High-Temperature Oxidation of Ta-Ti-(Nb)-Al-Cr-Mo Refractory High-Entropy Alloys. Part I: Isothermal oxidation

High-entropy refractory alloys (RHEAs) have attracted increasing interest in the scientific community due to their potential for applications in extreme temperature and oxidation conditions. Composed of multiple elements, they exhibit unexpected combinations of properties that positions them as potential substitutes to currently used materials. This study analyzed the oxidation of three RHEAs in the Ta-Ti-(Nb)-Al-Cr-Mo system, consisting in the TaTiAlCrMo equiatomic alloy, and two modifications of it, a non-equiatomic one, and one with partial substitution of Mo by Nb. They were exposed isothermally to 1000 °C for 20, 100, and 300 h under laboratory air. The equiatomic alloy gained less mass and formed uniform, thin, and adherent oxide layers, while the alloy with Nb showed a significantly increased mass gain, forming porous layers and reaching a breakaway oxidation stage; the non-equiatomic alloy showed apparently more irregular and thicker layers, but still exhibited a protective behavior. The predominantly formed oxides were the rutile-type (Cr,Ta,Ti)O 2 , and possibly Al 2 O 3 , Cr 2 O 3 , and Nb 2 O 5 . All alloys showed oxygen absorption in the substrate , which was more pronounced for the non-equiatomic alloy and the alloy with Nb. In general, it became clear that the chemical composition and microstructure of the alloys are crucial factors in defining oxidation resistance.