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

In undeformed superalloys, the disordered gamma -solid solution of nickel is hardened by coherently embedded small cuboids of the ordered gamma'-phase (Ni3Al). During high-temperature creep the gamma'-phase coalesces, coarsens and finally surrounds the gamma -phase, i.e., it becomes topologically the matrix. The kinetics of this so-called topological inversion during creep of the superalloy SRR99 at 980 degreesC and 200 MPa has been investigated quantitatively by analysis of scanning electron microscope images. The topological state of the gamma/gamma'-microstructure was characterized by the parameter R: the ratio of area densities of transverse terminations of gamma- and gamma'-lamellae. The topological inversion is explained by the formation of junctions connecting neighbouring gamma'-rafts and separating the gamma -phase. One reason is the generation of dislocations in the gamma -channels during primary creep. Another reason is the dissolution of gamma'-edges in the gamma -phase, which is more diffusionally penetrative. The released gamma'-forming atoms move along the interface towards dislocation concentrations, resulting in the formation of junctions between the gamma'-rafts. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.