The behaviour of high strength aluminum alloys under stress-corrosion conditions has been studied extensively in recent years. In the present investigation the effect of microstructures produced by various thermal treatments on hardness and tensile properties and also on the stress cor- rosion cracking behaviour of high purity AA7004 aluminum al- loy with and without Zr addition has been studied.

The following experimental techniques were used:​

1. hardness measurement
2. tensile testing
3. optical and electron metallography
4. slow strain rate testing
5. scanning electron microscopy
6. differential scanning calorimetry

Two alloys were used in this study, one without zirconi- um, alloy 4, and one containing 0.15 weight percent zircon- uim, alloy 5. Both alloys were solution heat treated at 773K [500°C] for 30 minutes, cooled to room temperature at three different cooling rates and aged at 423K [150°C] and 393K [120°C] for various lengths of time.

It was found that:​

1. The size of grain boundary precipitates and the width of the precipitate free zones increased as cooling rate from SHT decreased.
2. On aging at 423K [150°C] the grain boundary precipitates increased in size with aging time. However, the width of the precipitate free zones remained unchanged on aging.
3. The slow strain-rate test was a very simple and efficient technique used to establish the SCC susceptibility of aluminum alloys.
4. Susceptibility of AA7004 to SCC was most pronounced at the slowest strain-rate used, i.e. 4.1x10-7 s⁻¹.
5. Slow cooling rates from the solution heat treatment temperature and additions of zirconium aided the SCC resis- tance of this alloy in the naturally aged condition.
6. The cooling rate from SHT had no influence on the SCC susceptibility of material peak aged at 423K [150°C].
7. The SCC resistance of AA7004 aluminum alloy having a fully recrystallized structure was higher in the overaged than in the peak aged condition.
8. The degree of recrystallization was the most important parameter of those microstructural features influencing the SCC resistance of AA7004.
9. Aging at 423K [150°C] resulted in a higher SCC resistance than aging at 393K [120°C].
10. The tensile properties were inferior when the alloy was aged at 423K [150°C] rather than at 393K [120°C]).
11. The stress corrosion resistance of 7004 aluminum alloy, having a fully recrystallized structure, increases as the size of the grain boundary precipitates increases.