Effect of pre-ageing treatment on second nucleating of GPII zones and precipitation kinetics in an ultrafine grained 7075 aluminum alloy

To clarify the influence of a pre-ageing treatment on precipitation behaviors in the AA7075 aluminum alloys with ultrafine grain (UFG) structure, the alloys were pre-aged prior to a room temperature rolling process (PA + RTR). The precipitation kinetics in PA + RTR alloys during isothermal heat treatment at 100 °C were systematically investigated by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and Vickers hardness measurements compared to its counterpart without pre-ageing treatment (i.e. RTR). Results reveal the pre-ageing treatment can lead to a higher dislocation density (22.8 × 1014 m−2) and a larger concentration of Mg (~12.85 at.%) and Zn (~8.33 at.%) atoms in the vicinity of dislocation, which causes a considerable increase in the size and volume fraction of the precipitates in PA + RTR alloy. Specially, our study shows, for the first time, that many GPII zones come from second nucleating in PA + RTR alloy at 108 h, which may ascribe to the pre-ageing treatment. More importantly, the pre-ageing treatment not only makes both two hardness peaks of PA + RTR alloy appeared ahead of but also higher than those of RTR alloy, by virtue of the enhanced precipitation kinetics in PA + RTR alloy.