Submitted by Celso Bortolini Júnior null (celsobortjr@gmail.com) on 2016-03-14T21:15:08Z No. of bitstreams: 1 Dissertação Celso Bortolini Junior.pdf: 5854950 bytes, checksum: e97a433411ce86089561ffba7119ff02 (MD5) / Approved for entry into archive by Ana Paula Grisoto (grisotoana@reitoria.unesp.br) on 2016-03-15T13:34:55Z (GMT) No. of bitstreams: 1 bortolinijunior_c_me_guara.pdf: 5854950 bytes, checksum: e97a433411ce86089561ffba7119ff02 (MD5) / Made available in DSpace on 2016-03-15T13:34:55Z (GMT). No. of bitstreams: 1 bortolinijunior_c_me_guara.pdf: 5854950 bytes, checksum: e97a433411ce86089561ffba7119ff02 (MD5) Previous issue date: 2016-01-22 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Titânio e suas ligas são utilizados em implantes biomédicos devido a suas excelentes propriedades de volume, como resistência mecânica. Porém, tais propriedades estão diretamente ligadas à microestrutura, que por sua vez depende do processamento. O objetivo deste trabalho foi avaliar a microestrutura e a evolução da recristalização da liga Ti30Ta deformada a frio, após tratamento térmico. Os lingotes da liga Ti30Ta foram obtidos em um forno de fusão a arco voltaico. A liga foi tratada em um forno tubular sob vácuo a 950ºC por 24 horas para garantir a homogeneidade química e então foi forjada a frio em barras de 10 mm de diâmetro. As amostras foram solubilizadas a 900ºC por 2 horas e temperadas em água. Discos de 3 mm de espessura foram usados para análise. Para a avaliação da recristalização, foi realizado um tratamento térmico a 750ºC, 800ºC e 850ºC por 12 horas seguido de têmpera em água. A morfologia, composição e estrutura foram investigadas usando microscopia óptica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão e difração de raios X. As análises de microscopia e difração de raios X indicaram a presença apenas da fase martensítica α'' na amostra solubilizada e a presença das fases α'' e α' nas amostras recristalizadas. Os ensaios mecânicos mostraram que houve uma melhoria nas propriedades mecânicas após a recristalização, sendo a temperatura de 750° que teve maior influência, porém houve também um incremento no módulo de elasticidade, o que é indesejado. / Titanium and its alloys have been used in biomedical devices due their excellent bulk properties, such as mechanical strength. However, these properties are very sensitive of microstructures which depend of the processing. The purpose of this study was to evaluate the microstructure and evolution of recrystallization of the could-swaged Ti30Ta alloy after heat treatment. Ingots of Ti30Ta alloy were obtained in arc melting furnace. Alloys were treated at 950° C for 24 hours for chemical homogenization in vacuum tubular furnace and cold worked by swaging into rods with 10 mm in diameter. Samples where then solution treated at 900° C for 2 hours and water quenched. Discs with 10 mm in diameter and 3 mm in thickness were used for analysis. For recrystallization evaluation, heat treatment was carried out at 750º C, 800º C and 850º C for 12 hours followed by water quenching. The morphology, composition and structure were investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy and XRD (X-rays Diffraction Analysis). The microscopy and XRD analysis indicate the presence of the martensitic phase α'' for the solution treated sample and the presence of both α'' and α' phases for the recrystallized samples. Mechanical tests shows an increase in the mechanical strength to all the samples after recrystallization, with the temperature of 750° being the most impacting, however, it led an unwanted increase to the elastic modulus.

