A low-temperature ageing and irradiation study on 14YWT Nanostructured Ferritic Alloys for nuclear fusion applications

• Main Author: Bhojwani, Kris
• Other Authors: Roberts, Steven G.
• Published: University of Oxford 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.748699

14YWT Nanostructured Ferritic Alloys (NFAs) are candidate alloys for tokamak blanket applications. They differ from conventional steels as they contain dispersoids that are distributed across its matrix. These dispersoids pin dislocations and grain boundaries, and are responsible for the superior high temperature properties associated with these alloys. However, at ≤ 475°C, Fe-Cr based alloys are susceptible to the formation of α' precipitates. Their presence induces increases in hardness and embrittlement. In irradiation environments the kinetics of α' formation are enhanced. At these low-temperatures and under irradiation it is unknown whether 14YWT also forms α' precipitates, other secondary phases and tertiary intermetallics. It is also unknown whether segregation of alloying elements occurs too. This thesis describes an isothermal ageing study which sought to determine whether 14YWT NFAs are susceptible to the formation of α' precipitates, other secondary phases and tertiary intermetallics. This study aged samples of 14YWT NFA and 14WT non-NF at 475°C for 1, 3 and 8 months. 14WT non-NFA is a control variant of 14YWT containing no dispersoids. A second study was also completed that sought to study phase stability within 14YWT NFA ion-irradiated and neutron irradiated at lowtemperatures separately. Ion-irradiation was conducted using ⁵⁶Fe⁸⁺ to a peak dose of 0.74-1.49dpa, at 3.24-6.53x10^-5dpa s^-1 at 288°C. Neutron irradiation was conducted to 1.82dpa at 3.85x10^-7dpa s^-1 (E > 0.1MeV) at 288°C. Three variants of 14YWT were used in this thesis. They differed primarily in terms of their nominal concentrations of Ti and Y₂O₃, and are referred to as 14YWT1, 14YWT2, and 14YWT3. 14YWT1 was utilised in the isothermal ageing study, 14YWT2 was ion-irradiated and 14YWT3 was neutron irradiated. Post-experimental analysis for both studies primarily comprised of Atom Probe Tomography (APT), which provides nm-scale chemical resolution of alloying element distributions within the alloys. Microhardness indentation tests were also conducted on the isothermally aged alloys only, to determine whether any hardening occurred. Before APT experiments on the treated alloys could be conducted, an APT calibration experiment on 14YWT1 was completed. This sought to determine a combination of analysis parameters that produced datasets which were of a high quality. A dataset was deemed to be of a high quality if all of the following criteria was maximised: compositional measurement accuracy, signal-to-noise ratio and mass resolution. It was found that all three were satisfied when experiments were conducted at: a temperature of 55K, laser pulse energies of 0.4nJ, laser pulse repetition rate of 200kHz and an evaporation rate of 0.005 ions pulse^-1. APT characterisation of the aged 14WT showed α 0 precipitates had formed within the sample aged for 3 months and 8 months. This suggests an incubation period between 1 and 3 months. α' precipitate characteristics varied significantly between three experimental repeats in terms of: number density, radius, and precipitate Cr concentration. Average α' precipitate characteristics across three experimental repeats within the 14WT aged for 8 months are: 36 ± 32 x 10²²m^-3 , 1.38 ± 0.26nm and 52.17 ± 3.39wt.% respectively. 14YWT1 did not form any α' precipitates. No other secondary phases or intermetallics were observed to form in the aged 14WT and 14YWT1. Therefore 14YWT1 has a stable α phase and Cr is soluble in Fe under these conditions. Aged 14YWT1 grain boundaries (GBs) were seen to be enriched in Cr, W, Si and Mn. However, due to significant variation in the data, no trends as a function of ageing duration could be determined. NFA dispersoids were deemed to be stable after residing at 475°C for 8 months as no significant change in their radius and Y-Ti-O concentrations were observed. Microindentation tests revealed that after 8 months of ageing, 14WT had hardened by 32 ± 4kg mm^-2 . However, 14YWT1 had softened by 26 ± 10kg mm^-2 . APT characterisation of both the ion-irradiated 14YWT2 and neutron irradiated 14YWT3 showed that the matrix α phase did not contain any α' precipitates, other secondary phases or intermetallics. Therefore 14YWT2 and 14YWT3 are deemed to be stable under these irradiation conditions. Since little differences in the microstructures of the ion-irradiated and neutron irradiated occurred, ion-irradiation is thought to be a good analogue to fission neutron irradiation at these irradiation conditions.