Effect of cumulative plastic deformation on microstructure and strengthening mechanism of extruded Al-Zn-Mg-Cu alloy

Al-Zn-Mg-Cu alloy profiles are widely used as aircraft components because of lightweight and high strength. The section of thin-walled profile is easy to have unavoidable heterogeneous strain. However, the influence mechanism of different cumulative plastic strains (CPS) on the material properties at different cross-section locations needs to be further clarified. This study carried out an extrusion experiment to study the influence of CPS on the microstructure, properties, and strengthening mechanisms by TEM, SEM and XRD. Results show that the strength and micro-hardness are positively correlated with CPS, while the elongation exhibits an inverse correlation. The orientation relationships between different micro- and nano-scale precipitates and the matrix were characterized. The strengthening mechanisms under different CPS were clarified. It was found that the contribution of solid solution strengthening is 22.6 % under the maximum cumulative strain, while Orowan strengthening accounts for 20.3 %. In contrast, these contributions are 30.2 % and 11.9 % under lower cumulative strain, respectively. Specimens at low CPS have the precipitated phase bands parallel to the extrusion direction, while the high CPS induces smaller and dense precipitated phases in the matrix and grain boundaries, resulting in precipitate-free zones with a width of 200–500 nm at the sub-grain boundary, which reduces the strengthening effect at the grain boundary. The research findings can guide for optimizing the heat treatment processes of thin-walled profiles.