Papers by Author: Maya Radune

Abstract: Supersaturated titanium-aluminum nitride (Ti_1-xAl_xN) is a very attractive material for a wide range of applications due to its high oxidation and wear resistance accompanied by high strength, hardness, thermal conductivity and thermal shock resistance. Currently, its applications are limited to coatings obtained by physical or chemical deposition. Bulk materials based on Ti_1-xAl_xN may be fabricated by powder metallurgy approach using powders synthesized by high-energy ball milling (HEBM), which composition corresponds to supersaturated Ti_1-xAl_xN solid solution. In the present study, thermal stability of the supersaturated Ti_1-xAl_xN solid solution was investigated. According to the quasi-binary TiN-AlN phase diagram, constructed using density functional theory (DFT) analysis, the concentration ranges, where decomposition takes place through spinodal decomposition or through nucleation and growth, were determined. Experimental study on thermal stability of solid Ti_1-xAl_xN solution powder was conducted by means of differential scanning calorimetry (DSC), Brunauer-Emmited-Teller (BET) and XRD. The results indicated that spinodal decomposition of Ti_1-xAl_xN starts at 800°C, while at temperature higher than 1300°C regular decomposition (nucleation and growth) is occur.

Abstract: Taguchi’s method was applied to investigate the effect of the main HEBM parameters: milling time (MT), ball to powder weight ratio (BPWR) and milling speed (MS) on the dissolved AlN fraction in TiN. The settings of HEBM parameters were determined by using the orthogonal experiments array (OA). The as-received and milled powders were characterized by X-ray diffraction (XRD). The optimum milling parameter combination was determined by using the analysis of signal-to-noise (S/N) ratio. According to the analysis of variance (ANOVA) the milling speed is the most effective parameter and the optimal conditions for powder synthesis are: MT 20h, MS 600rpm, BPWR 50:1. The result of the experiment conducted under optimal conditions (AlN was completely dissolved during experiment) confirmed the conclusions of the statistical analysis.

Abstract: In the present study the modeling of the HEBM process is presented. The impact velocity, impact angle, rotation speed, mass of balls, ball-to-powder weight ratio and time of milling have been taken into account in order to calculate the energy transferred from the balls to the powder. Two different systems, namely, TiN-AlN and polysalicylic acid were experimentally investigated in order to confirm the validity of the model. The calculation results are in a reasonable agreement with the results of experimental research.

Abstract: A deterministic computational model of high-temperature heterogeneous re- action between metal and oxide melts has been developed. Transport of reagents and products of reaction occur simultaneously both by diusion and by laminar natural con- vection of the melting metal and oxide uxes. The convection-diusion equations have been numerically solved by a nite-dierences time-implicit discretization scheme. The model was implemented by program which had been written in C# language. The com- putations have been performed for desulfurization reaction between liquid steel and slag phases.The computed results agree well with the results which were found by experimen- tal methods.

Abstract: A mathematical model of sulfur transfer in a moving metal and slag melts has been developed. Sulfur is transported simultaneously both by diffusion and by laminar natural convection of the melting metal and oxide fluxes. The diffusion of sulfur is described by the 2nd Fick's law. We have computed the distribution of sulfur in the metal and slag phases and the concentration changes of sulfur in the volume of the metal and slag phases, both functions of space and time. Numerical results are provided to show the validity of this approach.

Abstract: The purpose of the present work is to investigate the mutual interaction between the melted metal and oxide phases with a small amount of sulfur. In this research, the following phases took part: metallic phase of Fe – C – S and slag CaO–Al2O3 –MgO–S. The mathematical model of sulfur diffusion in the metal and oxide is employed. The experiment was carried out at the temperature of 1773K. The result of the calculation is in qualitative agreement with the experiment. The proposed approach can be applied to the investigations of diffusion processes in molten metal and slag phases.