The following phase diagram (in which Liquid + Beta solid solution -----------→ Alpha solid solution) shows:

The peritectic reaction is an invariant reaction that occurs at a particular composition and temperature. The reaction during the cooling process can be written as: Liquid + β solid solution → α solid solution  Figure is the phase diagram for the Ag-Pt alloy system.  The α phase is silver-rich, the β phase is platinum-rich, and the two-phase (α-plus-β) region results from the limited solid solubility of less than approximately 12% Ag in Pt at 700°C (point F) of silver in platinum. Figure shows that the equilibrium solid solubility of Pt in Ag is approximately 56% at 700°C. The peritectic transformation occurs at point P, where the liquid composition at B and the platinum-rich β phase  (composition at point D) transform into the silver-rich α phase (composition at point P).