Lead-Tin Eutectic.

This alloy contains 62wt% tin and has the eutectic composition. The liquid metal solidifies at the eutectic temperature (183°C) as a fine dispersion of tin rich (97.5wt% Sn) and lead rich (81wt% Pb) phases. The solubility of lead in the tin rich phase decreases rapidly with decreasing temperature, as does the solubility of tin in the lead rich phase. At high magnification, fine precipitates of leaad and tin can be seen in each constituent of the eutectic.

This eutectic alloy is used for electrical solder. It has the lowest melting point of the lead-tin alloys and becomes molten as soon as it is heated above 183°C. It flows easily and solidifies as soon as it cools below 183°C. Compare this with the behaviour of plumber's solder