Quantitative study of nanoscale precipitates in Al-Zn-Mg-Cu alloys with different chemical compositions

The present work gives a quantitative study on the nanoscale precipitates in Al-Zn-Mg-Cu alloys with 6 different chemical compositions (all are in peak-aged state), by combining synchrotron-based small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, the size, shape, interprecipitate distance, volume fraction and number density of precipitates are extracted from SAXS data through model fitting. The results show that after peak-aging treatment, the average precipitate sizes in different alloys are close to each other, whereas the volume fraction and number density of nanoscale precipitates increase with the increasing content of Zn+Mg+Cu as well as the Zn/Mg ratio. The results also show that the alloy with a higher number density/volume fraction of nano-scale precipitates tends to have a higher mechanical strength.