Les alliages ODS (Oxide Dispersion Strengthened) sont des matériaux candidats pour la réalisation du gainage combustible des réacteurs nucléaires de Génération IV de type RNR-Na. Leurs propriétés mécaniques à haute température sont assurées par une dispersion d’oxydes nanométriques qui peut être modifiée sous l’effet de la température et notamment lors du passage par l’état liquide. Les procédés d'assemblage en phase solide sont donc préconisés. La méthode de soudage étudiée dans cette thèse est le soudage par résistance en bout. Une double approche simulation numérique-expérimentation a été mise en place pour étudier l’influence des paramètres opératoires sur le soudage et la soudure. L’ensemble des essais (expérimentaux et numériques) est réalisé selon la méthode des plans d’expériences factoriels complets. Un alliage ODS base fer à 20 % de chrome a été utilisé comme alliage d’essai.L’analyse de l’influence des paramètres opératoires sur le soudage est réalisée dans un premier temps. Les résultats montrent que l’étape de soudage peut se découper en trois phases. Dans une première phase, la température augmente au niveau des interfaces. Dans une seconde phase, le procédé de soudage est gouverné par le changement de section macroscopique lorsque le courant passe du bouchon massif à la gaine plus étroite. La température augmente alors dans la partie de gaine dépassant de l’électrode. Lorsque la température atteint une valeur suffisamment élevée, la partie de gaine dépassant de l’électrode se déforme, entrainant un affaissement général des pièces et constituant la troisième phase de l’étape de soudage. L’influence des paramètres opératoires sur les phénomènes physiques lors de l’opération d’assemblage est évaluée. Il est alors possible d’influer sur les sollicitations thermiques et mécaniques à l’aide des paramètres opératoires. L’analyse de l’influence des sollicitations thermomécaniques sur la soudure est réalisée dans un second temps. Sur les plages de paramètres opératoires étudiées, les résultats révèlent sur certaines soudures la présence de défauts de compacité ainsi qu’une modification de la microstructure et de la dispersion des oxydes. Les défauts de compacité peuvent être liés à des phénomènes thermiques et mécaniques au contact entre pièces. Les modifications de la microstructure sont liées à des phénomènes de recristallisation dynamique ou de fusion locale et donc à des phénomènes thermiques mais aussi mécaniques avec la présence de déformations importantes. Les modifications de la microstructure sont alors reliées à une modification de la dispersion d’oxydes. A l’aide des paramètres opératoires il est possible d’influer sur les températures et les déformations afin de limiter les modifications de la structure de l’alliage ODS ainsi que l’apparition de défaut de compacité. A l’aide de l’ensemble de ces résultats, la procédure de soudage est adaptée sur un alliage ODS à 9 % de chrome, nuance envisagée pour la réalisation des futurs gainages combustibles. L’influence des propriétés matériaux sur le soudage et la soudure est alors discutée en comparant les deux nuances ODS de compositions différentes mais aussi en comparant les résultats obtenus sur l’alliage ODS à 20 % de chrome avec un alliage non renforcé de composition semblable. / ODS steels (Oxide Dispersion Strengthened) are candidate materials for fuel cladding in Sodium Fast Reactors (SFR), one of the concepts at study for the forth generation of nuclear power plant. These materials have good mechanical properties at high temperature due to a dispersion of nanometer-sized oxides into the matrix. Previous studies have shown that melting can induce a decrease of the mechanical properties at high temperatures due to modifications of the nanometer-sized oxide dispersion. Therefore the fusion welding techniques are not recommended and the solid state bonding has to be evaluated. This study is focused on resistance upset welding.Welding experiments and numerical simulations of the process are coupled in this thesis. All the trials (experimental and numerical) are built using the experimental design method in order to evaluate the effects of the process parameters on the welding and on the weld. A 20Cr ODS steel is used in order to conduct the study.The first part is dedicated to the study of the influence of the process parameters on the welding. The simulation shows that the welding steps can be divided in three stages. First, the temperature of the contact between pieces increases. Second, the process is driven by the pieces geometry and especially the current constriction due to the thinness of the clad compare to the massive plug. Therefore, the heat generation is mainly located in the clad part out of the electrode leading to its collapse which is the third stage of the welding step. The evaluation of the process parameters influences on the physical phenomena (thermal, mechanical …) occurring during the welding step allow to adjust them in order to influence the thermal and mechanical solicitation undergone by the pieces during the welding process.The second part is dedicated to the study of the influence of the physical phenomena on the welds. In the process parameter range, some welds exhibit compactness defects or a modification of the microstructure and of the oxide dispersion. Compactness defects are related to thermal and mechanical phenomena occurring at the contact between pieces. The modification of the microstructure is related to dynamical recrystallization or to a local fusion. The dynamical recrystallization occurring in the clad due to high deformation and high temperature is linked to modification of the oxide dispersion.Using the effects of the process parameters on the welding and on the weld, it is possible to adjust the temperature and the deformation in order to avoid the compactness defects and the modification of the oxide dispersion. All these results are then apply to the welding of a 9Cr ODS steel which is a candidate alloy for the SFR fuel cladding. The effects of the material properties on the welding and the weld are then discussed by comparing the two alloy with a different chromium content but also by comparing results on the 20Cr-ODS with a material of similar chemical composition but without the oxide dispersion.

