Effect of heat treatment and different elements on the phase composition and fracture behavior of Nb-Si-Zr based alloy

In this paper, the effects of 1300 °C/30 h heat treatment on the microstructure and fracture behavior of Nb-16Si-20Zr-xm (xm=0, 4Cr, 2C, 4Hf, 4Ta, 0.5Sc) alloys were studied. Except for Nb-16Si-20Zr-2C alloy, the transformation from γNb₅Si₃ phase to αNb₅Si₃ phase occurs in other Nb-Si-Zr based alloys after heat treatment. There is a small amount of ZrC phase exists in the Nb-16Si-20Zr-2C alloy. After heat treatment, the segregation in the alloys decrease. In addition, the different phases are spheroidized and coarsened but the phases in Nb-16Si-20Zr-0.5Sc alloy are refine during the heat treatment. Except for Nb-16Si-20Zr and Nb-16Si-20Zr-2C, the room temperature fracture toughness of other Nb-Si-Zr based alloys is improved after heat treatment, which is related to the tetragonal crystal structure of αNb₅Si₃ phase and the spheroidization of the phase. Nb-16Si-20Zr-2C(HT) alloy has the highest fracture toughness mainly because of the precipitation strengthening of ZrC phase. There are many crack deflections and microcracks in the crack path of Nb-16Si-20Zr-2C(HT), and its fracture surface is not flat.