Shearing of γ′ precipitates by a〈112〉 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(112) through a simulated Ni-base superalloy. The driving force for dislocation dissociation reactions and formation of planar faults isincorporated 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.