Materials Science Forum Vols. 727-728

Abstract: After sintering, SmCo₅ magnets may present a surface layer depleted in Samarium. This happens because Samarium is much more susceptible for oxidation than cobalt. Besides, samarium has high vapour pressure. This Sm depleted zone is of the order of ~ 1 mm (in a 10 mm diameter magnet), and present oxygen content 1000 ppm higher than the center of the magnet. This layer is composed by SmCo_(5+x), which presents low coercivity (1-2 kOe). The occurrence of the Sm depleted layer is associated with kinks in the 2^nd quadrant of the hysteresis curve. After the removal of this layer, the hysteresis kinks disappear. The efficacy of different chemical etchings (Nital and chromic acid) to reveal microstructural constituents was discussed in detail. Nital is recommended for observation of magnetic domains under polarized light (Kerr effect), whereas chromic acid is more suitable to identify Sm₂Co₇ and other high Sm phases.

Abstract: An important process parameter in FeSi sintered alloys is the sintering temperature. If the sintering temperature can be reduced, the sintering process could be performed in less expensive furnaces. A densification model is here applied to experimental data. The model considers both grain size and density. After the model is applied to experimental data, the acquired model parameters allow the simulation of densification for other situations of temperature and heating rates. The model can be helpful to find suitable sintering temperatures in these alloys.

Abstract: Microstructural and magnetic properties of thermal aged NdFeB-based powders, recycled by means of the HDDR process, are studied. The time aging causes a reduction of the remanent magnetic moment per unit mass and intrinsic coercivity of the magnetic powders. On the other hand, it has been verified an increase of the magnetic polarization at µ₀H = 2 T after aging during 16 hours. The oxygen concentration is superior in the grain boundary region (Nd-rich) compared to that verified in the hard magnetic phase.

Abstract: The composite studied here consisted of a geopolymer matrix reinforced with quasicrystal powders. Quasicrystals are complex metal alloys with atypical structures. Due to their physical, mechanical and surface properties, quasicrystals have been widely studied. Nowadays quasicrystalline powders are proposed to be incorporated in ductile matrices. The geopolymers are inorganic polymers with excellent thermal performance and interesting adhesive properties. The quasicrystalline alloys were prepared by induction melting the constituents under argon atmosphere in a water-cooled copper crucible. In this work the adhesion of composites with 5, 10 and 15% of quasicrystal powder in aluminum joints was investigated. In addition, the influence of the curing time was taken in to account and tests were performed in samples with 7 and 28 days of cure. The results indicate an improvement in adherence with the inclusion of quasicrystal. The best adherence was obtained when the curing time was 28 days.

Abstract: This work aims the synthesis of nickel alloys (Ni-Cr-Al-C) with different amounts of carbon, in order to measure the hardness and analyze the chromium carbides distribution and size, as well as for gamma prime phase. It has been used the mechanical alloying technique in order to provide a fine dispersion of chromium carbides in the gamma prime phase matrix. The materials were characterized by x-ray diffraction, differential scanning calorimetry, scanning electron microscopy, energy dispersion spectroscopy, and microhardness test.

Abstract: Previous works studied the colloidal processing of nickel-silica and nickel-titania nanocomposites fabricated via slip casting. A rheological approach was used to characterize and optimize a 30 vol.% nickel aqueous suspension with up to 10 vol.% SiO₂ and 5 vol.% TiO₂ nanoparticles. In this work, the effect of mechanical activation of Ni-SiO₂ and Ni-TiO₂ nanocomposites on microstructural and mechanical properties was studied. For this aim Ni-SiO₂ and Ni TiO₂ slip-cast compacts were attrition milled for up to 12 hours. Green bodies of the mechanical-alloyed powders were obtained by cold pressing (300 MPa). Next, those green bodies were annealed at 700°C for 1 h, repressed at 700 MPa and sintered at 900°C for 1 h in flowing Ar/5%H₂ atmosphere. Porous and dense materials were characterized by SEM/FESEM, Archimedes densities and Vickers microhardness measurements. Mechanical alloying produces a remarkable improvement on microstructure homogenization, sintering densification and hardness comparing with slip-cast composites.

