Resumo em Inglês:

In this study, the hybrid corona-dielectric barrier discharge plasma treatment was employed to modify the physical, chemical and morphological characteristics of a half-knitted fabric composed of 92% polyamide 6.6 and 8% elastane (PA). These properties of the fabric were evaluated by the water contact angle, x-ray diffraction, infrared spectroscopy, scanning electron microscopy and atomic force microscopy techniques. In addition, the dyeing and washing processes were also investigated. A significant reduction of the contact angle was observed for plasma-treated PA. Infrared spectroscopy analyses indicated that C-H, N-H, and N-O groups in PA increased after plasma treatment, explaining the improved coloring strength for the plasma-treated samples when dyed with reactive and acid dyes. A better fixation of dye was also observed after the atmospheric plasma treatment. Furthermore, dyeing with a basic and acid dye caused the dyeability increases for the plasma-treated sample compared with the untreated sample.

Resumo em Inglês:

Abstract TiO2 immobilized in Sepiolite (TiO2/Sep) was successfully prepared by the sol-gel technique, with titanium isopropoxide as the precursor for the formation of TiO2 in the anatase phase calcined at 400 °C. The prepared samples were characterized by X-ray diffraction, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy coupled to energy dispersive spectroscopy, and thermogravimetric analysis. The results showed that TiO2/Sep structure was identified in all characterizations, showing the specific peaks, bands, mass loss, and morphology after the impregnation process. Photocatalytic experiments were performed under UV irradiation with various photocatalyst concentrations and pH effects in the reaction. The prepared samples presented 72% photocatalytic efficiency for eosin (EA) dye discoloration after 150 min under UV light. This efficiency was attributed to the radicals generated from the TiO2 and the high specific surface area, showing that TiO2/Sep is promising candidate in the degradation of organic pollutants.

Resumo em Inglês:

Abstract In this work, inorganic pigments were synthesized by solid state reaction, from the incorporation of components of exhausted alkaline batteries. For this, after disassembling the batteries, the components were subjected to water leaching, milling and heat treatment at high temperatures with an addition of 33% (in mol) commercial or recovered from own alkaline batteries anodes zinc oxide. The obtained pigments were characterized by X-ray diffractometry, scanning electron microscopy, laser diffraction particle size, and UV-Vis diffuse reflectance spectroscopy. It was concluded that incorporation of the components of batteries generally yielded brown pigments with a structure of the spinel type ZnMn2O4. SEM and particle size distribution analyses show that the addition of ZnO leads to a reduction in average grain size of the powders. The pigments when applied to color ceramic enamels, polymers and paints show good performance in the coloring of these products, with potential for commercial application.

Resumo em Inglês:

Abstract Shape memory alloys are generally produced by casting processes and are subsequently homogenized. However, to obtain semifinished products on an industrial scale, the ingots from the casting process must be hot worked. In particular, final bar and sheet products can be obtained by hot rolling process. During intense hot work, surface oxidation of the material and microstructural changes may cause modifications to its original thermomechanical properties. In this sense, the present work aimed to study the correlation of the superelastic behavior in a Cu-Al-Be-Cr alloy before and after subjecting it to the hot rolling thermomechanical process. Abnormal grain growth was observed for a hot rolled sample with 30% reduction in initial alloy thickness. This abnormal growth in relation to non-rolled alloy caused an increase in phase transformation temperatures, a reduction in residual strain, a reduction in induction stress and an increase in alloy superelasticity.

Resumo em Inglês:

Abstract Alumina (Al2O3) suffers from low fracture toughness and low bending strength which restrict its application in the industry for some advanced components. The incorporation of submicron SiC into Al2O3 matrix improves mechanical properties of the matrix. However, the high cost of SiC has delayed the industrial interest of synthesizing Al2O3-SiC composites. Rice husk, an agricultural waste material, is a potential source of low-cost SiC. Therefore, this study presents a simple approach to synthesizing SiC from locally sourced rice husk and using it to reinforce alumina. Rice husk was pyrolysed in a tube furnace under argon atmosphere at different temperatures (1000 °C – 1500 °C) and reaction times (60 – 120 min). Furthermore, Alumina powder was admixed with 5 – 20 vol% SiC derived from rice husk, and then sintered at temperatures between 1300 °C – 1600 °C by spark plasma sintering. Maximum yield of SiC was obtained from rice husk at 1500 °C and 120 min. Materials with theoretical densities higher than 95% were achieved for the sintered composites. The hardness of sintered composites reached a maximum of 20.2±1.4 GPa, while a maximum of 4.7±.7 MPa.m0.5 was obtained for the fracture toughness.

