Abstract
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.
Highlights
Hydrogen environmentally assisted cracking (HEAC) is a widespread problem affecting many high strength engineering alloys including steels [1], β-Ti alloys [2] and aluminium alloys
The microstructures of as-received AA7075-T651 and AA7449T7651 are shown in Fig. 4 and are notably different
The majority of grains in AA7449 contain an extensive substructure, showing a lower recrystallisation fraction. This difference can be associated to the difference in dispersoid alloying elements; the use of Zr instead of Cr for the recent AA7449
Summary
Hydrogen environmentally assisted cracking (HEAC) is a widespread problem affecting many high strength engineering alloys including steels [1], β-Ti alloys [2] and aluminium alloys. Structural high strength aluminium alloys from the 7xxx series are susceptible to HEAC in aqueous environments (including moist air and water-based ionic solutions) [7] In this case, the ingress of hydrogen to the alloy occurs via the reaction of the water/electrolyte with the alloy surface. The crack growth rate is controlled by the slowest step in the process of accumulating hydrogen at the critically stressed grain boundaries It may be limited by (a) the transport of hydrogen containing species to the alloy surface ( in gaseous environments), (b) the reaction rate at the alloy surface or (c) the diffusion rate of hydrogen within the alloy [11]. HEAC crack growth rates increase with temperature exponentially [7,12]
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