Deformation behaviour and development of microstructure during creep of a nickel-base superalloy

The creep behaviour and deformation microstructures of nickel-base superalloy C263 at 800degreesC have been investigated. The steady-state creep rate of the material follows the power law relation at stresses ranging from 120 to 250 MPa, with a power law exponent value of 5.5. Transmission electron microscope observation indicates that dislocation structures were not affected by the applied stresses used. Dislocation move mainly on parallel slip bands in a dissociated manner during steady state creep. When a unit matrix dislocation approaches to gamma/gamma' interfaces, it dissociated into a lending partial 1/3<112> and a trailing partial 1/6<112>. The leading partial dislocation shears both gamma matrix and gamma' precipitates, creating extended stacking faults while the trailing partial was pinned at the interfaces by high energy anti-phase boundary (APB). As the gamma' particles coarsened during prolonged high temperature creep, dislocation bowing, looping around gamma' particles and climbing became predominant mechanism. Furthermore, prolonged high temperature exposure resulted in microstructure instability at grain boundaries. eta phase began to precipitate from carbides at grain boundaries and grew in a lamella form with gamma phase. There was a gamma' phase depletion zone surrounding the eta phase. This has promoted grain boundary migration towards eta phase and consequently the frequent presence of eta phase along grain boundaries. Grain boundary migration away from eta phase has also been observed due to large plastic deformation near and the reduced carbide content locally at grain boundaries, Fracture surface observation revealed a predominantly intergranular fracture surface with a considerable amount of creep cavities.