Ion irradiation of tetrahedral amorphous carbon (ta-C) thin films induces a carbon phase transformation from the electrically insulating sp3 hybridization into the conducting sp2 hybridization. In this work, a detailed study on the electrical resistivity and the microstructure of areas, irradiated with several ion species at 30 keV energy is presented. Continuous ion bombardment yields a drastic drop of the resistivity as well as significant structural modifications of the evolving sp2 carbon phase. It is shown that the resistivity lowering can be attributed to the degree of graphitization in the film. Furthermore, the structural ordering processes are correlated with the ion deposited energy density. It is therefore revealed that the ion-induced phase transformation in ta-C films is a combination of sp3-to-sp2 conversion of carbon atoms and ion-induced ordering of the microstructure into a more graphite-like arrangement. All experiments were done with focused ion beam (FIB) systems by applying FIB lithography of electrical van-der-Pauw test structures. FIB lithography on ta-C layers is presented as a fast and easy technique for the preparation of electrically active micro- and nanostructures in an insulating carbon matrix.

The presence of the ordered $\gamma^{\prime}$ phase ($\text{Ni}_{3}\text{Al}$) in Ni-base superalloys is fundamental to the performance of engineering components such as turbine disks and blades which operate at high temperatures and loads. Hence for these alloys it is important to optimize their microstructure and phase composition. This is typically done by varying their chemistry and heat treatment to achieve an appropriate balance between $\gamma^{\prime}$ content and other constituents such as carbides, borides, oxides and topologically close packed phases. In this work we have set out to investigate the onset of $\gamma^{\prime}$ ordering in Ni-Al single crystals and in Ni-Al bicrystals containing coincidence site lattice grain boundaries (GBs) and we do this at high temperatures, which are representative of typical heat treatment schedules including quenching and annealing. For this we use the atomistic simulation methods of molecular dynamics (MD) and density functional theory (DFT). In the first part of this work we develop robust Bayesian classifiers to identify the $\gamma^{\prime}$ phase in large scale simulation boxes at high temperatures around 1500 K. We observe significant \gamma^{\prime} ordering in the simulations in the form of clusters of $\gamma^{\prime}$-like ordered atoms embedded in a $\gamma$ host solid solution and this happens within 100 ns. Single crystals are found to exhibit the expected homogeneous ordering with slight indications of chemical composition change and a positive correlation between the Al concentration and the concentration of $\gamma^{\prime}$ phase. In general, the ordering is found to take place faster in systems with GBs and preferentially adjacent to the GBs. The sole exception to this is the $\Sigma3 \left(111\right)$ tilt GB, which is a coherent twin. An analysis of the ensemble and time lag average displacements of the GBs reveals mostly `anomalous diffusion' behaviour. Increasing the Al content from pure Ni to Ni 20 at.% Al was found to either consistently increase or decrease the mobility of the GB as seen from the changing slope of the time lag displacement average. The movement of the GB can then be characterized as either `super' or `sub-diffusive' and is interpreted in terms of diffusion induced grain boundary migration, which is posited as a possible precursor to the appearance of serrated edge grain boundaries. In the second part of this work we develop a method for the training of empirical interatomic potentials to capture more elements in the alloy system. We focus on the embedded atom method (EAM) and use the Ni-Al system as a test case. Recently, empirical potentials have been developed based on results from DFT which utilize energies and forces, but neglect the electron densities, which are also available. Noting the importance of electron densities, we propose a route to include them into the training of EAM-type potentials via Bayesian linear regression. Electron density models obtained for structures with a range of bonding types are shown to accurately reproduce the electron densities from DFT. Also, the resulting empirical potentials accurately reproduce DFT energies and forces of all the phases considered within the Ni-Al system. Properties not included in the training process, such as stacking fault energies, are sometimes not reproduced with the desired accuracy and the reasons for this are discussed. General regression issues, known to the machine learning community, are identified as the main difficulty facing further development of empirical potentials using this approach.

