Showing papers in "Journal of Phase Equilibria in 1994"

Summary of constitutional data on the Aluminum-Carbon-Titanium system

Abstract: The constitution of the titanium-aluminum-carbon ternary system has been investigated combining critical evaluation of literature data with new experimental results. Three ternary phases occur in this system: Ti3AlC, Ti2AlC, and newly discovered Ti3AlC2. As analyzed by wet chemistry methods, all three phases are carbon deficient with respect to their “ideal≓ stoichiometry, which is based on the crystal structure formula. Ti2AlC and Ti3AlC melt incongruently at 1625 ± 10 ‡ and 1580 ± 10 ‡, respectively. Ti3AlC2 decomposes in the solid state. The two isothermal sections at 1000 and 1300 ‡ investigated experimentally are corroborated by thermochemical calculations. A projection of the liquidus surface is given, and a reaction scheme linking this liquidus projection with the isothermal sections observed is proposed.

A new thermodynamic description of the Cu-Zr system

Abstract: An optimized set of thermodynamic functions for the Cu-Zr system was obtained by the least squares method from phase diagram and thermodynamic data available in the literature. The excess Gibbs energies of the solution phases, liquid, and three terminal solid solutions, were described by the Redlich-Kister formula. All the intermediate compounds were treated as stoichiometric phases. The calculated phase diagram, as well as the thermodynamic properties vs compositions, agree well with the experimental values. The reliability of the optimized parameters was examined using μ-T plots.

Thermodynamic assessment of the copper-oxygen system

Abstract: The Cu-O system shows complete miscibility between the metallic liquid and the oxide liquid above ∼1623 K and a miscibility gap below that temperature. Because of the practical importance of the system, a wealth of experimental data exists, both on the phase diagram and on the thermodynamic properties. These data have been reviewed, and a consistent set of thermodynamic model parameters has been optimized. An ionic two-sublattice model was used to describe the liquid phase and was found to represent accurately the experimental data.

Binary alloy phase diagrams requiring further studies

Abstract: Binary Alloy Phase Diagrams , 2nd ed. 90Mas, covering ∼3000 systems and ∼2200 phase diagrams, is the most current compilation of binary systems. However, ∼500 of them include thermodynamically unlikely features. These problems are classified into more than 30 groups, and a few typical examples are shown for each group. If a phase diagram shows an improbable feature, it implies that either the phase diagram is erroneous or a very unique phase diagram situation is occurring in the system. In either case, it is worthwhile to investigate the system in more detail.

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the Al-Mg, Al-Sr, Mg-Sr, and Al-Mg-Sr systems

Abstract: All available thermodynamic and phase diagram data were critically assessed for all phases in the Al-Mg, Al-Sr, and Mg-Sr systems at 1 bar pressure from room temperature to above the liquidus temperatures. For these systems, all reliable data were simultaneously optimized to obtain a set of model equations for the Gibbs energy of the liquid alloy and all solid phases as functions of composition and temperature. The modified quasi-chemical model was used for the liquid. The Al-Mg-Sr ternary phase diagram was calculated from the optimized thermodynamic properties of the binary systems. Since no reliable ternary data were available, three assumptions were made: no ternary terms were added to the model parameters for the thermodynamic properties of the liquid, no ternary solid solutions are present in the system, and no ternary compound is present in the system. The calculated ternary phase diagram is thus a first approximation, which can be improved by the addition of new experimental data and can be used as a base for the calculation of phase diagrams of multicomponent systems.

The N-Zr (Nitrogen-Zirconium) system

Abstract: The assessed Zr-N phase diagram at one atmosphere is based primarily on the work of [56Dom], [59Sma], [62Sam], and [74Ero], and includes: (1) the liquid, L; (2) the bcc terminal solid solution (PZr) in which N has a maximum solubility of 5.0 at.% at 1883{degrees} C, the ({Beta}Zr) peritectic decomposition temperature; (3) the cph terminal solid solution ({alpha}Zr), which shows a maximum solubility of 24.7 at.% N at 1988{degrees} C where this phase decomposes peritectically; (4) the nonstoichiometric compound ZrN with a NaCl-type structure, which at 1988{degrees} C is stable in the composition range of {approx}40 to 50 at.% N (experimental data coupled to thermodynamic calculations indicate that ZrN melts congruently at {approx}48 at.% N and {approx}3670{degrees} C); and (5) the gas phase, G, constituted essentially by pure N{sub 2}.

