An alloy can contain different solid or liquid phases depending on composition and temperature. A phase diagram maps which equilibrium phases are stable under specified conditions.

Binary diagrams

A binary phase diagram uses composition along one axis and temperature along the other. Regions can contain one phase or mixtures of two phases.

Tie lines

Inside a two-phase region, a horizontal tie line intersects the phase boundaries at the equilibrium compositions of the two phases.

Lever rule

The relative amounts of the two equilibrium phases can be found from the distances along the tie line. If the overall composition lies between phase compositions $C_\alpha$ and $C_\beta$, the phase fractions follow the lever rule.

Equilibrium assumption

A phase diagram describes equilibrium or near-equilibrium states. Real cooling can be too rapid for diffusion to maintain equilibrium, producing metastable structures not read directly from the equilibrium diagram.

Phase diagrams connect alloy composition and thermal processing to the microstructures that determine material properties.