Ce travail de thèse concerne l’acier 16MND5 des cuves des Réacteurs à Eau Préssurisée (REP).L’étude vise à caractériser de manière quantitative la microstructure et à comprendre l’effet desparamètres influents (vitesse de refroidissement, température de revenu …) sur sa formation. D’unpoint de vue expérimental, l’étude est menée grâce à l’utilisation des techniques d’observationcomme la métallographie, le MET, MEB et l’EBSD, et la réalisation de traitements thermiquescontrôlés (dilatométrie notamment).Ces observations visent à identifier les différents constituants de l’acier et de déterminer lesgrandeurs quantitatives les caractériser (tailles, fractions volumiques des différentes phases ainsique leurs orientations cristallographiques).Nous avons dans un premier temps caractérisé de manière complète et quantitative, lamicrostructure de l’acier 16MND5 à l’état de réception (trempé et revenu). L’étude a permis demontrer qu’il existe un gradient de microstructure entre la peau interne et le coeur de l’épaisseur dela virole. La microstructure à coeur est constituée de ferrite proeutectoïde, de bainite et d’amas decarbures, tandis qu’en peau interne on est en présence de bainite et d’amas de carbures.Une campagne d’essais de dilatométrie a été réalisée afin de simuler la microstructure brute detrempe de l’acier 16MND5 et d’observer l’effet de la vitesse de refroidissement, des températuresde transformation sur la microstructure des aciers de cuve. Ces essais ont révélé que lamicrostructure de l’acier 16MND5 après refroidissement est constituée de bainite, d’îlots demartensite et d’austénite résiduelle (M-A) enrichie en carbone. Ce sont ces îlots M-A qui donnentnaissance aux amas de carbures lors du revenu final.L’effet de la composition chimique de cet acier a été étudié grâce à l’utilisation d’une tôle ayant unecomposition chimique plus riche en carbone et en éléments d’alliages. Ce travail a montré quel’augmentation des teneurs en carbone et en éléments d’alliages modifiaient la morphologie et lacristallographie de la ferrite.L’étude des comportements mécaniques des constituants présents en peau interne, quart épaisseur etmi épaisseur a été réalisée grâce à des essais de résilience sur des éprouvettes de mini charpy. Cetteétude a permis de montrer qu’à basse température (-120°C) la peau interne présente de meilleurespropriétés de résilience que le quart et la mi épaisseur de la virole. Des examens de la surfacelatérale des éprouvettes de Charpy ont montré que la présence de ferrite proeutectoïde favorise lagermination de micro-fissures de clivage. / Reactor pressure vessel is the second security barrier of the nuclear reactor and it is elaboratedwith a low carbon steel ( C-0.16%). Due to the large size of the ferrule there is temperatureand cooling rate gradient, which lead to microstructure gradient. To develop predictive modelit is necessary to well describe the microstructure which depend to the processing parameters.We focuse our work in investigating the effect of the processing parameters (cooling rate,chemical composition...) on the final microstructure during phase transformation.To get these informations and better describe the microstructure, we have used someexperimental technics like SEM, EBSD, TEM and Optical Microscopy. The steel used is alow carbon steel with a composition of C-0.16%, Mn-1.32%, Ni-0.72%, Mo-0.49%, Si-0.23%, Cr-0.23%, P-0.010%, S-0.004%. The material has been tempered in the range 635°C-660°C after cooling. Three positions have been chosen for examinations, because the coolingrate is not the same between the center and the edge of the material. The results of theobservations made in the different scale, indicate that the microstructure is mainly baniticwith bainitic ferrite and cementite precipitates. Examinations of the precipitates withextractive replicas in TEM reveal that morphology of cementite particles is complex they arecylinder-shaped particles, short bars particles and skeletal particles. OrientationsRelationships (OR) have been determined between ferrite and cementite particles with thinfoils in TEM by using Selected Area Diffraction, in a large number areas the Isaichev andBagaryatskii OR have been observed, a little Pitsch Petch OR have been obtained. The EBSDmap shows that the bainitic ferrite morphologies are both lath like and polygon. Themisorientations inside the laths are very small (0.5° misorientation point to point) andbetween laths we have the range 49-60° misorientation. The profile of misorientationsbetween point to point indicates higher frequency for the range 49-60°. Charpy test have beenperformed to analyze the effect of the microstructure on the fracture energy at lowtemperature. The results show that the fracture energy decreases when the content ofproeutectoid ferrite is high.

