Biblioteca Digital

O objetivo deste trabalho foi tentar correlacionar microestrutura e propriedades magnéticas de ímãs permanentes de alta energia do tipo Sm(CoFeCuZr)z para aplicações em temperaturas acima de 300°C. Avaliou-se a possibilidade de que o constituinte matriz da microestrutura dos ímãs para altas temperaturas fosse diferente do constituinte matriz dos ímãs convencionais. Com este propósito foram estudados três conjuntos de amostras: i) Dois ímãs comerciais para aplicações na temperatura ambiente, um da Electron Energy e outro da Vacuumschmelze. Eles foram caracterizados magneticamente e a sua microestrutura foi analisada por microscópio eletrônico de varredura dotado de análise química, com o objetivo de avaliar se os mesmos apresentavam dois microconstituintes presentes em algumas ligas de Sm(CoFeCuZr)z. ii) Uma liga comercial da Johnson Matthey (JM) que é utilizada para produzir ímãs permanentes de Sm(CoFeCuZr)z. Essa liga foi utilizada para o estudo e avaliação de ciclos térmicos como solubilização, tratamento isotérmico, rampa de resfriamento lento e tratamento térmico a 400°C, e o seu efeito nas propriedades magnéticas e na micro e nano estruturas. iii) Duas séries de ligas com seis amostras cada série produzidas em forno a arco voltaico no LMM-IFUSP...

A falha devido à ocorrência de fadiga térmica de materiais utilizados como ferramentas para trabalho a quente é identificada durante serviço e ocorre devido ao acúmulo de dano localizado. O aço AISI H13 é comumente utilizado em ferramentas para a conformação a quente devido à sua boa tenacidade à fratura e resistência ao desgaste, e considerável resistência à perda de dureza a quente. Este trabalho teve como motivação estudar a relação entre a microestrutura do aço H13 e os carregamentos térmicos e mecânicos, que possam levar à falha de ferramentas para forjamento a quente. Para este estudo, fez-se uso de meios computacionais (simulação numérica) aliados aos conhecimentos de caracterização microestrutural e do comportamento mecânico dos materiais. Nesta abordagem, elabora-se uma malha na microestrutura do referido aço no software OOF2®, do NIST, e as análises são feitas a partir da aplicação do método dos elementos finitos com o emprego do software Abaqus®. Com isso, torna-se possível examinar o efeito de aspectos microestruturais, como a influência dos precipitados, na ocorrência de tensões e de deformações na microestrutura de forma a obter um mapeamento de regiões críticas ao dano e à falha na ferramenta de forjar a quente. Os estudos são baseados e comparados com trabalhos já publicados...

The microstructure and dielectric properties of Nb-Mn or Sb-Mn codoped BaTiO3 compositions were investigated. Starting ceramics powders were prepared by Pechini method. The composites were sintered at 1310°C and 1330°C in an air atmosphere for two hours. The microstructure and compositional investigations were done with SEM equipped with EDS. Two distinguish microstructure regions are observed in Nb/0.05Mn doped BaTiO 3 ceramics sintered at low temperature. The first, large one, with grain sizes from 5-40 μm and the second region with small grain sizes from 1 to 5 μm. Sintering at higher temperature, independent of Mn content, enables to achieve a uniform microstructure with grains less than 6 μm. In Sb/Mn doped ceramics, for both sintering temperatures, bimodal microstructures with fine grained matrix and grains up to 10 μm is formed. The highest value of permittivity at room temperature and the greatest change of permittivity in function of temperature are observed in Nb/0.01Mn doped ceramics compared to the same ones in Sb/Mn doped ceramics. The greatest shift of Curie temperature towards lower temperature has been noticed in Sb/Mn BaTiO3 ceramics compared to others samples. In all investigated samples the dielectric loss after initially large values at low frequency maintains a constant value for f>3 kHz.

