Linking microstructure to irradiation defects in advanced manufacture of steels
Keywords: Steel
Neutron Irradiation
Ion Irradiation
Atom Probe Tomography

Embrittlement of reactor pressure vessel (RPV) steel during neutron irradiation is a well documented phenomenon. Hardening embrittlement in low-Cu RPV steels is known to be associated with the development of solute clusters, primarily containing Mn, Ni and Si. Understanding the microstructural changes, and the corresponding impact on mechanical properties, caused by irradiation is critical in predicting the safe operating lifetime of commercial fission reactors. Multiple factors influence how a steel may respond to neutron bombardment; notably, composition and existing microstructure. Another significant variable involved in dictating the microstructure of steels is the manufacturing process. However, there is limited research on how varying these processes influences the irradiation response of RPV steels.
To investigate the role of manufacturing route on irradiation response, this thesis considers four different SA508 steels, each with a unique manufacturing route: Hot Isostatic Pressing (HIP), conventional large forging (LF), and two trial forgings tempered for distinct durations. HIP is a novel method for the manufacture of structural nuclear components, with the benefits of net-shape casting and microstructural homogeneity. The viability of HIPed components for use in the nuclear industry is dependent on understanding the effect of the microstructure on the irradiation response of the steel. HIPed material also presents an opportunity to deduce the role of different microconstituents within the same specimen.
Atom probe tomography (APT) was chosen as the primary investigative tool as it is a proven method for investigating the development of potentially embrittling microstructural features with high chemical and spatial resolution. APT was employed to analyse the early-stage development of Mn-Ni-Si (MNS) solute clusters of the different steel types, following neutron (HIP and conventional forging) and self-ion (all four steels) irradiation. Specifically, samples produced via the various manufacturing routes were compared with respect to the observable solute segregation behaviours.
Neutron irradiation of the conventional forged (LF) and HIPed samples was conducted using the OPAL reactor at the Australian Nuclear Science and Technology Organisation (ANSTO). Two sets of irradiations were used to produce samples of approximately 0.1 dpa (≈ 6.7 × 1021 n/m2 ) and 0.2 dpa (≈ 1.3 × 1022 n/m2 ) respectively. APT analysis found that the HIPed ferritic and bainitic samples exhibited similar MNS clustering behaviour under neutron irradiation. After three-months of irradiation, the LF sample hardened to a lesser extent (Δσ𝑦 ≈ 157 ± 112 MPa) compared to the HIPed sample (≈ 490 ± 235 MPa).This slightly increased relative hardening may be a cause for concern when considering HIPed SA508 for nuclear applications.
A set of three Fe4+ irradiations enabled analysis of the three forged and the HIPed steels under varying dose and temperature. P atmospheres were found in the HIP and LF alloys after ∼0.1 dpa of damage at 300°C. The influence of manufacturing role was explored further, considering grain types, carbides and solute segregation to interfaces and dislocations. MNS clusters were found to grow in size as fluence increased, with additional Mn-rich solute clusters found after the 5 dpa irradiation.