Abstract: The effect of small addition of silver on a high-zinc super-high strength Al-Zn-Mg-Cu aluminum alloy was investigated. The mechanical properties of the investigated alloys with or without silver addition undergoing different heat treatment were tested by tensile test. Using transmission electronic microscope (TEM), scanning electronic microscope (SEM) and its energy dispersive spectrometry (EDS), the microstructures of both alloys were analyzed. The results show that small addition of silver promotes the formation of coarse constituent particles within the alloy and consumes hardening alloying elements of Zn and Mg, which may decrease the mechanical properties of the alloy. When the same solid-solution heat treatment and ageing process are employed to the silver-bearing alloy and the silver-free one, the tensile strength of the former alloy at peak-aged condition is decreased by about 40 MPa compared to the latter. However, the solution annealing temperature can be increased due to small addition of Ag. When the silver-bearing alloy is solutionized on a higher temperature and aged subsequently, the remained coarse particles in its microstructure can be solutionized into the matrix more sufficiently, and thus makes the alloy obtain a same level of the tensile strength as the silver-free one at peak-age hardening, while its elongation ratio slightly decreases. During ageing, trace addition of Ag enhances the formation of metastable phases and stablizes GP zones and metastable phases to a higher temperature. However, small addition of Ag prolongs the ageing time reaching the peak-age hardening for the studied alloy.

Key words: Al-Zn-Mg-Cu alloy; microstructure; mechanical properties; small addition of Ag