Effects of copper on the precipitation and RRA treatments in a high purity Al-6.0%Zn-2.75%Mg alloy were investigated by measuring hardness changes and crack growth rates. The microstructural changes in both the matrix and grain boundary have been investigated by means of transmission electron microscopy.
The metastable GPzone solvus temperature for pure ternary alloy is determined to be 143℃. The addition of 1.7% Cu into this alloy increased that temperature to above 160℃. It appears that the addition of copper significantly influences the incubation time of GPzone.
The RRA treatments on the T6 tempered sample improve stress corrosion resistance without sacrificing the original T6 strength in both Al-6.0%Zn-2.75%Mg and Al-6.0%Zn-2.75%Mg-1.7%Cu alloys.
Examination of the grain boundary microstructure exhibited that the RRA treatments result in a significant increase of the volume fraction of the grain boundary precipitates. The beneficial effect of the RRA treatments on the stress corrosion resistance is believed to be due to the increase in the volume fraction of grain boundary precipitates.
The addition of copper drastically increases the retrogression time within which the alloy recovers its original T6 strength after RRA treatments. This is believed to be due to the presence of higher density of particles in the copper containing alloy as compared to that in the ternary alloy because of different kinetics of dissolution and precipitation.
The addition of copper is also shown to increase significantly stress corrosion resistance. However, no significant increase in the volume fraction of precipitates was observed when copper was added. The reason for the improvement of stress corrosion resistance due to copper addition is believed to be the changes in the electro-chemical property of grain boundary precipitates.
