Made available in DSpace on 2014-06-11T19:32:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2004-07-21Bitstream added on 2014-06-13T19:21:33Z : No. of bitstreams: 1 lacerdajunior_w_dr_araiq.pdf: 2652658 bytes, checksum: 6bc345b5eed77cb8c4f1bc49a638de4f (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O objetivo deste projeto foi estudar o efeito da segregação de dopantes em cerâmicas à base de SnO2 obtidas pelo método de mistura de óxidos para verificar como a segregação dos dopantes para o contorno de grão influencia as propriedades elétricas que são, basicamente, uma propriedade do contorno de grão nas cerâmicas de SnO2. Estes resultados foram comparados com os resultados obtidos para as cerâmicas preparadas pelo método de precipitação com uréia. As características morfológicas dos pós foram correlacionadas às características elétricas e microestruturais das cerâmicas. As características de microestrutura e de segregação foram acompanhadas por microscopia eletrônica de varredura e por microscopia eletrônica de transmissão. As características elétricas incluíram medidas de impedância complexa e de tensãocorrente. Os valores de energia de barreira de potencial aumentaram com a concentração de Cr2O3 para os sistemas à base de SnO2. Observou-se que a resistividade elétrica, dos sistemas dopados com cromo, aumentou com sua concentração e este comportamento pode ser atribuído à diminuição do tamanho médio de grão que acompanha o aumento da concentração de cromo. A partir das concentrações ≥ 0,10 % mol de Cr2O3 observou-se forte diminuição nos tamanhos de grão das amostras. O circuito equivalente é do tipo ‘arco de Cole’ que apresenta um elemento de fase constante atribuído às depressões dos semicírculos. Através da aproximação de Mukae, calculou-se os valores de densidade de doadores de carga, densidade de estados de superfície e tensão por barreira. Observou-se que estes parâmetros aumentaram com a concentração de cromo e que os valores de coeficiente não-linear apresentaram tendência de acompanhar o aumento desses valores nos sistemas estudados. Os sistemas preparados pelo... / The objective of this project was to study the effect of dopant segregation in SnO2-based ceramics obtained by the method of oxide mixture in order to verify how the segregation of the dopants into the grain boundary influences the electrical properties which are, basically, a property of the grain boundary in SnO2 ceramics. These results have been compared with the results obtained for ceramics prepared by the precipitation method with urea. The powder morphological characteristics have been correlated to the electrical and microstructural characteristics of the ceramics. The microstructure and segregation characteristics have been followed by scanning electronic microscopy and transmission electronic microscopy. The electrical characteristics included complex impedance and current-voltage measurements. The values of potential energy barrier increased with the Cr2O3 concentration for the SnO2-based systems. It was observed that the electrical resistivity of the chromium doped systems increased with its concentration and this behaviour can be attributed to the reduction of the mean grain size, which accompanies the increase of chromium concentration. From concentrations ≥ 0,10 % mol de Cr2O3 it was observed a strong reduction in the grain sizes of the samples. The equivalent circuit is of the `Cole arc' type which presents a constant phase element attributed to the depressions of the semicircles. By using Mukae's approximation, the values of charge donors density, surface states density and barrier voltage were calculated. It was observed that these parameters increased with the chromium concentration and that the values of the nonlinear coefficient had a tendency to follow the increase of these values in the studied systems. The systems prepared by the method of conventional oxide mixture presented precipitates, in the grain ...(Complete abstract, click electronic access below)

