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 7 N01 alloy in our previous study, which is rather different from the general negative effects of pre-deformation on high-strength 7 XXX aluminum alloys. In order to explain the strengthening mechanism relating to the positive effect, in the present study, the microstructure of the aged 7 N01 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 ηtype of precipitates formed at small-angle boundaries, and the dislocation introduced by pre-deformation(residual work-hardening effect). By visualizing the distribution of the ηprecipitates through three-dimensional electron tomography, the 3 D microstructures of dislocation cells are clearly revealed. Proper combinations of ηprecipitates, GP-η′ precipitates and residual dislocations in the alloy are responsible for the positive effect of pre-deformation on its mechanical properties.