Effects of Retrogression Temperature on Microstructure and Properties of Al-Zn-Mg-Sc-Zr Alloy

The evolution of the microstructure and properties of Al-Zn-Mg-Sc-Zr alloy treated by retrogression,retrogression and re-aging（RRA） at different retrogression temperatures was studied by hardness, electrical conductivity,exfoliation corrosion tests and transmission electron microscope observation. The results show that with the increase of the retrogression temperature, the size and spacing of the precipitates of the retrogression and RRA samples gradually increase and the number decreases, and the precipitate free zone（PFZ） becomes wider, leading to a decrease in hardness and an increase in electrical conductivity. At the same retrogression temperature, compared with the retrogression sample, the number of intragranular precipitates of RRA sample（especially η’ phase） increases, and there is no significant difference in the grain boundary precipitates and PFZ, resulting in the increase of hardness and conductivity. At the retrogression temperature of 160 ℃,the hardness（167.5 HV） of the RRA sample shows little decrease, while the electrical conductivity（34.3%IACS） and resistance to exfoliation corrosion are improved. On balance, 160℃ is the best retrogression temperature for the Al-Zn-Mg-Sc-Zr alloy.