Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li
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THE surrounding environment, microstructure, stress intensity range (DK = Kmax - Kmin), and stress ratio (R = Kmin/Kmax) govern the fatigue crack propagation (FCP) rate (da/dN) and crack surface morphology for age-hardened Al alloys.[1–9] The present study focuses on legacy AA2024 and a modern Al-Cu-Li alloy denoted C47A. Previous experiments established strong increases in da/dN due to: (1) the evolution of precipitation-hardening microstructure (for AA2024),[6,7] and (2) water-vapor-saturated air and chloride-solution environments (each alloy)[6,7,10] relative to ultrahigh vacuum (UHV). Transgranular facet-like features were predominant for those AA2024 and C47A microstructures prone to slip localization, low DK loading, and both inert and moist environments, without indication of intergranular cracking.[6,7,10] Changes in the transgranular fatigue crack path and facet morphology correlated with differences in da/dN. Quantitative fracture surface analysis provides a means of understanding the governing fatigue damage mechanisms, particularly interactions of plasticity and environmental hydrogen.[4,8,9] YUNJO RO, Graduate Student, SEAN R. AGNEW, Associate Professor, and RICHARD P. GANGLOFF, Ferman W. Perry Professor and Chair, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 229044745. Contact e-mail: [email protected] Manuscript submitted October 13, 2006. Article published online November 2, 2007 3042—VOLUME 38A, DECEMBER 2007
A. Fatigue Fracture Surface Crystallography for Inert Environment The Al-Li based alloys exhibit tortuous faceted cracking in inert environments, indicative of a crystallographic mechanism, regardless of DK and temper.[2–4] Such tortuous faceted cracking in vacuum is attributed to intense slip localization along {111} planes, due to shearable d¢ (Al3Li) precipitates.[3,4,11–13] Verification of crystallographic cracking due to intense slip localization was based on several experimental approaches, including etch pit shape,[2,14–16] X-ray texture with singlesurface facet trace angle,[17] and electron backscatter diffraction (EBSD) with stereology.[4,5] The results from these techniques consistently indicate exact {111} or near-{111} crack facet planes, and thus slip plane or deformation band cracking is the likely fatigue damage mechanism.[1–5,11–16] This alloy-inert environment system is the reference for the present study and, moreover, a statistically valid characterization is required to determine whether the fatigue crack grows along {111} or near-{111} planes. Unlike the Al-Li class, the fatigue crack morphology for Al-Cu-Mg and Al-Zn-Cu-Mg alloys in inert environment depends on DK and temper.[18–22] Ductile striations or featureless ridges are commonly observed in the Paris regime for such alloys stressed in vacuum.[18–22] Although etch pit shape[21] and Laue X-ray back reflection techniques[23] indicated cracking parallel to {100} or {111}, these results are suspect, due to possible water vapor contamination. Crystallogra
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