Powder Metallurgy and Metal Ceramics (v.51, #5-6)

Effect of boron additives on the structure and properties of soft magnetic composites produced from nickel-clad iron powders by N. V. Boshitskaya; O. V. Vlasova; I. V. Uvarova; L. M. Appininskaya; O. I. Get’man (253-259).
The effect of boron additives on the structure, magnetic properties, and corrosion resistance of Ni– P-clad iron-based soft magnetic materials is studied. The iron powder (with a particle size of 250 to 350 μm) is clad with nickel–phosphorus through thermochemical reduction of nickel chloride by sodium hypophosphite, and boron (1 and 3 wt.%) is added to activate the formation of a liquid phase during high-temperature sintering and reduce the porosity of the prepared composites. It is shown that the cladding of iron powder with nickel–phosphorus increases its corrosion resistance by two points on the scale of ISO 11130:2010 and decreases the depth corrosion index from 0.6457 mm/year to 0.0269 mm/year, which is likely due to the high corrosion resistance of the nickel-phosphorus coating. Moreover, the cladding of iron powder with nickel–phosphorus substantially decreases (2–2.5 times) the magnetic loss in ac fields at a frequency of 50 Hz. The microstructure of the Fe–Ni–P–B composites with different boron contents is heterophase and consists of iron-based ferritic grains, pores, and a liquid phase based on the γ-Fe + Fe2B, α-Fe + Fe3P, and Ni + Ni3B eutectics. The boron content of the material should not exceed 1% because a decrease in the volume of the ferromagnetic component reduces the magnetic induction and permeability. The addition of boron to the clad iron powder increases the hardness and strength of the material produced.
Keywords: iron powder; nickel–phosphorus; cladding; boron; sintering

The method of granular dynamics is used to study the quasistatic uniaxial compaction of nanopowders with a particle size from 10 nm to several hundred nanometers. The interaction of individual particles includes Hertz elastic forces, Cattaneo–Mindlin friction forces, and van der Waals dispersion forces of attraction. The influence of the model cell size on simulation results is analyzed. The curves of uniaxial compression and elastic unloading in the “axial pressure–density” coordinates are plotted. The generalization of the traditional Hertz law in the range of relatively high strains is discussed.
Keywords: nanopowder; compaction; size effect; elastic unloading; Hertz law

Thermal tests and their effect on the micro- and macrostructure of nanocrystalline ZrO2 by S. P. Buyakova; V. V. Promakhov; S. N. Kulkov (267-272).
The paper examines the effect of thermal tests involving water quenching at 1000°C on structural and phase transformations in nanostructured ceramics based on ZrO2 partially stabilized by Mg+2 in high-temperature modifications. The ceramics have different amounts of magnesia in the ZrO2–MgO solid solution and, as a consequence, different ratios of high- and low-temperature modifications of zirconia. It is shown that the ZrO2–MgO solid solution decomposes regardless of its magnesia content with increasing number of thermal tests, but the ceramics remain resistant to thermal shock even in case of complete MgO removal, i.e., zirconia destabilization.
Keywords: thermal test; x-ray diffraction; high-temperature modifications of ZrO2

High fracture toughness of powder chromium sintered in magnesium vapor by I. G. Slys; N. P. Brodnikovskii; I. A. Kossko; M. E. Golovkova; Yu. Yu. Chernenko (273-281).
The fracture toughness of powder chromium sintered in magnesium vapor is higher by a factor of 53 than that of powder chromium sintered in hydrogen and by a factor of 5 than that of deformed low-alloy chromium VKh2K castings. This high fracture toughness is due to the skeleton formed of plastic interlayers of high-purity chromium. Chromium becomes highly ductile after fine purification in Cr–MgO alloys to remove interstitial impurities. The interlayers form on the surface of chromium powder particles under the refining action of magnesium vapor. Auger electron microscopy and data on fracture, chemical composition, and etching resistance lead to the conclusion that there are interlayers made of pure chromium. The high fracture toughness remains after annealing for 1 h at 1500°C.
Keywords: chromium; magnesium; activated sintering; refining; fracture toughness; crack resistance; plasticity

