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

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 A degrees C to 80 A degrees 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 similar to 150 nm at room temperature to similar to 400 nm at 313 K (40 A degrees 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 similar to 35 kJ/mol for temperatures above similar to 313 K (40 A degrees C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al2CuMg where E (a) is similar to 80 kJ/mol. For temperatures below similar to 313 K (40 A degrees C) E (a) is independent of copper content for stress intensity factors below similar to 14 MNm(-3/2), with a value similar to 80 kJ/mol but is sensitive to copper content for stress intensity factors above similar to 14 MNm(-3/2), with E (a) , ranging from similar to 35 kJ/mol for copper-free alloys to similar to 80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently similar to 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, AlH3, 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.