Effect of Nanocrystalline Powders on the Structure and Properties of Dispersion-Hardened Alloy TiC – 40% KhN70Yu

This article examines aspects of the effect of nanocrystalline powders of ZrO₂, Al₂O₃, W, WC, WC–Co, NbC, and Si₃N₄ on the combustion, structure, and physical and physico-mechanical properties of new dispersion-hardened electrode alloy TiC – 40%KhN70Yu. This heat-resistant hard alloy, based on titanium carbide and a nickel alloy, was obtained by self-propagating high-temperature synthesis (SHS). It is shown that the addition of a nanocrystalline powder decreases combustion rate, with the magnitude of the reduction depending on the specific surface of the addition. It was determined that the structure of the synthesis products is modified appreciably by introducing a mixture of nanocrystalline powders into the initial charge. Here, additions of ZrO₂ and NbC have a positive effect on the main physico-mechanical characteristics of the alloy (strength, hardness, fracture toughness). Nano-powders of Al₂O₃ and Si₃N₄ have a negative effect on the alloy's physico-mechanical properties. The addition of WC–Co increases the flexural strength of the material, while the addition of W and WC increases its fracture toughness.     

Effect of Mineralizing Agent on the Phase Composition of Nanocrystalline Powder in the System ZrO2 ― Y2O3 ― CeO2

The effect of mineralizing agent (AlF₃) on microstructural rebuilding and phase transformation that occurs during heat treatment in the range 600-1000°C for powder of the composition ZrO₂ 7 mole% CeO₂ 1 mole% Y₂O₃ is studied. The original nanocrystalline powder is prepared by high-temperature hydrolysis and sol-gel technology. It is established that during heat treatment there is decomposition of the solid solution based on T-ZrO₂ by a eutectoid reaction with formation of M-ZrO₂ and Ce₂Zr₃O₁₀. The compound Ce₂Zr₃O₁₀ is retained under these conditions at 1000°C. Anisotropic crystals of different chemical composition (M-ZrO₂, Ce₂Zr₃O₁₀, -Al₂O₃) form within the sintered matrix of a solid solution based on T-ZrO₂.     

Ti-Y2O3 Composites with Nanocrystalline and Microcrystalline Matrix

Mechanical milling of a Ti-2 pct Y₂O₃ powders mixture led to the synthesizing of a composite powder with a nanocrystalline Ti matrix having a mean crystallite size of 19 nm. Both the nanocomposite powder prepared through milling and the initial mixture of powders were consolidated by hot pressing under the pressure of 7.7 GPa at the temperature of 1273 K (1000 °C). The transmission electron microscopy (TEM) investigations of the bulk sample produced from milled powder revealed that Y₂O₃ equiaxial particles of less than 30 nm in size are distributed uniformly in the Ti matrix with a grain size in the wide range from 50 nm to 200 nm. The microhardness of the produced nanocrystalline material is 655 HV0.2, and it significantly exceeds the hardness of the microcrystalline material (the consolidated initial mixture of powders), which is equal to 273 HV0.2. This finding confirms that reducing the grain size to the nanometric level can have a beneficial influence on the hardness of titanium alloys. Dispersion hardening also contributes to the hardness increase.     

Change in the Physicochemical Properties of Nanocrystalline Powder Based on ZrO2 in the Presence of a Mineralizing Agent

The change in physicochemical properties of nanocrystalline powder of the composition ZrO₂ ― 3 mole% Y₂O₃ in the presence of aluminum fluoride is studied. The starting powder is prepared by a complex method including elements of hydrothermal synthesis and sol-gel technology. It is established that these conditions expand the temperature limits for the existence of ZrO₂ monoclinic solid solution. Transformation is connected with adsorption of fluorine at the ZrO₂ surface, diffusion in the solid phase, and a reduction in anion vacancy concentration.     

