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 σ -Al 7 Cu 2 Fe and T-Al 2 Mg 3 Zn occurred in the quenched Al–Zn–Mg–Cu alloys. When the Cu/Mg ratio approaches 1.64, the S-Al 2 CuMg 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.

中文翻译：

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Cu/Mg比对Al-Zn-Mg-Cu合金显微组织和力学性能的影响

详细讨论了不同两步时效 (T7X) 下不同 Cu/Mg 比的 Al-Zn-Mg-Cu 合金的微观结构演变与机械性能之间的关系。结果表明，在淬火的Al-Zn-Mg-Cu合金中出现了未溶解的第二相颗粒，包括σ -Al 7 Cu 2 Fe和T-Al 2 Mg 3 Zn。当 Cu/Mg 比接近 1.64 时，还观察到 S-Al 2 CuMg 相。尽管 Cu/Mg 比没有改变 Al-Zn-Mg-Cu 合金的析出顺序，但它改变了时效过程中的粗化速率和析出物尺寸分布。具有最低Cu/Mg比的Al-Zn-Mg-Cu合金具有最慢的粗化速率和最高的强度（655 MPa）。当 Cu/Mg 比值增加时，Al-Zn-Mg-Cu 合金的断裂形貌从典型的脆性断裂转变为以晶间为主的混合断裂模式或韧性断裂。此外，具有不同 Cu/Mg 比的 T79 时效合金具有相似的晶界沉淀 (GBP) 和无沉淀区 (PFZ)。具有较高 Cu/Mg 比的 T74 时效合金具有更粗的 GBP 和更宽的 PFZ。