Precipitation hardening, also called age hardening or dispersion hardening, is a
heat treatment technique used to
strengthen malleable materials, especially non-
ferrous alloys including most structural alloys of
aluminium and
titanium. It relies on changes in solid
solubility with
temperature to produce fine particles of an impurity
phase, which impede the movement of
dislocations, or defects in a
crystal's
lattice. Since dislocations are often the dominant carriers of
plasticity (deformations of a material under stress), this serves to harden the material. The impurities, in fact, play the same role as matrix substances in
composite materials. Just as the formation of ice in air can produce clouds, snow, or hail, depending upon the thermal history of a given portion of the atmosphere,
precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary
tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging.
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