Alloys used in fission and in future fusion reactors are subjected to extreme conditions including high temperatures, corrosive and intense radiation environments. Understanding the processes occurring at the microscopic level during radiation events is essential for the further development of them. As a prospective candidate material for new reactors, oxide dispersion strengthened (ODS) steels have shown good radiation resistance and the ability to trap He into fine scale bubbles, thus preventing swelling and preserving high-temperature strength. This thesis represents the findings obtained by performing computational studies of radiation effects in pure iron, Y-Ti-O systems and a simplified model of ODS using Molecular Dynamics (MD) and on-the-fly Kinetic Monte Carlo (otf-KMC) techniques. MD studies of radiation damage were carried out in a perfect body-centred cubic (bcc) iron matrix (alpha-Fe) in which yttria nanoparticles are embedded as a simplified model of an ODS steel. The results have shown how the nanoparticles interact with nearby initiated collision cascades, through cascade blocking and energy absorption. Fe defects accumulate at the interface both directly from the ballistic collisions and also by attraction of defects generated close by. The nanoparticles generally remain intact during a radiation event and release absorbed energy over times longer than the ballistic phase of the collision cascade. Also the nanoparticles have shown ability to attract He atoms as a product of fission and fusion reactions. Moreover, studies showed that He clusters containing up to 4 He atoms are very mobile and clusters containing 5 He or more become stable by pushing an Fe atom out of its lattice position. The radiation damage study in the Y-Ti-O materials showed two types of residual damage behaviour: when the damage is localized in a region, usually close to the initial primary knock-on atom (PKA) position and when PKA is directed in the channelling direction and creates less defects compared to the localised damage case, but with a wider spread. The Y2TiO5 and Y2Ti2O7 systems showed increased recombination of defects with increased temperature, suggesting that the Y-Ti-O systems could have a higher radiation resistance at higher temperatures. The otf-KMC technique was used to estimate the influence of the prefactor in the Arrhenius equation for the long time scale motion of defects in alpha-Fe. It is shown that calculated prefactors vary widely between different defect types and it is thus important to determine these accurately when implementing KMC simulations. The technique was also used to study the recombination and clustering processes of post-cascade defects that occur on the longer time scales.

Oxide dispersion strengthened (ODS) alloys have been developed with unique mechanical properties. However, in order to achieve commercial application an appropriate joining process is necessary which minimizes disruption to the alloy microstructure. Transient liquid phase (TLP) bonding is a promising joining method, but previous work has shown that the segregation of dispersoids within the joint region results in bonds with poor mechanical strengths. This research work was undertaken to further explore particulate segregation at the joint region when TLP bonding and to develop bonding techniques to prevent it. A Ni-Cr-Fe-Si-B interlayer was used to bond an alloy MA 758. The effects of parent alloy grain size, bonding temperature, and external pressure on the TLP bonding process were investigated. Three melting stages were identified for the interlayer, and the bonding temperature was chosen so that the interlayer was in the semi-solid state during bonding. This novel bonding mechanism is described and applied to counteract the segregation of Y203 dispersoids. The grain size of the parent alloy does not alter the particulate segregation behaviour. It is concluded that a low bonding temperature with moderate pressure applied during bonding is preferable for producing bonds with less disruption to the microstructures of the parent alloy. Joint shear tests revealed that a near parent alloy strength can be achieved. This study also shed some light on choosing the right bonding parameters suitable for joining the complicated alloy systems. A Ni-P interlayer was also used to bond the ODS alloy. Microstructural examination indicated that a thin joint width and less disruption to the parent grain structure were achieved when bonding the alloy in the fine grain state. The time for isothermal solidification was found to be shorter when compared with bonds made with the parent alloy in the recrystallized state. All these observations were attributed to the greater diffusivity of P along the grain boundaries than that of the bulk material. A high Cr content within the parent alloy changes the mechanism of the bonding process. The diffusion of Cr into the liquid interlayer has the effect of raising the solidus temperature, which not only accelerates the isothermal solidification process, but also reduces the extent of parent alloy dissolution.