Equilibrium Alloys Research Articles

Activity of phosphorus in liquid cobalt-phosphorus alloys in equilibrium with solid cobalt

Electrochemical measurements involving stabilized zirconia as a solid electrolyte were conducted in order to determine the activities of phosphorus in liquid {Co-P} alloys in equilibrium with solid cobalt at temperatures between 1573 and 1693 K. By using the present data for the two-phase region of Co(s) + {Co-P}(l), the free energy for the solution of diatomic gaseous phosphorus in pure liquid cobalt at 1 wt% solution was derived as:

Thermodynamics of the Al2O3-SiO2-TiOX Oxide System at 1873 K.

Isothermal phase relations for the Al 2 O 3 -SiO 2 -TO x system have been investigated under strongly reducing conditions, achieved by equilibrium between graphite and CO gas, P O2 =4.56×10 -16 atm at 1873 K, in equilibria with a saturating oxide tablet. Activities of Al 2 O 3 and SiO 2 were measured by the Knudsen effusion method. Activities of SiO 2 were also measured by the chemical equilibrium between the oxide and the Fe-Si-C alloy. Activity of TiO x was also estimated from the composition of the Si-Al-Ti alloy in equilibrium with the oxide assuming that the alloy conformed to a regular solution. In addition, steel composition equilibrated with the oxide of arbitrary composition has been calculated by the activities of each component of the oxide system obtained in the present study.

A first-principles approach to modeling alloy phase equilibria

This paper presents a brief overview of recent developments in the application of first-principles calculations to the study of bulk and interfacial thermodynamic properties and phase equilibria in alloys. Among the applications discussed are calculations of: bulk thermodynamic properties, phase boundaries, interfacial free energies, and precipitate morphologies. The article concludes by highlighting some further recent developments that are likely to lead to increasing applications of these modeling techniques.

Electrochemical Properties of Li3 N Dissolved in Molten LiCl at 900 K

Thermodynamic properties of dissolved in the molten LiCl salt at 900 K were explored using electrochemical methods. It was difficult to determine precisely the decomposition voltage of dissolved in the molten salt by cyclic voltammetry. The oxidation wave of ion could not be located with high accuracy. However, the lithium activity of the Pb-Li alloy in equilibrium with the molten salt containing dissolved under nitrogen atmosphere could be measured electrochemically with high accuracy using the reference electrode. Under the conditions used in this study, the potential of the Li-Pb electrode is equal to the decomposition voltage of The activity of in molten LiCl was determined for anionic fractions of ranging from to 0.028. The nitride ion concentration in the salt was determined by chemical titration. The activity coefficient of the at high dilution was found to be very low, around The activity coefficient increases sharply with composition and has a value of 0.25 at © 2001 The Electrochemical Society. All rights reserved.

High-temperature solution growth of intermetallic single crystals and quasicrystals

Solution growth continues to be one of the most powerful techniques for the production of single crystals for basic and applied research. It is a versatile technique that allows for the growth of congruently and incongruently melting materials with equal ease. The primary requirement for growth is that there be an exposed primary solidification surface in the appropriate equilibrium alloy phase diagram. In this paper, we will review some of the systems that we have applied this technique to over the past several years. These range from binary and ternary intermetallic compounds such as RFe2 (R=Rare earth) and RT2Ge2 (T=Ni and Cu) to the refractory RNi2B2C family of magnetic superconductors to several families of quasicrystals. In each case, specific nuances of the relatively simple growth technique will be discussed.

Formation of non–equilibrium alloys by high pressure melt quenching

Melt quenching under high pressure can promote the formation of metastable materials. High pressure accelerates the amorphization of Cu60Ti40 and Cd43Sb57.For an alloy systems having volume expansion after solidification, the higher the applied pressure, the lower the melting point and the higher the amorphization temperature, which promotes the formationof metallic glass. High pressure also enhances nucleation and suppresses grain growth, so solidification under high pressure can refine the crystal grains to form nanocrystalline alloys, such as Ti60Cu40,Cu70Si30 and Pd78Si16Cu6 alloys.

Structure of liquid Al–Mn–Cr alloys in equilibrium with the μAl4(MnyCr1−y) approximant phase

The structure of liquid Al88.5(MnxCr1−x)11.5 alloys (0≤x≤1) in equilibrium with the hexagonal μAl4(MnyCr1−y) approximant phase (0≤y≤1) was investigated by neutron scattering. The nature of the Mn/Cr substitution in these liquids was assumed to be isomorphic on the basis of preliminary results, showing that a complete Mn/Cr isomorphic substitution does occur in their primary solidified μ-phase. From the analysis of the partial pair distribution functions, as well as from the comparison between experimental and simulated structure factors at large Q values, we found that liquids exhibit a strong local icosahedral order similar to the one observed in Al–Mn and Al–Cr approximant phases, i.e. icosahedral clusters formed by one transition metal surrounded by 12 atoms, mainly Al.

