MULTIPHYSICS-BASED NUMERICAL MODELLINGOF FRACTURE INPOLYCRYSTALLINEMATERIALUNDER OXIDATION

Abstract

The quest for achieving higher efficiency in gas turbines has been supported by steady development in nickel-based superalloys owing to their superior mechanical properties at high temperatures and oxidative environments. The oxidation resistance of some Ni-based superalloys results from the formation of a continuous Al2O3 film on the surface and internally along the grain boundaries as a result of preferential diffusion of O and Al atoms at grain boundaries. However, oxidation also causes embrittlement of the grain boundaries, thus assisting crack growth and exposing new surfaces to oxidation. Moreover, the stresses ahead of the crack tip pull the diffusing elements, thereby further accelerating oxidation and fracture. Many numerical schemes have been proposed for better understanding of the concomitant creep-oxidation-diffusion processes. Most of them are phenomenological in nature with emphasis on oxide film stability or the interdependence of different processes for simplified coating-film-substrate systems. However, the aforementioned experimental investigations have clearly identified the vital role of material microstructure in the diffusion, crack growth and ultimate failure of the component. Some recent numerical studies have introduced material heterogeneity chemo-mechanical fracture but the complete kinematics and kinetics of oxidation-induced fracture is not considered. Hence, a comprehensive continuum theory of fracture of Ni-based superalloy with inherent microstructural heterogeneity is missing till date. In that context, a thermodynamically consistent multi-physics framework is presented to simulate the strongly coupled chemo-mechanical environment and the evolution of arbitrary fracture in polycrystalline structure of Ni-based superalloys. Specifically, a regular phase-field fracture variable is introduced for initiation and inter- or intra-granular propagation of cracks. An interface phase-field descriptor distinguishes between grain and grain boundary diffusivity. The proposed model employs a viscoplasticity algorithm to account for the high temperature micro-damage mechanisms causing crack nucleation on oxide surfaces, and partial release of residual stresses due to creep deformation. Thereafter, a finite element based numerical scheme is employed to solve the weak form of the coupled governing equations obtained using variational principle. The set of coupled finite element equations are solved for the displacement field, concentration field and phase-field fracture parameter in a staggered manner using a parallel computing architecture. The model will be applied to a polycrystalline Ni-based superalloy mechanical degradation under oxidation. The model is validated through several benchmark problems following which the failure of polycrystalline structures with and without oxidation-induced degradation is simulated. A parametric study will be performed to investigate the effect of grain boundary diffusion and grain size. The proposed model will help in making better design decisions related to the Ni-Al superalloy composition and microstructure to optimize its performance in the gas turbine service environment.