The effects of the Retrogression and Reaging (RRA) treatments on the strength and stress corrosion crack propagation in high purity Al-Zn-Mg alloy were studied by measuring hardness changes and crack growth rates as a function of stress intensity factor using the precracked C-T specimens.
Transmission electron microscopy was used to examine the microstructural changes in both the matrix and grain boundary.
The results show that the RRA treatment on the T6 tempered specimen results in a significant decrease of the crack velocity without sacrificing the original T6 strength.
The high strength of the RRA treated specimen, comparable to that of the T6 temper, is believed to arise from the relatively high overall concentration of matrix precipitates.
The presence of fine dispersion of semicoherent η’ transition phase is also believed to contribute to that effect.
Examination of the grain boundary microstructure show that the RRA treatments also results in a significant increase of the size of the grain boundary precipitates.
The beneficial effect of the RRA treatment on the susceptibility to SOC is believed to be due to the increase in the size of grain boundary precipitates.
This is due to the coarsening of the grain boundary precipitates during the retrogression treatment.
