Papers by Author: Claudemiro Bolfarini

Abstract: Metal matrix composites, in which crystalline Al was reinforced by particulates of the Al_87.5Ni₄Sm_8.5 amorphous alloy, were produced using cold pressing and hot extrusion processing. Controlled nanoprecipitation was used to improve the mechanical properties of the amorphous alloy. Amorphous melt-spun ribbons were produced by melt-spinning technique and then fragmented in fine powder by high-energy ball milling. Amorphous and pure aluminum powders were mixed in two different proportions: 85:15 and 70:30 (wt%) and homogenized by ball milling. Bulk samples were produced via cold pressing and hot extrusion. Controlled nanoprecipitation within the amorphous alloy was obtained by the correct choice of processing temperature. The composites were analyzed for reinforcement distribution, porosity content, microhardness and compression tests. The results showed that it was possible to control the precipitation by producing almost the same volume fraction of nanocrystals in each condition. Compression tests showed an improvement on the mechanical properties, which were correlated with the presence of the amorphous/nanocrystals reinforcement in the Al-matrix. The compression yield-strengths of the as-extruded composites were 192 and 310 MPa for 15% and 30% in volume of Al_87.5Ni₄Sm_8.5, respectively. These values are significantly higher than the typically found for the AA1100 wrought pure aluminum (180 MPa).

Abstract: Powder of Fe₇₂Nb₄Si₁₀B₁₄ (%at) glassy alloy was obtained by gas atomization in order to investigate the possibilities of amorphous phase formation due to the high cooling rates (10³ 10⁵ K/s) involved in this process. The ratio between the gas volumetric and the metal mass flow rates used was 1.0, and nitrogen (N₂) was used as the atomization gas. The powder, sieved in different granulometric size ranges, was characterized through: X-ray diffratometry (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Fe₇₂Nb₄Si₁₀B₁₄ (%at) bulk metallic glass (BMG) showed completely or partially glassy structure depending on the size range. The obtaining of powders with glassy structure that could be applied as shot penning powder particles and thermal spray feeding powder for metallic coatings or would make possible the production of bulk glassy materials by warm consolidation of such powders or even a millimeters thick deposit obtained by spray forming with glassy or metastable microstructure that would be very interesting considering applications as soft ferromagnetic parts.

Abstract: Bulk glassy alloys based on the Fe-Co-B-Si-Nb system have already achieved high levels of mechanical strength. The present work investigated the microstructural evolution of Fe_43.2Co_28.8B_19.2Si_4.8Nb₄ alloy during the spray forming and wedge mold casting processes, with emphasis on the formation of amorphous phase. The microstructure was evaluated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The region outer the spray deposit showed the formation of an amorphous structure with a thickness of ~2.5 mm, while that of the wedge-shaped sample exhibited a thickness of up to ~1.5 mm, suggesting that both processes show a promising potential for the production of bulk glass alloys.

Abstract: Samples of a 2Mg-Fe (at.%) mixture were produced by high energy ball-milling (HEBM) with ball to powder ratio = 20:1, in an argon gas atmosphere, in 190 ml vials (sample-1) to produce powders and in 300 ml vials (sample-2) to produce plates. Both samples were cold-pressed into preforms. The preforms were then extruded at 300°C at a ram speed of 1mm/min., with the following extrusion ratios: sample-1 at 3/1 to ensure porosity and sample-2 at 5/1 to increase the adhesion of the plates. The resulting bulks from samples 1 and 2 were hydrogenated for 24h in a reactor under 15 bar of H₂ to produce the Mg₂FeH₆ complex hydride, and at 11 bar of H₂ to produce both the complex hydride and MgH₂ hydride. In addition, sample-1 was severely temperature-hydrogen cycled to verify its microstructural stability and the influence of grain size on the sorption properties. XRD patterns showed Mg(hc), Fe(ccc) and Mg₂FeH₆ in both samples, and sample-2 also contained MgH₂ and MgO (attributed to processing contamination). DSC results demonstrated that the initial desorption temperature of sample-1 was lower than that of sample-2. However, sample-2 showed faster desorption kinetics, presenting a desorption peak about 73°C below that of sample-1. This could be attributed to the activation/catalyst effect of the MgH₂ hydride. The improvement in sorption properties was attributed mainly to porosity and to the type of employed catalysts.

