This work deals with fatigue properties and stability of microstructure of ultrafine-grained (UFG) copper prepared by severe plastic deformation by means of equal channel angular pressing (ECAP) method. The effect of different fatigue loading regimes and thermal exposition on microstructural changes was investigated and the fatigue lifetime curves were experimentally determined. The research attention was focussed on localization of cyclic plastic deformation and fatigue crack initiation in UFG structure. Experimental results indicate that after stress-controlled fatigue loading (both symmetrical and asymmetrical) the microstructure remains ultrafine; no grain coarsening was observed. Contrary to this, strain-controlled fatigue loading results in formation of bimodal structure. Grain coarsening was observed also after thermal exposition at 250 °C for 30 minutes. Annealing at lower temperatures does not result in grain coarsening or development of bimodal structure. Fatigue loading results in development of surface relief in form of cyclic slip markings. Their density, distribution and shape differ for particular fatigue loading regimes. Differences in crack initiation mechanism in low- and high-cycle fatigue region were found. Nevertheless, the characteristic feature for all loading regimes was stability of UFG microstructure in the region of cyclic slip bands and fatigue cracks.

This doctoral thesis summarizes the theoretical and experimental knowledge in the field of dissimilar weld joint of progressive austenitic and ferritic creep-resistant steels. The following materials were selected for the presented study: 316Ti stabilized austenitic stainless steel, martensitic 9-12 %Cr steel P92 and ferritic ODS steel MA 956. The main attention was focused on the long-term microstructural stability during high temperature exposure of heterogeneous joints of the austenite / ferrite type. The literature analysis critically evaluates the current state of knowledge in the field of microstructural stability of advanced creep-resistant steels weld joints. The practical experimental part was carried out in two directions. On the basis of the chemical composition phase equilibrium calculations were performed for each steel using the ThermoCalc software, giving the basic concepts about the dependence of the phase composition and the chemical composition of phases on temperature. In parallel with these calculations the laboratory joints 316Ti/P92 made by resistance welding and the MA 956/316Ti electron beam weld joints were prepared, analyzed in as-weld state and further annealed at different temperature conditions. Exposed joints were subjected to microstructure and phase analysis. The stability of the weld interface was mainly observed. Attention was also focused on the agreement between the calculation and experimental data in comparison with data published in the literature. Based on the calculations, experimental results and published data the suitability of the combination of materials is discussed in the thesis and reasoning about the behavior of studied weld joints during long-term high temperature exposure was formulated. Based on the results the expected degree of microstructural stability of 316Ti/P92 joint was confirmed, while the joints MA 956/316Ti were found to be unstable.

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<div>Several truck fires have occurred in recent years involving bridges with reinforced and prestressed concrete components. If the fire burns for a significant period of time (15 minutes or more), bridge inspectors and engineers must determine if the exposure to elevated temperature has reduced the strength and serviceability of the concrete components. Little guidance is available, however, correlating the results of field inspections with the actual condition of the reinforced/prestressed concrete elements. This dissertation presents the results of a research program conducted to develop rational guidance for inspectors and engineers to evaluate concrete bridge elements after a fire event and help them make informed decisions regarding the future status of the bridge. <br></div><div><br></div><div>The research program includes tests on portions of a reinforced concrete deck and three full-scaled AASHTO Type I prestressed girders acquired from a decommissioned highway bridge. In addition, six pretensioned concrete prismatic beam specimens with varying levels of prestress were fabricated and tested. The specimens had cross-sectional dimensions of 8 in. by 8 in. and were designed to simulate the bottom flanges of common I-shaped prestressed concrete bridge girders. The deck specimens and four (of the six) concrete beam specimens were subjected to elevated temperatures using radiation-based heaters. Two (of the six) prismatic specimens built in the laboratory were subjected to a hydrocarbon pool fire test conducted in the field for using approximately 135 gallons of kerosene. The concrete temperature profiles and the deformations of the specimens were measured using thermocouple trees and displacement transducers, respectively. Concrete samples were also cored and examined using various methods (DSC and