Materials Science Forum Vols. 498-499

Abstract: SmCo5 magnets are usually produced by powder metallurgy route, including milling, compaction and orientation under magnetic field, sintering and heat treatment. The samples produced by powder metallurgy, with grain size around 10 μm, are ideal for determination of intrinsic parameters. The first step for determination of intrinsic magnetic parameters is obtaining images of domain structure in demagnetized samples. In the present study, the domain images were produced by means of Kerr effect, in a optical microscope. After the test of several etchings, Nital appears as the most appropriate for observation of magnetic domains by Kerr effect. Applying Stereology and Domain Theory, several intrinsic parameters of SmCo5 phase were determined: domain wall energy 120 erg/cm2, critical diameter for single domain particle size 2 μm and domain wall thickness 60 Å. In the case of SmCo5, and also other phases with high magnetocrystalline anisotropy, Domain Theory presents several advantages when compared with Micromagnetics.

Abstract: The usual process for producing the high energy magnets based on rare-earth-transition metals as for example NdFeB, SmCo5 or Sm(CoFeCuZr)z involves powder metallurgy. In many cases, it is necessary the determination of anisotropy constants (K1 – first order and K2 second order) from polycrystalline samples. This is not the ideal situation because for more accurate determinations a single crystal is necessary. Nevertheless, in many cases it is very difficult, or not possible, obtaining a single crystal. Then, for these situations, the anisotropy constants can be evaluated from polycrystalline samples with uniaxial texture. In this study, the methodology for making such determination is described. It includes the measurement of Schulz Pole figure by X-Ray diffraction in a surface perpendicular to the c-axis, the axis of easy magnetization. The measured Pole figure can be adjusted with a Gaussian distribution f(q)=exp(-q2/2s2) or with a distribution of type f(q) = cosn q. A model to evaluate the remanence from quantitative metallography is also described. From these distributions, and using the microstructural model, it is possible to estimate the initial magnetization curves for polycrystalline samples, including the effect of the 2nd order anisotropy constant (K2) which produces a curvature in initial magnetization curve. With all these data it is finally possible to estimate the initial magnetization curves for single crystal samples (theoretical), and the anisotropy constants K1 and K2. The inadequacy of Sucksmith-Thompson plots for determination of anisotropy constants from polycrystalline samples is also commented. The described method can be used either for rare-earth transition magnets or for Barium or Strontium ferrites.

Abstract: In the present work is shown the results for solid interaction of a Cu-13.4Al-2Ni (wt%) elemental powders. Together with this mixture was placed tungsten carbide balls into a cylindrical vial under argon atmosphere. The weight ratio of the balls to powder was 20 to 1. A planetary ball mill (Fritsch Pulverizette 5) was used to perform MA at 250 rpm. The elemental powder was milled at 10min, 5 and 10 hours and a small amount of the powders were collected and analyzed via X-ray diffraction, using Cu Kα radiation (λ=0.15418 nm) and differential thermal analyses from 25 up to 450oC at a heating rate of 25oC min-1. The x-ray diffraction patterns showed that after 10h of milling the phase formation of the mechanical alloyed powders is composed of a Cu, full Cu(Al) solid solution and γAl9Cu4.

Abstract: Porous Ti-Nb alloys are promising candidates for biomedical applications. In the present study, alloy powders containing 60 wt-% Nb were prepared by high-energy milling of Nb, Ti, and/or TiH2 powders. The high-energy milling process was carried out in a planetary ball mill. The starting and as-milled materials were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental (Nb, and Ti) and TiH2 powder mixtures with composition Nb-40wt%Ti were mechanically alloyed for 2 to 30 h. The formation of a BCC Nb(Ti) solid solution by high-energy milling using elemental Ti powder to produce Nb-40Ti was observed after milling for 30 h. A HCP-Ti solid solution was formed after milling for 30 h due to the partial decomposition of titanium hydride powder mixture during high-energy milling.

