Production of nanocomposites by sintering iron-copper powder

Iron-copper nanopowder may be obtained by the reduction of hydroxides in hydrogen. The particles of this powder consist of iron cores within thin copper shells. Sintering of such powder produces a nanocomposite (density 6.9–7.1 g/cm³), whose microhardness (1300–1500 N/mm²) is more than triple that of regular Fe-Cu crystallites.     

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pet Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worm surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.     

Cobalt-Chromium Powder Alloys and Retention Coatings Made from Them for Orthopaedic Stomatology

An alloy of Co-Ni-Mo-Cr is prepared by diffusion impregnation with chromium from point sources of cobalt-nickel-molybdenum alloy powder synthesized by combined reduction. Optimum regimes are determined for preparing alloy powder by combined reduction of cobalt, nickel, and molybdenum oxides (reduction temperature 1325 K, isothermal soaking time 3 h) and diffusion impregnation with chromium (temperature 1373 K, isothermal soaking time 4 h). The adhesive strength of a facing coating with material of a dental prosthesis manufactured using the alloy developed is studied. It is shown that with application of retention layers the adhesive strength increases by a factor of three to five compared with using cast spheres (beads). __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 3–7, May–June, 2005.     

Liquid break-up in gas atomization of fine aluminum powders

Formation of powder particles has been studied in aconfined design atomizing nozzle. Liquid metal (AA 2014) is presented in the form of a thin film to the atomizing gas in this type of operation. Upon contacting, the film breaks up into large droplets of diameters up to 500 /im (primary break-up). These droplets undergo further disintegration in flight to produce the powder (secondary disintegration). Photographs taken using a high-speed flash indicate that this takes place by stripping. It is proposed that the fine range of the particles are the products of stripping break-up, while the coarse range are stable particles obtained when stripping stops and the remaining particle becomes too small to undergo further disintegration. Size distribution curves often contained two or more peaks providing support for different formation mechanisms for fine and coarse particles. All particles were dense and single droplets except for the very large ones (>55 μm)which had satellites of fine particles on the surface and showed porosity in some cases. Particles <30 μm in general were fully spherical, whereas larger ones also showed oblong features. No evidence was found for small particles agglomerating to produce large ones.     

烧结Nd-Fe-B磁体的温度稳定性缺陷分析

采用SEM、EDAX及粒度分析手段,研究了烧结Nd-Fe-B磁体开路磁通不可逆损失(hirr)异常偏高的原因。结果表明,磁体显微组织中晶粒尺寸分布不均匀及存在少量粗大晶粒,是导致其温度稳定性缺陷的主要因素,表现为少数产品hirr异常偏高。在磁体工业生产过程中,改进原料的铸锭和制粉技术,避免出现粗大粉末颗粒,减少极细粉末颗粒的数量,保证磁体显微组织精细均匀,是制备温度稳定性高的烧结Nd-Fe-B磁体的基本措施。     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

Preparation of finely-crystalline powders for tools steels

The possibility is examined of preparing finely-crystalline powders of tool steel R6M5F3 by the electric hydropulse method for treating molten metal before spraying. It is established that there is a reduction in gases and nonmetallic inclusions. As a result of this there is a reduction in the kinematic viscosity of the molten steel, and an increase in surface tension and the degree of undercooling. These factors promote the preparation of finely-crystalline powder with improved physical and production properties. Institute for Materials Science Problems, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 38–40. March–April, 1997.     

Morphology of dispersed nickel electrochemically deposited in solutions of the system NiSO4(NiCl2)-(NH4)2SO4-NH3·H2O

Morphology of dispersed nickel obtained by electrolysis in electrolytes (0.1–0.5 M) NiSO₄(NiCl₂)-0.5 M (NH₄)₂SO₄-NH₃·H₂O (to pH 8.0–8.5), which model the leaching solutions of secondary metal, is investigated. It is noted that the sediment of three structural levels is formed on the cathode at the current density 20–100 A/dm². These are (i) spherical particles, (ii) spherical agglomerates, and (iii) conglomerates of the particles of the second structural level. The shape of first-level elemental particles is governed by adsorption of the surface-active NH₃ molecules on the surface of crystalline nuclei. The second and the third levels of structuring emerge due to high surface energies of elemental particles. Spherical agglomerates are mechanically stable and determine the main characteristics of the nickel powder. The sizes of these agglomerates decrease as the cathode current density increases and the nickel concentration in electrolytes decreases, and lie in the limits 2–10 μm. Original Russian Text ? O.I. Kuntyi, 2007, published in Izvestiya VUZ. Tsvetnaya Metallurgiya, 2007, No. 1, pp. 20–23.     

