Effect of Platform Dimension on the Dendrite Growth and Stray Grain Formation in a Ni-Base Single-Crystal Superalloy
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IN the past decades, Ni-base superalloy single-crystal turbine blades have been extensively used in the modern gas turbine for aircraft propulsion and power generation. Compared with conventionally casting blades, they can be subjected to higher service temperature and possess superior mechanical properties due to the elimination of grain boundary (GB). However, owing to the complex geometry shape of turbine blades and the increased addition of refractory alloy content, the prevention of defect formation in modern turbine blades during the directional solidification (DS) process becomes increasingly challenging. One of the major defects encountered during the DS process is stray grain,[1–4] which is randomly oriented and easily forms high-angle boundaries (HABs) with primary grain. Since GB strengthening elements are absent in single crystal, HABs are very harmful for the mechanical properties and usually perform as the source for crack initiation. Therefore, many investigations[1–24] have X.B. MENG, Assistant Professor, J.G. LI, T. JIN, and X.F. SUN, Professors, and Z.Q. HU, Academician, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Z.Q. CHEN, Y.H. WANG, S.Z. ZHU, and X.F. BAI, Engineers, are with the Aviation Industry Corporation of China (AVIC) Shenyang Liming Aero-engine (Group) Corporation LTD., Shenyang 110043, P.R. China. F. WANG, Engineer, is with the Aviation Industry Corporation of China (AVIC) Guiyang Liyang Aero-engine (Group) Corporation LTD., Guiyang 550002, P.R. China. J. ZHANG, Professor, is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, P.R. China. Manuscript submitted June 11, 2011. Article published online December 1, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
been carried out to study the behavior of dendrite growth and the formation of stray grains. The platform of turbine blade, which is considered as the most sensitive site where stray grains tend to nucleate, has already been paid fully attention to. Napolitano and Schaefer[5] investigated the growth of dendrites and the formation of low-angle GBs in the platform. Newell and Souza et al.[9,10] thought the cumulative misorientation of low-angle GB in the platform results from plastic deformation of dendrite stems in response to contraction stresses during the DS process. Yang et al.[14] simulated the formation of stray grains in the platform by cellular automaton finite-difference model and found that increasing either the withdrawal rate or the inclination angle of isotherm is in favor of the nucleation and growth of stray grains. Ma et al.[24] simulated the thermal condition in the platform of turbine blades and proposed that the heat conductor technology could reduce stray grain formation in the platform. However, few studies are aimed at the effect of platform dimension on the formation of stray grains. Therefore, some studies should be performed in this field. In this article, it
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