Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li

A combined electron backscatter diffraction (EBSD)/stereology method successfully quantifies the orientation of fatigue crack surfaces for Al-Li-Cu and Al-Cu-Mg alloys stressed at low ΔK, in which deformation is localized in slip bands and cracking is highly faceted. The method orients features as small as ∼1 μm in complex microstructures. Vacuum fatigue facets align within 15 deg of up to four variants of {111} slip planes, governed by the distribution of crack tip resolved shear stress. The small fraction of precisely oriented {111} facets suggests that cracking involves complex intraband and multiple-band interface paths. Water vapor and NaCl solution affect a similar dramatic change in the crack path; near-{111} facets are never observed, at odds with mechanisms for H-enhanced slip localization and associated slip band cracking. Rather, two environmental crack facet morphologies, broad flat and repeating step, exhibit a wide range of orientations between {001} and {110}, as governed by crack tip resolved normal stresses. The repetitive stepped facets appear to contain areas parallel to {100}/{110} on the ∼1-μm scale, coupled with surface curvature consistent with a mechanism of discontinuous fatigue crack growth involving H-enhanced {100}/{110} cleavage and intermingled crack tip plasticity. Broad-flat faceted regions are parallel to a variety of planes, consistent with a mechanism combining high crack tip tensile stresses and H trapped at the dislocation structure from cyclic deformation, within 1 μm of the crack tip.