Precipitation hardening of Al-Cu

Age hardening raises the strength of alloys whose solid solubility decreases with falling temperature, by forming very fine precipitates that block dislocations. Al-4% Cu is the classic example (duralumin).

Conditions for age hardening

The phase diagram must show a solvus line, i.e., the solute (Cu) is soluble at high temperature (up to 5.65% at 548 °C) but only about 0.2% at room temperature.

Heat-treatment steps

 T
 |   solution  ______
 |   treat.   |      | (500-550 C)
 |            |      | quench
 |            |      `--.   ageing
 |            |           `----_____ (130-190 C, hours)
 +------------------------------------> time

1. Solution treatment: heat to about 520-540 °C, a single-phase $\alpha$ region, and hold to dissolve all Cu.
2. Quenching in water to room temperature: Cu is trapped in a supersaturated solid solution.
3. Ageing: hold at room temperature (natural ageing) or 130-190 °C (artificial ageing). Cu atoms diffuse and cluster to form precipitates.

Precipitation sequence

Supersaturated $\alpha$ $\rightarrow$ GP zones (Cu-rich discs, coherent) $\rightarrow$ $\theta''$ $\rightarrow$ $\theta'$ (semi-coherent) $\rightarrow$ $\theta$ (CuAl$_2$, incoherent, equilibrium).

Why strength increases

Coherent GP zones and $\theta''$ distort the surrounding lattice. Dislocations must cut through the particles or bow around them, which needs a higher stress. Maximum hardness occurs with a fine, dense dispersion of $\theta''$/$\theta'$.

Over-ageing

Long time or high temperature lets precipitates coarsen into large, widely spaced incoherent $\theta$ particles. Dislocations can easily bypass them (Orowan looping), so hardness and strength fall.