Optimum retrogression and reaging heat treatment of super-high strength Al-Zn-Mg-Cu-Zr alloy containing Sc

The effects of retrogression and reaging(RRA) treatment on the microstructures and mechanical properties of super-high strength Al-Zn-Mg-Cu-Zr alloy containing Sc were studied by transmission electron microscopy, tensile and conductivity tests.The results show that perfect mechanical and stress corrosion resistance properties can be gained if the alloy is treated by the RRA heat treatment of 120 ℃, 24 h pre-ageing, 180℃, 30 min retrogression and 120℃, 24 h re-ageing.Compared with the T6 sample, the strength of the alloy decreases slightly while the conductivity increases greatly.Treated by RRA, a great amount of fine and homogenized distributed precipitations η′ and few equilibrium phases η are observed inside the grains, while the equilibrium phases on the grain boundaries coarsen obviously, distributing discontinuously and isolated.The precipitation free zones are broadened than that of the T6 sample.The microstructure inside the grains is similar to that of the T6 sample while the microstructure of the grain boundary is similar to that of the duplex over-aged sample.