Les alliages de titane β métastables sont des matériaux de structure essentiels pour les applications aéronautiques de part leurs très bonnes propriétés mécaniques. En effet, ils présentent une résistance spécifique élevée, une bonne ductilité et forgeabilité et une excellente réponse aux traitements thermiques. Toutefois, il existe encore aujourd'hui à leur sujet des controverses et des questions ouvertes et ce, malgré les efforts pour comprendre les mécanismes d'évolution microstructurale au cours de traitements thermo-mécaniques et pour déterminer les phases en présence et leur contribution aux les propriétés mécaniques résultantes. Ce travail de thèse a pour objectif de déterminer la nature de la phase β et de caractériser la transformation β→α à haute et basse températures par des caractérisations fines en microscopie électronique à balayage et à transmission couplées à des mesures d'orientations cristallographiques et de composition chimique. L'alliage étudié est un Ti-5553 avec une microstructure initiale 100% β obtenue par mise en solution et trempe. Il a été démontré expérimentalement que la structure de la phase β métastable n'est pas purement cubique centrée. Les points de la phase β dans les clichés de diffraction présentent un allongement (streaking) et des points supplémentaires sont visibles aux positions de diffraction 1/2, 1/3 et 2/3. Par ailleurs, les images MET ont un aspect en moiré. A partir de ces résultats et de calculs crystallographiques, il a été prouvé que des déplacements atomiques sur les plans {110}β et {112}β forment une structure intermédiaire entre celle de la phase β parente et celles des phases α et ω, prouvant que la phase β a intrinsèquement initié une transformation. L'étude de la précipitation au cours du procédé thermomécanique dans le domaine α+β a révélé que des précipités α discontinus, équiaxes ou légèrement allongés (1~2μm) se forment aux joints β de forte et de faible désorientation mais rarement au coeur des grains β produisant ainsi une microstructure en "collier". La relation d'orientation de Burgers (ROB) entre les phases α et β est progressivement détruite par la déformation. L'écart à la ROB est plus marqué pour les précipités α qui se forment au joint de grains qu'à l'intérieur des grains. L'écart à la ROB augmente aussi avec la déformation, mais diminue avec la vitesse de déformation. Au cours des déformations en bas du domaine α+β, les précipités α ont une morphologie qui dépend de leur position. Au coeur des bandes de glissement, les grains α/β sont équiaxes et ne respectent pas la ROB. Entre les bandes de glissement, la microstructure est lamellaire où les phases α/β alternent et respectent la ROB. Dans ce dernier cas, une forte sélection de variantes a été observée: Seuls les deux ou trois variants favorisant l'accommodation de la déformation se sont formés. A titre de comparaison, dans l'état non déformé, les 12 précipités sont présents. La transformation β→α est retardée en cours de compression à haute température. Ceci est attribué à une compétition entre adoucissement et transformation de phase. Au contraire, celle-ci est favorisée au cours de la compression à plus basse température du fait que les défauts cristallins induits par la déformation jouent le role de sites de germination et que la croissance des précipités soit accéléré alors que l'adoucissement soit ralenti. Dans le Ti-5553, le mécanisme de déformation dominant est le glissement des dislocations. Dans les déformations en bas du domaine α+β, du glissement simple ou multiple avec deux ou trois systèmes de glissement activés. L'identification de ces systèmes a pu être effectuée par des analyses de traces. Cette thèse a résolu la nature de la phase β métastable et constitue un travail de référence pour l'étude de la transformation β→α au cours de traitement thermomécanique / Metastable β titanium alloys are important structural materials for aeronautical applications due to their high strength to density ratio, good ductility and workability and excellent hardenability. Despite the efforts in resolving the complex microstructural evolution related to thermomechanical processes and in gaining knowledge on the produced phases and their contribution to the resultant mechanical properties, there are still some controversial and unresolved issues. The aim of the present PhD work is to determine precisely the metastable nature of β phase and to characterize finely the characteristics of the β→α transformation during high and low temperature thermomechanical treatments. Investigations were performed on a Ti-5553 alloy with the single β phase initial microstructure obtained by solution treatment followed by quenching using scanning and transmission electron microscopy (SEM/TEM) coupled to crystallographic orientation measurements and chemical analyses. It was demonstrated experimentally that the structure of the β phase in the metastable titanium alloy is not “pure” body centered cubic. Diffraction diagrams presents streaking of the β diffraction spots and additional spots at the 1/2, the 1/3 and 2/3 diffraction positions. Also, striations are observed in TEM images. From this experimental evidence and crystallographic calculations, it was proved that atomic displacements on the {110}β and {112}β planes formed a structure between that of the parent β phase and that of the α or ω phase, demonstrating