L'évolution du contexte industriel pousse l'industrie du transport, et plus particulièrement le secteur automobile, à réaliser des gains de de masse. Ceci passe, pour partie, par le développement de nouvelles solutions en matériaux composites. Le présent travail de thèse est consacré à la caractérisation mécanique et à la modélisation micromécanique d'un nouveau matériau composite SMC renforcé de mèches de fibres de carbone. L'objectif est de constituer une première base de connaissances sur le comportement de ce SMC en fatigue. Les investigations expérimentales passent notamment par l'analyse de la microstructure, la caractérisation du comportement mécanique sous sollicitation quasi-statique et de fatigue ainsi que par l'analyse des modes de dégradations. L'approche multi-échelle développée prend en compte la microstructure du matériau aux deux échelles mises en évidence à travers deux homogénéisations successives par une méthode Mori-Tanaka. Cette stratégie de modélisation permet de relier le comportement des fibres et le comportement élasto-plastique de la matrice à travers une loi de comportement dédiée à celui du matériau composite, et enfin d'intégrer la distribution d'orientation des mèches induites par le procédé de thermocompression.Le modèle multi-échelle a été identifié par une méthode inverse à partir des bases de données expérimentales constituées lors des travaux. La loi constitutive globale, à l'échelle d'un volume élémentaire représentatif, a été implémentée dans la bibliothèque scientifique SMART+ en langage C++ et a été conçue pour être compatible dans le cadre d'analyse de structures par éléments finis. En régime non-linéaire intégrant l’endommagement. / The evolution of the industrial context is pushing the transport industry, and more specifically the automotive sector, toward better energy efficiency. This objective is partly achieved by the development of new composite material solutions. The current work is devoted to the mechanical characterization and the multiscale modeling of a new SMC composite material reinforced with short carbon fibers bundles. The objective is to build a first knowledge base on the behavior of this SMC under fatigue loading. Experimental investigations include the analysis of the microstructure, the characterization of the mechanical behavior under quasi-static and fatigue loading, as well as the analysis of the damage mechanisms. The proposed multiscale approach takes into account the microstructure of the composite at the two scales highlighted through two successive homogenizations by the means of a Mori-Tanaka based method. Such modelling strategy makes it possible to relate elasto-plastic behavior of the matrix through dedicated local constitutive laws and the behavior of the fibers to that of the composite material, and to integrate the orientation distribution of the bundles induced by the manufacturing process... The multiscale model was identified by the mean of an inverse computation method applied on the experimental results taken from the databases created during the works.The global constitutive law, computed at the scale of a representative elementary volume, has been implemented in the SMART+ scientific library in C++ language in such a manner to be compatible in a finite element analysis (FEA) framework.

abstract: This study employs a finite element method based modeling of cementitious composite microstructure to study the effect of presence of inclusions on the stress distribution and the constitutive response of the composite. A randomized periodic microstructure combined with periodic boundary conditions forms the base of the finite element models. Inclusion properties of quartz and light weight aggregates of size 600μm obtained from literature were made use of to study the effect of their material (including inclusion stiffness, stiffness of interfacial transition zone and matrix stiffening) and geometric properties (volume fraction of inclusion, particle size distribution of inclusion and thickness of the interfacial transition zone) on the composite. Traction-separation relationship was used to incorporate the effect of debonding at the interface of the matrix and the inclusion to study the effect on stress distribution in the microstructure. The