Variant of the plasticity condition for powdered solids

A plasticity criterion is proposed for powdered materials to account for the local character of slip and plastic deformation of the particles. The degree of localization of the contact interaction between the particles is quantitatively evaluated by using the relative fraction of the active volume, after M. Yu. Balshin. Good agreement is obtained between the theoretical and experimental data in describing of the process of the isostatic pressing of spherical powders of lead and tin. The effect of the density of the material and pressure on the form of the yield surface and the character of the volume deformation is examined. A method is presented for determination of the empirical parameters of the plasticity criterion. These parameters are evaluated for sand. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 4–10, March–April, 2000.     

New technologies in powder metallurgy and ceramics developed in Belarus. A survey

Basic trends in scientific and technical activity, new technologies in powder metallurgy, and new materials created at the Institute of Powder Metallurgy of the National Academy of Sciences of Belarus since the early 1990s through the present day are considered. The article reviews the contribution of Belarus scientists to the development of sintered functional materials and anti-friction composite materials, the development of technologies of drop forging of sintered billets, infiltration of powder compacts, warm pressing of powders and additional pressing of compacts, in the creation of porous powdered materials with anisotropic structure, porous-capillary materials, and industrial ceramic, in the use of technologies of powder metallurgy to fabricate diamond tools, and create materials and technologies of thermal deposition of coatings from powders. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), 118–128, January–February, 2006.     

Selection of Alloying Method to Produce Iron-Based Powder

There are different methods to produce low-alloyed iron-based powders. The properties of such powders depend on the method of alloying as well as on the characteristics of the basic grade of iron powder. A comparative experimental study to estimate the influence of the basic iron powders on low alloyed powder properties has been conducted. The method of co-reduction of powdered oxides that contain alloying elements and selected iron powder grades has been selected. Chemical and mass spectrometer analysis has shown satisfactory alloying element distribution. Tensile strength test of the sintered parts has shown that the best results are obtained for specimens produced from low alloyed powder of Indian “Blue Dus” concentrate as the basic iron powder grade. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 8–3, May–June, 2005.     

Use of a computerized simulation model of metal flow to calculate forging parameters

Conclusion The mathematical model that was developed and realized in the form of the computing system SPLENS (PRESS) can serve as an effective means of analyzing the flow of material in pressing with active friction. The model and the system can also be used to calculate control regimes that will ensure the desired physical properties in the pressed product. Moscow State Institute of Steel and Alloys. Translated from Metallurg, No. 10, pp. 31–33, October, 1998.     

Indentation of materials as a new method of micromechanical testing

The method of local loading (MLL) with a rigid indentor was used for the micromechanical testing of materials. This method enabled the construction of σ-∈ deformation curves, and determination of strength and plasticity characteristics, Young modulus, and work hardening. Considering the earlier described methods to determine fracture toughness, ductile-brittle transition temperature, and anisotropy of mechanical properties, MLL can be viewed as a universal method for investigating the mechanical properties of materials. The mechanical properties of brittle materials (including ceramics), thin coatings, phase components of composite materials and of inhomogeneous structures whose properties vary along the cross-section can be determined only by MLL. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp 85–93, July–August, 1999.     

Physicochemical Processes of Fabricating Ceramic Materials Based on Nanopowders of Zirconium,Yttrium, Cerium,and Aluminum Oxides

When studying nanoceramics, it is necessary to constantly keep in mind the closest interrelation of their fabrication method, structure, and properties. Nanoceramic materials are used in various branches of technology as structural and functional materials. Nanoceramics are also widely used in medicine. They are harmless, stable, and have great affinity to living organisms. ZrO2-based nanoceramics have a lower elastic modulus than other oxide materials. The specificity of their application lies in their high rupture strength, thermal shock resistance, and chemical stability at high temperatures. However, it is necessary to solve the problem of increasing the fracture toughness of ZrO2-based ceramic materials. The complex alloying of ZrO2 with yttrium and cerium oxides and the use of the Al2O3 additive leads to an increase in the fracture toughness and lowering of the negative effect of materials in the biological medium. In this work, the physicochemical properties of ceramic powders and materials of the ZrO2–2Y2O3–4CeO2–Al2O3 system synthesized by the chemical deposition of inorganic precursors when applying the sol-gel technology are considered based on scientific data and experimental studies. Alloying pure zirconium oxide by stabilizing Y2O3 and CeO2 oxides and thermal hardening of Al2O3 ensure the conservation of the tetragonal structure at room temperature, which makes it possible to retard and control the crack resistance of the material under the load. Investigations into the influence of the sintering temperature and aluminum oxide content on the microstructure and grain size, as well as physicomechanical properties of ceramic materials of compositions ZrO2–2Y2O3–4CeO2 + 1 wt % Al2O3 and ZrO2–2Y2O3–4CeO2 + 3 wt % Al2O3, are carried out.     

