PNNL

Abstract

Helium diffusion in metals is a complex process due to its very low solubility in solids and its ability to be trapped by vacancy type defects or impurities. The preferential positions and predominant migration mechanisms of He atoms depend on temperature, as well as the presence of other intrinsic or irradiation induced defects that can act as traps for He. It is generally accepted that grain boundaries provide fast diffusion paths for He. This work presents results of a systematic molecular dynamics study of the migration and diffusion mechanisms of He atoms in grain boundaries in alpha-Fe. Two grain boundaries, sigma11 {323} and sigma3 {112}, were used for the current investigations. The low-temperature (about 0 K) equilibrium structures of these grain boundaries were determined using standard molecular dynamics relaxation techniques, with a flexible border condition. The migration of He atoms were followed for 10 – 30 ns, at temperatures between 600 and 1200 K. The diffusion coefficient of He atoms using the mean square displacements of He atoms, and the effective migration energies were determined. We found that He atoms diffuse quickly in the sigma11 GB, where the binding energy of He atom is high, with one-dimensional behavior along specific directions, but a few directional changes were observed at higher temperatures. However, He atoms migrate one-dimensionally at low temperature, two-dimensionally at intermediate temperature and three-dimensionally at higher temperature in the sigma3 GB, where the binding energy of He atom is low. The different activation energies and diffusion mechanisms in these two representative grain boundaries suggests that the varying atomic structures of the grain boundaries are important for the diffusivity of He.