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

Abstract

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.

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

“…Consistent observations of these crack arrest markings will be hampered by the subsequent reactivity of the crack-wake surfaces in the local environment within the crack, leading to local solution pH increases, Table IX. The activation energies, E a , for SCC growth in Al-ZnMg-Cu alloys fall into the same two categories, as previously reported for SCC in moist air and distilled water, [95] with E a s of 20 through 40 and 80 through 100 kJ/mol. A summary of the activation energy, E a , data for SCC under region 1 (K = 10 to 12 MNm À3/2 ) and region 2 (K > 14 MNm À3/2 ) loading conditions at temperatures greater than and less than 313 K (40°C) for low-and high-copper alloys immersed in a 0.6 M sodium chloride solution and in distilled water, [95] respectively, is shown in Table XI.…”

“…However, reported crack arrest mark spacing and crack growth rates for SCC of high and low-copper-containing Al-Zn-Mg-Cu alloy in saline environments increase by a factor of 3 to 5 compared to those in distilled water or moist air as shown in Table X. Clearly the crack arrest mark spacings during the initial stages of a DCB stress corrosion test in a saline environment, before the crack growth rate increase, should be similar to those in [77,95] 6.6 to 6.9 6.9 to 7.2 6.9 to 7.2 --6.1 9 10 À9 FCP 0.5 M NaCl (7475-T651) [64,65] 2.7 -4.5 to 5.0 0.1 2 to 3 1.2 9 10 À8 FCP 1 M NaCl (7075-T651) [6,77] 3.0 to 3. Fig.…”

“…[95] (b) Time-lapse video studies of crack initiation and growth from notched AA7150-T6 samples immersed in 0.6 M NaCl, [96] have shown that initial crack growth (K = 5 to 18 MNm À3/2 ) occurs as a series of discreet steps with crack growth rates greater than 10 À6 m/s; (c) Crack propagation for AA7075-T651 in 0.6 M NaCl generated a greater number of electrochemical potential fluctuations (electrochemical noise) with a significant increase in the number of events at low frequencies, [97] which are consistent with the anticipated 10 through 20 second intervals between crack growth increments, based on the spacing between crack arrest marking spacing (200 through 400 nm [93,95] ) and the region 2 SCC growth rate (2 9 10 À8 m/s, Figure 1); and (d) Output from mini-load cells placed in bolts used during DCB SCC tests on AA7075-T651 totally immersed in 0.6 M NaCl showed perturbations in crack length-time measurements. [66,98] The experimental fractrographic evidence for discontinuous SCC growth it is less extensive than that for cracking in moist air or distilled water, [95] where crack arrest markings are retained on fracture surfaces. However, reported crack arrest mark spacing and crack growth rates for SCC of high and low-copper-containing Al-Zn-Mg-Cu alloy in saline environments increase by a factor of 3 to 5 compared to those in distilled water or moist air as shown in Table X.…”

“…[39,82,95] Candidate processes with activation energies in the ranges of 20 0.6 M NaCl 20 -4 7075-T651 76 to 172 [10] 3 9 10…”

“…[95] *Note that the activation energy was reduced to 15 to 20 kJ/mol when crack propagation was stimulated by anodic polarization. [9,47] **de Jong [7] data suggests E a is 80 to 85 kJ/mol under region 1 loading.…”