Investigation of fine scale precipitates in Al–Zn–Mg alloys after various heat treatments

Transmission electron microscopy and atom probe field ion microscopy have been used to study the effect of heat treatment at 150 °C and material composition on precipitation in Al–Zn–Mg alloys. Two Cu-free materials with different Zn:Mg ratios (1.7 and 3.2) and one material with low Zn:Mg ratio but with small additions of Cu have been investigated. Selected area electron diffraction patterns of Cu-free materials revealed the presence of GP II zones for all heat treatments used. The number of η particles increased for longer ageing times, but η′ is the dominant precipitate phase up to 6 h at 150 °C. In the material containing Cu, occurrence of GP I zones was also observed. One dimensional atom probe analyses showed that Zn:Mg ratio in the precipitates decreased with heat treatment time increase and that particle number density got lower. Investigation also revealed that Cu is incorporated into the precipitates. An attempt to estimate the contribution of Al excess into η′ due to ion trajectory aberration has been made. The influence of Cu and excess Zn on the precipitation process is discussed.