Improved understanding of environment-induced cracking (EIC) of sensitized 5XXX series aluminium alloys

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2017-01, Vol.682, p.613-621
• Main Authors: Holroyd, N.J.H., Burnett, T.L., Seifi, M., Lewandowski, J.J.
• Format: Article
• Language: English
• Subjects: Aluminum base alloys; Aluminum composites; Aluminum-magnesium alloys; Catalytic cracking; Computed tomography; corrosion cracking 3-D tomography; Crack propagation; Cracking (fracturing); Drying; Environment-enhanced-cracking; Exposure; Fracture mechanics; High resolution; Hydrogen embrittlement; Intergranular corrosion; Scanning electron microscopy; Slow strain rate; Strain rate; Stress; Stress corrosion cracking; Stress intensity factor; Stress intensity factors
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2016.11.088

Two mechanistically different modes of EIC have been identified using high-resolution X-ray computed tomography and scanning electron microscopy (SEM) in sensitized AA5083-H131 that had been pre-exposed to 0.6M NaCl prior to interrupted slow strain rate testing (SSRT) in the short transverse direction while exposed to laboratory air (50% RH). One mode, shown to propagate when local stress intensity factors are in the range of 4–12MNm‐3/2, is the well-known ‘classic’ form of intergranular stress corrosion cracking, Type-1 cracking, which would not initiate and propagate in dry air, irrespective of pre-exposure or sensitization. The second mode of cracking, identified presently as Type-2 cracking, is associated with sudden load-drops occurring after the UTS during SSRT. Type-2 cracking propagates at higher local stress intensity factors (above 12–15MNm-3/2) with significantly higher average growth rates than Type-1 cracking, involves the sudden simultaneous mechanical linkage of multiple fully-isolated regions of damage, pre-determined during pre-exposure to NaCl solution and generated during straining in laboratory air. Differences in the extent of Type-2 cracking for pre-exposed samples tested in ‘dry’ air compared to laboratory air (50% RH) were marked, with that in dry air being limited to isolated patches. High-resolution 3D tomography and detailed SEM has been used to distinguish these two mechanistically different modes of EIC. Implications for a role of hydrogen embrittlement during EIC are discussed.