Modeling of Cast Iron Solidification—The Defining Moments

There are many to contend that today human civilization has reached the age of engineered materials, yet the importance of iron castings continues to support the thesis that we are still in the Iron Age. Cast iron, the .rst man-made composite, is at least 2500 years old. It remains the most important casting material, with about 75 pct of the total world tonnage. This article is a review of the mathematical models that describe the fundamentals of solidification of iron-base materials, from the seminal articles by Jackson–Hunt (JH) and that of Old.eld, the first to attempt modeling of microstructure evolution during solidification, to the prediction of mechanical properties. The latest analytical models for irregular eutectics such as cast iron as well as numerical models with microstructure output (visualization models) are discussed. Because of space limitations, models for casting soundness are not included. This article is based on a presentation made in the symposium entitled “solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13–15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Dielectric characteristics of sintered silicon nitride materials

We have studied and summarized data on structure formation and dielectric characteristics of Si₃N₄-based materials obtained by hot pressing and activated sintering. We have measured the dielectric characteristics in the frequency range 1 kHz to 10 MHz. We have established that the level of the dielectric characteristics of the materials is significantly affected by the content of highly dispersed Si₃N₄ powder obtained by plasmochemical synthesis in the original mix. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 22–26, March–April, 2000.     

Three-sublattice model that takes into account the spin density anisotropy,short-range order,and dimensioned factor for the binary Fe-Cr(V,Mo) systems

The diffusion properties of the elements entering into the composition of austenitic and ferritic steels are analyzed as functions of the temperature and the time evolution of induced radioactivity. As compared to austenitic steels, ferritic steels are shown to have better diffusion properties and swelling resistance during operation at temperatures up to 650°C under irradiation conditions and can be used as structural materials in the core of a fast neutron reactor. Experimental and calculated (Calphad method) data on the Fe-Cr phase diagram and the thermodynamic properties and the experimental data on the short-range order in Fe-Cr alloys are analyzed, and it is concluded that they are conflicting and that models taking into account the short-range order should be developed. The results of quantum-mechanical calculations of the average magnetic moment for ferromagnetic (FM) iron as a function of the volume are analyzed and used to introduce a concept of partial magnetic moments of the iron atoms located in the first four coordination spheres (1–4 CS). The values of these moments are calculated. The concept of the partial magnetic moments of iron atoms agrees qualitatively with the experimental data on the spin-density anisotropy of the bcc lattice of pure iron. This concept is used to formulate a three-sublattice model for binary FM alloys of Fe-M systems (M is an alloying paramagnetic element). An extended cell whose sites contain 8 bcc cells and 16 atoms per cell and that is isomorphic to a DO₃-type crystal lattice is considered. A functional is constructed for the internal energy on the extended 16-atom cell. The dimensioned factor is taken into account in a self-consistent manner, by the expansion of the interaction energies of atoms of both components located in different CSs in the atom displacement with respect to ideal lattice sites. The dimensioned factor is taken into account in the three-sublattice model to obtain a set of equations of state for bcc FM binary iron-rich alloys in a 1–3 CS approximation. The obtained estimates demonstrate that the anisotropy of the distribution of magnetic moments in the bcc iron lattice is responsible for the appearance of a short-range order in bcc FM iron-rich Fe-Cr (V, Mo) binary alloys.     