stress distributions observed upon conducting a finite element analysis are caused due to the stiffness mismatch in both the quartz and the light weight aggregates as expected. The constitutive response of the composite microstructure is found to be in good conformance with semi-analytical models as well as experimental values. The effect of debonding throws up certain important observations on the stress distributions in the microstructure based on the stress concentrations and relaxations caused by the stiffness of the individual components of the microstructure. The study presented discusses the different micromechanical models employed, their applicability and suitability to correctly predict the composite constitutive response. / Dissertation/Thesis / Masters Thesis Civil Engineering 2016

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 ABAQUSR 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. / Esta tese desenvolvida dentro do acordo de cooperação internacional celebrado entre a Escola Politécnica da Universidade de São Paulo (EPUSP) e a École des Mines d\'Albi-Carmaux tem como tema principal a análise da influência da evolução microestrutural sobre o comportamento mecânico de chapa de liga de titânio - Ti-6Al- 4V sob condições superplásticas e trabalho a quente. O objetivo desta pesquisa é contribuir para o desenvolvimento de processos de conformação não convencional de chapas de ligas a base de titânio utilizadas na manufatura de componentes metálicos. Como objetivo específico, estabelecer uma correlação entre comportamento mecânico e a mudança microestrutural a partir de três tipos de ligas com diferentes tamanhos de grão iniciais (0.5, 3.0 e 4.9 ?m). Os testes foram realizados na faixa de temperatura de 700 a 950 °C combinados às taxas de deformação na faixa de 10-1 s-1 - 10-4 s-1. Para a metodologia, estabeleceu-se uma estratégia de ensaios mecânicos capaz de testar as hipóteses sobre o comportamento do material formuladas no início desta pesquisa. Em seguida, os ensaios mecânicos foram divididos em três partes. Na primeira, utilizou-se um simulador termomecânico modelo Gleeble 3800 para os ensaios a quente variando-se a taxa de deformação (??) entre 10-1 s-1 a 10-3 s-1 e temperaturas da ordem de 700 °C a 850 °C. Na segunda parte dos testes, priorizouse taxas de deformação mais lentas (10-2 s-1 - 10-4 s-1) e temperaturas mais elevadas (800 °C - 950 °C) objetivando atingir as deformações superplásticas do material, nesta etapa utilizou-se como equipamento uma máquina de tração modelo MTS 50kN com câmara de aquecimento acoplada. A terceira parte dos ensaios experimentais envolveu a conformação na condição superplástica por pressão hidrostática (Bulge test) realizadas no LEL-IPT de São José dos Campos. A partir da análise dos dados experimentais levantou-se os parâmetros introduzidos no modelo numérico de comportamento mecânico baseado na evolução da microestrutura da chapa testada permitindo a calibração do modelo numérico a partir das equações constituintes e finalmente introduzido no software de elementos finitos (ABAQUS 6.12) e construído a simulação numérica da conformação superplástica por pressão hidrostática. Os principais resultados indicaram uma forte correlação entre microestrutura inicial da conformação superplástica e a quente de onde se pode observar que tanto menor a microestrutura inicial maior será a quantidade do crescimento de grão. Os resultados da conformação superplástica de expansão multiaxial do domo hemisférico foram, então, comparados à simulação numérica permitindo confrontar os dados do modelo numérico do comportamento mecânico com a lei de comportamento estudada, o que possibilitou um melhor entendimento dos mecanismos da conformação plástica em condições de superplasticidade e também de trabalho a aquente do material.

