Powder Metallurgy and Metal Ceramics (v.53, #11-12)

Strain hardening during pressing of powder compacts is commonly described by evaluating the average strain rate intensity. Such data allow predicting the evolution of the average yield stress of powder particles considering the strain hardening of compact material. The accuracy of this approach is assessed by comparing the values of compacting pressure determined by evaluating the average yield stress and by numerically modeling the deformation of representative cells of a porous material. Determining strain hardening from the average strain rate intensity gives a qualitatively correct description of the variation in the compacting pressure. Quantitative differences are observed only at the beginning and at the end of the pressing process, when the strain rate distribution over powder particles becomes sharply nonuniform. Evaluation of the strain hardening of a porous sample is hardly effective without detailed data on the macroscopic yield behavior of powder during pressing. This shortcoming can be avoided by evaluating strain hardening using direct multiscale modeling, which does not require macroscopic constitutive equations in analytical form. The effect of strain hardening on the residual tensile stresses in a synchronizer ring is considered as an example. It is shown that high tensile stresses are responsible for cracks in the blank.
Keywords: powder; strain hardening; multiscale modeling

The effect of mechanical vibrations on the structure formation and mechanical properties of compacts during electron beam sintering is investigated. Using optical microscopy, continuous indentation, and X-ray diffraction, it was established that imposing mechanical vibrations decreases the average titanium grain size by a factor of 1.5–2 and induces residual compressive stresses in the sintered titanium compact. Due to the stresses, greater density, and smaller grains, the ductility and compressive strength of the compacts sintered under mechanical vibrations are higher than those of the compacts sintered in conventional way.
Keywords: titanium hydride; mechanical vibrations; stress; hardness; ductility; compressive strength

Water Vapor Transport Enhancement Through Isotactic Polypropylene by Incorporating Multiwalled Carbon Nanotubes by G. Bounos; K. S. Andrikopoulos; H. Moschopoulou; Th. Ioannides; K. Kouravelou; G. C. Psarras; G. A. Voyiatzis (634-642).
In the present work, the incorporation of multiwalled carbon nanotubes (MWCNTs) in isotactic polypropylene (i-PP) was examined in terms of water vapor transport. The structure of the composites, as well as the MWCNT dispersion, was characterized by scanning electron microscopy, Raman spectroscopy, thermal analysis, and X-ray diffraction. Furthermore, broadband dielectric relaxation spectroscopy was employed to study the electrical properties of the nanocomposites. The composite membranes were prepared by a melt mixing procedure. The incorporation of MWCNTs resulted in a 12-fold increase in the specific water vapor transmission rate. Water vapor transport properties were found to be limited by efficient obstructive nanotube agglomerate formation for high MWCNT loadings.
Keywords: mixed matrix membranes; carbon nanotubes–polypropylene composite; water vapor transport properties

Production processes for fine powders of tungsten, tungsten carbide, and WC–Co and sintering methods for nanosized WC–Co hard metals are reviewed. The properties, structure, and applications of nanosized WC–Co hard metals described in the literature are analyzed. It is established that, regardless of the process, the mandatory requirements for the production of fine hard metals are: (i) purity of starting materials, (ii) precise compliance with the process procedure, and (iii) careful control at all production stages, which exclude unwanted phases and defects in the structure.
Keywords: nanopowder; tungsten; tungsten carbide; WC–Co mixture; melt; nanosize; porosity; grain size; hardness; strength; fracture toughness; wear resistance

Structure and Properties of Sintered Silicon–Manganese Steels by L. A. Sosnovskii; G. A. Baglyuk; O. V. Vlasova; M. E. Golovkova (657-662).
The effect of the content of manganese silicide (MnSi1.77) additive in the original charge and sintering temperature on the structure and properties of sintered steels is investigated. It is shown that adding manganese silicide slightly decreases the density of sintered samples, but significantly increases the hardness and strength properties of the alloys. An electron microprobe analysis of sintered steels establishes that manganese is relatively uniformly distributed in iron particles, whereas the distribution of silicon is significantly different: its higher concentration is mainly seen along the grain boundaries.
Keywords: steel; powder; sintering; alloying; manganese silicide; density; hardness; strength; ligature

Structure, Phase Composition, and Wear Mechanisms of Plasma-Sprayed NiCrSiB–20 wt.% TiB2 Coating by A. P. Umanskii; M. S. Storozhenko; I. V. Hussainova; A. E. Terentiev; A. M. Kovalchenko; M. M. Antonov (663-671).
The structure, phase composition, and wear mechanisms of plasma-sprayed NKhTB20 coating (NiCrSiB–20 wt.% TiB2) are studied. To produce NKhTB20 composite powder, commercial PR-NKh16SR3 (NiCrSiB) powder was mixed with 20 wt.% TiB2 and the charge was pressed and sintered in vacuum at 1100°C for 30 min. During sintering, the components react to form chromium borides. The sinters were ground and classified into the particle size fraction –100+60 nm for plasma spraying. The plasma-sprayed NKhTB20 coating consists of a nickel-based matrix reinforced with titanium diboride and chromium boride grains. The friction and wear behavior of the NKhTB20 coating in dry friction against plasma-sprayed NiCrSiB and NKhTB20 coatings is examined. It is revealed that the NKhTB20/NiCrSiB friction pair has higher wear resistance than NKhTB20/NKhTB20. The contact surfaces of the NKhTB20/NKhTB20 friction pair are damaged under oxidative and abrasive wear mechanisms. Oxidative wear is the dominant mechanism for the NKhTB20/NiCrSiB friction surface. Complex oxide films form on the NKhTB20/NiCrSiB sliding surface and prevent it from damage.
Keywords: self-fluxing alloy; titanium diboride; plasma-sprayed coating; structure; wear resistance; wear mechanism

