Dimensionality of interstitial He migration in 〈1 1 0〉 tilt grain boundaries in α-Fe

In this work we study the migration of an interstitial helium atom in different 〈1 1 0〉 tilt grain boundaries (Σ19{3 3 1}, Σ9{2 2 1}, Σ3{1 1 1}, Σ3{1 1 2}, Σ11{1 1 3}, Σ9{1 1 4}) in α-Fe, with the misorientation angle varying in the range 26°–141°. Systematic molecular statics and molecular dynamics simulations were performed to characterize the interaction of He with the core of the grain boundaries (GB) and to estimate the diffusion coefficient, migration mechanism and effective migration energy. The simulations were performed in the temperature range 300–1000 K, applying a recently proposed set of interatomic potentials for Fe–He system by Juslin et al., specially fitted to the properties of He in bulk Fe. It was found that the migration of an interstitial He near the core of Σ3 〈1 1 0〉{1 1 1} GB is essentially three-dimensional in the investigated temperature range. Two-dimensional diffusion prevails in the case of Σ3〈1 1 0〉{112} and Σ9〈1 1 0〉{1 1 4} GBs and it changes to the three-dimensional one above ∼900 K. A completely one-dimensional diffusion along the tilt axis was observed in the core of Σ19〈1 1 0〉{3 3 1} and Σ9〈1 1 0〉{2 2 1} GBs. No core diffusion was detected at Σ11〈1 1 0〉{1 1 3} GB. Thus, only the release from and re-absorption at the GB core was seen in this GB. These results clearly demonstrate that the accommodation, migration mechanism and diffusivity of He is extremely sensitive to variations in the atomic structure of grain boundaries. Some of the results obtained are compared with previously published data, generated using early developed interatomic potentials. In general, the results obtained support and extend most of conclusions on He properties near grain boundaries drawn already, yet several important discrepancies are also found.