Journal of Phase Equilibria and Diffusion最新文献

The isothermal section of the phase diagram of the Mg-Co-Ga ternary system at 200 °C for the full composition range was built. The formation of five ternary compounds was observed. The crystal structures of all ternary compounds: τ₁—MgCo₂Ga₅ (own structure type, Pnnm space group), τ₂—Mg₃Co₂Ga₇ (C2/c space group, own structure type), τ₃—MgCoGa₂ (P2₁/n space group, own structure type), τ₄—Mg_1−xCo_2−yGa_x+y (x = 0.06, y = 0.64) (Cmcm space group, own structure type), τ₅—Mg_1−xCo_2−yGa_x+y (x = 0.10, y = 0.16) (R-3 m space group, own structure type) were investigated by means of single crystal diffraction. In addition to ternary compounds, solid solutions based on binary phases are formed in the system. The solid solution MgCo_2−xGa_x (x = 0-1, P6₃/mmc space group, MgZn₂ structure type) represent the largest area of homogeneity. The phase content of alloys was determined by SEM/EDX analysis.

Recently high entropy alloys (HEA) have shown remarkable potential due to their extraordinary properties and applications. HEAs are explored to create a new class of materials with an attractive set of properties that are difficult to achieve by conventional materials. Short-range ordering (SRO) is important in determining various materials properties at nanometer scale, such as phase stability. The relationship between SRO and phase stability can be understood through the enthalpy of mixing. Cluster expansion (CE) is often used to understand the relationship between the enthalpy of mixing and SRO parameters. Although exact, CE must be truncated in practice beyond some maximal-sized cluster, leading to truncation errors. In this work, as an alternative, a neural network is trained to understand the relationship between SRO and enthalpy of mixing among the various binary subsystems of NbTiVZr HEA. For training, a large pool of structures and their corresponding correlation functions (or SRO parameters) are generated using the alloy theoretic automated toolkit (ATAT) software for each subsystem. First-principles calculations are used to determine the enthalpy of mixing of these structures. This database is used to train a neural network and the predicted values of enthalpy of mixing from the trained neural network are found to be reasonably accurate and better than the corresponding CE model. The neural network approach is found to clarify the complex relationship between the enthalpy of mixing and SRO, especially when there is a limitation over the number of fitting parameters due to smaller size of databases.

The phase equilibria of the Cu₂S-SiS₂-GeS₂ system have been studied in the Cu₂S-Cu₈SiS₆-Cu₈GeS₆ composition area. Based on data obtained from differential thermal analysis, powder x-ray diffraction, and SEM-EDS techniques, the T-x diagram of the Cu₈SiS₆-Cu₈GeS₆ boundary system and two internal polythermal sections, as well as the isothermal section at 300 K of the phase diagram and the liquidus surface of the Cu₂S-Cu₈SiS₆-Cu₈GeS₆ system were constructed. The areas of primary crystallization and homogeneity of phases, the nature, and temperatures of invariant and monovariant equilibria were determined. Continuous solid solutions based on both crystalline modifications of the starting compounds of the Cu₈SiS₆-Cu₈GeS₆ boundary system, have been revealed, which are of interest as environmentally friendly functional materials. The temperatures and enthalpies of phase transitions of Cu₈SiS₆ and Cu₈GeS₆ compounds, and Cu₈Si_(1−X)Ge_XS₆ solid solutions were determined using differential scanning calorimetry. The entropies of phase transitions for end-member compounds were also calculated. It is shown that the heats and entropies of phase transitions of these phases are anomalously large in comparison with the thermodynamic functions of ordinary polymorphic transitions. Apparently, this is due to a significant increase in the degree of disorder in the cationic sublattice upon transition to the high-temperature ion-conducting phase. It has also been established that the heats of phase transitions of solid solutions are practically equal to the sum of the corresponding functions of the end-member compounds.

