Zr-2.5Nb is used as the pressure tube material for 2nd and 3rd generation CANDU reactors. The physical properties of pressure tubes in service, including strength, dimensional stability, and delayed hydride cracking resistance, are largely dependent upon the crystallographic texture of the hcp α-phase, whose texture is predominantly developed during the extrusion stage of manufacturing. During extrusion and subsequent cooling, the formation of α may occur by transformation of the bcc β-phase to α according to the Burgers relationship and influenced by variant selection – that is, a preference for one or more of the twelve possible orientations of the hcp lattice relative to the bcc lattice. Variant selection has been observed in other Zr alloys, including the heat-treated zone in pressure tube welds and the bulk texture of heat-treated pressure tubes. Further, it has been proposed as a possible explanation for texture characteristics in pressure tubes that are not explained by the deformation mechanics of extrusion. However, the criteria for variant selection are unclear. In this work, an understanding of the criteria for variant selection is developed through observations of the differing mechanisms at play during both directions of transformation, from α-β and β-α. Transformation via the Burgers relationship was confirmed; the existence of variant selection is also established. In thermal cycles to the β-regime, this selection manifests as the selection of a new (0002) variant, as driven by anisotropic thermal stresses generated during heating. Upon cooling, the high-temperature β texture is inherited by the α grains via the Burgers relationship; the magnitude of the texture maxima is driven by elastic transformation strains. Further thermal cycles to the β regime demonstrate texture memory, with some development of cubic symmetry due to grain growth during the hold in the β-phase. No texture changes are observed if samples are not heated fully into the beta regime. Finally, a study of the biasing effects of both residual and external stresses is discussed. While the external stress did not appear to be capable of biasing variant selection during either heating or cooling, some texture changes were observed, likely due to deformation at high temperature. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-09-23 20:44:24.784

Zirconium alloys are widely used by the nuclear industry as fuel cladding and structural materials. Many physical and metallurgical properties of zirconium alloys, that are important for their performance in nuclear reactors, are affected by crystallographic texture due to the strong anisotropy of individual crystals. Irradiation assisted growth is one example. Zirconium crystals deform anisotropically under irradiation, which in the presence of strong textures (like the ones observed in cold-rolled sheet) causes undesirable deformation of components during service. For this reason, the nuclear industry is interested in developing thermomechanical processes that produce random textures, taking advantage of the allotropic phase transformation undergone by zirconium, from the low temperature hcp alpha-phase to the high temperature bcc beta-phase. One of these processes is beta-quenching, which has showed certain success in weakening strong rolling textures. However, there is no consensus about the fundamental mechanisms involved.The aim of this work is to study the evolution of the texture of the zirconium alloy Zircaloy-2 during beta-quenching, in order to gain understanding on the mechanisms involved on texture development and evolution during the alpha-to-beta and beta-to-alpha phase transformations. Firstly industrially beta-quenched samples were characterised using well known techniques such as laboratory X-ray diffraction (LXRD) and electron backscatter diffraction (EBSD), which revealed a relationship between peak temperature and the inherited alpha texture. An in situ synchrotron X-ray diffraction (SXRD) experiment provided, for the first time, information of texture evolution of zirconium during rapid changes and at non-ambient conditions. Different peak temperatures and stress/strain conditions were tested. Detailed post mortem EBSD characterisation of samples studied in situ provided insight on the relationship between the microstructure and the texture. Finally, laboratory furnaces were used to beta-quench samples at very high temperature. It was found that there is selection of orientation variants during beta-quenching of zirconium, but while the selection during the alpha-to-beta transformation is almost negligible, depending on the texture evolution of the beta-phase (affected by grain growth and/or plastic deformation), diverse mechanisms of variant selection act during the beta-to-alpha phase transformation. The inherited textures observed result from the combination of these mechanisms. Some of the results of this work can be transferred to other systems such as titanium and the alpha-gamma-alpha phase transformation in steel.