Les aciers renforcés par dispersion d’oxydes (ODS) sont des matériaux candidats pourconstituer les gaines de combustibles des réacteurs nucléaires de IVème génération refroidis au sodium(RNR-Na). Le soudage par résistance en bout de ces gaines est étudié dans ce travail, avec pourobjectif la détermination et la compréhension des effets du procédé sur l’évolution de la microstructuredes aciers ODS à 9 % et 14 % de chrome à l’échelle des grains et des oxydes nanométriques. Uneapproche couplant caractérisations microstructurales sur soudures, simulation numérique et simulationphysique du procédé, à l’aide d’un simulateur thermomécanique Gleeble 3500, est adoptée. Le soudagepar résistance en bout impose localement des conditions de sollicitations sévères en termes dedéformation, vitesse de déformation et température. Ces sollicitations induisent un affinement de lamicrostructure dont l’origine peut être attribuée à un mécanisme de recristallisation dynamique (acier à14 % de chrome) ou à l’association de la recristallisation dynamique et des transformations de phase(acier à 9 % de chrome). Les conditions d’occurrence de la recristallisation dynamique sont étudiéessur ces matériaux. On montre notamment la possibilité d’une transition entre recristallisation dynamiquecontinue et discontinue sur l’acier à 14 % de chrome en fonction des conditions de sollicitation. Il estégalement observé que ces sollicitations thermomécaniques sévères induisent une augmentation de lataille des oxydes nanométriques associée à une diminution de leur fraction volumique. / Oxide dispersion strengthened (ODS) steels are considered as candidate materials for thedevelopment of fuel cladding for sodium-cooled fast reactors (SFR). Resistance upset welding of thecladding is studied in this work. The aim is to determine and to understand the process effects on themicrostructure of ODS steels with 9% and 14% of chromium at the scales of the grains and thenanometric oxides. An approach coupling microstructural characterization of welds, numericalsimulation and physical simulation of the process, using a thermomechanical simulator Gleeble 3500,is proposed. Resistance welding locally imposes severe thermomechanical conditions in terms of strain,strain rate and temperature. Refinement of the microstructure is noted and correspond to a dynamicrecrystallization mechanism (14 % Cr steel) or the combination of dynamic recrystallization and phasetransformations (9 % Cr steel). The conditions of occurrence of dynamic recrystallization are studied.The possibility of a transition between continuous and discontinuous dynamic recrystallization is shownfor the 14 % Cr steel according to the loading conditions. Such severe thermomechanical conditionsinduce an increase in the size of nanoscale oxides associated with a decrease of their volume fraction.

Made available in DSpace on 2016-08-18T18:19:27Z (GMT). No. of bitstreams: 1 LUCIENE ARAUJO NASCIMENTO.pdf: 1402629 bytes, checksum: b9ce204098b2c4fe0362b6792a692744 (MD5) Previous issue date: 2010-02-25 / FUNDAÇÃO DE AMPARO À PESQUISA E AO DESENVOLVIMENTO CIENTIFICO E TECNOLÓGICO DO MARANHÃO / Austenitic stainless steels have low economic value compared to titanium and its alloys, considered the most resistant to corrosion in biological environment, and for over fifty years are widely used in the manufacture of orthopedic implants, particularly in public health service. Currently the ISO 5832-1 steel (ASTM F 138) is the most used in the manufacture of orthopedic prosthesis, especially the temporary ones. However, because it is susceptible to localized corrosion when in contact with human tissue, it is being gradually replaced by austenitic stainless steel of low carbon and high nitrogen called ISO 5832-9. The same is already used extensively in Europe and the United States, while in Brazil it's use is more recent and smaller scale. This work investigates the behavior of dynamic recrystallization in an ISO 5832-9 steel through hot torsion tests and optical microscopy, for different conditions of temperature, strain and strain rate. It was found through the plastic flow curves of the studied material crystallizes dynamically and that the high value of its apparent activation energy for deformation can be attributed to the presence of a large amount of precipitated particles and nitrogen dissolved in the matrix. Micrographs confirmed that tests carried out at low temperatures reveal a strong retardation of dynamic recrystallization. Moreover, the presence