Effect of compressive stresses on deformation and contact phenomena in pressed fibrous materials during sintering

A study is made of the processes that control deformation and contact phenomena in porous fibrous metallic materials. It was established that sintering of fibrous materials which are subjected to small compressive stresses in the pressing direction makes it possible to completely suppress the volumetric growth of specimens, which is seen during pressing and sintering. Due to the reestablishment of interparticle contacts, the resulting specimens have a significantly higher level of mechanical properties. For example, the ultimate tensile strength of the specimens is increased by a factor of 1.5–2 compared to control specimens of the same porosity. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 29–34, January–February, 2000.     

Powder metallurgy of beryllium: The developments of the national scientific center “Khar’kov physicotechnical institute”

For the first time in the production of construction beryllium an integrated technology for the production of a new generation of high-purity isotropic materials has been developed and mastered. The technology combines processes of vacuum distillation of the metal, gas flow atomization of a melt, specialized processing of the powder prior to densification and hot isostatic pressing. A technology for the production of spherical powders of beryllium and beryllium-based alloys by atomization of a melt is also developed and employed for the first time. A technology is developed and optimal conditions of isostatic pressing of spherical powders are determined. Isotropic beryllium with high physico-mechanical characteristics is obtained. The properties of the materials thus obtained are studied. The strength of the new materials is 20%, and their plasticity two to three times greater, than commercial beryllium. It is shown that the high purity of the initial powder, the cellular structure, uniform distribution, and dispersity of the particles of beryllium oxide and segregations of secondary phases are the principal factors that govern the level of the properties of compact beryllium materials. __________ Translated from Poroshkaya Metallurgiya,Nos. 3–4(448), pp. 118–125, March—April, 2006.     

Vibrodeformation properties of construction, composite, and high-porosity sintered materials

The indices of ultimate cyclic strain ε₋₁ and vibrodeformability R_ε (as the product of ε₋₁ and the decrement of material vibrations at this level of deformation) are introduced. Data on the fatigue strength and vibration decrement of traditional structural materials based on titanium, aluminum, magnesium, iron, and copper, as well as composite condensed materials of microlaminate and dispersion-strengthened structure and sintered high-porous materials based on copper powders, fibers, and foamed copper are analyzed. This allowed us to establish advantages of sintered high-porous materials in comparison with compact ones, and to conclude that high-porous materials based on discrete fibers, and foamed metals based on traditional structural materials will have higher ε₋₁ and R_ε indices than those of compact matrices. As far as the index of vibrostrength R_σ is concerned, sintered high-porous materials will be inferior to the compact materials. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 87–98, July–August, 2006.     

Predicting the conductivity of biporous powder materials

The electrical conductivity of biporous permeable powdered materials is calculated on the basis of percolation theory. These materials are made with a pore-forming additive having a different ratio of particle size than does the metal phase. Calculations for nickel powder specimens as an example are in satisfactory agreement with the experimental data. Deceased. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 67–70, March–April, 2000.     

Texture formation in the cold rolling of pseudo-alloys

The formation of metallographic and crystallographic textures and the properties of the composite materials W - Cu, W - Ni - Fe, and Ag - Ni with a metal matrix were studied after 5–85% squcezing by rolling. It was established that crystallographic texture tended to be suppressed by the localization of deformation and formation of cracks along interfaces, the type of which, as well as the character of the structure-sensitive properties depended on the nature of the material and its processing history. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 88–96, January–February, 2000.     

Effect of Structural Anisotropy on Contact Properties in a Silver - Graphite Composite

We have studied the effect of structural anisotropy on the contact properties of silver –5 mass% graphite composite. We have established that the highest erosion resistance is typical of contacts with a structure in which the flake-shaped graphite particles are located perpendicular to the working surface. Welding resistance is higher in contacts with a structure in which the graphite particles are located parallel to the working surface of the contact. We have shown that for any arrangement of the graphite particles in the silver matrix, contacts with an anisotropic structure made from bars obtained by extrusion are distinguished by a higher erosion resistance compared with contacts prepared by single pressing - sintering - final pressing - annealing. The results obtained are of practical importance for designing electrical contacts to be used in low-voltage automatic switches. They make it possible to improve the operating characteristics of silver - graphite composite materials.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 101–105, January–February, 2005.     

