Disappearance of Ni₃Nb primary phase in hypereutectic alloy of Ni-24.19wt% Nb at abruptly decreasing growth rate under directional solidification

Directionally solidified microstructures of Ni-24.19wt% Nb hypereutectic alloy were investigated at the growth rate of 10 μm/s, and the abruptly decreasing growth rate from 10 to 1 μm/s. The results show that the microstructure consists of Ni₃Nb primary phase and eutectic (Ni + Ni₃Nb), in agreement with the theoretical calculation results at the growth rate of 10 μm/s. After the solidified distance of 20 mm, the growth of Ni₃Nb primary phase achieves the quasi-steady state. At the abruptly decreasing growth rate from 10 to 1 μm/s, the volume fraction of Ni, Nb primary phase firstly increases, attributing to the change of interface composition, and then decreases to zero. Finally, the off-coupled growth microstructure changes to the coupled growth eutectic due to the competitive growth of interface temperature between Ni₃Nb primary phase and eutectic (Ni + Ni₃Nb). The disappearance of Ni₃Nb primary phase does not simultaneously occur and mainly originates from the side to the center of the sample.