Effect of Stress-Aging Treatments on Precipitates of Pre-retrogressed Al-Zn-Mg-Cu Alloy

According to retrogression and re-aging treatment (RRA), a novel retrogression and stress-aging treatment (RSA) was put forward for Al-Zn-Mg-Cu alloys. The effect of stress-aging time and external stress on precipitates of pre-retrogressed Al-Zn-Mg-Cu alloy was investigated by transmission electron microscope (TEM). The observation shows that there is a large number of matrix precipitates (MPts) and slightly discontinuous grain boundary precipitates (GBPs) in the pre-retrogressed alloy. The effects of stress-aging time and external stress on precipitates of the retrogressed alloy are significant. With the increase of stress-aging time and external stress, the size of MPts increases and the density of MPts decreases. Meanwhile, the size of GBPs, the distance between GBPs and the width of precipitate free zone (PFZ) increase. Compared with the traditional retrogression and re-aging treatment, the retrogression and stress-aging treatments increase the size of MPts, narrows the width of PFZ and make the GBPs more discontinuous for the studied Al-Zn-Mg-Cu alloy.