Kinetics of the topological inversion of the gamma/gamma '-microstructure during creep of a nickel-based superalloy

In undeformed superalloys, the disordered gamma -solid solution of nickel i s hardened by coherently embedded small cuboids of the ordered gamma'-phase (Ni3Al). During high-temperature creep the gamma'-phase coalesces, coarsen s and finally surrounds the gamma -phase, i.e., it becomes topologically th e 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 topo logical state of the gamma/gamma'-microstructure was characterized by the p arameter R: the ratio of area densities of transverse terminations of gamma - and gamma'-lamellae. The topological inversion is explained by the format ion of junctions connecting neighbouring gamma'-rafts and separating the ga mma -phase. One reason is the generation of dislocations in the gamma -chan nels during primary creep. Another reason is the dissolution of gamma'-edge s in the gamma -phase, which is more diffusionally penetrative. The release d gamma'-forming atoms move along the interface towards dislocation concent rations, resulting in the formation of junctions between the gamma'-rafts. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.