Influence of Small Misorientation from <111> on Creep Properties of a Ni-Based Single Crystal Superalloy

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 degrees C 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 [(1) over bar 11] to [011], creep properties decrease more significantly compared with the deviation from [(1) over bar 11] to [001]. All samples deviate from < 111 > within 20 degrees exhibit poor strain hardening. While orientations toward [(1) over bar 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. 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 [(1) over bar 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 [(1) over bar 11] to [011], which lead to a significantly degradation on creep properties. While as for orientations along [(1) over bar 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.