Effects of Mg/Zn ratio and pre-aging on microstructure and mechanical properties of Al–Mg–Zn–Cu alloys

The alloy composition and aging process are significant factors determining the type of precipitates in Al alloys. In this work, the effects of Mg/Zn ratio and pre-aging process on the microstructure and mechanical properties of Al–Mg–Zn–Cu alloys were systematically investigated using transmission electron microscopy (TEM), hardness measurements and tensile tests. The results show that reducing the Mg/Zn ratio significantly increases the density of T′-Mg32(AlZnCu)49 precipitates, thereby enhancing and accelerating the aging response. A detailed comparison has been made between Al–Mg–Zn–Cu alloys aged by single-step and two-step artificial aging processes. With the same composition, the alloy exhibits a higher yield strength under the two-stage aging process, resulting from the higher precipitation strengthening effect (Orowan dislocation bypassing mechanism) due to the smaller size and higher density of precipitates. Furthermore, it is particularly interesting that there may be two evolution paths for GPII zones with (111)Al habit plane: one is that the GPII zones are dissolved into the matrix, and the other is that the GPII zones gradually evolve into the T′ precipitates. This work sheds some light on the precipitation sequence in Al–Mg–Zn–Cu alloys and is greatly helpful for designing age-hardening Al–Mg based alloys.