Resumo em Inglês:

Abstract The effects of the austenitizing temperature and the cooling rate upon the kinetic of athermal martensitic transformation in a microalloyed steel were evaluated. Considering the studied steel, the knowledge about these effects on the martensitic transformation has a great relevance for naval manufacturers and steel researchers. In this study, computational simulation was performed aiming to evaluate the phase’s stability. Specimens were submitted to quenching simulations in a dilatometer, considering four different austenitizing temperatures and four cooling rates. The results shown that the austenite chemical composition was not significantly affected by the austenitizing temperatures. Both the austenitic grain size and the cooling rate affected the martensitic transformation kinetics. The larger the austenitic grain size, the higher the Ms. The austenitic grain growth promoted a decrease in the required chemical energy to compensate the free energy increase associated with the lattice strain and the creation of new interfaces, leading to a lower austenite undercooling. An extrinsic effect of the cooling rate on the Ms was observed. For lower cooling rates, the carbide precipitation modified que austenite chemical composition, changing its stability and increasing Ms. A predictability equation, correlating the MS with the austenite grain size and the steel cooling rate, was proposed.

Resumo em Inglês:

Abstract Pseudoboehmite (AlOOH.xH2O) is one of the precursors of alumina. Its calcination produces different phases of alumina and at 1200º C the most stable phase, α-alumina, is formed. The synthesis of pseudoboehmite by the sol-gel process results in a product with high purity, high specific surface area, and high reproducibility. The aging of the pseudoboehmite at temperatures above room temperature results in pseudoboehmites with distinct properties. In this work we report the synthesis of pseudoboehmite by the sol-gel process, using aluminum nitrate and aluminum chloride as precursors, and investigate the effect of acetate ion on crystallinity, adsorption capacity, particle morphology, thermal properties and specific surface area. The X-ray diffraction data of the synthesized pseudoboehmites in the presence of the acetate ion revealed high quality nanoparticles.

Resumo em Inglês:

Abstract Resistance Seam Welding (RSEW) is commonly used in continuous steel processing for joining steel coils. Experimental studies focused on the welding process quality controls are justified by their wide application and influence on the performance of steelmaking processes. The present study focuses on the continuous annealing line welding process using as sample, a low carbon steel (0.07%C) with 0.21 mm thickness. The work proposes a correlation of thermal input, electric current parameter and thermogram monitoring during RSEW procedures. A methodology was developed to determine the real thermal profiles values by means of association of the thermograms with the sample emissivity curve. Three heat input levels were used, 3.64 J.mm-1, 5.09 J.mm-1, and 6.38 J.mm-1, respectively, for typical C-Mn steel. The thermal welding profile obtained experimentally exhibited linear correlation coefficients (R) of 0.99 with the electric current welding parameter and (R) of 0.99 with the calculated thermal input, respectively. The welds metallurgical characterizations results, expressed through the ductility and welds nuggets analysis, were conducted to correlate the welds quality with the welding thermal input. The Erichsen cupping test showed that the higher the thermal input corresponds to higher ductility of the welds. Nevertheless, the heat input of 5.09 J.mm-1 indicated better-balanced properties: ductility, weld nugget discontinuity, nugget size, and no expulsion of the melt material.

