High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that favour the primary creep regime. The study has revealed the detailed structure of extended a/2 < 1 1 2 > dislocations as they shear the gamma' precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a < 1 1 2 > dislocations was also found to distort the gamma/gamma' interface, changing its appearance from linear to a saw tooth pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a/2 < 1 1 0 > dissociated dislocations were also observed in the gamma channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the gamma' and intrinsic stacking faults in the gamma, and to make estimates of their energies. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.