Phase Diagrams

· The equilibrium micro-structure of a binary eutectic alloy depends upon its composition. This structure is shown for a eutectic Pb/Sn alloy.

· At (h) the eutectic alloy is a uniform liquid of Pb- 61.9 wt % Sn. · As the temperature is decreased it remains liquid until the eutectic temperature (183 C) is reached. At this point it separates into two solid solutions: α which is
Pb- 19.2 wt % Sn, and β with the composition Pb- 97.5 wt % Sn.
· The two phase zone has alternating lamella of α and β in its crystallites.

After: Callister, "Materials Science and Engineering," Wiley (1994)