Understanding the effect of deformation combined with heat treatment on age hardening of Al-Zn-Mg-Cu alloy AA7075

Al-Zn-Mg-Cu alloys obtain their exceptional high strength predominately relying on precipitation. Deformation can influence the evolution of precipitation and may potentially provide additional strength through work hardening. In this study, the coupling of dislocation and precipitation strengthening to achieve unique property combinations with strengths greater than those due to heat treatment alone was investigated on pre-aged AA7075 subject to a pre-age, deform, post-age cycle. The evolution of precipitates and mechanical properties were characterised to understand the contribution of each mechanism to the final strength. Small angle X-ray scattering (SAXS) was used to monitor the precipitate evolution during post-ageing, with the support from transmission electron microscopy (TEM) and isothermal calorimetry. The results revealed that uni-axial deformation to 10% strain had negligible effects on pre-existing precipitates but introduced sufficient dislocations to enable significant work hardening, which resulted in strengths greater than those obtained with a T6 heat treatment but with a sacrifice in further strain to failure. Post-ageing at 120 degrees C was found being effective on partially recovering dislocations and promoting precipitation strengthening. A simple model was applied to demonstrate that the time required to reach peak strength is determined by the competing effects of the enhanced kinetics of precipitation and dislocation recovery. The acceleration effect of prior deformation on the growth of precipitation during post-ageing is consistent with the expected effect of dislocations acting as the fast diffusion paths.

Automated summary

Al-Zn-Mg-Cu alloys achieve high strength mainly through precipitation strengthening. This study investigated the coupling of dislocation and precipitation strengthening in pre-aged AA7075 alloys to achieve strengths greater than those obtained with heat treatment alone. The results showed that deformation introduced sufficient dislocations for work hardening and post-ageing at 120 degrees C was effective in promoting precipitation strengthening. The time required to reach peak strength is determined by the competition between precipitation kinetics and dislocation recovery.