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No. of bitstreams: 1 TeseJRMH.pdf: 6415770 bytes, checksum: 1bdc924884c63f0003dce6138a5d2b07 (MD5) Previous issue date: 2016-10-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A study was made of the synthesis and processing of magnetoelectric particulate composites of strontium barium niobate (SrxBa1-xNb2O6, or simply SBN) as a lead-free piezoelectric phase and nickel ferrite (NFO) as a magnetic phase. The specific objectives of this work were: (i) to obtain SBN (TBT) and NFO phases without impurities during the synthesis of the powders and the densification of the ceramic bodies; (ii) to produce SBN-NFO composites of 0–3 connectivity, i.e., with a highly homogeneous distribution of ferrite phase in the SBN matrix; and (iii) to ensure mechanical interphase coupling based on high densification, without the occurrence of anomalous SBN grain growth. The Sr0.61Ba0.39Nb206/NiFe2O4 biphasic system (SBN61/NFO) was prepared with molar ratios of 50/50, 70/30 and 80/20 by means of in situ chemical synthesis using the Pechini method. The objectives of this work included an analysis and proposal of phase formation during the in situ synthesis of SBN/NFO grain growth, with high chemical homogeneity, a low degree of NFO phase percolation, and 0–3 connectivity. composites. The synthesized powders were analyzed by Rietveld refinement of their X-ray diffraction patterns. This enabled the quantification of spurious phases and revealed the effect of reducing these phases and the abnormal grain growth resulting from the increased proportion of NFO phase. However, the secondary phases could only be determined based on the hypotheses put forward in the proposal of the phase formation model of the SBN/NFO system. This proposed phase formation model enabled us to understand why SBN and NFO phases can be formed simultaneously during in situ synthesis by the Pechini method, and also to conclude that the presence of precipitates during the formation of the polymeric resin may be the cause of spurious phases. This allowed us to propose changes in the synthesis step in order to eliminate possible precipitates, which actually resulted in the reduction of the secondary phases in the tests of the 70/30 system. After sintering, magnetoelectric composites were obtained with a relative density of ~96%, without abnormal grain growth, with high chemical homogeneity, a low degree of NFO phase percolation, and 0–3 connectivity. / Neste trabalho estudou-se a síntese e o processamento de compósitos magnetoelétricos particulados de niobato de estrôncio e bário (SrxBa1- xNb2O6 ou, simplesmente, SBN), como fase piezoelétrica livre de chumbo, e de ferrita de níquel (NFO), como fase magnética. Os objetivos específicos do trabalho foram: obtenção das fases SBN (TBT) e NFO como únicas do sistema, tanto durante a síntese dos pós, quanto na densificação dos corpos cerâmicos; produção de compósitos SBN-NFO com conectividade 0-3, ou seja, com alta homogeneidade da distribuição da fase ferrita em uma matriz de SBN; e garantia do acoplamento mecânico entre as fases, a partir de alta densificação e sem a ocorrência de crescimento anormal de grãos da fase SBN. Neste trabalho adotou-se a síntese in situ pelo método Pechini para a preparação do sistema bifásico Sr0,61Ba0,39Nb206/NiFe2O4 (SBN/NFO), nas proporções molares 50/50, 70/30 e 80/20. Análise e proposta de formação de fases durante a síntese in situ de compósitos SBN/NFO foram incluídas nos objetivos deste trabalho. Os pós dos compósitos sintetizados foram analisados por refinamento estrutural dos perfis de difração de raios X, pelo método de Rietveld, o que permitiu evidenciar quantitativamente a presença de fases espúrias e o efeito de redução dessas fases, e do consequente crescimento anormal de grãos, pelo incremento da proporção da fase NFO. Contudo, a determinação das fases secundárias só pode ser concluída a partir das hipóteses levantadas na proposta do modelo de formação de fases desse sistema. Além disso, esse modelo permitiu explicar porque é possível a formação simultânea das fases SBN e NFO pela síntese Pechini in situ, e também concluir que a presença de precipitados na etapa de formação da resina polimérica poderia ser a causa da ocorrência das fases espúrias. Com isso, pode-se propor alterações na etapa de síntese, com a intenção de se eliminar possíveis precipitados, o que realmente resultou, para os ensaios realizados no sistema 70/30, em redução das fases secundárias e compósitos magnetoelétricos, após sinterização, com densidade relativa ~96%, sem crescimento anormal de grãos, de elevada homogeneidade química, com baixo grau de percolação da fase NFO e conectividade 0-3. / FAPESP: 2008/04025-0 / FAPESP: 2012/24025-0 / CAPES: Procad 2013 - 3012/2014

