Some alloys can be strengthened by creating a fine dispersion of second-phase precipitates inside a solid-solution matrix.

A typical precipitation-hardening treatment has three stages.

1. Solution treatment: heat the alloy into a temperature range where the relevant solute is highly soluble in one solid phase, allowing existing second-phase particles to dissolve.
2. Quenching: cool rapidly enough to retain a supersaturated solid solution rather than allowing the equilibrium precipitates to form immediately.
3. Aging: hold at a lower temperature so that controlled diffusion produces a fine distribution of precipitates.

Small coherent or partly coherent precipitates distort the surrounding lattice and obstruct dislocation motion. As particles grow, dislocations may need to cut through them or bow around them, depending on particle size, spacing and interface structure.

Strength therefore changes with aging time. Early aging produces too little precipitate; an intermediate condition often gives peak aging; prolonged exposure causes overaging, in which precipitates coarsen and become farther apart, reducing their effectiveness as obstacles.

Precipitation hardening is a direct example of the materials chain

$$\text{thermal processing}\rightarrow\text{diffusion and nucleation}\rightarrow\text{precipitate microstructure}\rightarrow\text{dislocation resistance}\rightarrow\text{strength}.$$