Journal of Phase Equilibria and Diffusion最新文献

The phase equilibria in the Al-Ta-V ternary system at 1000 °C and 1200 °C have been studied by using electron probe microanalysis and x-ray diffraction. τ-Al_2.9Ta_2.7V_1.4 phase was found in the Al-Ta-V ternary system in both isothermal sections. The addition of V stabilizes the Al₆₉Ta₃₉ phase at 1000 °C. The line compound Al₃(V,Ta) (D0₂₂-type) forms a continuous phase region from the Al-V side to the Al-Ta side at the two temperatures. Based on our experimental results, reported liquidus projection of the ternary Al-Ta-V system and thermodynamic data of the binary systems of Al-V, Al-Ta and V-Ta, a thermodynamic evaluation of Al-Ta-V system was carried out by CALPHAD method. A set of reliable thermodynamic parameters for the Al-Ta-V system was obtained. The current calculation results agree well with the available experimental data. The invariant reaction scheme of Al-Ta-V ternary system was presented. The present study could provide essential experimental and thermodynamic data for establishing a comprehensive Co-based superalloy database.

Phase equilibria of the Ti-Ga-Sn system have been determined at primary crystallization and at 1000 °C in the composition interval ~ 50-100 at.% Ti based on differential thermal analysis, x-ray powder diffraction, scanning electron microscopy and electron microprobe analysis. Partial liquidus and solidus projections, the melting diagram, a number of vertical sections, isothermal section at 1000 °C, as well as the reaction scheme (Scheil diagram) for the Ti-Ga-Sn system were constructed. A ternary compound Ti₅GaSn₂ (τ) (Nb₅SiSn₂-type structure, tI32-I4/mcm), found by us previously, forms by peritectic reaction L + Ti₂(Sn, Ga) + Ti₅(Sn, Ga)_3-4 ⇄ τ at 1500 °C and has a wide homogeneity range from 9 to 23.5 at.% Ga at solidus temperature and from 4 to 34 at.% Ga at 1000 °C, and located along constant composition of ~ 62.5 at.% Ti. D8₈-type compounds Ti₅Sn₃ and Ti₅Ga₄ form a continuous solid solution, denoted Ti₅(Sn, Ga)_3-4, at all investigated temperatures. Ga-poor part of it (below ~ 12.5 at.% Ga) forms by an interstitial mechanism, while in the interval above ~ 12.5 at.% Ga it is a substitutional phase. Isostructural compounds Ti₂Sn and Ti₂Ga also form a continuous solid solution Ti₂(Sn, Ga) at solidus temperatures, which decomposes with decreasing temperature. Meanwhile, at 1000 °C, one more continuous solid solution Ti₃(Sn, Ga) forms between isostructural compounds Ti₃Sn and Ti₃Ga.

A novel concept for diffusion couples based on iso-Gibbs free energy of mixing (iso-(Delta G^{M})) is proposed, in which the two terminal alloys of the couple have the same Gibbs free energy of mixing. A few iso-(Delta G^{M}) couples were assembled with FCC Fe-Ni-Cu alloys at 1000 °C. To enable the selection of the terminal alloys for iso-(Delta G^{M}) couples, iso-(Delta G^{M}) lines were constructed from the data on the thermodynamic activities of the constituent elements in FCC Fe-Ni-Cu alloys at 1000 °C. A few couples were also studied whose composition vectors were at about 90°, less than 90° and greater than 90° to one of the iso-(Delta G^{M}) couples. A strategy is also proposed and utilized to construct a near iso-Gibbs free energy (iso-(G^{soln})) couple. All but one iso-(Delta G^{M}) couples and also, the iso-(G^{soln}) couple were characterized by almost linear diffusion paths. All the iso-(Delta G^{M}) couples presented very low diffusion depths characterized in terms of effective penetration depth ((overline{x})), with the iso-(G^{soln}) couple exhibiting the lowest values of (overline{x}). This experimental work clearly reveals the definite role played by the difference in Gibbs free energies of terminal alloys in dictating the diffusion paths and the relative diffusion depths in the diffusion couples.

