Crystallography of Fatigue Crack Initiation and Growth in Fully Lamellar Ti-6Al-4V
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THE union of electron backscatter diffraction (EBSD) and quantitative tilt fractography[1–3] has provided an efficient method for relating crystallographic information obtained directly from a fracture surface (or a polished face that intersects it) to the spatial orientation of features on it. The direct method, in which EBSD patterns are collected directly from the as-fractured surface without additional preparation, has been reported to have an accuracy between 1[3] and 3 deg[4] when the spatial information and crystallographic information are obtained in the same microscope session without rotating the stage. This method is limited, however, by the necessity for a relatively flat fractured surface that was not accompanied by substantial plasticity during fracture. Thus, this method is often used to study cleavage facets[5] in steels or fatigue facets[2,3,6–8] in titanium alloys. The formation of facets in titanium alloys is of particular interest, because the crack initiation stage has a dominant contribution to total A.L. PILCHAK, formerly Graduate Research Associate, Department of Materials Science and Engineering, The Ohio State University, is currently Visiting Scientist, Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, OH 45433 and Research Scientist, Universal Technology Corporation, Dayton, OH 45432. Contact e-mail: [email protected] R.E.A. WILLIAMS, Graduate Research Associate, and J.C. WILLIAMS, Professor and Honda Chair, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted June 5, 2009. Article published online November 5, 2009 106—VOLUME 41A, JANUARY 2010
life during high-cycle fatigue loading. In addition, crack initiation during tests that include a dwell period is paramount to explaining the substantial reduction in life that accompanies this type of fatigue failure.[9] Previous studies have identified these facets as being parallel or nearly parallel to the basal plane.[2,3,6–8,10,11] The ‘‘nearbasal’’ orientations can be accounted for by the presence of steps, which are similar to river markings, on the facet surface.[12] A complete description of facets, however, also requires reporting the spatial orientation of the facet in addition to the crystallographic plane of fracture.[2,3] For example, dwell facets are most frequently oriented nearly perpendicular to the applied loading direction in an orientation that has low resolved shear stress on the basal plane, but high resolved normal stress. In contrast, facets formed during continuous cycling often form in grains the basal planes of which are more highly inclined to the loading direction.[12–14] While these experimental works have identified the role of the a-phase crystallographic orientation in crack initiation in titanium alloys, the b-phase orientation has not been given any attention despite its significant effect on the mechanical behavior of materials containing lamellar constituents.[15–19
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