3D characterisation of hydrogen environmentally assisted cracking during static loading of AA7449-T7651

• Published in: International journal of fracture Vol. 232; no. 1; pp. 93 - 116
• Main Authors: De Francisco, Unai, Beckmann, Felix, Moosmann, Julian, Larrosa, Nicolas O., Peel, Matthew J.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-11-2021; Springer Nature B.V
• Subjects: Aluminum; Automotive Engineering; Characterization and Evaluation of Materials; Chemistry and Materials Science; Civil Engineering; Classical Mechanics; Copper; Crack initiation; Crack propagation; Ductile fracture; Environmentally assisted cracking; Exposure; Fracture surfaces; Grain boundaries; Hydrogen embrittlement; Iron; Materials Science; Mechanical Engineering; Nucleation; Original Paper; Relative humidity; Shielding; Stress intensity factors; Striations; Synchrotron radiation; Synchrotrons; Transgranular fracture; X ray imagery; X ray microtomography; Yield stress; Hydrogen environmentally assisted cracking; AA7449-T7651; Synchrotron X-ray tomography; Static loading
• ISSN: 0376-9429; 1573-2673
• DOI: 10.1007/s10704-021-00595-y

In this investigation, synchrotron X-ray microtomography was used to perform 3D in situ observations of crack initiation and growth during hydrogen environmentally assisted cracking (HEAC) in tensile samples of AA7449-T7651. Two smooth tensile samples with a 1 mm diameter gauge section were held at a fixed displacement (
≈
30
% of yield stress) in warm, moist air (
≈
76
∘
C, 73% relative humidity). The samples were then imaged repeatedly using X-ray tomography until they fractured completely. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The samples were found to fracture within an environmental exposure time of 240 min. Some cracks in both samples nucleated within an exposure time of 80 min (33–40% of the total lifetime). Many cracks were found to nucleate both internally and at the sample surface. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct environmental exposure and the larger crack opening. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. Additionally, crack shielding from nearby cracks and the presence of coarse Al–Cu–Fe particles at the grain boundaries were also found to temporarily reduce the crack growth rates. After prolonged crack growth, the HEAC cracks displayed ductile striations and transgranular fracture, revealing a change in the crack growth mechanism at higher stress intensity factors.