Precipitation structure and mechanical properties on peak-aged Al–Zn–Mg alloys including different with some Zn/Mg ratios

Al–Zn–Mg alloys with different ratios of Zn/Mg have been investigated to understand the influence of the Zn/Mg ratios on age-hardening, microstructures and precipitates. Alloys containing higher ratios of Zn to Mg revealed higher peak hardness, ultimate tensile strengths, 0.2% proof stresses and higher number densities of precipitates per unit area. A square shaped T′-phase has been observed in the matrices of low Zn/Mg ratio alloys aged at 423 K with the aid of transmission electron microscope (TEM) images and its selected area electron diffraction (SAED) patterns, while SAED patterns of η′ and/or η-phase have been detected in high Zn/Mg ratio alloys. When aging temperature is lowered to 393 K, η′/η-phase have been again observed in the matrix, though no T′-phase was detected in low Zn/Mg ratio alloys. The aging condition in the present work has turned out to be on over aged condition for low Zn/Mg ratio alloys with coarse precipitates of T′-phase in the matrix, because those alloys attained lower peak hardness, tensile strength and higher elongation than high Zn/Mg ratio alloys which have fine η′/η-phase.