Powder Metallurgy and Metal Ceramics (v.53, #1-2)

The influence of equal-channel angular extrusion on the structure and mechanical properties of powder compacts with high initial porosity is studied. A compaction area whose size and density increase with the accumulated strain is discovered. After the third pass, the compaction area has 0.99 relative density, 845 MPa micro-hardness, 80 nm grains, and 530 MPa compression yield strength. Angular extrusion allows production of a powder material with high physical and mechanical characteristics from high-porous compacts in three passes.
Keywords: angular extrusion; copper powder; relative density; structure; grain size; micro-hardness; hardening coefficient; compression yield strength

Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y2O3–Al2O3 Diffusion Couple by O. I. Get’man; V. V. Panichkina; L. N. Paritskaya; V. V. Skorokhod; A. V. Samelyuk; Yu. V. Bykov; A. G. Eremeev (8-18).
The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al2O3–Y2O3 samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered Al2O3–5 vol.% ZrO2 layers, and yttria layers, such as sintered coarse-grained samples or fine Y2O3 powder layers on the Al2O3 surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, particularly to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al2O3–Y2O3 system, which involves both the change in the phase composition of the diffusion zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in Y2O3 oxide because of the unipolar diffusion of Al3+ ions to Y2O3. The validity of the proposed mechanism is confirmed by numerical evaluations.
Keywords: microwave heating; reaction diffusion; microwave effect; alumina and yttria; diffusion couple; microstructure; yttrium aluminum garnet (YAG); perovskite (YAP); monoclinic phase (YAM)

Rate-controlled sintering and spark plasma sintering are considered the most promising methods to produce dense nanoctructured ceramics. Refractory compounds are used to demonstrate the application of methods for controlling the densification rate and nonlinear heating and loading conditions to produce dense nanocomposites with 30–70 nm grains. The mechanical and tribological properties of ceramics with grains from 50 to 500 nm in size are compared. The effect of increase in the mechanical (5–15%) and tribological (to 50%) characteristics of nanocomposites consolidated by rate-controlled sintering and modified nonlinear spark plasma sintering is studied. Nanocomposites based on refractory nitrides and borides are regarded as promising materials for creating a new generation of cutting tools, as well as wear-resistant ceramics for wide application.
Keywords: TiN; TiCN; Si3N4 ; nanocomposites; sintering; spark plasma sintering; shrinkage

Cermet coatings based on Ni–Al alloys and titanium–chromium diboride are electrospark-deposited onto 40Kh steel. It is analyzed how the metal-to-refractory phase ratio in the electrode material and the deposition parameters influence the mass-transfer kinetics, thickness, and composition of the coatings. Higher metal content of the electrode material activates mass transfer and increases thickness of the coatings (up to 700 μm). The integrity, roughness, and thickness of the coatings can be controlled by varying electrospark deposition parameters and chemical composition of the electrodes.
Keywords: electrospark-deposited coating; mass-transfer kinetics; cermets; titanium–chromium diboride

Composite Coatings from Bioactive Calcium Phosphate Ceramics on Metal Substrates by N. D. Pinchuk; A. R. Parkhomei; A. A. Kuda; L. A. Ivanchenko (40-47).
Porous bioactive membrane-type calcium phosphate coatings on metal substrates are produced. Their microstructure and physicochemical properties are studied. Titanium (including porous) and medical stainless steel are used as metal substrates. The bioactive coatings are based on biogenic hydroxyapatite or synthetic calcium phosphates (calcium phosphate content was between 68 and 85 %) and sodium borosilicate glass. To add bacteriostatic properties to the coatings, some compositions were additionally doped with cerium dioxide (5 %). The surface microstructure, some mechanical properties, and in vitro behavior of the coatings in physiological solution are examined.
Keywords: bioactive coatings; hydroxyapatite; microstructure; solubility

