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

Abstract

The effects of in-situ rare earth oxide phase La2O3 on the microstructure and properties of NbSi based ultrahigh temperature alloys were investigated. The NbSi 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 KQ 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 (TmLa2O3= 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 NbSi 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.