Powder Metallurgy and Metal Ceramics (v.49, #3-4)

The paper examines the processes that occur during thermal treatment (sintering) of heterophase nanocrystalline ZrO2–Y2O3–CeO2–Al2O3 powders. It is shown that the disperse system is selforganized in various evolutionary ways. The common behaviors of ZrO2–Y2O3–CeO2–Al2O3 nanocrystalline powders are determined. The results offer possibilities for selecting initial nanocrystalline powders to develop fine-grained high-performance ceramics.
Keywords: ZrO2–Y2O3–CeO2–Al2O3 system; self-organization; evolution; nanocrystalline powder; zirconia

The paper considers the principle of superposition of density functions as an alternative to the available methods to analyze the grain-size characteristics of the charge with different powder fractions. These powder mixtures are used in the production of tool and ceramic materials.
Keywords: distribution density; tungsten carbide; histogram; size; superposition; fraction

Collective processes in liquid-phase reaction sintering by V. P. Solntsev; V. V. Skorokhod; T. A. Solntseva (141-146).
An experimental thermokinetic study of reaction sintering reveals that there are various irreversible paths in a nonequilibrium system. When the external temperature is in the contact melting range and the equilibrium condition of the mixture is in the solid-state range, the thermokinetic path is usually exponential with abrupt increase in the temperature and volume of the samples. Active compaction is observed in the case of thermokinetic oscillations
Keywords: nonequilibrium processes; reaction sintering; thermokinetics; rheological behavior; growth; shrinkage

Initial kinetics of microwave sintering of copper by D. N. Demirskii; A. V. Ragulya (147-152).
Model experiments for the initial stage of microwave sintering have revealed volume diffusion that can be accompanied by evaporation–condensation and surface diffusion. The volume diffusion is also confirmed by closer distances between the particles. The experimental data show that the mass transfer is intensified during microwave heating.
Keywords: microwave heating; sintering; initial sintering stage; diffusion; copper

Composites based on biogenic hydroxyapatite and Na2O–B2O3–SiO2 glass with the weight ratio 61 : 39 are produced. The effect of sintering temperature between 600 and 800°C on their basic properties is analyzed. To examine the effect of temperature, changes in the structure, linear and volume parameters, and density and porosity of the samples during sintering are studied. The pycnometric density of the composite is 2.904–3.009 g/cm3, apparent density 1.22–1.85 g/cm3, general porosity 36.2–59.5%, content of open porosity 0.14–0.93, and solubility in physiological solution (48 h) 0.38–1.69 wt.%. It is established that the pore structure of the composites can be controlled by variation in sintering temperature.
Keywords: hydroxyapatite; sintering; porosity; solubility

Anisotropy of transient high-temperature bending creep of single crystals by V. P. Matsokin; M. V. Milyi; I. N. Pakhomova; D. V. Matsokin (159-166).
The orientation dependence of the transient high-temperature creep rate of NaCl and KCl single crystals under nonuniform stresses is experimentally studied. The anisotropy of high-temperature bending creep of single crystals is ascertained by analyzing the distribution of bending stresses, the number of possible dislocations providing deformation, the reduced shear stresses causing sliding of dislocations, and the dislocation structure of single crystals after creep. It is shown that hindered dislocation sliding in the easy sliding system activates a new slip system at a certain temperature.
Keywords: creep; stresses; deformation; dislocation; anisotropy

A model for sintering of porous ceramic material is applied to study the densification behavior of a porous Al2O3 system that contains pores of different sizes. In the model, a sintering potential associated with the shrinkage of large macropores is considered separately from that of small pores. The predictions from the model are found to be in good agreement with the experimental data. The results show that consideration of the shrinkage of large macropores, which is traditionally assumed to be very stable, provides a more accurate prediction results for the densification of a multi-porous body.
Keywords: Al2O3 ; macropores; sintering; porous; constitutive model

Corrosion resistance of nanostructured TiB2 films in 3% NACl solution by A. S. Dranenko; V. A. Lavrenko; V. N. Talash (174-178).
The paper examines the corrosion behavior of amorphous TiB2 films 70–250 nm in thickness and amorphous–crystalline films with crystals 15–90 nm in size in 3% NaCI solution. It is shown that the corrosion resistance and passivation anodic potential increase with thickness of TiB2 amorphous films. It is also established that TiB2 films are oxidized through pitting corrosion. The corrosion instability of amorphous films is mainly due to their interaction with impurity (in particular, oxygen and carbon) structural inhomogeneities and of amorphous–crystalline films due to the interaction with amorphous–crystalline boundaries. The corrosion resistance of amorphous TiB2 films is approximately 4000 times higher than that of bulk powder material and 8 to 10 times higher than that of amorphous–crystalline films.
Keywords: thin films; electrochemical corrosion; nanostructure; borides