In this paper, the NaBr-RbBr binary system was computationally investigated. Experimental data including phase diagram and thermodynamic information are used as input data for the CALPHAD assessment. The liquid and solid solutions are treated as substitutional solutions and the solid solutions (NaBr, RbBr, both FCC type) were modeled as a single phase with a miscibility gap. Our calculations are in good agreement with the phase diagram data and experimental thermodynamic data available in the literature.

This article considers the features and fundamental difficulties of studying low-temperature phase equilibria associated with an exponential increase in the required duration of syntheses with a decrease in temperature. Methods for accelerating the achievement of equilibrium, including the use of salt solvents, are also considered. The results of phase equilibria studies in the SrF₂–LaF₃ system using sodium nitrate and in the ZrO₂–Sc₂O₃ system using sodium sulfate as fluxes are presented. The methods of extrapolation of phase diagrams to absolute zero temperature in accordance with the third law of thermodynamics are considered. Phase diagrams of the Au–Cu, Cu–Pd, Ni–Pt, and ZrO₂–Y₂O₃ systems are presented. Phase equilibria with plagioclase ordering are considered separately, and the phase diagram of the albite–anorthite (NaAlSi₃O₈–CaAl₂Si₂O₈) system is presented. As the temperature approaches absolute zero, the homogeneity region of labradorite shrinks to the compound NaCaAl₃Si₅O₁₆.

The Alkemade theorem goes back to a very fundamental paper on the graphical description of thermodynamic equilibrium problems from 1893 (van Rijn van Alkemade in Z Phys Chem 11: 289-327, 1893). It is one of the most helpful implements for the construction of the liquidus surface of ternary phase diagrams. In its original form, it allows to find the direction of falling or increasing temperature along the monovariant reaction lines forming the boundaries of the primary crystallization fields. The theorem is valid for systems with any number of phases; however, its geometrical construction rule is only defined for the case of stoichiometric phases and it is not clear how to apply the theorem in the case of phases with extended homogeneity ranges. Some examples from a ternary, transition-metal-based system containing phases with large homogeneity ranges are presented, and the usefulness and limits of applicability of the theorem are discussed.

The paper analyzes the works of the last decade on the study of ionic conductivity, thermoelectric, photovoltaic, photocatalytic, optical, etc. properties of the argyrodite family compounds (prototype Ag₈GeS₆), as well as phases and composite materials based on them. Considered works allow us to characterize them as valuable environmentally friendly functional materials with great potential for practical application. The main approaches which have been used in these works to enhance functional properties, as well as to improve the application capabilities of this class of substances have been analyzed. The importance of further systematic investigations on their design based on the “composition-structure-property” relationship has been noted. In the present review, special attention is paid to the analysis of works on phase equilibria in the corresponding systems since the information accumulated in phase diagrams is extremely important for optimizing the properties of compounds through targeted variation of composition and structure. Available data on the phase equilibria and thermodynamic properties of ternary and quaternary systems forming Cu/Ag argyrodite compounds and solid solutions based on them are presented and analyzed. It has been shown that anionic and Cu ↔ Ag substitutions in compounds lead to a strong decrease in their polymorphic phase transition temperatures and an expansion of the temperature-composition ranges of the existence of high-temperature ion-conducting phases up to room temperature and below. The possibility of replacing part of the chalcogen atoms in the compounds with halogens, and the Cu(Ag) atoms with elements of the zinc subgroup, which expands the range of argyrodite phases is also shown. The significance of expanding studies of phase equilibria and thermodynamic properties of these systems, especially five-component and more complex systems possessing a big possibility of the formation of high-entropy argyrodite phases, which have thermodynamic stability in a wide temperature-composition range and better applied characteristics has also been specified.