Effect of Environment on Fatigue Crack Wake Dislocation Structure in Al-Cu-Mg

Fatigue-induced dislocation structure was imaged at the crack surface using transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of naturally aged Al-4Cu-1.4Mg stressed at a constant stress intensity range (7 MPa√m) concurrent with either ultralow (~10⁻⁸ Pa s) or high-purity (50 Pa s) water vapor exposure at 296 K (23 °C). A 200-to-600-nm-thick recovered-dislocation cell structure formed adjacent to the crack surface from planar slip bands in the plastic zone with the thickness of the cell structure and slip bands decreasing with increasing water vapor exposure. This result suggested lowered plastic strain accumulation in the moist environment relative to the vacuum. The previously reported fatigue crack surface crystallography is explained by the underlying dislocation substructure. For a vacuum, \( \left\{ { 1 1 1} \right\} \) facets dominate the crack path from localized slip band cracking without resolvable dislocation cells, but cell formation causes some off-\( \left\{ { 1 1 1} \right\} \) features. With water vapor present, the high level of hydrogen trapped within the developed dislocation structure could promote decohesion manifest as either low-index \( \left\{ { 100} \right\} \) or \( \left\{ { 1 10} \right\} \) facets, as well as high-index cracking through the fatigue-formed subgrain structure. These features and damage scenario provide a physical basis for modeling discontinuous environmental fatigue crack growth governed by both cyclic strain range and maximum tensile stress.