Systematic aspects of the high-temperature thermochemistry of binary alloys and related compounds

Abstract: In order to obtain reliable information on the high- temperature thermodynamics of binary alloys and related compounds, the modern investigator has a range of different experimental methods at his disposal: vapor pressure measurements, the emf technique, solubility measurements, and high-temperature reaction calorimetry. It is now more than 40 years since we first analyzed these prob-lems in some detail. We recognized, of course, that reaction calorimetry alone cannot provide a complete picture of the thermodynamics of any system. On the other hand, we also knew that the en-thalpy contributions to the excess Gibbs energies of alloys usually outweigh the corresponding en-tropy terms. We concluded that, among these various experimental methods, high- temperature reaction calorimetry should be the most useful approach in any attempt to gain systematic thermo-dynamic information on alloy systems. We accordingly decided to pursue a program of research cen-tered on alloy high- temperature thermochemistry. A review is presented of the development of the modern high- temperature reaction calorimeter and of the application of calorimetric measurements to a wide range of binary alloy systems and related compounds.

Tie lines of the grain boundary wetting phase transition in the Al-Sn system

Abstract: Temperature dependencies of contact angles θ at the intersection of grain boundaries in Al bicrystals with solid Al/liquid Al-Sn interphase boundaries were studied. For this purpose, two Al bicrystals were grown with tilt {001} grain boundaries having misorientation angles 9 = 38.5‡ (near 19 coincidence misorientation) and ϕp = 32‡. These boundaries possess different energies. The tempera-turesTw of the grain boundary wetting phase transition for these two boundaries were determined, and the corresponding conodes in the two-phase area of the Al-Sn phase diagram were constructed. AboveTw, the contact angle θ = 0 and a layer of the liquid phase completely wets the grain boundary. The temperature of wetting transitionTw1= 6047± 1 ‡C, for the grain boundary with high energy (ϕp = 32‡) is lower thanTw2 = 6177 ± 1 ‡C for the grain boundary with low energy (ϕ = 38.5‡). Above the temperature interval where all the grain boundaries become wetted, the solid phase may exist only as isolated single crystalline “islands≓ in the “sea≓ of melted phase.

Application of differential thermal analysis to solid-solid transitions in phase diagram determination

Abstract: The differential thermal analysis (DTA) procedure is presented for the determination of equilibrium phase diagrams. The equilibrium solid-solid phase transition temperatures, which are needed in the construction of phase diagrams, cannot be obtained directly from the DTA data due to the superheating or undercooling during the actual DTA operation. Methods to extrapolate the equilibrium onset temperature for the solid-solid phase transitions from the DTA data are evaluated. Several practical problems in the DTA experiments and analysis are addressed.

Correlations between phase diagrams and thermodynamic data for metal hydride systems

Abstract: A series of correlations between the phase diagrams and enthalpic data are developed for metal hydride systems. The value of these correlations lies in their providing criteria for checking the consistency of the thermodynamic behavior for these systems. The correlations are illustrated using some experimental thermodynamic data and the accompanying phase diagrams for metal hydrides from the literature.

Metastable eutectic formation in Ni- Al alloys

Abstract: Directional solidification experiments have been carried out at low rates in alloys near the N13AI composition in the Ni-AI binary alloy system. Results confirm an earlier study at high rates showing that the eutectic in this system occurs between the β and γ′ phases. It is shown that under certain composition and growth conditions a metastable eutectic between the β and γ phases forms in these alloys. Analysis of the decomposition of this metastable eutectic into the γ′ phase and additional composition measurements are used to provide an improved version of the phase diagram near the N13AI composition.

On the Thermochemistry of the Rare Earth Compounds with the p-Block Elements

Abstract: A summary is given of the experimental work carried out to determine the heats of formation of the rare earth compounds with a number of elements of thep-block of the periodic table (in particular Al, In, Sn, Pb, As, Sb, and Bi). The experimental methods and the devices constructed to this end are briefly described and commented on. Some results recently obtained in the thermochemical investigation of binary (Pr-Al and Gd-Pb) and ternary (Ce-Ni-Al and La-Sb-Bi) alloys are presented.

The Cr-Te (Chromium-Tellurium) System

Abstract: The present assessment relied heavily on the experiments of [83Ips], which are of high quality with the quoted uncertainty in composition being ± 0.25 in at.% Te. The starting point is the phase diagram shown in Fig. 2 to which other experimentally observed thermal effects [79Gun] can be fitted. This diagram has two anomalies, namely, 1) a gap between the supposed solidus and the liquidus, and 2) a narrow two-phase region within a broad homogeneous field. To assimilate these anoma-lies, the existence of a highly disordered phase, redesignated as Cr1-xTe, is imagined at high temperature. This has led to the prediction of a eutectoid decomposition of Cr3Te4-h at 60.2 at.% Te at a temperature very close to 455 ‡. Further, in Fig.