The paper examines the structure of tool (corrosion-resistant) steel produced by spray forming followed by thermomechanical treatment. The optimal conditions for steel deformation are established. It is shown that the fine carbide component forms in melt quenching. As a result, there is no carbide inhomogeneity in alloyed tool steel after plastic deformation. The steel structure remains superfine after high-temperature (1090°C) hardening treatment. The bending strength of deformed hardened steel is 2500–2600 MPa, impact strength 230–200 kJ/ m2, and hardness 56–57 HRC.
Keywords: spray forming; plastic deformation (rolling); thermal treatment; structure; mechanical properties

Synthesis and characterization of porous Fe–25 wt.% Al alloy with controllable pore structure by P. Z. Shen; M. Song; Y. H. He; H. Y. Gao; J. Zou; N. P. Xu; B. Y. Huang; C. T. Liu (183-192).
Porous Fe–25 wt.% Al alloy is synthesized by reactive synthesis with Fe and Al powders as the raw materials. Various processing parameters such as the final sintering temperature, pressure during cold pressing, and powder size are investigated to control the pore structure of the alloy. It has been shown that the optimal final sintering temperature is around 950–1050°C. When the pressure is higher than 110 MPa, the variation of the pore structure is mainly caused by the change of the pore nature in the as-pressed compact. In addition, the pore structure of porous Fe–25 wt.% Al alloys depends on the size of the raw powders and the weight fraction of different-sized raw powders.
Keywords: intermetallics; powder metallurgy; iron aluminides; porous material

Production and properties of impact-resistant composites based on boron carbide and aluminum nitride by O. N. Grigor’ev; V. A. Kotenko; O. D. Shcherbina; N. D. Bega; T. V. Dubovik; V. I. Subbotin; T. V. Mosina; V. V. Lychko; I. L. Berezhinskii (193-200).
The dependence of the phase constitution and strength of B4C–AlN composites on the initial charge formulation and hot pressing temperature is studied. It is established that new reinforcing phases are synthesized during reaction sintering under pressure. The optimum conditions for producing B4C–AlN composites are determined. The composites have quite high strength margin and may be used as components of armor protection in military equipment.
Keywords: boron carbide; aluminum nitride; hot pressing; phase formation; composite; strength margin; armor protection

The Al2O3–HfO2–Y2O3 phase diagram. IV. Vertical sections by S. M. Lakiza; Ya. S. Tishchenko; V. P. Red’ko; L. M. Lopato (201-206).
For better presentation of the Al2O3–HfO2–Y2O3 phase diagram over wide ranges of temperatures and concentrations, three vertical sections are plotted to show interactions in the ternary system. The Y2O3 bisector shows the Y2O3-rich region of the Al2O3–HfO2–Y2O3 phase diagram and explains the mechanism of XHAB phase transformations for Y2O3 solid solutions. The 10 mol.% HfO2 (10H) isopleth shows the Al2O3–HfO2–Y2O3 structure in the region adjacent to the Al2O3–Y2O3 binary bounding system. The HfO2 bisector shows the structure of the HfO2-rich region and the mechanism of FTM phase transformations for HfO2 solid solutions.
Keywords: hafnia; alumina; yttria; phase diagram; vertical sections; eutectic materials

The CALPHAD method is used for the thermodynamic modeling of metastable phase transformations in the Fe–Mo system. The relative thermodynamic stability of metastable α and β Mn-like phases and stable phases of the system are assessed. The Gibbs energy of intermetallic compounds with a homogeneity range is described within the Compound Energy Formalism. The calculated versions of Fe–Mo metastable phase diagrams interpret correctly the phase relationships in high-speed solidification of 63Fe37Mo alloy.
Keywords: thermodynamic modeling; Fe–Mo system; metastable phase diagrams; Mn-like phases

Wetting of hydroxyapatite with biological fluids by N. V. Boshitskaya; A. D. Panasyuk (215-219).
The paper examines the wetting kinetics and mechanism of phase interaction between hydroxyapatite (HA) and biological fluids (blood plasma and physiological solutions) in relation to their composition and porosity. It is established that interaction of HA with blood plasma depends on HA porosity and the correlation of cohesion and adhesion between protein molecules and HA surface. The interaction of HA with physiological solutions depends on the porosity of the solid phase and the composition of Ringer–Locke and Ringer–Tirode solutions. The major factors that govern the phase interaction in HA–biological fluid systems are the porosity of the solid phase and the chemical composition and viscosity of biological fluids.
Keywords: wetting; hydroxyapatite; blood plasma; physiological solution; adhesion; cohesion; porosity

