Abstract: 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 {11$\bar{2}$1}<$\bar{11}$26> and/or {11$\bar{2}$2}<11$\bar{23}$> twin ORs (i.e., HCP-Ni₃Sn twins) hold independently on undercooling, whereas for α-Ni, the {111}<11$\bar{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.

Key words: Twin-assisted eutectic growth, Allotropy transformation, Nucleation mode, Anomalous eutectics, Undercooling