Powder Metallurgy and Metal Ceramics (v.51, #11-12)

Within the general framework of nonequilibrium thermodynamics, we develop a synergetic approach to examine, both theoretically and experimentally, nonlinear interactions and self-organization in reactive powder systems. Nonequilibrium physical and chemical powder systems prone to self-organization are described. A general principle behind the formation of dissipative objects acting as materials with nonlinear mechanisms of adaptation is stated. The capabilities of self-organization processes are demonstrated.
Keywords: reactive powder systems; nonlinear interaction processes; self-organization; thermokinetic paths; functional properties

Production of Co–Cr–Al–Y–Si powder alloys by N. I. Grechanyuk; K. A. Gogaev; V. G. Zatovskii (633-638).
A more effective process of producing the Co–Cr–Al–Y–Si powder with 40–100 mm particles has been developed. This process yields 70% of this powder fraction, while other existing processes provide 50–60%. The proposed process involves melting of ingots in an electron-beam unit and their subsequent fragmentation using a press and strip rolling mill. It is shown that energy consumption is lower during this mechanical grinding compared to the use of crushers and mills and is almost 20 times higher in atomization. The developed process is waste-free: 40 μm powder fraction is remetled in the ingot production process. The method of producing the Co–Cr–Al–Y–Si powder has been patented. The powder is used as a heat-resistant sublayer of thermal protection coatings for blades of gas turbine engines.
Keywords: electron-beam remelting; milling; grinding; press; strip rolling mill; size analysis

The paper presents a brief literature review on microstructure development in composites during liquid-phase sintering. Two mechanisms of microstructure coarsening in liquid-phase sintered systems are considered: dissolution–reprecipitation and coalescence. The data are systematized in the context of the Lifshitz–Slyozov–Wagner theory and the theory of coalescence at the final stages of liquid-phase sintering. The effect of various physicochemical factors, such as temperature, volume fraction, and time on microstructure development is explained in terms of quasichemical kinetics.
Keywords: liquid-phase sintering; composites; growth kinetics of solid particles; Lifshitz–Slyozov–Wagner theory; activation energy

Effect of Micrononuniform Heating of Powder in Field-Assisted Sintering on Shrinkage Kinetics by A. V. Kuz’mov; E. A. Olevskii; E. V. Aleksandrova (657-665).
The paper examines the micrononuniform distribution of temperature and its influence on the rheological behavior of a copper powder with oxidized spherical particles as an effective dispersion-porous medium through which an electric current is passed. The micrononuniform distribution of temperature needs to be considered to describe the temperature dependence of the rheological behavior of effective dispersion-porous medium during resistance heating.
Keywords: field-assisted sintering; oxide films; local resistance heating

Interdiffusion and Structural Changes in the Cr2O2–Al2O3(ZrO2) Diffusion Couple under Microwave Heating by O. I. Get’man; V. V. Panichkina; L. N. Paritskaya; P. Ya. Radchenko; A. V. Samelyuk; V. V. Skorokhod; Yu. V. Bykov; A. G. Eremeev (666-676).
The interdiffusion and microstructural evolution of the Cr2O3–Al2O3 (5 vol.% ZrO2) diffusion couple are studied in the temperature range 1600–1800°C under microwave heating (24 Hz) and, for comparison, under traditional heating using electron microprobe analysis and microscopic analysis. It is found that the concentration of chromium is distributed differently in Al2O3 in diffusion zones under microwave and traditional heating. This is due to greater contribution of grain-boundary diffusion to the effective diffusion flux under microwave heating. Bulk diffusion and average grain-boundary diffusion coefficients are calculated. The grain size in the diffusion zone toward Al2O3 is smaller after microwave heating. Traditional heating induces grain growth by recrystallization, whereas two processes, recrystallization and polygonization, are superimposed during microwave heating. The polygonization is due to the generation of dislocations under thermal stresses originating from nonuniform temperature distribution in the diffusion zone with variable concentrations of the components. The calculated bulk and grain-boundary diffusion coefficients can be used to predict the kinetics of various diffusion mass-transfer processes in Al2O3 and Cr2O3 oxides and their mixtures.
Keywords: Al2O3 ; Cr2O3 ; diffusion couple; microwave heating; interdiffusion; microstructure; bulk and grain-boundary diffusion coefficients

Structural Engineering of Powder Materials by Yu. N. Podrezov (677-686).
Three main areas in the engineering of structural powder materials related to the control of pore-space morphology, formation of perfect contacts, and optimization of solid-phase structure are discussed. The causes behind the effect of pores on the strength of both brittle materials and materials with high fracture toughness are analyzed. The formation of high-quality contact is analyzed in the context of structurization and evolution of interparticle boundaries at different production stages. The structural optimization of powder materials is discussed considering, as an example, the grain refinement and formation of nanostructured materials.
Keywords: contact formation; pore morphology; structure; strength; plasticity

