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 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.

Quantitative Electron Tomography for Accurate Measurement of Precipitates Microstructure Parameters in Al–Cu–Li Alloys

Mechanical theories show that properties of alloys are strongly dependent on the morphological parameters oftheir strengthening precipitates.However,accurate measurement of precipitates microstructure parameters is still a challenging task.In this article,we develop a quantitative electron tomography method by combining computer vision technology to accurately characterize the three-dimensional microstructure parameters,such as volume fractions,sizes and distributions,of the T_(1) and δ’/θ’/δ’ precipitates in Al-Cu-Li(-Mg) alloys.Since they have extremely large aspect-ratios in shape and large numbers in density upon formation in the Al matrix,these thin plate-like precipitates are difficult to be characterized quantitatively without the assistance of computer vision technology.It is shown that the property difference between two peak-aged states of the alloy can be well explained with the quantitative precipitate parameters correctly measured.Using these correct precipitate data,we also tested the validity of current mechanical models for projecting the contribution of precipitates to the strengths of the alloy,demonstrating that quantitative relations between strength and micro structure parameters still need to be refined.