Abstract: The mechanical alloying process provides alloys with extremely refined microstructure, reducing the need for alloying elements to grain growth restriction, as in casting techniques. The Cu-Al-Ni and Cu-Zn-Al alloys produced by casting may have the shape memory effect when plastically deformed at relatively low temperatures, returning to its original shape upon heating at a given temperature. This work aimed at the production of Cu-Al-Ni and Cu-Zn-Al alloys by mechanical alloying, followed by microstructural characterization and investigation of the shape memory effect by means of differential scanning calorimetry (DSC). Metal powders of Cu, Al, Ni and Cu, Zn, Al were processed in a SPEX high energy vibratory mill during 8 hours, with ball-to-powder weight ratio of 5:1. The milled products were characterized by X-ray diffraction. For each alloy, specimens with 8 mm diameter and 2 mm thickness were shapes by uniaxial pressing, sintered in a tube furnace with argon atmosphere, solubilized and then quenched in water. Samples were characterized by optical and scanning electron microscopy (SEM), Vickers hardness testing and DSC. An ultrafine microstructure was obtained in the Cu-Al-Ni alloy but the shape memory effect was not detected by DSC analysis because of second phase precipitation. The shape memory effect was not present in the Cu-Zn-Al alloy also, because of zinc oxidation during the sintering.

Abstract: High energy ball milling has been used like alternative route for processing of materials. In the present paper, the reduction of tungsten oxide by aluminum in order to obtain metallic tungsten was studied using a SPEX type high energy mill. A powdered mixture of WO₃ and metallic aluminum, weighed according to the stoichiometric proportion with an excess 10% Al, was processed with hardened steel utensils using a 1:6 powder-to-ball ratio. The processing was carried out with milling jar temperature measurement in order to detect the reaction type. The temperature evaluation indicated the self-propagating reaction occurrence by fast increase of the jar temperature after a short milling time. The tungsten oxide reduction was verified by X-Ray Diffraction (XRD) analysis and the milling products were characterized by Scanning Electron Microscopy (SEM). The results were slightly different from the literature due to the mill type and milling parameters used in the work.

Abstract: The present work reports on the structural evaluation of mechanically alloyed Ti-xZr-22Si-11B (x = 5, 7, 10, 15 and 20 at-%) powders. Milled powders and hot-pressed alloys were characterized by X-ray diffraction, electron scanning microscopy, and electron dispersive spectrometry. The Si and B atoms were preferentially dissolved into the Ti and Zr lattices during ball milling of Ti-xZr-22Si-11B (x = 7, 10, 15 and 20 at-%) powders, and extended solid solutions were achieved. The displacement of Ti peaks was more pronounced to the direction of lower diffraction angles with increasing Zr amounts in mechanically alloyed Ti-Zr-Si-B powders, indicating that the Zr atoms were also dissolved into the Ti lattice.

Abstract: This paper discusses on effect of molybdenum on the Ti₆Si₂B formation in mechanically alloyed and hot-pressed Ti-xMo-22Si-11B (x= 2, 5, 7 and 10 at%) alloys. High-energy ball milling and hot pressing were utilized to produce homogeneous and dense materials, which were characterized by scanning electron microscopy, X-ray diffraction, electron dispersive spectrometry, and Vickers hardness. The excessive agglomeration during milling was more pronounced in Mo-richer powders, which was minimized with the formation of brittle phases. Hot pressing of mechanically alloyed Ti-xMo-22Si-11B powders produced dense samples containing lower pore amounts than 1%. Ti₆Si₂B was formed in microstructure of the hot-pressed Ti-2Mo-22Si-11B alloy only. In Mo-richer quaternary alloys, the Ti₃Si and Ti₅Si₃ phases were preferentially formed during hot pressing. Oppositely to the ternary phase, the Ti₃Si phase dissolved a significant Mo amount. Vickers hardness values were reduced in hot-pressed Ti-xMo-22Si-11B alloys containing larger Mo amounts, which were dissolved preferentially in Ti solid solution.