O presente trabalho visa avaliar a microestrutura em chapas de um aço de alta resistência e baixa liga após soldagem por centelhamento e quantificar a resistência ao crescimento de trincas por fadiga ao longo do cordão de solda e da zona termicamente afetada, comparando-a ao comportamento do material base. O aço em estudo, recentemente desenvolvido pela Companhia Siderúrgica Nacional sob a designação RD480, foi fornecido na forma de chapas com 5,0 mm de espessura e soldado pelo processo de centelhamento na divisão de rodas e chassis da IOCHPE-MAXION. A avaliação microestrutural do aço após o processo de soldagem por centelhamento foi realizada nas regiões do metal de base, zona termicamente afetada e metal de solda, utilizando microscopia ótica com diferentes ataques químicos e microscopia eletrônica de varredura. A caracterização mecânica foi realiza com ensaio de tração, teste de dureza Vickers e ensaio de propagação de trincas por fadiga. Para este último foram adotados corpos-de-prova do tipo compacto C(T) e carregamento senoidal de amplitude constante com frequencia 10 Hz e razão de tensão R= 0,1 e as curvas obtidas deste ensaio foram avaliadas em relação a dois modelos matemáticos, o de Paris-Erdogan e o exponencial. Após o ensaio de propagação de trincas por fadiga, as superfícies dos corpos-de-prova foram avaliadas por microscopia eletrônica de varredura. Os resultados encontrados permitiram correlacionar a resistência mecânica e a taxa de crescimento da trinca com as características microestruturais resultantes do processo de soldagem. / This study aims to evaluate the microstructure of a high strength, low alloy (HSLA) steel after flash welding and to quantify the resistance to fatigue crack growth along the weld and heat affected zone, comparing it to the behavior of the base material. The steel under study, recently developed by the Companhia Siderúrgica Nacional under the designation RD480, was provided in the form of plates with 5.0 mm in thickness and welded by IOCHPE-MAXION, division of wheels and chassis. The microstructural evaluation of the steel after the flash welding process was performed at the base metal, heat affected zone and weld metal, using optical microscopy with different chemical attacks and scanning electron microscopy (SEM). The mechanical characterization was performed by means of tensile test, Vickers hardness measurement and fatigue crack growth (FCG) test. Compact Tension C(T) specimens were chosen for the fatigue tests, and the loading was sinusoidal with constant amplitude, frequency of 10 Hz and stress ratio R = 0.1. The crack propagation test results were summarized in terms of FCG rate (da/dN) versus stress intensity factor range (?K) curves. In order to describe the FCG behavior, two models were tested: the conventional Paris equation and a new exponential equation developed for materials showing non-linear FCG behavior. The fracture surfaces of the fatigued specimens were examined via SEM in the secondary electrons regime. The results allowed correlating the mechanical strength and crack growth rate with the microestrutural characteristics resulting from the welding process.

