  Co-base superalloys are considered as promising high temperature materials
besides the well-established Ni-base superalloys. However, Ni appears to be an
indispensable alloying element also in Co-base superalloys. To address the
influence of the base elements on the deformation behavior, high-temperature
compressive creep experiments were performed on a single crystal alloy series
that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr
content. Creep tests were performed at 900 {\deg}C and 250 MPa and the
resulting microstructures and defect configurations were characterized via
electron microscopy. The minimum creep rates differ by more than one order of
magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits
the overall highest creep strength. Several strengthening contributions like
solid solution strengthening of the $\gamma$ phase, effective diffusion
coefficients or stacking fault energies were quantified. Precipitate shearing
mechanisms differ significantly when the base element content is varied. While
the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys
develop extended APBs when the $\gamma^\prime$ phase is cut. This is mainly
attributed to a difference in planar fault energies, caused by a changing
segregation behavior. As result, it is assumed that the shearing resistivity
and the occurring deformation mechanisms in the $\gamma^\prime$ phase are
crucial for the creep properties of the investigated alloy series.