Phase transformations in Ti2Cu under destructive hydrogenation and recombination by T. I. Bratanich; V. V. Skorokhod; O. V. Kucheryavyi; L. I. Kopylova; N. A. Krapivka (220-226).
The thermodynamic possibility and conditions of Ti2Cu hydrogenation and recombination are assessed. The mechanism of Ti2Cu interaction with hydrogen between 293 and 973 K at pressure 1.0 MPa is studied. The interaction of Ti2Cu with hydrogen is thermodynamically possible in the destructive hydrogenation region over a wide temperature range. X-ray diffraction shows that destructive hydrogenation products are titanium dihydride and copper-rich intermetallic phases and copper depending on reaction conditions. According to the established mechanism of Ti2Cu destructive hydrogenation, hydrogen selectively interacts with titanium leaving Ti2Cu composition as follows: β-TiHy (bct-bcc) → TiH1.92 (fcc). The initial Ti2Cu composition is recombined from destructive hydrogenation products (titanium dihydride and copper) in vacuum and hydrogen. The minimum temperature of Ti2Cu vacuum recombination is 1050 K and ensures complete TiH2 decomposition. The recombination of Ti2Cu in hydrogen can occur at temperatures above 907 K.
Keywords: Ti2Cu; destructive hydrogenation; recombination; phase mechanism

Electrochemical and corrosion properties of composite electrolytic and galvanic Ni-based coatings. I. Electrochemical and corrosion properties of composite electrolytic coatings by V. K. Medyukh; V. A. Shvets; R. M. Medyukh; V. N. Talash; Yu. A. Guslienko; N. R. Medyukh; I. V. Uvarova (227-230).
The corrosion behavior of composite electrolytic nickel-based coatings in 3% NaCl solution is examined. The corrosion resistance of coatings as a foil and coatings on a steel substrate is compared. The corrosion properties of composite Ni–B and Ni–Ti electrolytic coatings are shown to be consistent with those of pure Ni.
Keywords: composite electrolytic coatings; corrosion characteristics; polarization curves; stationary potential

The paper examines the physicochemical features of the reduction of iron oxides (as multicomponent scale waste) in various gases: H2, CH4, CO, and generator gas containing 33% CO and 67% N2. At the initial stage of reduction, the activity of the gases decreases in the above sequence. It is analyzed how blocking of iron oxide surface with carbon formed in the conversion of methane and carbon monoxide influences the reduction processes. The deposition of carbon in these media is intensified when the reduction degree is higher than 70%. The thermodynamic dependencies of oxide reduction on the ratio of PCO2/Pco partial pressures are given. Noteworthy is high sensitivity of Fe3O4 and FeO reduction to the change in CO2 content.
Keywords: redox cycle; reduction mechanism; reducing gases; iron oxides; scale waste

The kinetics and mechanism of high-temperature air oxidation of powders and compact materials with different compositions in the AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) systems are studied using nonisothermal (up to 1500°C) and isothermal (1350 and 1670°C) thermogravimetry, DTA, XRD, petrography, and EMPA. During heating, Al4O4–x N x and Si2N2O oxynitrides, α-Al2O3 and α-SiO2 (crystobalite) oxides, and mullite-based Al2O3 ∙ SiO2 solid solution of the boundary composition form during the first oxidation stage, and 3Al2O3 ∙ 2SiO2 mullite forms at higher temperatures (>1350°C) in the upper scale layer and aluminum-nickel chromite spinel in the intermediate (barrier) scale layer. The AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramics may be regarded as hightemperature composites (HTCs) owing to their high corrosion resistance.
Keywords: composite ceramics; metal-ceramic composites; aluminum and silicon nitrides; hightemperature oxidation; mullite; nickel chromite; self-reinforced scale

Effect of magnetic treatment on the microstructure of NiAl–Re alloy by V. E. Oliker; E. N. Eliseeva; T. Ya. Gridasova; I. I. Timofeeva; A. V. Kotko (245-252).
The influence of the magnetic field on the dislocation structure of intermetallic NiAl–Re alloy is studied and is experimentally confirmed for the first time. Active displacement of dislocations is established. Rhenium inclusions perform stop functions in these conditions. In addition, Ni atoms move to the surface of the alloy and an Al-rich internal layer forms under the magnetic gradient. These results indicate that the new atomic configuration forms by diffusion due to jumps of Ni atoms to adjacent Ni vacancies in intermetallic NiAl compound.
Keywords: intermetallic NiAl–Re alloy; magnetic field influence; microstructure