Microstructure, Mechanical Properties and Oxidation Resistance of Nb-Si Based Ultrahigh-Temperature Alloys Prepared by Hot Press Sintering

Nb-Si based ultrahigh-temperature alloys with the composition of Nb-22Ti-15Si-5Cr-3Al (atomic percentage, at. %) were prepared by hot press sintering (HPS) at 1250, 1350, 1400, 1450 and 1500 °C. The effects of HPS temperatures on the microstructure, room temperature fracture toughness, hardness and isothermal oxidation behavior of the alloys were investigated. The results showed that the microstructures of the alloys prepared by HPS at different temperatures were composed of Nbss, βTiss and γ(Nb,X)₅Si₃ phases. When the HPS temperature was 1450 °C, the microstructure was fine and nearly equiaxed. When the HPS temperature was lower than 1450 °C, the supersaturated Nbss with insufficient diffusion reaction still existed. When the HPS temperature exceeded 1450 °C, the microstructure coarsened obviously. Both the room temperature fracture toughness and Vickers hardness of the alloys prepared by HPS at 1450 °C were the highest. The alloy prepared by HPS at 1450 °C exhibited the lowest mass gain upon oxidation at 1250 °C for 20 h. The oxide film was mainly composed of Nb₂O₅, TiNb₂O₇, TiO₂ and a small amount of amorphous silicate. The formation mechanism of oxide film is concluded as follows: TiO₂ forms by the preferential reaction of βTiss and O in the alloy; after that, a stable oxide film composed of TiO₂ and Nb₂O₅ forms; then, TiNb₂O₇ is formed by the reaction of TiO₂ and Nb₂O₅.