Abstract
The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150 °C have been investigated by using tensile tests, optical metallography, scanning electron microscopy, transmission electron microscopy and atom probe tomography analyses. With increasing Zn/Mg ratios, the ageing process is significantly accelerated and the time to peak ageing is reduced. T′ phase predominates in alloys of lower Zn/Mg ratios while η′ phase predominates in alloys with a Zn/Mg ratio over 2.86. Co-existence of T′ phase and η′ phase with a large number density is beneficial to the high strength of alloys. Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture. A strength model has been established to predict the co-strengthening effect of T′ phase and η′ phase in Al-Zn-Mg-Cu alloys, including the factors of the grain boundary, solid solution and precipitation.
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