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.