Laws of capillary transport in porous materials prepared from titanium alloy VT6 fibers

A study was made of the kinetics of capillary absorption of ethanol in highly porous (70–75%) materials prepared from discrete fibers of titanium alloy VT6 (specimen dimensions 330×20×(0.4–1.3) mm). The fibers, 3 mm in length and 20–140 μm in diameter, were obtained by rapid solidification from the melt. Capillary transport against the force of gravity up to the equilibrium height of capillary rise was investigated in an atmosphere of saturated ethanol vapor. Experimental data on the rate of absorption were analyzed with reference to the properties of the pore space structure—effective pore size, tortuosity of the pore channels, and free surface area. The laws of capillary transport of ethanol in porous materials composed of titanium alloy VT6 fibers, discrete copper fibers, and grade VTEM-2 titanium powder were compared. It was shown that, with regard to the speed of absorption, the advantage of fiber materials over those made from powder is attributable to the less convoluted pore channels in the former. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 67–74, January–February. 1997.     

Influence of random pore-type mesodefects on the strength of brittle materials

The effect of pore-type mesodefects on the mean bending strength of brittle materials is calculated by a statistical method. The combined effect of defect amount in a specimen is analyzed and so is the effect of their mean size on the mathematical expectation of the breaking stress. The breaking point is found as a function of the pore size at constant porosity and various degrees of material homogeneity. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 102–106, March–April, 1999.     

Structure and high-temperature strength of composite materials based on boron nitride

The structure and physicomechanical properties of composite materials based on boron nitride within which new phases (mullite and sialon) form during hot compaction are studied. It is established that the microstructure of composites is specified by their texture formation caused by the crystal morphology of boron nitride particles and it is almost independent of composite phase composition. It is shown that the main factor that affects strength is porosity. The dependence of strength on porosity is exponential in character. The strength of boron nitride-mullite and boron nitride-sialon composites is 110–140 MPa and at 20–1200°C it is almost unchanged. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 33–39, May–June, 2006.     

Effect of structure on the fatigue strength of dispersion-hardened Ni - 20% Cr - Al2O3 condensed materials

The effect of the mean free path between particles A and the average grain size D, and also the duration of cyclic loading N in the range (0.2–5)·10⁷ cycles, on the fatigue limit of dispersion-hardened Ni - 20% Cr -Al₂O₃ condensed materials is studied on the basis of fatigue curves. With a correlation coefficient of more than 0.9 these dependences on A^−1/2 and D^−1/2 correspond to an equation of the Mott—Stroh type. It is shown that particle boundaries affect the cyclic strength of the materials to a greater extent than grain boundaries up to a concentration of 1.1%. A change in the fracture mechanism occurs with a correlation coefficient of less than 0.9. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 112–120, January–February, 2000.     

Composite material based on nanocrystalline SiC reinforced with continuous SiC fibers

A paste of nanocrystalline particles was obtained by magnetron sputtering of a target consisting of silicon and graphite powders. The target and a silico-organic oil were evaporated together. Model specimens were fabricated from the paste and continuous SiC fibers. Interaction between the fibers and nanoparticles was investigated by measuring the internal friction of the model composite materials. It was found that sintering occurs in 16–20 h at 1473–1500 K for pastes containing nanoparticles with a diameter of =2–4 nm, and in 60–100 h for pastes with diameter =6–10 nm. The sinterng temperature is 230–270 K below the degradation temperature of the fibers. It was shown that it is possible to manufacture a composite material consisting of a nanocrystalline SiC matric reinforced with continuous SiC fibers. The bend strength of the composite was σ _B = (73 ± 9)·10⁷ Pat at 1500 K, and (64±12)·10⁷ Pa after thermal cycling at 1500 K for 100 h. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5/6(395), pp. 69–75, May–June, 1997.     

Permeability of porous materials from refractory compounds

Summary A study was made of porous materials from spheroidized powders of refractory compounds. A relationship is established linking permeability with porosity and the particle size of the powder from which the materials were prepared. An empirical formula is proposed for determining the dependence of the coefficient of permeability on powder particle size and component porosity under linear filtration conditions. The integral and differential pore radius distribution curves obtained show that such materials are relatively homoporous and exhibit little scatter in pore radius size.Translated from Poroshkovaya Metallurgiya, No. 1(49), pp. 27–30, January, 1967.     

Carboborosilicide and oxide composite coatings on carbon materials

Complex carboborosilicide and oxide coatings on carbon materials prepared by vacuum activated diffusion impregnation predominantly through a liquid phase, vacuum fusion, and enamelling are studied. It is established that coating formation from the vapor phase hardly reduces the original porosity of carbon materials. The complex protective coatings developed that are formed by vacuum activated diffusion impregnation through a liquid phase lead to a reduction in the original porosity and gas permeability of carbon materials thereby increasing their high-temperature strength and heat resistance at elevated temperatures. It is shown that higher heat resistance in the temperature range 1800–2000 °C is provided by multilayer coatings based on carbides and borides of silicon, titanium and zirconium, borosilicides of tungsten, hafnium, molybdenum, and also oxides of silicon, aluminum, zirconium, and hafnium. It is established that the protective properties of coatings depend markedly both on their composition, structure, preparation method and on the form of the original carbon materials. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 21–27, March–April, 2006.     

