The metals in the solid state can only dissolve other metals and nonmetals. As the pressure and temperature change, the solubility changes, this leads to the separation or dissolution of excess phases. If the alloy is in an unstable state, these processes can occur without changing the external conditions. Structural changes due to the limited solubility of the components in the solid state are widely used in the technique of heat treatment.
Let us consider the system whose components are bounded soluble in one another in the solid state. In this case, the field of the diagram below the solidus line is divided into regions of existence of alloys in single-phase and two-phase states (Fig. 1, c).
The lines ГВ and EД are called lines of limited solubility or saturation lines. The HV line characterizes the solubility of component B in component A. The solid solution B in A is designated as a-phase. The other phase can be the component of B, the solid solution of A in B, an intermediate phase of a constant or variable composition. In the above system, the β phase is a solid solution and the EД line characterizes the solubility of component A in this phase.
In Fig. 1, a, b shows the effect of the composition on the thermodynamic potentials of the α and β phases at different temperatures. At temperature Та, for example, alloy I is in the single-phase state of the α-solution, which ensures a minimum of the thermodynamic potential (Fig. 1, a). When cooled to the temperature Тб, the phase state does not change, but the solution becomes a saturated component of B. The tangent curve of the change in the thermodynamic potential of the α phase for a given composition of the alloy also applies to the β-phase curve.
When super cooled to temperatures below Tb, the solution is supersaturated by component B. The degree of super saturation can be defined as the difference in the concentrations of component B in the initial and saturated solutions. At the temperature Тв, for example, the degree of super saturation is ΔС = Св — Сг. Since the thermodynamic potential of the supersaturated solution (Z’в1, Fig. 1, b) is higher than the two-phase mixture α + β (Zв‘), the single-phase state of the alloy becomes metastable. The potential difference determines the thermodynamic stimulus of the process of separating the excess phase β from the supersaturated solution α. The embryos of the new phase, as in the case of polymorphic transformation, usually arise in a heterogeneous way on the defects present in the initial solution (at the boundaries of grains and sub grains, dislocations, inclusions of impurities, pores and cracks). The necessary supercooling (super saturation) depends on the difference in the specific volumes of the formed and initial phases, since the value of the strain energy is associated with it.
If the β phase nuclei in the alloy under consideration (Fig. 1, c) occur at the temperature Тв, their composition should be to the right of Сд. The fluctuation of the composition necessary for their formation is easier to create in sections enriched with component D. In an inhomogeneous solution, the β phase appears first in regions where, owing to segregation, more components B is present. As the β-phase crystal grows, the effect of surface tension is weakened and the crystal composition approaches stable Сд.