Structural modifications of grain boundaries due to vacancy segregation

Vacancies show strong tendencies to segregate towards grain boundaries (GB), where they interact with them. In general, this process changes the GB structure and reduces the vacancy concentration in the bulk which is relevant for non-equilibrium vacancies found e.g. in highly deformed materials or after radiation damage. Due to structure relaxations the vacancy can, at least partially, be filled after segregation. To assess such structural modifications, density functional theory simulation of the segregation of vacancies towards GBs in tungsten are performed. The simulations are carried out for two different coverages of the GB cell. In addition to the segregation energies also changes of the dimensions of the simulation cells and the stress are evaluated. This allows not only to simulate concentrations, but also to calculate shape changes due to the annealing of vacancies. The results show overall strong segregation tendencies and a correlation between the segregation energies and structural changes are observed. The obtained results are used to interpret dilatometric experiments where length-changes between severely deformed and well-annealed samples are compared. We find that the segregation of vacancies cannot fully explain the observed length changes and are likely to originate from other relaxations mechanisms.