The microstructure evolution and mechanical behavior during large strain of a 0.16%C-Mn steel has been investigated by warm torsion tests. These experiments were carried out at 685°C at equivalent strain rate of 0.1 s . The initial microstructure composed of a martensite matrix with uniformly dispersed fine cementite particles was attained by quenching and tempering. The microstructure evolution during tempering and straining was performed through interrupted tests. As the material was reheated to testing temperature, well-defined cell structure was created and subgrains within lath martensite were observed by TEM; strong recovery took place, decreasing the dislocation density. After 1 hour at the test temperature and without straining, EBSD technique showed the formation of new grains. The flow stress curves measured had a peculiar shape: rapid work hardening to a hump, followed by an extensive flow-softening region. 65% of the boundaries observed in the sample strained to ε = 1.0 were high angle grain boundaries. After straining to ε = 5.0, average ferrite grain size close to 1.5 μm was found, suggesting that dynamic recrystallization took place. Also, two sets of cementite particles were observed: large particles aligned with straining direction and smaller particles more uniformly dispersed. The fragmentation or grain subdivision that occurred during reheating and tempering time was essential for the formation of ultrafine grained microstructure.

In this work, we investigate the correlations between structural and rheological properties of emulsified aqueous sol and the porous microstructure of monolithic zirconia foams, manufactured by the integrative combination of the sol-gel and emulsification processes. Macroporous zirconia ceramics prepared using different amounts of decahydronaphthalene, as oil phase, are compared in terms of the emulsion microstructure and ceramic porosity. A combination of electrical conductivity, oil droplet diameter, and rheological measurements was used to highlight the key effect of the dynamic structural properties of the emulsion on the porosity of the ceramic zirconia foam. The minimization of drying shrinkage by appropriate sol-gel mineralization of the oil droplet wall enabled versatile and easy tuning of the ceramic foam microstructure, by fine adjustment of the emulsion characteristics. The foam with the highest porosity (90%) and the lowest bulk density (0.40 g cm-3) was prepared from emulsion with 80 wt% of decahydronaphthalene, which also showed a bicontinuous structure and elevated flow consistency. © The Royal Society of Chemistry 2013.

The microstructure and surface composition effects on the transpassivation behavior of untreated and heat treated at 500 ºC NiTi wires used for implant purposes were investigated by electrochemical techniques (open circuit potential and potentiodynamic polarization curves), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The potential at which the passive film breaks down (Eb) (transpassivation) was found to be strictly dependent on both the chemical composition of oxide covering layer and the microstructure of the biomaterial. It could be concluded that the disordered microstructure of untreated NiTi is highly resistant to transpassivation (Eb ~ 1.00 V/SCE), but the presence of a native oxygen rich oxide film onto it makes the dissolution easier (Eb ~ 0.00 V/SCE). Upon thermally treating, the microstructure becomes ordered and less resistant to dissolution (Eb ~ 0.12 V/SCE), but in this case the presence of an oxygen rich native film influenced Eb in a lower extent (Eb ~ 0.03 V/SCE).

Nickel-based alloy IN 625 is used to protect components of aircrafts, power generation and oil refinery due to an association of toughness and high corrosion resistance. These properties are associated with the chemical composition and microstructure of coatings which depend on the processing parameters and the composition of the component being protected. This paper assessed impact of dilution on the microstructure and properties of the Ni alloy IN 625 deposited by Plasma Transferred Arc (PTA) on two substrates: carbon steel API 5L and stainless steel AISI 316L. Differences due to the interaction with the substrate were maximized analyzing single layer coatings, processed with three deposition current: 120, 150 and 180 A. Correlation with a cast Nickel-based alloy sample contributed to assess the impact of dilution on coatings. Dilution was determined by the area ratio and Vickers hardness measured on the transverse section of coatings. Scanning electron and Laser confocal microscopy and X-ray diffraction analysis were carried out to characterize the microstructure. Results indicated the increasing dilution with the deposition current was deeply influenced by the substrate. Dilution ranging from 5 to 29% was measured on coatings processed on the API 5L steel and from 22 to 51% on the low thermal conductivity AISI 316L steel substrate. Differences on the microstructure and properties of coatings can be associated with the interaction with each substrate. Higher fraction of carbides account for the higher coating hardness when processing on API 5L whereas the low thermal conductivity of AISI 316L and the higher Fe content in solid solution contributed to the lower hardness of coatings.

