Effect of Pre-Deformation on Precipitation in Al-Zn-Mg-Cu Alloy

In this work, strengthening effects and evolution of precipitates in a pre-deformed Al-Zn-Mg-Cu alloy during ageing were investigated using Vickers hardness measurements, tensile tests, and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). It was found that all cold rolled conditions had higher mechanical strength than the non-deformed condition for all ageing times and that this effect increases at higher deformation ratios. It was also found that the non-deformed condition has a higher age hardening response than that of the cold rolled conditions. A homogeneous precipitate distribution was observed in the non-deformed condition, while the cold rolled conditions contained non-uniformly distributed precipitates due to the introduced dislocations. This led to larger precipitate sizes and a reduction in the precipitate number densities in the pre-deformed conditions. HAADF-STEM analysis revealed differences in the fraction of different precipitate types between the non-deformed and the cold rolled conditions. eta', eta 2, and disordered eta phase were observed in the non-deformed condition, while eta', eta 2 and the newly identified Y phase were observed in the cold rolled conditions. The disordered eta phase contained structural units of the eta 1 phase and was associated with reducing the lattice misfit between this phase and the Al matrix. Formation of the Y phase was related to an accelerated nucleation rate in the regions of high dislocation density.