Rapid solidification in undercooled Ni-18.7 at.%Sn eutectic melts was observed in-situ by a high-speed high-resolution camera and the microstructures were characterized in detail by electron backscattering diffraction. For the first time, the exact crystallographic orientation relations (ORs) between HCP-Ni₃Sn (α-Ni) subsets were analyzed. For HCP-Ni₃Sn, the $\left\{11\overline{2}1\right\}$$<\overline{1}\overline{1}26>$ and/or $\left\{11\overline{2}2\right\}$$<11\overline{2}\overline{3}>$ twin ORs (i.e., HCP-Ni₃Sn twins) hold independently on undercooling, whereas for α-Ni, the $\left\{111\right\}$$<11\overline{2}>$ twin OR is the case at low undercooling and would hold initially at intermediate and high undercooling. The roles of twinning and allotropy transformation (i.e., FCC-Ni₃Sn → HCP-Ni₃Sn) were integrated to reveal the formation mechanism of HCP-Ni₃Sn twins, and a reversed OR transition analysis was carried out for representative samples from low to high undercooling. Consequently, novel twin-assisted eutectic growth was found to occur all along. On this basis, we showed that the single nucleation mode of Herlach is followed, and speculated that primary and secondary coupled eutectic dendrite growth and un-coupled growth of α-Ni and FCC-Ni₃Sn might all be the origins of anomalous eutectics. This work would shed some lights on the long-time controversies about the nucleation mode and the formation mechanism of anomalous eutectics in undercooled eutectic alloys.

通过高速高分辨率相机原位观察过冷Ni-18.7 at.%Sn共晶熔体的快速凝固，并通过电子背散射衍射详细表征了微观结构。首次分析了 HCP-Ni ₃ Sn ( α- Ni) 亚群之间的精确晶体取向关系 (OR)。对于 HCP-Ni ₃ Sn，$\left\{11\overline{2}1\right\}$$<\overline{1}\overline{1}26>$和/或$\left\{11\overline{2}2\right\}$$<11\overline{2}\overline{3}>$孪晶 OR（即 HCP-Ni ₃ Sn 孪晶）独立地保持过冷，而对于α - Ni，$\left\{111\right\}$$<11\overline{2}>$双 OR 是在低过冷度下的情况，最初会在中度和高过冷度下保持。结合孪晶和同素异形体转变（即FCC-Ni ₃ Sn → HCP-Ni ₃ Sn）的作用，揭示了HCP-Ni ₃ Sn孪晶的形成机制，并对来自的代表性样品进行了逆OR跃迁分析。过冷度从低到高。因此，发现一直存在新的双辅助共晶生长。在此基础上，我们证明遵循Herlach的单形核模式，推测α -Ni和FCC-Ni _{3的初生和二次耦合共晶枝晶生长和非耦合生长。}Sn 可能都是异常共晶的起源。这项工作将为长期以来关于过冷共晶合金的形核模式和异常共晶形成机制的争议提供一些启示。