Pb-80wt%Sn.

The first phase to precipitate from the liquid metal during slow cooling is tin (Sn) rich and it forms as dendrites. This occurs below approximately 200°C. The tin concentration in the liquid decreases as the temperature falls until it reaches the eutectic composition (38wt% Pb) at the eutectic temperature of 183°C. The remaining liquid solidifies as a eutectic.

The tin rich dendrites are the large dark phase. The fine dispersion of tin rich (97.5wt% Sn) and lead rich (81wt% Pb) can be seen in the eutectic matrix.

Tin has no solubility for lead below 150°C, and at high magnification fine lead precipitates can be seen within the tin dendrites.