pre-phase transformation tendency. The study of the precipitation during thermomechanical processing at higher temperature in the α+β region revealed that discontinuous equiaxed or short rod shaped α precipitates (1~2μm) mainly form on the high angle and low angle β grain boundaries but seldom in β grain interiors, forming the “necklace” microstructure. The Burgers orientation relationship (BOR) between the α and β phases is destroyed gradually by the deformation. The BOR deviation of grain boundary α is larger than that of intragranular α. The deviation from the BOR increases both with the increasing strain and decreasing strain rate. During the deformation at the lower temperature in the α+β region, the α precipitates exhibit different morphologies: such as lamellar α, equiaxed α and irregular α depending on their localization. Within the slip bands, equiaxed α/β grains which do not respect the BOR are present. However, between the bands, lamellar α and β phases maintaining the BOR are distributed alternately. In that last case a strong variant selection is observed as only the two or three variants that form are those which can accommodate the macroscopic deformation. Comparatively, in absence of compression all 12 variants are formed. The β→α phase transformation is retarded during the hot compression at higher temperature region, which is attributed to the competition between softening and phase transformation. On the contrary, it is promoted during compression at lower temperature region due to the more inducted deformation defects acting as α phase nucleation sites and due to accelerating growth of α precipitates and retarded softening. Dislocation slip is the leading deformation mechanism for the Ti-5553 alloy. Under the lower temperature deformation condition, single or multiple-slip bands with two or three different activated slip systems would form during the hot deformation process. Identification of these slip systems have been done by trace analysis. These results provide new insights into the structural nature of β metastable phase and valuable reference for β→α phase transformation during thermo-mechanical treatment in metastable β titanium alloys

Les travaux réalisés dans cette thèse visent d’une part, à comprendre les évolutions structurales et microstructurales d’un composite à matrice titane et à renforts TiB au cours des différentes étapes d’élaboration par métallurgie des poudres et des traitements thermiques associés, et d’autre part, à établir des relations entre microstructures et propriétés mécaniques pour ce matériau. Les cinétiques de transformations des phases de la matrice et du renfort ont été caractérisées par DRX haute énergie in situ, au cours des différents traitements du cycle de fabrication. Des analyses de la microstructure par MEB, MEB EBSD et MET (EDX et EELS) complètent l’analyse par DRX. Il a été montré que les cinétiques de transformation de la matrice des composites sont fortement affectées (décalage d’environ 300°C vers les hautes températures de la température de transus ß) par le procédé de fabrication. Ce décalage a été associé à un enrichissement en éléments interstitiels dû au broyage mécanique des poudres et aux interstitiels présents dans les renforts TiB2 introduits pour former le TiB. L’étude in situ a aussi précisé la séquence de transformation du diborure de titane en borure TiB–B27 via la formation de la phase métastable TiB-Bf. Les analyses par MEB et MET ont permis d’atteindre et de discuter des évolutions morphologiques et spatiales des phases (matrice et borures) au cours des différents traitements, et de caractériser la composition chimique des borures. Une séquence de transformation du renfort est proposée. Enfin, des matériaux composites ont été élaborés et soumis à divers traitements thermomécaniques. Le lien entre les propriétés mécaniques statiques et les évolutions morphologiques des borures et de la matrice, comme de la texture des phases, a été abordé. Des traitements ont été proposés pour atteindre des propriétés optimales / The work done in this PhD thesis aims at the understanding of, on the one hand, the structural and microstructural evolutions of a TiB reinforced titanium matrix composite during the various steps and treatments of the powder metallurgy route used to produce it, and, on the other hand, the link between the microstructures and the mechanical properties for this material. The phase transformation kinetics, in the matrix and in the reinforcement, were characterised using in situ high energy XRD, during these treatments. Microstructural analysis, using SEM, SEM-EBSD and TEM (EDX and EELS) complete the XRD analysis. The matrix phase transformation kinetics were shown to be highly impacted by the processing route (a 300°C shift toward the high temperatures is found for the ß transus temperature). This shift has been linked with an increase in interstitial elements, coming from the powder mechanical alloying and from the interstitials in the TiB2 powder used to produce the TiB. The in situ study also helped in clarifying the transformation sequence of the TiB2 into TiB-27, via the formation of the metastable phase TiB-Bf. SEM and TEM analysis allowed to get access to and discuss the morphological and spatial evolutions of the phases (matrix and borides) during the various treatments and to characterise the chemical composition of the borides. A transformation sequence has been proposed. Finally, in a last part, composite materials were elaborated and submitted to defined heat treatments. The link between the static mechanical properties and the morphological and texture evolutions in the matrix and in the borides, was discussed. Some treatments were proposed to reach optimum mechanical properties