Orientador: Juno Gallego / Resumo: As superfícies desgastadas são normalmente revestidas com ligas metálicas resistentes ao desgaste abrasivo, nos quais durante a soldagem o metal de solda com matriz ferrosa é reforçado pela formação de carbonetos grosseiros durante a solidificação da poça de fusão. O propósito deste trabalho é investigar a aplicação de micro-ondas para o preparo de cavacos das ligas de titânio ASTM F67 e ASTM F136, visando a sua fragilização pela formação de uma fina camada de carbonetos que facilitam a moagem e a obtenção de pós finos. O pó obtido da reciclagem do titânio foi misturado com grafite, tornando possível a formação de carbonetos de titânio (TiC) durante a fusão da mistura sobre peças de aço-carbono ABNT 1020 com uma tocha a arco elétrico com proteção gasosa (GTAW). A aplicação de micro-ondas foi bem sucedida, permitindo a obtenção da mistura carbonetante com granulometria inferior a 45m. O metal de solda mostrou uma microestrutura ferrítica reforçada por partículas grosseiras TiC, que incrementaram significativamente a dureza do material. Os resultados obtidos indicam que a reciclagem de cavacos de titânio seja potencialmente interessante para o desenvolvimento de um consumível de soldagem para revestimentos resistentes ao desgaste abrasivo, mais econômico e ambientalmente sustentável. / Abstract: The worn surfaces are usually coated with abrasive resistant metal alloys, where the weld metal is reinforced by the formation of coarse hard carbides during the melt pool solidification. The aim of this work is investigate the irradiation of microwaves on scrap (chips) of ASTM F67 and ASTM F136 titanium alloys, becoming easier grinding of the residues for obtaining fine powder due to chips embrittlement by thin TiC coating. The titanium-rich recycled powder was mixed with graphite, making possible the formation of titanium carbides (TiC) during the melting of the mixture by Gas-Tungsten Arc Welding (GTAW) carried out on ABNT 1020 carbon steel pieces. The microwave application was very successful, allowing to obtain a fine carbide-former mixture with granulometry less than 45m. The weld metal have showed a microstructure composed by ferrite (BCC iron) reinforced by coarse dendritic TiC particles, which promoted significantly hardness increasing. The experimental results have indicate that microwave irradiation for titanium alloys chips recycling is potentially interesting for the development of a economical and environmentally sustainable consumables for abrasive wear resistant hardfacing. / Mestre

O aço SAE AISI 52100 com microestrutura martensítica revenida é utilizado para a fabricação de rolamentos como também na fabricação de outras peças e dispositivos para aplicações marinhas. A modificação deste aço com a adição de silício e manganês pode melhorar as propriedades mecânicas em média-alta temperatura. Este aço modificado ao Si e Mn apresenta grande resistência ao amolecimento durante o revenimento, o que pode ser explorado quanto ao comportamento em altas temperaturas. Para a realização deste trabalho, foi utilizado o aço AISI 52100 com composição química conforme estabelece a norma ASTM A295- 2014, e um aço experimental com 1,82% de Si e 1% de Mn baseado no aço AISI 52100. Após tratamento térmico de normalização, investigou-se os fenômenos de transformação de fase por meio de estudos microestruturais, mecânicos e corrosivos em diferentes temperaturas. A adição de Si e Mn junto ao tratamento de normalização conferiram melhor dureza, tenacidade ao impacto e maior resistência à tração a 25, 300 e 400 oC, porém diminuiu a resistência à corrosão comparada à do aço AISI 52100 na condição utilizada nas indústrias. Portanto, o aço modificado normalizado se mostrou adequado para utilização em maiores temperaturas que não exijam resistência à corrosão em solução de NaCl 3,5 %p. / SAE AISI 52100 steel with tempered martensitic microstructure is used for the manufacture of bearings as well as in the manufacture of other parts and devices for marine applications. The modification of this steel with the addition of silicon and manganese can improve the mechanical properties in medium-high temperature. This Si-Mn modified steel exhibits high resistance to softening during tempering, which can be exploited in high temperature behavior. For this work, it was used the AISI 52100 steel with chemical composition in accordance to ASTM A295- 2014 and an experimental steel with 1.82% Si and 1% Mn, based on AISI steel 52100. After normalization heat treatment, the phenomena of phase transformation were investigated through microstructural, mechanical and corrosive studies at different temperatures. The addition of Si and Mn together with the normalization treatment improve hardness, impact toughness and tensile strength at 25, 300 and 400 oC, but decreased the corrosion resistance compared to AISI 52100 steel in the condition used in the industries. Therefore, normalized modified steel proved to be suitable for use at higher temperatures that do not require corrosion resistance in 3.5%w. NaCl solution.

