Effect of Primary Dendrite Orientation on Stray Grain Formation in Cross-Section Change Region During the Directional So
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NTRODUCTION
IN recent years, as a result of the excellent high-temperature strength and creep strength of single-crystal superalloy, the fabrication of turbine blades from single-crystal Ni-based superalloy is a key technology used in advanced high-performance aero-engines and industrial gas turbines. The excellent properties of single-crystal superalloy can be attributed to the elimination of grain boundaries to limit creep ductility.[1] Nevertheless, in advanced aero-engines and industrial gas turbines, the blade design is increasingly complex and the blade dimensions are increased, making it more challenging to prevent the formation of grain defects. In the industrial production of Ni-based superalloys, the single-crystal casting is usually carried out using grain selectors as a result of which an off-axially orientated dendrite may be formed from the random nature of the grain selection during directional solidification.[2–5] Therefore, many types of grain defects, such as freckles,[6,7] low-angle boundaries (LABs),[8] and stray
WEIDONG XUAN, Materials Research Engineer, CHUANTAO LI and DENGKE ZHAO, Masters, BAOJUN WANG, Assistant Engineer, CHUANJUN LI, Associate Professor, and ZHONGMING REN, YUNBO ZHONG, XI LI, and GUANGHUI CAO, Professors, are with the State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P.R. China. Contact e-mails: [email protected], [email protected] Manuscript submitted April 11, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
grains,[9,10] may be present in the single-crystal casting, among which the formation of stray grains is a major defect during the preparation of single-crystal turbine blades. As a result of the random orientation of stray grains, high-angle boundaries (HABs) are easily formed, which are highly deleterious to the mechanical properties of single-crystal turbine blades. Stray grains usually originate from an undesirable nucleation at certain specified locations in the turbine blade, such as at platform ends or in shroud regions.[10–19] The formation of stray grains at platform ends or in the shroud regions of turbine blades has already been detected by several researchers.[8,10–19] Paul et al.[10] found that stray grains were often formed in the inner and outer shroud as a result of a curvature in the liquidus isotherm that led to a thermal undercooling during the solidification of single-crystal turbine blades. Gandin et al.[11,12] proposed a model to predict stray grain formation at platform ends during the preparation of single-crystal turbine blades. Napolitano and Schaefer[8] found that a low-angle boundary was easily formed in the platform region and proposed a tip-growth lattice model for predicting this phenomenon. Yang et al.[13] simulated the formation of stray grains in the platform region using a cellular automaton finite difference (CA-FD) model and found that the undercooling within this region increased with the increase of wit
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