Resumo em Inglês:

Abstract ZnAl2O4 spinel nanoparticles and CeO2/ZnO/ZnAl2O4 ternary nanocomposites were synthesized by a co-precipitation method. The structural, morphological, optical properties and chemical compositions of the products were analyzed respectively by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS) and X-ray fluorescence (XRF) spectroscopy. The optical band gap of ZnAl2O4 spinel nanoparticles was 3.220 eV. When 1.0 mmol Ce(NO3)3•6H2O was added to the synthesis reaction, the optical band gap of the obtained ternary nanocomposite was 3.170 eV. The influence of phase composition, optical band gap, oxygen vacancy and specific surface area on photocatalytic activity over CeO2/ZnO/ZnAl2O4 ternary nanocomposites was investigated. The CeO2/ZnO/ZnAl2O4 nanocomposite prepared with 1.0 mmol Ce(NO3)3•6H2O showed the lowest recombination rate of photoexcited electron-hole pairs, the narrowest optical band gap (3.170 eV) and the highest oxygen vacancy concentration or highest Urbatch energy (0.299 eV). These parameters produced the best photocatalytic activity toward methylene blue (MB) under UV irradiation. The CeO2/ZnO/ZnAl2O4 ternary nanocomposites exhibited better photocatalytic performance than pure ZnAl2O4 spinel nanoparticles and 100% degradation of aqueous MB solution was achieved within 60 min when using the CeO2/ZnO/ZnAl2O4 ternary nanocomposite photocatalyst synthesized with 1.0 mmol Ce(NO3)3•6H2O.

Resumo em Inglês:

Composites based on graphene oxide (GO), reduced graphene oxide (RGO) and zinc oxide (ZnO) with different mass ratios (8, 100 and 600) were synthesized through the hydrothermal method at 100 °C and used as reinforcement materials to commercial glass-ionomers cements (GIC). X-ray diffraction (XRD) characterization confirmed the graphite oxidation and ZnO formation as a wurtzite phase. Infrared spectroscopy analyses showed bands of oxygen-containing groups on the GO surface, which reduced after thermal treatment and RGO formation. D and G bands were observed in all samples synthesized, which presented morphology similar to flowers with a crystallite size of 18 nm. The effect on the mechanical properties of GIC after reinforcement with 0.1 wt% of RGO and 3 wt% of the composites was evaluated using a one-way analysis of variance (ANOVA). It was verified slight improvements in the hardness of GIC.

Resumo em Inglês:

Low-cost ceramic proppants were successfully prepared from natural bauxite and solid waste coal gangue via CaCO3 additive. 40 wt% of bauxite in raw materials was replaced by coal gangue, which significantly reduced the manufacturing costs. The apparent density, bulk density, acid solubility and breakage ratio of the proppant sintered at different temperatures were systematically investigated. The phase composition and morphological structure were determined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the amount of liquid phase affected the solid phase reaction velocity by changing sintering mechanism. When the sintering temperature was 1350 °C, the optimum size of the mullite crystal particles and the optimum amount of the liquid phase were observed and the samples exhibited the best performance.

Resumo em Inglês:

Abstract The motivation of this work is to show that the structural model, which was initially used to estimate the thermodynamic properties of binary silicate systems, can be also used to estimate the viscosity of binary and ternary silicate melts in terms of temperature and composition. The model links the viscosity to the internal structure of melts through the concentration of the oxygen bridges present in the slag. A previously proposed structural thermodynamic model was used to calculate the content of oxygen bridges. The viscosity model requires only three parameters to obtain a good agreement between experimental and calculated data for the SiO2−FeO binary system and for the SiO2−CaO−FeO, SiO2−MgO−FeO and SiO2−MnO−FeO ternary systems. The viscosity of ternary systems was calculated with the model while assuming a linear function of the parameters from binary systems; however, the content of the oxygen bridges was calculated using the thermodynamic model for ternary systems.

Resumo em Inglês:

Abstract Ti/Al/Mg/Al/Ti laminated composites were fabricated via hot-pressing at 350 °C, 400 °C and 450 °C successfully. The influences of interface morphology, diffusion zone, constraint effect on the mechanical properties were investigated. At Ti/Al interface, intermetallic compounds aren’t found. Whereas, they form at Al/Mg interface. With the increasing temperature, the bonding strength of Al/Mg interface doesn’t change linearly, and the maximum strength is obtained at 400 °C because of intermetallic compounds with appropriate thickness. With the increase of temperature, the hardness at both Ti/Al and Al/Mg interfaces increases owing to the solid solution and the intermetallic phases. Also, the ultimate tensile strength of LMCs increases with sacrificing the fracture elongation. The rule of mixture is used to predict the theoretical strength. It is found that the theoretical values are less than the measured, and the reasons may relate to interfaces in Ti/Al/Mg/Al/Ti laminated composites.

