Section 9.7 - A Coupled Diffusion-Viscoplastic Formulation for Oxidasing Multiphase Materials

Abstract

The oxidation of multiphase metallic alloys is an important factor leading to local microcrack formation at or near bimaterial interfaces. A proper description of the local deformation processes requires that the constitutive behavior of the materials be coupled with the dominant diffusion and oxidation mechanisms. This chapter uses the coupled diffusion-constitutive approach to describe the effect of the phase transformations caused by local internal and external oxidation processes on the constitutive behavior of multiphase metallic materials exposed to an oxidizing environment. The coupled oxidation-constitutive material framework accounts for the diffusion of oxidant species through a multiphase material and the subsequent oxidation of one of its phases (e.g., Al-rich or Si-based), and incorporates the effect that the local volumetric expansion of the newly formed oxide has in the generation of inelastic volumetric strains and residual stresses. It relies on the level of oxygen concentration at each material point to identify whether oxide precipitates have formed, in which case the material at that location is treated, through a self-consistent approach, as homogeneous equivalent material consisting of multimetallic phases and oxide products. Use of the coupled formulation in complex boundary value problems representative of service generally requires its numerical implementation into the finite element method.