Fabrication of Metal Powders for Energy-Intensive Combustible Compositions Using Mechanochemical Treatment: 2. Structure and Reactivity of Mechanically Activated Al–Modifier–SiO 2 Mixtures

The results of studying the particles of the aluminum–modifier–quartz composites after mechanochemical treatment (MCT) in a planetary centrifugal mill by various physicochemical methods are presented. Graphite (C), polyvinyl alcohol (PVA), and stearic acid (SA) are used as modifiers. To increase the dispersive ability of plastic metallic powders (metal–modifier), MCT was performed in the presence of quartz, the weight fraction of which in the composite varies from 5 to 20%. The largest grinding of aluminum particles is observed with an increase in the graphite content from 5 to 20% and SiO2 content from 5 to 10% in the composition of aluminum–modifier–quartz composites. The particle decreases, while that of crystallites increases with the MCT of the Al–SA–SiO2 system with an increased quartz content in the composite. The maximal imperfection of aluminum after MCT is characteristic of the Al–SA–5%SiO2 samples. An increase in the particle size and, correspondingly, a decrease in the specific surface with rather small crystallite sizes are observed after MCT for the Al–PVA–SiO2 composition. It is shown that the imperfection of the crystalline structure of aluminum particles increases with an increase in the quartz content in the system. The synthesized powder material is a composite formation of aluminum and quartz particles bound by polymer formed based on polyvinyl alcohol in this case. The MCT of the Al–modifier–SiO2 mixtures results in an increase in the powder activity both due to the accumulation and redistribution of defects in aluminum particles and due to the change of the surface structure due to the incorporation of modifying additives into the destructed oxide layer. The conceptual model of transformation of the surface layer and subgrain structure of aluminum particles during MCT is presented.     

The effect of Mo addition on the liquid-phase sintering of W heavy alloy

The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are liquid-phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process. formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology     

The production of very fine copper powder

Conclusions A study was made of the possibility of preparing copper powder by the method of reduction by titanium compounds in aqueous solutions, and it was established that, as a result of the reaction of titanium sulfate Ti₂(SO₄)₃ with copper sulfate CuSO₄ in a solution with an addition of 200–250 g/dm³ I₂SO₄, a very fine, pure copper powder with rounded particles is produced.Two methods of preparing copper powder were developed: one employing the gradual addition (drop by drop) of a saturated CuSO₄ solution to a Ti₂(SO₄)₃ solution (periodic method) and another employing the anodic dissolution of metallic copper in a Ti₂(SO₄)₃ solution (continuous method). In both cases, the process is carried out with the simultaneous electrochemical reduction Ti⁴⁺+eTi³⁺ on a lead or copper cathode.The periodic method is more suitable for the preparation of powders of extra-high purity, because it is easier to remove impurities from salts than from metals. The advantages of the continuous method comprise the possibility of utilizing various forms of copper scrap as anodes, absence of losses of titanium and copper salts, and possibility of performing the process automatically.The following have been developed: a procedure for the treatment and drying of powder, and methods of determining powder size and the degree of purity of the product. It is shown that, for determining the degree of oxidation of copper powder, it is possible to use a visual method of comparing the color of the product with that of specially prepared reference specimens.     

Interaction of a powder magnetic-abrasive tool with the surface of an articles in annular gaps

The effect of the shape of powder particles, their size, and the degree of irregularity on the characteristics of the actual contact of a magnetic-abrasive tool (MAT) with the surface of articles being machined is studied. It was established that the relative area of the actual powder contact with the surface is 4–10%. A model representation of an MAT is suggested in the form of tetrahedral quasistable formations and the features of powder tool formation within annular gaps in the form of quasistable volumes are described. Ukrainian National Technical University “Kiev Polytechnic Institute”. Translated from Poroshkovaya Metallurgiya. Nos. 7–8(408), pp. 117–121, July–August, 1999.     

Obtaining molybdenum powder by metallothermy of its compounds in a sodium chloride melt

The reduction conditions of the oxygen compound of molybdenum to metal powder using aluminum and magnesium in a sodium chloride solution are investigated. A thermodynamic evaluation of the reduction reactions is given. It is found that, in order to end up with a 95–97% yield of the powders, an excess of ∼30% of the reducing agent with respect to the calculated one is necessary. In this case, the content of impurity elements does not exceed 3%. The influence of obtaining conditions on the granulometric characteristics of the powder is established. It is shown that powders with a larger specific surface (64.4 × 10⁵ m⁻¹) are formed with the use of aluminum in concentrated solutions of the starting compound in the NaCl melt.     

Ferritic Fine Structure Characteristics in Hot Die Forged Powder Steels. I. Effect of Heating Temperature during Die Forging on Ferritic Fine Structure in Specimens of Powder Technical Grade Iron and Carbon Steels

We have studied the effect of the composition and heating temperature during die forging on the microstructure and fine structure of ferrite in powder iron and powder carbon steels. From the structural characteristics obtained for different planes of the blank, we used harmonic analysis of the x-ray line shape to establish the nonuniformity of deformation over the volume. We have established the heating temperature during die forging of powder technical grade iron and carbon steels, for which the deformation is close to uniform over the die forging volume: 1050 °C for technical grade iron and low-carbon steel, 1100 °C for carbon steel. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 108–119, May–June, 2005.     

Kinetics of formation of interparticle contacts in the laser sintering of single component metallic powders

The joining of two spherical particles of nickel alloy powder under the action of lager irradiation, and the lager sintering of thin layers of poured powder were investigated. It was determined that powder particles experience surface melting under the action of lager heating, and join by a welding mechanism Institute of Technical Acoustics, Belorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 54–62, January–February, 1997.     

