Powder Metallurgy and Metal Ceramics (v.50, #1-2)

The studies on deformation and diffusion processes during sintering are analyzed. Attention is paid to the effect of compressive and tensile stresses on shrinkage of silver, copper, and nickel powders. Superplastic deformation and displacement of particles as a whole are observed during shrinkage. Self-diffusion in transition metal carbides as well as their creep and neck growth during sintering are widely studied. The theory of effective diffusion coefficient is developed to be applied to diffusion-controlled processes in interstitial phases. The proposed relationships are shown to be applicable to creep and neck growth during sintering. Areas of further nanoscale studies are discussed.
Keywords: sintering; shrinkage; plastic deformation; diffusion creep; superplasticity; diffusion-controlled processes; effective diffusion coefficient; neck growth; carbides; nitrides; nanoparticles

Pressure sintering of powder materials by M. S. Kovalchenko (18-33).
Rheological models for pressure sintering of powder materials are described. The rheological models of deformable bodies and the associated dynamic deformation theory for porous bodies based on the energy conservation law enable a quantitative description of their densification under impulse and static hot pressing as well as hot forging using crank presses. The simulation of compaction of porous metals shows that the viscosity of the matrix, that forms the porous body, and the activation energy of viscous deformation dramatically decrease with increasing initial impact velocity. This promotes the compaction of the material to practically nonporous state and improves its mechanical properties.
Keywords: pressure sintering; powder materials; rheology; models; dynamics; kinetics; hot pressing; forging

The motion of a spherical inclusion in a fluid medium under the combined action of crossed electric and magnetic fields and thermocapillarity is studied theoretically. This motion (phoresis) can be qualified as an elementary motion of fine particles in suspensions, emulsions, aerosols, and liquid foamed materials. A term is introduced into the Stokes equation for a fluid inclusion, which takes into account the thermocapillary force acting on the inclusion due to temperature nonuniformity. Boundary conditions on the particle–medium surface are formulated properly. The solution to the system of equations (Stokes equations for the inclusions and medium, boundary conditions) provides an expression for the velocity of an inclusion in a viscous conducting medium under the combined action of thermocapillarity and electromagnetism. The expression for the velocity of phoresis in dimensionless categories provides the ratio between the viscosity and thermal conductivity of the particle and medium and contains a new dimensionless term, such as the ratio between the thermocapillary and electromagnetic effects (analogue of the Weber number). The formulas derived can be used to calculate the rate of transfer in conducting suspensions and emulsions containing various inclusions. The velocity of bubbles in molten metals (Cu, Al, Fe, Ag) is calculated to show that the effects of electromagnetism and thermocapillarity are comparable. The theoretical approach can be useful for developing a flexible method in the electrotechnology of fluid materials.
Keywords: phoresis; fluid inclusion; thermocapillary force; electromagnetic force; velocity of bubbles

The paper analyzes results on the initial spreading kinetics in binary metal systems, including those that involve the formation of intermediate phases and mass transfer through the interface. The drop flowing technique is used to study the effect of temperature, drop mass, and mutual saturation of the melt and solid substrate in binary systems of copper and tin with iron, cobalt, and nickel and also of tin with Ni3Sn and Ni3Sn2 intermetallics on the spreading rate. The proposed formal kinetic analysis of the spreading process using exponential time dependences for the motion of the three-phase boundary and for the difference between cosines of quasiequilibrium and dynamic contact angles permits satisfactory description of spreading in nonsteady-state conditions for the initial stages of the process and determination of its modes. The nature of phase interaction corresponds to phase equilibria in the studied systems with tin.
Keywords: spreading kinetics; initial stages; binary metal systems; intermediate phases; tin; copper; iron; cobalt; nickel

Approximate methods for calculating the effective elastic moduli of microheterogeneous materials, including multiphase ones, are considered. These methods are based on the analogy between the electrostatic field in dielectrics and the field of elastic stresses and strains in solids. Formulas are derived for the quantitative evaluation, with accuracy sufficient for materials science practices, of the effective elastic moduli of composites with different structures as a function of the elastic moduli, morphology, and volume fraction of the phases. The method is validated by calculating the elastic characteristics of bodies with plane slit-like defects, including powder compacts.
Keywords: effective elastic moduli; multiphase materials; composites; electrostatic and elastic fields; bodies with slit-like defects

The nanocrystalline material of an artificial heart valve sintered from 15 wt.% B4C with crystals <10 nm in size uniformly distributed in 85 wt.% carbon with particles about 10 nm in size has exceptionally high chemical stability in human blood plasma. The electrochemical interaction resulting from contact of the valve surface with a potential trace impurity (for example, iron) is experimentally modeled by polarization from an external current source to simulate an extreme corrosion event. The interaction kinetics is studied at 37°C using the method of anodic polarization curves. The elemental composition of interaction products is analyzed by emission spectroscopy using a DFS-13 spectrograph; the composition and thickness of the film layers formed on the valve surface during electrolysis are determined with quantitative Auger electron spectroscopy using a Riber LAS-2000 device. It is established that a nanocrystalline film 350 nm thick forms after 3 h electrolysis on the ceramic surface of the heart valve. The film contains to 94.0 at.% C and to 6.0 at.% N (including to 89.5 at.% C as nanocrystalline graphite and to 4.5 at.% C as nanocrystalline C3N4, as well as to 6.0 at.% N in C3N4) and an insignificant amount of sulfur and inclusions of boron and oxygen atoms. It is shown that the film results from the discharge of anions of corresponding α-amino acids (amino acid remains of complex blood protein chains) containing heterocycle rings.
Keywords: carbon–B4C composite; blood plasma; interaction mechanisms; electrochemical polarization; nanocrystalline film

