Improvement in the Mechanical Properties of Silicon Nitride Ceramic in High-Temperature Deformation Processes

Studies have been made on the changes in structure and properties of sintered materials: Si₃N₄ - 5 mass% Y₂O₃ - 2 mass% Al₂O₃, Si₃N₄ - 5 mass% Y₂O₃ - 5 mass% Al₂O₃, and Si₃N₄ - 40 mass% TiN on deformation in a high-pressure chamber of toroid type (pressure 4–5 GPa, temperature 1000–1600 °C), and also by direct extrusion with degrees of reduction of 55 and 72% (temperature 1750–1850 °C, pressure on the plunger 20–30 MPa). After pressure-chamber treatment, the materials have elevated mechanical characteristics: HV₁₀ ≈ 16.7 GPa, K_Ic up to 8.4 MPa · m^1/2 for the system Si₃N₄ - Y₂O₃ - Al₂O₃; and HV₁₀ ≈ 16.9 GPa, K_Ic up to 9.4 MPa · m^1/2 for Si₃N₄ - TiN. A structure feature is the small size of the coherent-scattering regions: 51 nm for Si₃N₄ and 65 nm for TiN in the system Si₃N₄ - TiN, and 33 nm for specimens in the system Si₃N₄ - Y₂O₃ - Al₂O₃. High-temperature extrusion results in a structure with β-Si₃N₄ grains elongated along the deformation direction. The anisotropic structure has K_Ic values in directions perpendicular to and parallel to the direction of extrusion of 11.5–12.0 MPa · m^1/2 and 7.5–7.8 MPa · m^1/2, respectively. The hardness after extrusion becomes 16.0 GPa.     

Dephosphorization equilibria between pure molten iron and CaO-saturated FeOn–CaO–CaF2 slags

Metal-slag refining reactions have been investigated to determine dephosphorization equilibria in steelmaking using CaO-saturated FeO_n–CaO–CaF₂ slags low in P₂O₅ content. Slag compositions have been optimized to achieve maximum partition ratios wt.%(P₂O₅)/wt.%[P] and minimum phosphorus contents in pure molten iron at 1 550 and 1 600°C. FeO_n–CaO–CaF₂ slags prove to be most effective dephosphorizers. Optimal slag compositions are between 10 and 20 wt.% FeO_n. Attempts were also made to present phosphate capacities fractions of free oxygen ions and theoretical optical basicities Λ as a function of the FeO_n content of slags. Dephosphorization capacity of FeO_n–CaO–CaF₂ slags is compared with that of FeO_n–CaO–SiO₂ and FeO_n–CaO–Al₂O₃ slags.     

Fabrication of W–20%Cu composite powder from copper and tungsten salts

Abstract

An investigation has been made to prepare homogeneous powders of CuWO₄ and WO₃ from ammonium paratungstate and copper nitrate to prepare nanosized W–Cu powder. Hence, a mixture of ammonium paratungstate and copper nitrate with predetermined weight proportion was made in distilled water; while the content of the beaker was being stirred at a certain speed to reach the desired composition of W–20 wt-%Cu. Mixture was heated to 80–100°C for 6 h. Also, pH range was adjusted at about 3–4. The mixture was then evaporated and dried in the air. To reach W–Cu composite powder, the precursor powders burned out at 520°C for 2 h in the air to form W–Cu oxide powder and then were ball milled and reduced in H₂ atmosphere to convert it into W–Cu composite powder. The resulting powders were evaluated using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis and differential thermal analysis techniques. The results showed that homogeneous powders of W–Cu with particle size of ～100 nm and a nearly spherical shape could be obtained by this process.     

Interactions in the Ca2SiO4-CaZrO3 system

X-ray analysis at 20–1200°C and differential thermal analysis have been used to refine the transformation temperatures and unit-cell parameters for crystalline modifications of dicalcium silicate. Phase equilibria have been identified in the Ca₂SiO₄-CaZrO₃ system at 1100–2500°C. The phase diagram for this system has been derived. The characteristic features are the eutectic form for the liquidus and the formation of narrow solid-solution regions based on various crystalline modifications of Ca₂SiO₄.     

Structure and Properties of B4C - SiC Composites

We have investigated how the composition, grain morphology, and method of preparing the starting mixture affect the processes that form the structure and phase composition of B₄C - SiC composites during hot pressing. We found that, depending on the composition of the initial powder mixtures, which is responsible for different mechanisms of consolidation of ceramic materials during hot pressing, the grain size of the main B₄C phase and its defect content as well as the nature of the SiC phase distribution within the material differ significantly. When B₄C - SiC composites with a low SiC content are made from initial B₄C - B₄Si - B - C powder mixtures those composites have a high cracking resistance because of their fine grain structure.__________Translated from Poroshkovaya Metallurgiya, Nos. 3–4(442), pp. 112–119, March–April, 2005.     

