PNNL

Abstract

he dimer method for searching transition states has been used to systematically study possible migration paths of vacancies, He interstitials and He-vacancy (He/V) clusters at sigma11 {323} and sigma3 {111} grain boundaries (GBs) in alpha-Fe. Vacancies trapped at the GBs diffuse along the GBs 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 GBs: directly along close-packed rows in the sigma3 GB, and in zigzag paths within the sigma11 GB. Also, dimer saddle point searches show that He interstitials can diffuse along the GBs with migration energies of 0.4-0.5 eV, similar to those of individual vacancies at the GBs, and the corresponding mechanisms are determined. The rate-controlling activation energy for migration of a He-divacancy cluster in the GBs determined using the dimer method is about 0.9 eV. This is comparable to the migration energy for a He-divacancy cluster in bulk alpha-Fe.