Coupling between diffusion and mechanics during the oxidation of zircaloy

Abstract
During the oxidation of zircaloy fuel cladding, large stresses develop in the oxide layer, due to the volumetric expansion related to phase change. The present paper deals with intermediate temperatures, typically 300-450°C. (1) The first section of the paper shows that the local geometry of interface must be taken into account to correctly describe the local stress field and explain the crack formation. For that purpose, finite element computations are performed with an undulated metal-oxide interface. (2) In a second section, a physical model is proposed to understand why the interface presents this wavy shape. It is assumed that this can be related to the heterogeneous diffusion process in zirconia. A numerical investigation is made using a 2D finite element model. The diffusion coefficient being much larger at the grain boundary, a self-organization occurs and produces undulations as observed by SEM. (3) As it is usually observed that pressure influences the diffusion coefficient, coupled computations involving both mechanics and oxidation are then shown.