As ligas de magnésio atraíram a atenção novamente nos últimos anos por causa de suas propriedades de baixa densidade, resistência à tração e rigidez específica. Por outro lado, a maior limitação para o uso de ligas trabalhadas é a baixa conformabilidade em temperatura ambiente devido à estrutura hexagonal compacta (HCP) das ligas. O presente trabalho de pesquisa teve como objetivo estudar o encruamento, recristalização e crescimento de grãos durante a laminação de liga magnésio AZ31B em alta e baixa temperatura, analisando a evolução da microestrutura, da textura e a variação das propriedades sensíveis à microestrutura. A liga AZ31B é sensível à taxa de deformação em alta temperatura, entretanto, a anisotropia é negativamente afetada na laminação a frio, portanto, apresenta uma melhor laminação na faixa de temperaturas de 200 a 300ºC, devido ao refinamento de grãos, causado pela recuperação e recristalização dinâmica. O estudo foi realizado em amostras de uma chapa de liga de magnésio AZ31B recristalizada (2 mm de espessura). Amostras foram deformadas por laminação em temperaturas diferentes (25, 100, 200, 250 e 300ºC) e com diferentes taxas de deformação. A caracterização microestrutural foi realizada com auxílio de várias técnicas complementares de análise microestrutural, tais como microscopia óptica, microscopia eletrônica de varredura, análise de raios-X por dispersão de energia, difração de raios X e microdureza Vickers. A deformabilidade e a ocorrência de recristalização dinâmica e crescimento de grãos mostraram forte dependência com as condições de laminação. Na laminação a frio, o refinamento de grão foi mais efetivo com baixas taxas de deformação (1,6 s-1) do que na laminação a quente. Entretanto, a intensa textura basal foi enfraquecida em temperaturas próximas a 300ºC e com taxas de deformação próximas a 3,5 s-1. A ductilidade das ligas pode ser melhorada em altas temperaturas de deformação, pelo refinamento dos grãos que produz a diminuição da fração volumétrica das regiões macladas e pelo aumento do número de sistemas de deslizamento, além do enfraquecimento da intensa textura basal, característica das ligas de magnésio. / Magnesium alloys have attracted the attention again in recent years because of their low density, their specific tensile strength and rigidity. However, the greatest limitation for the usage of wrought magnesium alloys is their poor formability at room temperature due to the hexagonal closed packed (HCP) crystal structure. The present research focused on study the work-hardening, recrystallization and grain growth during rolling of AZ31B magnesium alloy at low and high rolling temperature. It was made through the analysis of microstructure and texture evolution and variations of microstructure-sensitive properties. The AZ31 magnesium alloy is sensitive to strain rate at high temperature, meanwhile, the anisotropy is adversely impacted in cold rolling sheets. Thus, AZ31B magnesium alloy exhibits better workability in 200-300°C temperature range due to the grain refinement caused by dynamic recovery and dynamic recrystallization. This research was carried out on samples of recrystallized sheet (2 mm in thickness). Samples were deformed by rolling at different temperatures (25, 100, 200, 250 and 300°C), using different strain rates. Microstructural characterization was done by using several complementary techniques of microstructural analysis, such as optical microscopy, scanning electron microscopy, X-ray analysis by energy dispersive, X-ray diffraction and Vickers microhardness tests. A competition between dynamic recrystallization and grain growth depends on rolling conditions. Low strain rate (1,6 s-1) at cold rolling improved more effective in refining grains than warm rolling. Meanwhile, the intense basal texture was weakened at 300°C with a high strain rate of 3,5 s-1. The ductility of magnesium alloys can be greatly improved at high temperature, with a fine grain structure that causes the reduced volume fraction of twins, and an increase the number of slip systems, weakening the intense basal texture, rather characteristic for magnesium alloys.

