Enhancement of the rafting in Ni-based superalloys by controlling elastic misfit

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The elastic energies associated with the formation of raft structure of Ni-based superalloys has already been calculated and discussed by Nabarro et al. However, this involves an uncertain part about the method of approximation. Our calculation in the framework of purely elastic regime concerning a macroscopic shape deformation indicates that their conclusion is wrong owing to its method of approximation. Our calculation also leads the normal raft always forms regardless to the sign of the lattice misfit and external stress. This result does not agree with the experimental results and indicates that the mechanism of rafting cannot be explained by the calculation in the framework of purely elastic regime.When creep dislocations are induced before rafting, our calculation indicates that the dislocations can creep into so called normal channels but cannot creep into parallel channels. This anisotropic behavior of the motion of creep dislocations changes the spherical symmetry of the lattice misfit into the tetragonal symmetry which makes the morphology of raft as experimentally observed.The driving force for rafting in the framework of elastic regime is very small in comparison with other thermodynamic energies. So, it is difficult to explain the mechanism of rafting only by the elastic interactions. That in the framework of elastic-plastic regime where creep dislocations are taken into account is quite large and mainly depends on the lattice misfit. We conclude that the dominant factor for the choice of morphology of rafting and the elastic misfit is the secondary factor.