Plastic strain-induced rafting of γ′ precipitates in Ni superalloys: Elasticity analysis

Rafting in Ni-based superalloys is investigated by examining the dependence of elastic energy on the shape and orientation of γ′ particles. The precipitation misfit strain and the plastic strain occurring in the matrix are taken into account. The elastic energies associated with spherical or disc-shaped particles with different orientations are evaluated for some combinations of transformation strain and matrix plastic strain. The anisotropy in elastic constants as well as the difference in elastic constant between the particles and matrix are taken into account. It is found that the elastic energy becomes minimum when γ′ particles adopt a certain shape and direction with respect to the elongation axis 〈0 0 1〉. These results are in agreement with the rafted morphologies observed in crept superalloys. The onset of the shearing of rafted γ′ particles, leading to accelerated creep of the alloy, is also discussed by examining the stress state of rafted γ′ particles.