Dephosphorization equilibria between pure molten iron and CaO-saturated FeOn–CaO–SiO2 and FeOn–CaO–Al2O3 slags

Metal-slag refining reactions have been investigated to determine dephosphorization equilibria in steelmaking using CaO-saturated slags, low in P₂O₅–content, based on the systems FeO_n–CaO–SiO₂ and FeO_n–CaO–Al₂O₃. Slag compositions have been optimized with respect to basicity and oxygen potential to achieve maximum partition ratios wt.%(P₂O₅)/wt.%[P] and minimum phosphorus contents in pure molten iron at 1550, 1600 and 1700°C. Both slag systems prove to be effective dephosphorizers. Optimal slag compositions are around 10 wt.% SiO₂ near the CaO–3CaO · SiO₂ double saturation in the case of FeO_n–CaO–SiO₂ slags and at Al₂O₃ contents tending to zero in the case of FeO_n–CaO–Al₂O₃ slags. Attempts were also made to present phosphate capacities C_PO4^3?, fractions of free oxygen ions x_O^2? and theoretical optical basicities Λ as a function of the FeO_n content of slags.     

Structure Formation in Alloys of the Cr3C2–TiN System and the Properties of Materials Based on Them

The addition of 10-20% TiN to chromium carbide made it possible to obtain almost pore-free Cr₃C₂–TiN alloy specimens by hot pressing in the range from 1500 to 1850°C at a pressure of 35.7 MPa for 25 min. The 90% Cr₃C₂–10% TiN alloy had the maximum hardness (89 HRA) while the alloy containing 15% TiN had the maximum bending strength (480 MPa). Activation of solid-phase sintering is shown to cause a Cr₃C₂ solid solution to form in the titanium nitride. In alloys containing 50% or more TiN sintering is activated by the formation of a liquid phase based on the Ti–Cr–C–N eutectic.     

Experimental Verification of the Thermodynamics of Critical Aspects of the Carbothermic Reduction of Alumina. Part II: Thermogravimetric Study of the Reaction of Al2O3–C Mixtures and the Al2O3–Al4C3 Phase Diagram

The thermodynamics of several aspects of the carbothermic reduction of alumina have been examined. In Part I, the results of measuring the evolved CO from the reaction between Al₂O₃ and C mixtures were used to determine the temperature and carbon contents for carbide formation at alumina saturation and at carbide saturation in the Al₂O₃–Al₄C₃ system. In this part of the paper, results are presented for a thermogravimetric study of the reactions of Al₂O₃ with carbon, as well as those for the determination of the Al₂O₃ liquidus line and the Al₂O₃–Al₄O₄C eutectic in the Al₂O₃–Al₄C₃ phase diagram. The critical temperature for Al₂O₃ and C to react, producing gas at 1 atm., was in agreement with that predicted from thermodynamics and measured in Part I of this paper. However, the Al₂O₃ liquidus appeared to be steeper and the eutectic temperature lower than the predicted phase diagram.     

The Fracture of Ordered (Fe,Co)3V

Observations have been made of the fracture surfaces of ordered (Fe₂₂Co₇₈)₃V deformed at low strain rates (3.3 and 42 x 10^-3 s^-1) in tension at temperatures from 20 to 1000 °C, that is, above and below the order/disorder transformation temperature of 950 ± 10 °C. From 20 to 700 °C, transgranular fracture occursvia the nucleation, growth, and coalescence of micro voids; correspondingly, high ductilities are obtained (34 to 44 pct). From 700 to 960 °C, the fracture mode changes from transgranular to intergranular and the ductility falls to a minimum of 4 pct. At 1000 °C a third mode operates, consisting of localized dynamic recrystallization followed by boundary sliding of recrystallized grains; ductility now rises sharply. The underlying mechanisms are considered and are supported through the results of additional experiments performed at distinctly higher strain rates.     

Electrochemical synthesis of tetragonal oxide tungsten bronze nanofilms on platinum

We are the first to synthesize nanofilms of tetragonal oxide tungsten bronze (OTB) on a Pt(110) substrate by the electrolysis of the K₂WO₄–Na₂WO₄–WO₃ melt at 700 and 750°C. The composition and the morphology of OTB are shown to depend on the deposition potential and the WO₃ concentration in the melt. The laws of formation of tetragonal OTB films are discussed. The synthesized OTB samples are found to have a good thermal stability in the temperature range 20–800°C.     

