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 degrees 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 x 10(14) 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. (C) 2019 Published by Elsevier Ltd.