The paper examines the wear mechanisms of the VT-3 titanium alloy and TBKhN composite in fretting corrosion conditions. It is shown that the oxide film formed on the titanium alloy during friction fails and further wear occurs on the juvenile surface. This leads to seizure and removal of the material from the surface, indicating that adhesive wear prevails in friction of the titanium alloy. The TBKhN40 composite is characterized by oxidative wear. The oxide film with grain-oriented fine structure forms in the friction process through selective oxidation of refractory and metal components.
Keywords: fretting corrosion; composite; titanium–chromium diboride; wear resistance; wear mechanism; oxide film

Powder metallurgy methods are used to produce soft magnetic compact materials from amorphous and nanocrystalline alloys. Toroid-shaped samples are warm-pressed from nanocrystalline Fe73Si15B7.2Cu1Nb3 alloy powders and SFP-012A resin binder. The method does not require additional thermal treatment and the samples are ready to use. The key influence on the properties of soft magnetic powder materials is produced by ferromagnetic filling factor (FFF). The optimal experimental conditions to make samples with the minimum porosity and maximum FFF are determined. The density of the samples depending on compaction parameters and resin content is established.
Keywords: soft magnetic material; metal polymer; nanocrystalline powder; phenol formaldehyde resin; ferromagnetic filling factor; porosity; warm pressing

The interaction between nanosized powder of a solid solution of carbon in silicon carbide and iron oxide (as iron oxide pigment and disperse sintered iron ore concentrate) in high-temperature heating is studied. The quantitative phase composition of the products is analyzed. The potential reactions between silicon carbide and iron oxide are examined. It is established that the (SiC–C)–Fe2O3 powder mixtures interact to produce a heterophase structure through the reduction of iron and the formation of silicides (ε-FeSi, Fe2Si, Fe3Si, Fe5Si3), carbides (Fe3C, Fe7C3), and iron (α-Fe) as fine particles with homogenous microstructure.
Keywords: solid solution of carbon in silicon carbide; sintered iron ore concentrate; iron oxide; iron silicide; high-carbon iron carbide

The coefficients of thermal expansion of transition-metal borides, lanthanum hexaboride, and eutectic composites thereof are calculated using the a-priori pseudopotential method. To determine the energy of the electron–ion system as a function of the lattice parameters at nonzero temperatures, it is proposed that the energy scales of the system be compared at absolute zero and nonzero temperatures. The coefficient of thermal expansion of the composites shows an abrupt increase in the temperature range from 750 to 1000 K.
Keywords: a priori pseudopotential; electron–ion system; coefficient of thermal expansion; eutectic

Effect of the particle size of conductive inclusions on the structurization and resistivity of Si3N4–ZrC ceramics by I. V. Brodnikovska; M. V. Soltis; K. V. Kirilenko; I. V. Zvorskii; V. Ya. Petrovskii (307-315).
Heating elements with a resistive layer composed of Si3N4 and 13.5 vol.% ZrC with an average grain size varying from 1 to 46 μm are produced by hot pressing. The phase and grain-size composition and morphology of zirconium carbide grains are examined with X-ray diffraction, sedimentation analysis, and scanning electron microscopy. Microstructural images of the hot-pressed composites are taken with an optical microscope. Direct current resistivity is studied by voltmeter–ammeter method in the temperature range between 20 and 600°C. Different types of structure may form at different dielectric–conductor diameter ratios. These structures considerably differ in resistivity. The resistivity of the composites depends on the specific surface area of the conductive particles and effective cross-sectional area of the conductive cluster (resistivity increases by one order of magnitude with transition from statistical to matrix structure). It is shown that the structure and resistivity can be controlled by introducing a bimodal mixture of coarse and fine conductive particles. The matricity of the composite is evaluated by measuring its resistivity.
Keywords: silicon nitride; zirconium carbide; grain size of conductive particles; microstructure; resistivity; percolation; nondestructive monitoring

