Aging precipitation characteristics and tensile properties of Al–Zn–Mg–Cu alloys with different additional Zn contents

The increase in zinc content in Al–Zn–Mg–Cu alloys provides an effective method to enhance the strength. Transmission electron microscopy (TEM) and tensile test were employed to analyze the microstructure and tensile properties, respectively, of Al–9.3Zn–2.0Mg–1.8Cu alloy (9.3Zn alloy) and Al–9.8Zn–2.0Mg–1.8Cu alloy (9.8Zn alloy) with single- and double-stage aging states. The results showed that the two alloys possessed a closed strength under single-stage peak-aging treatment. As for double-stage aging treatment, 9.8Zn alloy had a higher strength values than 9.3Zn alloy under the same aging regimes. With the second step aging time prolonging, the strength gap was extended. The main precipitates for the two alloys with single-stage peak-aging state were GP zones and η′ phase, while there were a majority of η′ phases and η phases for the two alloys under typical double-stage over-aging state. Under double-stage over-aging state, the proportion of precipitate with large size for 9.3Zn alloy was larger than that for 9.8Zn alloy. Besides, a more obvious trend was revealed for the double-stage over-aging state. The gap of strength between the two alloys was explained by the difference of precipitation characteristics via interaction mechanism between precipitates and dislocations.