Solidification of Highly Undercooled Hypereutectic Ni-Ni3B Alloy Melt

The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔTp ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni3B dendrite, rod eutectic, and precipitates was obtained. If ΔTp < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni3B and L → Ni/Ni3B); if ΔTp ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni23B6, Ni23B6 → Ni/Ni3B, and Lr → Ni/Ni3B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as Lr → Ni/Ni3B; it also showed that this velocity decreases with increasing ΔTr, which can be explained by reduction of the residual liquid fraction and decrease of Ni23B6 decomposition rate.