Microstructure evolution of directionally solidified Nb-Si alloy

By taking the method of liquid-metal cooled directional solidification, alloys with a nominal composition of Nb-14Si-24Ti-10Cr-2Al-2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R=1·2, 6, 18 mm min ^-1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)_SS, (Nb,Ti) ₅Si₃, and Laves phase Cr₂Nb, and the former two components formed (Nb,Ti)_SS+(Nb,Ti)₅Si₃ eutectics. In addition, (Nb,Ti)₅Si₃ laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr2Nb gathered along the boundary between (Nb,Ti) ₅Si₃ and (Nb,Ti)_SS, and the overall alloy composition became homogenisation.