A geração de novas fontes de energia limpa, segura e renovável por meio da fusão nuclear envolve importantes desafios tecnológicos, dentre eles a pesquisa, caracterização e a fabricação de materiais avançados para os futuros reatores de fusão nuclear. Os aços ferrítico-martensíticos de reduzida atividade radioativa, em especial a liga Eurofer-97, destacam-se por apresentar uma combinação única de propriedades para esta aplicação. O objetivo desta Tese de Doutorado é avaliar a estabilidade microestrutural deste aço recozido numa ampla faixa de temperaturas. Cálculos termodinâmicos e testes de dilatometria foram usados para determinar as temperaturas de transformação de fase. A estabilidade microestrutural foi estudada por meio de recozimentos isotérmicos entre 200 e 1350oC após laminação a frio com reduções de 40, 70, 80 e 90%. A avaliação da estabilidade mecânica do aço Eurofer-97 foi realizada por meio de medidas de dureza Vickers. As principais técnicas utilizadas para caracterização microestrutural foram microscopias eletrônica de varredura e de transmissão, tomografia por sonda atômica e medidas de magnetização DC. Tanto a textura como a microtextura foram determinadas por meio de medidas de difração de raios X e de elétrons retroespalhados (EBSD). Recuperação, recristalização primária e crescimento anormal de grão ocorrem neste material recozido abaixo de 800oC. Acima desta temperatura, a transformação martensítica ocorre alterando bastante a micoroestrutura. A cinética de crescimento anormal de grão é alterada pela quantidade de redução a frio previamente aplicada. A hipótese proposta para explicar o crescimento anormal de grãos neste material baseia-se principalmente na vantagem de tamanho adquirida pelos núcleos de recristalização primária com diferenças de orientação médias superiores a 45º em relação aos vizinhos. Neste caso, o crescimento anormal de grão é responsável por fortalecer as componentes {111} e {111}, {001} e {110}. Acima de 800oC a transformação martensítica prevalece elevando a dureza Vickers e randomizando a textura deste aço. As características do produto transformado dependem tanto da temperatura de austenitização quanto do tamanho incial do grão ferrítico. Os dados de composição química das partículas estáveis após recozimento em temperaturas inferiores a 800oC foram usados para validar os resultados dos cálculos termodinâmicos obtidos via Thermo-Calc. / Clean, safe, and renewable energy sources such as nuclear fusion comprise important technological challenges, including research, characterization and manufacture of advanced materials for future fusion reactors. Modified ferritic-martensitic steels with reduced radioactive activity (RAFM), especially Eurofer-97 steel, are among worldwide references in the nuclear field for their unique properties. The scope of this Thesis is to evaluate the microstructural (thermal) stability in ferritic-martensitic Eurofer-97 after annealing within a wide range of temperatures. Themodinamic calculations as well as dilatometric tests were used to determine the main phase transformation temperatures. The microstructural stability of this steel was followed by isothermal annealing between 200 and 1350°C after cold rolling to 40, 70, 80 and 90% reductions in thickness. The mechanical stability in the Eurofer-97 was assessed by Vickers microhardness measurements. Representative samples for each metallurgical condition were characterized by scanning electron microscopy, transmission electron microscopy, atom probe tomography, and DC-magnetization tests. Both texture and microtexture were evaluated by X-ray diffraction and electron backscattered diffraction (EBSD) techniques. Recovery, primary recrystallization, and abnormal grain growth (secondary recrystallization) processes have been observed at temperatures below 800°C. The amount of abnormally grown grains depends on the amount of previous cold rolling. The hypothesis for the most probable mechanism responsible for abnormal grain growth is based on the advantage size acquired by nuclei with misorientations above 45º surrounding their neighboring grains, even in regions where primary recrystallization was incomplete. The texture developed after abnormal grain growth has components belonging to ?- and ?-fibers with predominance of {111}, {111}, {100} e {110} components. The martensite transformation takes place when this steel is annealed above 800°C causing an increase of hardness, significant changes in microstructure, and texture weakening. The martensitic sructure depends very much on both austenitization temperature and initial austenitic grain size. The results of chemical analyses of stable particles present in samples annealed below 800oC were used to validate the thermodynamic calculations provided by Thermo-Calc.