SEM) to correlate microstructure degradation (microcracking, dehydration of C-S-H, decomposition of calcium hydroxide, etc.) with the measured temperatures through the depth of the specimens. <br></div><div><br></div><div>To evaluate the residual loading-carrying capacities of prestressed concrete girders after being subjected to fire, a hydrocarbon pool fire test was performed on two decommissioned AASHTO Type I girders in the field. Load tests were then conducted on the prestressed girders under both ambient and post-fire conditions. After structural testing, material tests were also conducted on concrete cores taken from the girders to evaluate the post-fire concrete microstructure alteration. Furthermore, three-dimensional finite element models were developed to predict the residual load-carrying capacities and overall structural responses of prestressed concrete bridge girders after being exposed to fire. Results from the numerical models generally agree favorably with experimental observations and provide insights into the behavior of the specimens. A parametric study was performed using the benchmarked finite element models to expand the database and establish design recommendations further. Capacity influence lines for load-carrying capacities and structural stiffness were developed and discussed.</div><div><br></div><div>Based on the results from this research, guidelines for the post-fire assessment of prestressed concrete bridges are included in this dissertation along with a step-by-step checklist. Bridge inspectors can infer the extent of damage to prestressed concrete bridge girders in the event of a fire and develop a post-fire assessment plan cognizant of the findings. In most cases, no more than 1.0 in. of the concrete from the exposed surface undergoes material damage / deterioration due to loss of CH, cracking, and spalling. The impact on the strength of prestressed concrete girders is relatively minor based on experimental results. Their initial stiffness, however, will likely be reduced. <br> </div><br>

[pt] A demanda por veículos mais seguros e com baixo consumo de combustível vem levando a indústria automotiva a buscar novos materiais. A indústria do aço, ameaçada pela competitividade da indústria do alumínio, reagiu com uma série de novos aços de alta resistência. Dentre estes aços, os Aços Avançados de Alta Resistencia (AHSS) podem ser destacados. Esses aços podem ser divididos em 3 gerações, cada uma delas com suas vantagens e desafios. A primeira geração tem os aços mais baratos, geralmente com microestrutura ferritica/martensitica. A segunda geração possui os aços inoxidáveis austeníticos/ferríticos, com composição mais cara devido ao maior teor de liga, muitos deles apresentando efeito de plasticidade induzida por maclagem (TWIP). Entre essas duas gerações, uma terceira vem se sobressaindo, baseada em aços de composição mais barata, porém com processamento mais complexo, como os aços de tempera e partição (Quenching and Partitioning - Q and P). Como não existe apenas um único caminho para o sucesso, todas as três gerações vêm recebendo bastante atenção e pesquisa. Buscando atender parte da demanda do setor, a Companhia Siderúrgica Nacional (CSN), vem aprimorando seu portfólio de aços AHSS de primeira geração. Entre os aços de primeira geração, os mais utilizados hoje são os Dual-Phase (DP). No entanto, os aços DP apresentam alguns problemas, como a nucleação de vazios durante a deformação, o que é um fator limitante para suas propriedades mecânicas. Sua substituição por aços Complex-Phase (CP), com maiores quantidades de bainita e menos propensos a nucleação de vazio, vem sendo proposta. O processamento de aços de fase complexa envolve uma sequência de etapas de laminação a quente e a frio, seguidas de um tratamento térmico. Na presente tese, foi estudada a produção de um aço CP1200 em uma linha industrial de galvanização por imersão a quente. O ponto de partida foi um aço CP1100. Amostras deste aço laminado a frio foram analisadas por dilatometria de forma a se obter a curva CCT. Utilizando os dados dilatométricos, um novo tratamento térmico foi proposto e realizado nas instalações da CSN, produzindo com sucesso um aço CP 1200. Ensaios de tração, dureza, dobramento e expansão de buraco foram realizados para medir as propriedades mecânicas do novo aço. A caracterização microestrutural foi realizada por meio de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), microscopia de força atômica (MFA), difração de elétrons retroespalhados (EBSD) e microscopia eletrônica de transmissão (MET); a quantificação das micrografias foi realizada usando processamento digital de imagem e redes neurais. O aumento da propriedade mecânica foi atribuído ao aumento na fração de bainita, bem como de interfaces bainita-ferrita e bainita-martensita, que são menos suscetíveis a nucleação de vazios. / [en] The demand for safer and fuel-efficient vehicles leads the automotive industry to seek new and stronger materials. The steel industry, threatened by the aluminum competition, reacted with new and higher strength steels. Among the possible steels, the Advanced High Strength Steels (AHSS) can be highlighted. These steels