Solidification of undercooled Ni-32.5 wt% Sn eutectic alloy

Abstract

Bulk Ni-32.5 wt % Sn alloy was undercooled by purification. The solidification characteristics at different undercoolings and the various microstructures were studied. It was found that the two eutectic phases can grow in a matching form, to produce regular eutectic structures if the melt was undercooled by less than 10 K. If the melt was highly undercooled, the dendrite tip velocity of the Ni₃Sn phase was very much larger than that of Ni(α) phase during recalescence; as a result, the melt did not solidify in the regular eutectic morphology but in the dendrite cluster morphology with Ni₃Sn phase acting as the leading phase in the recalescence stage. The dendrite clusters subsequently developed into anomalous eutectic microstructures by ripening of Ni₃Sn dendrites and growth of Ni(α) phase between the Ni₃Sn arms. If the undercooling before nucleation was below 130 K, bulk melt of the original composition Ni-32.5 wt % Sn existed between the dendrite clusters after recalescence, which then solidified into regular eutectic microstructures. As a result, the structure of the sample consisted of regular eutectic zones and anomalous eutectic zones. If the undercooling before nucleation was above about 130 K, no bulk melts existed after recalescence, so the structures of the samples were completely anomalous eutectic.