Effect of Two-Step Aging on the Microstructure, Mechanical Properties, and Stress Corrosion Cracking Resistance of Ti- and Zr-Containing Al–Zn–Mg Alloys

The effects of Zn/Mg ratio, Cu addition, and Ti–Zr grain refiners on the microstructure, mechanical properties, precipitation behavior, and stress corrosion cracking resistance (SCCR) of Al–Zn–Mg alloys subjected to single and two-step aging treatments were investigated. Alloys with Zn/Mg ratios of approximately 1 and 2 were prepared with and without Cu addition, and Ti–Zr (ZM42HCTZr and ZM44TZr) were solution heat treated, followed by single aging or two-step aging using water quenching (W.Q.) and direct quenching (D.Q.) conditions. Mechanical properties were evaluated by hardness and tensile tests, SCC behavior was examined, and microstructural evolution was characterized by optical microscopy and transmission electron microscopy (TEM). Single aging resulted in the highest hardness due to the high number density of fine precipitates. However, two-step aging treatments improved the balance between strength, ductility, and SCCR. Among the investigated conditions, the two-step aging treatment with water quenching (W.Q.) condition provided the most favorable balance among strength, ductility, and SCCR. TEM observations revealed that ZM42HCTZr predominantly formed η′/η phases, whereas ZM44TZr formed both η′/η and T′/T phases. Pre-aging at 393 K promoted the formation of a high density of fine precipitates, while subsequent aging at 423 K caused precipitate coarsening and a reduction in precipitate number density. Furthermore, ZM42HCTZr exhibited a wider precipitate-free zone (PFZ) and higher precipitate number density than ZM44TZr.