Linking mechanical properties to microstructure in pre-deformed Al–Mg–Si(-Cu) alloys with variable Zn content

Pre-deformation prior to age hardening is an effective approach to increase strength in extruded 6xxx-series aluminum profiles. We systematically investigated how 0-35% pre-deformation combined with artificial ageing affects mechanical properties, precipitate characteristics, and phase evolution in AA6061, with and without 0.1 wt.% Zn. Adding Zn increased strength without reducing elongation in both the undeformed and the pre-deformed condition. For a given artificial ageing time and temperature, strength increased with deformation up to 15% strain before reaching a plateau. At higher strains, the additional work-hardening contribution appears to be counterbalanced by the faster coarsening of the precipitates. Transmission electron microscopy showed that Zn addition increased precipitate density without affecting size, raising the volume fraction. Pre-deformation also increased precipitate density, but a decrease in size resulted in a similar volume fraction as the undeformed condition. High-angle annular dark field scanning transmission electron microscopy revealed a transition from needle-like ordered phases to a mix of structurally disordered needle-like precipitates and lath-like phases. Strength modeling without phase specificity reproduced the behavior of the Zn-free alloy, including a 60 MPa contribution from work-hardening after recovery. While pre-deformation was shown to slightly reduce precipitate strengthening, work-hardening fully explained the net increase in strength.