The effect of pre-ageing/stretching on the ageing-hardening behavior of Al–Zn–Mg–Cu alloys correlated with Zn/Mg ratio

Pre-ageing and pre-stretching before artificial ageing can significantly affect the precipitation during the subsequent artificial ageing in Al–Zn–Mg–Cu alloys, and their effects are closely relevant to the alloy composition and Zn/Mg ratio due to the different predominate precipitates. The diverse effects of pre-ageing and pre-stretching on the mechanical properties and microstructures of Al–Zn–Mg–Cu alloys with five different Zn/Mg ratios have been systematically studied with transmission electron microscopy, hardness measurements and tensile tests. The results reveal that pre-ageing can effectively enhance the strength due to the improvement of number density of precipitates. Different pre-aging time causes similar improvement of strength but different elongation values in peak-aged alloys A2 (T′ phase strengthened) and A5 (η′ phase strengthened), which can be explained by different size and density of precipitates and PFZ width. The effects of pre-stretching on age-hardening behavior of the alloys with various compositions are significantly different. It results in a reduction in hardness in the lowest Zn/Mg ratio alloy A1 (T′ phase strengthened) due to accelerated coarsening of precipitates; on the contrary, it results in a remarkable improvement of the hardness of the highest Zn/Mg ratio alloy A5. Pre-stretching causes different strengthening effect in alloy A5, and the maximum enhancement takes place with 2% pre-stretching because the precipitates become denser and smaller. Additionally, both pre-ageing and pre-stretching are effective ways to reduce PFZ width by promoting the precipitation near the grain boundary.