Resumo em Inglês:

Abstract Diecastings of the A356 aluminum alloy were produced by rheo-diecasting (RDC) and High pressure die casting (HPDC), the microstructures of primary solidification, secondary solidification and eutectic Si of diecastings with different pouring temperature were explored and the mechanical properties of different parameters were tested. The result shows that the primary α-Al grains in RDC with self-inoculation method (SIM) are smaller and rounder than the dendrite structure in HPDC. During the RDC process, the amount of primary α-Al grains, average grain size and the lamellar spacing of the eutectic Si increase with the decrease of the melt treatment temperature. While the average grain size and the shape factor are gradually increasing with the increase of melt treatment temperature. As a result, RDC can significantly improve the mechanical properties of the A356 aluminum alloy compared with HPDC, the mechanical properties are optimal at 600°Cwith the tensile strength and elongation are 268.67MPa and 6.8%, respectively.

Resumo em Inglês:

Abstract In this study, arc stud welding process was employed for welding AISI 316 stainless steel studs to AISI 1060 high carbon steel plates. A disc of Ni powder prepared and used as a buffering layer to enhance the properties of welding area. Optical and scanning electron microscopy were used to examine the microstructure. Energy-Dispersive X-ray (EDX) and X-Ray Diffraction (XRD) tests were performed to analyse and identify elements and phases, respectively in the weld region. The results observed that Ni powder prevented the direct contact between the dissimilar base metals. Existing of Ni altered the microstructure of the weld zone and encouraged dendritic type over cellular. Hardness reduced in the weld region from 600 HV to 200 HV due to the effect of Ni powder which prevented the formation of brittle Fe-Cr phase.

Resumo em Inglês:

Abstract Silicon-rich silicon oxide (SRSO) film was prepared through the deposition of SiOx film by radio-frequency reactive magnetron sputtering from a Si target and the subsequent rapid thermal annealing treatment of SiOx film, and planar-type field emission cathodes based on SRSO film were fabricated. The surface morphologies and electron emission properties of the SRSO films deposited under different O2/Ar flow ratios were investigated. The experimental results show that a relatively high O2/Ar flow ratio during the deposition of SiOx film can lead to the formation of cluster structure in the SRSO film prepared, and the electron emission efficiency of SRSO film rises with the increase of electric field applied. An SRSO-film cathode fabricated under an O2/Ar flow ratio of 1:1 with a chamber pressure of 0.21 Pa has an emission current density of 65.61 μA/cm2 and a corresponding emission efficiency of 0.53% at a bias voltage of 18 V, and it exhibits relatively stable emission and fine emission uniformity.

Resumo em Inglês:

Abstract In this study, Mg-12Zn magnesium alloys alloyed with Al additions (0, 2, 4, 6, 8 and 10, wt.%) were fabricated by permanent mould casting. The Al content on their microstructure and mechanical properties were systematically examined with an optical microscope (OM), a scanning electron microscope (SEM), an X-ray diffractometer (XRD) and mechanical tests at room temperature. The experimental results indicate that the microstructure of the alloys is mainly composed of α-Mg and semi-continuous or continuous eutectic phases. A higher addition of Al (≥6%) causes the generation of the Mg17Al12 phases. Notably, the grain sizes of the alloys gradually decrease, whilst the partial morphology of some eutectic phases is modified into lamellar structure with increasing of Al addition. Mechanical properties characterization manifested that, the alloys with different Al additions reveal distinguishing tensile properties. Among them, the alloy with 4% Al provides an excellent mechanical properties, i.e., a UTS of 206 MPa and an EL of 7.92%, which is respectively higher 28 MPa and 1.08% than that of ZA120 alloy. The deterioration in the tensile properties for the higher Al-bearing alloys is possibly related to the lamellar structure, coarse and continuous net-work morphology and β-Mg17Al12 phases, respectively.