Submitted by Bruna Rodrigues (bruna92rodrigues@yahoo.com.br) on 2016-09-21T14:11:42Z No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T18:26:41Z (GMT) No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T18:26:47Z (GMT) No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) / Made available in DSpace on 2016-09-21T18:26:54Z (GMT). No. of bitstreams: 1 DissRDC.pdf: 7324446 bytes, checksum: 5e04b539c5f14da2f1e2fb0184498eb9 (MD5) Previous issue date: 2014-07-28 / Não recebi financiamento / The homogenizing heat treatment of aluminium billets has direct influence on extrusion process productivity, surface quality and mechanical properties of extruded shapes. The homogenizing process parameters that impacts on metallurgical properties of 6063 alloys are: temperature, time and cooling rate. The maximum recommended homogenizing temperature for 6063 alloy is 590ºC. The minimum homogenizing time is 2,25h. The minimum cooling rate recommended is 400ºC/h. As for extrusion logs homogenizing process of 6063 6” diameter at Sapa Aluminium Brasil the furnace is a process bottleneck for logs production, this work has investigated about the possibility of time reducing without negative impacts on billet metallurgical properties and without detrimental impacts on productivity of extrusion process and quality of its products. This investigation was done by billet metallographic analysis, cold and hot mechanical tests and extrusion/anodizing trials for different homogenizing time conditions. The results have shown that the time reducing did not cause negative impacts for metallurgical neither for mechanical properties and because of that no detrimental effects for extrusion performance and surface finish and anodized quality was observed. Hence, was possible to decrease 20 minutes on homogenizing time and gain 14,5% of productivity on heat treatment process. / O tratamento térmico de homogeneização de tarugos de alumínio tem influência direta na produtividade do processo de extrusão, na qualidade superficial e nas propriedades mecânicas dos perfis extrudados. As variáveis do tratamento térmico da liga 6063 que influem na qualidade metalúrgica são: temperatura, tempo e taxa de resfriamento. Para liga 6063, a máxima temperatura recomendada de homogeneização é de 590ºC. O tempo mínimo para a homogeneização é de 2,25h. A taxa de resfriamento mínima recomendada é 400ºC/h. Como para o tratamento térmico de tarugos de 6” há um gargalo no forno de homogeneização da Sapa Aluminium Brasil, investigouse neste trabalho possibilidades de redução de tempo no processo de homogeneização evitando impactos negativos na qualidade metalúrgica dos tarugos e/ou na performance dos tarugos no processo de extrusão. Tal investigação foi feita através de análises metalográficas, ensaios mecânicos a frio (dureza e tração) e a quente (torção) e testes práticos no processo de extrusão/anodização para amostras retiradas de tarugos homogeneizados em diferentes condições de tempo de homogeneização. Os resultados obtidos mostraram que a redução no tempo de homogeneização não impactou negativamente na qualidade metalúrgica dos tarugos e nem em suas propriedades mecânicas a frio e que as pequenas variações ocorridas mantiveram-se dentro dos limites de especificação e não reduziram a performance do processo de extrusão bem como as propriedades mecânicas e qualidade superficial dos perfis extrudados. Com isso, foi possível reduzir 20 minutos no tempo de tratamento térmico de homogeneização e gerar um ganho de produtividade de 14,5% no processo.

Made available in DSpace on 2016-12-08T15:56:17Z (GMT). No. of bitstreams: 1 Andre Olah Neto.pdf: 11111826 bytes, checksum: 36a7c3a3c11e61f18d8a74f06d619cc0 (MD5) Previous issue date: 2015-04-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In recent decades the automobile industry has made a great effort to deal with ecological and security challenges. To do so, it was necessary to develop vehicles which are lighter, more economical and have a greater intrusion resistance when subjected to a crash. This was made possible, among other actions, by the development of advanced high strength steels, associated with the use of new manufacturing processes. Inside this approach the use of the hot stamping and the emergence of 22MnB5 boron-alloyed steel, with high hardenability, stand up. The hot stamping operation has gained great importance for enabling the manufacture of strategic components of high complexity and high mechanical resistance, associated with reasonable toughness. In order to ensure its technological evolution this process has been widely studied by numerous authors, so that the phenomenon was better understood, allowing better control as well as the quality and reliability requirements involved in the stamped components. This focus led to the development of this work, whose main objective was to study the hot stamping process, evaluating the mechanical and thermal effects. To achieve this aim an experimental