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Why does Duralumin become stronger after aging?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

Duralumin gains strength through a process known as precipitation hardening, which occurs after the alloy is heat-treated and quenched.12

During the initial heat treatment, the alloy is heated to a high temperature to dissolve solute elements—such as copper or magnesium—into a supersaturated solid solution. By rapidly quenching the metal, these solutes are trapped in this unstable state rather than precipitating out.23

When the alloy is subsequently aged—either at room temperature or at a controlled low temperature—these trapped solutes form a dispersion of extremely fine precipitates within the crystalline matrix. These microscopic particles act as barriers that restrict the movement of dislocations, which are defects in the atomic arrangement responsible for metal deformation. By hindering the movement of these dislocations, the fine precipitates significantly increase the material's hardness and strength.23