Resumo em Inglês:

Abstract The compressive and tensile behaviors in a Ti nanopillar with a biphasic hexagonal close-packed (HCP) /face-centered cubic (FCC) phase boundary are theoretically researched using classic molecular dynamic simulation. The results indicate that the HCP/FCC interface and free surface of the nanopillar are the sources of dislocation nucleation. The plastic deformation is mainly concentrated in the metastable FCC phase of the biphasic nanopillar. Under compressive loading, a reverse phase transformation of FCC to the HCP phase is induced by the dislocation glide of multiple Shockley partial dislocations12<1¯21>under compressive loading. However, for tensile loading a large number of Lomer-Cottrell sessile dislocations and stacking fault nets are formed when the partial dislocations react, which leads to an increase in stress. The formation mechanism of a Lomer-Cottrell sessile dislocation is also studied in detail. Shockley partial dislocations are the dominant mode of plastic deformation behaviors in the metastable FCC phase of the biphasic nanopillar.

Resumo em Inglês:

Abstract The effects of deformation on transformation kinetics during continuous cooling and microstructure evolution of a medium carbon high strength steel were investigated by metallographic method, dilatometry and X-ray diffraction etc. The results show that ausforming accelerated the ferrite and martensite transformation, but significantly retarded the bainite transformation. In addition, the amount of retained austenite decreased first and then increased, while the carbon content in retained austenite increased with the accumulation of strain. The change trend of the average dislocation density in martensite with strain showed the opposite trend as that of the amount of retained austenite. Moreover, the martensite laths were refined by ausforming. The work provides the theoretical reference for the design of parameters of thermalmechanical controlled process (TMCP) in the production of medium-carbon high strength steels.

Resumo em Inglês:

Abstract The silicon solar cells achieved relatively low prices in the last years and to introduce a new structure in the PV industry, the amount of silicon per watt has to be reduced, requiring a cost-effective manufacturing process. The use of n-type solar grade silicon has the advantages of presenting higher minority carrier lifetime than p-type one and the absence of the boron-oxygen defects. The aim of this paper is to present the development of 100 μm thick n+np+ silicon solar cells with a selective p+ rear emitter formed by boron deposited by spin-on and an Al/Ag grid deposited by screen-printing. The firing temperature of Ag/Al (rear face) e Ag (front face) was optimized and the temperature of 840 °C produced the devices with higher efficiency. The solar cells presented efficiencies of 16%, achieving a low silicon consumption of 1.6 g/W, 40% lower than thick p-type devices produced by the same process.

Resumo em Inglês:

Abstract In this paper, high pressure heat treatment for Cu-Al alloy is carried out at the pressure of 5GPa and the heating temperature of 700°C for 15 minutes. The phase transformation temperature and time of α+γ2→ß in Cu-Al alloy are measured before and after 5GPa pressure treatment with the help of differential scanning calorimeter (DSC). The activation energy and Avrami exponent for phase transformation are also calculated. Based on the measurement, calculation and observation of the alloy’s microstructure, the effect of 5GPa pressure treatment on the phase transformation of α+γ2→ß in Cu-Al alloy is discussed thoroughly. The results show that 5GPa pressure treatment can decrease the phase transformation temperature and activation energy of α+γ2→ß in the alloy. It can also shorten the phase transformation time while increasing the Avrami exponent. These are beneficial to the phase transformation of α+γ2→ß in Cu-Al alloy since the microstructure of Cu-Al alloy gets refined and dislocation density increases after the high pressure heat treatment. Note that the high pressure process has less effect on the mechanism of phase transformation.

Resumo em Inglês:

Tissue Engineering aims to repair, regenerate or restore damaged tissues. Structures known as scaffolds can be manufactured in different ways and with various materials such as synthetic and natural polymers as well as inorganic materials. Additive Manufacturing (AM) has been used to produce porous scaffolds. Particularly, Selective Laser Sintering (SLS) produces materials and composites through a selective solidification of a variety of fine powders. AM-made biomaterials may be suitable for different affected or injured parts by stimulating the biological system around the implant. The present work aims to review relevant concepts concerning the nanostructure for regeneration of the bone tissue when in contact with the native tissue, as well as the suitable techniques and materials to fabricate it.

