Microstructural characterisation of proton-irradiated model reactor pressure vessel steels

Dickinson-Lomas, Alexandra Clare ORCID: 0009-0006-7745-3552 (2025). Microstructural characterisation of proton-irradiated model reactor pressure vessel steels. University of Birmingham. Ph.D.

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Abstract

Formation mechanisms of Mn-Ni-Si rich precipitates under irradiation in reactor pressure vessel (RPV) steels are not fully understood. This makes predicting radiation induced embrittlement in RPV steels challenging, leading to uncertainty around lifetime extensions of ageing reactors, and alloy design for new reactors. To understand and predict embrittlement, segregation behaviour of solute elements, and precipitation under irradiation must be understood. Proton irradiation experiments can be used to accelerate understanding of the mechanisms of solute segregation in radiation damage environments. In this work, three ferritic model alloys and an A508-3 reactor pressure vessel steel have
been irradiated using a 2.9 MeV proton beam, at approximately 350 °C, to a peak dose of 2 dpa. Microstructural characterisation has been carried out using a range of complementary techniques including: transmission electron microscopy (TEM), atom probe
tomography (APT), and small angle neutron scattering (SANS).
Radiation enhanced precipitation led to hardening by enhancing solute segregation, resulting in dense microstructures containing dislocation loops and precipitates. Mn, Ni and Si were observed to segregate to dislocation loops under irradiation in the Fe-Mn-Ni-Si alloy, and then to form precipitates at higher doses. Hardening was greater in the Ni containing alloy, corresponding with higher total number density of precipitates at all
doses; close to the Bragg peak it was 2 orders of magnitude higher in the Ni containing alloy at \(4.55 × 10^{22} m^{−3}\)compared with \(1.01 × 10^{20} m^{−3}\). Two new populations of precipitates were identified after irradiation in the Fe-Mn-Ni-Si model alloy, using TEM and APT; Mn-Ni-Si rich precipitates (non-stoichiometric Mn6Ni16Si7 G phase structures) and Mn-Si nitrogen rich precipitates (MnSiN2). The same cuboidal nitrides were observed in the Fe-Mn-Si alloy after irradiation, and their formation is attributed to nitrogen contamination of the vacuum. These nitrides were only observed close to the Bragg peak in the Fe-Mn-Ni-Si alloy, but were found at all doses in the Ni-free alloy. At higher doses, nitrogen ingress led to precipitation of Mn-Si nitrides inside Mn-Ni-Si precipitates, creating a Mn-Si-N core and Ni-rich shell structure, observed using STEM-EDX. This suggests that there is a lower barrier for radiation assisted nucleation of Mn-Ni-Si precipitates compared with Mn-Si nitrides. The increase in number of precipitates in the Ni containing alloy, compared with the sparsely distributed nitrides in the Ni free alloy, suggests that Ni enhances radiation induced segregation, clustering and precipitation.
Elevated diffusion rates produced by radiation damage led to segregation of Mn, Ni and Si to dislocation loops and the precipitation of Mn-Ni-Si phases. Ni has a significant synergistic effect with Mn and Si, on precipitation of secondary phases in a radiation environment, and is the main constituent element in secondary phases. The precipitation observed in model ferritic alloys is not comparable to neutron-irradiated RPV steel mi-
crostructures. However, observations of dislocation loops, segregation and precipitation in these samples contributes to understanding the interaction between Ni, Mn and Si solute elements in a radiation environment and the mechanisms of radiation mediated precipitation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
Chiu, Yu-Lung	UNSPECIFIED	orcid.org/0000-0001-7611-4774
Freer, Martin	UNSPECIFIED	orcid.org/0000-0001-5439-5579
Hewitt, Luke	UNSPECIFIED	orcid.org/0009-0009-0832-5372