5.2.3. Collapse and Topological Inversion

It is known that rafting usually occurs under mechanical loading at high temperature. Continuous creep deformation can cause the change of initial γ' precipitates of cubic shape to stable raft structures. However, further creep deformation may distort the stable raft structure and lead to topological inversion of γ/γ' phases, i.e., the γ' strengthening phase gradually surrounds the γ phase [94]. Incorporating damage parameters [8,19,80] and the change of misfit strain [84,95] in the phase-field models allows for the determination of the major factors contributing to the collapse and topological inversion of the γ/γ' microstructures.

The damage parameters can represent the change of the elastic constants of the γ' phase and/or the increase of the plastic shear rate during creep deformation. Some slip systems can cut into the γ' phase and distort stable raft structures [8,80] when plastic strain accumulated near the γ/γ' interface reaches a critical value. The results from the phase-field models with the variation of misfit strain reveal the decrease of misfit strain during the deformation, which makes the γ/γ' interface become semi-incoherent or incoherent. This trend destroys the stabilization mechanism in the γ channels and reduces the disjoining potential between adjacent γ' precipitates [95]. This is an essential step to achieve topological inversion, as observed experimentally.

Currently, the phase-field models with damage parameters are mainly under the framework of elasto-plastic deformation. It is a challenge to fully describe the three creep stages for the tensile creep at elevated temperature in addition to the analysis of the collapse and topological inversion of the γ/γ' microstructures and the microstructure evolution. The incorporation of the change of misfit strain in the phase-field models also enables the observation of the topological inversion of the γ/γ' microstructures, and the simulation results are in good agreement with those observed in long-term aging experiments [95].
