TEM investigation of the microstructure in a single crystal nickelbase superalloy creep tested in [011] orientation

Abstract
Quantitative TEM investigations are performed on [011] oriented single crystals of Nickelbase superalloys creep tested at 1033 K under a load of 680 MPa. Multiple slip in the matrix occurs on four 1/2 (110){111} slip systems from beginning on. Octahedral cross slip is not activated. Cube slip does not occur, whereas during secondary creep screw interfacial dislocation cross slip in the cube γ/γ'-interface. In the matrix phase, deformation is concentrated in the "roof" channels. Calculations show that this is the result of the superposition of coherency and external stresses in small matrix channels. In the stage of secondary creep, γ' precipitates are sheared by Shockley superpartials. Common shearing of matrix and γ' particles by Shockley superpartials has been analysed. Continued common shear on adjacent {111} planes finally causes damage by mechanical twinning. The high creep rate in the [011] orientation is considered to be caused by the stress concentration in roof matrix channels and the few activated slip systems.