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Neste trabalho, um estudo da influencia da seleÃÃo de variantes sobre a cristalografia apÃs a transformaÃÃo martensÃtica no Maraging foi feito. O estudo cobriu a transformaÃÃo sob deformaÃÃo elÃstica e tambÃm sob deformaÃÃo plÃstica. No aÃo Maraging, austenita se transforma em martensita eu uma temperatura em torno de 200ÂC, nÃo importando a velocidade de resfriamento. Para simular a transformaÃÃo durante a deformaÃÃo elÃstica, ensaios de traÃÃo foram feitos em um forno acoplado a uma mÃquina de ensaios universais com uma tensÃo aplicada abaixo do limite de escoamento do material. A amostra foi aquecida atà 850ÂC, o forno foi aberto e a amostra resfriada ao ar sob tensÃo constante. Para estudar a influencia da deformaÃÃo plÃstica antes da deformaÃÃo, amostras foram deformadas plasticamente na temperatura acima da Ms (temperatura de inÃcio da martensita), a forÃa externa atuando sobre a amostra foi removida e o material sofreu a transformaÃÃo martensÃtica por resfriamento ao ar. Figuras de pÃlos foram medidas por EBSD (Electron Back-Scatter Diffraction) em ambas as condiÃÃes e comparadas com as figuras de pÃlos calculadas assumindo os modelos de Patel-Cohen e Taylor-Bishop-Hill. A orientaÃÃo da austenita mÃe foi obtida por envelhecimento na temperatura de 650ÂC e pelo uso de grÃos austenÃticos reconstruÃdos matematicamente. Os resultados mostraram que o modelo de Patel-Cohen foi mais adequado para a deformaÃÃo elÃstica enquanto que Taylor-Bishop-Hill foi melhor para a deformaÃÃo plÃstica. / In this work a study of the influence of variant selection on the crystallography after martensitic transformation in Maraging was studied. The study covered both the transformation under elastic deformation and also during plastic deformation. In Maraging steel, austenite becomes martensite at a temperature around 200oC regardless of the cooling speed. To simulate the transformation during elastic deformation, a tensile test was performed in a furnace attached to a universal testing machine with an applied stress below the yield strength of the material. The specimen was heated to 850o C, the furnace was opened and the sample cooled in air under a constant stress. To study the influence of plastic deformation before transformation, samples were plastically deformed in a temperature above Ms (martensite start temperature), the external force acting on the sample was removed and the material was allowed to transform into martensite by cooling in air. Pole figures were measured by EBSD (Electron Back-Scatter Diffraction) in both conditions and compared with calculated pole figures assuming Patel-Cohen model and Taylor-Bishop-Hill model. The orientation of the parent austenite was obtained either by reversing the austenite by heating at 650  C and by using the mathematically reconstructed austenite grains. Results showed that Patel-Cohen model were more suitable to elastic deformation while Taylor-Bishop-Hill model was more appropriated to plastic deformation.

CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / O presente trabalho visou estudar o efeito da deformaÃÃo plÃstica prÃvia por laminaÃÃo e o efeito da deformaÃÃo por traÃÃo no processo de seleÃÃo de variantes durante a transformaÃÃo martensÃtica no aÃo inoxidÃvel austenÃtico AISI 301L. Foram comparadas amostras previamente laminadas e deformadas em ensaio de traÃÃo com amostras apenas deformadas plasticamente em traÃÃo. Aspectos relativos a macrotextura obtidos pela tÃcnica de difraÃÃo de Raios-x e microtextura obtidos por difraÃÃo de elÃtrons retro-espalhados (EBSD) foram estudados. A macrotextura analisou a evoluÃÃo das principais componentes de textura da fase martensÃtica a partir do cÃlculo das funÃÃes de distribuiÃÃo de orientaÃÃo cristalogrÃfica (FDOC). As figuras de pÃlos obtidas por EBSD para grÃos selecionados foram comparadas com figuras de pÃlos calculadas utilizando a teoria fenomenolÃgica da cristalografia da transformaÃÃo martensÃtica cuja sigla em inglÃs à (PMTC). O modelo de Patel-Cohen para simular a influÃncia da tensÃo aplicada durante o processo de transformaÃÃo martensÃtica foi utilizado. Os resultados mostraram que a textura da martensita induzida por deformaÃÃo à tÃpica de uma microestrutura cÃbica de corpo centrado (CCC). Mostraram tambÃm que as amostras apenas tracionadas seguiram o modelo de Patel-Cohen para a seleÃÃo de variantes. Jà as amostras prÃ-deformadas por laminaÃÃo apresentaram uma maior dispersÃo em relaÃÃo ao modelo principalmente as que tiveram posteriormente pouca deformaÃÃo plÃstica por traÃÃo. / The present work aimed to study the effect of the plastic prior strain for rolling and the effect of the strain for tensile test in the process of variants selection during the martensitic transformation