Materials Science Forum Vols. 727-728

Abstract: Some diamond tools use iron in their composition, and it is known that iron is a strong catalyst for the graphitization of diamonds. This graphitization occurs mainly during the processing of composite materials - conventional sintering or hot pressing, and during cutting operations. This work studies the influence of the behavior of the wear and adhesion, of iron-diamond composites, by considering the use of TiC coated diamonds. Samples were prepared by mixing powders of Fe (40 μm) and diamond (425 μm), and subsequent hot pressing at 35MPa/900°C, for a time of 3 minutes. It was evaluated the mechanism of wear, and the behavior of the samples during diametral compression test. It was used the cumulative times of 2, 6, 12 and 20 minutes during the testing of abrasion.

Abstract: The quasicrystals materials possess a combination of unusual properties, since they present a long range ordered atomic structure, which is not periodic. Because of this, these materials have been object of study of many researchers in the last few years. Currently, the research is focused on determining new techniques able to produce these materials in a large scale, as well as finding new utilities. One of the ways to do so, is to use quasicrystals as coating; another way is the fabrication of composites. In this paper, aluminum composites with the strengthening of quasicrystalline particles from the alloy Al_59,2Cu_25,5Fe_12,3B₃ in volumetric fractions of 6% to 20% were developed by high energy ball milling. The powders obtained by mechanical alloying was compacted at 300MPa, sintered and submitted to micro hardness tests. The characterization was made by X-ray diffraction and SEM. In the grinding we used a 2² factorial design with factors time and speed, and the hardness of composite as response. The specimens had an average hardness of 25.75GPa for reinforcement with 6% QC and 34.75GPa for reinforcement of 20% QC.

Abstract: Titanium alloys of Ti-Si-B system were manufactured by blended elemental powder method using Ti, Si and B powders as starting materials. It was found that uniaxial and isostatic pressing followed by hot pressing at around 1000°C, for 20 minutes, provided good densification of such alloys. The physicochemical studies were performed by means of scanning electron microscopy, X-ray diffraction, atomic force microscopy and microindentation/wear tests. The investigations revealed a multiphase microstructure formed mainly by α-titanium, Ti₆Si₂B, Ti₅Si₃, TiB and Ti₃Si phases. The phase transformations after pressureless sintering at 1200°C was also studied by X-ray diffraction for the Ti-18Si-6B composition. As stated in some other researches, these intermetallics in the α-titanium matrix provide high wear resistance and hardness, with the best wear rate of 0.2 mm³/N.m and the highest hardness of around 1300 HV.

Abstract: Dispersion strengthened aluminium composites have been prepared by mechanical alloying. At this work were studied the turning conditions to get in situ formation and dispersion of Al₄C₃ on 2024 alloy by graphite addition. The alloy matrix was obtained by attrition milling a mixture of starting powders; further additions of carbon (2,5; 5 and 10% wt) were performed by means of a planetary mill. Through an adequate sintering the reinforcement formation was showed by X-Ray diffraction analysis of powders with milling times next to 20 hours. The microhardness values appointed that mechanical properties were held even soft material addition (graphite) and improved by age hardening. Pin-on-disc test revealed the composite have low friction coefficient, due to lubricant carbon action and enough low volumetric wear due to high hardness of bulk Al₄C₃ reinforcement.

Abstract: Laser cladding is an adequate technique to fabricate Metal Matrix Composite (MMC) layers because of its focused high energy which allows the partial melting of hard ceramic reinforces particles like carbides. Thus, the wettability and gradual transition between metal and particle can be improved. However, metastable or new intermetallic phases can be formed during laser processing due to severe thermal cycle imposed to the clad with unknown properties in some cases. In this work our experience on microstructural analysis of Ti-MMC coatings acquired during the last five years is summarized. Special attention is paid on carbide dilution and secondary carbides formation mechanisms when TiC, SiC, Cr₃C₂, WC and B₄C are mixed with titanium alloys.

