Y. Aboura^a, A. Garner^a, R. Euesden^a, Z. Barrett^b, C. Engel^c , P. Prangnell^a, T. Burnett^a

a Department of Materials, The University of Manchester, M13 9PL, UK.

b Airbus UK, Pegasus House, Aerospace Ave, Filton Bristol, BS34 7PA, UK

c Airbus, Airbus Operations GmbH, Airbus-Allee 1, 28199 Bremen, Germany

Aluminium alloys based on Al-Zn-Mg-Cu compositions have been known to exhibit environmental assisted cracking (EAC) under aqueous and humid air conditions typically experienced in service [1], [2]. This has led the industry to refine the alloy compositions and develop heat treatment conditions that increase their resistance to EAC, culminating in the overaged temper T7xx [1], [3]. The latest generation 7xxx series alloys (AA7085 and AA7449) used in aerospace large component structures have been shown to be susceptible to humid air cracking in their overaged temper condition [4], even down to relative humidity levels of 20%. In contrast, the more established alloy AA7050 shows higher resistance to EAC in humid air when tested under the same conditions and temper. Testing of these alloys in warm humid environments (70 C, 85% Rh) has shown much longer crack initiation times, higher threshold KEAC and slower crack growth rates for AA7050, compared to AA7085 and AA7449[3], [5], [6]. EAC fractures in the latter were mainly intergranular and some transgranular with no evidence of extensive corrosion products [3], [5]. This discrepancy in cracking behaviour between the alloys in humid air has attracted research interest to understand the underlying reason for this phenomenon. Here, we present our interpretation of EAC in 7xxx series aluminium alloys based on multiscale correlative characterisation of specimens tested to failure under rising load (SSRT) conditions. High-resolution SEM – EDX/FIB and TEM coupled with 3D microstructural investigations were utilised to reveal detailed information on key aspects of initiation and propagation of EAC in humid air. Hydrogen-related EAC fracture features, including crack arrest marks (CAMs) and clean brittle intergranular facets, were observed on the fracture surfaces and compared between the different materials. The implications of these observations on the relative EAC performance of the alloys are discussed.

[1] N. J. H. Holroyd and G. M. Scamans, “Crack propagation during sustained-load cracking of Al-Zn-Mg-Cu aluminum alloys exposed to moist air or distilled water,” Metall. Mater. Trans. A Phys. Metall. Mater. Sci., vol. 42, no. 13, pp. 3979–3998, 2011, doi: 10.1007/s11661-011-0793-x.

[2] N. J. H. Holroyd and G. M. Scamans, “Stress corrosion cracking in Al-Zn-Mg-Cu aluminum alloys in saline environments,” Metall. Mater. Trans. A Phys. Metall. Mater. Sci., vol. 44, no. 3, pp. 1230–1253, 2013, doi: 10.1007/s11661-012-1528-3.

[3] E. Schwarzenböck et al., “Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air,” Corros. Sci., vol. 171, no. May, p. 108701, 2020, doi: 10.1016/j.corsci.2020.108701.

[4] E. aviation safety agency EASA, “Safety Information Bulletin: Environmentally assisted cracking in certain aluminium alloys 2018-04R2,” 2018.

[5] E. Schwarzenböck, L. Wiehler, T. Heidenblut, T. Hack, C. Engel, and H. J. Maier, “Crack Initiation of an Industrial 7XXX Aluminum Alloy in Humid Air analyzed via Slow Strain Rate Testing and Constant Displacement Testing,” Mater. Sci. Eng. A, vol. 804, no. August 2020, p. 140776, 2021, doi: 10.1016/j.msea.2021.140776.

[6] U. De Francisco, N. O. Larrosa, and M. J. Peel, “Hydrogen environmentally assisted cracking during static loading of AA7075 and AA7449,” Mater. Sci. Eng. A, vol. 772, no. August, 2020, doi: 10.1016/j.msea.2019.138662.