Improved toughness and oxidation resistance of Nb–Si based ultra-high temperature alloys by in-situ generated La₂O₃ phase

The effects of in-situ rare earth oxide phase La₂O₃ on the microstructure and properties of Nb–Si based ultrahigh temperature alloys were investigated. The Nb–Si based alloys were doped with varying amounts of La (0.05, 0.1, 0.2, 0.4 and 0.8 at.%). The La₂O₃ phase forms in situ during the melting process. The fracture toughness is greatly improved by the La₂O₃ phase, and the K_Q value of the 0.4La alloy is 27.1% higher than that of the base alloy. The improvement of fracture toughness is due to these three aspects: first, the high-melting-point La₂O₃ phase (T_m^La₂O₃= 2588 K) precipitates first and effectively refines the microstructure. As a result, the frequency of secondary cracking and crack bridging increases. Second, the formation of the La₂O₃ phase significantly reduces the issue of oxygen contamination. Third, the high-hardness La₂O₃ phase separates during the fracture process, reducing the constraints on the deformation of Nb–Si based alloys. Additionally, La₂O₃ has the lowest Gibbs free energy among all possible formed oxides. During the oxidation process, the continuous oxidation behavior of the La₂O₃ phase reduces the degree of oxidation inside the alloy. Therefore, the oxidation resistance is significantly improved.