Simulation of precipitate formation under neutron irradiation in low-alloy pressure vessel steels

Abstract

In high-Cu pressure vessel steels used for nuclear fission reactors, the formation of Cu rich precipitates (CRPs) substantially contributes to the reactor pressure vessel's (RPV) embrittlement following long periods of neutron irradiation and elevated temperatures. The RPV cannot easily be modified or replaced once reactor operation has begun making RPV embrittlement a significant factor in preventing reactor lifetime extensions, particularly given the RPV is a safety critical component. To limit CRP formation and embrittlement, strict limits are placed on the Cu contents of modern RPV steels. Even in reduced quantities Cu may still influence the late-life embrittlement of RPVs by assisting the formation of MnNiSi-rich complex precipitate phases. In this project we work to understand Cu nanoprecipitate morphologies in ferritic RPV steels, and how the surfaces of those nanoprecipitates interact with other solute species, using density functional theory (DFT). In our first manuscript we use DFT to calculate the interfacial energy densities (gamma) of six Fe-Cu interface orientations that may form the surface of a CRP's Cu nanoprecipitate core region. We identify the {110} orientation as possessing the lowest energy density, allowing us to infer that Cu nanoprecipitates are likely to take morphologies with surfaces dominated by this orientation. Using Wulff construction and optimisation techniques we identify low energy Cu nanoprecipitate morphologies based upon our DFT derived gamma values. Through these techniques we find that as Cu nanoprecipitates increase in size, their surfaces are increasingly comprised of non-{110} orientated interfaces allowing for more spherical geometries. Our second and third manuscripts demonstrate Ni possesses a reasonably strong attraction to Fe-Cu interfaces, which is significantly enhanced by the presence of vacancies on the interface. Si is seen to interact more weakly and less attractively than Ni with undecorated interfaces, with significant repulsion observed for higher interfacial Si concentrations. However, we see that the presence of vacancies and Ni on the interface can result in substantially more attractive Si segregation. These findings suggest it is likely that Ni segregates to the Fe-Cu interface initially after which co-segregation interactions draw Si to the interfacial region. With a sufficiently high vacancy density it may be possible for Si to segregate to Fe-Cu interfaces undecorated by Ni. These findings demonstrate that the formation of large mixed-solute shells around the Cu nanoprecipitate core region is likely to require some degree of co-segregation interaction and may be further enhanced by elevated vacancy densities.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Grace Burke (Supervisor) & Christopher Race (Supervisor)