can be divided into 3 generations, each one possessing advantages and challenges. The first generation includes more economical alloying and processing strategies, usually with a ferritic/martensitic microstructure. The second generation contains the more expensive, higher alloyed ferritic/austenitic stainless steels, using TWinning Induced Plasticity (TWIP). Between these two classes, a third generation is growing, based on more inexpensive compositions but with more complex processing, such as Quenching and Partitioning (Q and P). There is not only a single path to success, because of that, all three generations receive their fair amount of attention and research. Trying to fulfill part of the industry demand, the Companhia Siderúrgica Nacional (CSN) is conducting research on the first generation of AHSS steels. Among the first generation steels, Dual-Phase (DP) are the most used. However, DP steels present some problems, such as void nucleation during deformation, which is a limiting factor on their mechanical properties. Therefore, their replacement by Complex-Phase (CP) steels, with higher bainite amounts, and less prone to void nucleation, is welcome. The complex phase steels processing involves a sequence of hot-rolling and cold rolling steps, followed by a heat treatment. In the present thesis, the production of a CP1200 steel in an industrial hot dip galvanizing line was studied. The starting point was an industrial CP1100 steel. Samples of this cold rolled steel were analyzed by quenching dilatometry to obtain the CCT curve. Using the dilatometric data, a new heat treatment was proposed and done at CSN facilities, successfully producing a CP 1200 steel. Tension, hardness, bending and hole expansion tests were performed to measure the mechanical properties of the new steel. The microstructural characterization was done using light optical microscopy (LOM), scanning electron microscopy (SEM), atomic force microscopy (AFM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM); neural networks and digital image processing were used to quantify the obtained micrographs. The increase in tension and yielding strengths was explained based on the higher amounts of bainite as well as of bainite-ferrite and bainite-martensite interfaces, found to be less prone to void nucleation.

The high temperature microstructure of the solid phases within the electric arc furnace (EAF) slag has a large effect on the process features such as foamability of the slag, chromium recovery, consumption of the ferroalloys and the wear rate of the refractory. The knowledge of the microstructural and compositional evolution of the slag phases during the EAF process stages is necessary for a good slag praxis. In supplement 1, an investigation of the typical characteristics of EAF slags in the production of the AISI 304L stainless steel was carried out. In addition, compositional and microstructural evolution of the slag during the different EAF process stages was also investigated. Computational thermodynamics was also used as a tool to predict the equilibrium phases in the top slag as well as the amount of these phases at the process temperatures. Furthermore, the influence of different parameters (MgO wt%, Cr2O3 wt%, temperature and the top slag basicity) on the amount of the spinel phase in the slag was studied. In supplement 2, a novel study to characterize the electric arc furnace (EAF) slags in the production of duplex stainless steel at the process temperatures was performed. The investigation was focused on determining the microstructural and compositional evolution of the EAF slag during and at the end of the refining period. Slag samples were collected from 14 heats of AISI 304L steel (2 slag samples per heat) and 7 heats of duplex steel (3 slags sample per heat). Simultaneously with each slag sampling, the temperature of the slag was measured. The selected slag samples were studied both using SEM-EDS and LOM. In some cases (supplement 2), X-ray diffraction (XRD) analyses were also performed on fine-powdered samples to confirm the existence of the observed phases. It was observed that at the process temperature and at all process stages, the stainless steel EAF slag consists mainly of liquid oxides, magnesiochromite spinel particles and metallic droplets. Under normal operation and at the final stages of the EAF, 304L steelmaking slag contains 2-6 wt% magnesiochromite spinel crystals. It was also found that, within the compositional range of the slag samples, the only critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content. Petrographical investigation of the EAF duplex stainless steelmaking showed that, before FeSi-addition, the slag samples contain large amounts of undissolved particles and the apparent viscosity of the slag is higher, relative to the subsequent stages. In this stage, the slag also includes solid stoichiometric calcium chromite. It was also found that, after FeSi-injection into the EAF and during the refining period, the composition and the basicity of the slag in the EAF duplex steelmaking and EAF stainless steelmaking are fairly similar. This indicates that, during the refining period, the basic condition for the utilization of an EAF foaming-slag praxis, in both austenitic and duplex stainless steel cases, is the same. Depending on the slag basicity, the slag may contain perovskite and/or dicalcium silicate too. More specifically, the duplex stainless steel slag samples with a higher basicity than 1.55 found to contain perovskite crystals. / QC 20110413