in the austenitic stainless steel AISI 301L. There were compared samples previously rolled and deformed in tensile test with samples only plastically deformed in tensile test. Relative aspects to macrotexture obtained by the technique of diffraction of X-rays and microtexture obtained by Electron backscattered diffraction (EBSD) were studied. The macrotexture analysed the evolution of the principal texture components of the martensitic phase from the calculation of the orientation distribution function (ODF). The pole figures obtained by EBSD for selected grains were compared with pole figures calculated using the phenomenological theory of martensite crystallography whose acronym in English is (PMTC). The model of Patel-Cohen to simulate the influence of the tension applied during the process of martensitic transformation was used. The results showed that the texture of the deformation induced martensite is typical of a body-centered cubic microstructure. They showed also that the only tensile samples followed the model of Patel-Cohen for the variants selection. The prior deformed samples by rolling presented a bigger dispersal regarding model principally those who subsequently had little plastic deformation for traction.

Mainly two different morphologies of martensite can be obtained in steels depending on the amount of alloying elements. The first morphology, referred to as lath martensite, forms in low alloy, low carbon steels. It is, by far, the most extensively studied form of martensite due to its industrial applications. The second morphology of martensite, referred to as plate martensite, forms in highly alloyed and in high carbon steels and in particular in Fe-High Ni alloys. In this case, the transformation product is disc shaped and internally twinned. This morphology is the only form of martensite that has the potential to exhibit shape memory properties. It is therefore of great interest to understand the mechanisms of its formation. This is investigated in the present dissertation through the study of the martensitic transformation occurring in a Fe-30.5%Ni-0.155%C alloy. More precisely, the influence of stress and grain size on the crystallography of plate martensite is discussed in the general framework of the phenomenological theory of martensite crystallography. This theory allows associating a unique shape deformation to each orientational variant. In this way, the experimental observations carried out at different length scales by means of optical microscopy, EBSD and TEM can be used to infer the transformation path followed under different conditions. Firstly, the burst configurations of variants observed in coarse-grained austenite under stress free conditions are rationalized by considering the mechanical couplings between the variants. It is shown that self-accommodating and autocatalytic couplings are responsible for the formation of hierarchical configurations of variants. More precisely, the transformation is shown to occur through the alternate formation of perpendicular plate groups of variants. Self-accommodation is the dominant coupling between variants of the same plate group while autocatalytic couplings are responsible for the transfer of the transformation from one generation to the next. It is suggested that the plastic accommodation of the shape deformation plays a dominant role in propagating the transformation to a lower length scales. Secondly, the influence of a uniaxial stress state on the transformation is studied. It is seen experimentally that only the most favoured variants are systematically formed in coarse Cube grains while coarse non-Cube grains generally transform into plate groups of variants that are only moderately favoured by the stress. These observations are well explained by considering the interaction energy between the applied stress and the shape deformation associated with the transformation. Thirdly, the influence of the austenitic grain size on the transformation is also studied. A decrease in grain size is seen to decrease the martensite start temperature. For a grain size below about 10µm, the thermal transformation in liquid nitrogen is indeed suppressed in the present alloy. This observation is related to the increasing yield strength of austenite as the grain size is reduced. A noticeable change in the morphology of martensite also accompanies the decrease in grain size. Indeed, martensite forming in coarse-grained austenite is mostly lens shaped and partially twinned while it appears plate shaped and fully twinned in smaller grains. Furthermore, martensite forming in fine-grained austenite develops self-accommodating configurations suggesting that most of the transformation deformations are elastically accommodated in this case. This is believed to be related to the observance of a shape memory effect in the present alloy in its fine-grained condition. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

