Repositório RCAAP

Three high manganese TWIP steels were produced with stacking fault energies γSFE ranging from 20.5 to 42 mJ/m². The materials were mechanically tested in tension at temperatures and strain rates varying in the ranges of -50°C...80°C and 10-3 s-1...1250 s-1, respectively. Due to the temperature dependence of γSFE, also the mechanical behavior of TWIP steels reveals clear temperature dependence, determined by the prevailing deformation mechanism, i.e., dislocation slip, deformation twinning, or ε-martensite transformation. In addition to the 'ordinary' strain rate sensitivity, an increase in temperature due to adiabatic deformation heating contributes to the stacking fault energy (SFE) at high strain rates, shifting γSFE towards the dislocation slip regime and this way strongly affecting also the mechanical behavior. At stacking fault energies close to the transition between twinning and ε-martensite transformation, lowering the temperature can ultimately result in entering the ε-martensite transformation regime that may bring about further ductility.

The exceptional tensile strength of ramie fiber has motivated investigations on its application as reinforcement in polymeric composites. In this study the temperature variation of the dynamic-mechanical parameters of epoxy matrix composites incorporated with up to 30% in volume of ramie fiber were investigated by DMA tests. The parameters were the storage modulus, loss modulus and tangent delta. The investigation was conducted in the temperature from 20 to 200°C in an equipment operating in its flexural mode at 1 Hz under nitrogen. The results showed that the incorporation of ramie fiber tends to increase the viscoelastic stiffness of the epoxy matrix. It was also observed sensible changes in the structure damping capacity when the fraction of fiber is increased in the composite. These results indicate that the segmental mobility of the epoxy chains is affected by interaction with ramie fibers in the composite.

Digital Microscopy was employed to characterize the microstructure of fiber-reinforced composite tubes manufactured by filament winding. Optical Microscopy was used for void characterization while Scanning Electron Microscopy was used for fiber and layer analysis. Acquired images were assembled in mosaics to reveal the microstructure of different cross-sections of the sample. Image processing was employed to detect either voids or individual fibers and measure their size, shape and spatial distribution. Void spatial distribution was analyzed with two different methods - local analysis and the tessellation method - revealing different behaviors along different cross-sections. Fiber layers were automatically detected and their average winding angle and dispersion were analyzed.

Porous materials featuring cellular structures are known to have many interesting combinations of physical and mechanical properties. Some of them have been extensively used in the transportation field (i.e. balsa wood). Steel foams presented promising theoretical properties for both functional and structural applications in transportation, but processing of such a kind of foams is complex due to their high melting point. Recently a technique for processing Cu-based alloys open-cell foams through the molten metal infiltration of a leachable bed of amorphous SiO2 particles was proposed. A variation of the proposed technique that uses SiC particles as space holder is now presented and was recently successfully applied for dual phase steel foam processing. Results from a processing of dual phase DP500 steel foams, including some morphological, micro-structural and mechanical characterization, are here presented.

The fibers extracted from the piassava palm tree, scientifically known as Attalea funifera, are among the stiffest lignocellulosic fibers being considered for polymer composite reinforcement. Characterization of piassava composites have been carried out for different polymeric matrices and mechanical tests. In this work the tensile properties of DGEBA/TETA epoxy matrix composites reinforced with up to 30% in volume of continuous and aligned piassava fibers were evaluated. Tensile specimens post-cured at 60ºC for 4 hours were room temperature tested and the corresponding fracture analyzed by scanning electrons microscopy. The results showed a decrease in both the tensile strength and the elastic modulus of the composites up to 30% with an increase at 40% of piassava fibers to values above those of the pure epoxy. The fracture analysis revealed a weak fiber/matrix interface, which could account for the comparative low performance of these composite in tensile tests up to 30% of volume fraction. The relatively large amount of stronger piassava fibers accounts for the better performance of the composite with 40% in volume fraction.

The current interest for natural fibers as an environmentally correct composite reinforcement has motivated the investigation of new possibilities. For instance, the fibers extracted from the petiole of the buriti palm tree were recently found to have adequate mechanical properties to reinforce polymer composites. Therefore, the present work evaluates the tensile properties of polyester composites incorporated with thinner buriti petiole fibers for improved mechanical performance. Composites with up to 40% in volume of buriti petiole fibers embedded in orthophtalic polyester matrix were post-cured and then ruptured in tension. Fracture surfaces were analyzed by scanning electron microscopy. A marked increase in the tensile strength was found with the amount of buriti fibers. The fracture analysis revealed aspects of the bonding condition at the fiber/matrix interface, which could be associated with the composite performance.

