Microstructural morphology changes of the lead-tin eutectic alloy by different undercooling levels

Experimental work is described in undercoolings and microstructures of Pb-Sn eutectic alloy droplets emulsified in oil bath. By controlling experimental variables, the final droplet size produced was in the range of 10 to 300 μm, and measured undercoolings ranged up to about 70 K. Microstructures of the droplets depend strongly on the degree of undercooling before solidification. By increasing undercooling, three major microstructure transitions, which include the morphological changes of eutectic phase from lamellar to anomalous in lower undercooling level and of primary α-Pb phase from dendrite to equiaxed grains in higher undercooling levels, are observed. The phase change from eutectic to primary α-Pb dendrite plus eutectic is also observed. The physical mechanism of these microstructure transitions is analyzed on the basis of the Lipton-Kurz-Trivedi theory for dendrite growth and the Jackson-Hunt theory for lamellar eutectic growth.