The impact of oxygen impurity on phase relationships in the Zr-Mo-Fe system (Zr > 30 at.%) at 1000 °C was investigated by means of x-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis. An oxygen-stabilized ternary compound γ-Zr₂(Mo,Fe), derived from Ti₂Ni type Zr₄Fe₂O, with cell parameter of a = 1.2213 nm was observed to be present in the investigated Zr-Mo-Fe alloys. The phase relationships of the Zr-Mo-Fe (O) system at 1000 °C in the Zr-rich corner consist of 3 four-phase regions, i.e., [βZr + λ-Zr(Mo,Fe)₂ + γ-Zr₂(Mo,Fe) + ZrMo₂], [βZr + λ-Zr(Mo,Fe)₂ + γ-Zr₂(Mo,Fe) + Liquid], and [λ-Zr(Mo,Fe)₂ + γ-Zr₂(Mo,Fe) + Liquid + ZrFe₂]. The previously detected Zr₉Mo₄Fe compound was not observed in this work.

Vaporisation studies were carried out over the solid region of LiCl(cr), KCl(cr) and liquid region of LiCl-KCl-UCl₃ ternary salt system by using Knudsen Effusion Mass Spectrometry (KEMS) in the temperature range of 715–913 K. Monomeric and dimeric species were observed in the vapour phase in equilibrium with their respective salts, LiCl(cr) and KCl(cr). LiCl(g), Li₂Cl₂(g), KCl(g), K₂Cl₂(g), and UCl₃(g) were the neutral species observed in the equilibrium vapour over ternary salt. Partial pressure–temperature relations for vapour species were derived using in-situ calibration from pressure dependent equilibrium constants as well as using pure silver as external calibrant. Using p-T relations, various heterogeneous reaction equilibria that exist between condensed phase-gas phase and the dissociation equilibra of following gas phase reactions: Li₂Cl₂(g) = 2LiCl(g); K₂Cl₂(g) = 2KCl(g) were evaluated by using 2nd and 3rd law methods. Also, the enthalpies of pressure-independent reactions: LiCl(cr) + LiCl(g) = Li₂Cl₂(g); KCl(cr) + KCl(g) = K₂Cl₂(g) were evaluated by using 3rd law method. Knudsen effusion mass spectrometric studies on LiCl-KCl-UCl₃ ternary salt system were carried out for the first time.

Sb₂Te₃ is an important thermoelectric material. Co is a potential diffusion barrier. This study examined the electroplated Co/Sb₂Te₃ interfacial reactions at 300  °C, 400 °C and 500 °C. To provide fundamental information and for better understanding the reaction paths, the Co-Sb-Te phase equilibria isothermal sections at these three temperatures are proposed. No ternary compound is found. The 400 °C isothermal section contains (Co), (Sb), (Te), β-CoSb, β-Co_1−xTe, CoSb₃, δ-(Sb₂Te), γ-(SbTe), Sb₂Te₃ and a continuous solid solution γ-Co(Sb,Te)₂. At 500 °C, except for Te being molten, the phase relationships are similar to those at 400 °C. At 300 °C, although γ-CoSb₂ and γ-CoTe₂ have very significant mutual solubilities, they are two separate phases and do not form a continuous solid solution. In the Co/Sb₂Te₃ couple reacted at 300 °C, only one reaction phase, γ-CoTe₂ with Sb solubility, is formed. Similar results are found in the Co/Sb₂Te₃ couple reacted 400 °C, and the reaction phase is the continuous solid solution γ-Co(Sb,Te)₂. Two reaction phases, β-Co_1-xTe and γ-Co(Sb,Te)₂, are found in the couple reacted at 500 °C. The compositional ratio of Sb/Te in the reaction phase remained at 2/3 indicates that Co is the dominating diffusion species. The reaction phases are formed rapidly by Co penetration into the Sb₂Te₃ substrate. A peculiar phenomenon was found with the reaction layer that formed on the side of Co that was not in direct contact with the Sb₂Te₃ substrate which will be further investigated.

