The recent discovery of intermetallic L12 γ′ strengthened Co-base superalloys with superior high temperature strength provides a pathway for the development of new metallic structural materials for gas turbine engine applications.
In comparison to the well-known Ni-base superalloys with a negative lattice misfit between γ matrix and γ′ precipitates, Co-base superalloys possess a positive lattice misfit. To better understand the effects of positive misfit on directional coarsening, i.e. rafting, and creep properties, compressive creep tests were performed on a Ta-containing model Co-base single crystal superalloy. Different from previously reported creep tests on Co-base single crystal superalloys, two distinct creep rate minima are identified in the current experimental single crystal superalloy. The initial cuboidal γ′ morphology changes and successive rafting occurs at different creep stages. The microstructure and its change during creep regarding γ′ size and γ′ shape are explained with an emphasis on the elemental partitioning behavior, lattice misfit and internal stresses between γ and γ′ phases measured by atom probe tomography and diffraction experiments, respectively. The reason for the two creep minima will be presented with respect to the rafting behavior and possible elemental segregation to planar defects. The different microstructural change between Co-base and Ni-base single crystal superalloys during creep will be compared and the influence of the microstructure on the mechanical properties of Co-base superalloys will be discussed.