Neste trabalho, foi estudada a sinterização de Nanoestruturas de zircônia estabilizada com ítria, ZrO2-Y2O3. Pós de ZrO2-3%mol de Y2O3 com tamanhos nanométricos e área superficial específica de 16,2 m2/g, foram compactados uniaxialmente em pressões variando entre 12,3MPa e 73,5MPa. Nestas pressões, as amostras compactadas atingiram densidade à verde entre 33% e 44,3%. Os compactos foram sinterizados por dilatometria de 1.250ºC até 1.4000C e os resultados indicaram que a temperatura de início de densificação está próxima a 1.0000C, independente da pressão de compactação aplicada. Temperaturas da ordem de 1.4000C são necessárias para a densificação total do compacto. Comparativamente pós microestruturados de ZrO2-3%mol Y2O3 (área superficial de 7,0 m2/g), foram compactados a 73,5MPa e alcançou densidade a verde de 44,2%. Este material apresentou início da retração próximo a 1.2000C, sugerindo que o uso de pós nanoestruturados reduziu a temperatura de sinterização em 2000C. A sinterização convencional dos pós nanoestruturado foi realizada em temperaturas entre 1.2500C e 1.4000C, com patamares entre 2 e 16h. Os pós microestruturados foram submetidos à sinterização em temperaturas de até 1.6000C, com os mesmos patamares. Os resultados indicaram que em todas as condições de sinterização e indiferentemente do pó utilizado, apenas a fase ZrO2 tetragonal foi encontrada. Além disso, a densidade relativa (DR) dos nanopós variou entre 92%(1.2500C-0h) e superior a 99%(acima de 1.3500C- 4h). Análises microestruturais indicaram a presença de grãos refinados com tamanho médio de 0,18?m (DR=92%,) para nanopós sinterizados a 1.2500C-0h, e tamanho médio de 0,95?m (DR=100%, 1.4000C-16h). Os pós micrométricos apresentaram tamanho de grão médio de 0,39?m (DR=98,8%) para cerâmicas sinterizadas a 1.5300C-0h e tamanho de grão médio de 1,84?m (DR=100%) para cerâmicas sinterizadas a 1.600-0h. Os expoentes de crescimento de grão calculados foram da ordem de 2,8 e 2,3 para. nanoparticulas e microparticulas, respectivametne, indicando que mecanismo de difusão pelos contornos de grão foi preponderante nos materiais estudados. Os valores de energia de ativação para o crescimento de grão calculados foram de 141,3kJ/mol e 244,7kJ/mol, respectivamente, indicando que os pós micrométricos necessitam de maior consumo energético para promover o crescimento de grão. Os resultados são discutidos em função de associar os fenômenos de densificação e crescimento de grão com o tamanho das partículas utilizadas. / We studied the sintering of nanoparticles of yttria stabilized zirconia, ZrO2-Y2O3. Powders of ZrO2-Y2O3 3 mol% with nanometric sizes and specific surface area of 16.2 m2/g, were uniaxially compacted at pressures ranging between 12.3 MPa and 73.5 MPa. Such pressures, the compressed samples reached the green density between 33% and 44.3%. The compacts were sintered by dilatometry up 1.400ºC and the results indicated that the onset temperature of densification is next to 1.000ºC, regardless of the compaction pressure applied. Temperatures of 1.400ºC are required to complete the densification of the compact. Compared microparticulate powders of ZrO2-Y2O3 3 mol% (specific surface area of 7.0 m2/g) were compressed to 73.5 MPa and green density reached 44.2%. This material exhibited the beginning of the next retraction 1.200ºC, suggesting that the use of powder nanopatticulados reduced sintering temperature of 200ºC. The conventional sintering of nanoparticle powders was undertaken at temperatures between 1.400ºC and 1.250ºC, with levels between 2 and 16h. The microparticulate powders were subjected to sintering at temperatures up to 1.600ºC with the same levels. The results indicated that under all conditions, and sintering the powder used interchangeably, only tetragonal ZrO2 phase was found. Furthermore, the relative density (RD) of nanoparticles ranged from 92% (1.250ºC- 0h) and greater than 99% (above 1.350ºC-4h). Microstructural analysis indicated the presence of refined grains with average size of 0.18 ? m (DR = 92%) for nanoparticles sintered at 1.250ºC-0h, and average size of 0.95 ?m (DR = 100%, 1.400ºC -16h). The powders were micron average grain size of 0.39 ?m (DR = 98.8%) to the sintered ceramic 1.530ºC-0h and the average grain size of 1.84 ?m (DR = 100%) of sintered ceramics the 1.600 ºC-0h. The grain growth exponents calculated were the order of 2.8 and 2.3 for nanoparticles and microparticles, respectively, and indicating that the mechanism of grain boundary diffusion was predominant in the studied materials. The values of activation energy for the grain growth were calculated 141.3 kJ/mol and 244.7 kJ/mol, respectively indicating that the powder micrometric require more energy to promote grain growth. The results are discussed in terms of the associated phenomena of densification and grain growth in the particle size used.

