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

The gamma'/gamma 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 degrees-type rafting mechanism and the influence of orientation deviation on the gamma'/gamma evolution. The simulation results demonstrate that for the shear stress case, 45 degrees-type rafting appears under the external shear stress of 137 MPa. In 45 degrees-type rafting, the equivalent stress inhomogeneity in the gamma channel induces a directional diffusion of gamma'-forming elements into the low-equivalent-stress areas which are distributed along the diagonal directions. Based on the simulations, a model for the 45 degrees-type rafting mechanism has been proposed. In addition, the influence of orientation deviation on gamma'/gamma 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 degrees direction. Furthermore, the existence of orientation deviation accelerates the process of gamma' rafting. (C) 2018 Elsevier B.V. All rights reserved.