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[pt] Ferritas do tipo espinélio têm atraído atenção devido a suas propriedades magnéticas com possibilidade de aplicações em dispositivos spintrônicos e de memória magnética, além de aplicações em catálise. Este trabalho tem como objetivo sintetizar o espinélio de alumínio e ferro (FeAl2O4) pelo método de síntese reação de combustão e avaliar suas características estruturais e morfológicas bem como suas propriedades magnéticas devido a sua ampla possibilidade de aplicações. A síntese deste material foi feita utilizando soluções concentradas dos nitratos metálicos hidratados de ferro e alumínio, variando o tipo e quantidade de combustível no intuito de verificar sua influência na microestrutura do produto. Os combustíveis utilizados neste trabalho foram ureia e ácido cítrico, ambos em quantidade estequiométrica, dita ideal, calculada a partir da teoria química dos propelentes, além de reações com quantidades acima e abaixo do ideal. Uma síntese apenas utilizando os nitratos metálicos, sem combustível, foi feita para comparação. O produto final das diferentes sínteses foram caracterizados em termos de estrutura e composição por meio de difração de raios X (DRX), Microscopia Eletrônica de Varredura (MEV) e Microscopia Eletrônica Transmissão (MET). As propriedades magnéticas foram obtidas por magnetometria de amostra vibrante e a identificação das fases de ferro por espectroscopia Mössbauer. As análises indicaram que a amostra sintetizada sem combustível corresponde a óxidos de ferro e de alumínio, majoritariamente amorfo, com formação de partículas nanométricas de hematita (Fe2O3) e um comportamento paramagnético. As caracterizações das amostras produzidas com ureia apresentam maior cristalinidade em relação a sintetizada sem combustível, com a amostra em quantidade estequiométrica de ureia, resultando em um produto monofásico identificado como o espinélio FeAl2O4, misto. Já as amostras com excesso e deficiência de ureia tiveram como produto, além da formação do espinélio desejado, a formação de outra fase identificada como magnetita (Fe3O4), sendo todas as amostras sintetizadas com ureia manifestando comportamento ferromagnético. As amostras sintetizadas com ácido cítrico, em todas as condições, apresentaram um sistema trifásico constituído pelo espinélio, magnetita e hematita e mostraram comportamento também ferromagnético. Neste trabalho é mostrado que a síntese por reação de combustão se mostrou eficiente para produzir o FeAl2O4, alcançando melhores resultados utilizando ureia como combustível, em quantidade ideal. / [en] Spinel-like ferrites has attracted attention due to its magnetic properties with the possibility of spintronic and magnetic memory devices applications as well as catalysis applications. This work aims to synthesize aluminum and iron spinel (FeAl2O4) by combustion reaction synthesis method and evaluate its structural and morphological characteristics, as well as its magnetic properties due to its wide application possibilities. The synthesis of this material was made using concentrated solutions of hydrated iron and aluminum metal nitrates, varying the type and quantity of fuel in order to verify its influence on the product microstructure. The fuels used in this work were urea and citric acid, both in stoichiometric quantity, said ideal, calculated from the chemical theory of propellants, and reactions with quantities above and below ideal. A synthesis using only metal nitrates without fuel was made for comparison. The final product of the different syntheses were characterized in terms of structure and composition by Xray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Magnetic properties were obtained by vibrating sample magnetometry and the iron phase identification by Mössbauer spectroscopy. The analysis indicated that the sample synthetized without fuel corresponds to iron and aluminum oxides, mostly amorphous, with formation of nanometer hematite particles (Fe2O3) and a paramagnetic behavior. The characterizations of the samples produced with urea show higher crystallinity than the synthesized without fuel, with the sample in stoichiometric amount of urea, resulting in a single phase product identified as FeAl2O4 mixed spinel. The samples with excess and deficiency of urea had as product, besides the formation of the desired spinel, the formation of another phase identified as magnetite (Fe3O4), being all the samples synthesized with urea manifesting ferromagnetic behavior. The samples synthesized with citric acid, under all conditions, presented a triphasic system consisting of spinel, magnetite and hematite and showed also ferromagnetic behavior. In this work it is shown that combustion reaction synthesis was efficient to produce FeAl2O4, achieving better results using urea as fuel, in ideal quantity.