Interpretation in elastic regime for rafting of Ni-base superalloy based on the external-stress-free dimensional change due to internal-stress equilibration

The directional coarsening, so-called rafting, phenomenon in Ni-base superalloys is reexamined in the elastic regime, focusing on the evaluation of the external-stress-free macroscopic-shape change in the light of the Tien–Copley description [Metall Trans 1971:2;215]. The present parametric study using a simple but essential model explicitly demonstrates that creep dislocations are, in theory, not always required for the raft formation. The present result is consistent with the result of the Nabarro theory [Metall Mater Trans 1996:A27;513], and the two prior literature views are found to be equivalent. In the elastic framework, the trigger of rafting is the gain of the potential energy of an external load due to the dimensional change caused by the microstructural change. The argument based on the mechanical free energy indicates that the potential-energy change is much smaller than the strain energy released by the relaxation of the lattice mismatch, and the latter is found to be decisive for most of the rafting phenomena.