Improvement of fracture toughness of Nb-Si alloy by two-step heat treatment

Abstract

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·m1/2 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.