Microstructural development in undercooled lead- tin eutectic alloys

A model was developed to predict micro structural development in lead—61.9 wt pct Sn (eutectic) alloys which were undercooled 5 to 25 K below their equilibrium freezing temperature prior to being preferentially nucleated. While the initial solidification velocity rapidly increases with increasing undercooling, the model predicts it to quickly decrease, prior to 10 pct solid formation, after which growth continues near the equilibrium temperature. Experimentally, and in accordance with the prediction, the eutectic emanated from the nucleation site with an initially fine spacing that increased with distance. However, in contrast to the model, the eutectic grew outward in a spokelike manner with each arm surrounded by a globular structure, this being attributed to the difficulty of lateral nucleation. Microstructural uniformity was further compromised by equiaxed eutectic grains which grew ahead of the advancing interface in the now only slightly undercooled liquid. Consequently, while containerless techniques may ensure sample purity and permit processing of high-temperature materials, development of a continuously fine and uniformly aligned microstructure cannot be assumed.