Neste trabalho os efeitos da soldagem por fricção e mistura na resistência à corrosão da liga 2098-T351 foram avaliados e correlacionados com sua microestrutura. Neste sentido, técnicas eletroquímicas convencionais e locais (EIS, curvas de polarização, SVET, SECM e LEIS) em conjunto com técnicas de caracterização microestrutural (XPS, SEM, TEM, DSC, microdureza, perfilometria ótica) foram utilizadas. Devido a pouca informação encontrada na literatura sobre este material, uma caracterização prévia da liga na condição como recebida pelo fabricante foi realizada. Com isso, observou-se a existência da influência da preparação da amostra na resistência à corrosão da liga. Amostras na condição como recebida apresentaram maior resistência à corrosão do que as amostras polidas. Em seguida, a liga 2098-T351 foi caracterizada e sua resistência à corrosão comparada com a da liga convencional 2024-T3, amplamente usada na indústria aeroespacial. Os resultados mostraram diferentes mecanismos de propagação da corrosão para as duas ligas. Em termos de resistência à corrosão, a liga 2098-T351 apresentou potencial para substituir a liga 2024-T3 em termos de resistência a penetração do ataque localizado. Por fim, a reatividade das diferentes zonas de soldagem foi correlacionada com as respectivas características microestruturais. Os resultados mostraram que a região da junta soldada, que compreende a região da zona misturada e zona termomecânicamente afetada, foi mais resistente à corrosão do que a zona termicamente afetada e o metal base. A alta densidade da fase T1 (Al2CuLi) nestas ultimas regiões, comparadas à junta soldada foi considerada responsável por esse comportamento. / In this work, the effects of friction stir welding on the corrosion resistance of the 2098-T351 alloy were evaluated and correlated with its microstructure. In this sense, conventional and local eletrochemical techniques (EIS, polarization curves, SVET, SECM e LEIS) together with microstructural characterization techniques (XPS, SEM, TEM, DSC, microhardness, optical profilometry) were used. Due to the little information found in the literature on the 2098-T351 alloy a prior characterization of the alloy in the as received condition by the manufacturer was carried out. Thus, the influence of sample preparation on the corrosion resistance of the alloy was observed. Samples in the as-received condition showed higher corrosion resistance than the polished ones. Next, the the 2098-T351 alloy was characterized and its corrosion resistance compared to that of the conventional 2024-T3, widely used in the aerospace industry. The results showed different mechanisms of corrosion propagation for the two alloys. In terms of corrosion resistance, the 2098-T351 alloy presents potential to replace the conventional 2024-T3 alloy due to its resistance to corrosion attack penetration. Finally, the reactivity of the different FSW welding zones was correlated with their microstructural features. The results showed that the weld joint, which comprises the stir zone and the thermomechanically affected zone, was more resistant to corrosion than the heat affected zone and base metal. The high density of T1 phase (Al2CuLi) in these last zones, compared to the weld joint was considered responsible for this behavior.

Ligas de titânio do tipo ??compostas de elementos não tóxicos são materiais com potencial para aplicações biomédicas por apresentarem baixo módulo de elasticidade, efeito de memória de forma, biocompatibilidade satisfatória e boa conformabilidade. Para os biomateriais, o conhecimento das propriedades de fadiga é essencial para garantir uma alta confiabilidade para implantes ortopédicos e odontológicos. As propriedades mecânicas dependem diretamente do processamento termomecânico, da taxa de resfriamento imposta à liga e da composição química, que são fatores responsáveis pela determinação de sua microestrutura. Neste contexto, neste trabalho foi avaliado o comportamento em fadiga da liga Ti-35Nb-7Zr (% em p.) para aplicações biomédicas. Trata-se de uma liga de titânio do tipo ??e a motivação desse estudo baseou-se em complementar resultados de suas propriedades microestruturais e mecânicas obtidos em estudos anteriores realizados no DEMAR - EEL/USP. A liga foi produzida por fusão a arco a partir de materiais (Ti, Nb, Zr) de pureza comercial. A rota de processamento termomecânico envolveu as etapas de tratamento térmico de solubilização, forjamento rotativo a frio, tratamento térmico de recristalização. Um tratamento térmico ultrarrápido (Flash) foi adicionado para promover o refino de grãos e aumento da resistência mecânica. A caracterização microestrutural foi realizada por técnicas de microscopia óptica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, difratometria de raios X e medidas de dureza Vickers. As propriedades mecânicas foram investigadas por ensaios de tração uniaxial e ensaios de fadiga de alto ciclo por flexão rotativa de corpos de prova lisos e entalhados. Com relação ao conjunto de propriedades, os melhores resultados podem ser considerados para a condição recristalizada à 1000ºC/2h + flash. Os resultados obtidos neste trabalho confirmam a possibilidade de uso da liga Ti-35Nb-7Zr para aplicações biomédicas. / ?