Abstract: In the present work, we have processed 2Mg-Fe mixtures by reactive milling (RM) under hydrogen atmosphere to synthesize Mg2FeH6 phase in the powder form which were then systematically processed by High Pressure Torsion (HPT) to produce bulk samples. The bulk samples were characterized in terms of microstructural and structural analyses and of hydrogen desorption properties. The hydrogen sorption properties after HPT processing was evaluated in comparison with the Mg2FeH6 powder obtained by RM and with commercial MgH2. HPT processing of Mg2FeH6 can produce bulks with a high density of defects that drastically lower the activation barrier for hydrogen desorption. Therefore, the bulk nanocrystalline Mg2FeH6 samples show endothermic hydrogen decomposition peak at a temperature around 320°C. In addition, when compared with the Mg2FeH6 and MgH2 powders, the Mg2FeH6 HPT disks showed the same results presented by the Mg2FeH6 powders and certainly decreases the onset transition temperature by as much as 160°C when compared with the MgH2 powders.

Abstract: Severe plastic deformation (SPD) techniques are being considered as low cost processing routes for Mg alloys, aiming hydrogen storage applications. The main objective is to develop air-resistant materials, with lower specific surface area in comparison with ball-milled powders, but with still attractive H-sorption kinetics associated to the microstructural refinement. In this study, the effects of different SPD processing routes (high-pressure torsion, extensive cold rolling and cold forging) in the hydrogen activation behavior of Mg was evaluated. The results show that both microstructural and textural aspects should be controlled during SPD processing to obtain Mg alloys with good H-sorption properties and enhanced activation kinetics.

Abstract: Fe-6.5%Si alloy spray formed has the Curie temperature variation measured as a function of the parameters process. In this work the magnetic transition temperature was obtained by Differential Scanning Calorimeter although it can be also gotten by Vibration Samples Magnetometer. The process parameters controlled were gas pressure (P), tube diameter (T) and fly height (H) and the properties analyzed were coercive force, maximum permeability, power loss and Curie temperature. The chemical composition was determined using the Curie temperature. The microstructure of deposits was observed in different conditions of light microscopy, bright field, polarized light and dark field, and the grain size was measured according to ASTM 112-95. The process parameters combination P5T6H360 has produced the minimum power loss of 2.26 W/kg and maximum permeability of 9,000. The Curie temperature measured was 683°C, which indicates a Si content of 6.5wt%.

Abstract: Silicon-iron alloys with silicon content about 6.5wt.%Si offer a great potential for applications aiming reduction of core loss in electric parts. Deposits of the Fe-6.5wt%Si alloy produced by spray forming were annealed at temperatures between 400 and 1300oC, during 1h in vacuum. The grain size has a great importance to the magnetic properties. In the present work, it was analyzed the influence of the metallography parameters in order to get the best accuracy to determine the grain size according to ASTM 112-96. Chemical composition, time and temperature of specific etchings were modified and tested in different conditions of observation in light microscopy. Bright field, polarized light and dark field were used in the samples aiming to measure the grain size. The best etchings were Nital 10% and Marshall, both at room temperature. The results of grain measurement are presented in their relationship with the magnetic properties.

Abstract: Fe-Si alloys have excellent soft magnetic properties, specially around 12 at% Si. However, its industrial application is limited because of the lack of ductility, which causes cracking during rolling operations for the fabrication of thin sheets. The reason of the brittleness of the high silicon alloys is a disorder/order reaction at low temperatures. The aim of this work is to analyze the effect of the addition of Aluminum on the crystalline structure of Fe-Si alloys. Samples with a chemical composition of Fe88Si12 and Fe87Si12Al1 (at%) were prepared by Spray Forming. The structure was studied by means of X-ray diffraction and Mössbauer Spectroscopy. The presence of the DO3 and α- Fe phases were observed

Abstract: In the present work the crystallization process of an aluminum-based amorphous metal have been investigated. Rapidly quenched Al85Ce5Ni10 ribbon has been produced by melt-spinning. The amorphous structure evolution during heating has been studied by a combination of X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Thermograms obtained in continuous heating regime reveal a glass transition, Tg, resulting in a supercooled liquid temperature range of ∼16°C. Multiple crystallization events were observed by isothermal annealing of the as-quenched melt-spun ribbon at temperatures below Tg; precipitation of a metastable phase in the amorphous matrix has been observed. Further heating at increasing temperatures resulted in complete crystallization with α-Al and intermetallic compounds. Kinetics analyses indicate that crystallization occurs though nucleation and three-dimensional growth.