Effect of Nb Content on the Oxidation Behavior and Microstructural Evolution of Ti2AlNb-Based Alloys

Ti2AlNb-based alloys are potential structural materials for high-temperature applications due to their low density and superior specific strength. However, their widespread application is limited by relatively poor oxidation resistance above 700 °C. While Ti2AlNb-based alloys exhibit promising mechanical properties, their oxidation behavior remains inadequately characterized, particularly concerning the role of Nb content. In this study, the high-temperature oxidation behavior of Ti2AlNb-based alloys with different Nb contents was investigated at 800 °C in air. The results revealed a characteristic double-layered oxide structure consisting of an outer TiO2 layer and inner alternating TiO2-rich and AlNbO4-rich sublayers. Thermodynamic calculations confirmed the favorable formation of TiO2, Al2O3, Nb2O5, and AlNbO4 at high temperatures. However, the reaction between Nb2O5 and Al2O3 hinders the formation of a protective Al2O3 layer. Increasing the Nb content was found to replace Ti atoms, reducing the diffusion rate of oxygen and simultaneously decreasing the thickness of porous TiO2 regions. Nevertheless, the inadequate rate of aluminum diffusion inhibited adequate Al2O3 formation, leading to limited overall oxidation protection. These findings elucidate the composition–oxidation relationship in Ti2AlNb-based alloys and provide valuable insights for tailoring Nb and Al contents to achieve a balanced combination of mechanical properties and high-temperature oxidation resistance.