Formation and decomposition of C4F7 type calcium ferrites in superfluxed magnetite based pellets during oxidation and reduction

Abstract

In the current work the reactions of magnetite based pellets with large additions of calcite (3%CaO) during reduction have been investigated. This made it possible to use both X-ray diffraction (XRD) and scanning electron microscopy (SEM) to detect reaction phases that normally occur in very small amounts. The main binding phase in the pellets after oxidation was (CaO,MgO,FeO)₄(Fe₂O₃)₇, whereas the one commonly reported in the literature is (CaO)(Fe₂O₃)₂. During reduction at 500–700°C severe cracking occurred in these pellets, especially in the calcium ferrite phase. However, the decomposition of this phase began at 600°C, and therefore it is believed that the reason for the cracks is low strength of the phase itself, rather than weakness induced by reduction of the phase. Upon reduction of magnetite into wüstite at 800°C, the calcium began dissolving in the wüstite, and at 900°C porous calciowüstite had formed in the entire sample, except for some remaining magnetite left in the pellet cores.     

Effect of process parameters in the decomposition of a cryochemical salt product of the Bi - Pb - Sr - Ca - Cu - O system on the structure and properties of a ceramics

A study was made of the effect of the temperature and time of decomposition of a salt product on the phase composition, microstructure, and magnetic susceptibility of a Bi - Pb - Sr -Ca - Cu - O superconducting ceramics. Holding the powder at 570°C slows the formation of the high-temperature phases, while increasing the decomposition temperature to 840°C and increasing the time of decomposition at 815°C have the same effect on the concentrations of the phases Ca₂PbO₄, CuO, and (CaSr)₂CuO₃. The data obtained on magnetic susceptibility, together with the results of microscopic and x-ray spectral studies, made it possible to evaluate the effect of the morphology of the grains and the grain boundaries on the superconductivity of the sintered specimens. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 91–100, May–June, 2000.     

Microstructure and thermal stability of coated Si3N4 and SiC

Microstructure of coatings obtained by treating Si₃N₄ and SiC in Cr powders at 1273–1623 K has been studied employing XRD, SEM, AES and TEM. In accordance with thermodynamic calculations and kinetic consideration, the coatings have layered structures and contain metal-rich silicides and metal-rich nitrides or carbides. The microstructure of the coatings has been found to depend on the treatment conditions. The kinetics of the coatings growth obeys a parabolic growth law, the activation energies being close to the activation energies for self-diffusion of the corresponding metals. Thermal stability of the coated and uncoated Si₃N₄ and SiC in Fe-, Ni- and Co-based matrices has been studied and the coatings have been found to considerably improve the stability of Si₃N₄- and SiC-metal interfaces.     

Phase Equilibria in the Pr - Re - B System

X-ray diffraction methods have been applied to the component interaction in the system Pr - Re - B. Isothermal sections of the phase diagram have been constructed for 600°C (region of more than 33 at.% Pr) and 800°C (region less than 33 at.% Pr). The previously known borides Pr₂Re₃B₆, Pr₈Re_13–x B₁₂, Pr₇(Re₄B₄)₆ are confirmed, which have specific structural types. A new ternary compound has been found, Pr₃ReB₆ (structure not determined).__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 46–51, January–February, 2005.     

PRELIMINARY STUDIES OF THE FABRICATION OF UAl4–Al TEST-PLATES BY POWDER-METALLURGICAL TECHNIQUES

Abstract

The preparation of UAl₄ by induction melting and annealing (720°C, 993 K) yielded a product of restricted UAl₄ content (81 wt.-%). The UAl₄ phase has a variable stoichiometry (UAl_4.0–UAl_4.8), which leads to a slightly modified U–Al phase diagram. UAl₄–Al dispersions can be prepared by powder-metallurgical techniques, resulting in structures with higher UAl₄ content, improved phase-distribution rates, and specific particle sizes, compared with melting techniques. The thermal conductivity and thermal expansion of UAl₄–Al dispersions have been measured over the whole concentration range. No technological difficulties, by comparison with UAl₃-Al plate fabrication, were encountered. The production of UAl₄-Al test plates (33 vol.-% UA1₄ ? 35 wt.-% U) is thus feasible on a laboratory scale. The work reported is the final stage in a series of studies of uranium aluminide-aluminium dispersion fuels obtained by powder routes.     