Cobalt-Based alloys are largely applied to the surface of components as welded coatings. Carbides reinforced CoCrWC system is used to extend the service life under harsh environments involving wear and corrosion in different media. This work aims to evaluate the effect of deposition current on the microstructure and properties of PTA coatings. So, CoCrWC alloy (Stellite #6) was processed on AISI316L stainless steel plates with the following main arc current: 100, 120, 150, 180 and 200A. So, different interaction with the substrate must be expected and its effect on coatings features was evaluated. The geometry of single track coatings, dilution, formed phases and phase volume fraction was assessed by laser Confocal, scanning electron microscopy and X-ray diffraction analysis. Vickers hardness and wear tests were carried out to correlate microstructure to properties of coatings. Coatings showed microstructure composed by hypoeutectic dendrites of Cobalt solid solution and interdendrictic carbides. Dilution increased with deposition current from 11,8 e 56,5% which reduced the carbides fraction and increased the Cobalt solid solution areas, resulting in hardness decrease from 500 to 310HV0,5. Higher deposition current induced mass loss rate increase on pin-on-disc sliding wear tests...

Basic regularities of silicon nitride based materials microstructure formation and development in interrelation with processing conditions, type of sintering additives, and starting powders properties are discussed. Models of abnormal or exaggerated grain growth are critically reassessed. Results of several model experiments conducted in order to determine the most important factors directing the microstructure formation processes in RE-fluxed Si3N4 ceramics are reviewed. Existing data on the mechanisms governing the microstructure development of Si3N4-based ceramics are analyzed and several principles of microstructure tailoring are formulated.

Three kinds of X70 steels with the same chemical composition and different microstructures fabricated by varying processes were compared in aspect of the microstructures and mechanical properties using SEM, TEM and tensile testing machine. The experimental results showed that the steel 1 with acicular ferritic microstructure fabricated by thermal-mechanical processing with online accelerated cooling (TMCP) exhibited an excellent combination of strength and toughness, but provided high yield ratio of 0.85, low uniform elongation of 8.3% as well as low strain hardening exponent of 0.09, indicating poor deformability. In contrast to above steel 1, the steel 2 and steel 3 having ferrite-bainite dual-phase microstructure respectively fabricated by TMCP and intercritical annealing exhibited the improved deformability in terms of the low yield ratio of 0.69 and 0.68, high uniform elongation of 12.8% and 11.8%, and strain hardening exponent of 0.157 and 0.155. It is argued that the optimum properties combination of strength, toughness and deformability can best be achieved by obtaining a ferrite-bainite dual-phase microstructure. This kind of ferrite-bainite dual-phase pipeline steel is appropriate to transmitting oil and natural gas in seismic zone and permafrost.

Les polymères semi-conducteurs semicristallins sont utilisés au sein de diodes électroluminescentes, transistors ou dispositifs photovoltaïques organiques. Ces matériaux peuvent être traités à partir de solutions ou directement à partir de leur état solide et forment des agrégats moléculaires dont la morphologie dicte en grande partie leurs propriétés optoélectroniques. Le poly(3-hexylthiophène) est un des polymères semi-conducteurs les plus étudiés. Lorsque le poids moléculaire (Mw) des chaînes est inférieur à 50 kg/mol, la microstructure est polycristalline et composée de chaînes formant des empilements-π. Lorsque Mw>50 kg/mol, la morphologie est semicristalline et composée de domaines cristallins imbriquées dans une matrice de chaînes amorphes. À partir de techniques de spectroscopie en continu et ultrarapide et appuyé de modèles théoriques, nous démontrons que la cohérence spatiale des excitons dans ce matériau est légèrement anisotrope et dépend de Mw. Ceci nous permet d’approfondir la compréhension de la relation intime entre le couplage inter et intramoléculaire sur la forme spectrale en absorption et photoluminescence. De plus, nous démontrons que les excitations photogénérées directement aux interfaces entre les domaines cristallins et les régions amorphes génèrent des paires de polarons liés qui se recombinent par effet tunnel sur des échelles de temps supérieures à 10ns. Le taux de photoluminescence à long temps de vie provenant de ces paires de charges dépend aussi de Mw et varie entre ∼10% et ∼40% pour les faibles et hauts poids moléculaires respectivement. Nous fournissons un modèle permettant d’expliquer le processus de photogénération des paires de polarons et nous élucidons le rôle de la microstructure sur la dynamique de séparation et recombinaison de ces espèces.; Microstructure plays a crucial role in defining the optoelectrical properties of conjugated polymeric semiconductors which can be used in light harvesting and generating devices such as organic light emitting diodes...

