Powder Metallurgy and Metal Ceramics (v.56, #7-8)

On 1 July 2017, Valery Skorokhod, a brilliant scientist, an originator of the modern science of materials, Editor-in-Chief of Powder Metallurgy, Director of the Frantsevich Institute for Problems of Materials Science from 2002 to 2015, and Academician of the National Academy of Sciences of Ukraine, passed away. Valery Skorokhod devoted all his active life to studying the properties of materials and developing the scientific foundations of their formation. The modern materials science is a synthesis of knowledge based on very different scientific concepts. One can rarely find an expert who would have mastered this knowledge to the extent Valery Skorokhod did. The range of Skorokhod’s scientific interests was extremely diverse and included virtually all aspects relating to the behavior of materials and associated effects. Despite his busy activities, Valery Skorokhod found time to conduct his own research in seemingly distant areas of science. When Valery Skorokhod departed from life, several studies performed on his own and in collaboration with his colleagues were in their final stage. The Editorial Board of Powder Metallurgy offers some of them to the readers.
Keywords: modern science of materials; scientific foundations of materials formation; sintering theory

Mechanism for Improving the Mechanical Properties of Sintered Iron–Copper Composites Alloyed with Molybdenum by V. V. Skorokhod; O. I. Getman; V. V. Panichkina; P. Ya. Radchenko; O. I. Bykov; A. V. Samelyuk (370-378).
The structure of Fe–Cu composites after solid-phase and liquid-phase sintering was studied. It is shown that 2–10 wt.% molybdenum additions have an activating effect on the diffusion processes in densification, grain growth, and recrystallization, as well as on the amount and composition of copper and iron solid solutions. Molybdenum additions to 70 wt.% Fe–30 wt.% Cu composites simultaneously influence their strength and ductility properties. With increasing molybdenum content, the solubility of iron in copper decreases, promoting higher ductility of the composites, and the solid solutions of copper and molybdenum in iron preserve their strength characteristics. Solidphase sintering results in fine-grained FeCuMo samples with high relative density (up to 98.8%) and high ductility.
Keywords: composites; iron; copper; molybdenum; sintering; strength; hardness; ductility

A modified continuum model for the sintering of biporous materials is proposed. Along with the viscous flow of the matrix resulting from the sintering potential, the vacancy flow from small to large pores is considered. The capabilities of the modified theory to consider the above phenomena are used to predict and explain the effect of constraints on local porosity increase and instability of the sintering process. Potential ways for using the results obtained are identified.
Keywords: sintering; bimodal pore distribution; rheology; constitutive equations; vacancies

Kinetics of Reaction Interaction Between Molybdenum and Iron–Tin and Iron–Copper Melts by V. V. Skorokhod; V. P. Titov; M. I. Filippov (385-392).
The solubility of molybdenum in Fe–Cu and Fe–Sn melts and the growth kinetics of Mo6Fe7 layer at the molybdenum–melt interface at 1200°C are examined. The solubility of molybdenum in these melts is well described by the following equations: lgCMo = (–3.957 ± 0.176) + (25.77 ± 7.94) XFe –– (143.02 ± 77.92) XFe 2 (Fe–Cu melt) and lgCMo = (–2.783 ± 0.011) + (7.563 ± 0.111) XFe –– (10.844 ± 0.245) XFe 2 (Fe–Sn melt). The melt composition (atomic fractions) in the three-phase Mo–Mo6Fe7–melt equilibrium is established: 6.4 · 10–3 Fe, 1.64 · 10–4 Mo, and Cu being the rest (Fe–Cu melt); 0.046 Fe, 3.48 · 10–3 Mo, and Sn being the rest (Fe–Sn melt). The substantial difference between the growth rate constants in the melts, kFe–Sn >> kFe–Cu, at the same iron activity a Fe is attributed to the effect of admixtures (Cu and Sn) on the growth of the Mo6Fe7 layer. Data on the solubility of molybdenum in Fe–Cu and Fe–Sn melts and the growth kinetics of Mo6Fe7 in these melts are obtained for the first time.
Keywords: solubility; three-phase equilibrium; intermetallics; Mo6Fe7 layer growth; molybdenum; iron; copper; tin

The Role of Heat Transfer by Radiation in High-Temperature Reaction Synthesis of NiAl Intermetallide by K. M. Petrash; V. V. Skorokhod; V. P. Solntsev; T. O. Solntseva (393-398).
A thermal kinetic model is used to study the processes that occur in the Ni–Al powder reaction system in contact melting. The effect of control parameters on the nature of these processes is examined through a computer experiment. The kinetic dependences and maximum temperatures developing in the system in the synthesis of equiatomic intermetallide (NiAl) are compared for two heat transfer cases: radiation and thermal conductivity.
Keywords: nickel; aluminum; intermetallide; contact melting; thermal kinetic model; thermal radiation; thermal conductivity

