Partial mutual solubility in the solid state

4 Partial mutual solubility in the solid state. Fig. 2.9 shows different examples of binary systems for which there is still a complete miscibility in the liquid state, but only a limited mutual solubility in the solid state, depending on the temperature. The Ni-Au system, for instance, still has complete mutual solid solubility but only at high temperature, that is, by decreasing the temperature, de-mixing 

A similar behaviour (complete mutual solubility in the liquid state and partial solubility in the solid state) is presented also by the Pt-Au and Ba-Ca systems (Fig. 2.9(b) and (c)). Notice in the Pt-Au diagram the closeness (mainly for compositions near 40 at.% Au) between the melting and the de-mixing equilibria. 

The Ba-Ca diagram has a different appearance because Ca (in its high-temperature form) and Ba are completely mutually soluble, while only a partial solid solubility is observed for Ba in the low-temperature modification of Ca. 

---