Macrosegregation in undercooled Pb-Sn eutectic alloys

Abstract

A novel technique resulting in large undercoolings in bulk samples (23g) of lead-tin eutectic alloy is described. Samples of eutectic composition were processed with undercoolings ranging from 4 to 20 K and with cooling rates varying between 0.04 to 4 K/sec. The final macrostructure of undercooled samples depends on both the initial undercooling of the melt and the cooling rate. Gravity-driven segregation is found to increase with increasing undercooling. A eutectic Pb-Sn alloy undercooled at 20 K and cooled at 4 K/sec had a composition of about Pb-72 wt pct Sn at the top and 55 pct Sn at the bottom. Macrosegregation in these undercooled lead-tin eutectic alloys is shown to be primarily due to a sink/float mechanism caused by the difference in density of the solid and liquid phases and the undercooling and nucleation behavior of the alloy.