Resumo em Inglês:

Abstract Micro-arc oxidation (MAO) treatment with various voltages was carried out on Ti alloys in 10% H2SO4 aqueous solution to enhance surface wettability for biological application. The effects of applied voltage on the surface morphology, phase constituents, functional group, roughness and wettability of the MAO coatings were investigated comprehensively. Increasing applied voltages significantly change phase constituents, surface morphology and wettability of the MAO coatings. Higher applied voltages are favorable to form higher fraction of rutile phase, larger micro-pores and higher surface roughness. The apparent contact angle decreases slightly after treated at 80 V but it decreases sharply after treated at 160 V or 200 V. The significant change in wettability of MAO coatings is attributed to hydroxyl group content and morphology of the MAO coatings. Differences in hydroxyl group content and surface morphology result in different contact interface types between water and MAO coatings, which is attributed to air captured or no air captured interface models.

Resumo em Inglês:

Ultrasonic-assisted soldering, as a type of new welding method, is widely used in the field of electronic packaging. This research used the immersion method to study the dissolution behavior of copper in liquid tin and the growth of IMC at 513, 543, and 573 K with/without ultrasonic waves. The amount of copper dissolved and IMC layer thickness were measured and the dissolution activation energy of Cu/Sn was calculated. Experimental results indicated that Without ultrasonic waves, the amount of copper dissolved increased nonlinearly with immersion time. However, with ultrasonic waves, the amount of copper dissolved increased linearly with immersion time. The amount of copper dissolved in liquid tin increased considerably with ultrasonic waves, and the dissolution rate increased by 7–8 times. The thickness of the IMC layer decreased as the ultrasonic time and ultrasonic power increased. Meanwhile, the ultrasonic waves reduced the dissolution activation energy of the Sn/Cu system.

Resumo em Inglês:

The effect of laser remelting on the corrosion susceptibility and mechanical properties of Ti-35Nb-4Sn (mass%) alloy is investigated across the remelted track profile. In comparison to the cold-rolled microstructure of base metal, recrystallization of heat affected zone (HAZ) improved plastic energy absorption and pitting corrosion resistance, by the reduction of active sites for pit nucleation. In fusion zone, however, passivation was compromised by the formation of dendritic microstructure with Sn enriched interdendritic region, where preferential pitting corrosion occurred. Fusion zone was the most anodic region and presented a corrosion potential difference of 35 mV, in comparison to the adjacent and most cathodic HAZ. Electrochemical impedance analysis showed that spontaneous passivation was most effective in base metal, that developed a thicker and more compact passive film. Local increase in elastic modulus and microhardness of fusion zone shows that laser remelting is a viable processing route for the manufacturing of biomaterials with functionally graded properties.

Resumo em Inglês:

Abstract ISO 5832-1 stainless steel specimens were plasma nitrided at different nitrogen potentials. The main goal was to obtain the S-phase with different nitrogen concentrations and free of chromium-based precipitates. The control of nitrogen potential was made by pulsing the gas at predetermined times: 10/10, 05/15, 02/18, and 01/19, where the numbers represent the time in minutes that the nitrogen flow was kept on/off, respectively. For all pulsing conditions, the nitriding was carried out at 450 °C for 2 hours. After nitriding, specimens were characterized by means of optical (OM) and electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and microhardness. Results show that the nitrided layer thickness decreases with decreasing times of nitrogen pulse, and that lower times of nitrogen flow lead to lower precipitation of chromium nitrides. It can be thus concluded that the use of intermittent nitrogen flow is an alternative to control the nitrided layer in terms of thickness, hardness, and the amount of nitrogen present in phase ɣN (S-phase).

