The role of binding energies for phosphorus and sulphur at grain boundaries in copper

Abstract

In recent years it has been found that the segregation energy for phosphorus atoms to grain boundaries (GB) can vary strongly with atomic position in copper. This should be contrasted to the unique value for the segregation energy that has been used for property predictions of creep strength and ductility, which are crucial properties for copper canisters for final disposal of spent nuclear fuel. In the present paper, segregation energies for P and S atoms at different atomic sites for three symmetric tilt GB types: Σ3, Σ5 and Σ11 have been computed using density functional theory (DFT). We found that the segregation energies decrease with distance from the GB plane. However, for the GBs studied, the largest energies are not found at the GB planes, but instead at the first neighboring sites relative to the GB planes. The segregation energies are typically much larger for Σ5 than for Σ3 and Σ11. Subsequently, an effective segregation energy has been defined and it is used to predict the occupancy of P and S atoms at the GBs. Based on these data, the release of free energy during cavity formation has been computed. The release of free energy is lower for copper with phosphorus (Cu-OFP) than for phosphorus free copper (Cu-OF) indicating that cavity generation occurs more readily in the latter material. The data shows with good margin that creep cavitation in Cu-OFP does not lead to brittle creep rupture and that the creep ductility is quite satisfactory.