The Al2O3–HfO2–Gd2O3 phase diagram. II. Vertical sections by Ya. S. Tishchenko; S. M. Lakiza; L. M. Lopato (316-322).
For more complete description of the Al2O3–HfO2–Gd2O3 phase diagram, three vertical sections are constructed in a wide temperature and composition range. The Gd2O3 and HfO2 corner bisectors explain the mechanisms of Gd2O3 phase transformations, C ⇆ X ⇆ H ⇆ A ⇆ B, and interactions in the HfO2 corner of the system. The isopleth at 15 mol.% HfO2 (15H) shows the Al2O3–HfO2–Gd2O3 constitution in Al2O3- and Gd2O3-rich regions.
Keywords: ceramics; hafnia; alumina; gadolinia; interaction; vertical sections; eutectic materials

Components of the high-temperature heat capacity of REM silicides, Ln5Si3, and variation in their melting enthalpies in the Gd5Si3 → Ln5Si3 series are analyzed. A method for calculating the high-temperature thermodynamic properties of the compounds still to be experimentally studied is proposed. The experimental temperature dependences for the enthalpies of Ln5Si3 (Ln = Gd, Er, Lu) are used to calculate the melting enthalpies and temperature dependences of the heat capacity for other yttrium-subgroup silicides.
Keywords: enthalpy; heat capacity; thermodynamics; silicide

Thermodynamics of formation of HoGe3–x and HoGe2–y higher holmium germanides by L. V. Goncharuk; V. R. Sidorko; I. M. Obushenko (329-332).
The Gibbs energy, enthalpy, and entropy of formation of HoGe3–x (HoGe2.7) and HoGe2–y (HoGe1.8) from solid compounds are determined by emf measurement in the temperature range 770–965 K. The formation enthalpy of the higher holmium germanide is in good agreement with ∆ f H° (HoGe2.7) calculated from the limiting mixing enthalpy of Ho in liquid germanium on the assumption that $ varDelta {ar{H}_{Ho}} $ for solid alloys in the [HoGe2.7 + Ge] region remains the same as $ varDelta ar{H}_{Ho}^infty $ for liquid alloys.
Keywords: thermodynamic properties; Gibbs energy; enthalpy; entropy; holmium; germanium

The structural and phase mechanism and the rate of TiCu, Ti3Cu4, and Ti2Cu3 destructive hydrogenation (DH) are studied at 773 K under a hydrogen pressure of 1.0 MPa. The mechanism of destructive hydrogenation of the intermetallics consists in the formation of a hydrogen solid solution, selective hydrogenation of titanium, and subsequent formation of intermediate intermetallics and copper. The destructive hydrogenation products are nondestructive TiH1.9–Cu matrix composites. The rate of TiCu, Ti3Cu4, and Ti2Cu3 destructive hydrogenation is limited by the rates of hydrogen dissociation on the intermetallic surface and the diffusion of titanium hydride. The products of TiCu, Ti3Cu4, and Ti2Cu3 destructive hydrogenation are recombined in vacuum and hydrogen atmosphere.
Keywords: intermetallics; TiCu; Ti3Cu4 ; Ti2Cu3 ; phase mechanism; microstructure; destructive hydrogenation; recombination

The electrochemical oxidation of α-Al–Mg2Si–Sc alloys doped with small amounts of Zr and Mn in a 3% NaCl solution is studied using the potentiodynamic anodic polarization method, quantitative Auger electron spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and atomic absorption spectroscopy. It is established that there are two ranges of anodic potential (from stationary to –0.71 V and higher) and, accordingly, two stages of corrosion: (i) formation of protective films containing α-Al2O3, SiO2, and silicium solid phases in a protective film on the samples and passage of Mg2+ and SiO3 2– ions into solution and (ii) formation of lower magnesium silicide, MgSi, in addition to the solid phases. It is shown that zirconium and manganese serve as cathodic areas and substantially inhibit the corrosion of the alloys at the second (basic) stage (especially at Zr and Mn aggregations).
Keywords: aluminum; manganese; silicon; scandium; zirconium; alloy; electrochemical corrosion; polarization curves; cathodic areas

Effect of magnetic treatment on the microstructure and abrasive resistance of WC–Co detonation-sprayed coatings by V. E. Oliker; T. Ya. Gridasova; I. I. Timofeeva; E. F. Grechishkin; A. G. Gavrilenko (345-352).
The effect of magnetic field on the phase composition and mechanical properties of VK-9 and VK-15 hardmetal detonation-sprayed coatings is studied. It is established that the magnetic field induces phase and structural transformations in sprayed coatings, such as reinforcement of the Co matrix due to the dissolution of WC in Co and the formation of W x Co y C z binary carbides. Magnetic treatment reduces residual internal stresses in the coating, decreases the amorphicity of its structure, and increases abrasive wear resistance.
Keywords: hardmetals; abrasive wear; detonation-sprayed coating; magnetic treatment

