Segregation of Ni and Si to coherent bcc Fe-Cu interfaces from density functional theory

The formation of Cu rich precipitates (CRPs) is known to substantially contribute to the embrittlement of reactor pressure vessel steels with high Cu contents. CRPs are commonly observed to possess a Cu core surrounded by a shell of other segregating solute species like Mn, Ni and Si. Here we calculate the segregation energies of substitutional Ni and Si to {100}, {110} and {111} orientated coherent Fe-Cu interfaces. We find Ni is strongly attracted to all orientations of coherent bcc Fe-Cu interfaces whilst Si interacts in a more complex manner, with strong attraction only to the {100} orientation. In calculating these segregation energies we also explore different methods, concluding that for this system using an external reference bulk (ERB) is optimal. By performing additional calculations we were able to decouple the elastic and chemical contributions to segregation energy finding that for Ni the chemical contributions dominate whilst for Si the two contributions are more balanced. In conjunction with previous work we conclude that should Cu nanoprecipitate sizes influence segregation behaviours it is likely this will not be due to the strain state surrounding the Cu nanoprecipitate. It is more probable that this size dependence would originate from variation in the proportions of the specific orientations of coherent Fe-Cu interface that make up the Cu nanoprecipitate’s surface at different sizes.