Mechanical milling (MM) process as a novel technique for producing Al–B4C composite coatings on the surface of low carbon steel was investigated. The coating thickness, morphology, and cross-section microstructure of the composite coatings were analyzed with scanning electron microscopy (SEM). Also, Vickers microhardness and surface roughness of the coating layer were measured. It was found that a dense Al–B4C composite coating could deposit on the surface, particularly, if longer milling periods. The microstructure of Al–B4C composite coatings revealed that the distribution of B4C particles in the Al matrix was homogenous. Increasing milling time from 6 to 30 h resulted in increase of the microhardness values from 220 to 280 HV for the pre-interface layer (5 μm).
Keywords: coating; composite; microstructure; milling

Friction and Wear of TiN–Si3N4 Nanocomposites Against ShKh15 Steel by V. G. Kolesnichenko; O. B. Zgalat-Lozinskii; V. T. Varchenko; M. Herrmann; A. V. Ragulya (680-687).
Spark plasma sintering is used to produce dense silicon nitride nanoceramics with titanium nitride additions and nanofiber-strengthened composites. The sintered nanocomposites demonstrate quite high mechanical properties (HV ≈ 14–16 GPa, K Ic ≈ 4.8 MPa ∙ m 1/2 ) and low dry friction coefficient (f ≈ 0.65–0.68) and mass wear (~0.2–0.4 mg/km) against ShKh15 steel. The nanocomposite strengthened with 3.8 wt.% silicon nitride nanofibers shows the most balanced mechanical and tribotechnical characteristics.
Keywords: spark plasma sintering ; nanocomposite ; silicon nitride ; titanium nitride ; wear resistance ; friction

Laser ZrB2-Based Coating on Graphite by I. A. Podchernyaeva; A. D. Panasyuk; O. N. Grigoriev; G. A. Frolov; A. M. Bloshchanevich; D. V. Yurechko; M. A. Vasil’kovskaya (688-692).
A coating ≤50 μm thick is produced on graphite by laser melting of a ZrB2-based powder layer with additions of zirconium and molybdenum silicides in air. The main coating phases are ZrB2, SiO2, and ZrSiO4. The surface is formed by spherical particles 10–80 μm in size, representing a two-phase eutectic in the ZrSiO4–SiO2 system as needlelike crystals. Microhardness of the coating is 2.5 times higher than that of the substrate and reaches 15 ± 1 GPa.
Keywords: laser melting; graphite; coating; zirconium diboride

Thermodynamic Properties of Eu–In Alloys by V. V. Berezutskii; M. I. Ivanov; M. O. Shevchenko; V. S. Sudavtsova (693-700).
The mixing enthalpies of liquid binary Eu–In alloys (0 < x In < 0.66, 0.78 < x In < 1) are determined by isoperibol calorimetry at 1170–1300 K. The thermodynamic properties of the liquid Eu–In alloys are described in the entire composition range using the model of ideal associated solution. The thermodynamic activities of components in the Eu–In melts demonstrate negative deviations from the ideal behavior, and the mixing enthalpies are characterized by significant exothermic effects. The minimum value of the mixing enthalpy is −35.1 ± 0.5 kJ/mol at x In = 0.52 (T = 1300 K) and −41.2 ± ± 0.5 kJ/mol at x In = 0.50 (T = 1170 K).
Keywords: europium ; indium ; thermodynamics ; enthalpies ; melts ; phase diagram

Structural Engineering of Impregnated Dispenser Cathodes by O. I. Get’man; V. V. Skorokhod (701-721).
Targeted research efforts focusing on the properties and structure of impregnated dispenser cathodes (IDCs) have been monitored. The data are summarized in terms of scale structural hierarchy in inorganic materials to develop principles for controlling their formation in the design of high-emission and long-life IDCs. The performance of IDCs of different types is modeled using the materials science triad ‘chemical composition ↔ structure ↔ properties’ and the concept of structural hierarchical levels in IDCs. Basic structural levels in IDCs are determined: electronic, nanostructured, mesoscopic, microscopic, and macroscopic. Their structural elements are analyzed: electrons, emitter layer, film coating, matrix and emission material, and cathode structure. It is found out that the electronic level is the key one in the hierarchy of IDC structural levels; its effectiveness depends on the nanocrystalline, mesoscopic, and microscopic levels. The principles of structural engineering are developed for the design of high-emission and long-life IDCs and for the control of their formation at nanostructured and microscopic levels by variation in the chemical composition and structure.
Keywords: impregnated dispenser cathode (IDC) ; thermal emission ; life ; hierarchy of structural levels in IDCs ; emitter layer ; emission material ; metallic matrix

The paper continues the historical overview of technologies and applications of metals traditional for the Bronze Age that were in use from the early Middle to present times: gold, silver, tin, lead, mercury, and their alloys. Modern development of industries and artistic applications with these metals and their alloys are shown.
Keywords: gold; silver; tin; lead; mercury; amalgam; jewelry; electronics; electric and antifriction alloys; tin plating; tinplate

Erratum to: 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 (733-733).