Directional coarsening in nickel-base superalloys and its effect on the mechanical properties

During high temperature loading, the regular microstructure of nickel-base superalloys consisting of a γ-matrix (Ni) containing a large volume fraction of γ′-particles (Ni3Al) degrades. The cubic precipitates coarsen and elongate in a direction normal to the applied stress in a process called rafting. In this paper a phenomenological relation is proposed to describe the kinetics of this degradation process. Also, a relation is presented for the isotropic coarsening process that acts simultaneously and becomes dominant when the rafting is complete. The degradation affects the mechanical response of the material. The relevant mechanisms are discussed and formulations are developed to incorporate the degradation effects in an existing multiscale constitutive framework to describe the mechanical behaviour of nickel-base superalloys and in a recently postulated damage evolution equation. The capability of the model to simulate the response of the degraded material is demonstrated by comparing the computational results to experimental observations.