abstract: Fission products in nuclear fuel pellets can affect fuel performance as they change the fuel chemistry and structure. The behavior of the fission products and their release mechanisms are important to the operation of a power reactor. Research has shown that fission product release can occur through grain boundary (GB) at low burnups. Early fission gas release models, which assumed spherical grains with no effect of GB diffusion, did not capture the early stage of the release behavior well. In order to understand the phenomenon at low burnup and how it leads to the later release mechanism, a microstructurally explicit model is needed. This dissertation conducted finite element simulations of the transport behavior using 3-D microstructurally explicit models. It looks into the effects of GB character, with emphases on conditions that can lead to enhanced effective diffusion. Moreover, the relationship between temperature and fission product transport is coupled to reflect the high temperature environment. The modeling work began with 3-D microstructure reconstruction for three uranium oxide samples with different oxygen stoichiometry: UO2.00 UO2.06 and UO2.14. The 3-D models were created based on the real microstructure of depleted UO2 samples characterized by Electron Backscattering Diffraction (EBSD) combined with serial sectioning. Mathematical equations on fission gas diffusion and heat conduction were studied and derived to simulate the fission gas transport under GB effect. Verification models showed that 2-D elements can be used to model GBs to reduce the number of elements. The effect of each variable, including fuel stoichiometry, temperature, GB diffusion, triple junction diffusion and GB thermal resistance, is verified, and they are coupled in multi-physics simulations to study the transport of fission gas at different radial location of a fuel pellet. It was demonstrated that the microstructural model can be used to incorporate the effect of different physics to study fission gas transport. The results suggested that the GB effect is the most significant at the edge of fuel pellet where the temperature is the lowest. In the high temperature region, the increase in bulk diffusivity due to excess oxygen diminished the effect of GB diffusion. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2014

abstract: Hydrogen sulfide (H2S) has been identified as a potential ingredient for grain boundary passivation of multicrystalline silicon. Sulfur is already established as a good surface passivation material for crystalline silicon (c-Si). Sulfur can be used both from solution and hydrogen sulfide gas. For multicrystalline silicon (mc-Si) solar cells, increasing efficiency is a major challenge because passivation of mc-Si wafers is more difficult due to its randomly orientated crystal grains and the principal source of recombination is contributed by the defects in the bulk of the wafer and surface. In this work, a new technique for grain boundary passivation for multicrystalline silicon using hydrogen sulfide has been developed which is accompanied by a compatible Aluminum oxide (Al2O3) surface passivation. Minority carrier lifetime measurement of the passivated samples has been performed and the analysis shows that success has been achieved in terms of passivation and compared to already existing hydrogen passivation, hydrogen sulfide passivation is actually better. Also the surface passivation by Al2O3 helps to increase the lifetime even more after post-annealing and this helps to attain stability for the bulk passivated samples. Minority carrier lifetime is directly related to the internal quantum efficiency of solar cells. Incorporation of this technique in making mc-Si solar cells is supposed to result in higher efficiency cells. Additional research is required in this field for the use of this technique in commercial solar cells. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2014

abstract: Microelectronic industry is continuously moving in a trend requiring smaller and smaller devices and reduced form factors with time, resulting in new challenges. Reduction in device and interconnect solder bump sizes has led to increased current density in these small solders. Higher level of electromigration occurring due to increased current density is of great concern affecting the reliability of the entire microelectronics systems. This paper reviews electromigration in Pb- free solders, focusing specifically on Sn0.7wt.% Cu solder joints. Effect of texture, grain orientation, and grain-boundary misorientation angle on electromigration and intermetallic compound (IMC) formation is studied through EBSD analysis performed on actual C4 bumps. / Dissertation/Thesis / M.S. English 2013