Resumo em Inglês:

Composites including silicide of tungsten and boride of tungsten intermetallic compound as reinforced agents in alumina matrix have a good mechanical properties in low and high temperature (high friction resistance, strength, resin stance to creep and relatively high thermal shock resistance), moreover, they have chemical neutral with a high corrosion resistance in a high corrosive and high temperature environment. The purpose of this study is to investigate thermal analysis, phase and microstructural evaluation during synthesis of above mentioned composite by combustion aluminothermic processing in three systems 1-Al+Si+WO3, 2-Al+B2O3+WO3 and 3-Al+B2O3+Si+WO3. A Ball-milled starting material according to stoichiometric ratio was thermal analyzed (DTA-TGA) for each of above mentioned system. The XRD results never show any new phase during ball-milling even up to 10 hours. The forming of tungsten silicide (WxSiy) is in lower temperature in comparison with tungsten boride (WxBy) during the thermal analysis experiments. The presence of Si in the Al+B2O3+WO3 system facilitates the formation of tungsten borides. The microstructural observations show a uniform and dense distribution of silicide and boride of tungsten in the alumina matrix. Silicide phases are small grain with spherical morphology whereas; the boride phases are coarser and relatively elongated with irregular morphology.

Resumo em Inglês:

Abstract The δ is an important stable phase that replaces the metastable γ'' in the microstructure of Inconel 625 superalloy. The present work aims to investigate the kinetic of δ-phase precipitation in the microstructure of Inconel 625 nickel-based superalloy aged at 550, 625 and 725 °C up to 1000 h. Aging temperatures were chosen based on the thermodynamic simulation using JMatPro software which indicated that this phase is stable at temperatures below 905 °C. Microstructure of the heat-treated samples was analyzed via SEM/EDS and microhardness evolution was performed by Vickers microhardness tests. The results have shown that the samples aged at 550 °C (all times), 625 °C (all times) and 725 °C up to 200 h did not presented microstructural evidences of δ precipitation. However, samples aged at 725 °C for 500 and 1000 h clearly presented an acicular δ-phase precipitated mainly at the grain boundaries. It was noted changes in the microhardness at different aging time and temperatures probably due to the presence of γ'' and/or δ phases. The sample aged at 625 °C for 1000 h presented the highest microhardness value (333 ± 10 HV).

Resumo em Inglês:

Abstract Al2O3 (alumina) and MgO·Al2O3 (spinel) inclusions cause valve obstruction (clogging) in continuous casting and can deteriorate the quality of the final product. In this context, industrial heats of the bearing steel SAE 52100 was examined. Samples were collected in the final steps of the steelmaking process, both after vacuum treatment and during continuous casting. A scanning electron microscope (SEM) equipped with a energy-dispersive spectrometer (EDS) and automated particle characterization analysis was used to characterize the inclusions present in the steel samples. Thermodynamic calculations were performed with the commercial software FactSageTM 7.2. Based on thermodynamic predictions, parameters such as solid fraction, liquid fraction, MgO saturation point of the slags, content of dissolved elements in steel (Al, Mg, Ca,...) and the construction of a phase stability diagram were determined. The results in this study showed a tendency for increase in MgO content in the inclusions with the decrease of %FeO and SiO2 contents in the slag, an increase of binary basicity (%CaO/%SiO2). It is verified that the MgO contents in the slag were close to the saturation, increasing the probability for the formation of inclusions rich in MgO and/or spinel. On the other hand, stability diagrams confirm the formation of spinel inclusions for each of the heats analyzed. During the final step of the steelmaking process, there is a tendency for re-oxidation, which is verified by an increase in the density of inclusions (or total oxygen TO values).

Resumo em Inglês:

Abstract Zirconium (Zr) is an essential element in Mg-Zn and Mg-Zn-Y system magnesium alloys. In this study, an interesting phenomenon that the content of Zr element could influence the size and the morphology of the long period stacking ordered (LPSO) phases, which has never been reported by previous works before. The Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) magnesium alloys were fabricated by directional solidification, and the effects of the Zr content on the distribution characteristics of the bulk LPSO phases (18R) and the lamellar LPSO phases (14H) were investigated. The directional solidification technology showed good controllability in LPSO phase’s distribution, and the morphology of LPSO phases in Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) alloys were observed clearly. The results showed that the amount and the morphology of the 14H and 18R LPSO phases within grains continuously decreased with the Zr content increasing. The continuous 14H lamellar structure changed to discontinuous. In addition, Zr element exhibited purification ability on the grain boundaries and refined effect on the 14H and 18R LPSO phases. This can be attributed to the influence of Zr atoms on stacking fault energy (SFE) and the attraction of Zr atoms to Mg atoms.