Thermodynamic databases are essential to understanding alloy properties and guiding materials design. In this work, the mixing enthalpies of the liquid phase in the Gd-Te, Dy-Te and Ho-Te binary systems are calculated using Ab-initio Molecular Dynamics (AIMD) simulations and the thermodynamic parameters are determined using the CALculation of PHAse Diagrams (CALPHAD) method combined with the phase equilibrium data and thermodynamic data. The associated solution model is employed to describe the liquid phase of these systems. The sublattice model (Gd,Te)_0.0296(Gd)_0.4(Te)_0.5714, (Dy,Te)_0.0286(Dy)_0.4286(Te)_0.5714 and (Ho,Te)_0.5(Te)_0.5 are utilized to describe Gd₂Te₃, Dy₂Te₃ and HoTe, respectively. The line compounds, i.e. GdTe, DyTe, and DyTe₂, are modeled using the stoichiometric model. The calculated results can describe the experimental and thermodynamic information reported in the literature. In addition, the existence of a liquid-liquid separation in the Dy-rich side in the Dy-Te binary system is proposed in this work.

The parallel tangent method widely applied to predict the composition and driving force to form a nucleus from an oversaturated solution is extended in this paper. The parallel tangent method is shown to (i) Over-estimates the composition difference between the first nucleus and the parent phase, (ii) Neglects the composition dependence of interfacial energies and (iii) Neglects the composition dependence of probability to form embryos prior to nucleation. New model equations are developed here for the composition dependence of the interfacial energies and probability to form the embryos as function of nucleus composition at given matrix composition. The most probable composition of the first nucleus is found at the maximum of the driving force of nucleation extended by the new model equations. The success of the extended method is demonstrated for an Al-Fe liquid alloy with 0.3 w% of Fe to predict the first nucleating intermetallic phases upon cooling after nucleation of the fcc phase. It is shown that although the prediction based on the parallel tangent method contradicts experimental observations, the prediction based on our extended method agrees with them.

Graphical Abstract

The aim this work was the reinvestigation of phase equilibria in Bi-Te system. The as-cast and long-time equilibrated alloys of Bi-Te binary system were analysed by scanning electron microscopy, X-ray diffraction (XRD) and differential thermal analysis (DTA) methods. The primary solidified phases were identified for as-cast samples. The existence of the four intermetallic phases: Bi₂Te₃, BiTe, Bi₂Te, Bi₇Te₃ was confirmed. Three of them melt incongruently in peritectic reactions. The model parameters of all phases in this system were assessed by optimization using available data by Calphad method. The stabilisation of these phases in other ternary systems was discussed taking into consideration its future application in thermodynamic description of Ag-Bi-Te ternary system.

Zr metal bulks were isothermally reacted with B powder at 800-1630 °C for 2-16 h to form ZrB₂ layers on the Zr bulk surfaces. Diffusion kinetics of B element in association with Zr + B solid state reaction was investigated by means of XRD, SEM and EDS. Nano-sized ZrB₂ grains without formation of a discernible layer were detected on the Zr surface after reaction at 800 °C for 2 h. When the reaction temperature was increased, continuous layers consisting of faceted ZrB₂ grains were formed at temperatures ranging from 1200 to 1500 °C. The thickness of the ZrB₂ product layers showed a parabolic dependence on time t and an exponential dependence on temperature T via − 1/RT. The Zr + B reaction calculated a diffusion activation energy G = 152.0 ± 62.4 kJ/mol for B diffusion in ZrB₂. The diffusion coefficient of B element was experimentally determined to be (D{ } = 1743.4 times 10^{ - 12} {text{ exp}}left( {{raise0.7exhbox{${ - 152027.6}$} !mathord{left/ {vphantom {{ - 152027.6} {{text{R}}T}}}right.kern-0pt} !lower0.7exhbox{${{text{R}}T}$}}} right)).