Effects of Grain Boundary Morphology and Dislocation Substructure on the Creep Behavior of Udimet 71

Abstract

An extensive investigation of the effects of grain boundary morphology (GBM) and dislocation substructure (DS) on the creep behavior of a Udimet 710 alloy was carried out. The interactions between: (1) dislocations and transgranular particles, and (2) dislocations and grain boundary carbides were also investigated. A significant understanding of the strengthening mechanisms and the mechanisms of fracture in this alloy was achieved. Two types of specimens, one with straight grain boundaries (SGB) and the other with zigzag grain boundaries (ZGB), were obtained by specially designed thermomechanical processes. Specimens of each of these types were creep tested at 1123K under an initial stress of 35kg/mm2. Creep in a grain was primarily caused by dislocation climbing over y’ particles. The migration of grain boundaries was controlled by their gliding for the specimens with straight grain boundaries. However, for specimens with zigzag grain boundaries, creep in a grain was caused by both dislocations climbing over and dislocations cutting through y’ particles. The migration of grain boundaries was controlled by both gliding of grain boundaries and the “straightening” motion of grain boundaries in zigzag steps. Zigzag grain boundaries increased the creep rate in a grain and decreased the gliding rate of grain boundaries. This effectively retarded the coalescence and growth of cracks and greatly enhanced the fracture resistance in the tertiary stage of creep observed at constant load. The creep life and plasticity to failure were increased in specimens with zigzag boundary morphology.