Este trabalho tem como objetivo caracterizar as mudanças microestruturais durante o recozimento isotérmico de um fio de ferro trefilado a frio e de pureza comercial com ênfase no crescimento de grão. Os recozimentos foram realizados no intervalo de 823-1173 K por diversos tempos. As informações estatísticas do tamanho de grão foram obtidas por meio do método dos interceptos lineares. O mapeamento das orientações foi obtido por meio de difração de elétrons retroespalhados (EBSD). O material apresenta uma microestrutura ferrítica e totalmente recristalizada a partir de 873 K. Foram encontrados indícios de crescimento normal de grão a partir de 1023 K, porém este crescimento não evoluiu consideravelmente ao longo do tempo de tratamento. Os resultados obtidos a partir do método dos interceptos lineares demonstram que a distribuição de tamanho de grão segue uma curva próxima à log-normal. Nas amostras recozidas a partir de 1123 K, verificou-se a ocorrência de crescimento anormal de grão. O crescimento anormal de alguns grãos inicia-se na região central do fio, estendendo-se até uma região próxima à superfície do fio. Algumas modificações na dimensão da amostra e na atmosfera de recozimento foram realizadas a fim de se estudar a interação destas variáveis no desenvolvimento dos grãos anormais. Foram identificados indícios morfológicos de que o mecanismo para ocorrência de crescimento anormal foi o molhamento de contornos no estado sólido (solid-state wetting) que depois foram confirmados pelas análises de micro e mesotexturas. A análise de microtextura também revela que o material apresenta duas componentes preferenciais, as fibras || DT e || DT, onde DT é a direção axial de trefilação. A primeira orientação é a mais intensa, sendo que seu fortalecimento parece ser impulsionado pelo crescimento anormal de grão. A segunda é menos intensa e parece estar relacionada com as orientações dos grãos oriundos da recristalização primária. Aspectos teóricos e analíticos sobre o crescimento de grão são apresentados e associados aos resultados experimentais e à literatura. / This Dissertation aims follow the microstructural changes occurring during isothermal annealing of commercially-pure cold-drawn iron wire, with emphasys on grain growth phenomena. The grain size data were obtained by means of the linear intercept method. The orientations scanning were done by means of electron backscatter diffraction patterns (EBSD). Isothermal annealing was performed within the temperature range 823 - 1173 K for several times. The material displays full recrystallization at temperatures above 873 K. Normal grain growth was observed above 1023 K, although this growth did not evolve through longer annealing times. The results of the linear intercept method show the grain size distribution has a log-normal shape. For samples annealed above 1123 K, abnormal grain growth occurs. The first signs of secondary recrystallization appear close to the center of the wire, growing towards the surface. Changes in the wire diameter and annealing atmosphere were performed to assess the effect of these variables on abnormal grain growth. Results point out that abnormal grain growth is driven by solid-state wetting. These morphological observations were also confirmed by micro and mesotexture analyses. The microtexture also shows the presence of two major fiber texture components || WD and || WD, were WD is the axial wire-drawn direction. The first one is the most intense and its strengthening seems to be related with abnormal grain growth. The second one is weaker and it seems to be most related with small primary recrystallized grains. Theoretical and analytical features about grain growth are presented and discussed in light of literature and experimental results.