of serrated grain boundaries and nucleation of new grains in the old deformed grain boundaries, are strong indications that the recrystallization occurred by a necklace mechanism. The behavior of the average recrystallized grain size, DDRX, with temperature resembles the behavior of the austenitic grain growth in microalloyed steels. The presence of a minimum in the behavior of DDRX with the strain rate can be attributed to a minimum strain rate required to cause the greatest amount of dynamics precipitation. / Os aços inoxidáveis austeníticos possuem um baixo valor econômico quando comparados ao titânio e suas ligas, considerados os mais resistentes à corrosão em meio biológico, e há mais de cinquenta anos são amplamente utilizados na confecção de implantes ortopédicos, em especial no serviço de saúde pública. Atualmente o aço ISO 5832-1 (ASTM F 138) é o mais utilizado na confecção de próteses ortopédicas, principalmente as temporárias. Entretanto, devido o mesmo ser suscetível à corrosão localizada quando em contato com tecido humano, ele está sendo gradativamente substituído pelo aço inoxidável austenítico de baixo teor de carbono e alto teor de nitrogênio denominado de ISO 5832-9. O mesmo já é utilizado em larga escala na Europa e nos Estados Unidos, enquanto no Brasil a sua utilização é mais recente e em menor escala. Este trabalho investiga o comportamento da recristalização dinâmica em um aço ISO 5832-9, através de ensaios de torção a quente e microscopia ótica, para diferentes condições de temperatura, deformação e taxa de deformação. Foi constatado através das curvas de escoamento plástico que o material estudado recristaliza dinamicamente e, que o alto valor da sua energia de ativação aparente para a deformação pode ser atribuído à presença de uma grande quantidade de partículas de precipitados e ao nitrogênio em solução na matriz. Micrografias confirmam que ensaios realizados a baixas temperaturas revelam um forte retardamento da recristalização dinâmica. Além disso, a presença de contornos de grão serrilhados e nucleação de novos grãos nos antigos contornos de grãos deformados, são fortes indícios de que a recristalização ocorreu por mecanismo colar. O comportamento do tamanho de grão médio recristalizado, DDRX, com a temperatura, se assemelha ao comportamento do crescimento de grão austeníticos de aços microligados. A presença de um mínimo no comportamento do DDRX com a taxa de deformação pode ser atribuído a uma taxa de deformação mínima necessária para que ocorra a maior quantidade de precipitação dinâmica.

Les pièces métalliques constituant le circuit primaire des installations nucléaires sont élaborées par forgeage à chaud. Pendant ce procédé, les transformations microstructurales induites par la déformation et les recuits déterminent une partie des propriétés mécaniques des produits finaux. L’orientation de la microstructure lors du processus de fabrication nécessite une connaissance précise des mécanismes physiques qui opèrent dans le matériau. Dans le cas de la déformation à chaud de l’acier austénitique 304L, ces modifications microstructurales dépendent de la recristallisation dynamique discontinue (DDRX) et de la recristallisation post-dynamique (PDRX). L’objet de ce projet est : (i) l’étude de la DDRX et de la PDRX dans les conditions de déformation du procédé de forgeage, (ii) l’étude de l’influence d’un ajout de niobium sur ces mécanismes, (iii) la modélisation de ces mécanismes afin de prédire les caractéristiques de la microstructure (moyenne et distribution de la taille des grains) à l’issue d’un procédé multipasses. Dans le cadre de l’étude, les conditions de déformation rencontrées lors du forgeage à chaud sont reproduites à l’aide d’essais de torsion sur des matériaux modèles contenant des teneurs en niobium différentes. La caractérisation et la modélisation des microstructures a permis de comprendre les effets respectifs de la température, de la vitesse de déformation ainsi que de l’ajout de niobium sur les mécanismes de la DDRX et de la PDRX. Dans cette étude, une nouvelle approche semi-topologique de l’hypothèse champs moyens est développée afin de permettre la prédiction de distributions de la taille de grain cohérentes avec les données expérimentales. / Cooling system of nuclear power plants is constituted of metallic parts obtained by hot forging. Thus during the manufacturing process, the microstructural trans- formations induced by the deformation and annealing process define partially the mechanical properties