Ab-initio calculations of substitutional co-segregation interactions at coherent bcc Fe-Cu interfaces

Cu rich precipitates (CRPs) are commonly observed to form core-shell structures in neutron irradiated pressure vessel steels. The core region is typically composed of pure Cu whilst the shell is formed of other solute elements including Ni and Si. In this work we calculate the segregation energies for Ni and Si substitutional defects segregating to coherent Fe-Cu interfaces decorated with different types of point defect (vacancies and other solute substitutional defects). By comparing these values to those found for Ni and Si segregation to clean Fe-Cu interfaces we can establish how the presence of point defects on the interface influences solute segregation behaviours. Interestingly we find that the segregation of Ni to Si decorated interfaces and the segregation of Si to Ni decorated interfaces demonstrate a relatively strong co-segregation interaction. We additionally observe that both solute species experience more attractive segregation to vacancy decorated Fe-Cu interfaces. These findings suggest that mixed solute interactions and the presence of vacancies may both play an important role in assisting the formation of large solute shell regions in CRPs. We further observe that the presence of Ni and vacancies on coherent Fe-Cu interfaces both result in substantial reductions in interfacial energy density. These findings support experimental predictions and indicate both features may significantly contribute to CRP formation.