ODS steels for nuclear applications: thermal stability of the microstructure and evolution of defects

An approach to improve the performance of steels for fusion and fission reactors is to reinforce them with oxide nanoparticles. These can hinder dislocation and grain boundary movement and trap radiation-induced defects, thus increasing creep and radiation damage resistance. Steels containing these oxide particles are called ODS steels (Oxide Dispersion Strengthened). In the present thesis, two ODS steels containing 0.3 weight % of Y2O3 were studied: the 0.3% Y2O3 ODS Eurofer and the ODS 12 Cr steel. The main objectives of the work developed during these four years were: (i) evaluation of the thermal stability of the microstructure and of the oxide nanoparticles present in the steels; (ii) investigation of the effect of oxide nanoparticles on phase transformations and other microstructural processes, such as recovery and recrystallization; (iii) investigation of the interaction of oxide nanoparticles with defects intrinsic to the microstructure and (iv) development of the fundamental understanding of the behaviour of the steels prior to exposure to radiation.
The systematic characterization of microstructure of the two ODS steels was made, in their reference state and after 1 h annealing treatments at temperatures ranging from 573 K to 1600 K. The techniques used were Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Vickers hardness testing. The oxide nanoparticles present in the 0.3% Y2O3 ODS Eurofer steel were observed using Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT); the oxide nanoparticles in the ODS 12 Cr steel were analysed with TEM. The 0.3% Y2O3 ODS Eurofer steel has, in its reference state, an isotropic microstructure, without significant texture, composed of tempered martensite, residual ferrite and M23C6 carbides. The ODS 12 Cr steel does not form austenite at high temperatures and, therefore, its matrix is always ferritic, with TiC carbides located along grain boundaries. Because of consolidation by hot extrusion, the ferritic grains in the ODS 12 Cr steel are elongated and present <110>α-fibre texture. In the 0.3% Y2O3 ODS Eurofer steel the oxide nanoparticles are composed of Y, V and O; in the ODS 12 Cr steel, the nanoparticles are Y, Ti and O based. The addition of Ti is known for reducing the final oxide nanoparticle size and for conferring higher thermal stability to the particles. When the oxide nanoparticles remain refined at high temperatures, the Zener pinning force exerted by them also remains strong and the overall microstructure does not become coarser during exposure to elevated temperatures. The Y-V-O based nanoparticles in the 0.3% Y2O3 ODS Eurofer steel go through coarsening during annealing at 1400 K, which leads to the formation of a coarser microstructure upon cooling to room temperature and reduction in the Vickers hardness. In the ODS 12 Cr steel, a fraction of the Y-Ti-O nanoparticles becomes coarser only after 1 h annealing at 1573 K, which leads to a moderate degree of softening of the material.
Positron Annihilation Doppler Broadening (PADB) was used to investigate the thermal evolution of defects present in different ODS steels and their interaction with oxide nanoparticles. PADB results suggest that the oxide nanoparticles are able to trap thermal vacancies, formed in high concentrations during annealing at temperatures of 1400 K and above. The excess of thermal vacancies, trapped by the oxide nanoparticles, is retained in the microstructure upon cooling to room temperature. To further investigate this hypothesis, Thermal Desorption Spectroscopy (TDS) measurements were carried out in the ODS 12 Cr steel, in its as-received condition and after annealing at 1573 K for 1 h, after exposure to low-energy deuterium plasma. The deuterium uptake in the annealed condition was higher than that in the as-received state, and it could be related to the prior-trapping of thermal vacancies by oxide nanoparticles, which would be able, then, to accommodate more deuterium atoms. The ability to accomodate more deuterium atoms (or hydrogen, or helium, or other radiation-induced interstitials) could have positive effects on the performance of the steel during service, but mechanical testing is necessary to verify this influence.