apparatus was developed which allowed simulating the main thermomechanical aspects involved, such as the temperature, the conformation and the cooling. The purpose was to reproduce the conditions of the process and evaluate the influence of certain variables of the cooling speed on microstructure and on the final properties of the material, in order to study and understand some phenomena involved. This apparatus was composed of a heating furnace, an aluminum cooler, water cooled, operated at low pressure of closing and a control system, assembled on a mechanical testing 12 machine to promote the desired deformation. The experimental work was carried out in three stages. Initially, the hot plastic behavior of 22MnB5 steel was studied, evaluating the effect of temperature and strain rate on the mechanical characteristics, to determine the conditions for necking formation. In the second stage, the kinetics of phase transformation was studied, seeking to understand the effect of heating and cooling conditions on the cooling rate and on the final properties after quenching. In the last step, the plastic behavior on the kinetics of phase transformation, i.e., the effect of necking on cooling, was studied. The main objective was to show that the necking, depending on its intensity and geometry, generates the formation of a clearance between the cooler and the surface material, reducing the cooling rate to the point of affecting the mechanical properties in this region. Despite being localized, it can jeopardize the stamped component performance forming a fragile region of low mechanical strength and low toughness. It was concluded that hot plastic deformation undergone during the hot-stamping has a significant influence on the phase transformation, being necessary the proper control of process conditions so that the necking is also controlled, thus ensuring the structural homogeneity of the component and its performance. / Nas últimas décadas a indústria automobilística tem realizado um grande esforço em atender os desafios ecológicos e de segurança e para isto foi necessário desenvolver veículos mais leves, econômicos e com maior resistência à intrusão quando submetidos a um acidente. Isto foi alcançado, entre outras ações, através do desenvolvimento de aços avançados de elevada resistência mecânica, associado à utilização de novos processos de fabricação. Dentro deste enfoque se destaca dois aspectos, a utilização do processo de estampagem a quente e o surgimento do aço 22MnB5 de elevada temperabilidade ligado ao boro. A operação de estampagem a quente tem ganhado uma forte importância por possibilitar a fabricação de componentes estratégicos de elevada complexidade e elevada resistência mecânica, associada à razoável resistência ao impacto. No sentido de garantir sua evolução tecnológica este processo tem sido amplamente estudado por inúmeros autores, para que os fenômenos envolvidos pudessem ser mais bem entendidos, permitindo um melhor controle bem como o atendimento dos requisitos de qualidade e a confiabilidade envolvida nos componentes estampados. Com este enfoque desenvolveu-se este trabalho, cujo principal objetivo foi estudar o processo de estampagem a quente, avaliando os efeitos mecânicos e térmicos. Para este fim foi desenvolvido um aparato experimental, que permitiu simular os principais aspectos termomecânicas envolvidos, como a temperatura, a conformação e o resfriamento. O propósito foi o de reproduzir as condições do processo e avaliar a influência de determinadas variáveis sobre a velocidade de resfriamento, sobre a microestrutura e sobre as propriedades finais do material, no sentido de estudar e entender 10 alguns fenômenos envolvidos. Este aparato foi dotado de um forno de aquecimento, de um resfriador de alumínio refrigerado a água, operado a baixa pressão de fechamento e de um sistema de controle, montados sobre uma máquina de ensaios mecânicos para promover a deformação desejada. O trabalho experimental foi realizado em três etapas. Inicialmente foi estudado o comportamento plástico a quente do aço 22MnB5, avaliando-se o efeito da temperatura e da velocidade de deformação sobre as características mecânicas, determinando-se as condições para formação da estricção. Na segunda etapa foi estudada a cinética de transformação de fase, procurando-se entender o efeito das condições de aquecimento e do resfriamento sobre a velocidade de resfriamento e sobre as propriedades finais deste aço após têmpera. Na última etapa se relacionou o comportamento plástico sobre a cinética de transformação de fase, ou seja, o efeito da estricção sobre o resfriamento. O objetivo principal foi mostrar que a estricção, dependendo de sua intensidade e geometria, gera a formação de uma folga localizada entre a superfície do resfriador e do material, reduzindo a velocidade de resfriamento a ponto de afetar as propriedades mecânicas nesta região. Apesar de localizada esta folga pode comprometer o desempenho do componente estampado formando uma região de pouca resistência mecânica. Concluiu-se que a deformação plástica a quente sofrida durante a estampagem a quente apresenta uma significativa influência sobre a transformação de fase, sendo necessário o controle adequado das condições do processo para que a estricção também seja controlada, garantindo assim a homogeneidade estrutural do componente e o seu desempenho.

