Free Energy and Phase Diagrams

The chapter determines the conditions governing the effect of solute addition upon the solidus and liquidus temperatures and found that a more positive excess free energy for solid phase relative to the liquid phase results in a decrease in the “average melting temperature”. Further, the relations governing the distribution coefficient for solidus to liquidus compositions are derived. One origin of the eutectic phase diagram is shown to be the existence of a positive enthalpy of mixing in the solid state. The origin of intermediate phases in phase diagrams is explored, as well as the effect of metastability on solvus composition. The integral relation between free energy–composition curves for possible phases as a function of temperature and the phase diagram is demonstrated. Also, a strategy for the prediction of phase diagrams is out-lined. Finally, some aspects of coherent phase equilibrium are considered. The chapter also reviews the basic principles regarding heterogeneous chemical equilibria and then relates these principles to ternary phase diagrams and equilibrium between contacting compounds.