Microstructure and room-temperature fracture toughness of directionally solidified Nb-Si-Ti-Cr-Al-Hf alloy

The Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at%) was directionally solidified at 1750 °C with withdrawal rates of 0.24 mm/min, 1.2 mm/min, 6 mm/min, 18 mm/min, 30 mm/min, 50 mm/min, 70 mm/min and 100 mm/min, and subsequent heat treatment was carried out at 1375 °C for 10 h. The microstructures and room-temperature fracture toughness of the directionally solidified and heat treated samples were investigated. The results showed that instead of the microstructure consisting of NbSS dendrites, NbSS+Nb5Si3 eutectics and Cr2Nb+Ti-rich NbSS eutectics, the microstructure composed of NbSS dendrites, Nb3Si laths and Cr2Nb+Ti-rich NbSS eutectics was observed when the withdrawal rate was higher than 30 mm/min. After the heat treatment, the Nb3Si transformed into extremely fine NbSS and Nb5Si3, and the NbSS dendrites changed to be remarkably continuous. The room-temperature fracture toughness could reach a maximum of 15.7 MPa·m1/2 for the alloy withdrawn at 50 mm/min plus heat treatment. It is suggested that the microstructure with fine intermetallic phases (Nb5Si3 and Cr2Nb) and continuous NbSS would be of great benefit to the room-temperature fracture toughness.