Change in the Physicochemical Properties of Nanocrystalline Powder Based on ZrO2 in the Presence of a Mineralizing Agent

The change in physicochemical properties of nanocrystalline powder of the composition ZrO₂ ― 3 mole% Y₂O₃ in the presence of aluminum fluoride is studied. The starting powder is prepared by a complex method including elements of hydrothermal synthesis and sol-gel technology. It is established that these conditions expand the temperature limits for the existence of ZrO₂ monoclinic solid solution. Transformation is connected with adsorption of fluorine at the ZrO₂ surface, diffusion in the solid phase, and a reduction in anion vacancy concentration.

     

Refractory and ceramic materials

The paper examines the consolidation of 95 mole% ZrO₂-2 mole% CeO₂-3 mole% Y₂O₃ nanocrystalline powder in cold uniaxial double-action pressing, cold isostatic pressing (60 and 120 MPa), and sintering. Five starting powders are produced by processing a suspension after hydrothermal decomposition in different conditions. It is established that a homogeneous microstructure forms only in a material from the powder subjected to two homogenizing grindings. After cold uniaxial pressing and cold isostatic pressing, the sintered samples reach a relative density of 0.96 to 0.94. The bending strength is 600 to 660 MPa. The efficient consolidation of ceramics requires comprehensive processing of starting nanocrystalline powders to modify their morphology. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 7–8 (456), pp. 45–58, 2007.     

Change in Iron Powder Properties with Treatment in Rolls

Conditioned iron powders are prepared by carbothermal reduction of “Blue Dust” (India) ore concentrate followed by compaction treatment in rolling mill and a decarburizing anneal. The properties of powder prepared by grinding in a vibration and rolling mill are determined. The method makes it possible to control the bulk density of the powder from 1.75 to 2.75 g/cm³. Determination of the production properties of the powders obtained and powder NC.100.24 by the same procedure established satisfactory conformity of the results. The technological desirability of preparing conditioned iron powder by treatment in a rolling mill is demonstrated.__________Translated from Poroshkovaya Metallurgiya, Nos. 3–4(442), pp. 12–16, March–April, 2005.     

Features of solid solution formation with a fluorite type structure in the system ZrO2-HfO2-Y2O3 with different synthesis methods

X-ray phase, petrographic, and thermal analysis methods are used to study the properties of solid solutions with a fluorite type structure in the ternary system ZrO₂-HfO₂-Y₂O₃ in relationship to preparation method: mixing of the original powders followed by solid-phase sintering and melting in solar furnaces; hydrothermal synthesis of nanocrystalline powders followed by solid-phase sintering. It is shown that in specimens whose composition lies at the isoconcentrate of 10 mole% Y₂O₃ a single phase forms independent of preparation method, i.e. a solid solution with a fluorite type structure. The azeotrope, situated at the liquidus of the limiting binary system ZrO₂-Y₂O₃ in the region of fluorite-like solid solutions, affects the melting temperature of ternary solid solutions and the lattice parameters of specimens after melting in a solar furnace. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 3–11, January–February, 2006.     

Chemical heat treatment of powder materials based on iron

Diffusion chromizing and boriding of iron-base powder materials SP30, SP90, and SP90D3 are studied. The growth kinetics for diffusion layers are determined depending on diffusion impregnation conditions, and material composition and porosity. Comparative bending and tensile strength tests as well as wear- and heat-resistance tests for materials after sintering by standard conditions or boriding and chromizing are performed. It is shown that the mechanical properties of materials after different types of processing are approximately the same whereas the wear and heat resistance after diffusion impregnation increase markedly. This makes it possible to conclude that a combination of sintering and boriding (or chromizing) is possible under the following conditions: boriding at 1000°C for 2–3 h; chromizing at 1100°C for 3–4 h.Kiev Polytechnic Institute. Translated from Poroshkovaya Metallurgiya, No. 8(368), pp. 37–43, August, 1993.     