The fiber extracted from the husk of a coconut fruit, known as coir fiber, has been extensively investigated as a second phase incorporation into polymer composites. The moderate strength of the coir fiber usually does not represent reinforcement to relatively strong thermoset matrices such as polyester, epoxy and phenolic. However, a selection of thinner coir fibers and a post cure treatment of the composite could improve its mechanical performance. Therefore, this work investigated the tensile properties of post-cured polyester matrix composites incorporated with the thinnest coir fiber. Tensile specimens with up to 40% in volume of long and aligned coir fibers were tested and their fracture analyzed by scanning electron microscopy. A relatively improvement was found in the tensile properties for the amount of 40% of coir fiber. These results were compared with similar composites that were bend-tested. The fracture analysis showed a comparatively better fiber/matrix adhesion.

In this work the theoretical solutions based upon the upper-bound theorem recently proposed by Pérez and Luri [Mech. Mater. 40 (2008) 617] for the equal channel angular extrusion process (ECAE) are analyzed by performing a 25 central composite factorial analysis. The uniaxial mechanical properties of commercial pure aluminium are considered by assuming isotropic nonlinear work-hardening combined to von Mises and Drucker isotropic yield criteria to predict the ECAE load and the effective plastic strain. From the proposed 25 factorial analysis, the main parameters affecting the ECAE pressure may be ranked as: (1) Friction factor, (2) die channels intersection angle, (3) outer and (4) inner die corners fillet radii and lastly, (5) plunger velocity. Alternatively, the effective plastic strain is mainly controlled by the die channels intersection angle and, in a less extent, by the outer and inner die corners fillet radii.

The aim of the present work was to establish the relationship between dynamic behavior of Bisphenol A polycarbonate (BAPC) and degradation by gamma irradiation. The BAPC was exposed to 340 kGy dose and the molecular weight was evaluated by Size Exclusion Chromatography (SEC). A modified split Hopkinson pressure bar was used to measure stress-strain dynamic relations. The results showed little change in the dynamic behavior of irradiated BAPC in the highest strain rate used in this work, in spite of the high decrease in the molecular weight of irradiated BAPC. The lowest strain rate used in this work produced the highest change in the dynamic behavior of irradiated BAPC.

The corrosion characteristics and mechanical behavior of Al/SiCp/spinel composites prepared by reactive infiltration with fly-ash (FA) and rice-hull ash (RHA) -both with recycled aluminum- were investigated. MgAl2O4 is formed in situ during infiltration in the temperature range 1050-1150 °C for 50-70 min in argon atmosphere at a pressure slightly above to that of the atmospheric pressure. Results reveal that both FA and RHA help in preventing SiCp dissolution and the subsequent formation of the unwanted Al4C3. However, FA-composites are susceptible to corrosion via formation of Al4C3 by the interaction of native carbon in FA with liquid aluminum. The foremost corrosion mechanism in both types of composites is attributed to microgalvanic coupling between the intermetallic Mg2Si and the matrix. Microstructure and mechanical characterization show that FA- and RHA-composites possess mechanical properties that are significantly different and that this behavior is due to the original ash and MgAl2O4 morphologies. While RHA composites exhibit higher surface hardness than FA composites, the latter display a higher modulus of rupture.

The effect of particle size distribution of SiC particulate reinforcements coated with colloidal SiO2 on Young´s modulus of Al/SiCp/MgAl2O4 composites fabricated by reactive infiltration was investigated. Composites were prepared from porous preforms of silica-coated α- SiC powders of 10, 54, 86, and 146 μm, 0.6 volume fraction of reinforcements and particle size distribution from monomodal to cuatrimodal. Infiltration tests with the alloy Al-13.3Mg-1.8Si (wt. %) were carried out in Ar→N2 atmosphere at 1100ºC for 60 min. The composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition to density and residual porosity measurements, Young´s modulus was evaluated by ultrasonic techniques. Results show that with increase in particles size distribution, residual porosity decreases and density and Young´s modulus of the composites are improved, the latter from 185.39 ±3.6 to 201.93 ±2.3 GPa. This is attributed to the increased metal-ceramic interfaces and to an enhanced matrix-reinforcement load transmission.

The present work deals with grain refinement of medium carbon steel, having different initial microstructure, modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The microstructure development was analyzed in dependence of effective strain introduced (εef ~2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The cementite lamellae spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

In this work, thinky mixing method was used to disperse multi-walled carbon nanotubes (MWCNT's) and carbon nanofibers (CNF's) in SC-1 epoxy either in isolation or in combination with 3-roll shear mixing. To achieve better dispersion, MWCNT mixing with SC-1 resin directly or pre-mixed with a solvent and then mixed with SC-1 resin after evaporating the solvent. Dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), flexural tests, electrical conductivity tests and micrographic analysis were performed on neat, 0.2 and 0.4wt% MWCNT/CNF infused SC-1 epoxy to observe the loading effect on thermo-mechanical properties of composites. DMA results indicated improvement on storage modulus and glass transition temperature, Tg, while flexural results exhibited enhanced flexural strength and modulus with up to 0.4wt% MWCNT/CNF infused epoxy resin over neat. TGA results revealed improved residue content but almost constant decomposition temperature for nanophased resin compared to neat. However, these enhancements were observed only up to 0.2 wt. % loading after which the properties were seen to either reduce or not significantly improve. These results indicate that the methods used for dispersion is suitable for low weight percent loading only.

