Study of γ′ rafting under different stress states – A phase-field simulation considering viscoplasticity

The γ′/γ evolution in nickel superalloys under different stress states and stress values at 1223 K was simulated through the phase-field method to study the 45°-type rafting mechanism and the influence of orientation deviation on the γ′/γ evolution. The simulation results demonstrate that for the shear stress case, 45°-type rafting appears under the external shear stress of 137 MPa. In 45°-type rafting, the equivalent stress inhomogeneity in the γ channel induces a directional diffusion of γ′-forming elements into the low-equivalent-stress areas which are distributed along the diagonal directions. Based on the simulations, a model for the 45°-type rafting mechanism has been proposed. In addition, the influence of orientation deviation on γ′/γ evolution originates from the introduction of shear stress components. With increasing external stress, the shear stress component is increased, which causes the rafting direction to approach the 45° direction. Furthermore, the existence of orientation deviation accelerates the process of γ′ rafting.