Quantitative evaluation of precipitates in an Al–Zn–Mg–Cu alloy after isothermal aging

The evolution of microstructure parameters (precipitate size and volume fraction) for an Al–8.0 Zn–2.05 Mg–1.76 Cu alloy during isothermal ageing has been studied by synchrotron-radiation small angle X-ray scattering (SAXS) combining transmission electron microscopy (TEM). The results show that the precipitates are only a few nanometers even at higher temperature 160 °C up to 72 h (5.82 nm). The precipitate volume fraction reaches a plateau except ageing at 120 °C and the maximum is about 0.052–0.054 in the range 140–160 °C.Models describing the evolution of these two parameters with ageing temperature and time have been constructed for our further predicting the precipitate hardening. The coarsening of precipitate is consistent with LSW (Lifshitz–Slyozov–Wagner) model even in the initial stage where volume fraction is still varying. The activation energy of coarsening regime has been determined to be about 1.25 ± 0.02 eV.