Effect of continuous retrogression and re-ageing treatment on mechanical properties, corrosion behavior and microstructure of an Al-Zn-Mg-Cu alloy

The mechanical properties, electrical conductivity, corrosion behavior and precipitation evolution of an Al-Zn-Mg-Cu alloy during the continuous retrogression and re-ageing (CRRA) treatment with slow heating-cooling rates are investigated systematically. The results reveal that the partial dissolution of pre-existing precipitates (GP Ⅰ zone and η' phase), and the growth of remaining precipitates, as well as the formation of new η' phase occur in the retrogression heating process, which are similar to the phase transformation in the retrogression stage of the discontinuous retrogression and re-ageing (DRRA) treatment. The isothermal retrogression stage of the CRRA treatment plays a key role for precipitates coarsening. The alloy retrogressed at 190 °C for 10 min and re-aged (RRA190–10 alloy) obtains high strength and excellent corrosion resistance, as well as high electrical conductivity simultaneously. Furthermore, the thermodynamic stability of the precipitates in RRA190–10 alloy (η' phase + η phase) is greater than that of T6 alloy (GP Ⅰ zone + η' phase). The microstructure features related to the property changes are discussed.