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

Abstract EN

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 η' phase and the equilibrium η phase with a suspicion concerning the existence of T and S phases.

Microhardness measurements of the aged state at 226 °C confirm the existence of two types of phases: the metastable η’ phase and the equilibrium η phase (MgZn2).

The appearance of the first precipitates was followed by SEM.

The fine precipitates become larger (holdings at 243 and 266 °C) and the equilibrium η 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 η phase was observed in this type of alloy.