Microstructure evolution under [110] creep in Ni-base superalloys

Microstructure evolutions in Ni-base superalloys are investigated during [110] creep loading using 3D and 2D phase field simulations. A recently developed phase field model coupled to a crystal plasticity model based on dislocation densities is employed. The model uses a storage-recovery law for the dislocation density of each glide system and a hardening matrix to account for the short-range interactions between dislocations. We show that small misorientations of the tensile axis strongly modify the evolution: rafting is observed for small deviations, as opposed to a microstructure made of rod-like precipitates when loading is performed along a perfectly aligned [110] direction. Depending on the precise direction of the mechanical load, different evolutions are obtained accompanied by strong modification of the macroscopic creep behavior, explaining the variety of results observed experimentally. The relative role of inhomogeneous and anisotropic elastic and plastic driving forces is also investigated, plasticity being the main driving force for rafting in the considered case. In addition, our calculations show that the initial dislocation density slightly modifies the precipitates morphology but the creep curve is significantly impacted.