Formation of structure in powder bronze during electroplastic compression

We have used powder bronze to determine the laws governing formation of structure in powder materials during electroplastic compression (EC). We discuss the mechanisms of compression and the impact of the electroplastic compression process parameters on the formation of microstructure in the material. We identify regions in which varying the electroplastic compression parameters does not affect the structure of the resulting material. We determine the morphological characteristics of microstructures in which the original dendritic structure of the powder particles has undergone different levels of transformation into the equilibrium structure and the particles have different strength properties. __________ Translated from Metallurg, No. 5, pp. 66–67, May, 2007.     

Surface and Interface Energies of Complex Crystal Structure Aluminum Magnesium Alloys

Alloying Al with Mg can improve its structural properties but also can lead to the formation of grain-boundary precipitates of β-Mg₂Al₃ that lead to failure by intergranular fracture and corrosion. Simulating the properties of the β phase is difficult because it has a complex structure with more than 1000 atoms per unit cell. We approximate the experimental β structure by the β′ structure, which has about 300 atoms per unit cell, and we compute the fracture behavior of the material from density functional theory calculations of relevant surface and interface energies. We report also on experimental measurements of the orientation and fracture properties of the α-Al(Mg)–β-Mg₂Al₃ interface and compare them with the atomistic simulations. We have computed the surface energy of face-centered cubic α-Al with up to 10 at. pct Mg, as well as the decohesion energy of β′-Mg₂Al₃ and the interfacial decohesion energy between β′-Mg₂Al₃ and pure α-Al with geometry similar to that observed experimentally. We find that the β′-Mg₂Al₃ decohesion energy is nearly isotropic and is lower than the pure Al surface energy and the α-Al–β′-Mg₂Al₃ interface decohesion energy. This result is consistent with the experimental observations of fracture within the β phase rather than at the α-Al(Mg)–β-Mg₂Al₃ interface or within the α-Al(Mg) phase.     

Effects of raw material on the properties of partially alloyed powders and low-carbon materials based on them

Studies have been made on the effects of the raw material (cast iron and steel powders) on the properties of partially alloyed powders of grades D2, D2M, N2M, N2D2M, and N4D2M and also of materials based on them made by single pressing and sintering. When iron-bearing semifinished products are used, or the cast iron and steel powders, one can make partially alloyed powders with various compositions having good technological characteristics as well as materials based on them having good physical and technological properties.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 7–8, pp. 67–72, July–August, 1994.     

Mechanical properties of eutectic alloys β-NiAl + γ-Re and spray coatings

We have studied the structure and mechanical properties of eutectic alloys β-NiAl + γ-Re of the ternary system Ni-Al-Re. We have established that the best combination of mechanical characteristics, determined by local loading with a rigid indentor, is exhibited by the alloy containing 2.5 at.% Re, the structure of which consists of the eutectic β-NiAl + γ-Re. Rhenium inclusions can inhibit movement of cracks in the material, and also can play the role of traps for cracks. Brittle intercrystallite fracture is characteristic of the alloy consisting of one-phase intermetallic NiAl. Mixed fracture is typical of the eutectic alloy β-NiAl + γ-Re, with transcrystallite cleavage predominating. We have shown that plastic interlayers of a rhenium phase within the microstructure increase the crack resistance of a detonation coating made from eutectic alloys β-NiAl + γ-Re. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 78–87, March–April, 2006.     

Effect of carbon on volume changes during the sintering of an iron-chromium material

The effect of carbon added in the form of either cast-iron powder or graphite on the sinterability of chromium steel was investigated. The growth and instability of volume change during sintering produced by cast-iron additions was explained as a result of a thermally activated after-effect in the solid phase, and the formation of FeCr₂O₄ around particles of Kh30. Substitution of graphite for cast iron in the powder charge prevented oxidation of the chromium-rich particles, and resulted in low and stable shrinkage (0.1–0.8%) after sintering in the range 750–1250°C. Scatter in the degree of volume change was reduced from 1.5–2.0 to 0.1–0.3%. Iron-chromium material obtained with the use of graphite had improved strength properties as well as high produceability. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 38–42, May–June, 1998