Experimental investigation and thermodynamic optimization of the Y-Cu binary system

Abstract: The Y-Cu binary system was experimentally investigated by DTA and XRD. The DTA results provide evidence for the possible existence of a high-temperature phase transformation in the YCu2 compound. The XRD results confirm a range of solubility for the YCu6 phase. A thermodynamic description for all the phases was obtained by taking into account all experimental data in the literature. A new phase diagram is proposed.

Phase equilibria in Ti3Al-Nb alloys at 1000 °C

Abstract: Titanium aluminides are considered to be the future high-temperature structural materials for turbine applications. Major focus is on α2Ti3Al based and γTiAl based alloys. Niobium additions to Ti3Al alloys is found to improve the room-temperature ductility. Thus phase equilibria in Ti-Al-Nb system is of practical significance with regard to their processing and high-temperature phase stability characteristics. In the present research, four alloys with compositions Ti-22Al-12Nb, Ti-21A1-16Nb, Ti-20Al-20Nb, and Ti-25Al-25Nb (all in atom percent) were equilibrated at 1000 °C for 225 hours and then quenched in water. The quenched alloys were characterized for phase relations by optical microscopy, X-ray diffraction (XRD), and electron probe microanalysis (EPMA). Based on the phase analysis, the ternary isotherm of the Ti-Al-Nb system at 1000 °C was constructed on the Ti3Al-rich side. The orthorhombic Ti2AlNb phase was observed in the sample with Ti-25Al-25Nb composition signifying the presence of this phase at 1000 °C.

A tutorial on flow diagrams: A tool for developing the structure of multicomponent phase diagrams

Abstract: The tabular form of multiphase equilibria in temperature- or pressure-ordered sequence, often referred to as flow diagrams, shows the interrelations that exist among the various types or classes of equilibria in a system. These interrelations establish the structures of the various forms of phase diagrams. Flow diagrams have been used to simplify the presentation of complex ternary phase diagrams. However, they have been overlooked as a useful tool in predicting phase diagrams, in developing isotherms and isobars, and in teaching fundamentals of phase diagrams. In principle, flow diagrams can be applied to any system regardless of order.

Phase diagrams of Ti-Al-C, Ti-Y-O, Nb-Y-O, and Nb-Al-O at 1100°C

Abstract: To develop suitable coating materials for A12O3 fibers used in Ti-based metal-matrix composites, the phase diagrams of Ti-Al-C, Ti-Y-O, Nb-Y-O, and Nb-Al-0 at 1100 °C were studied. The phase equilibrium samples were prepared by hot isostatic press (HIP), annealed, and then examined by x-ray diffraction (XRD). In the Al-Ti-C ternary, the present study was concentrated on the Ti-rich corner involving Ti, TiC, α2Ti3A1, and the ternary Pphase (T3AlC). The results show that at 1100 °C, β-(Ti, Al) is in equilibrium with TiC, but not in equilibrium with the Pphase as suggested by Schuster et al. In the Ti-Y-O, Nb-Y-O, and Nb-Al-0 ternary systems, the present study shows that Ti-Y2O3, Nb-Y2O3, and Nb-Al2O3 are in equilibrium at 1100 °C.

Vapor Pressure Measurements of Arsenic and Arsenic Trioxide Over Condensed Phases

Abstract: The solid-vapor relations for arsenic in the temperature range from 680 to 840 K and the liquid-vapor relations for arsenic trioxide in the temperature range from 650 to 740 K were determined by direct vapor pressure measurements carried out with a quartz gauge. The resulting InP (total) vsT are : InP (atm) = 2545.1/T + 22.27 InT− 154.02 (for arsenic) and InP (atm) = −50983/T + 6.869 (for arsenic trioxide). Calculated enthalpy of vaporization (ΔHv,T0) for arsenic trioxide and enthalpy of sublimation (ΔHs,2980) for arsenic are 42.36 kJ/mol and 156.13 kJ/mol, respectively.

Thermodynamic description of dross formation when galvanizing silicon steels in zinc-nickel baths

Abstract: This paper proposes a thermodynamic description of liquid Fe-Ni-Zn alloys, which explains the dross formation observed when galvanizing iron alloys in a zinc bath with 0.1 wt.% Ni added. At 450 °C, dross particles are present at the ζ-FeZn13-liquid interface and are identified as the Γ2 2-Fe6Ni5Zn89 compound. At 480 °C, their shape and structure change drastically, and the Γ2-intermetallic compound is no longer observed. This behavior is related to the changes in the phase relationships in the Fe-Ni-Zn system between 450 and 480 °C.