Effect of Cr addition on the microstructure and room-temperature fracture toughness of Nb-Si based ultra-high temperature alloys

Four Nb-Si based ultra-high temperature alloys with compositions of Nb-22Ti-16Si-4Hf-3AlxCr (x = 0, 3%, 5% and 10%, atomic fraction) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450℃ for 50 h. The effects of Cr content on the microstructure and room-temperature fracture toughness of the alloys were investigated. The results showed that the Cr addition did not change the crystal structure of the silicide γ(Nb, X)₅Si₃. The amount of γ(Nb, X)₅Si₃ rose while that of the eutectic Nbss/γ(Nb, X)₅Si decreased with the increasing Cr content. Furthermore, a low melting point of three-phase eutectic composed of Nbss, (Nb, X)₅Si₃ and Cr₂(Nb, X) was observed in Cr-containing alloys and its amount gradually increased with the increasing Cr addition. After 1450℃/50 h heat-treatment, the original Nbss dendrites as well as eutectic colonies disappeared, and the microstructural uniformity was significantly ameliorated. The alloys with 0, 3% and 5% Cr were all composed of phases Nbss and γ(Nb, X)₅Si₃, while three-phase equilibrium of Nbss, γ(Nb, X)₅Si₃ and Cr₂(Nb, X) was observed in the alloy with 10% Cr. However, the room-temperature fracture toughness of the as-cast alloys exhibited a decreasing trend with Cr addition.