Powder Metallurgy and Metal Ceramics (v.50, #11-12)

The paper proposes a method to determine the kinetic parameters of temperature-controlled hot pressing from the current temperature and height of specimens. This method substantially reduces the scope of experimental work in comparison with the conventional method for studying isothermal hot pressing. The kinetics of the densification of boron carbide powder and its composites with 20, 50, and 80 wt.% Al2O3 and 2 wt.% Al is studied during nonisothermal hot pressing. It is experimentally established that the kinetics of densification is controlled by the creep mechanisms in the matrix that forms a porous body. The creep activation energy for the matrix is estimated to be 8.1 eV for boron carbide, 7.6 eV for B4C + 20% Al2O3 + 2% Al, and 6.6 eV for B4C + 80% Al2O3 + 2% Al. The densification of the composite with 50 wt.% Al2O3 is controlled by the weekly temperature-dependent viscous flow of the matrix, for which the estimated activation energy is 1.7 eV. At higher temperatures, the viscous flow of the matrix is controlled by nonlinear creep the activation energy of 4.6 eV, which may correspond to superplasticity. The structure, phase constitution, and some mechanical properties of the composites are studied.
Keywords: nonisothermal hot pressing; kinetics; boron carbide; composite materials; structure; mechanical properties

The hot forging of porous preforms with an axial hole in a semiclosed die with an internal flash gutter is numerically simulated. The distribution of deformation and density over the cross-section of the preform is substantially different at separate stages of the process. The maximum axial and radial strains are observed near the flash gutter. Enlarging the compensation slot over the optimum size causes residual porosity in some areas of the forging.
Keywords: hot forging; porosity; deformation; densification; numerical simulation

The effect of alternating current on the formation of phases and structure during the synthesis of TiN + TiB2 composite by spark-plasma sintering of TiH2 + BN powder mixtures employing a superposition of alternating and direct currents is studied. The sintering leads to composites with semiconductor and dielectric phases (Ti and BN oxides) if a direct current is used and to TiN + TiB2 composite with metallic conductivity if a superposition of direct and alternating currents is used. The alternating current intensifies the ion electromigration between particles during spark discharge. This accelerates the decomposition of α-BN by atomic hydrogen and, as a result, intensifies the formation of TiN + TiB2 composite.
Keywords: TiN + TiB2 ; spark-plasma sintering; superposition of alternating and direct currents; spark discharge

A method is developed for determining the temperature of the substrate in the area of electrospark deposition. The method is employed during experiments on deposition of two types of electrodes onto steel substrates: carbon steel electrodes and hard-alloy electrodes based on tungsten carbide with 6 wt.% cobalt. It is established that the dependence of substrate temperature on deposition time is defined by the initial structure of the substrate, electrode material, and pressing force on the electrode. Analysis of the data shows that cathode jets are the main working element of the electrospark deposition process.
Keywords: heating kinetics; substrate; electrospark deposition; coating; steel; hard alloy

The paper examines the structure of low-alloy high-speed spray-formed steel after plastic deformation and thermal hardening. It is established that spray-formed steel is satisfactorily deformed under hot plastic deformation (1100 ± 20°C) and subsequent deformation in the temperature range of solid-phase transformation (800 ± 20°C). The steel hardening temperature is 1190°C and heat resistance 620°C at HRC59. The bending strength of hardened deformed steel is 3300–3500 MPa and fracture toughness 230–250 kJ/m2 at 64–65 HRC.
Keywords: spray formation; plastic deformation (rolling); thermal treatment; structure; mechanical properties

Hot pressing of dysprosium hafnate and titanate pellets by V. S. Krasnorutskii; S. Yu. Saenko; N. N. Belash; I. A. Chernov; A. E. Surkov; N. D. Rybalchenko; F. V. Belkin (708-713).
The paper discusses the effect of quasiisostatic hot pressing conditions in compressible elastic media in the temperature range 1050–1850°C on the properties and phase composition of pellets made of dysprosium hafnate and dysprosium titanate, which are considered to be promising absorber materials for control rods of water-cooled water-moderated nuclear reactors. The hot pressing conditions that promote production of high-density radiation-resistant pellets are determined. The mechanisms of Dy2TiO5 and Dy2Hf2O7 phase formation are discussed. The microhardness of the absorber material after hot pressing at different temperatures is determined.
Keywords: absorber materials; dysprosium hafnate; dysprosium titanate; hot pressing; phase composition; density; microhardness