MgB2 is an intermetallic compound, has hexagonal crystal structure and is a clear example of two band superconductivity. Superconducting transition temperature (Tc) of MgB2 is at 39 K, which is twice that of Nb3Sn and four times that of Nb-Ti, the two most common commercial superconductors. The upper critical field (Bc2) of MgB2 is anisotropic but variable and can be enhanced by introducing structural disorder by e.g. C-doping and neutron irradiation. Recently Bc2(0) of 70 T in thin films and 37 T in bulk samples have been reported. The superconducting coherence length of MgB2 lies in the range of about 2-10 nm. Superconducting wires and tapes are hysteretic type II superconductors that are exposed to high magnetic fields and are in the critical (Shubnikov) state. Therefore, the critical current density (Jc (B,T)) is an exclusively important figure of merit for these superconductors. Most of the applied research on MgB2 is aimed at enhancing the Jc which reached about 1x105 Acm-2 (at 4.2 K and 12 T). In MgB2 the correlation of microstructure with the superconducting properties, in particular the critical current density requires powerful analytical tools. Critical current densities and electrical resistivities of different MgB2 superconductors differ by orders of magnitudes and the current limiting mechanisms have not been fully understood. Granularity of MgB2 is one significant reason for reduced critical current density and is introduced intrinsically by the anisotropy of Bc2 but also extrinsically by the microstructure of the material. This thesis can be subdivided as follows: (I) Development of the quantitative electron microscopy and spectroscopy methods essential for the microstructural analysis of MgB2 and thereby assess the performance of two energy-filtered TEMs (Zeiss 912 and Zeiss Libra 200FE). By quantitative electron microscopy and spectroscopy we mean a combined SEM and TEM analysis that covers various length scales from µm to nm. Contamination free sample preparation...

Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure&-property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels...

Broadly speaking, the research and design of coatings are generally studied by way of single-track deposits; otherwise, the development of a coated part entails the understanding of how multi-track welding influences the microstructure and properties of the surface. This study evaluated the effect of track overlap on the microstructure and properties of the CoCrMoSi Tribaloy T400 alloy coatings produced on AISI 316L steel substrate. The characterization was performed by scanning electron microscopy, hardness and X-ray diffraction. The correlation between the degree of overlap and performance of the coatings was assessed by wear tests. The single track deposits showed hypoeutectic microstructure as a result of high Iron, Chromium and Nickel content. From the second track on, the chemical composition was displaced back to eutectic and then to hypereutectic with overlap of 25 and 50%, respectively. The microstructure dictated the hardness of the coatings (527 – 701 HV0.5) and the lower mass loss rate was measured for hypereutectic with primary Laves phase.

Transformation-induced plasticity (TRIP) steels have excellent strength, ductility and work hardening behaviour, which can be attributed to a phenomenon known as the TRIP effect. The TRIP effect involves a metastable phase, retained austenite (RA), transforming into martensite as a result of applied stress or strain. This transformation absorbs energy and improves the work hardening rate of the steel, delaying the onset of necking. This work describes two distinct TRIP steel microstructures and focuses on how microstructure affects the RA-to-martensite transformation and the uniaxial tensile behaviour. A two-step heat treatment was applied to an aluminum-alloyed TRIP steel to obtain a microstructure consisting of equiaxed grains of ferrite surrounded by bainite, martensite and RA -- the equiaxed microstructure. The second microstructure was produced by first austenitizing and quenching the steel to produce martensite, followed by the two-step heat treatment. The resulting microstructure (labelled the lamellar microstructure) consisted of elongated grains of ferrite with bainite, martensite and RA grains. Both microstructural variants had similar initial volume fractions of RA. A series of interrupted tensile tests and ex-situ magnetic measurements were conducted to examine the RA transformation during uniform elongation. Similar tests were also conducted on an equiaxed microstructure and a lamellar microstructure with similar ultimate tensile strengths. Results show that the work hardening rate is directly related to the RA transformation rate. The slower transformation rate...

