Abstract: Based on the composition of Nb-16Si-22Ti-2Al-2Hf, 2 at% and 17 at% Cr (referred to as 2Cr and 17Cr alloys) are respectively added to replace Nb. Effects of Cr additions on phase constitution, microstructure, room temperature fracture toughness and high temperature strength are investigated, and failure mechanisms at room and high temperatures are analyzed. It is found that the microstructure of the as-cast and heat-treated (annealed at 1 375 ℃ for 100 h) 2Cr alloy is composed of Nbss and β-Nb₅Si₃, while that of the 17Cr alloy consists of the Nbss, β-Nb₅Si₃ and Laves Cr₂Nb with a C15 structure, and the spherical C15-Cr₂Nb precipitates are found in the β-Nb₅Si₃ of the heat-treated 17Cr alloy. As the Cr content increases from 2 at% to 17 at%, room temperature fracture toughness K_Q of the annealed alloy decreases from 14.32 MPa·m^1/2 to 10.30 MPa·m^1/2. Strength, as a function of the Cr content, is dependent on temperature. At room temperature and 1 150 ℃, the hardness or strength increases with the Cr content, whereas at 1 250 ℃ and 1 350 ℃, the strength decreases. For example, the 0.2% yield strengths σ_0.2 of the 2Cr and 17Cr alloys are 349 MPa and 387 MPa at 1 150 ℃, but 306 MPa and 74 MPa at 1 350 ℃, respectively.

Key words: Nb-Si alloys, phase composition, microstructure, fracture toughness, high temperature strength