Phase diagrams

Temperature-composition phase diagrams are introduced as maps of the regions of stability of binary systems at constant pressure, usually atmospheric pressure at sea level. Their construction is based on minimisation of the Gibbs free energy as a function of composition at a given temperature. The simple case of miscibility in the solid and liquid states over the full range of composition is discussed first. Eutectic and peritectic phase diagrams result from limited miscibility in the solid state. Intermediate phases, or ordered alloys, usually occur in narrow ranges of composition in phase diagrams, and this is also explained in terms of free energy composition curves. Each phase diagram is shown to obey the phase rule discussed in the previous chapter.