  In this study, the effect of rhenium (Re) addition on microstructural
evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented.
Addition of Re significantly influences the {\gamma}' precipitate morphology,
the {\gamma}/{\gamma}' lattice misfit and the {\gamma}/{\gamma}'
microstructural stability during long term aging. An addition of 2 at.% Re to a
Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900{\deg}C for 50 hours,
reduces the {\gamma}/{\gamma}' lattice misfit by {\approx} 40% (from +0.32% to
+0.19%, measured at room temperature) and hence alters the {\gamma}' morphology
from cuboidal to round-cornered cuboidal precipitates. The composition profiles
across the {\gamma}/{\gamma}' interface by atom probe tomography (APT) reveals
Re partitions to the {\gamma} phase (K_Re=0.34) and also results in the
partitioning reversal of Mo to the {\gamma} phase (K_Mo=0.90) from the
{\gamma}' precipitate. An inhomogeneous distribution of Gibbsian interfacial
excess for the solute Re ({\Gamma}_Re, ranging from 0.8 to 9.6 atom.nm-2) has
been observed at the {\gamma}/{\gamma}' interface. A coarsening study at
900{\deg}C (up to 1000 hours) suggests that the coarsening of {\gamma}'
precipitates occurs solely by an evaporation--condensation (EC) mechanism. This
is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in
some of the Ni-Al based and high mass density Co-Al-W based superalloys, where
{\gamma}' precipitates coarsen by coagulation/coalescence mechanism with
extensive alignment of {\gamma}' along <100> directions as a sign of
microstructural instability. The {\gamma}' coarsening rate exponent (K_r) and
{\gamma}/{\gamma}' interfacial energy are estimated to be 1.41 x 10-27 m3/s and
8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys.
