Understanding the environmentally assisted cracking (EAC) initiation and propagation of new generation 7xxx alloys using slow strain rate testing

Mechanisms of Environmentally Induced Crack Initiation in Humid Air for New-Generation Al-Zn-Mg-Cu Alloys

“…1b) was consistent for all the primary cracks in both alloys under the same test conditions (See Ref. [42]), although the overall EIC performance was worse for AA7449 compared to AA7085, which had a shorter time to failure in the 4PB tests, as has already been reported [2,9,42]. Fig.…”

“…Under a tensile load, this can lead to intergranular cracking caused by hydrogen embrittlement. The Mg containing (Mg(Zn,Al,Cu) 2 ) η-phase, which precipitates copiously on grain boundaries in 7xxx alloys (GBs) with a high area coverage, can anodically dissolveif fully immersed -or react with water vapour, or thin surface adsorbed water layers [9], and is thought to be the main reactant in this process. Many metallurgical factors are known to affect the EIC performance of 7xxx alloys which are related to their bulk alloy composition and thermomechanical history, including; the composition and distribution of the grain boundary precipitates (GBPs) and…”

Environment-Induced Cracking of High-Strength Al-Zn-Mg-Cu Aluminum Alloys: Past, Present, and Future

“…EIC performance data for these 3 rd Generation high strength 7xxx series alloys within the patent literature, typically are presented as the minimum stress levels for survival after conventional ASTM G47 EIC testing under alternate immersion in 0.6M NaCl [127,130], and further testing to outdoor marine environments [127,130] and high humidity at 80 °C [131]. The slow strain rate testing (SSRT) method used to characterize the influences of homogenization, solution heat-treatment, thermal aging and alloy microstructure, strength and toughness on EIC performance typically employ nominal strain rates well above the ~10 -7 /s, needed to provide information on EIC initiation and initial growth behavior [132,133]. High nominal strain rates during SSRT provides insufficient time at stresses around the yield stress (Region 1 in Figure 10) for EIC incubation/initiation processes to activate and promote crack growth before plastic deformation processes trigger local inhomogeneous deformation and the onset of mechanical failure (Region 2 in Figure 10).…”

“…It is clear from their results, when assessed in terms of UTS and plastic elongation ratios for tests conducted in the test environments relative to mechanical testing (Table 8), the extreme brittleness of the 10% Zn alloy renders it unsuitable for EIC assessment using SSRT, no evidence of EIC initiation is provided for AA7075-T6 in any of the test environments, and EIC had initiated in the 8.5% Zn alloy strained in the chloride containing environments and may have in all the other test environments. Using conventional SSRT to assess the EIC propensity of commercial aluminum alloys is not recommended and ideally should only be conducted on tensile samples strained in the through-thickness (short-transverse) direction, using extremely low nominal strain rates (≤ ~10 -7 /s) and for particularly resistant tempers should involve pre-exposure to an appropriate environment ahead of testing [132]. Use of pre-cracked fracture mechanics type test specimens during EIC studies on 3 rd Generation 7xxx series alloys has increased during the last decade, particularly in China using DCB test specimens [138] and a test method very similar to ASTM G139 [137], Table 9 and Figure 11.The reduced EIC performance for 3 rd versus 2 nd Generation alloys is shown by minimum mechanical driving forces (K IEIC ) to sustain EIC crack growth rates above around 10 -11 m/s being consistently lower for 3 rd Generation alloys, and crack propagation rates for alloys in equivalent tempers under similar mechanical driving forces being consistently higher for 3 rd Generation alloys, Figure 11b, Table 9.…”

The Cracking of Al-Mg Alloys Welded by MIG and FSW under Slow Strain Rating

“…Slow strain rate testing (SSRT) with subsequent fracture analysis can also determine the nature of the cracking. Aboura, Y et al [ 36 ] and Wang, S. et al [ 37 ] studied the 7xxx alloys, while Li, M. [ 38 ] studied the 8xxx aluminum alloy while determining their corrosion resistance. Holroyd, N.J.H.…”