Resumo em Inglês:

The phenomenological formalism, which yields Fick's Laws for diffusion in single phase multicomponent systems, is widely accepted as the basis for the mathematical description of diffusion. This paper focuses on problems associated with this formalism. This mode of description of the process is cumbersome, defining as it does matrices of interdiffusion coefficients (the central material properties) that require a large experimental investment for their evaluation in three component systems, and, indeed cannot be evaluated for systems with more than three components. It is also argued that the physical meaning of the numerical values of these properties with respect to the atom motions in the system remains unknown. The attempt to understand the physical content of the diffusion coefficients in the phenomenological formalism has been the central fundamental problem in the theory of diffusion in crystalline alloys. The observation by Kirkendall that the crystal lattice moves during diffusion led Darken to develop the concept of intrinsic diffusion, i.e., atom motion relative to the crystal lattice. Darken and his successors sought to relate the diffusion coefficients computed for intrinsic fluxes to those obtained from the motion of radioactive tracers in chemically homogeneous samples which directly report the jump frequencies of the atoms as a function of composition and temperature. This theoretical connection between tracer, intrinsic and interdiffusion behavior would provide the basis for understanding the physical content of interdiffusion coefficients. Definitive tests of the resulting theoretical connection have been carried out for a number of binary systems for which all three kinds of observations are available. In a number of systems predictions of intrinsic coefficients from tracer data do not agree with measured values although predictions of interdiffusion coefficients appear to give reasonable agreement. Thus, the complete connection has not been made, even for binary systems. The theory has never been tested in multicomponent systems. An alternative path to understanding diffusion behavior in multicomponent systems is presented which is based upon a kinetically derived version of the flux equations. While this approach has problems of its own, it has the potential for providing a new range of insights into the process, and for devising simple models for predicting composition evolution in multicomponent systems.

Resumo em Inglês:

We are currently studying microstructural responses to diffusion-limited coarsening in two-phase materials. A mathematical solution to late-stage multiparticle diffusion in finite systems is formulated with account taken of particle-particle interactions and their microstructural correlations, or "locales". The transition from finite system behavior to that for an infinite microstructure is established analytically. Large-scale simulations of late-stage phase coarsening dynamics show increased fluctuations with increasing volume fraction, Vv, of the mean flux entering or leaving particles of a given size class. Fluctuations about the mean flux were found to depend on the scaled particle size, R/<R>, where R is the radius of a particle and <R> is the radius of the dispersoid averaged over the population within the microstructure. Specifically, small (shrinking) particles tend to display weak fluctuations about their mean flux, whereas particles of average, or above average size, exhibit strong fluctuations. Remarkably, even in cases of microstructures with a relatively small volume fraction (Vv ~ 10-4), the particle size distribution is broader than that for the well-known Lifshitz-Slyozov limit predicted at zero volume fraction. The simulation results reported here provide some additional surprising insights into the effect of diffusion interactions and stochastic effects during evolution of a microstructure, as it approaches its thermodynamic end-state.

Resumo em Inglês:

For thirty years it has been known that certain compositions of Na2O-CaO-P2O5-SiO 2 glasses will form a mechanically strong, chemical bond to bone. These materials have become known as bioactive glasses and the process of bonding is called bioactive fixation. Bioactive glasses are widely used clinically in the repair of bone defects. Recent research at the Imperial College Tissue Engineering Centre has now established that there is a genetic control of the cellular response to bioactive materials. Seven families of genes are up-regulated when primary human osteoblasts are exposed to the ionic dissolution products of bioactive glasses. The gene expression occurs very rapidly, within two days, and includes enhanced expression of cell cycle regulators. The consequence is rapid differentiation of the osteoblasts into a mature phenotype and formation of large three-dimensional bone nodules within six days in vitro. These cell culture results correlate with extensive human clinical results using the same bioactive material. The new genetic theory of bioactive materials provides a scientific foundation for molecular design of new generation of resorbable bioactive materials for tissue engineering and in situ tissue regeneration and repair. Application of this theory to the synthesis of bioactive foams for tissue engineering of bone is described.