This paper proposes a method for increasing the heat resistance of VT6 titanium alloy from 400 to 900 °C through complex saturation of its surface with aluminum and chromium in powder mixtures. The diffusion coatings formed on the alloy consist of titanium aluminides: Al3Ti, Al2Ti, AlTi, and AlTi3, which show high thermal stability ensured by adequate aluminum content required to create a surface alumina film. The outer coating Al3(Ti, Cr) layer contains 4.3–9.3 wt.% Cr, which may induce the formation of protective Cr2O3 oxide besides Al2O3. Increase in the microhardness of the VT6 alloy by a factor of 2 to 2.5, resulting from complex aluminum–chromium saturation, compared with the initial state should enhance its wear resistance.
Keywords: titanium alloy; VT6; diffusion coating; aluminum and chromium saturation; heat resistance; oxidation; microhardness

(Ti, W)C–Ni cermets with different contents of Mo2C were produced by the spark plasma sintering (SPS) method. The grain size (GS), composition of ceramic phases, and mechanical properties of the sintered cermets were investigated. The amount of Mo2C had a significant influence on the microstructure and mechanical properties of as-prepared cermets. GS and fracture toughness (KIc) were decreased as a result of increasing the amount of Mo2C. By increasing the amount of Mo2C, the transverse rupture strength (TRS) and hardness (HRA) were enhanced. However, above 10 wt.%, the TRS was reduced. The conventional black cores observed by field-emission scanning electron microscopy (FE–SEM) in backscattered electron imaging (BSE) in (Ti, W)C–Ni cermets will be partially turned into some white cores which contain higher Mo, except for Ti and W elements, when content of Mo2C reaches ~15 wt.%. Batch mechanical tests indicate that cermets with some white cores have refined microstructure and higher hardness, but relatively lower transverse rupture strength (TRS) and fracture toughness (KIc) at room temperature.
Keywords: (Ti, W)C–Ni cermets; microstructure; mechanical properties; spark plasma sintering (SPS)

The friction and wear of detonation-sprayed Cr–Si–B coatings in high-temperature friction conditions are studied. The optimal choice of Cr–Si–B composition for spraying of wear-resistant coatings subjected to friction at high temperatures is justified. It is noted that doping elements in certain concentrations and spraying parameters have positive influence on the structure and properties of multicomponent coatings. It is shown that the introduction of silicon and boron promote the formation of complex doped high-temperature compounds with increased wear resistance. The maximum microhardness corresponds to the Cr–Si coatings with ~25 % silicon content. Besides, the mechanical properties of the material are improved by additional doping with ~ 12 % boron. In turn, the acetylene–oxygen mixture supplied at 22/27 L/min promotes constant spraying parameters, unchanged chemical composition, and stable properties of the coatings. The Cr–Si–B coatings show appropriate structural adaptability, which minimizes the friction and wear parameters, at 5.0 MPa loading and a sliding speed of 1.5 m/sec up to 700 °C. Metallographic analysis and strip chart recording of the samples indicate that the friction surfaces are free of visible defects and individual cold-welded regions are located in thin-film surface layers. The Cr–Si–B coatings exhibit high adhesion, mechanical characteristics, and wear resistance at elevated temperatures, which correspond to the properties of heat-resistant high alloys. Current physical and chemical analysis methods are employed to study the structure and properties of thin-film surface structures. It is determined that the mechanical, physical, and chemical properties combined in the Cr–Si–B coatings provide wide opportunities for their application in high-temperature wear conditions.
Keywords: detonation-sprayed coating; wear resistance; surface layer; structural adaptability; temperature

