High-temperature oxidation behaviour of Al_xFeCrCoNi and AlTiVCr compositionally complex alloys

Compositionally complex alloys (CCAs), also termed as high entropy alloys (HEAs) or multi-principal element alloys (MPEAs), are being considered as a potential solution for many energy-related applications comprising extreme environments and temperatures. Herein, a review of the pertinent literature is performed in conjunction with original works characterising the oxidation behaviour of two diverse Al-containing alloys; namely a lightweight (5.06 g/cm³) single-phase AlTiVCr CCA and a multiple-phase Al_0.9FeCrCoNi CCA (6.9 g/cm³). The thermogravimetric results obtained during oxidation of the alloys at 700 and 900 °C revealed that both alloys tended to obey the desired parabolic rate law. Post-exposure analysis by means of electron microscopy indicated that while the oxide scale formed on the AlTiVCr is adherent to the substrate, the scale developed on the Al_0.9FeCrCoNi displays a notable spalling propensity. This study highlights the need for tailoring the protective properties of the oxide scale formed on the surface of the CCAs.