Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time. The precipitation-hardening process involves three basic steps: solution treatment, quenching and aging.
a) Solution treatment: where all the solute atoms are dissolved to form a single- phase solution.
b) Quenching: rapid cooling across the solvus line to exceed the solubility limit. This leads to a supersaturated solid solution that remains stable (metastable) due to the low temperatures, which prevent diffusion.
c) Aging: precipitation heat treatment where the supersaturated solution is heated to an intermediate temperature to induce precipitation and kept there for some time.