Sintering behavior and strength properties of stomatological porcelain—Titanium nickelide composite materials

The sintering behavior and strength properties of stomatological porcelain—titanium nickelide composites were investigated. The dependence of porosity, linear and volumetric changes, phase composition and structure on sintering time and temperature, component concentrations, and initial porosity have been determined. It was shown that density changes are the result of a competition between shrinkage and volumetric growth. An optimal regime of sintering to ensure open porosity and high strength is recommended. Siberian Physico-Technical Institute, Tomsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 31–36, July–August, 1998.     

Effect of compressive stresses on deformation and contact phenomena in pressed fibrous materials during sintering

A study is made of the processes that control deformation and contact phenomena in porous fibrous metallic materials. It was established that sintering of fibrous materials which are subjected to small compressive stresses in the pressing direction makes it possible to completely suppress the volumetric growth of specimens, which is seen during pressing and sintering. Due to the reestablishment of interparticle contacts, the resulting specimens have a significantly higher level of mechanical properties. For example, the ultimate tensile strength of the specimens is increased by a factor of 1.5–2 compared to control specimens of the same porosity. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 29–34, January–February, 2000.     

Structure formation during sintering in the presence of a liquid phase in Cr-Cu-metal of the iron family systems. II. Systems Cr-Cu-Ni(Co, Fe)

Features of structure and phase formation in Cr-Cu-Ni(Co, Fe) systems during liquid-phase sintering in a vacuum of (2–4) · 10⁻³ Pa at 1200 and 1270°C are studied. It is shown that with introduction of elements of the iron family into molten copper there is dispersion of refractory particles and formation at the boundary with the readily melting component of a transition layer whose structure and composition are determined by the nature of the addition. Growth of refractory particles during liquid-phase sintering of composites of the system Cr-Cu-Ni is controlled by diffusion of elements into the solid phase of the transition layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 19–25, July–August, 2006.     

Microstructure of W - Ni - Sn Composites in Relation to Amount of Liquid Phase on Sintering

Microstructure transformation is examined for W - Ni - Sn composites made by liquid-phase sintering at 1200 °C for 1 h; the liquid phase was a eutectic containing 67.5 mass% Ni and 32.5 mass% Sn. There is a nonmonotone dependence of the tungsten grain size in the sintered composites on the volume proportion of liquid phase in the range 6.7–36.8%. The mean grain size in relation to volume proportion of liquid phase in liquid-phase sintering is calculated from rheological sintering theory. The form of the empirical function f(v) is quantitatively demonstrated, which determines the rate of tungsten grain growth on liquid-phase sintering and is dependent on the proportion of matrix in the composite and the size of the liquid phase layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 53–57, July–August, 2005.     

Powder sintering with application of an electric current and periodic mechanical pulses

Electric-discharge sintering of metal powders and powder composites is accompanied by anomalously high rates for compaction, and alloy and phase formation. Application of low-frequency pressure pulses on powders and powder mixtures during the concluding stage of electric-discharge sintering leads to even greater intensification of compaction and alloy formation. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(412), pp. 105–109, March–April, 2000.     

Densification and structure formation in the sintering of composite materials based on nitrides of the IV–V group transition metals. III. Sintering of composite materials in the (VN, TaN)—Cr systems

An investigation was made of densification and structure formation in composite materials in the systems (VN, TaN)—Cr during sintering in argon. It was shown that the shrinkage of these materials during liquidphase sintering is insufficient to provide dense composites (residual porosity was 35–40%). This is attributed to the low thermodynamic stability of VN and TaN, and the rapid evolution from these of nitrogen which accumulates in closed pores. Processes of heterodiffusion and alloy formation also have a negative effect on densification. Exchange reactions between chromium and the nitride-forming metals lead to the formation of a large quantities of intermetallics which embrittle the composite materials. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 13–18, January–February, 1999.     

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. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 13–20, May–June, 2000.     

Selective laser sintering. I. Principles, continual model

he main thermophysical and rheological processes accompanying selective laser sintering are considered. A mathematical model of laser sintering is proposed. The results of computer experiments with the model applied to multilayer composites containing titanium nitride, diboride and carbide, as well as molybdenum, nickel, and steel are discussed. It is shown that densification depends on the heating regime and initial porosity of the composites. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 16–26, July–August, 1998.     

