The dimer method for searching transition states was used to systematically study possible migration paths of vacancies, He interstitials and He-vacancy (He/V) clusters at Σ11<110> {323} and Σ3<110> {111} grain boundaries in α-Fe. Vacancies trapped at the grain boundaries diffused along the grain boundaries with migration energies much less than that within the perfect crystal. Long-time dynamics simulations of diffusion pathways reveal that vacancies migrate one-dimensionally along specific directions in both grain boundaries: directly along close-packed rows in the Σ3 grain boundary, and in zigzag paths within the Σ11 grain boundary. Also, dimer saddle point searches show that He interstitials could diffuse along the grain boundaries with migration energies of 0.4–0.5eV, similar to those of individual vacancies at the grain boundaries, and the corresponding mechanisms were determined. The rate-controlling activation energy for migration of a He-divacancy cluster in the grain boundaries determined using the dimer method was about 0.9eV. This was comparable to the migration energy for a He-divacancy cluster in bulk α-Fe.

Migration of Vacancies, He Interstitials and He-Vacancy Clusters at Grain Boundaries in α-Fe. F.Gao, H.L.Heinisch, R.J.Kurtz: Journal of Nuclear Materials, 2009, 386-388, 390-4