Orientador: Antonio José Ramírez Londoño / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-17T12:50:46Z (GMT). No. of bitstreams: 1 UnfriedSilgado_Jimy_D.pdf: 31862670 bytes, checksum: f798572d3f744b9a4460a24f795ffa4d (MD5) Previous issue date: 2010 / Resumo: A fratura por queda de ductilidade (FQD) é um tipo de falha que ocorre em temperatura elevada e que afeta adversamente diversos materiais metálicos com estrutura cristalina cúbica de faces centradas (CFC), tais como, ligas de Níquel, Cobre e aços inoxidáveis. Na FQD é observada uma forte redução da ductilidade e a ocorrência de fratura intergranular no intervalo de temperatura homologa entre 0,4 e 0,8, e sob aplicação de esforços de tensão. Diversos autores têm proposto alguns fenômenos metalúrgicos, tais como o escorregamento ao longo dos contornos de grão e a precipitação de carbonitretos e carbonetos, como fatores que influenciam o comportamento da FQD em estrutura bruta de solidificação de ligas de Níquel. Não obstante, o mecanismo fundamental operante neste tipo de fratura ainda não está totalmente esclarecido. Neste trabalho é estudado o mecanismo operante na FQD em função do papel da precipitação de carbonitretos e das características microestruturais na estrutura bruta de solidificação de ligas Ni-Cr-Fe endurecidas por solução sólida. Foram fabricadas e avaliadas cinco ligas experimentais baseadas na composição da liga 690 com e sem adições de Nb, Mo e Hf, as quais foram projetadas com o suporte do método Calphad. A avaliação da FQD foi realizada a través da aplicação iterativa de técnicas de caracterização baseadas em microscopia eletrônica, combinada com a determinação experimental da energia de falha de empilhamento (EFE) usando radiação sincrotron e o uso de ensaio termomecânico in situ em temperatura elevada acoplado a um microscópio eletrônico de varredura (MEV), cujos resultados permitiram realizar mapeamento da deformação a partir de imagens digitais. O teste in situ facilitou o acompanhamento, em tempo real, do fenômeno de FQD dentro do intervalo de temperaturas entre 500 °C e 1000 °C, evidenciando a ocorrência de escorregamento ao longo dos contornos de grão, o que por sua vez permitiu a identificação das etapas do fenômeno com suas respectivas características. Nas ligas experimentais com adições de Nb e Hf foram obtidos contornos de grão fortemente ondulados devido à alta freqüência de precipitados primários intergranulares finos. A adição de Mo nas ligas experimentais juntamente com as adições de Nb e Hf contribuíram para uma forte diminuição da EFE. Os contornos de grão ondulados foram relacionados com o bloqueio mecânico do escorregamento que ocorre ao longo dos mesmos, como sugerido por diversos autores. A presença de Mo na rede cristalina e a baixa EFE contribuíram na restrição da mobilidade de discordâncias em temperaturas elevadas. As características anteriores foram relacionadas com o aumento da resistência a FQD. Finalmente, baseado na evidencia experimental obtida neste trabalho é proposto um mecanismo fundamental de ocorrência da FQD em estruturas brutas de solidificação por soldagem de ligas Ni-Cr-Fe similar ao mecanismo de fluência sem domínio da difusão com escorregamento ao longo dos contornos de grão proposto por Rachinger. / Abstract: Ductility-dip cracking (DDC) is a high temperature fracture phenomenon, which affects several face centered cubic (FCC) metallic materials, such as Nickel alloys, Copper alloys, and stainless steels. DDC is observed as a drastic reduction of ductility that leads to intergranular fracture at homologous temperature range between 0,4 and 0,8 under tensile stress. Diverse theories related to the grain boundary sliding and to carbides and carbonitrides precipitation were proposed to describe DDC behavior in Ni-alloys; however, the fundamental mechanism of DDC is not clear yet. In this work is investigated the fundamental mechanism of DDC, as well as the role of carbonitride precipitates and metallurgical characteristics on this phenomenon in aswelded solid-solution strengthened Ni-Cr-Fe alloys. Experimental alloys were designed by means of Calphad methodology using the alloy 690 chemical composition as the start point. Five compositions with Nb, Mo and Hf additions were subsequently fabricated and evaluated. The DDC evaluation was performed using electron microscopy characterization techniques, experimental measurements of stacking fault energy (SFE) using synchrotron radiation, and a scanning electron microscopy thermo-mechanical in situ test that allows a strain mapping from digital images. The in situ test has allowed obtaining at real time information about of DDC phenomenon on the temperature range between 500 °C and 1000 °C. Evidences of grain boundary sliding (GBS) were obtained through high temperature