Resumo em Inglês:

We have developed a method to obtain polymer hybrids with control of inorganic content and molecular configuration. Hybrid poly(methyl methacrylate) (PMMA) was obtained via atom transfer radical polymerization (ATRP) initiated by octafunctional cubic silsesquioxanes using CuCl as catalyst. Two different ATRP initiators obtained by attaching specific chemical groups to the cubic silsesquioxane were employed to produce two types of hybrids with different architectures. While a-bromide ester groups generated a star-shaped polymer, the benzyl chloride groups produced longer and more linear polymeric chains containing the cubic silsesquioxane. These results were explained on the basis of the low initiation efficiency of benzyl chloride groups for PMMA, leading to polymers with molecular weights (Mn) and polydispersities (PDI) higher than the expected ones.

Resumo em Inglês:

Porous hydroxyapatite manufactured by foaming of aqueous ceramic suspensions and setting via gelcasting of organic monomers was tested for in vivo biocompatibility in rabbit tibia for a period of 8 weeks. The foams provide tortous frameworks and large interconnected pores that support cell attachment and organisation into 3D arrays to form new tissue. The HA foam implants were progressively filled with mature new bone tissue and osteoid after the implanted period, confirming the high osteoconductive potential and high biocompatibility of HA and the suitability of foam network in providing good osteointegration. No immune or inflammatory reactions were detected.

Resumo em Inglês:

Thermodynamic calculations of the Fe-Cr-N System in the region of the Gas Phase Equilibria have been compared with experimental results of maximum nitrogen absorption during nitriding of two Martensitic Stainless Steels (a 6 mm thick sheet of AISI 410S steel and green powder compacts of AISI 434L steel) under N2 atmospheres. The calculations have been performed combining the Fe-Cr-N System description contained in the SGTE Solid Solution Database and the gas phase for the N System contained in the SGTE Substances Database. Results show a rather good agreement for total nitrogen absorption in the steel and nitrogen solubility in austenite in the range of temperatures between 1273 K and 1473 K and in the range of pressures between 0.1 and 0.36 MPa. Calculations show that an appropriate choice of heat treatment parameters can lead to optimal nitrogen absorption in the alloy. It was observed in the calculations that an increased pressure stabilizes CrN at expenses of Cr2N - type nitrides.

Resumo em Inglês:

In the past forty years considerable progress has been achieved on the knowledge of human blood as a non-Newtonian shear-thinning suspension, whose initial state, that is at rest (stasis) or at very low shear rates, has a gel-like internal structure which is destroyed as shear stress increases. The main goal of this communication is to describe the role of geometrical aspects during RBC (red blood cell) aggregate formation, growth and compaction on naturally aggregate (porcine blood) and non-aggregate (bovine blood) samples. We consider how these aspects coupled with tension equilibrium are decisive to transform red cell linear roleaux to three-dimensional aggregates or clusters. Geometrical aspects are also crucial on the compaction of red blood cell aggregates. These densely packed aggregates could precipitate out of blood- either as dangerous deposits on arterial walls, or as clots which travel in suspension until they block some crucial capillary.

Resumo em Inglês:

Powder metallurgy techniques have been used to produce controlled porous structures, such as the porous coatings applied for dental and orthopedic surgical implants, which allow bony tissue ingrowth within the implant surface improving fixation. This work presents the processing and characterization of titanium porous coatings of different porosity levels, processed through powder metallurgy techniques. Pure titanium sponge powders were used for coating and Ti-6Al7Nb powder metallurgy rods were used as substrates. Characterization was made through quantitative metallographic image analysis using optical light microscope for coating porosity data and SEM analysis for evaluation of the coating/substrate interface integrity. The results allowed optimization of the processing parameters in order to obtain porous coatings that meet the requirements for use as implants.

Resumo em Inglês:

Geometric modeling is an important tool to evaluate structural parameters as well as to follow the application of stereological relationships. The obtention, visualization and analysis of volumetric images of the structure of materials, using computational geometric modeling, facilitates the determination of structural parameters of difficult experimental access, such as topological and morphological parameters. In this work, we developed a geometrical model implemented by computer software that simulates random pore structures. The number of nodes, number of branches (connections between nodes) and the number of isolated parts, are obtained. Also, the connectivity (C) is obtained from this application. Using a list of elements, nodes and branches, generated by the software, in AutoCAD® command line format, the obtained structure can be viewed and analyzed.

