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

The relationship between microstructure evolution and mechanical performance of Al-Zn-Mg-Cu alloys with varying Cu/Mg ratios was discussed in detail under different two-step aging (T7X). The results showed that the undissolved second-phase particles including sigma-Al7Cu2Fe and T-Al2Mg3Zn occurred in the quenched Al-Zn-Mg-Cu alloys. When the Cu/Mg ratio approaches 1.64, the S-Al2CuMg phase was also observed. Although the Cu/Mg ratio did not change the precipitation sequence of the Al-Zn-Mg-Cu alloys, it changed the coarsening rate and precipitate size distribution during aging process. The Al-Zn-Mg-Cu alloy with the lowest Cu/Mg ratio has the slowest coarsening rate and the highest strength (655 MPa). The fracture morphology of Al-Zn-Mg-Cu alloys changes from the typical brittle fracture to the intergranular-dominated mixed fracture mode or ductile fracture when the Cu/Mg ratios increases. Besides, the T79-aged alloys with varying Cu/Mg ratios have similar grain boundary precipitates (GBPs) and precipitate-free zones (PFZs). The T74-aged alloys with higher Cu/Mg ratio have more coarser GBPs and wider PFZs.