Effect of multi-stage aging treatments on the precipitation and mechanical properties of Al-Zn-Mg alloys

The mechanical properties of Al-Zn-Mg alloys via multi-stage aging treatments were studied to explore the strength-toughness trade-off. The microstructures of aged alloys were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated that large numbers of GP zones and some part of eta' phases were formed during the low-temperature (90 degrees C) aging at the third-stage aging, which consumed considerable solid solubility. This prevents fast growth and coarsening of precipitates at the fourth-stage aging (150 degrees C). Finally, the mixed precipitation state of mainly eta' phases and some eta phases formed. The grain boundary phase grew insignificantly, which is beneficial to decrease the strength gap between intragranular and grain-boundary. The intragranular equilibrium eta phase is beneficial to pin the movement of dislocation, which avoids fracture caused by accumulation of dislocations around the large second-phase and on the grain boundaries. Although the strength of materials decreased slightly, the impact toughness of materials significantly increased due to the positive effect of the intragranular equilibrium eta phase. Compared with the conventional retrogression and re-aging (RRA) treatment (120 degrees C/24 h + 185 degrees C/105min + 120 degrees C/12 h), the designed four-stage ageing treatment (FSA) (120 degrees C/24 h + 185 degrees C/105min + 90 degrees C/12 h + 150 degrees C/12 h) resulted in the decrease of tensile strength and yield strength by only 3% and 4%, respectively, but contributed to the increases of elongation and impact toughness by 15% and 17%, respectively.