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第17914号 / エネ博第286号 / 新制||エネ||59(附属図書館) / 30734 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 小西 哲之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Les alliages ODS (Oxide Dispersion Strengthened) sont des matériaux candidats pour la réalisation du gainage combustible des réacteurs nucléaires de Génération IV de type RNR-Na. Leurs propriétés mécaniques à haute température sont assurées par une dispersion d'oxydes nanométriques qui peut être modifiée sous l'effet de la température et notamment lors du passage par l'état liquide. Les procédés d'assemblage en phase solide sont donc préconisés. La méthode de soudage étudiée dans cette thèse est le soudage par résistance en bout. Une double approche simulation numérique-expérimentation a été mise en place pour étudier l'influence des paramètres opératoires sur le soudage et la soudure. L'ensemble des essais (expérimentaux et numériques) est réalisé selon la méthode des plans d'expériences factoriels complets. Un alliage ODS base fer à 20 % de chrome a été utilisé comme alliage d'essai.L'analyse de l'influence des paramètres opératoires sur le soudage est réalisée dans un premier temps. Les résultats montrent que l'étape de soudage peut se découper en trois phases. Dans une première phase, la température augmente au niveau des interfaces. Dans une seconde phase, le procédé de soudage est gouverné par le changement de section macroscopique lorsque le courant passe du bouchon massif à la gaine plus étroite. La température augmente alors dans la partie de gaine dépassant de l'électrode. Lorsque la température atteint une valeur suffisamment élevée, la partie de gaine dépassant de l'électrode se déforme, entrainant un affaissement général des pièces et constituant la troisième phase de l'étape de soudage. L'influence des paramètres opératoires sur les phénomènes physiques lors de l'opération d'assemblage est évaluée. Il est alors possible d'influer sur les sollicitations thermiques et mécaniques à l'aide des paramètres opératoires. L'analyse de l'influence des sollicitations thermomécaniques sur la soudure est réalisée dans un second temps. Sur les plages de paramètres opératoires étudiées, les résultats révèlent sur certaines soudures la présence de défauts de compacité ainsi qu'une modification de la microstructure et de la dispersion des oxydes. Les défauts de compacité peuvent être liés à des phénomènes thermiques et mécaniques au contact entre pièces. Les modifications de la microstructure sont liées à des phénomènes de recristallisation dynamique ou de fusion locale et donc à des phénomènes thermiques mais aussi mécaniques avec la présence de déformations importantes. Les modifications de la microstructure sont alors reliées à une modification de la dispersion d'oxydes. A l'aide des paramètres opératoires il est possible d'influer sur les températures et les déformations afin de limiter les modifications de la structure de l'alliage ODS ainsi que l'apparition de défaut de compacité. A l'aide de l'ensemble de ces résultats, la procédure de soudage est adaptée sur un alliage ODS à 9 % de chrome, nuance envisagée pour la réalisation des futurs gainages combustibles. L'influence des propriétés matériaux sur le soudage et la soudure est alors discutée en comparant les deux nuances ODS de compositions différentes mais aussi en comparant les résultats obtenus sur l'alliage ODS à 20 % de chrome avec un alliage non renforcé de composition semblable.

The diploma thesis is focused on determining mechanical properties of alloys produced by powder metallurgy methods. Two alloys were studied, namely a highly entropic CrMnFeCoNi alloy and an oxide dispersion-strenthened FeAlO alloy. Both alloys were made in laboratory amount of test material and miniature test specimens were used to measure their mechanical properties. The main goal of the diploma thesis was to develop a crack preparation methodology, to examine and to evaluate the properties of miniature test specimens for materials with low levels of resilience. In the analytical part of the thesis, tensile and deformation properties and fracture toughness of both alloys were measured. For the highly-entropic CrMnFeCoNi alloy, the thesis focused on the effect of the strain rate on the properties of the alloy. For the oxide dispersion-strenthened FeAlO, the effect of temperature in the 23-600 °C temperature range on its mechanical properties was studied.