-type titanium alloys composed of non-toxic elements are materials with potential for biomedical applications because they have low elastic modulus, shape memory effect, satisfactory biocompatibility and good workability. For biomaterials, knowledge of fatigue properties is essential to ensure high reliability for orthopedic and dental implants. The mechanical properties depend directly on thermomechanical processing, alloy cooling rate and chemical composition, which are responsible for the determination of its microstructure. In this context, this work was evaluated the fatigue behavior of the Ti- 35Nb-7Zr alloy (wt. %) for biomedical applications. It is a ?-type alloy and the motivation of this study was based on complementing the results of its microstructural and mechanical properties previously obtained at DEMAR-EEL/USP. The alloy was produced from materials of commercial purity (Ti, Nb and Zr) by arc meling. The thermomechanical processing route consisted the steps of solubilization heat treatment, cold rotary forging, recrystallization heat treatment. A ultrafast thermal treatment (Flash) was additioned to promote the grain refinement and to increase mechanical strength. The microstructural characterization was done by optical microscopy, scanning electron microscopy, transmission electron microscopy, X ray diffraction techniques and Vickers microhardness tests. The mechanical properties was investigated by uniaxial tensile tests and rotary bending high cycle fatigue tests of smooth and notched specimens. With respect to the set of properties, the best results can be considered for recrystalized condition at 1000ºC/2h + flash. The results obtained in this work confirm the possibility of using the Ti-35Nb-7Zr alloy for biomedical applications.

A liga de alumínio AA 6351 é uma liga endurecível por precipitação coerente do sistema Al-Si-Mg, contendo adicionalmente Mn e Fe. Neste trabalho foi realizada a caracterização microestrutural durante o processamento de um tubo extrudado de parede grossa da liga AA 6351. Foram utilizadas várias técnicas complementares de caracterização microestrutural, tais como microscopia óptica de luz polarizada, microscopia eletrônica de varredura eletrônica (MEV), espectroscopia de raios X por dispersão de energia (EDS), difração de elétrons retroespalhados (EBSD), medidas de dureza e de condutividade elétrica. Foram analisadas as principais características dos grãos e precipitados presentes. Após solidificação, foi constatada a presença de uma distribuição relativamente homogênea de grãos com diâmetro médio de aproximadamente 100 ?m e de uma fase intermetálica com morfologia alongada e rica em Al, Fe, Mn e Si. Durante a extrusão, o tamanho de grão inicial foi reduzido para cerca de um quinto do diâmetro inicial e a fase alongada, provavelmente ?-Al(FeMn)Si, foi fraturada e redistribuída e as partículas resultantes da fragmentação foram alinhadas na direção de extrusão. Foi detectada e analisada uma zona periférica de grãos grosseiros nas proximidades das superfícies externa e interna do tubo. Tratamentos térmicos complementares realizados na faixa de 400 a 550ºC em amostras retiradas do tubo na condição T6 e medidas de condutividade elétrica revelaram que a dissolução de partículas associada à queda de condutividade elétrica tem início por volta de 440ºC. / The aluminum alloy AA 6351 is a precipitation hardening alloy of the Al-Mg-Si system, additionally containing Mn and Fe. In this work the microstructural characterization during processing of a thick wall extruded tube of the AA 6351 was carried out. Several complementary techniques of microstructural characterization were used, such as polarized light optical microscopy, electronic scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), backscattered electron diffraction (EBSD), hardness and electrical conductivity measurements. The main characteristics of the grains and precipitates were analyzed. After solidification, the presence of a relatively homogeneous grain distribution with an average diameter of approximately 100 ?m and of an intermetallic phase with elongated morphology rich in Al, Fe, Mn and Si was observed. During extrusion, the initial grain size was reduced to about one-fifth of the initial diameter and the elongated phase, probably ?-Al (FeMn) Si, was fractured and redistributed and the resulting particles of the fragmentation were aligned in the extrusion direction. A peripheral coarse grain zone was detected and analyzed in the vicinity of the outer and inner surfaces of the tube. Complementary thermal treatments performed in the range of 400 to 550ºC in samples taken from the tube in the T6 condition and electrical conductivity measurements revealed that the precipitate particles dissolution associated with the electrical conductivity drop starts at around 440ºC.