Titanium alloys are strong candidates for the aerospace industry and biomaterial applications because of their low density, high strength-to-weight ratio and very high strength even at temperatures up to 600°C. Like many other engineering alloys, titanium alloys are prone to strong preferred crystallographic orientation development during thermomechanical processing. Part of the titanium processing route is to heat treat the material above the β transus for the purpose of homogenization and associated phase transformation. This heat treatment dramatically affected the microstructure and texture evolution. Theoretically, such heat treatment should result in a nearly random texture if all variants during α→β→α phase transformation are active. In reality, significant textures are observed after such a heat treatment process. The present project aims at developing a detailed understanding of the root cause for this relatively strong texture by means of EBSD and in-situ neutron diffraction studies. The effect of β grain growth on variant selection during β to α phase transformation has been investigated by using two variants of Ti-6Al-4V with and without 0.4 wt% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate a similar starting microstructures and crystallographic textures. Subsequently, both materials were solution heat treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Detailed EBSD and in situ neutron diffraction analysis were carried out to study microstructure and texture evolution. The variant selection calculation suggests that more variant selection occurred in convectional material with a large grain size compared to material with yttrium addition. In situ measurements showed that β texture strengthened significantly above the β transus with increasing β grain size. There was no significant variant selection during α→β transformation; variant selection noticeably increased during β→α transformation with increasing β grain size. Additional interrupted cooling experiments followed by EBSD analysis showed early nucleation of α variants with a 'butterfly morphology' from β grain boundaries that have a pair of β grain with a common <110> pole. These observations suggest reduced nucleation energies for α formation in such circumstances allowing extensive growth of these α variants into unoccupied β grains making it a dominant variant. The influence of rolling temperatures (i.e. at 800 ºC and 950 ºC) to produce different starting texture, on texture evolution and variant selection during α→β→α transformation was also investigated. Laboratory X-ray, EBSD and in-situ neutron diffraction texture analyses were carried out. Even though the transformation texture is stronger at 800 ºC, the degree of variant selection is stronger in materials rolled at950 ºC compared to material rolled at 800 ºC. Here, the enhanced variant selectionfor the material rolled at 950 ºC was related to the different β texture. It is suggested that the combination of a particular β texture components promote variant nucleation that can increase the likelihood of having β grain pairs with a common <110> pole.

Les propriétés d’emploi des pièces matricées en alliage de titane sont fortement dépendantes des conditions de matriçage (déformation, température de déformation, etc.) ainsi que des conditions de traitement thermique. Nos objectifs furent de caractériser la microstructure et la texture de l’alliage Ti6246 pour comprendre l’influence des paramètres du procédé sur le développement de la microstructure et les conséquences sur l’anisotropie des propriétés mécaniques.En particulier, nous nous sommes intéressés à l’évolution de la texture des grains β; ainsi qu’à la précipitation de la phase α; en fonction du taux de déformation. Des observations EBSD ont permis d’étudier les conditions de germination croissance de la phase α; au niveau des joints de grains β/β et les conditions de sélection des variants d’orientation de la phase αWI observées sur des échantillons fortement déformés. Des observations MEB couplées à un algorithme d'analyse d'image spécialement développé ont permis de déterminer l’influence des paramètres du cycle thermomécaniques sur les caractéristiques microstructurales de la phase αp (fraction de phase, taille…).Les propriétés mécaniques ont été caractérisées à partir d’essai de traction pour déterminer l’influence de l’anisotropie microstructurale induite par la mise en forme. L’observation des surfaces de rupture a mis en évidence le rôle des liserés αGB sur l’amorçage et la propagation des fissures.Les cartographies d’orientation ont montré des variations de propriétés élastiques à l’échelle des grains β; en relation avec la précipitation de la phase α. Il a ainsi été possible de relier les augmentations du bruit ultrasonore rétrodiffusé avec les évolutions de texture et les sélections de variant induites par le forgeage