of the final products. A sharp knowledge of the physical mechanisms generated within the material is required to handle the microstructure. In the case of hot deformation of 304L austenitic stainless steel, the microstructural modifications depend on the discontinuous dynamic recrystallization (DDRX) and the post-dynamic recrystallization (PDRX). The aim of this project is: (i) the study of the DDRX and the PDRX under the conditions of deformation inherent in the forging process, (ii) the study of the influence of niobium addition on these mechanisms, (iii) the modeling of these me- chanisms in order to predict the microstructure characteristics (mean grain size and distribution) following a multipass process. As part of the research, the deformation conditions experienced during the hot forging process are replicated through torsion tests with model materials containing various niobium concentrations. Characterization and modeling of microstructures enable to understand the respective e ects of temperature, strain rate as well as niobium addittion on the DDRX and PDRX mechanisms. In this study, a new topological approach of mean-field hypothesis is developed in order to allow the prediction of realistic grain size distributions.

No description available.

In this study polycrystalline NiAl has been subjected to torsion deformation. Torsion has been used because of its characteristics. By this deformation mode high shear strains (gamma = 18 in this study) can be imposed on the sample. The deformation conditions are well-defined because of the local deformation mode, which is simple shear. Due to the monoclinic sample symmetry one half of the pole figure is needed in order to obtain the complete texture information, which is more than is needed e.g. by extrusion or rolling. Therefore, texture analysis might be more sensitive with respect to texture components. Furthermore, torsion deformation is characterized by being inhomogeneous in terms of the amount of shear strain and shear strain rate along the sample radius. The shear strain gradient makes the analysis of different stages of deformation on the same sample (i.e. under the same deformation conditions) possible. Another characteristic being special for torsion is that samples change their length, although no axial stress is applied. This effect is known as Swift effect and will be analyzed in detail. The deformation, microstructure and texture development subject to the shear strain are studied by different techniques (Electron Back-Scatter and High Energy Synchrotron Radiation). Beside the development of microstructure and texture with shear strain, the effect of an initial texture as well as the deformation temperature on the development of texture and microstructure constitute an important part of this study. Therefore, samples with three different initial textures were deformed in the temperature range T = 700K – 1300K. The development of the microstructure is characterized by two different regimes depending on the deformation temperature T. For T up to 1000K, continuous dynamic recrystallization (CDRX) takes place. This mechanism leads to the deformation-induced dislocations forming low angle grain boundaries (LAGBs) or being incorporated into them and the successive transformation of these boundaries into high angle grain boundaries (HAGBs) by a further increase of their misorientation. The predictions of this model were compared with the experimental results. The shear stress – shear strain curves are characterized by a peak at low strains, which is followed by softening and a steady state at high strains. This condition is fulfilled for a number of samples, but especially <111> oriented samples do not show a softening stage at low temperatures. Grain refinement takes place for all samples and the average grain size decreases with temperature. The predicted LAGB decrease is in best agreement with the experiments at the lowest temperatures (T = 700K and 800K). Deviations from the model can be explained by the temperature dependence of the grain boundary mobility. For temperatures T > 1000K, discontinuous dynamic recrystallization (DDRX) occurs, by which new grains form by nucleation and subsequent growth. The texture is characterized by two components, {100}<100> (cube, C) and {110}<100> (Goss, G). The intensity of G increases with temperature, while that of C decreases independent of the initial orientation. Both components have their maximum deviated about the 1 axis. The deviation