The Microstructural Observation in Precipitations of Peak-Aged Al–Zn–Mg Alloys with Various Zn/Mg Ratios

The precipitation behavior of peak-aged Al–Zn–Mg alloys with various Zn/Mg ratios was investigated to clarify the relationship between aging microstructure and mechanical response using (scanning) transmission electron microscopy (STEM/TEM). The results revealed that both aging temperature and Zn/Mg ratio significantly influence the type, size, and density of precipitates, thereby determining the alloy hardness. At higher temperatures, coarser and lower-density precipitates led to reduced hardness, while finer and denser precipitates formed at lower temperatures enhanced hardening. For alloys with Zn/Mg < 1.0, the T′/T phases dominate strengthening, whereas η′/η phases are predominant for Zn/Mg > 2.0. Moreover, this study identifies a novel type of precipitate aligned along [110] Al in high-Mg alloys aged at 120 °C, which has not been reported previously and may contribute additional strengthening. These findings provide new insight into the compositional and thermal control of precipitation mechanisms in Al–Zn–Mg alloys.