Microstructure investigation of a new type super high strength aluminum alloy at different heat-treated conditions

As a structural material with low density and high strength, super-high strength aluminum alloys have a future for wide application. However, its poor stress corrosion resistance (SCC) restricts further development. In present, retrogression and re-ageing (RRA) treatment, which can improve both strength and SCCR of 7XXX series alloy, is a best method to solve this problem. The effect of RRA treatment on the microstructure evolution of a new type low frequency electric-magnetic casting Al-9.0Zn-2.45Mg-2.2Cu-0.15Zr alloy was investigated using DSC and TEM Technologies. The results show that the typical microstructure of the alloy at T6 condition is characterized by both fine η' and GP zone homogeneously distributed in the matrix and continuous η particles occurred on the grain-boundary. After RRA treatment, the matrix precipitations are mainly fine and dispersed η' and η phases, being coarser and more stable than that from T6 temper. While, the grain-boundary microstructure is very close to that resulting from T73 temper. High retrogression temperature and long retrogression time leads to a more stable microstructure after re-ageing.