Comparative investigation of high temperature oxidation behavior of single-phase equi-atomic Ti-Zr-Hf-Nb alloys: From binary to quaternary

Refractory multi-principal element alloys (RMPEAs) have emerged as promising candidates for high temperature structural applications due to their exceptional strength retention at elevated temperatures. However, their practical implementation has been limited by oxidation resistance concerns under extreme operating conditions. This work presents a systematic investigation of the oxidation mechanisms of equi-atomic TiNb, ZrNb, TiZrNb, TiHfNb and TiZrHfNb alloys through combined experiments and first-principles calculations. Isothermal oxidation at 1000℃ for 60 min revealed distinct behavior between binary alloys and RMPEAs. All RMPEAs exhibited significantly higher initial surface reaction rates compared to binary counterparts, attributed to the stronger oxygen adsorption at multi-element sites confirmed by first-principles calculations. Subsequently, RMPEAs formed constant thin oxide layer with severe progressive internal oxidation. In contrast, binary alloys formed external oxide scales that grow continuously, leading to progressive oxide thickening with stable internal reaction zone (IRZ) thickness. Detailed microstructural analysis revealed that microvoids formed in the external oxide layer, and vertical cracks developed in the IRZs of RMPEAs. Both features accelerated inward oxygen diffusion. This oxygen ingress, combined with high intrinsic oxygen solubility of constituent elements, led to severe internal oxidation in RMPEAs. These findings provide critical insights into the oxidation dynamics of RMPEAs, highlighting key challenges and offering guidance for future alloy design to enhance high-temperature stability.