Influence of mechanical activation on electrical properties of barium–zinc–titanate ceramics sintered at 1100°C by N. Obradovic; S. Filipovic; M. Mitric; V. Pavlovic; V. Paunovic; D. Kosanovic; I. Balac; M. M. Ristic (714-718).
Starting mixtures of BaCO3, ZnO, and TiO2 powders are mechanically activated in a planetary ball mill for various periods of time. The powders obtained are sintered isothermally at 1100°C for 120 min. Non-isothermal sintering process is followed by measurements with a sensitive dilatometer, as well. Results of microstructure characterization using SEM analyses along with DTA analyses are provided. These results are correlated with the values of electric resistivity, capacitance, and loss tangent of the samples, and it is found that with increasing milling time, the increase in values of electrical properties is noticed.
Keywords: milling; sintering; SEM; DTA; ceramics

Composites based on TiB2–SiC with a nickel–chromium matrix by M. S. Storozhenko; A. P. Umanskii; V. A. Lavrenko; S. S. Chuprov; A. D. Kostenko (719-725).
New TiB2–SiC composite materials with a nickel–chromium matrix are obtained. The process parameters of hot pressing of these advanced materials are optimized and their structure and mechanical properties are studied. The tribotechnical parameters and wear mechanism of these composites are established. The wear mechanism consists in the formation of TiO2–B2O3–SiO2 protective films on friction surfaces. It is shown that the amount of the metal matrix (Ni–20 % Cr) influences the thickness of oxide films and, hence, the wear rate. The composite with 40 % metal matrix has high wear resistance owing to the formation of a protective film with the optimal thickness (120 nm) on friction surface.
Keywords: composite material; titanium diboride; silicon carbide; hot pressing; sintering; structure; wear resistance; wear mechanism; high-temperature oxidation

Based on constitution of the solidus surface of the Ti–TiRh–AlRh–Al partial system and on metallography, X-ray diffraction, electronic microprobe, and differential thermal analyses of its ascast alloys, the liquidus surface projection of the system is constructed onto the concentration triangle for the first time and the processes occurring in the crystallization of its alloys are studied. This has given an opportunity to construct the melting diagram of the Ti–TiRh–AlRh–Al partial system for the first time. Its liquidus surface is completed with 13 surfaces of primary crystallization of solid solutions based on components and phases based on binary and ternary compounds. In the Ti–TiRh–AlRh–Al partial system, there are 13 invariant four-phase equilibria involving liquid as well as nine invariant three-phase equilibria, eight of them being eutectic and one peritectic.
Keywords: liquidus surface; component; equilibrium; melting diagram

Mixing enthalpies of liquid nickel–gallium alloys by V. S. Sudavtsova; N. V. Kotova; V. G. Kudin; L. A. Romanova; N. I. Usenko (740-743).
The partial mixing enthalpies of melt components in the Ni–Ga system are measured using hightemperature isoperibolic calorimetry at 1770 ± 5 K in a wide composition range. The partial mixing enthalpy of gallium in liquid nickel is –96 kJ/mol. for infinite dilution, while the same function of nickel in liquid gallium is –75 kJ/mol. The integral mixing enthalpy of liquid alloys in this system is calculated from partial enthalpies for the entire composition range (∆m H min = –32 kJ/mol. at xNi = = 0.5). The mixing enthalpies ∆m H of Ni–B (Al, Ga, In) binary alloys are compared. It is shown that the indicated systems are arranged in the Ni–In → Ni–Ga → Ni–B → Ni–Al series according to the increase in interaction energy of dissimilar components. The ∆m H min values of liquid Ni–B(Al, Ga, In) alloys are compared with the values of electrochemical and size factors.
Keywords: calorimetry; nickel; boron; gallium; indium; mixing enthalpies

