Materials Science. Non-Equilibrium Phase Transformations.
Vol. 5 (2019), Issue 2

Table of Contents

  • Fundamental properties of crystalline nano-structures caused by mechanical and thermodynamical excitations

    pg(s) 32-36

    Mechanical or thermodynamic excitations in solid state physics – phonons, cause all fundamental physical properties of materials and always are present, regardless of what is the main carrier of transport properties and ordering (for example, in electroconductivity, it can be electrons / holes, ions, etc., and in magnetism – magnons). In particular, phonons play a different and more subtle role in low-dimensional nano-scale samples, because they, due to the confinement effects, influence the creation of completely unusual and altered characteristics in relation to large (bulk) samples of exactly the same material. Therefore, the possible phonon spectra and states in model of crystal nanostructures: ultrathin films, nano-wires and quantum dots were founded in the paper. The most noticeable phenomenon is the consequence of the dimensional quantization, but also the shape of the boundary surfaces, as well as the presence of the environment surrounding the nano-pattern. In addition to the analysis of the microscopic properties of the phonon subsystem, the calculation of the temperature dependence of the thermal capacity and entropy of these nano systems was also calculated and performed by comparisons with the same for bulk structure.

  • Stimulation of the crystallization processes of casts of petrurgical materials obtained by the gas counter-pressure casting method

    pg(s) 37-37

    Test recipes of petrurgical materials, based on basalt raw materials and technological additives were developed. The influence of the additives used, their quantities and the mode of thermal treatment on the crystallization processes in the experimental samples was investigated. Products with microcrystalline structure and predominant crystalline phase of augite were obtained. By applying the gas counter-pressure casting method and a set of precision casting molds, products with a variety of mechanical and physicochemical characteristics were obtained. The optimal technological conditions for the production of petrurgical materials, suitable for the making of technical equipment with application in different industries, were established experimentally.

  • Semi-automatic MAG welding of alloyed machine-building steels

    pg(s) 40-43

    The present study explores the possibility of semi-automatic MAG welding of three steels with carbon equivalents of 0,40%, 0,50% and 0,60%. The impact of the welding speed on the crack formation is determined by a structural analysis of the TIZ (Thermal Impact Zone). A three-factor experiment was conducted to determine the impact of the factors on the obtained hardness in the TIZ. The regression equation was obtained and the isolines were outlined showing the impact of these factors

  • Determination of the tribological characteristics of heavy loaded frictive pairs based on density modifications of boron nitride

    pg(s) 44-46

    In this work the constructive solution of the friction node is proposed, which provides the possibility to obtain reliable information about the tribological characteristics of the materials for work in heavy loaded slip supports. The method of determining the expediency of application in friction pairs of different materials is considered, which is based on certain shapes and sizes of the contact spot and the state of the friction track on the counter bodies. The efficiency of polycrystalline superhard materials application based on the wurtzite boron nitride in heavy loaded friction pairs due to an increase in the antifriction characteristics of the pair by the formation of graphitelike boron nitride on the surface of a thin film В2О2 as a result of the reverse phase transformation of dense phases (cBN + wBN) → gBN is shown.

  • Obtaining the stress – strain behaviour of stainless steel at elevated temperatures

    pg(s) 47-50

    In several metal forming processes, relation between stress and effective strain of the material must be known for obtaining important forming parameters such as forces and work, required for metal forming processes. Dependence of the flow stress on the effective strain is called the flow curve and is influenced by strain rate and temperature. In order to reach high quality and full functionality of the product, the characteristics of formed material must be determined as precisely as possible and flow stress is one of the main characteristic of the metal materials. In general, flow curves are determined by experiments such as tensile test, upsetting test and torsion test. The proper choice of testing method depends on the metal forming process to be simulated.
    In this paper we present the experimental measurements of flow stress for stainless steel at elevated temperatures by using torsion test. In the experimental work a torque, temperature and number of twists of test pieces were measured. Also, the influence of forming parameters such as strain, strain rate and temperature on flow stress was analysed. Experimental results are presented in form of tables and diagrams. Finally, from experimental data the regression model was obtained for successful prediction of flow stress of stainless steel at elevated temperatures. The method of regression analysis allows with relatively small number of experiments the accurate information about the influence to mathematical model of the process.

  • High performance metal-matrix composite coatings

    pg(s) 51-53

    The effect of the initial structure of the fillers on the structure formation of the dissolution-and-diffusion type interfaces in macroheterogeneous composite coatings with a metal binder has been investigated. It has been shown that by combining phases in the filler structure, which differ in resistance to molten binder during infiltration, it is possible to attain an increased resistance of composite coatings to dry friction, abrasive, gas abrasive, and corrosion wear. The compositions of composite coatings to strengthen the surface of the parts of machine-building industry have been recommended.

