In the present work, an ODS 12 Cr steel was characterized using Electron Microscopy techniques, in an as-received condition and after annealing treatments between 773 K and 1573 K. Results show a complex microstructure, with the presence of fine Y–Ti–O nanoparticles dispersed in the matrix. After annealing at 1573 K, the average diameter of Y–Ti–O nanoparticles increases from ~ 4 to ~ 7 nm and partial recrystallization occurs. The trapping behavior of deuterium in the steel in its as-received state and annealed at 1573 K was investigated. Samples were exposed to low-energy deuterium plasma and analyzed with thermal desorption spectroscopy, after waiting times of 1 day and 25 days. The samples measured 1 day after exposure released a higher total amount of deuterium than the ones measured after 25 days. The effect of waiting time is explained by the release of deuterium, at 300 K, from sites with low activation energy for detrapping, E_d. In the as-received condition, part of the deuterium detrapped at 300 K was re-trapped by high-E_d sites. For the samples in the annealed condition, the redistribution of deuterium from low-E_d to high-E_d sites was not observed, but the total amount of deuterium released was higher.

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Oxide Dispersion Strengthened (ODS) steels are potential candidate materials for application as structural components of fission and fusion reactors, known for their high thermal stability, high resistance to creep and to radiation-induced damage. These attractive properties result from the presence of the fine and highly thermally stable yttrium‑oxygen (Y-O) based nanoparticles, which exert a strong Zener pinning force to hinder the grain boundary movement, and are able to pin dislocations and trap radiation induced defects. In the present work, the effect of annealing at 1400 K on the microstructure and oxide nanoparticles in a 0.3% Y₂O₃ ODS Eurofer steel was assessed. The material was characterized with Scanning Electron Microscopy, Transmission Electron Microscopy and Atom Probe Tomography in a reference condition and after annealing at 1400 K, followed by cooling at different rates. The results showed that the average diameter of the oxide nanoparticles increases from 3.7 ± 0.01 nm to 5.3 ± 0.04 nm, after annealing at 1400 K for 1 h. The particles present a well-known core/shell structure, with a core rich in Y, O and V and a shell rich in Cr. The effect of the increase in oxide nanoparticle size on the microstructure is discussed in terms of the Zener pinning force.

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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. @en

An approach to improve the performance of steels for fusion 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. The present work investigates the thermal stability of the microstructure and the evolution of defects in a 0.3% Y₂O₃ dispersed Eurofer steel. Samples were annealed for 1 h under vacuum, from 600 K to 1600 K, followed by cooling inside the furnace. Electron Microscopy techniques and Vickers Hardness were used to characterize the microstructure and evaluate its thermal stability. Positron Annihilation Spectroscopy Doppler Broadening (PASDB) was used to monitor the evolution of defects, such as dislocations and vacancies, and their interaction with Y-O based nanoparticles. Several types of events take place simultaneously in the material, due to its initial deformation caused by mechanical alloying, the presence of oxide particles and austenitic phase transformation. Annealing up to 1000 K shows that the Y-O based nanoparticles keep the microstructure refined. Upon cooling from 1200 K (above A_c3), martensite forms with an equiaxed morphology, instead of the conventional lath form, due to the pinning of prior-austenite grain boundaries by the oxide nanoparticles. Annealing at 1400 K and 1600 K results in the progressive coarsening of Y-O based nanoparticles and their loss of ability to pin grain boundaries. PASDB shows that annealing up to 1200 K leads to an overall decrease in defect concentration, mainly due to recovery of dislocations. After annealing at 1400 K and 1600 K, PASDB indicates the formation of a different type of positron trap. The hypothesis is that, at these temperatures, clusters of thermal vacancies are trapped by the oxide nanoparticles, accumulating at their interfaces with the matrix and being retained in the material upon cooling to room temperature.

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