O. I. Get’man

Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y2O3–Al2O3 Diffusion Couple

The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al2O3–Y2O3 samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered Al2O3–5 vol.% ZrO2 layers, and yttria layers, such as sintered coarse-grained samples or fine Y2O3 powder layers on the Al2O3 surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, particularly to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al2O3–Y2O3 system, which involves both the change in the phase composition of the diffusion zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in Y2O3 oxide because of the unipolar diffusion of Al3+ ions to Y2O3. The validity of the proposed mechanism is confirmed by numerical evaluations.

Use of powders of the tungsten—Rhenium alloys for the preparation of impregnated cathode skeletons I. Densification and formation of porous structure in sintering of powders of tungsten-rhenium alloys

The effect of phase composition, fineness, and morphology of W-Re powders with different rhenium content on compaction, and formation of porous structure has been studied. Fine particle powders with a specific surface of 5–8 m2/g, mechanical mixtures of tungsten and rhenium powders, and W-Re alloy powders were used. Local nonuniform compaction is observed during sintering of powder mixtures and alloy powders. It was established that alloy powders and intermetallic powders have lower sintering activity.

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.

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.

Structural Engineering of Impregnated Dispenser Cathodes

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.

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.

Effect of boron additives on the structure and properties of soft magnetic composites produced from nickel-clad iron powders

The effect of boron additives on the structure, magnetic properties, and corrosion resistance of Ni– P-clad iron-based soft magnetic materials is studied. The iron powder (with a particle size of 250 to 350 μm) is clad with nickel–phosphorus through thermochemical reduction of nickel chloride by sodium hypophosphite, and boron (1 and 3 wt.%) is added to activate the formation of a liquid phase during high-temperature sintering and reduce the porosity of the prepared composites. It is shown that the cladding of iron powder with nickel–phosphorus increases its corrosion resistance by two points on the scale of ISO 11130:2010 and decreases the depth corrosion index from 0.6457 mm/year to 0.0269 mm/year, which is likely due to the high corrosion resistance of the nickel-phosphorus coating. Moreover, the cladding of iron powder with nickel–phosphorus substantially decreases (2–2.5 times) the magnetic loss in ac fields at a frequency of 50 Hz. The microstructure of the Fe–Ni–P–B composites with different boron contents is heterophase and consists of iron-based ferritic grains, pores, and a liquid phase based on the γ-Fe + Fe2B, α-Fe + Fe3P, and Ni + Ni3B eutectics. The boron content of the material should not exceed 1% because a decrease in the volume of the ferromagnetic component reduces the magnetic induction and permeability. The addition of boron to the clad iron powder increases the hardness and strength of the material produced.