experiments, consequently allowing the recognition of DDC stages characteristics. Wavy grain boundaries were obtained in Ni-Cr-Fe alloys with Nb and Hf additions due to the high frequency and homogeneous distribution of fine intergranular primary precipitates. Mo, Nb, and Hf additions contributed for a perceptible SFE reduction. Several authors suggested that wavy grain boundaries block GBS, while the Mo presence in the crystal lattice leads to SFE reduction, which is related to the restriction of dislocations mobility at high temperature and to the increase of DDC resistance. Finally, a new fundamental mechanism of DDC is proposed based on experimental evidences for as-welded structures of Ni-Cr-Fe alloys, which is similar to the creep mechanism without diffusion, involving grain boundary sliding mechanism proposed by Rachinger. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Os aços de alta resistência e baixa liga são amplamente utilizados em dutos transportadores de óleo e gás e, atualmente, o desenvolvimento de novos projetos de liga e o uso de técnicas altamente avançadas de fabricação e processamento dos aços se tornaram essenciais para obtenção de estruturas que resistam aos danos provocados por H, principal motivo de falha de oleodutos e gasodutos em meios ricos em H2S. No presente trabalho, avaliou-se o efeito de características microestruturais na difusividade do H em dois aços grau API X65, com diferentes teores de Mn. Uma das chapas ainda está em fase experimental de desenvolvimento, tem baixo teor de Mn e foi produzida para aplicação em ambientes sour. A outra chapa tem alto teor de Mn, já é usada comercialmente há alguns anos e foi desenvolvida para trabalho em ambientes doces. Os dois materiais passaram por caracterização microestrutural nas três seções da chapa: longitudinal e transversal à direção de laminação e do topo da chapa (paralela à direção de laminação). Após a caracterização, amostras de cada seção dos aços foram submetidas a ensaios de permeabilidade ao H; o aço baixo Mn passou por análises de EBSD (Difração de Elétrons Retroespalhados), para determinação de textura. O aço baixo Mn tem microestrutura homogênea ao longo da espessura da chapa, composta por ferrita refinada e pequenas ilhas de perlita. O aço alto Mn, por sua vez, apresenta microestrutura heterogênea ao longo da espessura, formada por bandas de ferrita e perlita, com marcada presença de segregação central de elementos de liga. Os ensaios de permeabilidade ao H mostraram que os coeficientes de difusão efetiva do H, Deff, do aço baixo Mn são ligeiramente superiores aos do aço alto Mn. Outros dois importantes parâmetros que foram calculados para os dois aços são a concentração de H na sub-superfície do material, C0, e o número de traps por unidade de volume, Nt. Contrariando expectativas, o aço baixo Mn apresentou maiores valores de C0 e Nt do que o aço alto Mn. Ensaios preliminares de dessorção térmica realizados nos dois aços mostraram os mesmos resultados: o aço baixo Mn aprisiona mais H do que o aço alto Mn. Estes resultados contraditórios de C0 e Nt foram atribuídos à presença de nanoprecipitados de microadições de liga no aço baixo Mn, não detectáveis por microscopia óptica e eletrônica de varredura. Ainda, para os dois aços, os valores de Deff variaram em função da seção analisada da seguinte maneira: Deff longitudinal ? Deff transversal > Deff topo. Para entender melhor o comportamento anisotrópico da difusão do H nos dois aços calculou-se um novo coeficiente de difusão, que foi chamado de coeficiente de difusão no estado estacionário, Dss. O Dss considera que todos os traps do aço estão saturados, permitindo, assim que se avalie somente o efeito de obstáculos físicos à difusão do H. No aço alto Mn, o Dss variou da mesma maneira que o Deff: Dss longitudinal ? Dss transversal > Dss topo; este comportamento foi atribuído ao bandeamento presente no material. No aço baixo Mn, o Dss variou de forma diferente do Deff: Dss transversal > Dss longitudinal >= Dss topo, indicando que a difusão do H pode ser auxiliada por contornos de grão enquanto os traps estão sendo saturados, e que a textura cristalográfica pode influenciar a difusão após o estado estacionário ser atingido. / High strength low alloy steels are widely applied as pipelines for crude oil and natural gas transportation and, currently, new approaches to alloy design, in addition to the use of advanced steelmaking and processing techniques, have become essential for obtaining structures that resist to