dans le domaine β. / Mechanical properties of closed die forging titanium alloys are very sensitive to variations of processing parameters (forging amount, forging temperature, etc.). The claims of this study were to characterize the microstructure and texture of Ti6246 alloys in order to understand the process parameters' impact on microstructure and their consequences on mechanical properties’ anisotropy.We investigate the impact of the forging amount on the β texture and α phase nucleation. The study of α nucleation and growth at β/β grains boundaries and variant selection in αWI phase conditions is carried out by EBSD measurements. SEM observations associate to an image analysis algorithm specially conceived give a good description of the thermo-mechanical processing parameters’ effects on the αp morphology (fraction, size …).Thanks to tensile testing, mechanical properties have been measured in order to evaluate the effects of microstructure anisotropy induced by forging. αGB-layers at β-grain boundaries dominate fracture toughness (cracks nucleation and cracks propagation) as observed on crack path profiles.Variations of elastic properties at β grains scale linked to α phase precipitation have been revealed by crystallographic orientation maps. Therefore the increasing of backscattered ultrasound noise can be linked to the texture evolution and the variant selections induced by β-forging.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19709号 / 工博第4164号 / 新制||工||1642(附属図書館) / 32745 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 乾 晴行, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Les alliages de zirconium sont utilisés notamment dans les assemblages de combustible nucléaire pour leur transparence aux neutrons ainsi que pour leur tenue mécanique et leur résistance à la corrosion. La connaissance de leur microstructure et de son évolution est nécessaire pour maîtriser les différents traitements thermomécaniques de la gamme de transformation qui comporte plusieurs trempes depuis le domaine bêta. Cette microstructure présente, à l’issue d’une trempe, des lamelles dites de Widmanstätten. Ces dernières soit se disposent parallèlement entre elles (platelets parallèles), soit se croisent en vannerie. Ces morphologies jouent sur l’étape suivante de filage ; en effet, les platelets parallèles défavorisent la fragmentation des lamelles. Une méthode a été mise en place pour quantifier ces morphologies.Lors de la transformation bêta vers alpha, un grain peut générer 12 orientations alpha (variants). Les paramètres qui influencent leur sélection sont encore peu connus. Le modèle proposé minimise la déformation moyenne lors de la transformation. D’abord analytique, il a été ensuite implémenté numériquement afin d’aborder des effets tels que la relaxation d’Eshelby, l’anisotropie élastique, une contrainte extérieure ou le voisinage. En parallèle, la sélection expérimentale a été quantifiée au moyen original de l’EBSD et des fractions des variants locales dans un ex-grain bêta. La confrontation des résultats expérimentaux avec le modèle a permis de le valider en partie et de déterminer la contrainte de trempe à la surface des éprouvettes ainsi que son effet sur la sélection de variants. / Zirconium alloys are frequently used in nuclear fuel assemblies. They are chosen for their low neutron absorption, their mechanical properties and their corrosion resistance. A better understanding of the microstructure evolution of these alloys should allow a better control of their process of fabrication. During processing, several quenches, from the beta to the alpha domain take place. The resulting microstructures are lamellar and are called Widmanstätten microstructures. These lamellae are either disposed in parallel or in crisscross and are named “parallel platelets” and “basketweaves”, respectively. These various morphologies have a significant impact on the extrusion; basketweaves facilitate grain fragmentation unlike parallel platelets. In this thesis project, a methodology was developed in order to quantify these morphologies.During the phase transformation, one beta grain can generate 12 different orientations of new alpha grains. The parameters which can influence variants selection are not yet well-known. The model proposed in the present study is based on the minimization of the mean elastic energy of the system during the phase transformation. First results were obtained analytically. Then, additional effects such as the Eshelby relaxation, the elastic anisotropy and the external strain were implemented numerically. In parallel, each alpha variant was quantified within a former beta grain by EBSD analysis. The comparison between the model and the experiments helped to partially validate the model as well as determine the quench strain on the surface of the sample. It was then possible to study the effect of quench strain on the variant selection.