Hydrogen environmentally assisted cracking during static loading of AA7075 and AA7449

Some newer 7xxx aluminium aerospace alloys seem to be more sensitive to hydrogen environmentally assisted cracking (HEAC) in moist air than older alloys. This investigation compares the relative propensity of new (AA7449) and old (AA7075) alloys to cracking during static loading in warm, moist air (80∘C, 85% relative humidity). The surface stress was held below yield via 4-point bend tests performed using small rigs that permitted ongoing monitoring for small scale surface cracking. Both alloys exhibited HEAC but large cracks formed much more quickly in AA7449 and at lower stresses. The AA7449 alloy rapidly formed cracks at surface stresses as low as 200 MPa, where one sample nucleated a crack greater than 5 mm after only 704 h of exposure. In contrast, AA7075 samples at 250 MPa did not form macroscopic cracks greater than 5 mm within 1600 h of exposure. The importance of many microstructural features and the differences in crack morphology of both alloys were analysed using optical and electron microscopy. Crack propagation in AA7449 was found to be facilitated by the ability of cracks to grow via tortuous paths and overcome barriers, such as triple junctions and unfavourably oriented grain boundaries. This resulted in fewer, much longer cracks in this alloy for the same load and environmental conditions.