L'Inconel 718 est un superalliage base nickel très utilisé pour produire les disques de turboréacteurs. Typiquement, une gamme de forgeage à chaud se compose de plusieurs étapes de déformation et de recuit. La présence des particules de seconde phase (particules de phase delta dans l'Inconel 718) permet en principe de limiter la croissance de grains pendant les étapes de recuit grâce au phénomène d'ancrage de Zener. Néanmoins, l'hétérogénéité microstructurale (distribution des particules, écrouissage, composition chimique) peut favoriser une croissance anormale des grains pendant le recuit. Ce phénomène est connu industriellement sous la terminologie d'"éclatement de grains". Les objectifs de la thèse étaient d'identifier les mécanismes responsables de l'éclatement des grains qui peut survenir durant les étapes de recuit sur les pièces forgées en Inconel 718, de les modéliser, et de simuler numériquement le phénomène. Les mécanismes physiques à l'origine du phénomène sont d'abord étudiés expérimentalement grâce à la caractérisation (par MEB et EBSD) des pièces forgées. L'influence des particules de seconde phase et de l'énergie stockée (estimée par des mesures de désorientations intragranulaires) est notamment étudiée. A partir des observations réalisées, une explication est proposée : le phénomène apparaît lorsque les forces motrices pour la migration des joints de grains dépassent la force de freinage de Zener ; ceci peut se produire lorsque la microstructure contient de l'énergie stockée, distribuée de manière hétérogène. Des essais de torsion à chaud sont mis en place pour reproduire, en laboratoire, le même phénomène, étudier la sensibilité aux paramètres thermomécaniques, et tester les hypothèses émises concernant les mécanismes. Les mécanismes ainsi identifiés comme responsables de l'éclatement de grains sont enfin simulés au moyen d'un modèle numérique en 2D. Le modèle numérique en champ complet est basé sur la méthode des éléments finis, et utilise le formalisme level-set pour décrire les joints de grains. La simulation de l'évolution microstructurale prend en compte à la fois les forces motrices des joints de grains liées à la capillarité et à l'énergie stockée, et l'interaction des joints de grains avec les particules de seconde phase. Ainsi, l'effet de la distribution de l'énergie stockée (estimée à partir de données expérimentales) a pu être étudié numériquement dans des microstructures avec particules. / Inconel 718 is a nickel base superalloy commonly used to manufacture the rotating disks of turbojet engines. Such disks are generally produced by hot forging, which involves a sequence of different deformation and annealing steps. The presence of second phase particles (delta phase in Inconel 718) is commonly exploited to limit grain growth during annealing via the Zener pinning phenomenon. Nonetheless, microstructure heterogeneity (with regards to second phase particles, hardening, texture and chemical composition) can lead to inhomogeneous grain growth during annealing. The objectives of this PhD work were to understand, model and simulate numerically the phenomenon of inhomogeneous grain growth that can occur in Inconel 718 turbine disks during the annealing steps of hot forging sequences. The physical mechanisms which may explain the occurrence of the phenomenon are investigated experimentally by performing SEM and EBSD analyses of Inconel 718 industrial pieces. The focus is placed on the influence of second phase particles and strain energy (estimated from intragranular misorientations) on the occurrence of the phenomenon. From those observations, it is inferred that the phenomenon occurs when the grain boundary driving forces overcome the Zener pinning forces; this is achieved when stored energy is present and heterogeneously distributed. Moreover, hot torsion tests are carried out to reproduce the phenomenon in laboratory, to evaluate its sensibility to thermomechanical parameters and to test the previously postulated mechanism. The validity of this mechanism is finally demonstrated by modelling numerically the phenomenon in 2D. The full field numerical model is based on a level set description of the grain boundaries in a finite element context. Microstructure evolution is simulated explicitly taking into account Zener pinning, capillarity and stored energy driven grain growth in a single framework. The effect of strain stored energy distributions (estimated from experimental data) in pinned microstructures is investigated focusing on the conditions leading to inhomogeneous grain growth.