Made available in DSpace on 2015-05-08T14:59:43Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 3419531 bytes, checksum: 47a7d7cdb7277fb0c7f884e30a96df42 (MD5) Previous issue date: 2012-09-14 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work was evaluated the influence of multiple quenching in the peak temperatures of phase transformation (PTPT) in the alloy Cu-11.8Al-0.6Be-0.5Nb-0.27Ni (%wt), as well as the influence that deformation applied in temperatures above Ms, at nominal composition Cu-11.8Al-0.6Be-0.5Nb-0.27Ni, Cu-11.8Al-0.55Be-0.5Nb-0.27Ni and Cu-11.8Al-4Nb-2.16Ni-0.5Be, would have on the residual deformation. The alloys were melted, homogenized during 12h by 850ºC and machined using wire electroerosion. Then, the samples were quenched in water at room temperature and subsequently analyzed by optical microscopy, scanning electron microscopy, using energy dispersive x-ray, differential scanning calorimetric analysis and x-ray diffractometry. For samples quenched successively, it was found a large change in PTPT after 22 quenching, there is no reverse transformation in this range. From the 34th quenching, the PTPT remained constant around 420ºC and severe changes in your micrographs were detected. Nevertheless, there was no change in Cu/Al able to change the PTPT. Alloys with containing nominal composition 0.4% and 0.2%Be indicated strong influence of the Be in the PTPT. When analyzed by x-ray diffractometry, the sample with 0.2Be indicated the presence of β' and γ' phases, when aged by 530ºC. For quantifying the residual deformations, the samples were subjected to static tensile and loading/unloading tests. When subjected large deformation and temperature near Ms, the results showed a great residual deformation, whereas small deformations with temperatures above Af showed not to be viable. The alloy Cu-11.8Al-4Nb-2.16Ni-0.5Be when tractioned, showed excessive weakness even after treatment of solubilization. / Neste trabalho avaliou-se a influência de têmperas múltiplas, nas temperaturas de pico da transformação de fase (TPTF) e na microestrutura da liga Cu-11,8Al-0,6Be-0,5Nb-0,27Ni (% em peso), bem como a influência que deformações aplicadas, em temperaturas a partir de Ms, às ligas de composição nominal Cu-11,8Al-0,6Be-0,5Nb-0,27Ni, Cu-11,8Al-0,55Be-0,5Nb-0,27Ni e Cu-11,8Al-4Nb-2,16Ni-0,5Be, teriam nas deformações residuais. As ligas foram fundidas, homogeneizadas durante 12h a 850ºC e usinadas via eletroerosão à fio. Em seguida, os corpos de prova foram temperados em água a temperatura ambiente sendo posteriormente analisadas via microscopia óptica, microscopia eletrônica de varredura, utilizando-se energia dispersiva de raios-x (EDS), análise calorimétrica diferencial de varredura (DSC) e difratometria de raios-X. Para as amostras cicladas termicamente, verificou-se que após 22 têmperas uma mudança significativa nas TPTF ocorre, não havendo a partir deste intervalo transformação reversa. A partir da 34ª têmpera, as TPTF permaneceram constantes em torno de 420ºC e as micrografias indicaram mudanças severas em suas microestruturas. Não obstante, não se verificou alteração na relação Cu/Al capaz de alterar as TPTF. As amostras contendo composição nominal de 0,4% e 0,2% de Be indicaram que as ligas estudadas são fortemente influenciadas pela presença do Be. Quando analisada por difratometria de raios-x, a amostra com 0,2Be indicou a presença das fases β e γ , quando submetida a tratamento de envelhecimento a 530ºC. Para a quantificação das deformações residuais, os corpos de prova foram submetidos aos ensaios de tração estática e de carregamento/descarregamento. As amostras submetidas a deformações próximas as de ruptura e com temperatura de ensaio próximo a Ms mostraram resultar em deformações residuais de maiores intensidades, enquanto quedeformações de pequena magnitude, com temperaturas acima de Af, mostraram não serem viáveis. A liga de composição nominal Cu-11,8Al-4Nb-2,16Ni-0,5Be, quando ensaiada sob tração, mostrou fragilidade excessiva mesmo após tratamento térmico de solubilização.

L'objectif de cette thèse était de synthétiser des particules de carbonate de calcium (CaCO3) poreuses pour applications industrielles comme charge dans du papier à cigarette, ainsi que pour l'encapsulation d’'arômes. Nous avons cherché à maîtriser les paramètres de synthèse pour obtenir de la vatérite de taille contrôlée. Nous avons étudié sa transformation à haute température et dans l’'eau, car ce polymorphe est métastable. La transition de phase vatérite/calcite a été étudié par DRX et imagerie par diffraction des rayons X cohérents qui permet d’accéder à l’'image en 3D des particules. Nous avons montré que la vatérite de taille 1 à 2µm présentant 20% de porosité peut être synthétisée de façon reproductible. Les particules préparées ont été introduites comme charge dans du papier à cigarette pour évaluer l’'impact de nouvelles formes de CaCO3 sur les propriétés physiques du papier ainsi que sur la réduction des certains composées nocifs contenus dans la fumée. Nous avons développé l’analyse conjointe de l’'absorption et de la diffraction des rayons X pour estimer la charge réelle introduite ainsi que la porosité des papiers. Nous avons démontré que l’'utilisation de CaCO3 sous forme des sphères poreuses permet d’'augmenter la diffusivité du papier et de réduire l’'émission de CO dans la fumée principale.L’encapsulation d'arômes par la co-cristallisation et l'inclusion moléculaire dans le carbonate de calcium a été aussi étudiée. Nous avons montré que CaCO3 