Precipitation Kinetics: Nucleation, Growth and Coarsening

Supersaturation drives precipitation

Solution treatment and quenching retain solute above equilibrium. Ageing allows the alloy to reduce free energy by forming a second phase.

Nucleation barriers

Precipitates form more readily at defects or interfaces because heterogeneous nucleation lowers the interfacial-energy penalty.

Growth and strengthening

Particle size, spacing, coherency and volume fraction determine whether dislocations cut precipitates or bow around them.

Over-ageing and coarsening

At long time or high temperature, larger particles grow at the expense of smaller ones. Spacing increases and strength usually falls.

Engineering takeaway

The optimum ageing condition is a microstructural state, not simply a time on a heat-treatment sheet.