Evolution of precipitate microstructures during the retrogression and re-ageing heat treatment of an Al–Zn–Mg–Cu alloy

Using a combination of experimental techniques, including anomalous small-angle scattering and atom-probe tomography, the evolution of precipitate microstructures during the different steps of retrogression and re-ageing (RRA) heat treatments of an Al-Zn-Mg-Cu alloy has been systematically evaluated. Quantitative information on the morphology, scale and chemistry of the precipitates provide new insight into the mechanisms at work during this process. It is shown that both the final chemistry and precipitate size distribution are different in the final RRA temper compared to classical heat treatments, with the presence of small clusters nucleated during the re-ageing step, and an average precipitate composition richer in Cu, together with a matrix enrichment in Zn, related to the difference in diffusivity between the two solute atoms. The mechanisms of precipitate evolution during the reversion and re-ageing steps are discussed in light of the influence of the process parameters.