Characterizing the Local Primary Dendrite Arm Spacing in Directionally Solidified Dendritic Microstructures
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DEVELOPING an enhanced understanding of mechanical behavior in materials relies upon sufficiently characterizing microstructure details at the relevant length scales that contribute to this behavior. Moreover, to truly enhance the predictive capability of processing– structure–property models that aim to improve material performance requires a quantitative stereological description of the relevant microstructure features and, thereby, the material itself. Predictive models that effectively capture the linkage between processing and properties (through microstructure) can be utilized within an integrated computational materials engineering (ICME) approach to design materials and accelerate their insertion into application. The focus of the present work is on single-crystal nickel-based superalloys, which are used in turbine blades within the high temperature section of the modern turbine engine.[1,2] In single-crystal nickel-based superalloys, there are a number of length scales of MARK A. TSCHOPP, Senior Research Scientist, is with the Lightweight and Specialty Metals Branch, Materials and Manufacturing Science Division, Army Research Laboratory, Aberdeen Proving Ground, MD 21005, and also with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759. Contact e-mail: [email protected] JON D. MILLER, Senior Materials Research Engineer, is with the Air Force Research Laboratory, Wright-Patterson AFB, Dayton, OH 45433. ANDREW L. OPPEDAL, Postdoctoral Associate, is with the Center for Advanced Vehicular Systems, Mississippi State University. KIRAN N. SOLANKI, Assistant Professor, is with the School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287. Manuscript submitted June 14, 2013. Article published online September 17, 2013 426—VOLUME 45A, JANUARY 2014
microstructure that contribute to mechanical behavior, ranging from the c0 precipitates to pores and eutectic particles to the dendrites themselves. At the largest microstructure length scale in directionally solidified single-crystal microstructures, the features of interest are the dendrites; many features at lower length scales (e.g., eutectic particles, precipitates, etc.) or at similar scales (e.g., porosity, freckle defects, etc.) are strongly associated with the dendrite arm spacing and morphology.[3–7] The solidification morphology associated with dendrite arm spacing has been described in the literature.[8,9] Historically, the primary dendrite arm spacing (PDAS) has been found to correlate with processing (e.g., solidification rate)[7,10–14] as well as with properties (e.g., creep strength, fatigue properties).[15,6] For instance, Lamm and Singer[6] produced single-crystal nickel-based microstructures (PWA 1483) with a varied range of different dendrite arm spacings (250 to 600 lm) and found that decreasing the mean dendrite arm spacing was associated with an increased high-cycle fatigue life. The fatigue cracks were found to originate at shrinkage porosity and the largest pores
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