Kinetics Versus Thermodynamics
This technique exploits the phenomenon of supersaturation, and involves careful balancing of the driving force for precipitation and the thermal activation energy available for both desirable and undesirable processes.
Nucleation occurs at a relatively high temperature (often just below the solubility limit) so that the kinetic barrier of surface energy can be more easily overcome and the maximum number of precipitate particles can form. These particles are then allowed to grow at lower temperature in a process called aging. This is carried out under conditions of low solubility so that thermodynamics drive a greater total volume of precipitate formation.
Diffusion's exponential dependence upon temperature makes precipitation strengthening, like all heat treatments, a fairly delicate process. Too little diffusion (under aging), and the particles will be too small to impede dislocations effectively; too much (over aging), and they will be too large and dispersed to interact with the majority of dislocations.
Read more about this topic: Precipitation Hardening