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In this paper, the changes in microstructure during high temperature exposure with and without applied stress have been studied. In the absence of applied stress, the factors associated with microstructural instability of the alloy include the coarsening of gamma', the formation of sigma and mu phase, the precipitation of P phase from sigma phase, the formation of M(23)C(6) from decomposed MC, and the precipitation of M(23)C(6) in gamma matrix. The sigma phase nucleates in the gamma matrix or on M(23)C(6) carbides that form during aging, with the last stage of growth involving interactions with gamma'. The nucleation and growth of sigma phase in the absence of applied stress are discussed in terms of structural analysis, activation energy, and elemental distribution. In the condition of under applied stress, when the superalloy exhibits a rafted gamma' morphology after long-term exposure, the microstructure is free of sigma phase until the samples are plastically strained to fracture; however, a larger number of small new nucleated gamma' precipitates are observed in gamma channels when gamma' particles remain cubic shape with sharp corners under the condition of high applied stress. In addition, the alloy shows formation of sigma phase in this situation. The formation of sigma phase as well as the formation of small spherical gamma' is discussed in terms of energy dynamics. (C) 2008 Elsevier B.V. All rights reserved.