hydrogen damage, which is the main cause of pipelines failure in H2S-rich environments. The main objective of the present work is to evaluate the influence of microstructural features on hydrogen diffusivity in two API X65 steels, with different Mn contents. One of the steel plates has been recently developed for usage in sour environments, is on its experimental stage and has a low Mn content. The other one is a commercial plate steel, with high Mn content, developed for sweet applications. Both steel plates were characterized in its three sections, in relation to the rolling direction: longitudinal, transverse and top surface of the plate (parallell to the rolling direction). After that, samples obtained from each section of the plates were submitted to hydrogen permeation tests; the low Mn steel was also analysed with EBSD, for texture determination. The low Mn steel presents a homogeneous microstructure through plate thickness, composed of refined ferrite and small pearlite islands. The high Mn steel has a heterogeneous microstructure through the plate thickness, composed of ferrite and pearlite bands, and presents centerline segregation. Hydrogen permeation tests showed that the Deff obtained for the low Mn steel sections are slightly higher than for the high Mn steel. Another two important parameters that were calculated for both steels are the subsurface hydrogen concentration, C0, and the number of traps per unit volume, Nt. Contrary to what was expected, the low Mn steel presented the higher C0 and Nt values. Thermal dessorption spectroscopy analysis confirmed that the low Mn steel traps more H atoms than the high Mn one. These results, along with the similar Deff values, were related to the presence of nanoprecipitates of microalloying elements, that cannot be detected via optical and scanning electron microscopy. Additionally, also for both steels, the Deff values varied in function of the analyzed section as it follows: Deff longitudinal ? Deff transverse > Deff top. In order to better understand this anisotropic behavior, a new diffusion coefficient, which was called diffusion coefficient at the steady state, Dss, was determined. Dss considers that all the trapping sites are saturated, enabling, thus, the evaluation of physical obstacles to H diffusion. For the high Mn steel, the Dss varied in the same matter as the Deff: Dss longitudinal ? Dss transverse > Dss top; this behavior was associated with the microstructural banding present in the material. For the low Mn steel, the Dss exhibited a different behavior: Dss transverse > Dss longitudinal >= Dss top, suggesting that H diffusion can be aided by grain boundaries while the trapping sites are being filled and that crystallographic texture may play its role after the steady state is reached.

Ce travail est consacré à l'étude de l'ingénierie des joints de grains dans les superalliages à base de Nickel pour disques de turbines aéronautiques. L'ingénierie des joints de grains a montré son efficacité dans l'amélioration des propriétés en fatigue et en fluage dans certains alliages de cuivre et de nickel. Cette amélioration est obtenue en réalisant une succession de déformations à température ambiante séparées par des traitement thermiques. Ce traitement a pour objectif de modifier la distribution du réseau de joints de grains. L'objectif de l'étude est d'adapter ces traitements au procédé du forgeage à haute température des superalliages. Une étude expérimentale est menée pour mettre en évidence l'influence des paramètres de forgeage sur les caractéristiques du réseau de joints de grains. La caractérisation de ce réseau est faite grâce à la réalisation de cartographies d'orientations cristallographiques par EBSD. Une méthode de caractérisation innovante basée sur la discrétisation des cartographies en réseaux de points triples est proposée. Cette méthode permet d'analyser la connectivité du réseau de joints de grains en vue d'une corrélation avec les propriétés mécaniques. / This work is dedicated to the study of Grain Boundary Engineering in Ni-based superalloys for aircraft turbine disk. The grain boundary engineering has proven its efficiency for the improvement of the fatigue and creep properties of copper and nickel alloys. This improvement is achieved by performing a succession of room temperature deformations interspaced by heat treatments to modify the distribution of the grain boundary network. The aim of the study is to adapt these processes to high temperature forging of superalloys. An experimental study is achieved to highlight the influence of forging parameters on the grain boundary network. The characterization of the grain boundary network is made through the completion of crystallographic orientation mapping by EBSD. An innovative characterization method based on the discretization of orientation maps in a triple junction network is proposed. This method allows to analyze the connectivity in the grain boundary network with the objective of a correlation with mechanical properties.