Shearing of γ′ precipitates by a〈1 1 2〉 dislocation ribbons in Ni-base superalloys: A phase field approach

The phase field model of dislocations has been used to study the propagation of dislocation ribbons with an overall Burgers vector of a〈1 1 2〉 through a simulated Ni-base superalloy. The driving force for dislocation dissociation reactions and formation of planar faults is incorporated into the free energy functional using periodic functions specially fitted to ab initio γ-surface data. The model shows that the mechanism of cutting of the γ′ precipitates by these ribbons exhibits significant dependence on stress magnitude, orientation and precipitate shape. In the case of mixed screw–edge ribbons a change of shearing mode is observed, from stacking fault shear to anti-phase boundary shear, when the applied stress approaches the yield of the material. This transition is absent in pure edge ribbons.