Resumo em Inglês:

Theoretical determination of the ground-state geometry of Si clusters is a difficult task. As the number of local minima grows exponentially with the number of atoms, to find the global minimum is a real challenge. One may start the search procedure from a random distribution of atoms but it is probably wiser to make use of any available information to restrict the search space. Here, we introduce a new approach, the Assisted Genetic Optimization (AGO) that couples an Artificial Neural Network (ANN) to a Genetic Algorithm (GA). Using available information on small Silicon clusters, we trained an ANN to predict good starting points (initial population) for the GA. AGO is applied to Si10 and Si20 and compared to pure GA. Our results indicate: i) AGO is, at least, 5 times faster than pure GA in our test case; ii) ANN training can be made very fast and successfully plays the role of an experienced investigator; iii) AGO can easily be adapted to other optimization problems.

Resumo em Inglês:

This paper discusses the effects of carbonation on the microstructure of Portland cement concrete for long-term durability applications. A class C40 concrete (characteristic compression strength between 40 MPa and 44 MPa on the 28th day, according to Brazilian standard NBR 8953) was chosen for the experimental study of the carbonation effects, from which test samples were molded for accelerated test under a 100%-CO2 atmosphere after physical and mechanical characterization. It was observed that carbonation provoked a reduction of 5% to 12% of the concrete open porosity accessible to water. Flexural strength values obtained after the carbonation tests revealed a decrease of 12% and 25% in relation to the values obtained before tests on the 28th and 91st days, respectively.

Resumo em Inglês:

The results of an experimental investigation on the microcracking of high-performance concrete subjected to biaxial tension-compression stresses are presented. Short-term static tests and microcracking mapping were performed on 12.5 cm square by 1.25 cm thick plates. Strain controlled tests were executed in a biaxial testing machine constructed at the University of Texas. The primary variables studied were the deformations and the ultimate stress level at each stress ratio as well as the microcracking patterns and total crack lengths. For the microcracking study, the plates, after straining, were impregnated by an epoxy and then examined under a microscope. Microcracks were classified into simple and combined cracks, since this distinction allows for a much better representation of the microcracking process. A simple crack is either a bond or mortar crack where a combined crack contains both of these. For all stress ratios tested, the stress-strain behavior was directly related to the internal microcracking pattern. In all cases, the failure was directly related to the formation and propagation of the combined cracks.

Resumo em Inglês:

In this work we analyzed the "residual" performance of Portland cement concretes heat-treated at 600 °C after cooling down to room temperature. Concretes with characteristic compressive strength at 28 days of 45 MPa and of 60 MPa were studied. The heat-treatment was carried out without any imposed load. We measured the residual compressive strength and modulus of elasticity. The geometry of the structure was described by mercury intrusion porosimetry and nitrogen sorption tests. We observed a decrease of residual compressive strength and modulus of elasticity, with the raise of heat-treatment temperature, as a result of heat-induced material degradation. The results also indicated that the microstructural damage increased steadily with increasing temperature. Based on the results of this experimental work we concluded that residual mechanical properties of concrete are dependent of their original non heat-treated values.

Resumo em Inglês:

In this paper, a finite element model including both material heterogeneity and size effects is presented. The concrete is considered as a statistical combination of constituent phase with different properties (aggregate, mortar and interface material). The material point response is based on a combination of the random occurrence of the solid phases in the structural volume as well as on the differences of structural response due to the size effect. Such combination allows for higher or lower heterogeneity corresponding to smaller or larger structural size. Simulations of the material heterogeneity and associated size effect in a computationally efficient and simple manner show good qualitative agreement with available experimental results for the three-point bending and Brazilian split tests.

Resumo em Inglês:

Aluminophosphate sieves with AFI structure substituted by Ti (denominated TAPO-5) have been synthesized hydrothermally. These materials were characterized by X-ray diffraction (XRD), chemical analysis (ICP), scanning electronic microscopy (SEM), ultraviolet diffuse reflectance spectroscopy (DRS-UV) and thermogravimetric analysis (TGA). XRD results showed the materials have good TAPO-5 crystallinity, although DRS-UV spectra indicated anatase phase as contamination. TGA analysis showed mass loss in the range of high temperatures, which can be attributed to protonated template decomposition. This indicates the existence of structural charge as a consequence of Ti incorporation into AFI structure

Resumo em Inglês:

Cu and Fe species formed during the preparation of Cu/ and Fe/ZSM-5 catalysts by ion exchange were studied. XRD, SEM, H2-TPR, DRS-UV-VIS, EPR, Mössbauer Spectroscopy (MÖSS) and chemical analysis (AAS) were used to sample characterization. Cu/ZSM-5 catalysts, irrespective of their Si/Al ratio and Cu content, showed a reduction peak at around 210°C, which was attributed to the reduction of Cu+2 to Cu+1. The reduction peak of Cu+1 to Cu0 shifted to higher temperatures with the increase of Si/Al ratio or with the diminution of Cu/Al ratio, evidencing that isolated Cu cations present a higher interaction with the zeolite structure. The MÖSS data showed the presence of Fe+3 species in charge-compensation sites and a higher content of hematite (Fe2O3) in the catalysts prepared in aqueous medium. The EPR analysis also evidenced the Cu+2 and Fe+3 presence in Cu and Fe/ZSM-5 catalysts, respectively.