Os objetivos deste trabalho foram: a) realizar a análise microestrutural de cerâmicas odontológicas e relacioná-la aos valores de resistência à flexão obtidos, b) avaliar o efeito de dois tipos de envelhecimento (ciclagem mecânica e ciclagem em autoclave) sobre a resistência à flexão desses materiais e c) avaliar se a configuração do espécime (discos versus barras) é capaz de alterar o efeito da ciclagem mecânica sobre a resistência à flexão. Cinco materiais foram utilizados: zircônia tetragonal policristalina estabilizada por ítrio (YZ), alumina policristalina (AL), alumina infiltrada por vidro/reforçada por zircônia (ICZ) e duas porcelanas (VM7 e VM9). Espécimes em forma de barra (2 x 4 x 16 mm) e disco (12 mm x 2 mm) foram confeccionados conforme as recomendações dos fabricantes. A densidade foi determinada pelo método de Arquimedes, as constantes elásticas foram determinadas pelo método do pulso-eco ultrassônico. Microscopia óptica e eletrônica de varredura foram utilizadas para determinar a fração e tamanho médio de poros e partículas e a análise de EDS para a realização de análise química dos microconstituintes. O efeito do envelhecimento por ciclagem mecânica (YZ, AL e ICZ: 80 N por 105 ciclos e VM7 e VM9: 45 N por 104 ciclos, a 2 Hz) ou por ciclagem em autoclave a 134°C, pressão de 2,2 kgf/cm2 durante 5h, foi avaliado nos espécimes em forma de barra e comparados com um grupo controle, não ciclado. O efeito do envelhecimento por ciclagem mecânica foi comparado entre os espécimes em forma de barra e disco para as porcelanas estudadas. Houve diferenças significativas entre os valores de resistência em função do material, causadas pelas diferenças em sua microestrutura, sendo que as porcelanas apresentam baixos valores de resistência (VM7: 69,4 MPa e VM9: 64,7 MPa), enquanto que a AL e a ICZ apresentaram valores intermediários (462,8 MPa e 408,6 MPa, respectivamente) e a YZ apresentou o maior valor de resistência à flexão (869 MPa). Após a ciclagem mecânica, somente o material AL apresentou redução estatisticamente significativa da resistência à flexão (389,2 MPa). A ciclagem em autoclave não resultou em degradação significativa da resistência de nenhum dos materiais testados. A configuração do espécime (barra ou disco) não alterou o efeito da ciclagem mecânica sobre a resistência das porcelanas testadas (nenhum material apresentou queda da resistência após ciclagem). Entretanto, nos espécimes em forma de disco a resistência à flexão da VM7 foi significativamente maior que a da VM9 (69,6 MPa e 57,6 MPa, respectivamente), enquanto que nos espécimes em forma de barra não houve diferença estatística entre os valores de resistência das duas porcelanas (VM7: 61,1 MPa e VM9: 68,8 MPa). / The aims of this study were: to perform a microestrutural analysis of dental ceramics and correlate the results with the flexural strength obtained; to evaluate two aging process (mechanical cycling and autoclave cycling) on the flexural strength of the materials and to evaluate the specimen configuration (bar or disc) on the flexural strength of mechanical cycling aged specimens. The five ceramics tested were: yttria-stabilized zirconia (YZ), polycrystalline alumina (AL), glass-infiltrated alumina/zirconia reinforced (ICZ) and two dental porcelains (VM7 and VM9). Bars (2 x 4 x 16 mm) and discs (12 mm x 2 mm) were produced according to the manufactures instructions. Density was obtained by Arquimedes method and the elastics constants were obtained by ultrasonic pulse-echo method. Optical and Scanning Electron Microscope were used to determine the fraction and size of both pores and particles while the chemical composition was obtained by EDS. Aging effect on flexural strength after mechanical or autoclave cycling was evaluated on bar specimens and compared to a control group, which was not cycled. The specimen configurations (bars or discs) were compared after mechanical cycling for porcelains. Microestrutural differences between material resulted on flexural strength values statistically different, porcelains presented lower values of flexural strength (VM7: 69,4 MPa and VM9: 64,7 MPa), while AL and ICZ presented intermediate flexural strength (462,8 MPa and 408,6 MPa, respectively) and YZ presented the highest flexural strength (869 MPa). Only AL presented a significant lower flexural strength after mechanical cycling (389,2 MPa). Autoclave cycling did not influence the flexural strength of the studied materials. Also, the specimen configuration did not influence the flexural strength values before or after mechanical cycling. It should be noted that disc specimens of VM7 presented significantly higher flexural strength values than VM9 (69,6 MPa and 57,6 MPa, respectively), however for bar specimens no statistical difference was noted between the mean strength of both porcelains (VM7: 61,1 MPa and VM9: 68,8 MPa).

