Microstructural evolution and property variation of Nb-Ti-Si-Cr-Ta alloys regulated by tantalum content

Five multi-component NbSi based ultrahigh temperature alloys with different Ta contents (0, 1, 2, 4, and 8 at.%) were prepared via vacuum non-consumable arc melting. The effects of Ta content on the microstructure, nanoindentation hardness, room-temperature fracture toughness, compressive strength at 1250 °C, and oxidation resistance at 1250 °C of the alloys were systematically investigated. The results indicate that the alloys containing 0 and 1 at.% Ta consist of primary γ(Nb,X)₅Si₃ and Nbss/γ(Nb,X)₅Si₃ eutectic structure. By contrast, the alloys with 2, 4, and 8 at.% Ta feature a multi-phase microstructure composed of primary γ(Nb,X)₅Si₃, primary α(Nb,X)₅Si₃, and Nbss/γ(Nb,X)₅Si₃ eutectic structure. Ta addition suppresses the formation of γ(Nb,X)₅Si₃, while promoting the formation of α(Nb,X)₅Si₃. The addition of Ta presents an overall beneficial effect on the comprehensive properties of the alloys. The solid solution strengthening effect derived from Ta addition enhances the nanoindentation hardness of Nbss and primary α(Nb,X)₅Si₃ blocks, and also improves the compressive strength at 1250 °C of the alloys. The alloy with 2 at.% Ta exhibits the optimal compressive strength of 419.7 MPa. For alloys with higher Ta contents (4 and 8 at.%), a continuous Ta₂O₅ layer impregnated with amorphous silicates is formed in the outer layer of the oxide scales during oxidation, which reduces the number of oxygen diffusion pathways and hinders the inward diffusion of oxygen, thereby effectively improving the oxidation resistance of the alloys.