Abstract: In this work shake milling were used to mechanically activate Nb – Al powder mixtures at different relative proportions (Nb80Al, Nb65Al, Nb54Al e Nb42Al). All milling process parameters were unchanged, e.g., powders mass, ball/powder mass ratio, balls diameter, quantity and kind of process control agent. Uniaxially compacted cylindrical pellets of milled powders were vacuum reacted. After a two-step degassing treatment (290°C for 0.5 h and 400°C for 4 h), samples were heated at 30°C/min. Ignition and combustion temperatures were measured by a thermocouple inserted in a hole drilled into the pellets. The microstructure of milled powders and reacted pellets were characterized by X-ray diffraction and SEM analysis. Bulk density of the pellets was measured by water immersion (Archimedes). The results showed a decrease of both ignition and combustion temperature with mechanical activation as seen by comparison with reacted pellets of the same composition not mechanically activated (simple mixtures). By increasing the heating hate the completeness of the reactions were improved. The lower the aluminum contents the lower the ignition and combustion temperatures and also the densification. The decrease on ignition temperature was caused by a more effective dispersion (and so more activation) attained by samples with lower aluminum content.

Abstract: This work analyses the production of Al based composites with particulate reinforcement, via mechanical alloying. Composites were produced by mixing Al and NbAl3 powders by high energy mechanical alloying, under liquid nitrogen atmosphere, followed by cold pressing and hot sintering; and by controlling NbAl3 phase precipitation in liquid Al (in situ formation of the reinforcement). Results on composite produced from powders showed better distribution and incorporation, besides finer dispersion of particles in the matrix when mechanical alloying is employed. In this case, high dispersion on particulate phase was found despite predominance of small particles; there are no evidence of interface formation. When composites are produced by in situ formation of NbAl3 intermetallics, results showed that the formation of the reinforcement directly from the liquid matrix and the peritectic reaction between NbAl3 and liquid Al, provide a perfect reinforcement/matrix interface. Products showed good mechanical properties, good wear behavior and reduced thermal expansion.

Abstract: Porous structures are applied as coatings in order to improve surgical implants bone fixation by allowing the mechanical interlocking of the pores and bone. Sintered titanium porous coatings have been used for surgical implants because they have a strong attachment of the coating to the substrate. This works reports the processing and characterization of titanium porous coatings and foam samples, for surgical implants applications. Pure titanium powders mixed with urea as a binder was used for the porous coatings and foam samples. A rod shape of Ti-6Al-7Nb alloy P/M sample was used as substrate. Coatings surfaces were analyzed via scanning electron microscopy and the porosity characterization was made by quantitative metallografic analysis. It was found that coating porosity can be controlled by adjusting the binder percent addition and powder sizes. Sintered samples exhibited a microstructure with micropores and inteconnected macropores which is suitable to be used in surgical implants.

Abstract: A two-dimensional fluidynamics model for turbulent flow of gas in cyclones is used to evaluate the importance of the anisotropic of the Reynolds stress components. This study presents consisted in to simulate through computational fluid dynamics (CFD) package the operation of the Lapple cyclone. Yields of velocity obtained starting from a model anisotropic of the Reynolds stress are compared with experimental data of the literature, as form of validating the results obtained through the use of the Computational fluid dynamics (Fluent). The experimental data of the axial and swirl velocities validate numeric results obtained by the model.

Abstract: The effect of quenching heat treatments on the microstructure of cast and worked commercial steel AISI M2 and hot isostatically pressed as well as IPEN vacuum sintered AISI T15 have been studied. The quenching treatments were carried out from 1160, 1185, 1210 and 1235°C. The average grain size, volume fraction and average size of the primary carbides M6C and MC were determined by scanning electron microscopy and by using the digital image analysis method "Quantikov". An increase in average austenite grain size with increase in quenching temperature was observed for the AISI M2 commercial steel and vacuum sintered specimens. In the case of the AISI T15 steel specimens, increase in quenching temperature did not result in an increase in average grain size, both for the commercial and the vacuum sintered steels. The average size and volume fraction of M6C and MC carbides remained unaltered with increase in quenching temperature.