50wt%Sn-50wt%Pb alloy.

This alloy has a composition close to the eutectic. When cooled slowly from the molten state, it begins to solidify at approximately 210°C by the nucleation and growth of lead rich dendrites. The remaining liquid becomes tin rich with decreasing temperature until it solidifies as the eutectic at the eutectic temperature (183°C).

The lead rich dendrites are the large dark phase. At high magnifications, the fine dispersion of lead rich and tin rich phases in the eutectic can be seen.

The solubility of tin in the lead rich dendrites decreases rapidly below the eutectic temperature, and tin precipitates can be seen within the dendrites.

Compare it with the 80wt%Pb-20wt%Sn alloy.