  Predicting the mechanical failure of parts in service requires understanding
their deformation behavior, and associated dynamic microstructural evolution up
to the near-atomic scale. Solutes are known to interact with defects generated
by plastic deformation, thereby affecting their displacement throughout the
microstructure and hence the material mechanical response to solicitation. This
effect is studied here in a polycrystalline Ni-based superalloy with two
different Nb contents that lead to a significant change in their creep
lifetime. Creep testing at 750C and 600 MPa shows that the high-Nb alloy
performs better in terms of creep strain rate. Considering the similar initial
microstructures, the difference in mechanical behavior is attributed to a phase
transformation that occurs along planar faults, controlled by the different
types of stacking faults and alloy composition. Electron channeling contrast
imaging reveals the presence of stacking faults in both alloys. Microtwinning
is observed only in the low-Nb alloy, rationalizing in part the higher creep
strain rate. In the high-Nb alloy, atom probe tomography evidences two
different types of stacking faults based on their partitioning behavior.
Superlattice intrinsic stacking faults (SISF) were found enriched in Nb, Co, Cr
and Mo while only Nb and Co was segregated at superlattice extrinsic stacking
faults. Based on their composition, a local phase transformation occurring
along the faults is suggested, resulting in slower creep strain rate in the
high-Nb alloy. In comparison, mainly SISF enriched in Co, Cr, Nb and Mo were
found in the low-Nb alloy. Following the results presented here, and those
available in the literature, an atomic-scale driven alloy design approach that
controls and promotes local phase transformation along planar faults at 750C is
proposed, aiming to design superalloys with enhanced creep resistance.