Effect of inhomogeneous deformation on the electronic structure of SnO2 and Sn x Sb1–x O2 phases by V. I. Ivashchenko; B. M. Rud; A. G. Gonchar; L. A. Ivashchenko; O. O. Butenko (353-362).
The electronic structures of various phases in the Sn–Sb–O2 system under pressure and under tetragonal, monoclinic, and orthorhombic deformation are studied. Calculations are performed using the first-principles pseudopotential method. It is established that SnO2 undergoes the following phase transitions under pressure: rutile–pyrite (17 GPa) and pyrite–fluorite (138 GPa). It is also found that doping SnO2 with Sb leads to a shift of the Fermi level to the conduction band and to additional resonant states below the valence band. Inhomogeneous deformation of Sn x Sb1–x O2, x = 1.00; 0.94; 0.88, at δ ≤ 0.2 causes a stress up to 6.2 GPa, depending on the strain. An analysis of the density of electron states in the bandgap of the deformed structures shows that the gap broadens under tetragonal deformation and narrows under orthorhombic and monoclinic deformation. The theoretical results obtained should be taken into account in interpreting the piezoresistive properties of Sn–Sb–O2-based thick films.
Keywords: high-pressure phases; molecular dynamics; phase transitions; mechanisms of stabilization of metastable phases

Effect of CaF2 surface layers on the friction behavior of copper-based composite by K. Konopka; T. A. Roik; A. P. Gavrish; Yu. Yu. Vitsuk; T. Mazan (363-367).
The paper studies the effect of the process parameters for the production of new copper-based bearing composite materials doped with nickel and molybdenum, with CaF2 additions (DN5M3KF9) as a solid lubricant, on the structurization and tribotechnical properties at high-speed friction (bearings perform at a sliding speed of 6 m/sec and a pressure up to 38.5 MPa in air). The study focuses on the distribution of CaF2 in the composite with a Cu–Ni–Mo matrix, its role in self-lubrication of the material, and the behavior of CaF2 in the friction area under extreme operating conditions. It is shown that the solid lubricant is evenly distributed over the contact surfaces as it piles up over the entire friction area during high-speed friction. It is established that the tribofilms formed in the presence of the CaF2 solid lubricant provide high wear resistance of copper-based materials.
Keywords: high-speed bearings; solid lubricant; copper; process parameters; operating conditions; friction films; wear resistance

A design of an injection molding machine for the fabrication of complex-shaped parts from mixtures of thermoplastics and micro- and nanosized powders is presented. This machine provides uniform temperature distribution in the barrel and formation of parts with required qualitative characteristics. The machine was used to produce a pilot batch of paint spray nozzles from micro-and nanosized powders.
Keywords: injection molding; thermoplastic; barrel; ram; mixer; gate; nozzle; mold

Graded porous titanium scaffolds fabricated using powder metallurgy technique by X. P. Fan; B. Feng; Y. L. Di; J. X. Wang; X. Lu; J. Weng (372-377).
Graded porous titanium scaffolds with a pore size of 300–400 μm are prepared by sintering. Their mechanical properties and porosity are investigated. Compared with common titanium scaffolds, the mechanical properties of graded specimens can be improved by introducing graded structure. The graded porous titanium scaffold consists of inner, middle, and outer layers. When the outer layer porosity of the graded specimen is increased from 67 to 72%, the Young’s modulus of the graded specimen increases from 3.29 to 4.72 GPa, meanwhile the compressive strength increases from 131.9 to 165 MPa, respectively. These results suggest that the graded porous titanium scaffold has potential application for tissue engineering scaffolds under load-bearing conditions.
Keywords: titanium; sintering; foams; mechanical properties; graded structure

Erratum to: The Co–Ga–Si ternary system at 870 K by P. Ya. Lyutyy; A. O. Fedorchuk (378-378).