Quantifying the Effect of Pre-deformation on Microstructure-Property Relationships in an Overaged Al-Zn-Mg-Cu Alloy

The strength contributions from various microstructural features in an overaged Al-Zn-Mg-Cu alloy under different levels of pre-deformation have been quantitatively investigated. The results reveal that dislocations induced by pre-deformation promote the nucleation of the equilibrium eta phase and accelerate the growth of eta' precipitates during ageing, leading to a decrease in strength with higher levels of pre-deformation. The dislocations introduced by low levels of pre-deformation (<= 6%) are unable to form subgrain boundaries during subsequent heat treatment. The pre-deformation only increases the aspect ratio of subgrains with negligible effect on the strength contribution of grain boundaries. Quantitative calculations demonstrate that the precipitate strengthening is the primary contributor to the alloy's overall yield strength, accounting for over 49.4%. As pre-deformation increases from 2% to 6%, the strength reduction caused by coarsening of precipitates (similar to 91 MPa) is significantly higher than the change in strength contribution due to increased dislocation density (similar to 21 MPa), resulting in a lower yield strength.