摘要
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-Ni_(3)Sn(α-Ni)subsets were analyzed.For HCP-Ni_(3)Sn,the{1121}<1126>and/or{1122}<1123>twin ORs(i.e.,HCP-Ni_(3)Sn twins)hold independently on undercooling,whereas forα-Ni,the{111}<112>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-Ni_(3)Sn→HCP-Ni_(3)Sn)were integrated to reveal the formation mechanism of HCP-Ni_(3)Sn twins,and a reversed OR transition analysis was carried out for rep-resentative 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 ofα-Ni and FCC-Ni_(3)Sn 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.
基金
This work was financially supported by the National Natural Science Foundation of China(No.51975474)
the Fundamental Research Funds for the Central Universities(No.3102019JC001)
Haifeng Wang is very grateful to Prof.D.M.Herlach(1949-2022)for his inspiring suggestions and continuous support on the work of rapid solidification.
