Microstructure optimization of directionally solidified hypereutectic Nb-Si alloy

Nb-16Si-24Ti-10Cr-2Al-2Hf alloy was directionally solidified with withdrawal rates of 1.2, 6, 18, 36 and 50 mm/min and then heat treated at 1400, 1450 and 1500 C with withdrawal rate of 50 mm/min for 10 h. The effects of withdrawal rate and heat treatment temperature on the microstructure were studied. The microstructure of directionally solidified alloy was composed of the primary Nb₅Si₃, Nbss/Nb₅Si₃ eutectic cells and Cr₂Nb, which distribute paralleled to the growth direction. The microstructure becomes more refined with the increasing withdrawal rate, accompany with the evolution of eutectic cells morphology. After heat treatment, Nbss phase connects and forms a continuous matrix, and the Cr₂Nb phase becomes smaller and distributes more dispersedly. After heat treatment at 1450 C for 10 h, the alloy achieves balance between the optimization of microstructure and alleviation of solute segregation.