Interfacial Dislocation Networks and Creep in Directional Coarsened Ru-Containing Nickel-Base Single-Crystal Superalloys

Abstract

Mechanisms of creep deformation in nickel-base superalloy single crystals in the directional coarsening regime have been studied in alloys with large variations in γ-γ′ 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 γ-γ′ 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 γ′ 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.