This problem deals with the PbSn phase diagram shown in the

This problem deals with the Pb-Sn phase diagram shown in the figure below. Assume a

constant pressure phase diagram.

a.) What is the reaction/change that occurs when cooling (at equilibrium) through point D? Express as a name or equation.How many degrees of freedom does the point D have?

Compositicn (a1% Sn) 60 100 327°C Liquid 500 232° 400 183 C 18.3 61.9 97.8 300 100 100 20 60 (Pb) Compcsit on (at% Sn) (Sn) Figure 2: The Pb-Sn phase diagram

Solution

a) At point D a eutectic reaction is occuring because two solids ( alpha and Beta) are generated from a single Liquid phase (L) .

      L   -->   alpha(A) + Beta (B)     (Eutectic Reaction)

    The point D has zero degrees of freedom, all 3 phases (alpha, beta and liquid) coexist in equilibrium, changing either temperature or compostion from the point represented by D will cause one or more of the phases to disappear.