is larger for grains containing the C component and decreases with temperature. Grains containing the G component have the smaller deviation, which decreases with temperature and strain. Texture simulations based on the full constraint Taylor model under the assumption of {110}<100> and {110}<110> slip were done with the experimental <110> and <111> fibres as well as a theoretical <100> fibre and a {100}<100> single orientation (ideal as well as rotated about the torsion axis). The G component is predicted by the simulations and is therefore a deformation texture. However the C component does not appear in the simulation. It therefore must originate by different mechanisms. For the non-<100> oriented samples, possibly nucleation is responsible for the formation of C oriented nuclei. Simulations with single orientations lead to the conclusion, that the ideal C orientation rotates about the 1 axis, while other C orientations, which are rotated about the torsion axis, increasingly converge towards the G component with strain. A single G orientation on the other hand is stable against such a rotation and is therefore the most likely steady state texture. Based on these results it is proposed, that ideally C oriented nuclei rotate until an orientation is reached into which they grow. These new grains are further rotated up to a critical angle, at which a part of them disappears either by adjacent grains or new C oriented nuclei. The recrystallization texture for T > 1000K is most likely the C component as well. Torsional creep of NiAl is characterized by a stress exponent, which depends on temperature and an activation energy, which is stress dependent. A model incorporating both dependencies is proposed and applied to the creep data. It is shown that these equations are able to describe the experimental findings. Thus creep of NiAl based on this model is dominated by non-diffusional processes such as cross slip of <100> screw dislocations for T  1000K. For T > 1000K the stress exponent and the activation energy are in a region, which according to previous reports is rather dominated by dislocation-climb controlled creep. The Swift effect, due to which samples change their axial dimension during torsion without applied axial stress, is observed for NiAl. It is strongly related to the texture development and in the case of NiAl the C component is identified as being responsible for shortening, whereas the G component leads to lengthening as long as it is not aligned with the shear system. Both tendencies can be explained based on the active slip systems. Simulations fail to predict the experimental observation, because the C component is not present. HESR and EBSD were compared with respect to local texture measurements. It was concluded depending on the average grain size HESR has an advantage in terms of grain statistics. For DDRX samples however, both methods are limited. Local texture inhomogeneities can be better detected using EBSD, whereas for an overall local texture information HESR is better suited.

No description available.

The current study focus on investigating a relationship between processing variables and microstructure evolution mechanism in 304L stainless steel subjected to hot torsion. The Gleeble 3800 with Mobile Torsion Unit (MTU) is utilized in the current study to conduct hot torsion test of 304L stainless steel. Samples are rotated at 1100℃ in the shear strain rate range of 0.02s-1 to 4.70s-1 and the shear strain range of 0.5 to 4. Orientation imaging microscopy (OIM) technique is used to collect and analyze the microstructure. At low strains (≤1) and strain rate (0.02s-1), average grain size remains relatively constant, but the lengths of DSs and LABs increase within grains. These are characteristics of the dynamic recovery (DRV). With increasing strain and strain rate, the lengths of DSs, LABs and HABs increase, accompanied by the decrease of average grain size. Subgrains with HAB segments are observed. These are characteristics of continuous dynamic recrystallization (CDRX). At strain rates greater than or equal to 0.94s-1, the fraction of deformation texture is about 3 times higher than that of rotated cube texture. The average grain size increases relative to that at a strain rate of 0.20s-1, accompanied by the increase of twin length per area. This indicates that grain growth take place after CDRX. Sigma phase is not observed in the current study due to the lack of static recrystallization (SRX) and the higher cooling rate.

