Mechanism of anomalous eutectic formation in the solidification of undercooled Ni–Sn eutectic alloy

Anomalous eutectics in the solidification structure of undercooled Ni–18.7 at.% Sn eutectic alloy were examined by optical metallography and electron backscattered diffraction. It was revealed that α–Ni particulates are, in principle, randomly distributed in the anomalous eutectics in the undercooling range investigated. Another eutectic phase, β–Ni₃Sn, is well orientated at low undercoolings but gradually becomes inconsistent in orientation as undercooling increases, accompanied by an increasing number of grain boundaries in it. As the solidification structure changes from a mixture of anomalous eutectics plus lamellar eutectics to full anomalous eutectics beyond a critical undercooling of 130 K, however, misorientation in the β–Ni₃Sn phase disappears completely from the measurement area. Partial remelting of the primary solid was proposed to be the origin of the anomalous eutectic formation, and quantitative analyses were performed.