Hydrogen partitioning in pure cast aluminum as determined by dynamic evolution rate measurements

A statistical analysis of the porosity in 99.995 wt pct pure commercially available cast aluminum has been correlated with real time hydrogen evolution data obtained in an ultrahigh vacuum furnace in order to estimate the hydrogen partitioning in the aluminum. The dynamic technique employed permitted the detection and separation of hydrogen evolved from solid solution, hydrogen released by the rupture of large pores, and gases desorbed from the aluminum surface. Results of the statistical analysis indicate average pore diameters in pure cast aluminum extend from less than 1 to over 400 μm. Interdendritic pores having diameters greater than 25 μm constitute over 98 pct of the pore volume. The overall volume fraction of pores was determined to be 0.71 pct. Compared to vacuum remelted rolled aluminum, the porosity resulted in a reduction of ultimate tensile strength of 13 pct and a reduction in yield strength of 21 pet. The evolution of hydrogen from the aluminum was observed to occur by large hydrogen pressure pulses due to the rupture of pores near the surface and by a smooth steady desorption from solid solution. The rupturing pores were observed visually and found to occur both in the solid state and after melting. A substantial change in slope of the desorption curve following the pulse train suggests the pores are the primary sources of hydrogen in the bulk. Analysis of the pore and pulse size distributions indicates more than 99 pct of the bulk hydrogen is partitioned in pores greater than 25 μm. Pressures within the larger pores (≈270 μm) were determined to be about 2.4 atm at room temperature. Hydrogen content in the large pores was found to be as high as 2 × 10¹⁶ molecules. The total hydrogen content in the pores and in solid solution was determined to be 6.3 × 10¹⁷ atoms/cm³ (0.43 cm³/100 g). Measurements on commercially available 99.9995 wt pct cast aluminum indicate the total hydrogen content to be 4.8 × 10¹⁷ atoms/cm³ (0.33 cm³/100 g).     

Numerical Simulation of Heat Transfer in Porous Metals for Cooling Applications

Porous metals have low densities and novel physical, mechanical, thermal, electrical, and acoustic properties. Hence, they have attracted a large amount of interest over the last few decades. One of their applications is for thermal management in the electronics industry because of their fluid permeability and thermal conductivity. The heat transfer capability is achieved by the interaction between the internal channels within the porous metal and the coolant flowing through them. This paper studies the fluid flow and heat transfer in open-cell porous metals manufactured by space holder methods by numerical simulation using software ANSYS Fluent. A 3D geometric model of the porous structure was created based on the face-centered-cubic arrangement of spheres linked by cylinders. This model allows for different combinations of pore parameters including a wide range of porosity (50 to 80 pct), pore size (400 to 1000 µm), and metal particle size (10 to 75 µm). In this study, water was used as the coolant and copper was selected as the metal matrix. The flow rate was varied in the Darcian and Forchheimer’s regimes. The permeability, form drag coefficient, and heat transfer coefficient were calculated under a range of conditions. The numerical results showed that permeability increased whereas the form drag coefficient decreased with porosity. Both permeability and form drag coefficient increased with pore size. Increasing flow rate and decreasing porosity led to better heat transfer performance.     

Effects of particle shape on technological properties of gas-sprayed high-speed steel powders

Gas-sprayed R6M5F3 high-speed steel powder has been examined for the effects of grain size and shape for the initial powder and for the powder deformed by rolling, as regards the poured density and strength of compacts. The particle size and shape have been determined with a specialized SIAMS analysis suite. The changes in the particle shape factors for the rolled powder reduce the poured density and increase the strength of compacts. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 12–18, January–February, 2006.     

Deformation of porous fibrous TiNi materials

The paper examines the strain of porous fibrous TiNi materials resulting from the shape memory effect. The most intensive growth of green compacts in heating is observed at 35 to 50°C and stops at 90°C. It is shown that the strain reaches 250% for fibrous TiNi compacts; the porosity thus increases by a factor of 1.4 to 1.8. It is established that flexible-rigid particulate bonds and shape memory effect in combination cause a manifold increase in the strain as compared with powder TiNi or fibrous tungsten. Sintered materials showed mainly the effect of superelasticity. The total shape recovery is close to 100% at a strain of 10 to 20% and sharply decreases at a strain greater than 30%. The most probable cause is the Ti₄Ni₂O_x phase. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 38–43, 2007.     

