Oxidation resistance of Al-containing refractory high-entropy alloys

Refractory high entropy alloys (RHEAs) hold the promise of superior mechanical properties at high temperatures but are plagued by a lack of oxidation resistance. In this work, the role of Al additions (4.8, and 13 at.%) to HfNbTaTiZr is explored as a means of improving protective oxide scale formation in RHEAs. Oxide formation was resolved using STEM-EDS/EELS, precession electron diffraction, and computational predictions of formation energy. Our results show that oxides with large negative formation energies (i.e., HfO2/ZrO2) always formed near the metal/oxide interface while Al2O3 and oxides with less negative formation energies formed near the surface. The addition of Al prevented pesting in both alloys and formed Al2O3 with 13 at.% Al. While the Al2O3 formed was not continuous, we report the lowest threshold yet to prevent pesting with only 4.8 at.% Al added. These findings provide guidance to future alloy development of Al2O3-forming alloys.