Formation mechanism of anomalous eutectic in highly undercooled Ni-Sn eutectic alloy

Applying glass flux combined with cyclical superheating, high undercooling were achieved in Ni-32.5%Sn eutectic alloy melt. The anomalous eutectic was observed in the as-solidified microstructure. The volume fraction of the anomalous eutectic increases with undercooling, and when undercooling was exceeding a critical value, the as-solidified microstructure was totally occupied by anomalous eutectic. In terms of competitive nucleation and growth between α(Ni) and Ni₃Sn, and remelting of α(Ni) dendrites, the forming mechanism of anomalous eutectic was interpreted. Subjected to high undercooling, rapid primary solidification leads to the growth of primary dendrites, thus resulting in rapid recalescence and subsequent remelting of the primary dendrites. Then secondary phase is solidified from the remained liquid, and surrounds the primary dendrites fragments, so the anomalous eutectic is formed.