Artificial aging effect on precipitation and age-hardening in an Al-Zn-Mg-Cu alloy

In the present work, the influence of heat treatment on the mechanical properties and precipitation of the different phases in Al-Zn-Mg-Cu alloy was studied. The precipitation sequence and the mechanism of structural hardening have been followed using the differential scanning calorimetry (DSC), the microhardness measurements, the X-ray diffraction (XRD) and the scanning electron microscopy (SEM). Five calorimetric effects have been recorded and correspond to the different phases precipitating in this system of alloys and which are: the GPI zones and / or VRC (or GPII), the intermediate eta' phase and the equilibrium eta phase with a suspicion concerning the existence of T and S phases. Microhardness measurements of the aged state at 226 degrees C confirm the existence of two types of phases: the metastable eta' phase and the equilibrium eta phase (MgZn2). The appearance of the first precipitates was followed by SEM. The fine precipitates become larger (holdings at 243 and 266 degrees C) and the equilibrium eta phase continues to grow inside and on the grain boundaries to become a large spherical and/or lamellar precipitate. The X-ray diffraction confirmed that only the equilibrium eta phase was observed in this type of alloy.