Revisiting the Hierarchical Microstructures of an Al–Zn–Mg Alloy Fabricated by Pre-deformation and Aging

Pre-deformation before aging has been demonstrated to have a positive effect on the mechanical strength of the 7N01 alloy in our previous study, which is rather different from the general negative effects of pre-deformation on high-strength 7XXX aluminum alloys. In order to explain the strengthening mechanism relating to the positive effect, in the present study, the microstructure of the aged 7N01 alloy with different degrees of pre-deformation was investigated in detail using advanced electron microscopy techniques. Our results show that, without pre-deformation, the aged alloy is strengthened mainly by the η′ type of hardening precipitates. In contrast, with pre-deformation, the aged alloy is strengthened by the hierarchical microstructure consisting of the GP-η′ type of precipitates formed inside sub-grains, the ηp type of precipitates formed at small-angle boundaries, and the dislocation introduced by pre-deformation (residual work-hardening effect). By visualizing the distribution of the ηp precipitates through three-dimensional electron tomography, the 3D microstructures of dislocation cells are clearly revealed. Proper combinations of ηp precipitates, GP-η′ precipitates and residual dislocations in the alloy are responsible for the positive effect of pre-deformation on its mechanical properties.