High strength aluminum alloys were aged at 383 K. Their aluminum alloys were made clear to consist of two major substructures with different scale; metastable phase with a few nm size precipitated in the matrix and the quaternary rod-like intermediate phase with a few micrometer length containing Mn. The remarkable strengthening is dominantly attributed to the precipitation hardening due to fine metastable phase. In order to get further high strength, we have carried out systematic investigation using the various Al–Zn–Mg–Cu based alloys with different compositions of Zn and Mg. By a synchrotron radiation small angle scattering technique, the structure change of the alloys during isothermal aging was examined. The change of Vickers hardness during isothermal aging, could be well expressed as a function of the square root of particle size multiplied by volume fraction of metastable precipitates, suggesting the coherency strain model for the interaction of precipitate with dislocation. The coherency strain for each alloy was experimentally estimated.
