Observations on the faceted initiation site in the dwell-fatigue tested ti-6242 alloy: Crystallographic orientation and
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I. INTRODUCTION
AUTOMATED electron backscattered diffraction (EBSD) techniques have been used in prior studies to obtain orientation images for a wide variety of materials.[1–5] In this study, the unique potential of automated EBSD techniques, when coupled with serial sectioning experiments, in characterizing the crystallographic orientation of the local regions near the fracture surface has been demonstrated. It is also shown that one set of crystallographic orientation data can be presented as various orientation maps to understand the crystallography as well as the slip activity associated with the crack-initiation site and its surrounding regions. As an example of this novel approach, the fracture in a near-& titanium alloy has been investigated with a special emphasis on understanding the micromechanisms of crack-initiation under dwell-fatigue loading conditions. The near-& titanium alloys, such as Ti-6242, are known to exhibit a significant reduction in fatigue life under dwellfatigue conditions when compared to normal-fatigue (continuous cycling) test conditions.[3,5–8] Furthermore, the orientation of the facets comprising the dwell-fatigue crack initiation site has been observed to be near basal.[4,9–11] A large faceted dwell-fatigue crack initiation site has been V. SINHA, Research Associate 1-Engineer, M.J. MILLS, McDougal Professor of Engineering, and J.C. WILLIAMS, Honda Chair and Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] J.E. SPOWART, Materials Engineer, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted June 6, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
observed for Ti-6242 alloy with a high level of microtexture.[3,5] In the current study, a representative dwell-fatigue tested specimen of an &/' forged Ti-6242 alloy has been characterized. To determine the crystallographic orientation of the grains at and around the dwell-fatigue crack initiation site, the polishing was done directly onto the fracture surface with the plane of polish being perpendicular to the loading axis. Serial sectioning and EBSD techniques were used to obtain the orientation images at different depths below the fracture surface. These detailed characterizations and analyses were carried out to describe the crystallographic orientation of the dominant (i.e., the one that leads to eventual specimen failure) faceted initiation site, and associated features such as Schmid factor for prism and basal slip. These experiments also permitted an evaluation of the microtextured region size effects associated with the dominant crack initiation site. In prior studies, other researchers have reported that small crack growth is strongly dependent on the local microstructural conditions at the crack tip.[12,13] Our results suggest that the growth of small cracks is arrested when the crack tip approaches a region where the basal plan
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