Papers by Keyword: Entropy Production

Abstract: The process of crystal growth in a metastable multicomponent melt has a high speed of the solidification front, which captures atoms of some other components. As a result of such a growth, at the surface of the growing crystal the effect of “impurity capture” is observed, and the concentrations of components significantly deviate from the local equilibrium. Under such conditions, the conventional physico-chemical methods for description of processes at the interfacial surface become inapplicable. Therefore, a new variational approach was applied for an integrated description of diffusion and thermal processes at the phase interface. The growth rate of crystal nucleus in a metastable melt was obtained, using the methods of non-equilibrium thermodynamics. The developed approach allows estimation of the degree of metastable effects influence on a crystal growth rate.

Abstract: The conditions are substantiated for the loss of thermodynamic stability of a tribosystem and for its adaptation with a decreasing wear rate at the moving frictional contact of parts from commercially pure titanium with an ultrafine-grained structure produced by equal-channel angular pressing. The regularities of the influence of the structure's dispersion degree and the friction contact's temperature on the tribotechnical characteristics of ultrafine-grained materials are established theoretically and experimentally.

Abstract: A consistency between the Darken method and the Onsager representation for cross diffusion in multicomponent system is shown. The justification is made by defining new sets of forces and fluxes linearly interrelated by a symmetric matrix of phenomenological coefficients. For the first time, the system of the components having various molar volumes is treated in this way. It is shown that the transformation leaves the entropy production unchanged. As an example, the entropy production for interdiffusion in the ternary Co-Fe-Ni diffusion couple is calculated and compared with mixing entropy.

Abstract: The Kirkendall effect appears due to the unbalanced diffusion fluxes causing the vacancy flux. There are several numerical methods that allow to predict the position of Kirkendall plane after the diffusion couple annealing. In this work for the first time the entropy density distribution is used to estimate the trajectory of the Kirkendall plane. The entropy density distribution is calculated with use of the bi-velocity method, which combines: (1) the volume continuity, (2) the conservation of mass, (3) momentum and (4) entropy-density. The method is applied to simulate the diffusion in Ni-Pd diffusion couple.

Abstract: Interdiffusion plays a significant role in the formation and stability of metallic joints and coatings. It is also of critical importance in designing advanced materials. Because commercial alloys are usually multicomponent, the key target is prediction of a complex morphology of the diffusion zone which grows between the alloys, alloy-coating, etc. In a two-component system, the diffusion zone can be composed of single-phase layers of the intermetallic compounds and solid solutions. The evolution of the composition and thicknesses of such layers are fairly well understood and consistent with the phase diagrams. The situation is qualitatively different in multicomponent systems. For example, the diffusion zone in a ternary system can be composed of single-and two-phase sublayers. Their number and thicknesses depend on the initial conditions, i.e. composition, component diffusivities and geometry of the system. The usual way of presenting the sequence of the layers and their compositions is by drawing a diffusion path which is, by definition, a mapping of the stationary concentrations onto the isothermal section of the equilibrium phase diagram. The diffusion path connects initial compositions of the diffusion couple and can go across the single-, two-and three-phase fields. It starts at the composition of one alloy and ends at the other. The possibility of mapping the concentration profiles onto the ternary isotherm has been postulated in one from the seventeen theorems by Kirkaldy and Brown [] for the diffusion path. The detailed presentation of all theorems was recently done by Morral []. Here we remind the reader only of the chosen ones (shown in italics).

Abstract: In this article, sensitivity analysis was performed using bidirectional single method with shell-and-tube heat exchanger as the basis and the entropy production in the working process of heat exchanger as target, to explore the optimizing direction for heat exchangers with the objective to reduce entropy production. First, the differential element analysis method was used in a case study of the entropy production of the heat transfer process - including the three heat transfer processes of convective heat exchange inside and outside the pipes and heat conduction across the pipe wall and the flow process - the fluid flowing process inside and outside the pipes, and the typical process parameter - dimensionless inlet heat exchange temperature difference, operation parameter - fluid flow rate inside the pipe and structural parameters - the heat transfer pipe inner diameter and length were used as characteristic parameters, to obtain the sensitivity coefficients under the conditions of the example, being respectively 0.95, 0.3, 0.3 and 0.38. The study in this article can provide some support to the energy efficiency evaluation of heat exchangers.

Abstract: Dynamic superplasticity of aluminium alloys is considered from positions of the theory of nonequilibrium phase transitions. Evolution of the open nonequilibrium system by which temperature-rates process in materials is modelled, is investigated within the framework of worked out defining relations with attraction of thermodynamic functions of the response. It is shown, that to peak of superplasticity there corresponds a maximum of a specific thermal capacity. The entropy production in optimum thermomechanical conditions of superplasticity aspires to a minimum which corresponds to formation of qualitative ultrafine-grained structure. With use of the Focker-Plank equation mechanisms of deformation characteristic for superplasticity and boundary conditions are analyzed. It is shown, that at superplasticity the main is the mechanism grain-boundary sliding, and in metastable conditions are added diffusionary processes. Influence of the specified processes in conditions of superplasticity becomes prevailing.

Abstract: The suppression criterion of the binary phase growth due to addition of a third component is considered. In this case the analysis of the two possible criteria of the first phase growth are considered: first – kinetic criterion based on the balance of components fluxes and second - thermodynamic criterion which is based on the maximal rate of the entropy production principle. We demonstrate that in the case of a model system the thermodynamic criterion lead to a bigger value of the critical thickness of the phases which are suppressed by the growth of the investigated phase.

Abstract: A model of discontinuous precipitation in binary polycrystalline alloys at low temperatures is presented. The proposed approach allows independent determination of the main parameters, including the interlamellar distance, the maximum velocity of the phase transformation front, and the concentration step at this boundary. This is achieved by using a set of equations for: 1) the mass transfer in the moving interphase boundary; 2) the balance of the entropy fluxes at the phase transformation front, and 3) the maximum rate of the free energy release under constraint of entropy balance. Concepts of mobility and linear interrelation between the driving force and velocity are not used explicitly. Comparison of the model calculation with the experimental results for the Pb-Sn system at different supersaturations is provided.

Abstract: A model of alloying in the three-layer thin-film system at the low temperature is constructed. Solid solution formation takes place as a result of the diffusion-induced grain boundary migration (DIGM). The unknown parameters are determined from the set of the equations for: (1) grain boundary diffusion along the moving planar phase boundary; (2) the entropy balance in the region of the phase transformation moving with constant velocity; (3) the maximum rate of the free energy release. We consider the model system with complete solubility of the components. The main parameters are self-consistently determined using thermodynamic and kinetic description in the frame of the regular solution model. The model allows determining the concentration distribution along the planar moving phase boundary, its velocity, the thickness of the forming solid solution layer and the limiting average concentration in this layer.