Effect of Cooling Rate on Precipitation Behavior of Al–7.65Zn–2.59Mg–1.95Cu Alloy with Minor Elements of Zr and Ti

We investigate the effect of cooling rate on the precipitation behavior during cooling from solution treatment temperature and post-aging of a high-strength Al–7.65Zn–2.59Mg–1.95Cu–0.11Zr–0.04Ti extruded alloy. Solution treatment at 450 °C caused the partial dissolution and disintegration of η phase, along with a partial recrystallization of Al grains. The formation of fine L1₂-type Al₃Zr/Al₃(Zr,Ti) (~ 20 nm) and relatively large Ti-rich dispersoids (~ 100 nm) took place during extrusion and/or solution treatment processes. The slow cooling from solution treatment temperature (0.3 °C/min) caused the precipitation of η phases on coarse Al₃(Zr,Ti) particles (formed during solidification), Ti-rich dispersoids (formed during extrusion/solution treatment), grain boundaries, and grain interiors, thereby resulting in negligible aging responses during post-aging at room and elevated temperatures. During fast cooling at 850 °C/min, however, the η phases did not precipitate and thus the Al matrix remained supersaturated, leading to significant aging responses by the formation of GP zones and metastable η″/η′ precipitates.