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

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 neigh- bouring 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.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.