Navegando por Autor "Botta, Walter Jose"
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Artigo 2Mg–Fe alloys processed by hot-extrusion: influence of processing temperature and the presence of MgO and MgH2 on hydrogenation sorption properties(Elsevier, 2011-06) Peres, Maurício Mirdhaui; Lima, Gisele Ferreira de; Garroni, Sebastiano; Baró, María Dolors; Surinach, Santiago; Kiminami, Claudio Shyinti; Botta, Walter Jose; Peres, Maurício Mirdhaui; Jorge Junior, Alberto Moreira2Mg–Fe alloy powder produced by high-energy ball milling was processed by hot extrusion at temperatures of 200 ◦C and 300 ◦C to produce bulk samples. The alloys were hydrogenated for 24 h under hydrogen pressures of 24 bar (to produce the Mg2FeH6 phase) and 15 bar (to produce a mixture of MgH2 + Mg2FeH6 phases), respectively. After the hydrogenation treatments, the complex hydride Mg2FeH6 was identified in both conditions, while the MgH2 and MgO phases were observed only after extrusion at 200 ◦C. Desorption temperatures varied with the extrusion conditions; extrusion at 300 ◦C resulted in a desorption onset temperature about 68 ◦C lower than that of samples extruded at 200 ◦C, and about 200 ◦C lower than that of commercial MgH2. Extrusion at the lower temperature did not change the number of stored defects (point defects, dislocations, voids, stacking faults, vacancies and others) produced in the milling process and increased the preferential sites for hydride nucleation, increasing the hydrogen storage capacity. The presence of MgO produced the beneficial effect of grain boundary pinning, but delayed the onset temperature of desorption. The combined presence of MgH2 and Fe after hydrogenation at 15 bar seems to play a catalytic role that considerably hastened the Mg–H reactions and increased the desorption kinetics. However, the desorption kinetics in both conditions was still lowArtigo Nanoquasicrystalline Al–Fe–Cr–Nb alloys produced by powder metallurgy(Elsevier, 2013-11-15) Peres, Maurício Mirdhaui; Audebert, Fernando E.; Galano, Marina L.; Rios, C. Triveño; Kasama, H.; Kiminami, Claudio Shyinti; Botta, Walter Jose; Bolfarini, ClaudemiroNano-quasicrystalline Al–Fe–Cr based alloys produced by rapid solidification processes exhibit high strength at elevated temperatures. Nevertheless, the quasicrystalline particles in these systems become unstable at high temperature limiting the industrial applications. In early works, it was observed that the use of Nb or Ta increases the stability of the Al–Fe–Cr quasicrystalline phase delaying the microstructural transformation to higher temperatures. Thus, these nano-quasicrystalline Al-based alloys have become promising new high strength material to be used at elevated temperatures in the automotive and aero-nautical industries. In previous works, nano-quasicrystalline Al–Fe–Cr–Nb based alloys were obtained by rapid solidification using the melt-spinning technique. In order to obtain bulk alloys for industrial applications other fabrication routes such as powder production by gas atomization followed by compaction and extrusion are required. In the present work, the production of Al–Fe–Cr–Nb based alloys by powder atomization at laboratory scale was investigated. The powders obtained were sieved in different ranges of sizes and the microstructures were characterised by means of X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive of X-ray analysis. Mechanical properties have been measured by compression tests at room temperature and at 250 C. It was observed that a very high temperature is required to produce these alloys by gas atomization; the icosahedral quasicrystalline phase can be retained after the atomization in powder sizes typically under 75 lm, and also after the extrusion at 375 C. The extruded bars were able to retain a very high strength at elevated temperature, around 60% of the yield stress at room temperature, in contrast with the 10–30% typically obtained for many commercial Al alloys