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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.

A series of Co-Sn alloys was solidified. It was found that the as-cast microstructure of the Co-20.5at.%Sn alloy, which on the basis of literature reports has long been thought to be the eutectic composition, contains much primary phase besides lamellar eutectics. The cooling curve of this alloy contains two exothermic peaks. In contrast, when a Co-24.0at.%Sn alloy was solidified, an entire lamellar eutectic was obtained in the as-cast microstructure. Also, the thermal analyses indicate that there is only one solidification reaction in this alloy. It is suggested that the actual eutectic composition of the Co-Sn system is around Co-24.0at.%Sn, and the corresponding eutectic temperature is about 1385 K.

A series of Ni-P alloys (Ni-rich) have been synthesized and solidified. It is found that the solidification microstructure of the Ni-19.0 at.%P alloy that has long been regarded as the eutectic composition contains many primary phase dendrites besides rod eutectics. In contrast, only rod eutectic microstructure appears in the samples of Ni-19.6 at.%P, and this alloy shows the lowest end temperature of melting among all the compositions investigated. The results indicate that the actual eutectic composition at the Ni-rich part of the Ni-P system is Ni-19.6 at.%P. The eutectic temperature is determined to be about 1163 K by the differential scanning calorimetry analysis.