Micromechanical modeling of the viscoplastic behavior of a porous polycrystal: application to an irradiated austenitic stainless steel

Austenitic stainless steels employed as internals in pressurized water reactor vessels may nucleate intragranular voids when exposed to prolonged irradiation and high temperature. The voids, almost spherical in shape, modify the mechanical behavior of the material. This work explores three different approaches in order to model visco-plasticity of voided single crystals. The first approach consists in idealizing the voided crystal as a hollow sphere assemblage made of crystalline material. The second approach consists in idealizing the voided crystal as a sequential laminate of infinite rank obeying an isotropic lamination sequence. The third approach consists in idealizing the voided crystal as a periodic medium with a complex unit cell, and computing the mechanical fields numerically via a Fast Fourier Transform algorithm. Then, the estimates for porous single crystals are used to model the visco-plasticity of voided polycrystals via a double up scaling process. Finally, in order to apply the present model to an irradiated austenitic stainless steel, the constitutive material parameters are identified with numerical simulations on periodic unit cells where locally the constitutive behavior is described by a phenomenological model especially devoted to this irradiated austenitic stainless steel, taking account of the evolution of irradiation defects. As a general rule, this work aims at delivering innovative, high-performance modeling tools, applicable to a wide variety of crystalline materials together with irradiated austenitic stainless steels. (author)