Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air

Schwarzenböck, Elisabeth; Ollivier, E.; Garner, Alistair; Cassell, Alexander; Hack, T.; Barrett, Zak; Engel, Christian; Burnett, Tim L.; Holroyd, N.J.H.; Robson, Joseph D.; Prangnell, P.B.

doi:10.1016/j.corsci.2020.108701

Cited by 42 publications

(20 citation statements)

References 42 publications

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“…Previous 4-point bend samples by the authors found that most cracks in AA7075-T651 and some in AA7449-T7651 nucleated in the proximity of coarse intermetallic particles (Mg 2 Si and Al-Cu-Fe stringers) (De Francisco et al 2020). Similarly, Schwarzenböck et al found that most cracks in static tensile samples formed at coarse Cu and Fe rich intermetallic particles near the sample surface (Schwarzenböck et al 2020). Al-Cu-Fe particles have also been established as crack initiation sites of 7xxx alloys under other loading conditions.…”

Section: Crack Initiation At Intermetallic Particlesmentioning

confidence: 84%

“…The cracking behaviour and fracture rates during HEAC can differ significantly between nominally identical samples subjected to equivalent loading conditions (De Francisco et al 2020;Schwarzenböck et al 2020). This is due to the statistical nature of HEAC, particularly during the crack initiation and microstructurally short cracking stages.…”

Section: Future Workmentioning

confidence: 99%

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

et al. 2021

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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 ($$\approx 30$$ ≈ 30 % of yield stress) in warm, moist air ($$\approx 76\,^\circ $$ ≈ 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.

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Section: Crack Initiation At Intermetallic Particlesmentioning

confidence: 84%

Section: Future Workmentioning

confidence: 99%

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

et al. 2021

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“…When

≤ 10 −9 mm/s, the SCC test was stopped. It is worth mentioning that the crack growth rate is generally estimated from sequential measurements averaged on both sides of the DCB specimen [ 28 ]. Therefore, both sides should be characterized spontaneously.…”

Section: Methodsmentioning

confidence: 99%

The Localized Corrosion and Stress Corrosion Cracking of a 6005A-T6 Extrusion Profile

Sun

Guan

et al. 2021

Materials

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In the present work, the localized corrosion and stress corrosion cracking (SCC) behaviors of a commercial 6005A-T6 aluminum extrusion profile was studied comprehensively. The velocity of crack growth in self-stressed double-cantilever beam (DCB) specimens under constant displacement was estimated, which also provides insight into the local microstructure evolutions at the crack tips caused by the localized pitting corrosion, intergranular corrosion (IGC), and intergranular SCC. Characterizations of local corrosion along the cracking path for a period of exposure to 3.5% NaCl were revealed via optical microscope (OM), scanning electron microscope (SEM), and electron backscatter diffraction (EBSD). The typical features of the pits dominated by the distribution of precipitates included the peripheral dissolution of the Al matrix, channeling corrosion, intergranular attack, and large pits in the grains. The discontinuous cracking at the crack tips indicated the hydrogen-embrittlement-mediated mechanism. Moreover, the local regions enriched with Mg2Si and Mg5Si6 phases and with low-angle grain boundaries presented better SCC resistance than those of the matrix with high-angle grain boundaries, supporting a strategy to develop advanced Al–Mg–Si alloys via interfacial engineering.

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“…7xxx series aluminium alloys, Al-Zn-Mg-(Cu), are used as high performance structural materials -AA7075 for instance carries an ultimate tensile strength 572MPa in the peak aged T6 temper [1]. However, material degradation arising from a collection of phenomena generally labelled as environmentally assisted cracking (EAC) has been a key challenge in the development of these alloy systems [2]; for alloys used by the aircraft industry, EAC is an intergranular phenomenon [3]. Considerable study within metallurgy has delivered two fundamental causes of EAC, namely anodic dissolution (AD) and hydrogen embrittlement (HE), and the problem has been recorded in a variety of environments where the relevant atomospheric conditions are reportedly humidity and chloride content [4].…”

Section: Introductionmentioning

confidence: 99%

Modelling Hydrogen Embrittlement using Density Functional Theory: A theoretical approach to understanding environmentally assisted cracking in 7xxx series aluminium alloys

2020

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The effects of H segregation to a Σ11 symmetric tilt Al grain boundary are investigated using atomistic simulations, as part of a wider study on cracking in 7xxx series alloys. Density functional theory based simulations of uniaxial straining of grain boundaries containing 11 different concentrations of H were performed under the cohesive zone fracture mechanics framework. The theoretical strength of grain boundaries is shown to be supressed by H segregation, and the cause of this is attributed to the prevention of the formation of Al ligaments across grain boundaries. Segregated concentrations of relevant alloying elements (Zn, Mg, and Cu) show minimal impact on the H embrittlement process investigated, namely H enhanced decohesion (HEDE). Further modelling, of H transport and grain boundary precipitates, is required to confirm the validity of the HEDE mechanism in the case of 7xxx alloys.

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Environmental cracking performance of new generation thick plate 7000-T7x series alloys in humid air

Cited by 42 publications

References 42 publications

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

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

The Localized Corrosion and Stress Corrosion Cracking of a 6005A-T6 Extrusion Profile

Modelling Hydrogen Embrittlement using Density Functional Theory: A theoretical approach to understanding environmentally assisted cracking in 7xxx series aluminium alloys

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