Microstructure evolution of directionally solidified Nb-Si alloy

摘要

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)_5Si_3, and Laves phase Cr_2Nb, and the former two components formed (Nb,Ti)_(ss) + (Nb,Ti)_5Si_3 eutectics. In addition, (Nb,Ti)_5Si_3 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 Cr_2Nb gathered along the boundary between (Nb,Ti)_5Si_3 and (Nb.Ti)ss, and the overall alloy composition became homogenisation.