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

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 38; no. 12; pp. 3042 - 3062
• Main Authors: RO, Yunjo, AGNEW, Sean R, GANGLOFF, Richard P
• Format: Journal Article
• Language: English
• Published: New York, NY Springer 01.12.2007; Springer Nature B.V
• Subjects: Aluminum alloys; Applied sciences; Crystallography; Exact sciences and technology; Fatigue; Grain boundaries; Grain size; Materials science; Mechanical properties; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microstructure; Precipitation; Shear stress; Transmission electron microscopy; Precipitation; Aluminium base alloys; Magnesium alloy; Mechanical properties; Fatigue; Microstructure; Precipitation hardening; Fatigue crack; Crack propagation
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-007-9344-x

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. [PUBLICATION ABSTRACT]