Correlations between intergranular stress corrosion cracking, grain-boundary microchemistry, and grain-boundary electrochemistry for Al–Zn–Mg–Cu alloys

Knight, Steven Peter; Birbilis, Nick; Muddle, Barrington Charles; Trueman, Antony; Lynch, S.P.

doi:10.1016/j.corsci.2010.08.024

Cited by 269 publications

(121 citation statements)

References 13 publications

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“…[5,102] Recent work suggested a relationship between intergranular stress corrosion crack growth rates (region 2) and the initial open-circuit electrochemical potential (OCP) for brittle intergranular fracture surfaces exposed to an aqueous saline solution for peak-and overaged AA7075 and AA7079 alloys, with higher crack growth rates being associated with more negative OCPs. [102] Arguments stemming from these observations maybe strengthened by: (a) obtaining further evidence the simulated intergranular fracture surface generated by mechanical fast fracture at a low temperature is representative of an intergranular sustained-load fracture surface and (b) the use of an aqueous test solution more representative of the local environments known to exist in crack-tip regions of propagating stress corrosion cracks, which are not simple aerated aqueous saline solutions, but acidified de-aerated environments containing a significant concentration of dissolved aluminum species. [62,104] Other studies have indicated that OCP and pitting potentials for Al-Zn-Mg(-Cu) alloys exposed to aqueous saline solutions at room temperature increase logarithmically with an alloy's copper content.…”

Section: Mechanistic Implicationsmentioning

confidence: 99%

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Holroyd¹,

Scamans

2011

Metall Mater Trans A

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Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10°C to 80°C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from~150 nm at room temperature to~400 nm at 313 K (40°C). A re-evaluation of published data reveals the apparent activation energy, E a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently~35 kJ/mol for temperatures above~313 K (40°C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al 2 CuMg where E a is~80 kJ/mol. For temperatures below~313 K (40°C) E a is independent of copper content for stress intensity factors below~14 MNm À3/2 , with a value~80 kJ/mol but is sensitive to copper content for stress intensity factors abovẽ 14 MNm À3/2 , with E a , ranging from~35 kJ/mol for copper-free alloys to~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH 3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.

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Section: Mechanistic Implicationsmentioning

confidence: 99%

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Holroyd¹,

Scamans

2011

Metall Mater Trans A

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“…High stress-corrosion cracking strength and low stresscorrosion cracking plateau velocity could be attained using the T7 temper ( Ref 12,13). The present study evaluated the effects of processing factors, such as deformation at cryogenic temperatures, on the mechanical properties and corrosion resistance of 7075-T73 alloy samples with and without anodization and sealed treatment.…”

Section: Introductionmentioning

confidence: 99%

Effects of Cryogenic Forging and Anodization on the Mechanical Properties of AA 7075-T73 Aluminum Alloys

Shih

Liao

Hsu

2016

J. of Materi Eng and Perform

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In this study, high-strength AA7075 alloy samples were cryogenically forged after annealing and then subjected to solution and aging treatments. The cryogenically forged 7075-T73 alloy samples displayed equiaxed fine grains associated with abundant fine precipitates in their matrix. Compared with conventional 7075-T73 alloy samples, the cryogenically forged samples exhibited an 8-12% reduction in tensile strength and an increased fatigue strength and higher corrosion resistance. The fatigue strength measured at 10 7 cycles was 225 MPa in the bare samples; the strength was increased to 250 MPa in the cryogenically forged samples. The effect of anodization on the corrosion resistance of the bare samples was improved from (E corr ) 20.80 to 20.61 V.

