is strongly influenced by temperature and time. In addition, accelerated oxidation, known as pesting, was observed to occur at 700 °C for all alloys; while, partial spallation was observed at 800 °C for the Al₁ and Al_1.25 alloys. Detailed analyses of oxidation kinetics have been carried out for the oxidation test series at 900 °C. The mechanism behind disintegration was investigated and attributed to accelerated internal oxidation followed by the formation of voluminous Nb₂O₅, TiNb₂O₇, and fast-growing AlNbO₄, and is also thought to be related to the partial evaporation of V₂O₅.