Resumo em Inglês:

Mixed metal oxides with perovskite-type structure show a great potential to be used in catalysis, electrocatalysis and electronic ceramics. Perovskites oxides catalysts with the composition LaNiO3, LaNi(1-x)Fe xO3 and LaNi(1-x)Co xO3 (x = 0.4 and 0.7) have been synthesized by the precipitation method to be used in the methane reforming to produce hydrogen and synthesis gas. The compounds were characterized by X-ray diffraction, thermogravimetric and differential thermal analysis, inductively coupled plasma atomic emission spectroscopy, surface area measurements, energy dispersive X-ray spectrometry coupled to scanning electron microscopy and temperature programmed reduction. The results showed that a suitable combination of the preparation method with calcination variables (time and temperature) could result in oxides with the desired structure and with important properties at the application point of view in heterogeneous catalysis.

Resumo em Inglês:

An Al-5SiO2 (5 wt% of SiO2) aluminium matrix fiber composite was produced where the reinforcement consists of fossil silica fibers needles. After being heat-treated at 600 °C, the original fiber morphology was retained but its microstructure changed from solid silica to an interconnected (Al-Si)/Al2O3 interlaced structure named co-continuous composite. A technique of powder metallurgy, using commercial aluminium powder and the silica fibers as starting materials, followed by hot extrusion, was used to produce the composite. The co-continuous microstructure was obtained partially or totally on the fibers as a result of the reaction, which occurs during the heat treatment, first by solid diffusion and finally by the liquid Al-Si in local equilibrium, formed with the silicon released by reaction. The internal structure of the fibers was characterized using field emission electron microscope (FEG-SEM) and optical microscopy on polished and fractured samples.

Resumo em Inglês:

The attainment of finer ferrite grain structure in low carbon is object of interest because significantly higher yield strengths and lower ductile-brittle transition temperatures can be predicted at the same time with ultra-fine grain sizes. This work verified the microstructural evolution of a low carbon 0.15%C-1.39%Mn steel after an ice brine quenching from 1200 °C followed by warm rolling and intercritical annealing at 800 °C at different times. To compare the final microstructure, a first group of specimens were heated at temperatures in the range from 660 °C to 800 °C during 30 min and quenching in ice brine. After quenching from 1200 °C, a second group of specimens were laboratory warm rolled at 700 °C and annealed at 800 °C, for 1, 60, 120 and 180 min, following air cooling or quenching. The final microstructure of all specimens was analyzed by quantitative metallography using optical and scanning electron microscopy. The initial steel grain size condition was 120 µm. The specimens, after the whole processing cycle achieved a microstructure with ferrite grain size between 1 µm and 1.5 µm. The ferrite grain size values changed until 50% for samples warm rolled and heat treated between the first and last annealing time.

Resumo em Inglês:

Thermogravimetric measurements in H2/H2S environments for up to five hours were carried out to determine the sulphidizing behavior of Fe-20Cr and Fe-20Cr-0.7Y alloys. These measurements helped to determine the mechanism of sulphidization in the early stages. The reaction products were characterized using a scanning electron microscope coupled to an energy dispersive analyzer and by X-ray diffraction analysis. The addition of yttrium influenced the morphology of the sulphide layer and the reaction kinetics.

Resumo em Inglês:

A nuclear reactor pressure vessel steel was submitted to different annealing heat treatments aimed at simulating neutron irradiation damage. The obtained microstructures were mechanically tested with subsequent metallographic and fractographic characterization. The relevant microstructural and fractographic aspects were employed in the interpretation of the mechanical behavior of the microstructures in both quasi-static (J-R curve) and dynamic (Charpy impact) loading regimes. A well defined relationship was determined between the elastic-plastic fracture toughness parameter J-integral and the Charpy impact energy for very most of the microstructures.

