Microstructure evolution of Ni-NbC alloy under high-temperature gradient directional solidification

Directionally solidified microstructures of Ni-10.6wt% NbC alloy were investigated at various withdrawing velocities. The results show that at the given temperature gradient of 300 K/cm and the withdrawing velocity in a range of 5 μm/s to 20 μm/s, the solidified interface morphology of Ni-10.6% NbC alloy is planar. As the withdrawing velocity increases to 50 μm/s, the cellular eutectic microstructure forms. NbC phase with the tilobar and rod morphologies gradually transfer to the plate morphology with the increase of the withdrawing velocity. When the withdrawing velocity changes abruptly from 2 μm/s to 10 μm/s, the regular eutectic of Ni-10.6% NbC could keep; While from 2 μm/s to 50 μm/s and 100 μm/s respectively, the cellular eutectic and primary dendrites of Ni are obtained in directional solidification microstructure. In addition, the solidified microstructure is refined by increasing the withdrawing velocity abruptly compared to the constant withdrawing velocity due to the nucleation of NbC phase ahead of the interface, which decreases the fiber spacing of NbC.