Effects of the retrogression and reaging(RRA) treatments on the strength and susceptibility to stress corrosion cracking(SCC) of the commercial 7075 Aluminum alloy were studied by measuring hardness changes and crack propagation rates as a function of stress intensity using double cantilever beam specimens. Transmission electron microscopy was used to determine the matrix and grain boundary of T6 tempered specimens.
As a result of the RRA treatment, SC crack velocity is significantly decreased to an extent that it is nearly comparable to that in the T73 tempered specimen. But the strength is comparable to the maximum strength of the T6 tempered specimen. The high strength of the RRA structure, comparable to that of T6 case, is believed to arise from the fact that it contains relatively high overall concentration of particles. The presence of significant amount of coherent η' transition phase in this structure is also considered to contribute to that effect.
The RRA treatment also results in the significant increase. Microstructural study shows that the RRA structure contains grain boundary precipitates with much larger sizes than that in the T6 structure, resulting in the increase of line fraction of grain boundary particles together with the decrease in the number of grain boundary particles. This effect is believed to be responsible for the much enhanced SCC resistance as a result of the RRA treatment.