In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed. / text

Cemented carbides are composite materials consisting of a hard carbide and a ductile binder. They are powdermetallurgically manufactured, where liquid-phase sintering is one of the main steps. The most common cemented carbide consists of WC and Co and it is widely used for cutting tools. Two of the most important factors controlling the mechanical properties are the WC grain size and the grain size distribution and thus it is of great interest to understand the grain growth behavior. In this thesis the grain growth during sintering at 1430 °C is studied both experimentally and through computer simulations. The grain growth behavior in cemented carbides cannot be explained from the classical LSW-theory. The WC grains have a faceted shape necessitating growth by 2-D nucleation of new atomic layers or surface defects. A new model based on 2-D nucleation, long-range diffusion and interface friction is formulated. Three powders having different average sizes are studied and both experiments and simulations show that a fine-grained powder may grow past a coarse-grained powder, indicating that abnormal grain growth has taken place in the fine-grained powder. Fine-grained powders with various fractions of large grains are also studied and it is seen that a faster growth is obtained with increasing fraction of large grains and that an initially slightly bimodal powder can approach the logaritmic normal distribution after long sintering times. The grain size measurements are performed on 2-D sections using image analysis on SEM images or EBSD analysis. Since the growth model is based on 3-D size distributions the 2-D size distributions have to be transformed to 3-D, and a new method, Inverse Saltykov, is proposed. The 2-D size distribution is first represented with kernel estimators and the 3-D size distribution is optimized in an iterative manner. In this way both negative values in the 3-D size distribution and modifications of the raw data are avoided. / QC 20110426

Made available in DSpace on 2016-08-18T18:19:30Z (GMT). No. of bitstreams: 1 Eden Santos Silva.pdf: 8647632 bytes, checksum: 49c35fcae19f13234278dca9aa0bc215 (MD5) Previous issue date: 2007-07-16 / Austenitic stainless steels present high resistance to corrosion under some aggressive medium, being for this widely used in the industrial sector. These steels, when submitted to determined temperature intervals can be sensitized due to the precipitation of chromium carbides in the grain boundaries, becoming subject to intergranular corrosion. When alloy elements are added to the steel base, turning it stabilized, they can prevent sensitization, however modifying completely the behavior of the grain growth of these steels. This work investigates the behavior of the austenitic grain growth and the degree of sensitization of stainless steel austenitic AISI 321 and AISI 347, when submitted to annealing thermal treatments, between 800 and 1200° C, and to sensitization treatments at 600° C, for different times of exposition, respectively. The sensitization degree was determined qualitatively through electrochemical attack and quantitatively by the double cycle potentio-kinetics reactivation test (DLEPR). The grain growth behavior in function of the temperature disclosed that, for temperatures below the grain growth critical temperature, Tcg, due to the pinning of the grain boundaries by precipitated particles, the grain growth is completely annulled. For temperatures above Tcg, due to the dissolution and to the coarsening of particles, occurs abnormal grains growth. At high temperatures, the grains become coarse and the grain growth resembles the growth of normal grains. The Tcg varies with the austenitização time, due to the increased coarsening of precipitated particles, with the increase of time. The estimated values for the TiC and NbC particles critical ratio, on the order of, on around 10 nm, are in excellent accord with literature data. Previous annealing treatments, carried out for 800 and 900° C, had been efficient in preventing the sensitization of the steel AISI 321. AISI 347 has proved to be more efficient than AISI 321 in accomplishing this task when used at high temperatures. / Os aços inoxidáveis austeníticos apresentam alta resistência à corrosão sob vários meios agressivos, sendo por isto largamente utilizados no setor industrial. Estes aços quando submetidos a determinados intervalos de temperatura podem sensitizar, devido à precipitação de carbonetos de cromo nos contornos dos grãos, tornando-se susceptíveis à corrosão intergranular. Quando elementos de liga são adicionados ao aço base, tornando-o estabilizado, podem prevenir a sensitização, porém alteram completamente o comportamento do crescimento de grão destes aços. Este trabalho investiga o comportamento do crescimento de grão austenítico e o grau de sensitização dos aços inoxidáveis austeníticos AISI 321 e AISI 347, quando