Effect of Zn/Mg Ratio on the Microstructure and Coarsening Resistance of Al–Zn–Mg Alloys Aged at 150 °C

The role of the Zn/Mg ratio in regulating microstructure, precipitation evolution and coarsening resistance in Al–Zn–Mg-based alloys was investigated by microhardness testing, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) during isothermal aging at 150 °C. Three alloy compositions were designed with Zn/Mg ratios of 1.6, 2.4, and 3.9, whereas the combined Zn and Mg level was fixed at 6.0 wt%. All alloys exhibited a typical age-hardening response, whereas the maximum peak hardness was obtained at an intermediate Zn/Mg level rather than at the two extremes. The optimal composition (Zn/Mg = 2.4) reached 137 HV, which is attributable to the formation of the finest precipitates (~3 nm) and the highest number density. Moreover, this alloy exhibited the smallest hardness loss (ΔH = 19 HV) after prolonged aging (192 h). TEM analysis indicated that this alloy exhibited the lowest coarsening rate constant, Kr = 0.43 at 192 h. Furthermore, the variation in Zn/Mg ratio affected grain boundary precipitation, leading to a minimized PFZ width at Zn/Mg = 2.4 while maintaining a similar discontinuous distribution of grain boundary precipitates among the alloys. Overall, tailoring the Zn/Mg balance offers an effective strategy to achieve refined precipitates, improved coarsening resistance, and enhanced mechanical performance with superior thermal stability.