Grain boundary (GB) segregation has important implications in mechanical properties of materials. Despite extensive studies of GB segregation of various solute elements and impurities in copper alloys, little is known about the segregation tendency of vacancies. Here we present a computational study of monovacancy site preference at three high-angle low-index symmetric tilt GBs in fcc Cu: Σ5, Σ9 and Σ11. The computations are performed using the Vienna ab-initio simulation package (VASP), employing projector-augmented wave pseudopotentials and a semi-local exchange-correlation potential (PBE) of density functional theory (DFT). To characterize the asymmetry of local coordination at potential segregation sites, we introduce an atomic site symmetry-quantifying parameter (σ) based on the difference in surface area between the Voronoi polyhedron and the corresponding Wigner-Seitz sphere. We show that this parameter accurately quantifies changes in symmetry at each GB site relative to the symmetry in the bulk fcc lattice. Furthermore, the value of σ is found to exhibit certain correlations with the coordination number (Cn) and with the volume expansion (Vx) at the different lattice sites near the three studied GBs. The GB that has the largest distortions relative to the bulk is the Σ5, closely followed by the Σ9, while the Σ11 has a decrease in symmetry which is only ¼ of the other two. At Σ5, changes in site Cn imply changes in the site symmetry to extents that do not occur for the other two GBs.

 For 100% of the sites at Σ5 and 50% of the sites at Σ9 and Σ11, the formation of vacancies is more favorable than in the bulk. However, the vacancy formation energy lowering at Σ11 is very weak, which implies that vacancies will occur at this GB with a similar frequency as in the bulk. These results show that the transferability of vacancy formation energies and vacancy distribution data between different GBs is nontrivial.