The Relationship Between Hardness and Microstructure in Zn/Mg Ratio-Controlled Al–Zn–Mg Alloys Aged at 120 °C

Al–Zn–Mg alloys are widely recognized for their high strength-to-weight ratio, with the primary strengthening precipitates being the η/η′ and T/T′ phases. In this study, Al–Zn–Mg alloys with Zn/Mg molar ratios of 0.17, 0.40, 0.75, 1.3, 2.5, and 6.0 were systematically investigated after aging at 120 °C. η′/η precipitates predominantly strengthened alloys with high Zn/Mg ratios, whereas T′/T precipitates dominated those with low Zn/Mg ratios. In contrast, alloys with an intermediate Zn/Mg ratio (Zn/Mg ≈ 1.3) exhibited a balanced coexistence of η′/η and T′/T phases, resulting in the highest hardness among the six alloys. In addition, novel precipitates were observed, with their length increasing as the Zn/Mg ratio decreased. However, because these novel precipitates constitute only a small fraction of the total precipitate population, their direct contribution to the overall hardness remains unclear and warrants further investigation.