COLUMNAR TO EQUIAXED TRANSITION IN TIN-LEAD ALLOYS.

Tin-lead alloys were solidified directionally and the position of the columnar to equiaxed transition determined on vertical sections of the ingots. The columnar length increases with decreasing lead concentration and increasing heat transfer coefficient. A mathematical model of heat flow was used to determine local temperatures, temperature gradients, and velocities in the solidifying alloy. The transition occurred when the temperature gradient in the melt at the liquidus temperature was 0. 11 degree C/mm for Sn 10 wt pct Pb, 0. 10 degree C/mm for Sn 5 wt pct Pb, and 0. 13 degree C/ mm for Sn 15 wt pct Pb. The position of the transformation was independent of melt superheat for the conditions considered.