Electrochemical polarization and passive film of a cryogenic and non-magnetic steel in aqueous solution

The corrosion behaviour of the austenitic steel Fe-23Mn-4Al-5Cr-0.3C in different aqueous solutions of pH-0.8 to 15.3 and the corrosion protection mechanism induced by adding Al or Al and Cr have been investigated by electrochemical measurements and AES/XPS analysis. The corrosion behaviour of Fe-Mn base steel have been compared with those of mild steel, cryogenic 9 % Ni steel, stainless steels 1Cr13 and 1Cr18Ni9Ti. The addition of manganese to mild steel is very detrimental to the corrosion resistance. Fe-25Mn steel passivates with difficulty even in such neutral aqueous electrolytes as 1 M Na₂SO₄ solution. The addition of 5 % aluminum to Fe-25Mn steel confers passivity to the steel in neutral or oxidizing, chloride-free solution. The addition of 5 % Cr to Fe-Mn-Al steel further improves resistance to corrosion. The passivity of Fe-23.5Mn-4Al-5Cr-0.3C steel in aqueous electrolytes tested is superior to that of 9 % Ni steel and approximate to that of 1Cr13 stainless steel. The corrosion resistance is probably imparted by a thin barrier film of oxides. The outer part of the passive film formed on the surface of Fe-23.5Mn-4Al-5Cr-0.3C steel in 1 M Na₂SO₄ solution is enriched in Al³⁺, Cr³⁺ and Fe³⁺, and this means that the film is probably made up of a mixture of Al₂O₃, Cr₂C₃ and Fe₂O₃.     

The extraction of iron(III) from aqueous sulphate solutions by primene sulphate

A study of the extraction of iron(III) from aqueous sulphate media by the primary amine Primene 81R at 50°C has been made. The compound extracted from the organic phase with Primene 81R sulphate in kerosene has been isolated and recrystallized from a methanol-acetone mixture and has been shown to have the stoichiometric formula 3(RNH₃)₂SO₄·(Fe(OH)SO₄)₂ represents the alkyl groups associated with the amine. It has been shown that the extraction of iron(III) occurs by an adduct formation reaction between primary amine sulphate molecules and the species IFe(OH)SO₄I₂ from the aqueous phase. On the basis of the experimental data and spectral studies a dimeric structure is suggested for the extracted complex.     

Formation of highly porous ceramic based on silicon nitride

The formation of the structure of a porous ceramic based on Si₃N₄ has been investigated. It has been found that the structure can be controlled over a wide range of porosities. Materials based on a consisting of a single fraction silicon nitride of grain size 3–5 μm, with the addition of a fine-grained pore agent have most uniform, hard, and developed porous structure. A comparative evaluation of the properties of material based on SiC and Si₃N₄ showed that silicon nitride materials of the same porosity are stronger and that small micropores can be formed in them. That is of fundamental importance in the development of materials—catalyst carriers for chemical production and various kinds of filtering devices. With the results of the investigations general technological recommendations can be made for producing ceramics with specific structures.     

Slag corrosion of dolomite-carbon refractories

Seven industrial doloma refractory samples, coming from three European suppliers, and with different carbon, or graphite, or binder content, are subjected to the action of a slag required for the desulfurization of the steel in the secondary metallurgy at 1600°C. Three tests have been carried out within two induction furnaces. The corrosion depth, at the slag-steel interface, has been measured; the graphite-containing samples display a better resistance than the graphite-free pitch or resin-bonded refractories. Among the graphite-containing samples correlations have been found with the carbon content and with the resistance to oxidation by CO–CO₂ at 1100–1200°C. The slag-refractory interface has been investigated by X-ray diffraction, optical microscopy and electron microprobe analysis; the observed phases are CaO, MgO, Ca₂SiO₄, Ca₃SiO₅ and Ca₁₂Al₁₄O₃₃ (outside graphite and iron). The corrosion mechanism is linked to the reaction of slag with lime, followed by infiltration of the refractory and dispersion of periclase grains in the slag.     

Production of Co–Cr–Ti Composite and Investigation of Mechanical Properties

Intermetallic materials such as Co₂Ti, Cr₂Ti are among advanced technology materials that have outstanding mechanical and physical properties for high temperature applications. Especially creep resistance, low density and high hardness properties stand out in such intermetallics. The microstructure, mechanical properties of %64Co–%32Cr and %4Ti powders were investigated using specimens produced by tube furnace sintering at 800–1200 °C temperature. A composite consisting of ternary additions, a metallic phase, Ti, Cr and Co have been prepared under Ar shroud and then sintered in tube furnace. XRD, scanning electron microscope, were used to characterize the properties of the specimens. Experimental results carried out for composition %64Co–%32Cr–%4Ti at 1200 °C suggest that the best properties as 182.09 HV and 5,584 g/cm³ density were obtained at 1200 °C.