Modeling the effects of helium-vacancy clusters on the stress-strain response of a grain boundary in iron by a mechanistic finite element approach informed by molecular dynamics data

The effects of helium (He)-vacancy clusters on the stress-strain behavior of polycrystalline iron (a-Fe) are investigated by a mechanistic finite element (FE) approach using a continuum damage mechanics (CDM) description of the material behavior informed by molecular dynamics (MD) data. First, MD analyses of a single crystal (loading normal to {332} plane) and a bicrystal system containing a Sigma 11 <110>{332} grain boundary (GB) were performed to compute the uniaxial tensile response of an Fe single crystal and a system with a GB. MD results were then used in FE analyses of the same systems to identify parameters for the CDM constitutive relations for the crystal and the traction-separation law for the GB depicted by cohesive elements. Next, a 3D FE model of an alpha-Fe bicrystal system with an imperfect GB subjected to uniaxial tensile loading was developed. This model includes an equivalent hollow sphere under internal pressure in the middle of the GB to model the effects of pressurized He bubbles at 5 K, room temperature (RT) and 600 K on stress, strain and damage distributions. The radius of the equivalent sphere was determined assuming the presence of two vacancies in the system. Finally, MD stress/strain data of the same bicrystal system with He -vacancy clusters were compared to the corresponding FE results to validate this mechanistic approach that appears to be efficient in terms of computational time. FE model predictions of system strength and fracture strain are in fairly good agreement with the MD results at all three temperatures. Our results show that small and highly pressurized He-vacancy clusters reduce GB strength and fracture strain more significantly at 5 K than at RT and 600 K. (C) 2019 Elsevier B.V. All rights reserved.