Papers by Author: Claudio Shyinti Kiminami

Abstract: In this work, the microestrutural characterization and mechanical properties of atomized Al-9Si-3Cu alloy powders and extruded samples are presented. The microstructure was evaluated by a combination of X-ray diffraction, optical microscopy and scanning electron microscopy. The mechanical properties of extruded samples were also characterized by tensile test and hardness measurements. The results revealed that the powder particles and the extruded samples are constituted by α-Al, intermetallic and metastable phases. The extruded samples obtained by the use of smaller atomized particles show lower ductility than with larger particles. The same behavior was observed with low extrusion temperature than with high temperatures. It was also observed minor variations in the yield strength and hardness with variation in the size of the powder particles.

Abstract: In the present contribution, atomised Sn-3.0Ag-0.7Cu powder alloy with a size of less than 250 μm was first compacted and then microwave and conventionally sintered at 175 °C for 15 and 120 minutes, respectively. Despite the very different processing times applied, the degree of porosity was very similar, i.e., around 7%. The as-atomized microstructure of the conventionally sintered samples was barely altered with β-Sn dendritic matrix comprising Cu₆Sn₅ and Ag₃Sn intermetallic particles. The microwave sintered Sn-3.0Ag-0.7Cu sample exhibited a combination of fine and very coarse intermetallic particles, with the presence of well-developed needle-like Ag₃Sn, and its dendritic pattern disappeared completely. The Vickers hardness of both samples was measured and found to be consistent with their microstructures.

Abstract: Glassy overspray powders of Ni₅₉Nb₃₅Sn₆ (at%) bulk metallic glass (BMG) obtained by spray forming were used in order to produce coatings on AISI 1020 mild steel substrate by laser cladding of the pre-placed powders. Different laser parameters, resulting in a variation of the power density, PD (J/mm2), were tested with a Yb fiber laser (up to 500 W). Gas atomized powders, suction cast sample trough copper mold casting and the laser clad tracks were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and coatings were subjected to measurements of Vickers microhardness. Atomized powder obtained showed no crystalline phases formation up to 425 μm, indicating good glass forming ability (GFA) of Ni₅₉Nb₃₅Sn₆ (at%) alloy. Microstructure characterization confirmed maximum glassy dimension of t_c =1mm for the Ni₅₉Nb₃₅Sn₆ (at%). Laser cladding track showed nanocrystalline phases embedded in a glassy matrix with Vickers microhardness ranging from 336 to 1184 HV.

Abstract: Shape memory alloys (SMAs) are a class of material that undergoes a reversible shape change after a plastic deformation. The recovery of the original shape is possible due to a structural transformation upon heating to a critical temperature. The shape memory effect is related to a martensitic-austenitic transformation from a phase with a low symmetry (martensite) to a high-temperature phase (parent phase) [1]. Cu-based shape memory alloys have the advantage of large thermal and electrical conductivities and the system Cu-Al-Ni alloys are quite attractive due to better stabilisation against aging phenomena [2].

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.