The objective of this dissertation is to demonstrate the use of in situ TEM mechanical testing to find mechanical properties of as received, self-ion, and proton irradiated Fe-9%Cr ODS. The desire to work at small scale in the characterization of irradiated materials to reduce costs and improve throughput, require the development of novel methods to assess mechanical properties in volume-limited irradiation damage layers. Yet at these micrometer or nanometer scales, the mechanical properties can begin to be impacted by size effects. In this work micropillar compression, cantilever bending, lamellae indentation, and clamped beam fracture testing is conducted on ion-irradiated Fe-9%Cr ODS to find yield stress, elastic modulus, flow stress, and fracture toughness. Micropillars in compression allow us to define a minimum sample dimension, which approaches the obstacle spacing of the material, at which size effects are observed. This relationship between sample dimension and obstacle spacing defined through micropillar compression is extended to a new testing geometry, cantilever bending, and material property, flow stress. Lessons learned during the cantilever bending informed the clamped beam design for conducting fracture testing on a ductile engineering alloy at micrometer scales. Finally, lamellae indentation was conducted to link qualitative observations of the microstructure under load with literature strength of obstacle values. By combining an understanding of the microstructure of irradiated Fe-9%Cr ODS and the in situ TEM technique, one can find the bulk-like mechanical properties of ion irradiated Fe-9%Cr ODS.

The thesis focuses on exploring the sub-grain structure in stainless steel 316L prepared by additive manufacturing (AM). Two powder-bed based AM methods are involved: selective laser melting (SLM) and electron beam melting (EBM). It is already known that AM 316L has heterogeneous property and hierarchy structure: micro-sized melt pools, micro-sized grains, nano-sized sub-grain structure and nano-sized inclusions. Yet, the relation among these structures and their influence on mechanical properties have not been clearly revealed so far. Melt pool boundaries having lower amount of sub-grain segregated network structures (Cellular structure) are weaker compared to the base material. Compared with cell boundaries, grain boundaries have less influence on strength but are still important for ductility. Cell boundaries strengthen the material without losing ductility as revealed by mechanical tests. Cellular structure can be continuous across the melt pool boundaries, low angle sub-grain boundaries, but not grain boundaries. Based on the above understanding, AM process parameters were adjusted to achieve customized mechanical properties. Comprehensive characterization were carried out to investigate the density, composition, microstructure, phase, magnetic permeability, tensile property, Charpy impact property, and fatigue property of both SLM and EBM SS316L at room temperature and at elevated temperatures (250°C and 400°C). In general, SLM SS316L has better strength while EBM SS316L has better ductility due to the different process conditions. Improved cell connection between melt pools were achieved by rotating 45° scanning direction between each layer compared to rotating 90°. Superior mechanical properties (yield strength 552 MPa and elongation 83%) were achieved in SLM SS316L fabricated with 20 µm layer thickness and tested in the building direction. Y2O3 added oxide dispersed strengthening steel (ODSS) were also prepared by SLM to further improve its performance at elevated temperatures. Slightly improved strength and ductility (yield strength 574 MPa and elongation 90%) were obtained on 0.3%Y2O3-ODSS with evenly dispersed nanoparticles (20 nm). The strength drops slightly  but ductility drops dramatically at elevated temperatures. Fractographic analysis results revealed that the coalescence of nano-voids is hindered at room temperature but not at elevated temperatures. The achieved promising properties in large AM specimens assure its potential application in nuclear fusion. For the first time, ITER first wall panel parts with complex inner pipe structure were successfully fabricated by both SLM and EBM which gives great confidence to application of AM in nuclear industry. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.</p>

Dans le cadre du développement de matériaux de gainage combustible pour les réacteurs à neutrons rapides refroidis au sodium (RNR-Na), cette étude s’intéresse au développement d’un procédé innovant d’élaboration d’aciers renforcés par dispersion d’oxyde (aciers ODS) par mécanosynthèse qui doit éviter l’apparition de précipités de tailles micrométriques indésirables dont la présence dégrade drastiquement les propriétés mécaniques. Ce nouveau procédé consiste à introduire directement des nanoparticules d’oxydes de structure pyrochlore Y2Ti2O7 dans une poudre métallique Fe-Cr via un broyage. Pour cela, un procédé de synthèse des oxydes de structure pyrochlore Y2Ti2O7 par co-broyage des poudres nanométriques Y2O3 et TiO2 a d’abord été mis en place au laboratoire. L’optimisation des conditions de broyage a permis d’obtenir une poudre nanostructurée de nature voulue et de grande pureté. Grâce au produit obtenu, la gamme de fabrication innovante d’aciers ODS proposée a pu être développée puis validé, à petite échelle puis à l’échelle semi-industrielle. Ce nouveau procédé a été utilisé pour étudier l’impact des conditions de broyage et de la composition chimique sur les caractéristiques