On the character of nanoscale features in reactor pressure vessel steels under neutron irradiation

Nanostructural features that form in reactor pressure vessel steels under neutron irradiation at around $290°C$ are responsible for significant hardening and embrittlement. It is well established that the nanostructural features can be separated into well formed precipitates and matrix features comprised of point defect clusters complexed with solutes, which may also include regions of solute enrichment that are not well formed precipitates. However, a more detailed atomicscale understanding of these features is needed to better interpret experimental measurements and provide a physical basis for predictive embrittlement models. The overall objective of this work is to provide atomic-level insight into the character of the nanostructural features and the physical processes involved in their formation.; One focus of this work has been on modeling cascade aging; defined as the evolution of self-interstitial and vacancy defects spanning from their spatially correlated birth in displacement cascades over picoseconds to times on the order of $>105$ seconds, when defect populations have built up to steady-state values and no longer have a geometric correlation. During cascade aging, the self-interstitial and vacancy fluxes are responsible for radiation enhanced diffusion, resulting in wellformed precipitates, and are a direct source of matrix defect features. Many-bodied molecular-statics energy relaxation methods have been used to investigate the structure and energetics of self-interstitial and vacancy clusters. The characterization reveals that self-interstitial clusters form as highly kinked, prismatic, perfect proto dislocation loops and vacancy clusters form as faceted three-dimensional clusters. Molecular dynamics simulations of self-interstitial cluster migration reveal that they undergo easy one-dimensional glide, probably due to the presence and easy motion of intrinsic kinks. Our study of the structural characteristics and mobility of the self-interstitial clusters leads to the conclusion of limited vacancy-interstitial recombination during cascade aging. Kinetic Lattice Monte Carlo (KLMC) simulations have been used to provide insight into the coupled vacancy and solute clustering during cascade aging. The simulations reveal that cascade vacancy-solute clustering may play a role in the formation of dilute atmospheres of solute enrichment and serve to enhance the formation of manganese-nickel rich precipitates at low copper levels. KLMC simulations have also been used to study copper diffusion and precipitation in dilute Fe-Cu alloys and reveal that highly correlated copper cluster migration mechanisms occur in these steels. The primary result is that larger clusters, with 5-30 vacancies, form in cascades with thermal lifetimes up to $7.1\times 105s$.; The nanostructural features have been experimentally characterized using small angle neutron scattering (SANS). This work describes the comprehensive single variable experimental studies which characterize the evolution of copper and/or manganese nickel rich precipitates (CRP and/or MNP) as a function of key embrittlement variables. Post-irradiation annealing is a method of ameliorating irradiation embrittlement in these steels, and a set of SANS experiments have been performed which characterize the precipitate evolution during post-irradiation annealing (PIA) at 400 and $450°C$. The character of matrix defect features cannot be obtained from SANS experiments alone, but have been studied in irradiated low copper steels which do not contain well-formed precipitates.; In summary, this work has provided a semi-quantitative analysis of cascade aging in reactor pressure vessel steels under neutron irradiation, with an emphasis on the character of the nano-scale features...