Resumo em Inglês:

A nuclear reactor pressure vessel steel was submitted to different quenching and tempering heat treatments aimed at simulating neutron irradiation damage. The obtained microstructures were mechanically tested and submitted to metallographic and fractographic survey. The relevant microstructural and fractographic aspects were employed in the interpretation of the mechanical performance of the thermally embrittled microstructures. A well defined correlation was determined between the elastic-plastic fracture toughness parameter J-integral and the Charpy impact energy, which was achieved for some of the Q&T microstructures.

Resumo em Inglês:

To study the effect of the annealing temperature on the structure and magnetic properties of a 2%Si non-oriented steel cold rolled samples were submitted to final annealing in the temperature range of 540 °C to 980 °C in hydrogen atmosphere. The samples had received cold rolling reduction of 75% to a final thickness of 0.50 mm. Recovery and recrystallization resulted in significant improvement of magnetic properties, with decrease of iron loss (W1.5) and increase of polarisation (J50) and relative permeability (µ1.5). On further grain growth, after recrystallization, there was simultaneous decrease of iron loss, polarisation and relative permeability. Texture evolution on grain growth accounts for the observed decrease of J50 and µ1.5. The beneficial effect of increasing grain size on core loss overcomes the detrimental effect of texture resulting in decrease of W1.5.

Resumo em Inglês:

The continuing development of stainless steels has resulted in complex steel compositions with substantial amounts of alloying elements. The benefits of such additions invariably come attached to unavoidable disadvantages. One of the most critical item is the potential microstructural instability of the material. Alloying elements may be in a supersaturated solid solution, in which the precipitation of carbides, nitrides, borides and intermetallic phases occurs in a wide range of temperatures. In order to dissolve the mentioned precipitates, solution annealing is commonly performed. However, at the temperature range in which this treatment is carried out, the onset of abnormal grain growth can occur. The interaction between the dissolution of these second-phase particles and the occurrence of abnormal grain growth is investigated in this work. This study also shows that the thermodynamics and the kinetics of dissolution of precipitates may be used to predict whether abnormal grain growth takes place.

Resumo em Inglês:

Duplex stainless steels, with ferritic-austenitic microstructure, have excellent mechanical properties and corrosion resistance. However, when duplex stainless steels are exposed to temperatures between 600 and 1000 °C, some phase transformations can occur such as chromium nitrides precipitation, chromium carbides precipitation and the sigma phase formation. The formation of such compounds leads to loss in both corrosion resistance and fracture toughness. The negative effects of the formation of chromium nitrides, carbides and the sigma phase are due to the chromium depletion in the matrix. The phase transformations cited above occur initially at ferritic-austenitic interfaces and at the grain boundaries. The aim of this work is to identify and characterize the phase transformations, which occur when aging heat treatments are carried out at temperatures at which the kinetics is the fastest for the reactions mentioned. At first, the samples were annealed at 1100 °C for 40 min. The aging heat treatments were then carried out at 850 °C for 6, 40 e 600 min. Microstructural characterization was done by using optical microscopy with different etchings, in order to identify each phase formed in the duplex stainless steel during aging heat treatments. The toughness was also evaluated by using Charpy impact test. Impact tests show that loss of toughness was related to phase transformations.

Resumo em Inglês:

Structural parameters as grain size, dendritic and cellular spacings, segregated products, porosity and other phases are strongly influenced by the thermal behavior of the metal/mold system during solidification, imposing a close correlation between this and the resulting microstructure. Several unidirectional solidification studies with the objective of characterizing cellular and dendritic spacings have been developed in large scale involving solidification in steady-state heat flow. The main objective of this work is to determine the thermal solidification parameters during the cellular/dendritic transition as well as to compare theoretical models that predict cellular and primary dendritic spacings with experimental results for solidification situations in unsteady-state heat flow. Experiments were carried out in a water cooled unidirectional solidification apparatus and dilute alloys of the Sn-Pb system were used (Sn 1.5wt%Pb, Sn 2.5wt%Pb and Sn 5wt%Pb). The upper limit of the Hunt-Lu cellular growth model closely matched the experimental spacings. The lower limit calculated with the Hunt-Lu dendritic model best generated the experimental results. The cellular/dendritic transition was observed to occur for the Sn 2.5wt%Pb alloy over a range of analytical cooling rates from 0.28 K/s to 1.8 K/s.