Effect of Zn content on the microstructure, mechanical performances, and corrosion resistance of ultra-high strength Al-xZn-2.8Mg-1.0Cu alloy with high Zn content

For Al-Zn-Mg-Cu alloys, it is essential to find a critical value of Zn content that optimally balances the comprehensive performances. In this study, advanced electron microscopy techniques such as EBSD, SEM, TEM and APT as well as comprehensive property tests such as tensile, EXCO, electrochemical test are used to investigate the influence of high Zn content on the microstructure and properties of Al-xZn-2.8Mg-1.0Cu alloys. The Zn content has no significant effect on the grain size of the as-cast alloy, and a higher Zn concentration result in a lower over-burning temperature. After enhanced solid solution treatment, the excess Zn atoms in high Zn content alloys fail to fully dissolve into the matrix due to the solid solution limit and exist in the form of insoluble second phases (AlZnMgCu phase). The strengthening phase in most alloys is the η′ phase, but there is a clear indication of co-precipitation of the η′ and T′ phases in the alloy with a low Zn content. As the Zn content is increased, the volume fraction and size of precipitates exhibits a notable increase followed by a period of stabilization. Consequently, the alloy strength increases rapidly from 705 MPa to 771 MPa and then remained stable. In contrast, the elongation continuously decreases, which is attributed to the large amount of insoluble second phases. The alloy's resistance to exfoliation corrosion and electrochemical corrosion deteriorates as Zn content increases. The Zn content of 10.6 wt% is considered to be the optimal composition exhibiting good comprehensive performance.