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 degrees 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 eta' 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 beetling 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. MST/4264.