Nanocrystalline Powders Based on ZrO2 for Biomedical Applications and Power Engineering

An 80 mass% ZrO₂ ― 20 mass% Al₂O₃ powder was produced using a complex method which integrates sol-gel technology and hydrothermal synthesis. The specific surface areas of the powder varied from 39 to 5.3 m²/g depending on the thermal treatment conditions. Metastable F-ZrO₂ formed after powder annealing at 400°C. The phase transformation F-ZrO₂ → T-ZrO₂ (traces of M-ZrO₂) occurred under powder thermal treatment from 700 to 1000°C. Only Θ-Al₂O₃ was detected under experimental conditions. The powder was characterized by sintering activity. Operating the processes under powder thermal treatment in the ZrO₂ ― Y₂O₃ ― CeO₂ ― Al₂O₃ system will allow one to produce a variety of ceramic microstructures from fine-grained to “self-reinforced.” These powders can be used in manufacturing surgical cutting tools as well as in ceramic passive bioimplants and solid electrolytes for fuel elements.     

Protection of carbon materials from high-temperature gas corrosion

A study is made of composite carbon -borosilicide on carbon materials prepared by gas phase deposition, diffusion impregnation, liquid-phase impregnation, and fusion. It is shown that muldlayer coatings prepared from carbon-borosilicide materials, particularly silicon and titanium carbides, and molybdenum, tungsten, and hafnium borides and silicides have greater heat resistance in the temperature range 1500–2000°C in air. Their protective properties are strongly dependent on composition, coating structure, and preparation methods as well of the grade of the original carbon materials.NNTs Kharkov Physicotechnical Institute. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(384), pp. 47–50, March–April, 1996. Original article submitted October 17, 1994.     

Synthesis of ZrO2-HfO2-Y2O3-Sc2O3 Nano-Particles by Sol-Gel Technique in Aqueous Solution of Alcohol

Zirconium dioxide is a significant material withgood physicochemical characteristics . It is used inmanyfields such as energysources ,material and envi-ronment . Cubic phase zirconium dioxide doped withyttria (YSZ) is alreadyin wide use in solid oxygenfu-…     

Oxidation of Cr-Ni coatings on diamond

The kinetics of the oxidation of chromium and chromium-nickel coated diamond powders in air at 750 and 850°C was studied by means of interrupted weighing. The coatings and metallized powders were subjected to x-ray phase analysis. Decrease of the powder mass confirms that permeation of the mass with volatile oxides predominates over the increment due to the weight of the absorbed oxygen in the oxides NiO and Cr₂O₃. The weight loss is caused by the burning away of both the coating (oxidation of chromium and its carbides to CrO with formation of CO and CO₂) and the diamond. Carbon from the diamond is also removed with CO and CO₂ as a result of successive reduction of chromium oxide by carbon, which diffuses to the oxidized layers, and further reduction of the carbides by the same mechanism. The higher the oxidation temperature and the thinner the coating, the greater the contribution of these reactions is to the processes responsible for the decrease in weight of the metallized diamonds. It is also encouraged by diffusion of carbon to the scale formed and by simultaneous diffusion of oxygen to the coating. The reactions of primary oxidation of chromium contained in the nickel solid solution, reduction of nickel by carbon and chromium, and dissolution of chromium in the reduced nickel contribute significantly to the oxidation of chromium-nickel coatings.Translated from Poroshkovaya Metallurgiya, No. 7(367), pp. 62–66, July, 1993.     

Formation of Polycrystalline Boron Carbide B4C with Elevated Fracture Toughness

We have studied the processes of formation of high-density polycrystalline boron carbide B₄C during sintering under high pressure (5.5 GPa) and at high temperature (2200 K). Study of the structure by transmission electron microscopy showed that under the indicated conditions for sintering B₄C powders with original particle size range 2–80 µm, processes of primary recrystallization occur, as a result of which interlayers of nanodispersed grains (10–50 nm) are formed in the near-surface regions of the original coarse grains. The microhardness of such material is 41 GPa, and the fracture toughness is 7.2 MPa · m^1/2.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 90–101, January–February, 2005.     