With the rapid development in electronic packaging due to product miniaturisation, the size of solder joints is decreasing considerably, thus the failure of solder interconnects induced by electro-migration (EM) and thermo-migration (TM) became a reliability concern. The incorporation of foreign reinforcement can effectively improve properties of the solder alloys. However, this presents an imperative need for a further investigation to elaborate the underlying fundamentals associated with the reliability of reinforced solders. In this study, the Sn-Ag-Cu (SAC) based solder alloy powders as matrix were incorporated with Fullerene (FNS), TiC and Ni-coated graphene (NG) reinforcements to form composite solders through powder metallurgical method. These composite solders were then characterised in terms of their microstructure, physical property, solderability, followed by a systematic investigation of their performance under isothermal ageing, current stressing and large thermal gradient, respectively. The results showed that three types of reinforcements were successfully incorporated into the solder matrix; with all reinforcements added being embedded in the solder matrix or around the intermetallic compounds (IMC). The average loss of FNS and TiC particles in the solders was approximately 80% after the initial reflow, while this was only 40% for NG particles. It has been observed that β-Sn and Ag3Sn in the SAC solder alloys can be refined by adding appropriate amount of FNS and TiC, which is beneficial to the wettability with a reduced coefficient of thermal expansion (CTE) with the minimal influence on the melting point and electrical resistivity of solder alloys. For the SAC alloys without reinforcements, obvious extrusion of interfacial IMC at the anode was present after 360 hours of current (1.5×104 A/cm2) stressing, while the changes of surface profiles of all reinforced solders were unnoticeable. Under the current stressing regimes, a continuous increase of interfacial IMCs at the anode of the original SAC alloys was observed, but decreased at the cathode with stressing time. For the composite solders, both anode and cathode showed a continuous growth of interfacial IMCs; the growth rates of IMCs at the anode were greater than that at cathode. In addition, NG and TiC were found to be most effective to retard the growth of Cu3Sn IMC under current stressing. A gradient in hardness across the stressed SAC joints was present, where it was harder at anode. However, no such obvious gradient was found in SAC/FNS and SAC/NG solder joints. FNS and NG were proven to be beneficial to prolong the service life of solder joints up to approximately 7.6% and 10.4% improvements, respectively. Thermal stressing made the interfacial IMC in the original SAC joints to grow at the cold end considerably; causing serious damage at the hot end after 600 hours under temperature gradient of 1240K/cm stressing; a large number of IMCs, cracks and voids appeared in the SAC solder joints. However, a uniform increase of IMCs at both sides in the composite solders was observed without apparent damages at the interfaces under the same thermal stressing conditions, indicating an effective reduction of the elemental migration in the reinforced solders. Although there were also some voids and IMCs formed in the composite solder joints after a long-term thermal stressing, the integrity of the composite solder joints was enhanced compared with the SAC alloys. During thermal stressing, the dissolution rate of Cu atom into the SAC solder joints was estimated to be 3.1×10-6 g/h, while the values for SAC/FNS, SAC/NG and SAC/TiC were only 1.22×10-6 g/h, 1.09×10-6 g/h and 1.67×10-6 g/h, respectively.

Les contraintes environnementales imposent de trouver des solutions pour limiter les émissions gazeuses (gaz à effet de serre) ainsi que d’éliminer les matériaux nocifs dans les produits de consommation. L’étude de solutions alternatives devient donc un point clé au développement des produits futurs. Dans ce contexte, les véhicules électriques et hybrides sont en forte progression sur le marché mais leur développement commercial reste fortement lié à leur fiabilité, plus particulièrement à celle des organes contrôlés par l’électronique de puissance. Notre étude s’inscrit dans ce cadre et se focalise sur la pâte d’argent micrométrique, matériau candidat au remplacement des solutions à base de plomb pour le report des puces. Si la pâte d’argent est assez bien caractérisée pour ses propriétés électriques, peu de données sont reportées dans littérature quant à ses propriétés mécaniques, pourtant indispensables pour appréhender la durée de vie des assemblages complets. Dans la thèse, une méthode originale d’élaboration des échantillons a été développée. Basée sur les recommandations d’utilisation donnée par le fournisseur, elle a été adaptée afin de d’obtenir des échantillons massifs dont la microstructure brute et après vieillissement en température est identique à celle des brasures réelles. Après élaboration, le taux porosité, compris entre 15 % et 20 %, n’évolue pas au cours de