peut être utilisé comme matrice d’'imprégnation d'arômes avec une efficacité d’'encapsulation de plus que 55%. Les particules aromatiques ont été après ajoutées dans le papier pour évaluation sensorielle. / The aim of this thesis was to synthesize porous calcium carbonate (CaCO3) particles for industrial applications as fillers for cigarette paper as well as a matrix for flavour encapsulation. We show that we can control the fabrication of porous particles of vaterite with a given size by tuning the parameters of synthesis. After the synthesis, the stability of vaterite in aqueous solution and at high temperature was studied. The phase transition was analyzed by XRD and coherent X-ray diffraction imaging that allows us to have a 3D-image of the particles. Finally, particles of 1-2 μm size with 20% porosity were reproducibly synthesized. Prepared vaterite particles were introduced as a filler in cigarette paper, with the goal to evaluate their impact on the physical properties of papers as well as on the reduction of some harmful compounds during the smoking. It was demonstrated that the use of vaterite can increase the diffusivity of paper and reduce the CO emission in the mainstream smoke. We also show that the use of X-ray absorption and diffraction can provide an estimation of the filler fraction and porosity of the papers in a non-destructive way. The encapsulation of flavours in CaCO3 particles was performed by co-crystallization and molecular inclusion. It was demonstrated that CaCO3 can be used as a matrix for flavour impregnation with more than 55% of encapsulation efficiency. Flavoured particles was added in paper for sensory evaluation. We shown that it is possible, to flavour the final product with flavoured calcium carbonate particles.

Neste trabalho, propõe-se um modelo numérico-computacional representativo dos processos de tratamento térmico, que seja uma ferramenta eficiente e forneça meios para um entendimento efetivo do mecanismo de geração de tensões residuais durante a têmpera de aços. Foram investigados os fenômenos térmicos, mecânicos e de transformação de fase observados na têmpera, bem como o acoplamento entre esses três fenômenos. O modelo utiliza o Método dos Elementos Finitos (MEF) e o programa ABAQUS®, além de rotinas numéricas em FORTRAN responsáveis pela resolução do problema termo-mecânico-microestrutural acoplado. A utilização de sub-rotinas, que implementam uma alteração na formulação (matemática e numérica) do programa de Elementos Finitos, permite incluir no modelo as informações presentes em uma curva CRC (curva de resfriamento contínuo) do aço SAE 4140, implementando o cálculo de deformações da peça simulada de modo incremental e cumulativo. Os resultados mostram que a utilização das sub-rotinas desenvolvidas neste trabalho permitiu implementar, conjuntamente com o programa ABAQUS®, o cálculo das frações volumétricas, durezas, distorções e tensões que surgem em um tratamento térmico de têmpera, simulando as transformações martensítica, perlítica, bainítica e ferrítica. Os resultados dos modelos foram equivalentes aos relatados pela literatura, principalmente no que se refere às durezas e tensões associadas a cada transformação de fase. Em particular, os resultados indicam que a transformação martensítica está sempre associada à formação de tensões compressivas. Ensaios experimentais foram realizados a fim de validar os modelos computacionais propostos, utilizando-se um teste Jominy adaptado e instrumentado, de modo a permitir a amostragem da variação de temperaturas no material. Ensaios metalográficos permitiram correlacionar as frações volumétricas transformadas durante a têmpera do corpo de prova Jominy aos valores calculados pelo modelo numérico acoplado. / The objective of this work is to analyze residual strains and stresses and volumetric expansion due to phase transformations that occur during quenching of a steel body, as well as to predict these phase transformations. The coupled thermo-mechanical-phase transformation problem was analyzed, specifically in terms of the quenching process. Different computational models were presented, based on the finite element software ABAQUS® and on the use of FORTRAN subroutines. The continuous-cooling-transformation (CCT) diagrams of SAE 4140 steel are represented differently in each model, depending on the transformed phases and correspondent volumetric expansion. These subroutines include information from the CCT diagrams of SAE 4140 into a FORTRAN code. The subroutine calculates all the microstructures resulting from quenching (ferrite, pearlite, bainite, and martensite), depending on cooling rate. The numerical analysis conducted in this work provided results in terms of the temperature and stresses developed during quenching. The properties determined in this work are hardness, yield strength, volumetric fraction and distortion. Hardness has been predicted by the use of analytical equations. The finite element analyses were able to explain and reproduce phenomena observed during quenching of a steel cylinder. In particular, numerical results indicated that martensite formation is always related to a compressive stress field. The results of the models are in qualitative agreement with data provided by literature, particularly, in relation to the stresses originated by each different phase transformation during quenching process. Experimental testing was conducted, based on the analysis of the quenching of a Jominy probe, in order to validate the computational model developed in this work.