Cette thèse s’est déroulée dans le cadre d’une cotutelle internationale entre l’IMT - École des Mines d’Albi-Carmaux et l’Ecole Polytechnique de l’Université de Sao Paulo (EPUSP). Elle a pour but de contribuer à l’étude du comportement mécanique des alliages de titane Ti6Al4V, et plus spécialement dans le domaine du formage superplastique et du formage à chaud. L’objectif général de ce doctorat est de contribuer au développement de procédés de formages non conventionnels des alliages de titane pour applications aéronautiques. C’est pourquoi, en fonction des équipements disponibles sur les deux sites, les travaux de recherche se sont déroulés soit à l’Ecole des Mines soit à l’EPUSP. Cette thèse adresse la problématique scientifique des interactions entre le comportement mécanique dans le domaine du formage superplastique et de formage à chaud d’une part et la microstructure initiale et son évolution dans le domaine de sollicitation d’autre part. Pour cela une stratégie d’essais et de caractérisation a été développée et suivie. Les essais incluent des essais mécaniques uniaxiaux à haute température sur différents alliages Ti6Al4V présentant des microstructures initiales différentes (taille de grain 0,5; 3,0 et 4,9 μm). Une attention particulière a été portée à l’évolution microstructurale avant essai - c’est- à-dire durant sa montée en température et la stabilisation thermique de l’échantillon – et durant l’essai. Les conditions d’essai ont été choisies de façon à couvrir le domaine du formage à chaud et du formage superplastique, température de 700°C à 950°C et vitesse de déformation entre 10-1 s-1 et 10-4 s-1. La croissance de grain dépend de la microstructure initiale mais aussi de la durée de l’essai en température (croissance statique) et de la vitesse de déformation (croissance dynamique). Afin d’améliorer la validité du modèle des observations microstructurales de taille de grain sont effectués après les essais mécaniques par micrographie optique et Microscope Electronique à Balayage. Un modèle de comportement unifié a été introduit de façon à être capable de couvrir toute la plage de température et de vitesse de déformation : écrouissage cinématique non linéaire, sensibilité à la vitesse de déformation et loi de croissance de la taille des grains sont inclus dans le modèle. Afin de pouvoir valider le modèle, il a été introduit dans le code de simulation ABAQUS®. Les résultats des simulations (en particulier déformation macroscopique et taille de grain locale) ont été comparés, pour l’un des matériaux de l’étude, aux résultats d’un essai de gonflage axisymétrique de tôle. Pour obtenir un cycle de contrôle simple, les essais effectués au laboratoire de l’IPT/LEL à Sao José dos Campos au Brésil ont été opérés à vitesse de déformation constante. Les résultats montrent une très bonne corrélation avec les prédictions et permettent de conclure à une validation du modèle de comportement développé dans la thèse dans des conditions industrielles de formage de l’alliage de titane. / This thesis was developed in the frame of a Brazil-France cooperation agreement between the École des Mines d'Albi-Carmaux and the Polytechnic School of Engineering of the University of Sao Paulo (EPUSP). It aims to contribute to the study of the mechanical behaviour of Ti6Al4V alloys especially in terms of superplastic forming. The general objective of this research is to develop non-conventional forming processes for new titanium alloys applied to aerospace components. Therefore, in accordance of the equipment’s available in the two groups, the work will be conducted either at the Ecole des Mines d'Albi-Carmaux and either at EPUSP. This thesis aims to answer questions such as what are the implications in relation to the microstructural and mechanical behaviour of these alloys during superplastic and hot forming in order to establish a behaviour law for these alloys based on titanium. This requires a good knowledge of the properties of materials used in the superplastic and hot forming domain to control the parameters governing the phenomenon of superplasticity or high temperature plasticity. For this, a testing strategy and characterization methodology of those new titanium alloys was developed. The tests include high temperature uniaxial tensile tests