  The microstructural and compositional evolution of intergranular carbides and
borides prior to and after creep deformation at 850 $^\circ$C in a
polycrystalline nickel-based superalloy was studied. Primary MC carbides,
enveloped within intergranular $\rm \gamma'$ layers, decomposed resulting in
the formation of layers of the undesirable $\rm \eta$ phase. These layers have
a composition corresponding to Ni$_{3}$Ta as measured by atom probe tomography
and their structure is consistent with the D0$_{24}$ hexagonal structure as
revealed by transmission electron microscopy. Electron backscattered
diffraction reveals that they assume various misorientations with regard to the
adjacent grains. As a consequence, these layers act as brittle recrystallized
zones and crack initiation sites. The composition of the MC carbides after
creep was altered substantially, with the Ta content decreasing and the Hf and
Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions
on the stability of MC carbides. By contrast, $\rm M_{5}B_3$ borides were found
to be microstructurally stable after creep and without substantial
compositional changes. Borides at 850 $^\circ$C were found to coarsen,
resulting in some cases into $\rm \gamma'$-depleted zones, where, however, no
cracks were observed. The major consequences of secondary phases on the
microstructural stability of superalloys during the design of new
polycrystalline superalloys are discussed.