Alloy 617 displays a ductility dip during straining at exactly 1000˚C, leading to brittle fracture. A sudden decrease in ductility appearing during Gleeble hot ductility tests of Ni-based superalloys is a well-known phenomenon, while its cause is unknown. Many mechanisms have been established as possible contributors to the issue, and in later years not one, but the simultaneous presence of several of these mechanisms were confirmed as the cause. The ductility dip leads to solid state cracking and a specific solid state cracking phenomenon known as ductility dip cracking is specifically common in Ni-based superalloys. Ductility dip cracking is identified by intergranular cracks and the occurrence of specific precipitates, among other things. This work investigates the possibility that the decreased ductility is due to ductility dip cracking. Furthermore, other possible explanations are investigated. Visual examination was conducted through LOM, SEM and chemical analysis using EDS technique. Combined with thermodynamic calculations, the existence of Cr-rich M23C6 carbides, Ti(N,C) and Mo-rich particles, most likely M3B2, were confirmed. Further, it is established that the ductility dip is related to the lack of dynamic recrystallization at 1000˚C. It is not confirmed that the ductility dip in alloy 617 is due to ductility dip cracking. / Nickelbaslegeringen 617 uppvisar en minskning i duktilitet under Gleeble-dragprovning vid exakt 1000˚C vilket leder till sprött brott. En plötslig sänkning av duktiliteten vid varmdragning av Ni-baserade superlegeringar är ett välkänt fenomen, dock är orsaken inte fastställd. Många mekanismer har bekräftats som bidrag till problemet och under de senaste åren har den simultana närvaron av fler av dessa mekanismer bekräftats som orsaken. Sänkningen i duktilitet leder till sprickbildning i fast fas och en specifik typ av sprickbildning känd som ”ductility dip cracking” är speciellt förekommande i Ni-bas legeringar. Denna identifieras bland annat genom intergranulära sprickor och närvaron av specifika utskiljningar. Detta arbete undersöker möjligheten att duktilitetssänkningen beror på  ”ductility dip cracking”. Dessutom undersöks fler tänkbara förklaringar. Visuell granskning genomfördes via LOM och SEM och analys av sammansättningar via EDS-analys. I kombination med termodynamiska simuleringar blev förekomsten av Cr-rika M23C6 karbider, Ti(N,C) och Mo-rika partiklar, troligtvis M3B2, bekräftad. Fortsatt är det bekräftat att duktilitetssänkningen är relaterat till avsaknaden av rekristallisation vid 1000˚C. Det är inte bekräftat i detta arbete att duktilitetssänkningen i legering 617 beror av ”ductility dip cracking”.

In the present work, deformation behavior, texture and microstructure evolution of commercially pure titanium (CP Ti) are investigated by electron backscattered diffraction (EBSD) after compression tests at elevated temperatures. By analysing work hardening rate vs. flow stress, the deformation behaviour can be divided into three groups, viz. three-stage work hardening, two-stage work hardening and flow softening. A new deformation condition map is presented, dividing the deformation behavior of CP Ti into three distinct zones which can be separated by two distinct values of the Zener-Hollomon parameter. The deformed microstructures reveal that dynamic recovery is the dominant deformation mechanism for CP Ti during hot working. It is the first time that the Schmid factor and pole figures are used to analyse how the individual slip systems activate and how their activities evolve under various deformation conditions. Two constitutive equations are proposed in this work, one is for single peak dynamic recrystallization (DRX), the other is specially for CP Ti deformed during hot working. After the hot compression tests, some stress-strain curves show a single peak, leading to the motivation of setting up a DRX model. However, the examinations of EBSD maps and metallography evidently show that the deformation mechanism is dynamic recovery rather than DRX. Then, the second model is set up. The influence of the deformation conditions on grain size, texture and deformation twinning is systematically investigated. The results show that {10-12} twinning only occurs at the early stage of deformation. As the strain increases, the {10-12} twinning is suppressed while {10- 11} twinning appears. Three peaks are found in the misorientation frequency-distribution corresponding to basal fiber texture, {10-11} and {10-12} twinning, respectively. A logZ-value of 13 is found to be critical for both the onset of {10-11} compressive twinning and the break point for the subgrain size. The presence of {10-11} twinning is the key factor for effectively reducing the deformed grain size. The percentage of low angle grain boundaries decreases with increasing Z-parameter, falling into a region separated by two parallel lines with a common slope and 10% displacement. After deformation, three texture components can be found, one close to the compression direction, CD, one 10~30° to CD and another 45° to CD. / QC 20100819