Prospects of Improving the Tribotechnical Characteristics of Titanium Composites by A. G. Kostornov; O. I. Fushchich; T. M. Chevychelova; V. T. Varchenko; A. D. Kostenko (687-696).
Titanium-based composites containing MoS2, MoSe2, CaF2, and BN solid lubricants are synthesized. Their tribological characteristics are investigated without lubrication at different sliding velocities (0.5, 1, 2, 4, 6, and 15 m/sec) and low pressures (0.27, 0.54, 0.8, 1.1, 1.35, 1.47, and 2.7 MPa) in air. In view of the high friction coefficient and large wear, the composite materials cannot be proposed as antifriction ones for operation at small sliding velocities and low pressures. The titanium-based composites are promising as antifriction materials at an increased sliding velocity (15 m/sec) and low pressures when their friction coefficients range from 0.3 to 0.36 and wear ranges from 1.91 to 68.3 mg/km. In dry friction at a high sliding velocity in air, the temperature of their working surface increases resulting from the formation of titanium oxides and then a dense secondary lubricating microheterogeneous film. The composition and structure of the secondary films differ from those of the starting materials and determine their antifriction properties and friction performance.
Keywords: composite material; antifriction material; composition; structure; sliding velocity; pressure; temperature; friction coefficient; wear; secondary structural films

The papers presents results of studies focusing on the development and characterization of ultrahigh-temperature structural ceramics, ceramics with high wear resistance and impact penetration resistance, and cermets and coatings based on nonoxide refractory compounds. The phase and structure formation of the materials is studied in interrelation with their mechanical and service properties. The materials are intended to perform as wear-resistant, shock-resistant, and structural parts (at temperatures above 1600°C in corrosive environments) for machines and units in various engineering fields.
Keywords: nonoxide refractory compounds; structure; mechanical properties; oxidation; impact penetration resistance; ceramics; cermets

Microstructural Design of Ceramics by G. S. Oleinik (709-723).
A methodological approach to the design of ceramics with predetermined composition and microstructure is proposed. Its fundamental idea is to implement the accumulated knowledge into existing technologies or develop novel science-based technologies. The choice of a known microstructure and the design of a novel one is based on the knowledge accumulated so far on material structurization and properties as follows: knowledge systematization – analysis – generalization – establishment of regularities. The ideology of designing a novel material with predetermined microstructure consists in implementing the following sequence of stages: knowledge-based choice of microstructure and sintering conditions → manufacture of a pilot sample of the material (prototype) → study of the properties → optimization of the microstructure and properties → technology. An algorithm is proposed for designing a material with required microstructure, which can be achieved through coordinated structural transformations during sintering, whose evolution determines the formation of planned microstructural elements, their crystalline morphology, size, content, and distribution throughout the volume. An example is provided for designing the microstructure of silicon carbide material with a SiC–B4C eutectic binder.
Keywords: microstructure; design; science; technology; ceramics; formation

Microstructural Design of Bioinert Composites in the ZrO2–Y2O3–CeO2–Al2O3–CoO System by A. V. Shevchenko; E. V. Dudnik; V. V. Tsukrenko; A. K. Ruban; V. P. Red’ko; L. M. Lopato (724-733).
It is shown that a scientifically sound approach to each stage of producing ZrO2-based bioinert implants (from the synthesis of starting powders to their sintering) is a necessary condition for promoting the optimum structure and high mechanical properties. Conditions for producing bioinert implants with regular, laminar, and highly porous microstructures are found. The research results serve as a scientific basis for microstructural design of various bioinert implants in the ZrO2–Y2O3–CeO2–Al2O3-CoO system.
Keywords: ZrO2–Y2O3–CeO2–Al2O3–CoO system; nanocrystalline powder; transformation hardening; laminate; porous composite; bioimplant

Effect of the Structural State of Graphitic Materials on Their Phase Transformations under Shock Compression by A. V. Kurdyumov; V. F. Britun; V. V. Yarosh; A. I. Danilenko (734-742).
The effect of structural disordering of graphitic materials on their phase transformations into dense modifications of carbon under shock compression conditions (P shock = 30 GPa, T shock = 3000 K) is studied. It is shown that a lower degree of three-dimensional ordering of initial structure (P3) first decreases and then increases the overall yield of dense phases, reaching the maximum at P3 = 0. The content of lonsdaleite simultaneously decreases and that of the dense amorphous phase (C am) increases. The results obtained are attributed to gradual change of the predominant martensite transformation mechanism to predominant diffusion-controlled mechanism, and also to the fact that the metastable phases (lonsdaleite and C am) that form at the initial transformation stage partly transform into a stable diamond phase.
Keywords: graphitic material; shock compression; diamond; lonsdaleite; dense amorphous phase; nanostructure