The Effects of the Withdrawal Rate and Heat Treatment on the Microstructure of Directionally Solidified Nb-14Si-24Ti Alloy

In this work, an Nb-14Si-24Ti alloy (at.%) was prepared by liquid-metal-cooled directional solidification (DS) and then heat treated (HT) at 1450 degrees C for 100 h. The effects of the withdrawal rate and heat treatment on the microstructure were investigated. The results show that the DS samples consisted of Nb-SS dendrites, Nb3Si blocks and eutectic Nb-SS/Nb3Si. As the withdrawal rate increased, the microstructure became finer, the volume fraction of eutectic Nb-SS/Nb3Si increased and the morphology of eutectic cell transformed from petaloid to regularly arranged granular morphology. The microstructure of the HT samples consisted of Nb-SS and Nb3Si. The metastable phase of Nb3Si did not discompose through the eutectoid reaction. The dendritic Nb-SS connected through diffraction and the secondary dendrite arm got coarser. The boundary of eutectic Nb-SS/Nb3Si became blurry and the black Ti-rich phase of the DS samples in this region disappeared. The Nb3Si connected to form the continuous matrix. The granular Nb-SS of eutectics spheroidized and the lamellar Nb-SS of eutectic interconnected and became more bending distributed uniformly in the Nb3Si matrix.