Experimental study of the thermophysical properties for aluminum-magnesium alloy AMg3

The thermal diffusivity (a), the thermal coefficient of linear expansion (α), the isobaric heat capacity (cp ) and the fusion enthalpy (ΔH) of aluminum-magnesium alloy AMg3 were investigated by laser flash method, dilatometric method and method of differential scanning calorimetry in the temperature range of 300–773…1000 K. The thermal conductivity (λ) has been calculated from the measurement results. The estimated errors of the obtained data were 2–5%, 3–5%, 2–3% and (1.5–2.0)⋅10-7 K-1 for a, λ, cp and α, respectively. Approximation equations and a table of reference values for the temperature dependence of the studied properties have been obtained.

Contact Interaction of Steam Turbine Inner Casing Elements During Plastic Deformation

A structure’s material plasticity influence on the pattern of contact interaction of its elements during operation is studied. The stress-strain state problem for the inner casing of a steam turbine high-pressure cylinder operating at supercritical steam parameters (over 240 atm and 565 °C) is solved. The problem is solved by using a finite-element software package. A model of thermoplasticity with kinematic and isotropic hardening is considered. In carrying out the study, experimental strain curves were used for the materials of the connection. The main dependencies used in solving the problem are given. The method of solving the thermal contact problem of interaction of flange connector elements in the conditions of plasticity is based on the application of a contact layer model. To be able to take into account changes in the load from the fastening in the process of combined strain of both the fastening and the casing, first proposed is a method of the three-dimensional modeling of the thermal tightening of the fastening of the horizontal casing connector by applying the linear coefficient of linear expansion of the material. The proposed approach allows modeling the stress of the initial tightening of studs by specifying a fictitious change (decrease) of the coefficient of linear expansion of a stud given as a separate body in the calculation scheme. The magnitude of the specified change in the coefficient of linear expansion is determined from the relationship between the stress of the initial tightening in the stud and the required, for its creation, elongation, which is implemented in the calculation scheme in the presence of different values of linear expansion of both the stud and the casing. To conduct the numerical experiment, an ordered finite-element grid of the casing design was constructed. A 20-node finite element was used in the construction of the casing grid and the fastening. The effect of force loads and the temperature field, in which the structural element under consideration is operated, is taken into account. An analysis of the results of distribution of equivalent stresses and contact pressure during operation is carried out. The difference between the obtained results and the results of solving the problem in the elastic formulation is noted.

About the titanium foil using in the brazed construction of small windows for the transmission of radiant energy

The aim of this work was to miniaturize the design of a brazed joint of a thin non-metallic disk with a hollow thin-walled metal cylinder for operation as part of a vacuum device in a wide temperature range. For brazing a non-metallic disk with a glass tube-shaped device body, when choosing a metal filler, the fragility of glass, its low mechanical strength in bending and tensile and a lower thermal coefficient of linear expansion than metal fillers must be taken into account. Therefore, the use of low-temperature plastic solders based on tin, indium, lead etc., which are alloyed with titanium, is preferable. However, the disadvantages of these fillers include a significant decrease in mechanical strength when the brazed unit is heated to relatively low temperatures, especially if there is a gas inside the device under excessive pressure. The option of brazing a leucosapphire disk with a glass tube was also considered. A design has been proposed to determine how a disk is connected to a titanium body. This design has increased the heat resistance of the leucosapphire lens, which is connected to the body of the product made of titanium alloy or covar. Such connection can be used as pyrometer windows installed directly in the body of an internal combustion engine or gas turbine engine. The main feature of this development was the creation of a brazed joint structure in which the shell covering the disk is made of non-metal, in this case of leucosapphire, made of titanium foil with a thickness of 100 and in some cases 50 μm. Brazing modes were set and brazed windows were made. Tests on the vacuum density after different modes of thermal cycling of brazed samples showed high performance of this structure of the brazed joint. It was shown that the use of titanium foil makes it possible to obtain high-quality heat-resistant brazed joints. The relatively small stresses that lead to plastic deformation of the foil make it possible to increase the inconsistency of the thermal coefficient of linear expansion (TCLE) of materials that were brazed. It shout be noted that the foil material can also be other metals which had chemical active towards non-metallic materials or their components, for example, zirconium, niobium, tantalum, etc. Keywords: structures of brazed joints, brazing of non-metals, non-metallic materials, leucosapphire, glass tube, adhesive-active filler metals, fillers, titanium.

Determination of the temperature coefficient of linear expansion of materials based on silicon carbide

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INFLUENS OF NON-OXID ADDITIVS ON THE PROPERTIES OF GLASS-CERAMIC COATING FOR PARTS MADE OF HEAT-RESISTANT ALLOYS (review)

A review of the scientific and technical literature is presented, which provides examples of improving glass-based coatings by introducing non-oxide modifying additives. The main groups of chemical compounds that are used to improve the properties of glass-ceramic coatings are considered. Such substances include compounds based on boron, silicon and rare earth metals. It is concluded that these additives have a positive effect on the following properties of coatings: heat resistance, heat resistance, mechanical strength, coating formation temperature, temperature coefficient of linear expansion and adhesion strength to the protected material.

