Formation of Sliver Defect in Ni-Based Single Crystal Superalloy
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Nickel-based single crystal (SX) superalloys have been widely used in turbine blades for their exceptional hightemperature properties.[1,2] However, casting defects, such as stray grains,[3] freckle,[4] low angle boundary,[5] and sliver[6,7] are potential threats to the performance of the SX superalloys. At present, very limited information on the formation mechanism of sliver has been reported comparing to that of the other defects. Sliver is observed as strip-like contrast in SX. It may extend along the directional solidification (DS) direction after formation.[6–8] Previous works confirmed that the origin of sliver is primarily associated with the deformation of dendrites in the mushy zone.[6,7] There have been numerous indirect observations of dendrite
YAQI HUANG is with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China and also with the School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China. JIAN SHEN, DONG WANG, GUANG XIE, YUZHANG LU, LANGHONG LOU, and JIAN ZHANG are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences. Contact e-mail: [email protected] Manuscript submitted July 22, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation in various materials.[9] The deformation has been linked to gravity,[10] elastic deformation,[11] c¢ precipitation,[12] and solute field ahead of the solidliquid interface.[13] During directional solidification, the dendrite deformation is possibly caused by: (I) Interdendritic fluid flow (convection) induced by alloying or processing conditions during solidification. The strong segregation in directional solidification of SX superalloy often results in the vertical fluid convection at a certain stage of solidification.[14] Both remelting at the neck of dendrite arms during coarsening, solute enhancement, or recalescence[15–17] and mechanical interactions[18,19] have been considered to cause the deformation or fragmentation of dendrites; (II) Thermal contraction during solidification. Aveson et al.[7] proposed that the sliver arose from the high stresses in the constricted channel due to different thermal contraction between mold and metal. Similar misorientation accumulated during DS has also been reported recently in a third generation SX superalloy.[20] Sun et al.[6] believed that the lateral sliver defects on the platform generated due to the high contraction stresses around the connections of platform and the SX body. To date, the direct observation of dendrite deformation in the mushy zone is still lacking, and therefore, the exact reason leading to the deformation and the formation mechanism of sliver are still unclear. In this study, SX castings were produced, with a detailed X-ray computed tomography (XCT) and electron backscattered diffraction (EBSD) characterization. The formation mechanism has been explored based on the experimental evidence of dendrite deformation in the mushy zone. The nominal composition of the superall
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