submetidos a tratamentos térmicos de solubilização, entre 800 e 1200 °C, e a tratamentos térmicos de sensitização a 600 °C, para diferentes tempos de exposição. O grau de sensitização foi determinado qualitativamente através de ataque eletroquímico e quantitativamente pelo teste de reativação potenciocinética de ciclo duplo (DLEPR). O comportamento do crescimento do grão em função da temperatura revelou que, para temperaturas inferiores à temperatura crítica de crescimento de grão, Tcg, devido ao aprisionamento dos contornos dos grãos por partículas de precipitados, o crescimento do grão é completamente anulado. Para temperaturas acima da Tcg, devido à dissolução e ao coalescimento das partículas, ocorre o crescimento anormal dos grãos. A altas temperaturas os grãos tornam-se grosseiros e o crescimento do grão assemelha-se ao crescimento do grão normal. A Tcg varia com o tempo de austenitização, em face ao aumento do coalescimento das partículas de precipitados com o aumento do tempo. Os valores estimados para o raio crítico das partículas de TiC e NbC, da ordem de 10 nm, encontram-se em excelente acordo com os dados da literatura. Prévios tratamentos térmicos de solubilização, realizados para 800 e 900 oC, foram eficientes em prevenir a sensitização do aço AISI 321. O aço AISI 347 apresenta-se mais eficiente que o AISI 321 quando usado em altas temperaturas.

A geração de novas fontes de energia limpa, segura e renovável por meio da fusão nuclear envolve importantes desafios tecnológicos, dentre eles a pesquisa, caracterização e a fabricação de materiais avançados para os futuros reatores de fusão nuclear. Os aços ferrítico-martensíticos de reduzida atividade radioativa, em especial a liga Eurofer-97, destacam-se por apresentar uma combinação única de propriedades para esta aplicação. O objetivo desta Tese de Doutorado é avaliar a estabilidade microestrutural deste aço recozido numa ampla faixa de temperaturas. Cálculos termodinâmicos e testes de dilatometria foram usados para determinar as temperaturas de transformação de fase. A estabilidade microestrutural foi estudada por meio de recozimentos isotérmicos entre 200 e 1350oC após laminação a frio com reduções de 40, 70, 80 e 90%. A avaliação da estabilidade mecânica do aço Eurofer-97 foi realizada por meio de medidas de dureza Vickers. As principais técnicas utilizadas para caracterização microestrutural foram microscopias eletrônica de varredura e de transmissão, tomografia por sonda atômica e medidas de magnetização DC. Tanto a textura como a microtextura foram determinadas por meio de medidas de difração de raios X e de elétrons retroespalhados (EBSD). Recuperação, recristalização primária e crescimento anormal de grão ocorrem neste material recozido abaixo de 800oC. Acima desta temperatura, a transformação martensítica ocorre alterando bastante a micoroestrutura. A cinética de crescimento anormal de grão é alterada pela quantidade de redução a frio previamente aplicada. A hipótese proposta para explicar o crescimento anormal de grãos neste material baseia-se principalmente na vantagem de tamanho adquirida pelos núcleos de recristalização primária com diferenças de orientação médias superiores a 45º em relação aos vizinhos. Neste caso, o crescimento anormal de grão é responsável por fortalecer as componentes {111} e {111}, {001} e {110}. Acima de 800oC a transformação martensítica prevalece elevando a dureza Vickers e randomizando a textura deste aço. As características do produto transformado dependem tanto da temperatura de austenitização quanto do tamanho incial do grão ferrítico. Os dados de composição química das partículas estáveis após recozimento em temperaturas inferiores a 800oC foram usados para validar os resultados dos cálculos termodinâmicos obtidos via Thermo-Calc. / Clean, safe, and renewable energy sources such as nuclear fusion comprise important technological challenges, including research, characterization and manufacture of advanced materials for future fusion reactors. Modified ferritic-martensitic steels with reduced radioactive activity (RAFM), especially Eurofer-97 steel, are among worldwide references in the nuclear field for their unique properties. The scope of this Thesis is to evaluate the microstructural (thermal) stability in ferritic-martensitic Eurofer-97 after annealing within a wide range of temperatures. Themodinamic calculations as well as dilatometric tests were used to determine the main phase transformation temperatures. The microstructural stability of this steel was followed by isothermal annealing between 200 and 1350°C after cold rolling to 40, 70, 80 and 90% reductions in thickness. The mechanical stability in the Eurofer-97 was assessed by Vickers microhardness measurements. Representative samples for each metallurgical condition were characterized by scanning electron microscopy, transmission electron microscopy, atom probe tomography, and DC-magnetization tests. Both texture and microtexture were evaluated by X-ray diffraction and electron backscattered diffraction (EBSD) techniques. Recovery, primary recrystallization, and abnormal grain growth (secondary recrystallization) processes have been observed at temperatures below 800°C. The amount of abnormally grown grains depends on the amount of previous cold rolling. The hypothesis for the most probable mechanism responsible for abnormal grain growth is based on the advantage size acquired by nuclei with misorientations above 45º surrounding their neighboring grains, even in regions where primary recrystallization was incomplete. The texture developed after abnormal grain growth has components belonging to ?