In this study, two-step heat treatment is conducted to adjust the microstructure and mechanical properties of Nb-Si alloy with addition of Y. The time and temperature of second-step heat treatment are studied to optimize the process. According to the microstructural observation and the tests of mechanical properties, it is confirmed that the two-step heat treatment can significantly improve the fracture toughness (about 30.3%) of Nb-Si alloy without sacrifice of tensile strength which also increases slightly. The improvement can be attribute to the adjustment of microstructures by the two-step heat treatment: Compared to the silicide after single-step heat treatment, the volume of silicide after two-step heat treatment reduces by 40.6%; The silicides with sharp corners tend to be spheroidized by the two-step heat treatment. The proper micro-cracks introduced by the two-step heat treatment are also the reason for improvement of fracture toughness which can promote the crack deflection and crack bridge to toughen the Nb-Si alloy. The optimal fracture toughness about 32.6 MPa·m^1/2 can be obtained after the first-step heat treatment at 1450 °C for 12 h and the second-step heat treatment at 1100 °C for 10 h.

本研究中，通过添加Y进行两步热处理以调节Nb-Si合金的组织和力学性能。研究了第二步热处理的时间和温度以优化工艺。通过显微组织观察和力学性能测试，证实了两步热处理可以显着提高Nb-Si合金的断裂韧性（约30.3％），而不会牺牲拉伸强度，拉伸强度也略有增加。这种改善可以归因于两步热处理对显微组织的调节：与单步热处理后的硅化物相比，两步热处理后的硅化物体积减少了40.6％；具有尖角的硅化物倾向于通过两步热处理而球形化。通过两步热处理引入适当的微裂纹也是提高断裂韧性的原因，其可以促进裂纹挠曲和裂纹桥以使Nb-Si合金增韧。最佳断裂韧性约为32.6 MPa·m在第一步在1450°C下进行12小时的热处理，在第二步在1100°C下进行10 h的热处理后，可获得^1/2的结果。