Effect of Hydrogen Heat Treatment of Aluminum and Silicon Nitrides Powders on Mechanical Properties of Ceramics by I. A. Morozov; V. V. Skorokhod; R. O. Morozova; A. I. Itsenko; O. V. Bezdorozhev (399-406).
The effect of hydrogen heat treatment on the chemical composition and morphology of aluminum and silicon nitrides powders for producing ceramics with enhanced operation properties is investigated. It is shown that the hydrogen heat treatment leads to decrease in the content of admixture elements, loosening of aggregates of powder particles and, as a consequence, activation in compacting. The resulting ceramics have enhanced mechanical properties, contrary to the samples produced of nitride powders with no hydrogen treatment.
Keywords: hydrogen heat treatment; aluminum nitride; silicon nitride; admixtures; hydrogen; sintering; mechanical properties

Nanocrystalline Powders in ZrO2–Y2O3–CeO2–Al2O3–CoO System for Microstructural Design of ZrO2-Bazed Color Composites by E. V. Dudnik; V. V. Tsukrenko; M. S. Glabai; A. K. Ruban; V. P. Red’ko; A. I. Khomenko (407-415).
The changes in the physical–chemical properties of ZrO2–Y2O3–CeO2–Al2O3–CoC nanocrystalline powders, produced by hydrothermal synthesis combined with mechanical mixing, are investigated. It is found out that, in the presence of cobalt compounds, the temperatures of phase transformations of ZrO2-based solid solution decrease. The powders are characterized by increased activity to sintering. AMIS software is used for processing the research results on the powder morphology. The flame atomic absorption spectrometry is used to confirm that the probability of cobalt wash-out from ZrO2–Y2O3–CeO2–Al2O3–CoO composites does not exceed 0.2 mg/l. The research results will be used for the microstructural design of blue ZrO2-based composites.
Keywords: ZrO2 ; Al2O3 ; CoAl2O4 ; nanocrystalline powder; phase transformations; color composites; microstructural design

The density distribution over the radius of porous permeable hollow cylinder produced by radial isostatic compacting is analyzed and calculated. The factors promoting an inhomogeneous density distribution in porous permeable materials during this type of compacting are determined by analytical calculations. It is proved that the optimal pore distribution across the section of porous permeable material providing maximum operating properties can be obtained by using the compacting procedure that allows relating the structural properties of the material with principal operating modes.
Keywords: radial isostatic compacting; distribution density; porosity; stress field; stress deviator

GOST 25280–90 (CMEA 6741–89, ISO 3927–77) “Metal powders. Method for Determination of Compressibility” is considered. Given the deficiencies revealed, an alternative method for determining the compressibility allowing for the maximum pressure during ejecting of briquettes from cylindrical dies is proposed. The updated comparative method for determining the density includes several stages: (i) compacting lubricant-containing mixtures in lubricated and unlubricated dies and (ii) recompacting lubricated briquettes in a die with bigger diameter. The proposed method can be used for commercial compacting of powders.
Keywords: powders; density; porosity; lubricant; die; pressure; ejection

The publications of the effect of electric, magnetic, or complex electromagnetic fields applied in addition to well-known conventional processing of WC–Co powder composites are analyzed. Two types of composite processing are considered: conduction and induction heat treatment. The basic properties of equipment used by techniques for processing the above composites are studied based on the existing equipment. The effect of some features of the structure of materials on their properties is analyzed. A special attention is paid to structure defects, such as cracks and the continuity of brittle (WC) and ductile (Co) phases. Their effect on the strength, wear resistance, elasticity, fracture toughness, and other properties of hard metals is considered in details. This study provides a number of recommendations for eliminating or minimizing the impact of defects on the structure and properties of powder composites.
Keywords: electric discharge sintering; graphite die with surface hardening; electro-convective dissolution in liquid metal; fracture toughness of hard metal; cracks; influence by separate electric and magnetic fields

Model of Reactions for the Synthesis of Turbostratic Boron Nitride Nanoparticles from Urea by V. V. Garbuz; V. A. Petrova; L. S. Suvorova; T. A. Silinska; L. M. Kuzmenko (445-447).
A chemical model for the formation of nanosized turbostratic boron nitride powder from urea is proposed. The mechanism is based on the interaction of metaborate acid O=B—OH with urea (NH2)2CO. When OH groups are replaced by NH2 groups, H2O and CO2 are released. Metaboryl amide O=B—NH2 formed in the replacement process is studied. It is found that dehydration of the compound releasing >B—N< radicals promotes the subsequent formation of turbostratic boron nitride.
Keywords: chemical model of formation; turbostratic boron nitride; metaboryl amide; nanoparticles