des matériaux produits, et a permis d’établir le lien entre conditions d’élaboration, microstructure et propriétés mécaniques. Cette étude permet d’abord de valider le mode d’introduction des renforts sous la forme Y2Ti2O7 comme une méthode efficace pour l’obtention d’aciers ODS performants. De plus, elle révèle l’importance des conditions d’élaboration, et en particulier du broyage, sur la microstructure et les propriétés mécaniques des aciers ODS. Les résultats mis en avant ouvrent des perspectives prometteuses en ce qui concerne l’optimisation des conditions d’élaboration des aciers ODS. / This study concerns the development of an innovative manufacturing process for oxidedispersion strengthened steel (hereafter referred to as “ODS”) by mechanosynthesis. As part of materials development for Sodium-cooled Fast Reactors (SFR), the aim is to prevent the growth of undesirable micrometric precipitates which can drastically degrade steel’s mechanical properties. This new process introduces Y2Ti2O7 pyrochlore oxide nanoparticles directly into Fe-Cr metallic powder through mechanical milling. To achieve this, a process has been set up to synthesize Y2Ti2O7 pyrochlore oxides via mechanical alloying of nanosized Y2O3 and TiO2 powders. Optimization of the milling parameters enabled the production of a pure nanostructured powder of the desired nature. Thereafter, the innovative ODS steel manufacturing process was validated on a small and then semiindustrial scale. This new process was used to study the impact of milling parameters and chemical composition on the produced material’s features and enabled the establishment of a link between milling conditions, microstructure and mechanical properties. This study first enables the validation of the method of introducing precipitates in the form of Y2Ti2O7 oxides as an efficient way to obtain competitive ODS steel. Moreover, it reveals the importance of the features of the manufacturing process, particularly milling conditions, on the microstructure and mechanical properties of ODS steel. The results reveal promising perspectives concerning the features of ODS steel manufacturing.

Reduced activation oxide dispersion strengthened (ODS) steels are candidate alloys for use in fusion reactor systems and are fabricated by mechanically alloying yttrium oxide to a reduced activation ferritic steel powder. The product is consolidated at high temperature by hot isostatic pressing (HIP), producing a dispersion of nanometre sized oxide particles throughout the ferritic microstructure. These particles have been shown to both improve the high temperature mechanical properties of the alloy and provide trapping sites for helium gas. The use of these particles to sequester helium is of particular significance in the development of a structural ODS steel for fusion reactor systems. A fusion power reactor, based on the ITER design, is expected to produce over 2000 appm transmutant helium in any steel components exposed to the core neutron flux. At these gas concentrations, conventional steels undergo severe swelling and embrittlement, motivating the development of materials capable of managing helium accumulation. This thesis investigates the use of the oxide particle dispersion in sequestering helium introduced by ion implantation. An initial characterisation of a model Fe-14Cr-0.25Y₂O₃ (wt&percnt;) system was completed using high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). This demonstrated the efficacy of the production methods and the gas trapping capabilities of the oxide particles via argon gas, introduced during the mechanical alloying process. The subsequent consolidation of a full set of Fe-14Cr-3W-0.2Ti-0.25Y₂O₃ (wt&percnt;) ODS alloys at 1150&deg;C, 1050 &deg;C and 950 &deg;C produced a systematic variation in the density of the particle dispersion. The characterisation of these materials using APT provided an insight into the consistent Y₂Ti₃O₅ particle chemistry found in each consolidation, and identified a stoichiometric shift from Y₂Ti₃O₅ to YTiO2 following short term annealing periods at 1000&deg;C. Though further work is required, this shift is thought to be consistent with a thermodynamically mediated transition of the metastable clusters to stable oxide particles. Following implantation with 2000 appm helium and examination under TEM, the helium bubble and particle densities were found to be closely correlated thus providing evidence for an association between the particles and the gas bubbles. Controlling the helium bubble density via the particle dispersion demonstrates the potential use of processing temperature in controlling how helium accumulates in an implanted ODS microstructure. The effects of both bubble and particle densities on mechanical properties were investigated further using nanoindentation methods. Significant local variation in the hardness of the ODS steels was found to result from the bimodal grain size distribution of the material. By using only those measurements taken from large grained regions of the ODS, the grain refinement and particle hardening effects could be deconvolved and used to quantify particle hardening using a dispersed barrier model. The significant hardening effects with helium addition observed in the reference alloys were found to be almost entirely absent from the ODS systems, though anomalous softening in the 950&deg;C consolidation indicated a potentially unexpected interaction between the bubble and particle populations. A possible explanation for this anomaly and a proposal for further work to establish its origin is discussed.