P. Ya. Radchenko

Properties of molybdenum powders obtained by reduction in moving beds

We have studied the process of decomposition of ammonium paramolybdate and reduction of the molybdenum oxides in moving beds with rotation of the working chamber. We have shown that agglomerated disperse molybdenum powders are formed under these conditions. Movement of powder beds during reduction ensures continuous contact between the oxides being reduced and the hydrogen, rapid removal of water vapor from the reaction zone, and establishment of nearly kinetic reduction conditions. As a result the metallic molybdenum powders are strong, highly porous agglomerates, similar to the MoO3 particles in size and shape. The powders have high bulk (as-poured) density and good flowability, and their dispersity depends only on the reduction temperature.

Interdiffusion and Structural Changes in the Cr2O2–Al2O3(ZrO2) Diffusion Couple under Microwave Heating

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.

Nature of changes in the microstructure and properties of fine-grained iron-copper composites under heat treatment

We examine the nature of the changes that occur in the microstructure and properties of fine-grained iron-copper composites with 30 mass % (27.3 vol. %) Cu during solid-phase heat treatment and when passing through the melting point of copper. Quantitative studies of the microstructure were made during sintering of mixtures of the highly dispersed powders of the initial metals and during heating of sintered high-density fine-grained specimens. The process of microstructure transformations during liquid-phase sintering and heating of high-density fine-grained composites above the melting point of copper was found to have three stages: recovery of the crystal structure and formation of large-angle boundaries in the Fe component, an increase in Fe grain size, and formation of solid solutions by mutual diffusion of components; penetration of the liquid phase along Fe grain boundaries with a decrease in grain size because of disintegration; and a secondary growth of Fe grains and formation of a Cu matrix structure or, more likely, a matrix structure of solid solution of Fe in Cu begins to form.

Segregation of solutes to the grain boundaries of tungsten-copper alloyed pseudoalloys

The authors examine the effect of silicon additions on the redistribution of solutes in the struture of a tungsten network in tungsten-copper pseudoalloys and determine their ductility characteristics in tensile loading. The fracture surfaces of the specimens were examine using a JAMP10S Auger microprobe, and the tensile test was carried out using the standard procedure. Data show that the introduction of silicon greatly reduces the content of carbon and oxygen both in the volume and at the grain boundaires of tungsten.

Mechanism for Improving the Mechanical Properties of Sintered Iron–Copper Composites Alloyed with Molybdenum

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.

COPPER HARDENING WITH FINE IRON PARTICLES

The production of sintered bulk Cu–(5–25% Fe) pseudoalloys retaining the α-iron phase is studied. The pseudoalloys with a relative density of 97.5–98.1% are prepared by sintering a powder mixture of the metals reduced from their nanosized oxides. The microstructure of the composites represents a copper matrix with α-iron phase inclusions of about 20–200 nm in size, formed by 6.5–16 nm nanocrystallites. When iron content is 5 and 10%, the composites show matrix microstructure in relation to copper, providing 47–52% electrical conductivity and promoting higher hardness (to 1380 MPa) through precipitation hardening of the copper matrix by iron nanoparticles. The composites acquire matrix-statistical microstructure with increasing iron content.

The Structure and Properties of Powder Copper Hardened by Fine Tungsten Particles

The Cu–W pseudoalloys with 2–10 vol.% of nanosized tungsten particles (30–40 nm) are studied. The bulk samples with relative density up to 99.1–99.6% are produced by shock compaction. The introduction of nanosized tungsten particles increases the pseudoalloy strength to 2.7HCu with insignificant reduction in plasticity (δ = 13%) and conductivity (to δ = 0.875% IACS for the pseudoalloy with 10 vol.% W). The microstructures of the Cu–W pseudoalloys are analyzed.

Effect of heat treatment on the plasticity of molybdenum-doped iron–copper pseudoalloys

The effect of heat treatment on the mechanical properties and microstructure of Fe–30% Cu pseudoalloys doped with 10% Mo is studied. The samples were produced by compacting mechanically alloyed metal powder mixtures and subjecting them to solid-phase sintering (SPS) and liquid-phase sintering (LPS) at 1000 and 1130°C, followed by quenching and tempering. It is shown that doping Fe–Cu pseudoalloys with molybdenum increases the density of the compacts after both SPS and LPS (residual porosity about 1%). The interdiffusion of all the three components promotes the formation of stable heterophase fine-grained microstructure which prevents grain growth and improves the plasticity of the FeCuMo pseudoalloys. Heat treatment increases the strength of FeCuMo and does not affect its high plasticity. The FeCuMo samples produced by SPS and LPS show optimum values of ultimate strength (683–694 MPa and 741–752 MPa), elongation (12.1–12.4% and 8.2–9.4%), and contraction (24.0–25.9% and 12.5–19.7%) after heat treatment.

Consolidation processes in the sintering of fine-grained iron-copper pseudo-alloys

The properties of fine-grained iron-copper pseudo-alloys (ICPA) and consolidation processes that occur during their sintering are studied. The grains are no larger than 0.5 µm. It was established that the specimens undergo shrinkage, not growth, when fine iron-copper mixtures are sintered within the range 600–1130 °C. This occurs as a result of active consolidation of the dispersed powder mixture, shortening of the diffusion paths, and the active formation of solid solutions based on iron and copper. Sintered ICPAs have a stable fine-grained microstructure with a maximum grain size of 0.5 µm only when they are sintered and treated in the solid phase. A coarse-grained structure is formed when ICPAs are heated to temperatures at which a liquid phase appears. Fine-grained ICPAs are also characterized by high hardness (up to 240–260 HB).

Effect of oxygen on the liquid-phase sintering of very fine tungsten-copper powder mixtures

A study was made of the processes of oxygen removal accompanying the sintering of blanks from W-30% Cu (TC) powder mixtures and of the effect of this refinement on their densification. It has been established that at 1100-1200/sup 0/C, oxygen-saturated copper does not wet tungsten. With a rise in temperature from 800-1000/sup 0/C, the solubility of hydrogen in copper increases threefold, and at the same temperature, the coefficient of diffusion of oxygen in copper is slightly smaller. A reduction in oxygen content to 0.03% in the sintering in hydrogen of parts of wall thickness greater than 10mm takes place only after the copper has melted.