on several Ti6Al4V alloys showing different initial grain sizes. Special focus was made on the microstructural evolution prior to testing (i.e. during specimen temperature increase and stabilization) and during testing. Testing range was chosen to cover the hot forming and superplastic deformation domain. Grain growth is depending on alloy initial microstructures but also on the duration of the test at testing temperature (static growth) and testing strain rate (dynamic growth). After testing microstructural evolutions of the alloys will be observed by optical micrograph or SEM and results are used to increase behaviour model accuracy. Advanced unified behaviour models where introduced in order to cover the whole strain rate and temperature range: kinematic hardening, strain rate sensitive and grain growth features are included in the model. In order to get validation of the behaviour model, it was introduced in ABAQUS numerical simulation code and model predictions (especially macroscopic deformation and local grain growth) were compared, for one of the material investigated, to axisymmetric inflation forming tests of sheet metal parts, also known as bulge test. To obtain a simple control cycle, tests performed at IPT/LEL laboratory in San José Dos Campos in Brazil were operated with a constant strain rate. Results show a very good correlation with predictions and allows to conclude on an accuracy of the behaviour models of the titanium alloys in industrial forming conditions.

Methods for and computed results of including the physics and spatial attributes of microstructures are presented for a number of materials applications in devices. The research in our group includes applications of computation of macroscopic response of material microstructures, the development of methods for calculating microstructural evolution, and the morphological stability of structures. In this review, research highlights are presented for particular methods for computing the response in: 1) ferroelectric materials for actuator devices; 2) coarse-graining of atomistic data for simulations of microstructural evolution during processing; 3) periodic and non-periodic photonic composites; and 4) re-chargeable battery microstructures. / Singapore-MIT Alliance (SMA)

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

Diffusion tensor imaging (DTI) is a specialist MRI modality that can identify microstructural changes or abnormalities in the brain. It can also be used to show fibre tract pathways. Both of these features were used in this thesis. Firstly, standard imaging analysis techniques were used to study the effects of mild, repetitive closed head injury on a group of professional boxers. Such data is extremely rare, so the findings of regions of brain abnormalities in the boxers are important, adding to the body of knowledge about more severe traumatic brain injury. The author developed a novel multivariate analysis technique which was used on the same data. This new technique proved to be more sensitive than the standard univariate methods commonly used. An important part of diagnosing and monitoring brain damage involves the use of biomarkers. A novel investigation of whether diffusion parameters obtained from DTI data could serve as bio-markers of cognitive impairment in Parkinson's disease was conducted. This also involved developing a multivariate approach, which displayed increased sensitivity compared with any of the component parameters used singly, and suggested these diffusion measures could be robust bio-markers of cognitive impairment. Fibre tract connectivity between regions of the brain is also a potentially valuable measure for diagnosis and monitoring brain integrity. The feasibility of this was investigated in a multi-modal MRI study. Functional MRI (fMRI) identifies regions of activation associated with a particular task. DTI can then find the pathway of the fibre bundles connecting these regions. The feasibility of using regional connectivity to interrogate brain integrity was investigated using a single healthy volunteer. Fibre pathways between regions activated and deactivated by a working memory paradigm were determined. Though the results are only preliminary, they suggest that this line of research should be continued.