Structuring on sintering in the presence of liquid phase in Cr-Cu-iron group metal systems. I. Cr-Cu system

Structuring has been examined for Cr-Cu composites under conditions of impregnation and subsequent liquid-phase sintering at 1200°C in a vacuum of (2–4) · 10⁻³ Pa with reduced and electrolytic chromium powders. The size distribution for the particles of the refractory component in the microstructure containing the reduced chromium on liquid-phase sintering for 60 min corresponds to a logarithmic normal distribution; the distribution parameters are sensitive to the volume fraction of refractory particles. The calculated values for the dihedral angle are close to one of the modes of the distribution for the dihedral angles in the microstructure for specimens made of electrolytic chromium (115°). At 1200°C, the equilibrium Cr_s-Cu_l system obeys the condition . This indicates the probability of formation or preservation of framework structure elements during the liquid-phase sintering, which are observed by experiment in specimens containing reduced chromium. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 3–9, May–June, 2006.     

Densification kinetics of W-Fe-Sn pseudoalloys in liquid phase sintering

The densification kinetics in liquid phase sintering of W-Fe-Sn powder composites containing 90 wt.% of a refractory component and 10 wt.% of a low-melting component is strongly dependent on the iron content in the melt. As the iron content in composites increases, the concentration dependence of their densification shows a maximum. Samples containing more than 1.5 wt.% Fe grow intensively because of the formation of W₆Fe₇ intermetallide whose decomposition temperature is higher than that of liquid phase sintering. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 22–29, 2007.     

Investigation into the Influence of the Structure Dispersion and Homogeneity on the Properties of Powder Metastable Austenitic Carbide Steels and Diamond Tools

Diffusion and homogenization in “iron (5 μm–nickel (5 μm or 50 nm)” powder systems of various degrees of dispersion during sintering (900 and 1000°C), as well as spark plasma sintering, are investigated using the Matano–Boltzmann method. Calculated diffusivities in pairs of micron powders sintering without applying pressure (900°C, 6 h) and by the spark plasma method (900°C, 5 min) in these systems are equal to 7 × 10–10 cm2/s. It is shown that the use of nanodispersed nickel powder in diffusion pairs based on finely dispersed iron powder promotes a twofold increase in diffusivity at 900°C in contrast to the pair with the microdispersed nickel powder. Constants in the Ivensen sintering kinetics equation are calculated for the “iron–nickel” powder systems, by which the factors activating sintering of these systems are established. The dependences of the structural phase composition and physicomechanical properties of carbide steels of the Fe(base)–14 wt % Ni–8 wt % TiC system on the sintering temperature in range t = 900–1200°C and structure dispersity and homogeneity are determined. The dependences of the grain size, porosity, hardness, microhardness, fracture toughness, and bending ultimate strength on the sintering temperature are shown. Dependences of tribotechnical properties on the degree of homogeneity of the solid solution and volume of the phase transformation of metastable austenite into deformation martensite during abrasive friction turn out similar for carbide steels and diamond tools based on carbide steel. Optimal values of the variation coefficient of the nickel concentration in austenite and carbide steels of the same chemical composition but with different degrees of dispersity, which provide the maximal volume of the austenite decomposition and high values of the diamond-tool grinding coefficient, turn out equal to 5 in both systems, but the sintering parameters are different. It is shown that the physicomechanical properties of the studied systems depend on the structure porosity and dispersity, while tribotechnical properties depend on the structural homogeneity of steels.     

Compositional dependence of mechanical and wear properties of electroconductive ceramics

Some experimental works carried out at the Research Institute for Ceramics Technology (IRTEC) and National Physics Laboratory (NPL) on electroconductive ceramics are reviewed. The tests were mainly devoted to characterize mechanical and wear properties of electroconductive particulate composites based on silicon nitride and alumina matrices with reinforcement of nitrides, carbides, and borides. The focus of the study was to ascertain the influence of the sintering techniques and the amount and quality of particle reinforcement on the mechanical and wear performances of the composites with respect to the matrix. The experimental results indicate that,besides the electroconductivity, some mechanical properties can be greatly enhanced by the introduction of a specific amount of intermetallic particles. Unfortunately, the refractoriness of the starting matrix is at the same time reduced, making the composites more sensitive to the temperature effects. Research Institute for Ceramics Technology, National Research Council, Italy. Published in Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 32–41, March–April, 1999.     

