Acta Metall Sin  2019, Vol. 55 Issue (9): 1204-1210    DOI: 10.11900/0412.1961.2019.00094
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Influence of Small Misorientation from <111> on Creep Properties of a Ni-Based Single Crystal Superalloy
HU Bin,LI Shusuo,PEI Yanling,GONG Shengkai(),XU Huibin
School of Materials Science and Engineering, Beihang University, Beijing 100191, China
Abstract

Single crystal nickel-based superalloys have been widely used in high temperature structural materials applications including blade parts of aero-engines and gas turbines due to their excellent mechanical properties in service. Although commercial single crystal superalloy blades are in [001] orientation, misorientation deviations are inevitable in industrial productions and work blades frequently have to endure complex stress states caused by their complicated shapes and temperature gradients. Therefore, it is of great significance to study the creep behavior of single crystal superalloys with different orientations for the design of engine blades. The anisotropic creep properties of a nickel-based single crystal superalloy with different orientations near <111> were investigated under 760 ℃ and 650 MPa. It is found that specimens with the smallest deviation from <111> orientation exhibit best creep strength because of the relatively low Schmid factors of both {111}<110> and {111}<112> slip systems. With the increase of orientation deviate from [$1ˉ$11] to [011], creep properties decrease more significantly compared with the deviation from [$1ˉ$11] to [001]. All samples deviate from <111> within 20° exhibit poor strain hardening. While orientations toward [$1ˉ$11]-[001] boundary have a distinct incubation creep stage with relatively low initial creep rate. Further dislocations and lattice rotation analysis showed that the dominant slip systems are {111}<110> for specimens with minimum deviations. The stress is almost uniformly distributed in three γ matrix channels, which lead to a homogeneous deformation behavior. As the orientation deviation increases, {111}<112> slip systems begin to play a leading role during creep process. While the generation of <112> dislocations is closely related to the reaction and decomposition of <110> dislocations. Specimens on [$1ˉ$11]-[011] boundary have coplanar double slips for {111}<110> slip systems resulting in a high initial creep rate and poor strain harding. Meanwhile, Schmid factors of {111}<112> slip systems increase rapidly with the increase of orientation deviation from [$1ˉ$11] to [011], which lead to a significantly degradation on creep properties. While as for orientations along [$1ˉ$11]-[001] boundary, Schmid factors increase in a relatively gentle way with the number of dominant slip systems reduced from 6 to 2. Multiplication of dislocations and the formation of <112> dislocation ribbons are impeded, resulting in a comparatively long incubation creep stage.

 ZTFLH: TG132.3
Fund: Supported by National Natural Science Foundation of China(51771007、51671015);National Key Research and Development Program of China(2017YFA0700700)
Corresponding Authors:  Shengkai GONG     E-mail:  gongsk@buaa.edu.cn
 Fig.1  Orientations of specimens A~E, plotted within the standard triangle of the stereographic projection Table 1  Creep properties of Ni-based single crystal superalloy with different orientations after creep test at 760 ℃ and 650 MPa Fig.2  Creep curves of specimens A~E with different orientations after creep test at 760 ℃ and 650 MPa Fig.3  Schematic and SEM image of specimen A Fig.4  Lattice rotations of specimens B~E after creep rupture (a) and SEM images of specimen C after creep test at 760 ℃ and 650 MPa with distances from fracture surface 1 mm (b), 3 mm (c), 6 mm (d) (Square areas in Figs.4b and c show the twin structures and arrow in Fig.4c indicates the direction of applied stress) Fig.5  TEM images for different creep stages in specimens A, B and D after creep test at 760 ℃ and 650 MPa(a) specimen A after creeping 20 h (b) specimen B after creeping 10 h (c) specimen B after creeping 35 h (d) specimen D after creeping 5 h Table 2  Schmid factors of {111}<110> and {111}<112> slip systems for specimens B~E Fig.6  Maximum Schmid factors of {111}<110> and {111}<112> slip systems for different orientations deviate from <111>(a) {111}<110> deviate along [$1ˉ$11]-[001] and [$1ˉ$11]-[011] boundary(b) {111}<112> deviate along [$1ˉ$11]-[001] and [$1ˉ$11]-[011] boundary