Role of size and cooling rate in quenched droplet of SnBi eutectic alloy

Employing the solvent-encapsulation remelting and quenching (SERQ) process combined with the Newtonian cooling model, the droplets were quenched at different temperatures, by which the role of size and cooling rate in droplet solidification of SnBi eutectic alloy can be investigated separately. Undercooling is shown to be the key factor in determining the solidified microstructure evolution from “Regular + Irregular” (RI) to “Totally Irregular” (TI). Direct influence of the size and cooling rate is not observed, but the undercooling of droplets plays a decisive role in the formation of solidification structure. Under the present experimental conditions, the undercooling increases from 34.7 K to 51.5 K as the droplet size reduces from 34 μm to 20 μm, with an increment of 16.8 K. In contrast, only a negligible increment of 0.4 K is obtained as the cooling rate is enhanced from 4.08 × 104 K/s to 1.23 × 105 K/s with respective undercoolings of 51.1 K and 51.5 K. Obviously, a significant increase in undercooling can be achieved by reducing the droplet size during rapid solidification.