Abstract: In the production of metallic saw blades segments impregnated with diamond particles, there are two basic functions of the metal-based matrix. First, to hold tightly the particles and, second, to display wear rate compatible with the diamond loss. The most common matrix materials are composed of either cobalt or iron, nickel, copper, bronze alloys and tungsten alloys. This paper investigated the microstructure and mechanical properties of Ni-Fe-Cu-Sn alloys as the metallic matrix of saw blade segments. It was found that the Ni content significantly influences the microstructure and mechanical properties of the alloy. Information on the diamond bonding efficiency of the Ni-Fe-Cu-Sn alloy as a cutting tool matrix were disclosed.

Abstract: Cubic boron nitride (cBN) composites with hardmetal such as metal turning and rocks drilling, WC-Co, matrix are alternative substitutes for similar diamond composites in high speed machining operations and rocks and drilling. Although it possesses related technical properties close to diamond, cBN has the inconvenience of reacting with hardmetals. This deteriorates the composite properties. A possible solution to this negative effect is to apply high pressures during the sintering operation. In this work, powder mixtures of cBN with a hardmetal (WC15%Co) were subjected to pressures up to 7.0GPa and temperatures up to 1400°C for 40 seconds. The results showed significantly higher values of hardness and compressive strength as well as an improved wear resistance.

Abstract: Metal-ceramic composites are recently being used as electrode materials in solid oxide fuel cell (SOFC), which have received much attention as alternative energy sources. In this work cermets of Cu-Ni-Ag/Y₂O₃ were synthesized with different quantities of Ni and Ag. Various samples were prepared from pure precursors by conventional powder metallurgy processing and sintered at 800°C. The characterization by differential scanning calorimetry (DSC) and thermal gravimetric analyze (TGA) indicate that the presence of Y₂O₃ could increase the corrosion resistance of the metallic copper alloys. The copper crystalline structure presents internal strengths and possibly crystallites with different cell parameters due to the effect of thermal and mechanical treatments in presence of Y₂O₃. The highest electrical conductivity and highest hardness were obtained for the composition with 5% Y₂O₃ and 5% nickel, which increases the hardness and could contribute to the higher conductivity through a precipitation mechanism.

Abstract: Composites are versatile materials owing to their extensive possibilities of improving mechanical properties by distinct matrix reinforcements. In particular, it has been shown that diamonds are very effective reinforcement for metal matrix composites (MMC). In this work, the properties and microstructure of plain Al matrix composites reinforced with up to 50 wt.% of nanodiamonds as well as Cu, W, Al MMCs reinforced with 1% nanodiamonds were investigated. Composites were fabricated by processing metallic powders of Cu, W and Al as well as nanodiamond particles at sintering conditions of 2 GPa of pressure and 700°C for times up to 5 minutes. As compared to the other nanodiamond reinforced MMCs, the investigated composite presented improved microhardness and reduced wear in abrasive tests.

Abstract: Quasicrystalline (QC) materials represent a new class of alloys differing from amorphous and crystalline materials due to quasicrystalline periodicity and therefore unusual properties. Applications of quasicrystals range from surface coatings, thin films to reinforcements of ductile matrix composites such aluminum and, more recently, polymers. Quasicrystalline alloys show fundamentally different behavior compared to crystalline alloys even when their compositions are very similar, including low friction coefficient, high hardness and high brittleness. Due to this brittle behavior there are some limitations with respect to what methods can be used to process QC materials restricting their applications to powder form. One of the techniques for particle size reduction is mechanical milling which, however may lead to destabilization of the icosahedral phase. Therefore, there is a need to study the stability of quasicrystalline alloys during this comminution process. In the present study, AlCuFeB alloys were milled with the aid of a grinding agent that helps reduce overheating and thus controlling the QC powder stability. It was found that QC phase was destabilized after 10 h while the addition of a grinding agent led to milling times of 20 h without destabilizing the QC phase.