Halo formation in directional solidification of Ni-Ni₃Nb hypereutectic alloy

The pulling velocities under directional solidification were calculated and compared with experimental results for various growth temperatures of different phases in Ni-24.19% Nb hypereutectic alloy. When the pulling velocity is 1 μm/s, coupled growth eutectic was obtained in experiment, in agreement with the calculated result of 1.1 μm/s at the temperature gradient of 160 K/cm in liquid. As the pulling velocity increases to 2 μm/s, primary β-Ni₃Nb plate dendrites and coupled growth eutectic microstructure appeared. At the pulling velocity of 10 μm/s, halo α-Ni dendrites formed from the Ni-rich solute neighbor to primary β-Ni₃Nb phase, and followed by the growth of coupled eutectic. The critical pulling velocity required to form the halo α-Ni dendrites was calculated as 16 μm/s by Nave et al.'s model [Acta Materialia, 50(2002) pp. 2837-2849]. This matches well with the experiment result of 10 μm/s. The calculation also shows that α-Ni haloes will not appear at the composition below 23.35% Nb in the directionally solidified Ni-Ni₃Nb hypereutectic alloys.