Bars of Titanium Grade 2 were subjected to deformation by Equal Channel Angular Pressing, both at room temperature and at 300°C. Additionally, some specimens were cold rolled up to 70% reduction. From tensile tests, data such as yield, maximum strength, elongation and area reduction were obtained. Results show that the best strength - ductility combination is produced by four passes followed by cold rolling. Finally, the corrosion behavior was assessed following ASTM F2129 and no noticeable difference between the starting material and the ECAP-deformed was detected.

This paper presents a study on the compressive behavior of steel fiber-reinforced concrete. In this study, an analytical model for stress-strain curve for steel fiber-reinforced concrete is derived for concretes with strengths of 40 MPa and 60 MPa at the age of 28 days. Those concretes were reinforced with steel fibers with hooked ends 35 mm long and with aspect ratio of 65. The analytical model was compared with some experimental stress-strain curves and with some models reported in technical literature. Also, the accuracy of the proposed stress-strain curve was evaluated by comparison of the area under stress-strain curve. The results showed good agreement between analytical and experimental data and the benefits of the using of fibers in the compressive behavior of concrete.

Dynamic behavior of hydrogen desorption from pure iron with a body-centered-cubic lattice and Inconel 625 with a face-centered-cubic lattice was examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates and cyclic stresses. Hydrogen desorption rarely increased under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, reached its maximum, and then decreased gradually with increasing applied strain for both pure iron and Inconel 625. This desorption behavior is closely related to hydrogen dragging by moving dislocations. The thermal desorption analysis results showed that the amount of desorbed hydrogen differed at each strain rate. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.

The main purpose of this paper is the thermodynamic study of non-metallic inclusion formation in the CC tundish for SAE 1141 steel. The specific purposes are: 1) obtaining inclusions as function of steel composition and casting temperature. 2) establishing steel chemical composition to form less harmful inclusions to the SAE 1141 steel castability. Simulations using the commercial software FactSage and databases were carried out. Results showed both different solid oxides and liquid phase formation in inclusions by varying calcium content in the steel. Thus, it was possible: 1) to determine both the inclusion composition as a function of aluminum and calcium content of SAE 1141 steel. 2) to establish a range of calcium content in which inclusions are formed predominantly by liquid phase. 3) to calculate percentage of liquid and solid phases in inclusions, and oxides composition as well.

The issue of adiabatic shearing is discussed in this work and a new interpretation is given to some failure phenomena which are usually termed as adiabatic shears. We propose that only a few materials undergo a truly inherent failure which is due to thermal softening at the shearing zone and that the interplay between microvoids, cracks and narrow shear bands should be taken into account through the temperature rise at the front of advancing cracks. Also, the size of the plastic zone ahead of a crack plays an important role in determining the brittleness of a given specimen and should be taken into account when specimens of different sizes are tested. Experimental results for several alloys in the Kolsky bar system support our approach.

This work aimed to study the microstructural evolution of commercial aluminum alloy AA7050 in the solution treated condition (W) processed by equal channel angular pressing - ECAP. The analyses were made considering the effects of process parameters as temperature (Tamb and 150°C), processing route (A and B C) and number of passes. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for microstructural characterization, and hardness tests for a preliminary assessment of mechanical properties. The results show that the refining of the microstructure by ECAP occurred by the formation of deformation bands, with the formation of dislocations cells and subgrains within these bands. The increase of the ECAP temperature led to the formation of more defined subgrains contours and intense precipitation of η phase in the form of spherical particles. The samples processed by Route B C present a more refined microstructure.

Titanium nitride films were formed on the surface of Ti-6Al-4V discs by plasma nitriding (glow discharge) in different N2:H2 atmospheres at several substrate temperatures. In this study the influence of the process parameters on dynamic micro-hardness were investigated. Grain sizes of the nitride films, determined with X-Ray Diffraction, were related to the nitriding parameters. TiNx stoichiometry was determined with Nuclear Reaction Analysis and showed a correlation to substrate temperature during the nitriding process. Micro-hardness measurements were taken on the nitrided surfaces. Grain sizes increased for a particular gas composition of 60%N2+40%H2 where hardness was lowest.