The effects of environment (in air and 3.5 wt. 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. 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. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct Supplementary Information The online version contains supplementary material available at environmental exposure and the larger crack opening. Many cracks were found to nucleate both internally and at the sample surface. Some cracks in both samples nucleated within an exposure time of 80 min (33-40% of the total lifetime). The samples were found to fracture within an environmental exposure time of 240 min. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs.

The samples were then imaged repeatedly using X-ray tomography until they fractured completely. 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 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.

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