Microstructure evolution of Ni-Ni₃Nb alloy at an abruptly changing velocity under directional solidification

Considering the effect of non-steady-state solidification process on the solidified eutectic microstructure, Ni-Ni₃Nb hypereutectic alloy was employed to investigate the microstructure evolution at an abruptly changing velocity during directional solidification. The results show that at the pulling velocity jumping from 1 to 10 μm/s, the eutectic lamellar spacing is refined by the process of Ni phase continuously splitting. The relation between the lamellar spacing (A) and the pulling velocity (V) agrees with λV^1/2-18 μm^3/2s^-1/2. As the pulling velocity decreases abruptly from 10 to 1 μm/s, Ni phases are merged in the long non-steady-state transition zone. When the pulling velocity increases abruptly from 5 to 15 μm/s, Ni₃Nb phases keep stability. However, some of Ni phases grow unstably, resulting in the regular eutectic microstructure transiting to the irregular eutectic microstructure.