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 degrees C caused the partial dissolution and disintegration of eta phase, along with a partial recrystallization of Al grains. The formation of fine L1(2)-type Al3Zr/Al-3(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 degrees C/min) caused the precipitation of eta phases on coarse Al-3(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 degrees C/min, however, the eta phases did not precipitate and thus the Al matrix remained supersaturated, leading to significant aging responses by the formation of GP zones and metastable eta ''/eta precipitates.Graphical Abstract