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