Oxidation behaviour of Ti-V-Cr-Nb-Ta and Al-Ti-V-Cr-Ta refractory high entropy alloys: Effects of Nb and Al substitutions

• Oxidation kinetics of four RHEAs in air at 1000 °C were investigated. • Replacing Nb with Al improved oxidation resistance in RHEAs in the study. • Al-containing RHEAs formed a textured oxide layer enhancing oxidation resistance. • Nb-containing RHEAs formed porous oxides with limited protection to further oxidation. Refractory high entropy alloys (RHEAs) are candidate materials for nuclear and other high-temperature applications, due to their high-temperature strength, good irradiation resistance, and high melting temperatures. However, a significant issue with current RHEAs is that those that exhibit good mechanical properties tend to show poor oxidation resistance, and vice versa. In this paper, the oxidation kinetics of four RHEAs (20Ti-20 V-20Cr-20Nb-20Ta, 25Ti-25 V-5Cr-20Nb-25Ta, 20Al-20Ti-20 V-20Cr-20Ta, and 14Al-24Ti-24 V-14Cr-24Ta alloys) were investigated in an air atmosphere at 1000 °C. As-prepared and oxidised samples were characterised by a combination of state-of-the-art microscopy techniques. By replacing Nb with Al, the two Al-containing alloys were observed to form a less porous oxide microstructure, showing significant improvement in their oxidation resistance. As a result of oxygen/nitrogen ingress during oxidation and associated phase-segregation at high temperatures, the hardness of the underlying metal matrix of the RHEAs increased by approximately 5 GPa.