Formation of Lateral Sliver Defects in the Platform Region of Single-Crystal Superalloy Turbine Blades
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le-crystal (SX) nickel-base superalloys are used as blade materials in advanced aircraft and industrial gas turbine due to their excellent mechanical properties at elevated temperatures that approach melting.[1] Two different techniques are applied to fabricate these SX blades: the grain selection technique[2–4] and the seeding technique.[5] Under the extreme conditions, any crystal imperfection (misorientation defects) may provide sites for crack initiation and propagation, and thus limit the mechanical properties severely.[6,7] The common crystal imperfections include recrystallized grains,[8,9] stray grains,[10–13] freckles,[14,15] low-angle boundaries (LABs),[7,16–19] surface scale,[20,21] and sliver defects.[22–24] In the past, many investigations have been performed on
DEJIAN SUN, LIN LIU, TAIWEN HUANG, WENCHAO YANG, CHUANG HE, ZHUORAN LI, JUN ZHANG, and HENGZHI FU are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, China. Contact emails: [email protected]; [email protected] Manuscript submitted September 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the freckles, stray grains, and recrystallized grains. In contrast, sliver defects are less well understood. Sliver defects belong to the low- to medium-angle misorientation defects. These defects often occur at features with a reduction in cross-sectional area, where the dendrite deformation in the mushy zone is more remarkable due to the relatively stronger shrinkage driven flow and larger differential thermal contraction between the mold and the dendrites.[22,23] Furthermore, Huo et al. also identified the sliver defects in the extended cross-sectional platforms and suggested that these were due to the thermal contraction stress.[24] Once formed, the sliver defects would extend along the casting axial direction. After solution heat treatment (SHT), sliver defects’ higher degree of dendrite deformation could further lead to the formation of recrystallization, resulting in severe degradation of mechanical performance.[8,9,22] However, no related articles have reported the lateral sliver defects, which propagate along the casting lateral direction (perpendicular to the axial direction) as opposed to the casting axial direction. This work, for the first time, provides evidence of lateral sliver defects within the platform base of SX superalloy turbine blades. Moreover, the characteristics of misorientation profiles are also investigated and compared. Finally, the formation mechanism of lateral sliver defects is analyzed based on the predicted stress contour. Ni-based superalloy DD403 was used in the current experiments with the nominal composition (wt pct) of 9.5Cr, 5.0Co, 3.8Mo, 5.2W, 5.9Al, 2.1Ti, and balance Ni. The simplified turbine blade containing three platforms with various platform lengths, used for all the experiments, is presented in Figure 1(a). The detailed casting dimension could be obtained from a previous article.[25] The designed SX component was cast using a modified Bridgman direction
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