- and ?-fibers with predominance of {111}, {111}, {100} e {110} components. The martensite transformation takes place when this steel is annealed above 800°C causing an increase of hardness, significant changes in microstructure, and texture weakening. The martensitic sructure depends very much on both austenitization temperature and initial austenitic grain size. The results of chemical analyses of stable particles present in samples annealed below 800oC were used to validate the thermodynamic calculations provided by Thermo-Calc.

Este trabalho tem como objetivo caracterizar as mudanças microestruturais durante o recozimento isotérmico de um fio de ferro trefilado a frio e de pureza comercial com ênfase no crescimento de grão. Os recozimentos foram realizados no intervalo de 823-1173 K por diversos tempos. As informações estatísticas do tamanho de grão foram obtidas por meio do método dos interceptos lineares. O mapeamento das orientações foi obtido por meio de difração de elétrons retroespalhados (EBSD). O material apresenta uma microestrutura ferrítica e totalmente recristalizada a partir de 873 K. Foram encontrados indícios de crescimento normal de grão a partir de 1023 K, porém este crescimento não evoluiu consideravelmente ao longo do tempo de tratamento. Os resultados obtidos a partir do método dos interceptos lineares demonstram que a distribuição de tamanho de grão segue uma curva próxima à log-normal. Nas amostras recozidas a partir de 1123 K, verificou-se a ocorrência de crescimento anormal de grão. O crescimento anormal de alguns grãos inicia-se na região central do fio, estendendo-se até uma região próxima à superfície do fio. Algumas modificações na dimensão da amostra e na atmosfera de recozimento foram realizadas a fim de se estudar a interação destas variáveis no desenvolvimento dos grãos anormais. Foram identificados indícios morfológicos de que o mecanismo para ocorrência de crescimento anormal foi o molhamento de contornos no estado sólido (solid-state wetting) que depois foram confirmados pelas análises de micro e mesotexturas. A análise de microtextura também revela que o material apresenta duas componentes preferenciais, as fibras || DT e || DT, onde DT é a direção axial de trefilação. A primeira orientação é a mais intensa, sendo que seu fortalecimento parece ser impulsionado pelo crescimento anormal de grão. A segunda é menos intensa e parece estar relacionada com as orientações dos grãos oriundos da recristalização primária. Aspectos teóricos e analíticos sobre o crescimento de grão são apresentados e associados aos resultados experimentais e à literatura. / This Dissertation aims follow the microstructural changes occurring during isothermal annealing of commercially-pure cold-drawn iron wire, with emphasys on grain growth phenomena. The grain size data were obtained by means of the linear intercept method. The orientations scanning were done by means of electron backscatter diffraction patterns (EBSD). Isothermal annealing was performed within the temperature range 823 - 1173 K for several times. The material displays full recrystallization at temperatures above 873 K. Normal grain growth was observed above 1023 K, although this growth did not evolve through longer annealing times. The results of the linear intercept method show the grain size distribution has a log-normal shape. For samples annealed above 1123 K, abnormal grain growth occurs. The first signs of secondary recrystallization appear close to the center of the wire, growing towards the surface. Changes in the wire diameter and annealing atmosphere were performed to assess the effect of these variables on abnormal grain growth. Results point out that abnormal grain growth is driven by solid-state wetting. These morphological observations were also confirmed by micro and mesotexture analyses. The microtexture also shows the presence of two major fiber texture components || WD and || WD, were WD is the axial wire-drawn direction. The first one is the most intense and its strengthening seems to be related with abnormal grain growth. The second one is weaker and it seems to be most related with small primary recrystallized grains. Theoretical and analytical features about grain growth are presented and discussed in light of literature and experimental results.

