Journal of Phase Equilibria and Diffusion最新文献

The phase diagram of the Fe-Zn binary system was evaluated based on the CALPHAD method with reference to the latest experimental data. The solubility ranges of the intermetallic compound phases, Γ-Fe₄Zn₉, Γ₁-Fe₁₁Zn₄₀, δ_1k-FeZn₇, δ_1p- Fe₁₃Zn₁₂₆, and ζ-FeZn₁₃ were modeled considering their structures consisting of Zn₁₂ icosahedra with Fe at the center (Fe₁Zn₁₂ clusters) as well as glue-like Fe and Zn atoms, and the miscibility gap between the δ_1k and δ_1p phases was also taken into account in the present calculations. The solubility of Fe in the liquid and (ηZn) phases that was confirmed as dozens of times larger than the values reported in the earlier literature could be calculated by introducing Fe₁Zn₁₂ associates to these solution phases. Consequently, all phase equilibria were adequately reproduced by the thermodynamic models and parameters revised in the present study.

Faceted morphology is common in the microstructures resulting from solid-state phase transformations in a wide range of crystalline materials. This study explains the faceted interfaces based on the concept of singular interfaces, which are characterized by key interfacial structures at two levels: the singular dislocation structure and the preferred state existing between the dislocations. It identifies interface geometries required by these structures at two stages: before and after dislocation generation. Methods to determine the interface geometries are reviewed. These methods enable quantitative interpretation of phase transformation crystallography features, including the interface orientations and the orientation relationship between the two phases, irrespective of whether these features are described as rational or irrational. The agreement achieved across different systems indicates the crucial role of geometric matching in the development of phase transformation crystallography. An example is provided for an illustration of the application of the two-stage approach, especially with an analysis in reciprocal space using a superimposed diffraction pattern.

The In-Sn binary alloy system exhibits several unusual features that challenge crystallographic and thermodynamic expectations. We combine first principles total energy calculation with simple thermodynamic modeling to address two key points. First, we evaluate energies along the Bain path to interpret the discontinuous transition between the phases α-In (Pearson type tI2) and β-In₃Sn (also Pearson type tI2) that are identical in symmetry. Second, we demonstrate that the solid solution phases β-In₃Sn and γ-InSn₄ (Pearson type hP1) exist at high temperatures only, and they exhibit eutectoid decompositions at low temperatures.

The creep properties of fully lamellar γ/α₂ titanium aluminides can be significantly improved by alloying with Nb, Ta or Zr. While the influence of these alloying elements on the γ-phase has already been examined, their diffusivity and strengthening properties in the α₂-phase are still lacking. In order to study the effect of Nb, Ta and Zr in α₂-Ti₃Al, the alloys Ti-33Al, Ti-33Al-5Nb, Ti-33Al-5Ta and Ti-33Al-5Zr were investigated using a diffusion couple approach and strain rate jump tests. The results show that Zr diffuses the fastest, followed by Nb and Ta. Furthermore, these alloying elements also increase the strength compared to a binary Ti-33Al alloy, from which Zr leads to the highest strength increase followed by Ta and Nb. The lower diffusivity of Ta becomes increasingly important at higher temperatures and lower strain rates resulting in a higher strengthening potential than Nb and Zr under such conditions.

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.

Abstract

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.

Abstract

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.