Structure and properties of sintered iron–boron–carbon alloys with different carbon contents by S. G. Napara-Volgina; G. A. Baglyuk; L. M. Orlova; V. K. Kud; A. I. Khomenko; A. D. Kostenko (67-72).
The structure and mechanical and tribotechnical properties of iron–boron–carbon alloys produced from a mixture of iron, boron carbide, and graphite powders by single pressing and sintering are used to examine heterophase wear-resistant materials containing from 0.8 to 2.4% boron and from 0.7 to 1.6% fixed carbon. The wear resistance of the materials depends on the ratios of boron and carbon in them and on their total content and substantially exceeds the wear resistance of ShKh15 steel chosen as a reference sample.
Keywords: sintered material; boron carbide; wear resistance; boron; carbon; eutectic; iron

Highly porous 17-4 PH stainless steel having porosities in the range of 39–82% with an average pore size of around 700 μm was successfully fabricated using space holder technique in powder metallurgy. Irregular carbamide particles were used as a space holder material. The final porosity was directly related to the added fraction of carbamide. The specimens were sintered at either 1300°C or 1350°C for times of 60 and 90 min in hydrogen atmosphere. In this porosity range, Young’s modulus and compressive strength of the specimens before aging treatment found to be in the range of 0.17–5.34 GPa and 24–290 MPa, respectively and decreased with increasing porosity. 0.5 wt.% boron addition to the 17-4 PH steel powders lowered the sintering temperature and time. The relationship between the mechanical properties and the relative density of porous 17-4 PH steel was found to obey the power law relation.
Keywords: porous materials; powder metallurgy; space holder; 17-4 PH stainless steel; sintering

The Al–Cr–Fe phase diagram. I. Phase equilibria at subsolidus temperatures over composition range 58–100 at.% Al by V. G. Khoruzha; K. E. Kornienko; D. V. Pavlyuchkov; B. Grushko; T. Ya. Velikanova (83-97).
Based on transmission and scanning electron microscopy, x-ray diffraction, electron microprobe and differential thermal analyses, the solidus surface of the ternary Al–Cr–Fe system is constructed for the first time on the concentration triangle over composition range 58–100 at.% Al. Four ternary compounds, D3, O1, H, and ε, with decagonal, orthorhombic base-centered, hexagonal, and orthorhombic primitive lattices participate in phase equilibria on the solidus surface. Solid solutions based on aluminum and binary compounds as well as ternary phases form 12 single-phase surfaces, 25 ruled surfaces of two-phase equilibria bounding two-phase regions, and 14 three-phase isothermal planes corresponding to invariant four-phase equilibria on the solidus surface.
Keywords: solidus surface; compound; ruled surface; isothermal plane

Mixing enthalpies of liquid alloys and thermodynamic assessment of the Cu–Fe–Co system by M. A. Turchanin; L. A. Dreval; A. R. Abdulov; P. G. Agraval (98-116).
The mixing enthalpies of ternary Cu–Fe–Co liquid alloys are studied by the calorimetric method at 873 K and x Co = 0–0.55. Thermodynamic assessments of the Fe–Co and Cu–Fe–Co systems are carried out with the CALPHAD method. A set of self-consistent thermodynamic parameters of the phases is obtained using data on the mixing enthalpies established in this study and published information on phase transformations. Isothermal and vertical sections of the phase diagram, liquidus and solidus surfaces, and metastable miscibility gap for overcooled liquid alloys are calculated. The undercooling degrees for the metastable liquid phase separation and temperature–composition ranges of the formation of the supersaturated solid solutions during the liquid quenching have been assessed.
Keywords: calorimetry; mixing enthalpy; thermodynamic properties; thermodynamic modeling; phase diagram; alloys of the Cu–Fe–Co system

The thermodynamic possibility of interaction between products of LaNi5, TiFe, Ti2Ni, TiNi, Ti2Cu, TiCu, Ti3Al, TiAl, Mg2Ni, Mg2Cu, and ZrMn2.8 destructive hydrogenation and their interaction with gases (nitrogen, ammonia, methane, water vapor, carbon dioxide and oxide) is studied. The recombination of intermetallics from hydride phases and metals is thermodynamically favored in the temperature range suitable for inverse hydrogen accumulation. The minimum temperature for the recombination of intermetallics in hydrogen is directly proportional to the titanium content of an intermetallic and products of its destructive hydrogenation. The recombination of Ti3Al proceeds in hydrogen at 1.0 MPa and 1073 K from products of its destructive hydrogenation. The thermodynamic possibility for the transformation of interacting products from destructive hydrogenation of titanium-based intermetallics into noninteracting ones by replacement of titanium hydride by titanium nitride, carbide, or oxide during interaction with nitrogen or ammonia, methane or carbon, CO2, CO, or H2O is proved. Heat-resistant TiH2–TiN–Ti3Al–TiAl composite is obtained by annealing the products of Ti3Al destructive hydrogenation (TiH x –Ti3Al–TiAl2–TiAl) in nitrogen at 2.0 MPa and 1073 K.
Keywords: recombination of intermetallics in hydrogen; heat-resistant nanocomposite