A Novel Method for Direct Synthesis of WC-Co Nanocomposite Powder

In this study, a novel method, termed dielectric-barrier-discharge-plasma (DBDP) assisted ball milling and low-temperature carburization, was used to synthesize WC-Co nanocomposite powder. X-ray diffraction, scanning/transmission electron microscopy, and differential scanning calorimetry were used to characterize the microstructure of powders. Starting from W, Co, and graphite powder mixtures, the DBDP-milled W-C-10Co powder exhibited a flakelike morphology with very fine lamellar structure. The WC-Co composite powder was synthesized at 1273 K (1000 °C), which is much lower than the requisite temperature for the conventional carburizing method. The obtained WC-Co composite powder had a nanocomposite microstructure in which fine WC particles were bounded by homogenously distributed Co phase, and the WC crystals had a slablike morphology with a planar size of about 200 nm and <100-nm thickness. The combinational effect of the milling and the plasma in the DBDP milling caused a unique fine flakelike morphology and high-density interfaces in the W-C-10Co powder mixture, which is responsible for the reduced carburization temperature and the nanocomposite structure of WC-Co powder.     

Effect of carbon on volume changes during the sintering of an iron-chromium material

The effect of carbon added in the form of either cast-iron powder or graphite on the sinterability of chromium steel was investigated. The growth and instability of volume change during sintering produced by cast-iron additions was explained as a result of a thermally activated after-effect in the solid phase, and the formation of FeCr₂O₄ around particles of Kh30. Substitution of graphite for cast iron in the powder charge prevented oxidation of the chromium-rich particles, and resulted in low and stable shrinkage (0.1–0.8%) after sintering in the range 750–1250°C. Scatter in the degree of volume change was reduced from 1.5–2.0 to 0.1–0.3%. Iron-chromium material obtained with the use of graphite had improved strength properties as well as high produceability. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 38–42, May–June, 1998     

Magnesiothermal Synthesis and Consolidation of the Multicomponent Powder Ceramics in the Zr–Si–Mo–B System

This work is devoted to the fabrication of ZrB2–ZrSi2–MoSi2-based composite powder ceramics by self-propagating high-temperature synthesis (SHS) according to the scheme of magnesium thermal reduction from oxide feedstock and its subsequent hot-pressing (HP) consolidation. The combustion of reaction mixtures has rather high adiabatic temperatures in a range of 2060–2120 K and burning rates in a range of 8.3–9.4 g/s. The end product yield of the magnesiothermal reduction is 34–38%. The resulting powder contains 13–47% ZrB2, 21–70% ZrSi2, 2–32% ZrSi, and 10–18% MoSi2, depending on the composition of the initial reaction mixture; has high structural homogeneity; and consists of composite polyhedral particles with an average size on the order of 8 μm. The structure of ceramics consolidated by HP from the SHS powder is homogeneous and includes needle ZrB2 grains distributed in the ZrSi2 matrix, MoSi2 inclusions of various morphologies, and inclusions of ZrSiO4 silicate distributed over the ZrSi2 grain boundaries. The HP-formed samples have a high degree of homogeneity of the chemical composition and residual porosity of 2.5–7.4%.     

Particle interaction in the atomization of high-speed steel

Conclusions Liquid, solid, and semisolidified particles in an atomized metal-gas spray experience numerous collisions, which exert a strong influence on the end structure of the bulk of the resultant powder. Very fine, rapidly solidified particles colliding with larger drops act as foreign inner centers of solidification. Collisions between previously solidified particles and drops of smaller weight are accompanied by a sharp increase in cooling rate during solidification (to 10⁶–10⁷ deg C/sec) and the formation on the particles of shells with a very fine-columnar structure (without any visible-signs of primary carbide evolution).Translated from Poroshkovaya Metallurgiya, No. 3(255), p. 7–13, March, 1984.     

Microstructure and tensile properties of Fe3Al produced by combustion synthesis/hot isostatic pressing

Combustion synthesis was carried out in a hot isostatic press (HIP) in order to prepare near-theoretical density Fe₃Al and Fe₃Al + Cr alloys from elemental powder mixtures. The microstructures and room-temperature tensile properties of these materials were studied in the as-synthesized condition and after heat treatment. As-synthesized materials exhibit a fine, equiaxed grain structure with grain sizes typically less than 10 μm. Yield and ultimate tensile strengths were found to be significantly higher than what has been reported for conventionally processed materials having similar composition. Although lower ductility was generally observed, elongations exceeding 5 pct were obtained in heat-treated Cr-containing alloys. Fracture occurred predominantly by transgranular cleavage. The strengthening of these materials is attributed to the fine grain size resulting from combustion synthesis/HIP processing. Transmission electron microscopy (TEM) revealed the presence of two distinct populations of aluminum oxide particles in the material. Nanometer-sized oxides exist within grains that likely formed at prior iron particle boundaries, and a high density of larger oxides exist along grain boundaries that probably originated from surface oxides on the aluminum powder. The presence of the grain boundary oxides was qualitatively observed to provide resistance to grain growth. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.