Diffusion, Trapping, and Dissociation Behaviours of Helium at the Σ5 Grain Boundary in Tungsten: A Molecular Dynamics Study

Abstract

Tungsten (W) is a promising candidate for plasma-facing materials in future fusion devices because of its excellent properties. Quantitatively understanding and describing the behaviours of helium (He) in polycrystalline W is important for determining the long-term structural evolution of these materials under extreme operating conditions. However, there exist some intrinsic difficulties in the determination of the key parameters to describe various dynamical processes via static methods. Therefore, it is necessary to extend new models based on molecular dynamics (MD) simulations to abstract the dynamical parameters. In this study, the diffusion, trapping, and dissociation behaviours of He at the Σ5<100>(310) symmetric tilt grain boundaries (GBs) in W were systematically investigated using MD simulations. The key parameters, including the diffusion coefficients of He in bulk W (bulk He) and He trapped by GB (GB-He), the dissociation coefficients of GB-He, and absorbing probability of He by GB were firstly identified using mean square displacement, a coarse-grained model, and a continuous diffusion theory (CDT)-based model. The temperature effect on the diffusivity of GB-He and the dissociation coefficient of GB-He can be described by the Arrhenius relationship, but the temperature effect on the diffusion of bulk He is non-Arrhenius. The diffusion of GB-He is significantly anisotropic; the GB is a strong sink for interstitial He and acts as an inhibitor of He diffusion. The migration of He in the GB can be frequent before the dissociation of GB-He when the temperature increases. The trapping behaviour of He by the GB is described by one CDT-based model, which means that GB can be modelled as an absorbing layer that absorbs He atoms with one probability when the He atoms migrate to the impurity-free zone near the GB plane. This work not only presents a systematic study on the interaction between He atom and GB in W, but also provides some extended models to abstract the key dynamical parameters describing the behaviours of He atom near the GB in W and these abstracted transport parameters for the long-term simulation methods, such as kinetic Monte Carlo and rate theory method. Furthermore, this work gives a good example of supporting the long-term simulation methods by offering transport coefficients from MD simulations.