The microstructural instability of a hot corrosion resistant superalloy during long-term exposure

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 γ′, the formation of σ and μ phase, the precipitation of P phase from σ phase, the formation of M23C6 from decomposed MC, and the precipitation of M23C6 in γ matrix. The σ phase nucleates in the γ matrix or on M23C6 carbides that form during aging, with the last stage of growth involving interactions with γ′. The nucleation and growth of σ 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 γ′ morphology after long-term exposure, the microstructure is free of σ phase until the samples are plastically strained to fracture; however, a larger number of small new nucleated γ′ precipitates are observed in γ channels when γ′ particles remain cubic shape with sharp corners under the condition of high applied stress. In addition, the alloy shows formation of σ phase in this situation. The formation of σ phase as well as the formation of small spherical γ′ is discussed in terms of energy dynamics.