Kinetics of the topological inversion of the γ/γ′-microstructure during creep of a nickel-based superalloy Original Research Article

In undeformed superalloys, the disordered γ-solid solution of nickel is hardened by coherently embedded small cuboids of the ordered γ′-phase (Ni3Al). During high-temperature creep the γ′-phase coalesces, coarsens and finally surrounds the γ-phase, i.e., it becomes topologically the matrix. The kinetics of this so-called topological inversion during creep of the superalloy SRR99 at 980°C and 200 MPa has been investigated quantitatively by analysis of scanning electron microscope images. The topological state of the γ/γ′-microstructure was characterized by the parameter R: the ratio of area densities of transverse terminations of γ- and γ′-lamellae. The topological inversion is explained by the formation of junctions connecting neighbouring γ′-rafts and separating the γ-phase. One reason is the generation of dislocations in the γ-channels during primary creep. Another reason is the dissolution of γ′-edges in the γ-phase, which is more diffusionally penetrative. The released γ′-forming atoms move along the interface towards dislocation concentrations, resulting in the formation of junctions between the γ′-rafts.