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“…However, it is well known that over-aging in Al-Zn-Mg-Cu alloys can improve the resistance to IGC and SCC, partly because more Cu incorporates in the Mg(Zn,Al,Cu) 2 precipitates at the grain boundaries for the over-aged sample. 18,21 Moreover, a Cu-rich Al alloy was shown to exhibit lower SCC growth rate than a Cu-lean Al alloy. 13 From the discussion above, it can be seen that the role of Cu in the IMPs, the matrix solid solution and the grain boundary precipitates on the corrosion behavior is complicated.…”

Section: Role Of Almentioning

confidence: 99%

“…The higher Cu content in the η-phase resulting from overaging is considered to be beneficial to the resistance to intergranular corrosion (IGC) and stress corrosion cracking (SCC). 18,21 The goal of the present study is to establish the correlation between microstructure evolution, electrochemical behavior and pitting corrosion damage under the salt-spray environment in commercial Cu-rich AA7055 and an experimental Cu-free Al-Zn-Mg alloy with different aging conditions, and to further investigate the role of Cu in the pitting corrosion. The microstructure evolution and electrochemical behavior of the two Al alloys in various aging conditions were studied by transmission electron microscope (TEM) and polarization measurements.…”

mentioning

confidence: 99%

Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

Wang

Huang

et al. 2015

J. Electrochem. Soc.

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The corrosion behavior and microstructure of AA7055 alloy with a high Cu content, and a Cu-free Al-Zn-Mg alloy in various aging conditions were studied. Polarization measurements and ASTM B117 salt-spray exposure were used to assess the corrosion characteristics. Transmission electron microscope and atom probe tomography were used to evaluate the microstructure and the composition of precipitates. The matrix composition of AA7055 changed with aging because of precipitation. Thus, the galvanic relationship between Al 7 Cu 2 Fe particles and the surrounding matrix correspondingly changed, which affected the susceptibility to pitting corrosion attack. The susceptibility to pitting corrosion damage of AA7055 was found to decrease according to: over-aged samples > under-aged samples > peak-aged sample. However, aging had only a small influence on pitting corrosion damage for the Cu-free Al-Zn-Mg alloy. Furthermore, despite the more active breakdown potential of the Cu-free Al-Zn-Mg alloy, this alloy exhibited less severe pitting corrosion damage than AA7055, in particular for the over-aged samples. This was attributed to the weaker galvanic coupling between Al 3 Fe particles and the surrounding matrix. Although Cu in the intermetallic particles is detrimental to pitting resistance, the galvanic effect can be reduced by increasing the matrix Cu content. High-strength precipitation-hardenable aluminum alloys are commonly used for structural components of aircraft due to their high strength-to-density ratio.1 However, they are also very susceptible to pitting corrosion, in particular for Navy aircraft subjected to extended periods of salt water spray and/or salt fog.2 These corrosion pits can act as initiation sites for corrosion fatigue cracks and reduce the fatigue life, which is a significant damage mechanism in aging aircraft structures. 2,3From a microstructural viewpoint, initiation of localized pitting corrosion is often associated with coarse cathodic intermetallic particles (IMPs), such as Al 7 Cu 2 Fe, Al 3 Fe, Al 2 Cu, and Al 2 CuMg, which are common in 2xxx and 7xxx series Al alloys.4-9 A number of studies have been performed on electrochemical characteristics of these particles. 4,8,9 In general, they act as local cathodes and sustain oxygen reduction at a high rate to drive the pitting corrosion attack at the particle-matrix interface. In addition, a few studies have related the precipitate size to metastable and stable pitting corrosion in Al-Cu-Mg and Al-Zn-Mg-Cu alloys.10-13 Ralston et al. 11,12 suggested that there was a critical S-phase precipitate size range from ∼3 -8 nm to trigger pitting events. Aged samples with smaller precipitate size had pitting resistance similar to as-quenched samples. The susceptibility to pitting corrosion was greatly enhanced after prolonged aging to create precipitates larger than this critical size range. Gupta et al. 13 reported that a critical precipitate size existed for a transition in electrochemical behavior of 7xxx series Al alloys. For Cu-rich AA7150, increased ...

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Correlations between intergranular stress corrosion cracking, grain-boundary microchemistry, and grain-boundary electrochemistry for Al–Zn–Mg–Cu alloys

Cited by 269 publications

References 13 publications

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Effects of Cryogenic Forging and Anodization on the Mechanical Properties of AA 7075-T73 Aluminum Alloys

Effect of Cu Content and Aging Conditions on Pitting Corrosion Damage of 7xxx Series Aluminum Alloys

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