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14574号 / 工博第3042号 / 新制||工||1453(附属図書館) / 26926 / UT51-2009-D286 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 粟倉 泰弘, 教授 杉村 博之, 教授 田中 功 / 学位規則第4条第1項該当

The aim of this thesis is to investigate the effects of additives, reactive elements and impurities, on the lifetime of thermal barrier coatings. The thesis consists of a number of studies on interface adhesion, impurity diffusion, grain boundary sliding and cleavage processes and their impact on the mechanical behaviour of grain boundaries. The effects of additives and impurity on interface adhesion were elaborated by using total energy calculations, electron localization and density of states, and by looking into the atomic separations. The results of these calculations allow the assessment of atomic level contributions to changes in the adhesive trend. Formation of new bonds across the interface is determined to improve the adhesion in reactive element(RE)-doped structures. Breaking of the cross interface bonds and sulfur(S)-oxygen(O) repulsion is found responsible for the decreased adhesion after S segregation. Interstitial and vacancy mediated S diffusion and the effects of Hf and Pt on the diffusion rate of S in bulk NiAl are studied. Hf is shown to reduce the diffusion rate, and the preferred diffusion mechanism of S and the influence of Pt are revealed to be temperature dependent. Finally, the effects of reactive elements on alumina grain boundary strength are studied. Reactive elements are shown to improve both the sliding and cleavage resistance, and the analysis of atomic separations suggest an increased ductility after the addition of quadrivalent Hf and Zr to the alumina grain boundaries.

Turbine components operate under severe loading conditions and at high and varying temperatures that result in thermal stresses in the presence of temperature gradients created by hot gases and cooling air. Moreover, static and cyclic loads as well as the motion of rotating components create mechanical stresses. The combined effect of complex thermo-mechanical stresses promote nucleation and propagation of cracks that give rise to fatigue and creep failure of the turbine components. Therefore, the relationship between thermo-mechanical stresses, chemical composition, heat treatment, resulting microstructure, operating temperature, material damage, and potential failure modes, i.e. fatigue and/or creep, needs to be well understood and studied. Artificial neural networks are promising candidate tools for such studies. They are fast, flexible, efficient, and accurate tools to model highly non-linear multi-dimensional relationships and reduce the need for experimental work and time-consuming regression analysis. Therefore, separate neural network models for γ’ precipitate strengthened Ni based superalloys have been developed for predicting the γ’ precipitate size, thermal expansion coefficient, fatigue life, and hysteresis energy. The accumulated fatigue damage is then estimated as the product of hysteresis energy and fatigue life. The models for γ’ precipitate size, thermal expansion coefficient, and hysteresis energy converge very well and match experimental data accurately. The fatigue life proved to be the most challenging aspect to predict, and fracture mechanics proved to potentially be a necessary supplement to neural networks. The model for fatigue life converges well, but relatively large errors are observed partly due to the generally large statistical variations inherent to fatigue life. The deformation mechanism map for 1.23Cr-1.2Mo-0.26V rotor steel has been constructed using dislocation glide, grain boundary sliding, and power law creep rate equations. The constructed map is verified with experimental data points and neural network results. Although the existing set of experimental data points for neural network modeling is limited, there is an excellent match with boundaries constructed using rate equations which validates the deformation mechanism map.