PNNL

Abstract

The effects of helium (He) bubbles 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 the single crystal (loading normal to {332} plane) and bicrystal lattice ?11 {332} orientation were performed to compute the uniaxial responses of the a-Fe single crystal and a crystal with the grain boundary (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 a-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 and room temperature (RT) 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 bubbles were compared to the corresponding FE results to validate this mechanistic approach that appears to be very efficient in terms of computational time. FE model predictions of GB strength and fracture strain are in fairly good agreement with the MD results at both temperatures. Our results show that small, highly pressurized He-vacancy clusters reduce GB strength and fracture strain more substantially at 5 K than at RT.