High temperature oxidation behavior and mechanism of AlCrNbTaTi refractory high entropy alloys

The isothermal oxidation behavior of single-phase body-centered cubic AlCr_0.5NbTa_xTi_1-x (x = 0, 0.5, 1) alloys was investigated in air for 100 h at 800–1200°C. The average grain size decreased from 615.05 to 249.86 μm with increasing Ta content. Oxidation kinetics followed a near parabolic law at 800°C, shifting to near-linear kinetics at 1000–1200°C. A temperature-dependent variation in Ta effectiveness was observed. At 800°C, AlCr_0.5NbTa₁Ti₃ exhibited slightly higher mass gain and oxidation rate constant k (0.534 mg/(cm²·h)), which could be attributed to the grain refinement of Ta, providing diffusion pathways for oxygen along grain boundaries. At 1000 and 1200°C, Ta addition improved oxidation, with k values 0.924 and 2.851 mg/(cm²·h) of AlCr_0.5NbTa₁Ti₃, respectively. Results indicated that Ta-containing alloys form mixed AlTaO₄/CrTaO₄ oxide scales at 1000–1200°C, enhancing scale continuity and limiting oxidant ingress compared to Ta-free alloys, while TiN formation was observed beneath the scale after high-temperature exposure. These findings provide multi-temperature kinetic metrics and scale-evolution evidence for evaluation of Al–Cr–Nb–Ta–Ti refractory HEAs for high-temperature applications.