Fast calorimetric scanning of micro-sized SnAgCu single droplet at a high cooling rate

The undercooling of the single micro-sized droplet of Sn-3.0Ag-0.5Cu (wt%) alloy has been studied via the newly developed fast calorimetric scanning technique, by which the fast heating and cooling treatment for a single droplet can be realized, with the maximum heating or cooling rate being 1×10⁴ K/s. Owing to the nearly spherical shape of the single droplet upon heating and cooling and the resultant geometric stability, the influence of the droplet size on the solidification process could be eliminated. As a result, the puzzled issue on how to separate the mutual effects of droplet size and cooling rate in the field of rapid solidification has been primarily solved, making it possible to study separately the effect of droplet size and cooling rate. Meanwhile, the in-situ observation on deep undercooling could be actualized in this condition, differing from that obtained only by theoretical calculation. The results showed that the undercooling was increased with the increasing cooling rate, and the maximum in-situ measured undercooling reached 116.9 K. The undercooling of the single droplet, however, was increased abruptly when cooled at the rate of 2×10³ K/s. The undercooling increased slightly as the cooling rate was increased continuously to 1×10⁴ K/s, implying the infeasibility for gaining large undercooling only by increasing the cooling rate. Supported by the Robert Bosch Foundation (Grant No. 32.5.8003.0025.0/MA01), the National Natural Science Foundation of China (Grant No. 50571057), the National High-Tech Research and Development Program of China (“863” Project) (Grant No. 2006AA03Z339) and the Shanghai Rising-Star Program (Grant No. 06QA14020)