Mixing Enthalpies of Melts and Thermodynamic Assessment of the Cu–Fe–Cr System by M. A. Turchanin; A. A. Bondar; L. A. Dreval; A. R. Abdulov; P. G. Agraval (70-90).
The mixing enthalpies of ternary liquid alloys in the Cu–Fe–Co system have been studied by calorimetric method at 1873 K and x Cr = 0–0.45. The phase transformations in the Cu–Fe–Cr system have been examined using methods of physicochemical analysis. The thermodynamic assessment of the ternary system has been carried out in the framework of the CALPHAD method. The set of self-consistent parameters for thermodynamic models of the phases has been obtained taking into account experimental data of the present work and information on phase transformations. The isothermal and vertical sections of the phase diagram, liquidus and solidus surfaces, as well as metastable miscibility gaps of supercooled liquid alloys have been calculated. The degrees of supercooling for metastable liquid phase immiscibility have been assessed. The composition ranges for the formation of alloys with special structures have been predicted. The temperature–composition ranges for the formation of supersaturated solid solutions during liquid quenching have been evaluated.
Keywords: calorimetry; mixing enthalpy; thermodynamic properties; thermodynamic modeling; phase diagram; Cu–Fe–Cr alloys; core-type structure; dispersed droplet structure

Phase Transformations in Ti3Sn and Ti2Sn Intermetallics During Hydrogenation by T. I. Bratanich; O. V. Kucheriavyi; V. V. Skorokhod; L. I. Kopylova; M. O. Krapivka (91-97).
The sequence of phase transformations in Ti3Sn and Ti2Sn intermetallics during their interaction with hydrogen is studied. The mechanism whereby Ti3Sn hydride phases form is clarified. It is shown that Ti3Sn reacts with hydrogen at 373–1173 K through hydrogen ordering of the initial intermetallic structure, subsequent amorphization, and restructuring during the formation of hydride phases in the following sequence: hcp Ti3Sn → diamond Ti3SnH0.8 → hcp Ti3SnH0.8–0.9 → fcc Ti3SnH0.9–1.0 → bcc Ti3SnH1+x. A new hydride phase, Ti3SnH1+x (bcc, a = 0.538 nm), with high hydrogen capacity compared to the known fcc Ti3SnH hydride, which decomposes when heated to 1073 K, has been found. Destructive hydrogenation of Ti3Sn at 298–1173 K and hydrogen pressures of 1.0–6.0 MPa is not revealed. It is found that intermetallic Ti2Sn reacts with hydrogen through destructive hydrogenation to form a solution of hydrogen in β-titanium, hydride of a face-centered tetragonal solid solution of tin in titanium, and intermetallic Ti5Sn3.
Keywords: Ti3Sn; Ti3SnH1+x new hydride phase; Ti2Sn; destructive hydrogenation

Using the theory of plasticity for porous bodies and finite-element computer simulation, the stress–strain state around spherical and conical indenters has been studied for different mechanical properties of indented material. Porosity distribution near the indenter depending on the properties of the porous body and on its friction against the indenter has been analyzed. Load–displacement curves with respect to tendency to dilatancy and contact friction are shown. The development and shape of bulks around the indenter have been studied.
Keywords: indentation; plasticity of compressible media; spherical indenter; conical indenter; dilatancy

Mathematical Formalism of Pore Stereology of Powder Materials by V. M. Mel’nik; V. D. Rud’; Yu. A. Mel’nik (107-112).
A method for analyzing the stereology and morphology of pore space in powder materials is described. The method is designed to employ scanning electron microscopy. The wavelet transform is proposed for filtering noise distortions. Scanning electron microscopy images are used for morphological analysis of the structure. The method employs discrete Voronoi diagrams.
Keywords: ceramics; scanning electron microscopy; wavelet; discrete Voronoi polygons

It is shown that the effectiveness of fluidized-bed reactors is limited by the low reaction rate at low temperatures rather than by the small size of particles. Low-temperature hydrogen reduction is associated with low gas utilization and high energy consumption of the gas treatment system. Temperature cannot be increased since coalescence of iron particles causes destruction of the fluidized bed, but mechanical activation allows accelerating the reaction without increase in the process temperature. Since sulfur and phosphorus bond with iron in the reduction of iron oxides, iron and magnetite can be separated from iron sulfides and phosphides using difference in the Curie points for different iron compounds and magnetic separation at ~500 °C. The possibility to create high-temperature magnetic separators for fine materials is pointed out.
Keywords: iron powder; mechanical activation; fluidized bed; solid-phase reduction