Fatigue and Compression Characteristics of Al 6061–B 4 c Composite Materials

Aluminum matrix composites reinforced with boron carbide are a kind of materials that are widely used because of high strength, low density, and improved tribological properties. In this study, mechanical properties of Al 6061–B4C composites reinforced with B4C of three different particle sizes were investigated. In the Al 6061–B4C composite materials, produced by the powder metallurgy methods (extrusion of billets obtained by sintering at temperature of 550°C under pressure of 450 MPa), the change of mechanical properties such as hardness, compressive strength, and fatigue life, related to B4C particle size and the applied heat treatment mode (aging at 180°C for 5 h), were investigated. The hardness of the materials is increased with B4C grain size and the heat treatment. After the heat treatment, the fatigue life of Al 6061–B4C (3 μm) material increases slightly, while that of the composite materials decreases with larger size of B4C reinforcement. The fatigue life of the composite materials reinforced with a larger grain size B4C is reduced by heat treatment. While the compression test data of untreated composite materials were similar to each other, the heat treatment increased these values in all samples. The highest increase in the compression strength was observed in the composite reinforced with 17 μm sized B4C. The addition of graphite reduces the deformation ability of the composites.     

Simulating the temperature pattern at the contact between two metal particles during rolling with induction heating

The temperature pattern is simulated at the contact between two metal particles during pulse sintering with rolling. Mathematical expressions are given for the solid-phase and liquid-phase processes. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 5–12, May–June, 2000.     

Compaction and grain growth in alumina ceramics under microwave sintering

The laws governing the transformations of the porous and grain structures that occur in Al₂O₃-based ceramics during microwave sintering are investigated. These processes are very similar to analogous processes that occur during coherent compaction of powders. Under microwave sintering the ceramic preforms retain a wide range of porosity and microstructure for a long time. As a result strong capillary forces exist throughout the process, causing active compaction. On the basis of the experimental data we suggest that the heating rate and the bulk mode of the heating are not the only factors that accelerate the consolidation of the ceramics under microwave sintering. Institute of Problems in Material Science, National Academy of Sciences of Ukraine, Kiev. Institute of Applied Physics, Russian Academy of Sciences, Nizhnii Novgorod. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 8–13, July–August, 1997.     

Kinetics of interparticle contact formation during the laser sintering of two-component powders

The kinetics of interparticle contacts formation during the liquid-phase sintering of unlike materials under the action of laser irradiation was investigated. Trios of particles arranged in a row according to the schemes glass—polymer glass, metal—polymer—metal, and metal—glass—metal were sintered. The processes of particle approach and liquid neck formation between particles were studied. Possibilities for the manufacture of powder components with polymer and glass binders by the use of selective laser sintering followed by heat treatment are considered]. Institute of Technical Acoustics, Academy of Sciences of Belarus', Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 37–41, May–June, 1999.     

Device for electric-pulse sintering of powders under pressure

The construction and energy parameters of an experimental device for electric-pulse sintering of powder under pressure are described. The properties of material prepared from Nibon-20 powder by electric-pulse sintering technology are given. Translated from Poroshkovaya Metallurgiya. Nos. 1–2(411), pp. 125–128. January–February, 2000.     

Analysis of structure parameters for compacted porous materials. II. Integrated and local compaction during sintering of porous bodies

Integrated and local compaction of porous bodies during sintering are analyzed and an algorithm is developed for calculating the structure parameters of sintered materials. Accurate quantitative estimates are obtained for the efficiency of porous material sintering in different structural models. The most precise procedure is based on a model developed by the author for the structure of porous materials and an approximation according to which Voronoi polyhedra are considered to be unchanged with a decrease in system porosity, and pore volume decreases as a result of an apparent increase in particle diameter. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 106–111, July–August, 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 carbon on volume changes during the sintering of an iron-chromium material

The effect of carbon added in the form of either cast-iron powder or graphite on the sinterability of chromium steel was investigated. The growth and instability of volume change during sintering produced by cast-iron additions was explained as a result of a thermally activated after-effect in the solid phase, and the formation of FeCr₂O₄ around particles of Kh30. Substitution of graphite for cast iron in the powder charge prevented oxidation of the chromium-rich particles, and resulted in low and stable shrinkage (0.1–0.8%) after sintering in the range 750–1250°C. Scatter in the degree of volume change was reduced from 1.5–2.0 to 0.1–0.3%. Iron-chromium material obtained with the use of graphite had improved strength properties as well as high produceability. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 38–42, May–June, 1998