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 theηp precipitates through three-dimensional electron tomography,the 3 D 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.
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