Oxidation Resistance of Ti-Al-Cr-Nb-Based High-Entropy Alloys in Air at 1073 K

The effects of prolonged exposure to air at 1073 K on Al20Cr25Nb20Ti20Zr15 and Al20Cr25Nb19-Ti20Zr15Y1 (at.%) high-entropy alloys (HEAs) were investigated in this work. Combined scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM-EDS) analysis revealed that scales containing all major elements in the systems are formed during the oxidation process. Thermogravimetric analysis carried out under the above-mentioned corrosive conditions indicates an initial parabolic kinetics course in the case of the sample with 1 at.% Y addition. However, after a certain period of time, the correlation between mass gain per unit surface area and time becomes linear. In the other case, oxidation proceeds according to the parabolic rate law for the entire process duration. The addition of 1 at.% Y decreases the parabolic rate constants by 1 order of magnitude, thereby improving the chemical stability of the studied Al-Cr-Nb-Ti-Zr system. This is confirmed by visual evaluation, as well as SEM-EDS cross-sectional analysis. Additionally, x-ray diffraction studies indicate that a multiphase oxide scale is formed on the metallic core of both samples. This means that selective oxidation does not occur and all constituent elements took part in the reaction. Taking all of the above into account, it can be concluded that more research is required to fully understand and improve the corrosion resistance of Ti-Al-Cr-Nb-based HEAs.