En usinage, la zone de coupe présente des conditions de température, des cinétiques thermiques, des déformations et des pressions extrêmes. Dans ce contexte, être capable de relier les variations des conditions de coupe (vitesse de coupe, avance, lubrification, usure, outil,…) à l’intégrité de la surface usinée constitue un objectif scientifique majeur. Cette thèse s’intéresse au cas du tournage finition du 15-5PH (acier inoxydable martensitique utilisé, entre autre, pour la fabrication des pièces de rotor d’hélicoptère ainsi que les pompes et les vannes de circuit primaire de centrale nucléaire) et s’inscrit dans le cadre du projet MIFSU (Modélisation de l’Intégrité et de la Fatigue des Surfaces Usinées).Dans un premier temps, le comportement du matériau a été étudié afin d’alimenter les simulations d’usinage. Des essais de dilatométrie libre ont été conduit afin de calibrer les cinétiques d’austénitisation du 15-5PH pour des vitesses de chauffe élevées (jusqu’à 11000 °C/s). Les paramètres du modèle de changement de phase de Leblond ont alors été identifiés. De plus, des essais de compression dynamique (dε/dt allant de 0.01 à 80 /s et ε > 1) ont été réalisés pour calibrer une loi de comportement élasto-plastique aux grandes déformations avec une sensibilité à la vitesse de déformation. Ces essais ont aussi permis de mettre en évidence des phénomènes de recristallisation dynamique et leurs influences sur la contrainte d’écoulement du matériau. Un modèle de recristallisation dynamique a donc également été mis en œuvre.En parallèle, un modèle numérique de prédiction de l’intégrité des surfaces tournées a été construit. Ce modèle repose sur une méthodologie dite « hybride » (développée au cours de la thèse Frédéric Valiorgue pour l’acier AISI 304L) qui consiste à supprimer la modélisation de l’outil de coupe et de la formation du copeau, et à remplacer l’impact thermomécanique de ces derniers sur la surface usinée par des chargements équivalents. Une étape de calibration de ces chargements a donc été réalisée à travers des essais de coupe orthogonale et de frottement (étude de sensibilité des efforts d’usinage, du coefficient de frottement et du coefficient de partage thermique) aux variations des paramètres de coupe.Enfin, les résultats des simulations numériques de tournage portant sur la prédiction des changements de microstructure (austénitisation et recristallisation dynamique) ainsi que des contraintes résiduelles ont été comparés aux résultats issus d’une campagne d’essais de chariotage. / During machining, extreme conditions of pressure, temperature and strain appear in the cutting zone. In this thermo-mechanical context, the link between the cutting conditions (cutting speed, lubrication, feed rate, wear, tool coating…) and the machining surface integrity represents a major scientific target. This PhD study is a part of a global project called MIFSU (Modeling of the Integrity and Fatigue resistance of Machining Surfaces) and it focuses on the finish turning of the 15-5PH (a martensitic stainless steel used for parts of helicopter rotor). Firstly, material behavior has been studied in order to provide data for machining simulations. Stress-free dilatometry tests were conducted to obtain the austenitization kinetics of 15-5PH steel for high heating rates (up to 11,000 ° C/s). Then, parameters of Leblond metallurgical model have been calibrated. In addition, dynamic compression tests (dε/dt ranging from 0.01 to 80/s and ε > 1) have been performed to calibrate a strain-rate dependent elastoplasticity model (for high strains). These tests also helped to highlight the dynamic recrystallization phenomena and their influence on the flow stress of the material. Thus, recrystallization model has also been implemented.In parallel, a numerical model for the prediction of machined surface integrity has been constructed. This model is based on a methodology called "hybrid" (developed during the PhD thesis of Frédéric Valiorgue for the AISI 304L steel). The method consists in replacing tool and chip modeling by equivalent loadings (obtained experimentally). A calibration step of these loadings has been carried out using orthogonal cutting and friction tests (with sensitivity studies of machining forces, friction and heat partition coefficients to cutting parameters variations).Finally, numerical simulations predictions of microstructural changes (austenitization and dynamic recrystallization) and residual stresses have been successfully compared with the results of an experimental campaign of turning.