Materials Science Forum Vols. 727-728

Abstract: The development of technological processes for obtaining small size diamond powder is of industrial interest as basic products for roughing and finishing surfaces, like in the polishing of ornamental rocks. Therefore, this work investigates the influence of zinc, as a doping agent, in association with the graphite to diamond transformation, which occurs during high pressure and high temperature synthesis in the presence of Ni-Mn as a catalyst-solvent metallic alloy. Diamond synthesis was carried out at 4.7 GPa of pressure and 1300°C using a reactive mixture with 1:1 ratio of graphite and Ni-Mn alloy powders doped with up to 6% of Zn. The results indicated that the highest diamond yield was obtained for 0.5% while the lowest yield for 6% of Zn. Regardless the Zn content , the diamond crystal were produced with 212/150 μm of granulometry.

Abstract: Titanium is an increasingly used material on industry. Based on Ti and titanium alloys characteristics, one of the most favorable processing methods is the powder metallurgy. Attending to microstructure, alloys showing β phase have singular characteristics of low elastic modulus, good specific properties and high corrosion resistance, which make β-Ti alloys very appropriated for specific uses covering a width list of fields including aeronautics or biomaterials. At this work, it has been obtained and characterized different β-Ti alloys. The processing technique has been optimized by means of an initial blender elemental followed by basic powder metallurgy. Microstructural and mechanical characterization of the studied alloys has been achieved. Results show that these kinds of alloys can be produced by this technique and the obtained properties are really interesting for a wide variety of applications.

Abstract: The performance of biomaterial scaffolds for bone tissue engineering, as porous titanium implants, is strongly dependent of its structural features. A reliable structural characterization of this kind of implant is very important. The most of image analysis techniques just supplies 2D information about the structure of specimens. X-ray microtomography is imaging technique that can produce 3D images of samples, however, stochastic models can also estimate properties of porous materials in 3D. This work presents the evaluation of a 3D model (using a truncated Gaussian method) in comparison to 3D microtomography volume, both from a titanium scaffold sample. In order to compare, geometrical parameters were measured for both 3D volumes. By the results, the truncated Gaussian 3D method reproduced a model with similar values to the microtomography volume, showing a good agreement among data, which suggests the use of this technique to estimate physical parameters of titanium scaffolds

Abstract: Mean stress on fatigue strength of Ti-35Nb-7Zr-5Ta, used in the manufacture of orthopedic prostheses, was evaluates. Samples of Ti-35Nb-7Zr-5Ta were pressureless, sintered and tested using microhardness (Vickers) and four point bending fatigue conditions. Characterization was carried out using optical microscopy, scanning electron microscopy, EDS analysis, oxygen analysis and density. The microstructural analysis shows low densification after sintering (almost 86% of theoretical) and some precipitates of α phase and presumably ω phase at the grain and grain boundaries. The bending fatigue resistance limit reached (Se (10⁶) = 90 MPa), due to, presumably, the influence of the medium-high porosity, α phase precipitates at grain boundaries as well as the high oxygen content in the material after sintering (0.96 %). In this case, Kwofie model best explains the influence of mean stress on fatigue strength of this alloy.

Abstract: The microstructure and electrochemical properties of a La_0.7Mg_0.3Al_0.3Mn₀.₄Co₀.₅Ni_3.8 hydrogen storage alloy have been studied. The anode was prepared using a mixture of the ingot alloy in the as-cast state with carbon black and polytetrafluoroethylene (PTFE) as a binder. A Ni (OH)₂ electrode was used as the cathode of the square-type test cell. A separator was used together with a 6M KOH electrolyte. Microstructure and phase composition of the alloy have been investigated using inductively coupled plasma atomic emission spectrometry (ICP-AES), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). A niobium-containing alloy has also been included for a comparison.

Abstract: In this work, the effect of sintering atmosphere on the corrosion resistance of sintered titanium has been evaluated in 0.9 % aqueous NaCl solution to simulate physiological environment. Corrosion tests were performed on titanium porous sintered under vacuum and vacuum plus dynamic argon. The results showed better passive properties associated to the titanium sintered under argon plus vacuum atmosphere than to the vacuum sintered titanium. The better corrosion resistance of the argon plus vacuum sintered titanium was attributed to the formation of a thin passive film on the titanium surface during sintering due the low oxygen content present in this atmosphere.

Abstract: The influence of adding 6 wt% (NbC) niobium carbide on the sintering temperature and microstructure of high speed steel - AISI M₂ (0.87% C, 5.00% Mo, 6.00% W, 4,00% Cr, 2.00% V and Fe bal.) powder was studied. The starting material was obtained by vacuum melting followed by atomization in water. The samples were axially cold compacted in a cylindrical matrix and then vacuum sintered at 1250 and 1350 °C. Dilatometry and differential scanning calorimetry measurements indicated an increase in sintering temperature with addition of niobium to the AISI M₂ steel. Optical and scanning electron microscope observations revealed a microstructure with uniformly distributed niobium carbides.

Abstract: In this work, the corrosion resistance of passivated PIM 316L stainless steel specimens was evaluated in 1M H2SO4 + 2 ppm HF solution at room temperature during 28 days of immersion. Passivation was carried out in HNO₃ and H₂SO₄ solutions. The electrochemical behavior of the passivated specimens was assessed through electrochemical impedance spectroscopy and anodic polarization curves. Scanning electron microscopy (SEM) was employed to observe the surface of the specimens before and after the passivation treatments. The results pointed to a strong influence of the passivation conditions on the corrosion resistance of the specimens.

Abstract: In this present work Ti-13Nb-13Zr alloy was produced by PM using planetary ball mill with zirconium oxide grinding bowl and balls to reduce contamination. The effect of milling time upon microstructure and microhardness was studied. Powders have been produced by hydrogenation of Ti, Nb and Zr at 1MPa. Milling speed was 200 rpm during 90 to 360 min. Sintering was carried out at 1150°C during 10h. Powder size distribution was analyzed using CILAS equipment and chemically characterized by X-Ray Fluorescence (XRF). Microhardness was determined by means of a Vickers microhardness tester. Microstructure and phases were analyzed employing scanning electron microscopy (SEM) and X-Ray diffraction.

Abstract: The oxidation resistance at high temperature of the sintered stainless steels highly depends on their porosity and composition. In this work, 9 different powder mixes of gas atomized (ga) and water atomized (wa) particles were prepared. wa ferritic AISI 434L was used as base material, mixed in different amounts (5-20% by wt.) with ga austenitic AISI 316L or ga ferritic AISI 430L. All materials were vacuum sintered at two temperatures (1175 and 1225 °C). Thermogravimetric curves of the materials at 800 °C were obtained. Logarithmic curves are used to fit the experimental mass gains of PM steels. The addition of ga powder to the wa powder reduces the kinetic constant (k) in a remarkable way. The value of k depends on the amount of small pores that become blocked by the oxides when the exposure at high-temperature takes place. For similar porosity values, pure ferritic alloys show slower oxidation kinetic than duplex stainless steels.