Dislocations in a Ni-based superalloy during low temperature creep

The nature and variety of the dislocations passing through the two-phase γ γ ′ microstructure of Ni-based superalloys is key to the properties of these materials. The chemistry, size and arrangement of the precipitates greatly affects the nature of these dislocations. We present High Angle Annular Dark Field (HAADF) TEM observations of the structure of dislocations entering, passing through the γ ′ precipitates in the single-crystal superalloy CMSX-4 R©. The creep deformation of the sample was interrupted after 8 hours at 750C and 750 MPa, a critical stage just as secondary creep was being established, and shows a range of defects in both not always those predicted by the Schmid factor for the deformation geometry. We show that dislocations in the γ γ ′ interfaces have a significant effect on the structure of the interface and that they combine to stacking faults which cut through the γ ′. The implications of these observations for secondary creep are discussed.