Mechanics of sintering materials with bimodal pore distribution. III. Kinetics of sintering combined with various forms of loading and adhesion conditions

A study has been made on the effects of loading scheme and conditions restricting macroscopic strain on the work-hardening kinetics and strain accumulation in the solid state in sintering materials with bimodal pore size distributions. Active loading intensifies the reduction in the small pores. The greatest effect comes from combining sintering with hydrostatic compression. At the same time, kinematic constraints (partial or complete adhesion in surfaces) substantially retards the shrinkage of large pores, which means that the porous structure can be controlled. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 10–15, May–June, 2006.     

Effect of heat treatment on the dispersity of Sn(IV)—Sb—O powders and the porosity of thick-film gas sensors based on them

A study is made of how the granulometric composition and porosity of powders of solid solutions in the system Sn(IV)−Sb−O is affected by the conditions of precipitation of mixtures of tin hydroxide and antimony hydroxide and the heat-treatment temperature. Powders of tin and antimony hydroxides have a microporous structure and a high (≥200 m ²/g) specific surface. Heat treatment above 870 K forms Sn_1−xSb_xO₂ solid solutions, this being accompanied by an increase in the size of the particles and transformation of the microporous structure to a mesoporous structure. An increase in the antimony content of the solid solutions helps form finer powders. A examination is made of the parameters of the pore structure of bulk specimens of semiconductor gas sensors obtained by heat-treating mixtures of powders of solid solutions and ultrafine clay. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 111–116, May–June, 1999.     

Structural and hydraulic characteristics of porous metals from bound gauzes

Conclusions By changing the wire diameter from 30 to 90 m, the number of wires in a strand from 1 to 40, and the porosity from 0.35 to 0.77, it is possible to obtain PGMs from bound lastingweave gauzes covering wide ranges of structural and hydraulic characteristics. The coefficient of variation of the mean pore size of PGMs ranges from 0.02 to 0.17, and that of the maximum pore size from 0.043 to 0.18, in the porosity range 0.35–0.77 for materials from 1-, 3-, and 6-wire strand gauzes of wire diameters 50 and 70 ym. The coefficient of variation of the coefficient of permeability of PGMs changes from 0.02 to 0.11 in the porosity range 0.40–0.77.Translated from Poroshkovaya Metallurgiya, No. 8 (260), pp. 42–45, August, 1984.     

Prediction of the filtering and mechanical properties of porous cold-worked stainless steel

Conclusions Additional cold plastic working makes it possible with a decrease in porosity and pore size to increase the tortuosity of the pore channels, to increase the strength, and to reduce the allowable bend radius of the sheet. Based on the mathematical models obtained, it is possible to establish the stainless-steel powder particle size and the compacting rolling method for providing the specified combination of filtering and mechanical properties.The coefficient of permeability of porous sheet may be calculated from the parameters of the internal structure of the pores using the capillary model of the porous medium.Cold compacting rolling of porous sintered sheet increases the sensitivity of the material to stress concentration.Translated from Poroshkovaya Metallurgiya, No. 9(261), pp. 88–91, September, 1984.     

Effect of porosity and grain size on the mechanical properties of hot-pressed boron carbide

Conclusions An investigation was carried out into the effect of porosity in the range 2–46% and grain size in the range (5–140)·10^–6 m on the mechanical properties of boron carbide. It is shown that the level of mechanical properties of boron carbide produced by hot pressing from powders synthesized from the elements is 1.5 times higher than that of boron carbide produced by other methods. Increasing the porosity of boron carbide to 46% decreases its strength by a factor of six or seven. Increasing the grain size to 140·10^–6 m has the same effect. An analysis is made of equations describing the dependence of the mechanical properties of boron carbide on porosity and grain size. The constants of these equations have been determined.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 63–67, January, 1982.     

Fabrication of Lotus-Type Porous Carbon Steel Slabs by Continuous Casting Technique in Nitrogen Atmosphere

Lotus-type porous carbon steel slabs with long cylindrical pores aligned in one direction were fabricated by the continuous casting technique in a mixture gas of N₂ 0.8 MPa and Ar 1.7 MPa or in N₂ 2.5 MPa at various transfer velocities from 2.5 mm·min⁻¹ to 20 mm·min⁻¹. The pore size in lotus carbon steel fabricated in the mixture gas of nitrogen and argon was small and homogeneous, whereas the pore size in nitrogen had bimodal distribution depending on the transfer velocity. The large pores were observed mainly at the edge of the slab, which are considered to be merged of several inclined pores. The porosity depended on nitrogen partial pressure, which is explained by Sieverts’ law. The hardness of lotus carbon steel matrix increased, which was attributed to the solid-solution of nitrogen.     

High-Temperature (1023 K to 1273 K [750 °C to 1000 °C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by ~0.1 wt pct, reduces its as-cast prior β grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C) and in the strain rate range of 10^–3 to 10⁺¹ s^–1. True stress–true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K [900 °C to 1000 °C]) and a lower strain rate (10^–2 to 10^–3 s^–1) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10⁰ to 10⁺¹ s^–1), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.