vieillissements représentatifs des conditions d’utilisation réelles. Seule la morphologie des pores évolue, pour laquelle un grossissement, respectant les cinétiques du mûrissement d’Ostwald, et une évolution de la distribution spatiale est observée. Les propriétés mécaniques sont une fonction de la densité à l’état brut. Après vieillissement des échantillons massifs, si les propriétés élastiques ne varient pas (à densité constante), la dispersion des propriétés plastiques sont reliées à la modification de la distribution spatiale des pores. L’évolution des propriétés élastiques sur les échantillons représentatifs des brasures est attribuée aux mécanismes de relaxation des contraintes d’élaboration. Une fois celles-ci relaxées, les propriétés sont identiques pour les deux états (massif et couche mince) et sont donc intrinsèques au matériau. / The requirement for reducing the use of harmful materials in convenience goods hasprompted investigation into alternative solutions. Along with the need to drastically limit theemission of greenhouse gases, the increase of electric or hybrid vehicles in the market reliesmostly on their dependability with a specific focus on reliability of the embedded powerelectronics. The study of alternative materials to lead (Pb) or Pb-based alloys for die bondingis a critical step towards realising an environmentally friendly solution. Micrometric silverpaste was chosen as a candidate because of its excellent electrical properties. However, fewdata are available in the literature concerning its mechanical properties, mandatory to modelthe entire electronic system for service life assessment. The processing route, based on thesintering of micrometric powder, provides a material with significant porosity that is knownto alter the mechanical properties when compared to the dense material. In this thesis, anoriginal processing route was developed in order to obtain bulk samples with the samemicrostructure of real solder joints either before or after ageing. The mechanical properties vsdensity was established prior to or after aging. After aging, the elastic properties do not varywhile the dispersion observed for the plastic properties is connected with the microstructureevolution. The evolution of the elastic properties on the representative samples of solder jointsafter aging is attributed to the mechanisms of stress relaxation. Once these relaxed, theproperties are identical for both states (bulk and thin layer) and are thus intrinsic in thematerial.

This dissertation investigates microstructure/property relations in ultrahigh carbon steel (UHCS) with the aim of improving toughness while retaining high hardness. Due to high C contents (ranging from 1 to 2 wt%), UHCS exhibit high strength, hardness, and wear resistance. Despite this, applications for UHCS are currently limited because they typically contain a continuous network of proeutectoid cementite that greatly reduces ductility and toughness. In previous research, thermomechanic processing had seen considerable success in breaking up the network. However, the processing is difficult and has thus far seen very limited industrial application. Chemical modification of the steel composition has also seen some success in network break-up, but is still not well understood. There have been relatively few fundamental studies of microstructure evolution in UHCS; studies in the literature typically focused on lower C steels (up to 1 wt% C) or on cast irons (>2.1 wt% C). Thus, this work was undertaken to gain a better understanding of microstructural changes that occur during heat treatment and/or chemical modification of UHCS with a focus on the distribution of proeutectoid cementite within the microstructure. This dissertation is composed of eight chapters. The first chapter presents an introduction to phases found in UHCS, descriptions of research materials used in each chapter, and the hypotheses and objectives guiding the work. The second chapter describes a study of the microstructure found in a 2C-4Cr UHCS before and after an industrial-scale austenitizating heat treatment that increased hardness and toughness and also produced discrete carbide particles in the matrix. The third chapter establishes and demonstrates a metric for measuring connectivity in carbide networks. The fourth chapter describes a series of heat treatments designed to investigate kinetics of spheroidization and coarsening of carbide particles and denuded zones near cementite network branches in 2C-4Cr UHCS. The fifth chapter describes an additional series of heat treatments comparing coarsening kinetics in 2C-1Cr and 2C-4Cr UHCS. Lowering the Cr content caused clustering of cementite particles near grain boundaries, in contrast to the denuded zones observed in the higher Cr UHCS. The fifth chapter details four in situ confocal laser scanning microscopy heat treatments of 2C-4Cr UHCS. The seventh chapter investigates the effects of a 2wt% Nb addition on 2C-4Cr UHCS. The eighth and final chapter summarizes the findings of all the experiments of the previous chapters and revisits the objectives and conclusions.