The Cu–Co–Fe system in CO oxidation reaction in the presence of hydrogen by V. L. Veselovskii; V. K. Yatsimirskii; O. V. Ishchenko; S. V. Gaidai; B. G. Mischanchuk; M. I. Zakharenko (744-748).
The Cu–Co–Fe oxide system is studied in the CO oxidation reaction in the presence of hydrogen. It is shown that the presence of hydrogen in the reaction mixture contributes to the transformation of the Cu2(OH)3NO3 phase into the superfine CuO phase, which is more active in CO oxidation in the presence of hydrogen, compared with pure CuO. It is determined that the oxidation of H2 begins only after almost complete CO conversion into CO2 for the samples containing the Cu2(OH)3NO3 phase.
Keywords: oxides of transition metals; catalytic oxidation of carbon monoxide in the presence of hydrogen

Electrochemical oxidation of ZrB2–MoSi2 ceramics in a 3% NaCl solution by V. O. Lavrenko; V. A. Shvets; V. M. Talash; V. A. Kotenko; T. V. Khomko (749-753).
Electrochemical oxidation of ZrB2 and MoSi2 refractory compounds as well ZrB2–MoSi2 ceramics in a 3% NaCl solution is studied using the method of polarization curves, quantitative AES, and SEM. It is established that the highest corrosion resistance is observed in the case of composites with a small amount (~5–10 wt.%) of MoSi2 due to their stable passivation at comparatively low anodic potentials. It is shown that the corrosion rate of a molten ZrB2 sample is 90% lower than that of a hot-pressed one.
Keywords: ceramics; zirconium diboride; molybdenum disilicide; corrosion; kinetics of anodic oxidation; phase composition; oxide film

Structural studies of rare-earth activated cubic boron nitride micropowders by E. M. Shishonok; J. W. Steeds; A. V. Pysk; E. O. Mosunov; O. R. Abdullaev; A. S. Yakunin; D. M. Zhigunov (754-767).
Rare-earth (RE) activated cBN-based materials were produced as cBN–Tb, cBN–Ce, cBN–Eu, and cBN–Tm micropowders with 0.1 at.% RE in HPHT conditions. Their X-ray diffraction spectra are compared with those of a standard unactivated cBN micropowder. It is established that structurally cBN–RE materials are interstitial solid solutions based on interstitial solid solutions because there is initial distortion in the unactivated cBN due to its nonstoichiometry. The atomic displacements in the cBN crystal lattice caused by interstitial RE ions are calculated from XRD reflection intensities. The displacements correlate with the cBN lattice distortions evaluated from the regularly nonlinear relationship between the cBN–RE lattice parameters (calculated for each reflection) and the Nelson–Riley function, unlike the unactivated cBN micropowder where this relationship is linear. The degree of nonlinearity depends on the RE content of cBN and the size of RE ions and can correlate with larger shifts of the (111), (220), and (331) reflections toward smaller angles, compared with the shift of (331). The XRD and Raman spectra of cBN micropowders with 0.1 at.% RE of the same kind reveal no stacking faults in the crystal lattice. However, it is not improbable that such faults might form in cBN in which the concentration of RE of one kind is higher than 0.1 at.% or there are different kinds of RE ions. The latter is supported by the TO–LO splitting in the Raman spectra of cBN–Ce–Tb (~0.1 at.% RE) micropowders, which has never been observed in cBN. The RE concentration in cBN is no higher than 0.1 at.%, which is less than that (10 at.% Er) in cBN nanopowders achieved so far by another scientific group using the plasma method. Therefore, studies aimed at obtaining light-emitting materials based on cBN will be continued to reach higher luminescence of RE ions in cBN. Since no data have been previously provided on the crystal structure of light-emitting materials based on RE-activated cBN, it is concluded that structurally new cBN-based materials have been produced.
Keywords: cubic boron nitride; rare-earth elements; XRD spectrum; Raman spectrum; light-emitting properties of cBN

Permeable microporous materials (average pore diameter <10 μm and porosity 50–90 %) made from superfine (2, 5, and 10 μm in diameter) metal fibers are produced by fiber metallurgy methods. The structural parameters and hydrodynamic characteristics of the materials are determined and used to evaluate their filtration, capillary, and reinforcing properties. Cleanable fine filters and porous electrodes for electrochemical cells made from such materials are designed.
Keywords: superfine metal fibers; microporous materials; porosity; pore size; permeability; filtration and capillary properties