STUDY OF THE MAIN PROPERTIES OF GLASS FIBER LAMINATES FOR PRINTED CIRCUIT BOARDS AND ASSESSMENT OF THE INFLUENCE OF CLIMATIC INFLUENCES ON THE INDICATORS OF THEIR DIELECTRIC CONSTANT

In this paper, the main properties of glass fiber laminates – dielectric composite materials for printed circuit boards are considered: glass transition temperature, temperature coefficient of linear expansion, ultimate strength in bending, water resistance. The influence of climatic influences (increased ambient temperature, low ambient temperature, increased air humidity, the effect of temperature changes, the effect of salt fog) on the dielectric constant of glass fiber laminates has been evaluated.

Mechanical and Thermal Properties of Harmonic Structured Composites by MM/SPS Process

Harmonic structured composites consist of a low fraction metal region like network and a dispersed another major metal region like island. The harmonic structured composites were produced via mechanical milling (MM) followed by spark plasma sintering (SPS), and its mechanical and thermal properties were investigated in detail. Microstructural observation of the MM powders and SPS compacts was achieved using scanning electron microscopy (SEM). The mechanical properties of the harmonic structured composites were evaluated using results of the Vickers hardness and the tensile tests. The thermal properties of a part of the harmonic structured composites were evaluated using results of thermo-mechanical analysis and laser flash method. High speed steel / mild steel harmonic structured composite exhibited high strength and enough ductility in spite of the trade-off relationship between strength and ductility. In addition, the high speed steel / mild steel harmonic structured composite also demonstrates a superior wear properties and low hardness simultaneously. On the other hand, molybdenum / copper harmonic structured composite demonstrate low coefficient of linear expansion and enough thermal conductivity compared to the conventional copper / molybdenum particle dispersed composite. The coefficient of linear expansion and thermal conductivity are the trade-off relation in this composite. In summary, the harmonic structure control is effective for improvement of the trade-off mechanical and thermal properties in the composite.

SIZE DEPENDENCES OF LINEAR EXPANSION AND VOLUME ELASTICITY OF MONO- AND BIMETALLIC NANOCLUSTERS

Проведена серия молекулярно-динамических экспериментов по охлаждению разупорядоченных металлических наночастиц Au, Cu, Al,Ti и биметаллических наносплавов Au - Cu и Ti - Al с использованием потенциала сильной связи. Получены размерные зависимости коэффициента линейного расширения и модуля упругости для моно- и биметаллических частиц. В первом приближении размерная зависимость коэффициента линейного расширения обратно пропорциональна соответствующей зависимости для температуры плавления наночастицы, что коррелирует с аналитической моделью. Молекулярно-динамические результаты предсказывают более умеренный относительный рост коэффициента линейного расширения, по сравнению с аналитической моделью. Установлено, что модуль упругости увеличивается с уменьшением размера наночастиц. A series of molecular dynamics experiments on cooling disordered Au,Cu, Al,Ti metal nanoparticles and Au - Cu, Ti - Al bimetallic nanoalloys using the tight-binding potential have been performed. The size dependences of the temperature coefficient of linear expansion and the elasticity modulus for mono- and bimetallic particles are obtained. In the first approximation, the size dependence of the linear expansion coefficient is inversely proportional to the corresponding dependence for the melting temperature of a nanoparticle, which correlates with an analytical model. Molecular dynamics results predict a more moderate relative increase in the linear expansion coefficient compared to the analytical model. It was found that the modulus of elasticity increases with decreasing the nanoparticle size.

Calculation of thermal expansion, glass transition temperature and glass density in the system RO–Al2O3–B2O3–SiO2 (where RO=BaO, SrO, CaO, MgO, ZnO)

Glasses of the system RO–Al2O3–B2O3–SiO2 are used as a base for the fabrication of heat-resistant nonmetallic materials and general-purpose products. The purpose of this work is to develop mathematical models for calculating the temperature coefficient of linear expansion, glass transition temperature and density as a function of the composition of glass in the oxide system RO–Al2O3–B2O3–SiO2 where RO=BaO, SrO, CaO, MgO, ZnO. The disadvantage of the known models is that the range of their application is limited by the quantitative content of components in the glass. At the same time, an increase in the sample size of experimental compositions made it possible to obtain more accurate mathematical models for calculating these properties. The glasses included in the experimental sample are distinguished by a wide range of temperature coefficient of linear expansion (from 30 to 10510–7 К–1). The glass transition temperature of these glasses is within the range of 580–7100C, which allows a reasonable approach to the choice of temperature regime for the formation of the structure vitreous and glass-ceramic materials for different functional purposes. The mathematical models were developed with the use of the experimental and statistical method. The obtained mathematical models are adequate to the experimental data and allow calculating the thermal expansion, glass transition temperature and density of glasses; the mean-square deviations of temperature coefficient of linear expansion, glass transition temperature and density being 1.910–7 К–1, 16.00C and 0.06 g cm–3, respectively. Their accuracy is sufficient for the development of basic glass compositions for various functional purposes.