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Mechanisms of creep deformation in nickel-base superalloy single crystals in the directional coarsening regime have been studied in alloys with large variations in c-c¢ lattice misfit and phase composition, achieved by Ru additions and variable levels of Cr and Co. Interfacial dislocation spacings established by long-term annealing experiments under no externally applied stress indicate that the experimental alloys have high-temperature lattice misfits ranging from near-zero to as large as -0.65 pct. Variation in misfit influences the stress-induced directional coarsening (rafting) behavior during creep deformation at 950°C and 290 MPa. In postcreep deformed material, the density of excess dislocations (defined as the dislocations beyond those necessary to relieve the lattice misfit) at the c-c¢ interfaces varied with alloy composition, with the most creep-resistant alloy containing the highest excess interfacial dislocation density. In the directional coarsening creep regime, continued deformation requires shearing of the c¢ rafts and is strongly influenced by the resistance of the precipitates to shearing as well as the interfacial dislocation structure. A preliminary model for creep in the rafting regime is developed.

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