Co-precipitation of T′ and η′ phase in Al-Zn-Mg-Cu alloys

Abstract The ageing precipitates and strengthening effects of Al-Zn-Mg-Cu alloys with different Zn contents were investigated. The results showed that increasing Zn contents can significantly improve the age-hardening response and influence the type of precipitates as well. The low Zn-containing alloy (1.70 at.%) is strengthened by nano-scale T′ phases in the peak aged condition. However, the microstructure of the high Zn content (2.34 at.%) alloy in the peak aged condition consists of a higher density of globular T′ phases and a small proportion of plate-like η′ phases. The results of atom probe tomography revealed that clusters formed at the early stage of ageing with a Zn/Mg atomic ratio higher than 1.3 transform into η′ phases and these clusters with a lower Zn/Mg ratio transform into T′ phases. The composition of a T′ phase varies with its size, where the concentrations of Zn and Mg increase with its increasing radius, while the concentration of Al presents an opposite trend.