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Age Hardening

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Precipitation Hardening
Precipitation hardening, or age hardening, provides one of the most widely used mechanisms for the strengthening of metal alloys. The strongest aluminum alloys (2xxx, 6xxx and 7xxx) are produced by age hardening.
In order for an alloy system to be able to be precipitation-strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. The precipitation-hardening process involves three basic steps: solution treatment, quenching and aging.
The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed second-phase particles within the original phase matrix in a process known as precipitation or age hardening. The precipitate particles act as obstacles to dislocation movement and thereby strengthen the heat-treated alloys. Many aluminum based alloys, copper-tin, certain steels, nickel based super-alloys and titanium alloys can be strengthened by age hardening processes.
In order for an alloy system to be able to be precipitation-strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. E.g: Al-4.5% Cu, Al-6% Zn-2.5%Mg, Cu-2%Be, Ni-17%Cu-8%Sn, Ti-6%Al-4%V.

Stages:
Solution treatment: A suitable alloy is heated to a temperature at which a second phase (Usually present in small quantities) dissolves in the more abundant phase. The metal is left at this temperature until a homogeneous solid solution is attained.
Quenching is the second step where the solid is rapidly cooled forming a supersaturated solid solution of αSS which contains excess of second phase and is not an equilibrium structure. The atoms do not have time to diffuse to potential nucleation sites and thus θ precipitates do not form.

Aging is the third step where the supersaturated is heated below

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