Abstract
In this work, we propose a new crystal plasticity finite element model to simulate the mechanical behavior of austenitic stainless steel subjected to operational conditions typical for internal structures of light water reactors. In particular, we address the coupling of (i) hydrogen and (ii) irradiation defects (Frank loops) with mobile dislocations on microscale to study the emergence of irradiation hardening, plastic strain localization and embrittlement on macroscale. The constitutive equations accounting for neutron-irradiation and hydrogen-concentration effects are derived theoretically for the austenitic stainless-steel single crystals within the crystal plasticity framework. A crystalline lattice is equipped with two types of residing hydrogen atoms: normal interstitial lattice sites and trapping sites attributed to the gliding dislocations. Both hydrogen site concentrations are assumed to be in equilibrium during the mechanical loading. In this respect, the total deformation gradient tensor is multiplicatively decomposed into three distinct configurations: the elastic (volumetric), hydrogen (hydrostatic) and plastic (isochoric) parts. The combined effects of hydrogen concentration and irradiation damage are studied within a polycrystalline aggregate in a series of uniaxial tension simulations. The proposed model, based on dislocation dynamics inferred mechanisms, is able to capture the neutron-irradiation-induced hardening followed by softening due to the formation of defect-free channels on each slip plane during plastic deformation. The effect of hydrogen is manifested in a higher yield stress, due to activation of immobile dislocations, and in a reduced work hardening, in accordance with the increase of dislocation mobility observed experimentally. The overall softening response, accompanied with plastic strain localization, is predicted ”at smaller applied strains for higher neutron fluences and higher hydrogen concentrations” due to combined effects of clear channel formation and increased dislocation mobility. In this regime, the embrittlement is predicted for austenitic stainless steel.