  • Investigation of the structure and some of phase transformations in wear-resistant cast alloys

    pg(s) 54-56

    The study of phase transformation processes in wear-resistant alloys from Fe-Cr, Fe-Ni-Cr, Fe-Cr-Mn base systems has not only a fundamental, but also a practical significance, especially with regard to alloys with a chemical composition with difference from the conventional one – this is an interesting scientific and applied direction. The importance of this type of researches is motivated by fact, that in the available literature there is insufficient data on the structure and properties of non-standard iron-based materials, with variation amount of alloying elements (as well as additional alloying or modification) not only in the cast state, but also after thermal or plastic treatment. That`s why, the aim of the present study is to obtain data on alloys with the most mass application that have increased carbon and reduced chromium content relative to stainless deformable steels.

  • Vibro-acoustic approach for registration and evaluation of technical deviations in aluminum castings

    pg(s) 57-60

    Superficially masked internal discontinuities in the aluminium casting during the manufacturing process occurs often. As a result of dynamic loadings in the process of operation, it is possible the occurrence of leakage in the machines. The application of classical methods of non-destructive testing of internal discontinuities in parts with relatively complex geometric dimensions requires a significant resource or is not always possible.
    A vibro-acoustic study for express diagnosis of internal discontinuities in aluminium castings is shown in the work. The resonance behaviour of a part depends on its specific shape and material properties of the elastic medium. This allows diagnostic approaches for registration of different technical deviations, estimating an equivalent size of internal discontinuities, to be created.
    Classical theory of resonance is used for scientific justification of study. It is suitable in spatial parts of complex shape the theoretical results to be obtained for each particular construction by means of the constructive model in CAD environment. The possible technical deviations are created in the geometric model by software. The model is divided into elementary parts and a detailed description of their properties and the elastic connections are described by the Finite Element Method (FEA). This allows diagnostic models for registration of discontinuities, incl. for assessing its equivalent size in a particular section of the casting to be created rapidly.
    A study of an aluminium part by a universal acoustic apparatus is shown in this work. The samples are separated into factory conditions of „suitable” and „unsuitable” after machining of the joining dimensions. Typical areas of occurrence of discontinuities in the casting process are defined. The „suitable” samples are divided according to the specific elastic characteristics of the material used. Insignificant scattering (2-6 Hz) of resonant frequencies in the range 20 Hz to 20 kHz is measured. Discontinuities in the typical areas of registration in the real parts are formed successively in the CAD model. The resonances are calculated in FEA. Diagnostic signs for registration and evaluation of an equivalent size of discontinuity according to its disposal are created. Discontinuities of irregular shape are created in part of the „suitable” samples and the resonances are registered again; the changes of resonances are determined. The principle consistency between theory and experiment is assessed. A good compliance is obtained.
    The work may be used to assess the technical condition of castings by manufacturers of such parts.

  • Materialscience – additive of material and technologies

    pg(s) 61-64

    This paper proposes the idea of a “small volume” in which to consider complicated processes to create structures in phase transitions of first and second order in the foundry. The small volume is chosen based on the classical theory of crystallization and its use for quantum mechanics. A numerical solution of Stefan-Schwarz’s task was presented by obtaining the temperature field of solidification of a composite cast in a squeeze casting. This little volume we proposed for a good possibility in the direction for a theoretical possibility of hybridization of production and technology.

  • Influence of EB radiation on the mechanical properties of organic bentonites-HIPS nanocomposites

    pg(s) 65-66

    The effect of EB radiation on the mechanical properties of HIPS matrix nanocomposites is discussed in this paper. Two different types of clays were used (calcic Cuban and sodic Brazilian bentonites) as reinforcement material, in 5 and 7% additions regarding the weight of the material. All clays used were subjected to organic modification. Composite material pellets were prepared in a hot extrusion process adding the clays aided by a vibrating dispenser. Mechanical test samples, for tensile and impact tests, according to ASTM standards, were elaborated in an injection molding machine. Half of the samples were irradiated at 600KGy and seven days later, they were tested. The rest of the specimens were tested from their direct injection state. It was shown that an addition of up to 5% of clay as reinforcement in a matrix of HIPS achieved increases in tensile strength over 30% compared with the unreinforced polymer. Additions of higher amounts of clay (7%) represented a decrease on mechanical strength, compared with the 5% addition. Electron beam irradiated samples showed a considerable increase in the tensile strength with respect to this strength in pure material. Increases between 48 and 56% were achieved. In all cases elongation and impact strength decreased with the aforementioned increase in tensile strength..