Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air

Abstract

The hydrogen environmentally-assisted cracking (HEAC) behaviour of new generation (new-gen) Al-Zn-Mg-Cu thick-plate alloys (AA7449, AA7085, and AA7037), under accelerated humid warm-air exposure, has been compared to the aerospace industry benchmark alloy AA7050 in over-aged T7x type tempers. Constant load time-to-failure and DCB crack growth tests have been performed at a relative humidity of 85 %, at 70 °C. It has been found that in constant load tests the failure time of the new generation alloys can be reduced by at least an order of magnitude and in some cases cracking was observed in only a few days at stress levels of 85 % of yield. Analysis of the failed fractured specimens has confirmed that the fracture behaviour is predominantly intergranular, and highly localised to high angle grain boundaries. The progression from surface initiation to self-propagating cracks occurred much more rapidly in the new higher Zn content alloys, but with less prior surface chemical attack. In DCB v-K tests stage II crack growth rates were also found to be significantly higher (by a factor of 6–20 times) in the new-gen alloys and the K1HEAC threshold stress intensity was less than half that for AA7050-T7651. Under long crack growth conditions, propagation thus required a higher mechanical driving force and crack growth stagnated more readily in the AA7050-T7651 benchmark material, compared to in the new generation alloys. The results have been discussed in the context of the potential influences of differences in the alloy’s chemistries and microstructures.