Effect of Zn/Mg ratio on microstructure and mechanical properties of Al-Zn-Mg alloys

The influence of Zn/Mg ratio on microstructure and mechanical properties of Al-Zn-Mg alloys with the same Zn + Mg contents has been investigated through microhardness test, electrical conductivity test and transmission electron microscopy (TEM). In this study, Zn/Mg ratio was changed from 1.61 to 3.93 with Zn + Mg content of 6 wt%. The results showed that the Vickers hardness of the alloy with the Zn/Mg ratio of 2.38 aged at 120 ℃ for 48 h was highest. Through TEM observation, strengthening precipitates in investigated alloys were mainly spherical T′ phases and a few plate-like η′ phases. With the increase of Zn/Mg ratio, T′ phase fraction in alloys decreased from 88% to 81%, and the corresponding η′ phase fraction increased from 12% to 19%. As aging at 120 ℃ from 48 h to 168 h, the coarsening degree of precipitates of alloy A and alloy C were both greater than that of alloy B. When the Zn + Mg content was 6 wt%, the alloy with the Zn/Mg ratio of 2.38 had optimum microstructure and mechanical properties.