Evolution of Sliver Defect in Ni-Based Single Crystal Superalloy
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NI-BASED single crystal (SX) superalloy is widely applied in the hot section of advanced aero-engine and gas turbines due to the excellent mechanical performance at elevated temperature.[1,2] With the increasing content of the refractory elements in SX alloy and the complexity in geometry of the blade, the occurrence of casting defects in SX blade, such as stray grains,[3,4] freckle,[5,6] low angle boundary[7,8] and sliver References 9 through 11 becomes rather frequent during directional solidification (DS). However, compared with other defects, sliver defect is less well understood in the past. Sliver is observed as strip-like contrast in SX and belongs to the low to medium angle misorientation defects.[10] It has been reported that tensile properties and creep rupture lives decreased with the increase of misorientation angle of a low angle grain boundary in SX superalloys.[12,13] One would also expect that sliver defect reduces the properties as it has similar structure to that of the low angle grain boundary, although no information has been reported so far in the open YAQI HUANG is with the Superalloys Division, Institute of Metal Research, ChineseAcademy of Sciences, Shenyang 110016, P.R. China and also with the School of Materials Science andEngineering, University of Science and Technology of China, Hefei 230026, P.R. 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 January 19, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
literature. Therefore, it is important to understand the initiation and evolution of this defect in SX castings. It is generally accepted that the initiation of sliver is related to the dendrite deformation in mushy zone References 9 through 11. Researcher studied the sliver generated from the narrow channel on the top of the seed in SX casting and found that high stresses in the constricted channel induced the sliver defects. The dendrites were loaded with both bending moments and torques owing to different thermal contraction between mold and metal.[10] Furthermore, the formation of lateral sliver defects on the platform was also examined by Sun et al.[9] It is concluded that the defects generated due to the high contraction stresses around the connections of platform and the SX body. Recently, detailed X-ray computed tomography (XCT) and electron backscattered diffraction (EBSD) characterization revealed that the origin of sliver was attributed to the localized deformation of 1 to 2 dendrites at the diverging boundary near the mold wall and thermal contraction force played an important role in sliver formation.[11] Unfortunately, the evolution of sliver defect after its formation is still not well understood except that sliver can extend along the casting axis (parallel to DS direction) after formation.[10,11] The change of orientation and competitive growth between sliver and matrix dendrites have
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