Refractory High-Entropy Alloys (RHEAs) are regarded as perspective materials for high temperature applications primarily due to their attractive mechanical properties even at very high temperatures. However, a severe disadvantage of most RHEA is their low oxidation resistance stemming from the poor oxidation behaviour of refractory metals. A new concept pursues improving the oxidation resistance by enabling the formation of protective Cr-Ta-based oxide scales on RHEA. Experimentally, it was shown that these oxides can provide oxidation resistance competitive to that of commonly accepted Al₂O₃ and Cr₂O₃ or SiO₂.

While the oxidation mechanism of the equiatomic Cr-Ta-based oxide forming alloy Ta-Mo-Cr-Ti-Al in air was identified, the role of nitrogen is still unknown. First experiments showed increased oxidation kinetics if Ta-Mo-Cr-Ti-Al is exposed to a nitrogen-free Ar-20%O₂ atmosphere for 48h at 1000°C. Instead of a protective Cr-Ta-based oxide scale, a thick oxide scale with a more complex layered microstructure was observed.

Three possible mechanisms with respect to the role of nitrogen plays in the oxidation of Ta-Mo-Cr-Ti-Al are discussed in this study: (i) nitrogen reduces the density of oxygen-vacancies density (ii) nitride precipitations at the oxide/metal interface retard the inward oxygen diffusion and (iii) high solubility of nitrogen in the alloy reduces the oxygen transport into the metallic substrate. Corrosion experiments of the alloy studied in different atmospheric conditions support the last hypothesis. It was concluded that nitrogen represents one of the key players in the formation of protective Cr-TA-based oxides on RHEA.