Formation mechanism of anomalous eutectic and microstructure evolution in highly undercooled solidification of Ni-30%Sn alloy

Eutectic solidification involves many important metals and inorganic non-metallic materials. To date, there still exist a large controversy on the formation mechanism of anomalous eutectic under the nonequilibrium rapid solidification. In this work, adopting glass flux method combined with cyclical superheating, the recalescence behaviors and microstructure evolution in highly undercooled solidification of Ni-30%Sn hypoeutectic alloy were investigated. It is found that, there is not an obvious recalescence process in the alloy melts with the low undercooling. With increasing the melt undercooling, the solidification microstructure experienced a gradual phase evolutions from α-Ni dendrite + (α-Ni+Ni₃Sn) lamellar or feathery eutectic to completely (α-Ni+Ni₃Sn) anomalous eutectic, which led the recalescence process to occur from twice to once in the cooling process. Through analyzing the nucleation behaviors of α-Ni and Ni₃Sn phases and the relationships between their growth velocities and the melt undercoolings, and the variation of their crystalline fraction in rapid recalescence process, the formation mechanism of anomalous eutectic was explained. The formation of anomalous eutectic in highly undercooled Ni-30%Sn hypoeutectic alloy should be attributed to the following two reasons: in the large melt undercooling, the complete coarsening and remelting of the pre-formed refined α-Ni dendrite skeleton occured in the subsequent recalescence process, and then the dendrite fragments were surrounded by the precipitated Ni₃Sn phase, which eventually led to the formation of anomalous eutectic. With the undercooling increased, the primary single-phase dendritic growth will change to the two-phase dendritic growth, the coarsening and remelting of two-phasic dendrites occured in the subsequent larger recalescence process, which also led to the formation of anomalous eutectic.