Solid–Solid Solutions

Consider the temperature-composition phase diagram of two mutually insoluble solids, A and B, whose liquids are completely miscible. At lower temperatures, the diagram shows a two-phase mixture of pure A and pure B.

However, at higher temperatures, a single-phase liquid solution of A and B forms.

As the system cools from point a₁ to a₂, pure solid B begins to separate from the solution, enriching the remaining liquid in A. Further cooling to point a₃, causes additional separation of solid B and further enrichment of the liquid phase in A.

At point a₄, the liquid fraction decreases and eventually solidifies, forming a two-phase system of pure A and B, with composition e₂.

The isopleth at e₂ represents the eutectic composition - a mixture with the lowest melting point.

A liquid of this composition freezes at a single temperature, while the corresponding solid melts at the lowest temperature of any mixture without changing composition.

At the eutectic composition, as the mixture cools and begins to solidify, the temperature remains constant. This region of constant temperature is known as the eutectic halt.