Structural Transformations on the Surfaces of Epoxy Composite–Steel Tribological Pair by P. P. Savchuk; V. P. Kashytsky; O. L. Sadova; O. M. Lyushuk (448-455).
The modern problems of tribotechnical material science are considered; in particular, the selforganization in polymer composites has been studied. It is established that steady-state selective transfer reduces the wear rate of an epoxy composite material. It is confirmed that it can be used in partial or zero lubrication conditions. The self-organization processes that occur in polymer composites during tribological interaction determine the suitability of a system to be used as a selflubricating material. In friction, the determining factor is not only the initial material structure, but also the surface layer structure formed according to the load–speed operating mode.
Keywords: self-organization; structure; tribological contact surface; epoxy composite; friction interaction; counterface

Isothermal Section of the Al2O3–TiO2–Er2O3 Phase Diagram at 1400°C by Ya. S. Tishchenko; S. M. Lakiza; V. P. Red’ko; E. V. Dudnik (456-462).
The nature of phase equilibria in the Al2O3–TiO2–Er2O3 system at 1400°C is established and shown in the isothermal section of the phase diagram at this temperature. The interaction in the system is determined by compound Er2Ti2O7, which participates in equilibria with most phases of the system and determines its triangulation. No new phases or appreciable solubility regions based on components and binary compounds are found in the system. Ternary eutectic points are expected in the three-phase regions and binary eutectic points in the quasibinary sections.
Keywords: ceramics; titania; alumina; erbia; interaction; isothermal section; phase diagram

Theoretical bases for calculation of metastable phase transformations with participation of supercooled melts, such as immiscibility and glass formation, are considered. A dataset of the model parameters for the thermodynamic properties of phases in the Cu–Fe–(Ti, Zr, Hf) systems is presented for performing corresponding calculations in the framework of the CALPHAD method. The diagrams of metastable phase transformations with participation of supercooled liquid alloys and terminal solid solutions are calculated. The composition regions of metastable immiscibility of supercooled ternary liquid alloys are presented. The composition regions for producing rapidly quenched amorphous Cu–Fe–(Ti, Zr, Hf) alloys are theoretically estimated. The production of ternary immiscible rapidly quenched amorphous alloys in the composition region with x Me ≈ 0.1–0.2 is predicted. Bulk amorphous alloys are expected to be produced in the composition region with x Me ≈ 0.3–0.6.
Keywords: CALPHAD; alloys of copper and iron with titanium; zirconium; and hafnium; rapidly quenched and bulk amorphous alloys; immiscible rapidly quenched amorphous alloys

Interface Evolution of YG11C/42CrMo Joint Brazed with BCu64MnNi Filler Metal by Haibin Geng; Jinglong Li; Jiangtao Xiong; Changbin Shao; Fusheng Zhang (473-480).
An erosion often occurs, when brazing cemented carbide with Cu–Mn based filler alloy, therefore, the binder phase erosion and interface evolution are systematically investigated. When heating, the binder phase Co dissolves into the molten filler and the erosion product acquires liquid state. Heavy WC particles successively submerge into the molten filler, while the erosion layer becomes incompact. With increasing brazing temperature, the tensile strength increases to the maximum 589MPa at 970 °C with holding for 300 sec, and then decreases to 431 MPa at 1010 °C. The microscopic fracture morphology analysis reveals that the erosion product possesses poor wettability against WC particles. When long-time heat preservation at 970 °C, a structure gradient layer is locally formed, because the de-bonded WC particles submerge into the liquid filler metal successively, which promotes the thermal stress release and obtaining high-strength joint.
Keywords: induction brazing; hard metal; binder erosion; BCu64MnNi filler; Cu–Mn–Co ternary compound; interface evolution; gradient structure; property

Structural Defects of Initial Crystallization Areas in Single-Crystalline Turbine Blades by J. Krawczyk; A. Tondos; W. Bogdanowicz; R. Paszkowski (481-486).
The as-cast single-crystalline turbine blades made of CMSX-4 superalloy were studied. The blades were obtained by the Bridgman technique at a withdrawal rate of 5mm/min. The as-cast samples were prepared by cutting the blade root with a fragment of selector. The dendritic structure of obtained samples was studied by Scanning Electron Microscopy. The crystal orientation and lattice parameters were analyzed by Ω-scan mapping method. Additionally, the X-ray diffraction topography was applied. It was found that the most structural defects are created in the areas where a change in the shape and dimensions of the blades occur. Even minor changes in their geometry and the unevenness of mold walls may also affect the formation of defects.
Keywords: CMSX-4 superalloy; single-crystalline turbine blades; structural defects; X-ray topography