The present study aims to identify and characterize the development of microstructure and deformation characteristics of steel grades in semi-solid state which is affected by the change in morphologies of microstructure at high temperature. Thixoextrusion tests with different combinations of forming temperature and forming speed were performed. It was identified that several process parameters, such as initial billet and die temperatures or forming speed, affect thermal exchanges thereby influencing the microstructure evolution and material flow. Furthermore, 2D and 3D microstructure characterization was performed on the same sample which was partial remelted and quenched. Reconstructed 3D images were compared with the ones obtained with a Scanning Electron Microscope and an Energy Dispersive Spectrometry system. The good agreement between 2D SEM observations and 3D X-ray microtomography results makes these two techniques efficient to characterize steels in the semi-solid state.

The microstructure investigation and flow behavior during thixoforging of M2 steel parts were investigated. Partial remelting was performed at processing temperatures ranging from 1290 ◦C to 1340 ◦C corresponding to a liquid fraction range between 10% and 30% (according to differential scanning calorimetry measurements and quantitative image analyses). A conventional microstructure for thixoforming process was obtained: spherical solid grains surrounded by liquid phase. The microstructure across the heated billets was relatively homogeneous with bigger grain size near the surface. Successful thixoextrusion for producing parts was finally achieved at processing temperatures. By investigating the microstructure and load-displacement curves, different mechanisms in various forming stages were proposed.

The microstructure plays a crucial role for steel semi-solid forming process, and particularly for the steel thixoforging process, since it determines the thixotropic flow behavior of materials in the semi-solid state. Therefore, it is necessary to well understand the microstructure evolution during high speed heating and forming. Classically, it is investigated on a solid material quenched from semi-solid state by 2D characterization techniques. However, the semi-solid microstructure could probably not be preserved in the solid state by quenching due to complicated phase transformations or high diffusion rate of alloying elements during cooling, especially at low liquid fractions. In order to avoid this, a new in situ technique - high temperature Confocal Laser Scanning Microscopy (CLSM) - was developed and used for studying the microstructure evolution directly at high temperature. The present study aims at providing an experimental investigation of the microstructure evolution on several steel grades (M2, 100Cr6 and C38LTT) during heating from the as-received state to the semi-solid state (heating rate: ~200°C/min) and finally cooled to the solid state (cooling rate: ~200°C/min). It has been found that the temperature sensitivity of liquid fraction (ΔT/ Δfl) of these grades is much different. In addition...

We present a simple microstructure model of financial returns that combines (i) the well-known ARFIMA process applied to tick-by-tick returns, (ii) the bid-ask bounce effect, (iii) the fat tail structure of the distribution of returns and (iv) the non-Poissonian statistics of inter-trade intervals. This model allows us to explain both qualitatively and quantitatively important stylized facts observed in the statistics of microstructure returns, including the short-ranged correlation of returns, the long-ranged correlations of absolute returns, the microstructure noise and Epps effects. According to the microstructure noise effect, volatility is a decreasing function of the time scale used to estimate it. Paradoxically, the Epps effect states that cross correlations between asset returns are increasing functions of the time scale at which the returns are estimated. The microstructure noise is explained as the result of the negative return correlations inherent in the definition of the bid-ask bounce component (ii). In the presence of a genuine correlation between the returns of two assets, the Epps effect is due to an average statistical overlap of the momentum of the returns of the two assets defined over a finite time scale in the presence of the long memory process (i).; Comment: 31 pages + 19 figures

Ferrite have found a variety of uses in electronic and communication engineering. Mn-Zn ferrites (Mn1-XZnxFe2O4 ) are widely used as filter core materials over a range of frequencies varying from several hundred Hz to several MHz. There are many other applications such as in television receivers as deflection yokes and E.H.T. cores etc. The development of a ferrite suitable for a particular application is an interesting scientific problem and technological challenge. The properties of ferrites are determined by a number of intrinsic properties and their interaction with the ceramic microstructure. Impurities, present in or added to the raw materials used for processing ferrites, play an important role in determining the properties of the ferrites. The cost of ferrites is very much related to the purity level of the raw materials used. It is, therefore, both scientifically and economically important that the behaviour of ferrites is studied with additions of controlled amounts of impurities commonly present in the raw materials cheaply available. Silica (Si0 2) is commonly found in the raw materials and is also, to some extent, contributed by atmospheric dust. In order to determine the tolerance of SiO2 as an impurity in the raw materials and during processing...