Effect of the method of production of porous powder metallurgy materials on their capillary properties (review)

The unique features of liquid infiltration in capillary porous powder metallurgy materials (CPPM) are discussed. The effect of the production method on the maximum height and rate of capillary rise of a liquid in CPPM is clarified. It is shown that in materials with a uniform pore structure, and the same average pore size, the maximum value of H_max is attained when the form factor of the initial powder particles is a minimum. The highest values of liquid infiltration rate at closely similar heights of capillary rise (within 5–7%) were obtained in CPPM with a variable pore distribution prepared from mixtures of powders with dtiferent particle sizes (pseudo fluidization method), or in CPPM with a biporous structure prepared by the sintering of a previously oxidized powder, by compacting metal powder with a pore forming agent, as well as by the cathodic deposition of a porous copper structure from electrolytic solution.Scientific Research Institute for Powder Metallurgy, Minsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 30–39, May-June, 1996. Original article submitted July 14, 1994.     

Effect of separate and combined milling of Cu and TiB2 powders on the electrical and mechanical properties of Cu–TiB2 composites

The present work compares the properties of the Cu–TiB₂ composites prepared by varying the mechanical milling conditions. The Cu–TiB₂ composites were processed using Cu–TiB₂ powders combined milling, a powder mixture consisting of separately milled Cu & TiB₂ and a powder mixture prepared by the combination of separate and combined milling. The hardness and flexural strength of the combined milled powders were found to be maximum, despite of their lower sintered density. The separately milled powders achieved excellent electrical properties combined with moderate hardness and flexural strength. The properties of composites processed using the combination of separate and combined milling laid in between the two conditions of combined and separate milling.     

Mechanism and sintering kinetics of moistened Y—Ba—Cu—O powders

The influence of preliminary moistening of YBa₂Cu₃O_7−x powder on the kinetics of sintering and recrystallization, and the evolution of element and phase compositions in the bulk and on grain boundaries, was studied by the methods of scanning electron microscopy, x-ray microprobe analysis, and x-ray diffractometry. It was established that moistening the powder leads to a change of the sintering mechanism from bulk diffusion (for unmoistened powder) to diffusion through solid layers in the grain boundary and subsurface regions of moistened powder, formed from the interaction products of YBa₂Cu₃O_7−x with the moist atmosphere. The volume diffusion coefficients in YBa₂Cu₃O_7−x, and the diffusion coefficients in the solid layers, were calculated from the sintering kinetics data. The diffusion coefficients in the solid layers were 2–3 orders of magnitude hither than the volume diffusion coefficients. This results in more rapid sintering of the moistened than the unmoistened powders. Kharkov University. Institute of Monocrystals, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 1–2, pp. 50–54, January–February, 1997.     

Formation of the phase composition of tungstenless hard alloy powder with mechanochemical synthesis

Carbon and nickel are studied for their effect on the formation of phase composition of tungsten-free hard alloy powder prepared by mechanochemical synthesis. It is established that the phase formation of this powder is governed by the initial titanium carbide stoichiometry. If titanium carbide has a nonstoichiometric composition, the alloy structure corresponds to the three-phase equilibrium TiC_1–x-Ni₃Ti-Ni, and otherwise it corresponds to the equilibrium TiC-Ni.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 21–24, November, 1992.     

Control over the particle size of stainless-steel powders

Conclusions Replacing some of the metallic iron by ferric oxide, Fe₂O₃, in the charge for the preparation of Kh18N15 and Kh23N28 stainless-steel powders by the calcium-hydride reduction process increases (two- to threefold) the yield of the fine fraction (-0.063 mm) without affecting the apparent density of the powders (1.4–1.85 g/cm³) or the sponge shape of their particles. At the present time, stainless-steel powders are being manufactured using a charge in which 20–30% of metallic iron has been replaced by the oxide Fe₂O₃.Translated from Poroshkovaya Metallurgiya, No. 2 (134), pp. 1–8, February, 1974.