Influence of copper addition on precipitation kinetics and hardening in Al–Zn–Mg alloy

The influence of the addition of copper on the precipitation behaviour of an Al–Zn–Mg alloy at 160°C has been investigated. Phase transformation has been investigated by differential scanning calorimetry, the precipitation kinetics by small angle X-ray scattering and transmission electron microscopy, and the mechanical properties by microhardness testing. The addition of copper results in an increase in the stability of Guinier–Preston zones formed at room temperature, and in a decrease in the nucleation temperature for the η′ phase. It also results in a decrease in the apparent diffusion coefficient for precipitation, and in an increase in the strengthening ability of precipitates. As a consequence, the copper bearing alloy shows a much higher maximum strengthening ability, and a lower sensitivity to the heating rate to the aging temperature. However, the higher supersaturation of the copper bearing alloy results in a higher quench sensitivity, and at slow quench rates the ternary alloy shows a higher strengthening potential. The experimental data are quantitatively compared with the results of a precipitation model, describing the evolution of precipitate size, volume fraction, and yield stress, and values for physical parameters of the precipitation and strengthening mechanisms are obtained.