Polycrystalline geological materials are not normally single phase materials and commonly contain second phases which are known to influence the grain size and mechanical properties of bulk material. Despite the well documented significance of second phases, there are relatively few detailed systematic experimental studies of the effect of second phases on isostatic high temperature grain growth in geological materials. Grain growth is a process that is fundamental to our understanding of how rocks behave in the lower crust / upper mantle where grain size is considered to play an important role in the localization of deformation in addition to determining the strength of materials at these pressure and temperature conditions. Furthermore, the effect that the spatial distribution and grain size of the second phases have on the mechanical properties of rocks is generally acknowledged, but it is not well constrained. Spatial variation is particularly significant in geological systems where a strength contrast exists between phases. With these two things in mind, a two-part study is presented in which the influence of a pore second phase on the microstructural evolution of halite during grain growth (Part I), and the influence of a calcite second phase on the mechanical behaviour of two phase calcite + halite aggregates (Part II), is investigated. In Part I, high temperature (330 °-600 °C), high confining pressure (200 MPa) isostatic grain growth experiments were carried out on 38-125 μm reagent grade halite (99.5%+ NaCl) powder over durations of 10 secs up to 108 days. After hot-pressing, the halite displays a foam texture. Some porosity remained along the grain boundaries, the size and distribution of which appears to impact significantly on the resulting grain size, growth mechanism and kinetics of halite grain growth. Halite grain growth was found to be well described by the normal grain growth equation: d^(1/n)-d0^(1/n)=k0(t-t0)exp(-H/RT) where t is the duration of the growth period, t0 is the time at which normal growth begins, d is the grain size, d0 is the grain size at t0, k0 is a constant, H is the activation enthalpy for the growth controlling process, R is the universal gas constant,T is temperature and n is a growth constant. At 330 °-511 °C, the data is best described by n = 0.25 indicating growth controlled by surface diffusion around pores that lie on the grain boundaries. An activation enthalpy of 122±34 kJ/mol was obtained using the grain size data from these data sets. At 600 °C the data is best described by n = 0.5, suggesting that a transition to interface controlled growth takes place between 511 °C and 600 °C. To investigate the impact of porosity, the Zener parameter (Z = pore size/pore volume fraction) was determined for individual grains in 10 samples. A general trend of increasing with increasing halite grain size is observed, indicating pore elimination keeps pace with pore accumulation in the growing grains. In some samples, the largest grains display a decrease in the Zener parameter corresponding with an increase in pore volume fraction. These grains are interpreted as having experienced a short-lived, abnormal growth phase shortly after t0 during which pore accumulation outpaced pore elimination. A model of pore controlled grain growth is proposed with a view to explaining these observations. In Part II, calcite + halite aggregates of constant volume fraction (0.60 calcite : 0.40 halite) and varying calcite clast size (6 μm 361 μm) were axially deformed to <1% bulk strain at room temperature in a neutron diffraction beamline. Elastic strain and stress in each phase was determined as a function of load from the neutron diffraction data. The strain (and stress) behaviour correlates well with the microstructural parameters: 1) halite mean free path and 2) calcite contiguity. Both phases behaved elastically up to aggregate axial stresses of 20-37 MPa, above these stresses the halite yielded plastically while the calcite remained elastic. Once yielding began, the rate of enhanced load transfer from halite to calcite with increasing applied load decreased with halite mean free path and increased calcite with contiguity. A Hall-Petch relationship between halite mean free path and aggregate yield stress was observed.