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

Abstract: 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. Graphical Abstract: [Figure not available: see fulltext.].