Neutron diffraction and phase evolution of the mechanically alloyed intermetallic compound ζ-FeZn13

High-energy ball milling with subsequent annealing is used to synthesize the intermetallic compound ζ-FeZn13. The mechanically alloyed phase in the as-milled state is determined to be nonequilibrium, or metastable. Transmission electron microscopy (TEM) studies show a highly defective microstructure with undefined grain areas, and the alloy can be described as a mechanical mixture of elemental Fe and Zn, based on neutron diffraction measurements. Characteristic stages associated with its transformation to the equilibrium state are identified based on differential scanning calorimetry (DSC) measurements. The activation energies corresponding to these stages are 128, 202, and 737 kJ/mole, respectively, with increasing transformation temperatures. The first stage is related to limited atomic diffusion or rearrangements, such as recovery, during thermal treatment, while the second stage depicts continued recrystallization and long-range atomic diffusion leading to a stable phase formation. The third and final stage marks structural decomposition of the equilibrium structure due to phase transition. Neutron diffraction of the equilibrium alloy confirmed that the structure is C2/m, with lattice parameters of a=13.40995 A, b=7.60586 A, c=5.07629 A, and β=127 deg 18 minutes. The atomic positions of Fe and Zn compared well to reported values.

Physicochemical Studies of Lithium Nitride in Molten LiCl–KCl

Thermodynamic properties of lithium nitride in the molten LiCl-KCl eutectic salt were explored using electrochemical methods at temperatures between 723 and 900 K. Attempts to measure the decomposition voltage of Li 3 N, ΔE decomp , dissolved in the molten salt by cyclic voltammetry were not sufficiently precise because the oxidation wave of N 3- ion could not be located with high accuracy. As an alternative method, lithium activity of Pb-Li alloy in equilibrium with the molten salt containing dissolved Li3N under nitrogen atmosphere was measured electrochemically. Under the conditions used in this study, the electrochemical potential of the Li-Pb electrode with reference to Li/Li + electrode is equal to the ΔE decomp , and could be measured with higher accuracy. With this method, the activity of Li3N in molten salt was determined for anionic fractions of N 3- ranging from x N3- = 3 × 10 -4 to 0.015. The nitride ion concentration in this salt was determined by chemical analysis, and compared with nominal values. The activity coefficient of Li 3 N at high dilution (x N3- < 10 -3 ) was found to be very low, around 10 -2 to 10 -4 at 723 K. The activity coefficient increases sharply with composition and has a value in the range 10 -1 to 1 when x N3- is around 10 -3 at temperatures investigated in this study.

X-ray diffraction, magnetization and Mössbauer studies of nanocrystalline Fe–Ni alloys prepared by low- and high-energy ball milling

Fe–Ni alloys were prepared both by low- and high-energy ball milling processes. Structure and magnetic properties were studied by using X-ray diffraction, differential scanning calorimetry, Mössbauer spectroscopy and magnetization measurements. Mechanical treatment influenced the magnetic properties of Fe–Ni alloys as compared with the equilibrium alloys. Reduction of grain size resulted in the increase of magnetization. Invar anomaly for 35 at% Ni was not detected.

Transformations and fine magnetic structure of mechanically alloyed Fe–Ni alloys

Structure and phase composition as well as fine magnetic structure of mechanically alloyed (MA) Fe 100− x Ni x compositions ( x=10, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 50, 60, 70, 80, 90 at%) were investigated by Mössbauer spectroscopy and X-ray diffractometry. The concentration ranges of existence of single-phase solid solutions for samples prepared by MA technique were extended comparing with the equilibrium alloys. Specialities of fine magnetic structure of as prepared and annealed MA samples were found to be in a good agreement with their phase composition.

Compositional ordering in SiGe alloy thin films

We have performed surface x-ray diffraction experiments on ${\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ films grown on Ge(001) substrates. The results for our thinnest film (eight monolayers) show the compositional order at the initial stages of growth. This ordering is observed underneath the surface $(2\ifmmode\times\else\texttimes\fi{}1)$ dimer reconstruction, and was previously predicted by calculations on the equilibrium alloy surface. This initial surface compositional ordering is also consistent with the ultimate structure for the bulk of these films. Measurements from thicker alloy films show an increase in the average order parameter with increasing thickness, but then a decrease with the thickest film of 1000 \AA{}.

Thermodynamic estimation on the reduction behavior of iron-chromium ore with carbon

The thermodynamics for reduction of iron-chromium ore by carbon is discussed. The thermodynamic properties of iron-chromium ore were evaluated from our previous work on the activities of constituents in the FeO·Cr2O3-MgO·Cr2O3-MgO·Al2O3 iron-chromite spinel-structure solid solution saturated with (Cr, Al)2O3, and those of the Fe-Cr-C alloy were estimated by a sublattice model. The stability diagrams were drawn for carbon reduction of pure FeO · Cr2O3, (Fe0.5Mg0.5)O·(Cr0.8Al0.2)2O3 iron-chromite solid solution, and South African iron-chromium ore. The evaluated stability diagrams agreed well with the literature data. It was concluded that the lowest temperature for reduction of FeO · Cr2O3 in the iron-chromium ore was 1390 K and a temperature higher than 1470 K would be necessary to reduce Cr2O3 in MgO·(Cr,Al)2O3 in the prereduction process of iron-chromium ore. The composition of liquid Fe-Cr-C alloy in equilibrium with iron-chromium ore was also estimated under 1 atm of CO at steelmaking temperature. The predicted metal composition showed reasonable agreement with the literature values.

Copper removal from carbon-saturated molten iron with Al2S3-FeS flux

Copper removal from carbon-saturated molten iron to Al2S3-FeS flux was experimentally investigated in the temperature range from 1473 to 1573 K. The maximum copper distribution ratio between the Al2S3-FeS flux and the ferrous alloy was about 28 at the composition where the molar ratio of Al to Fe in the flux was around 2. The distribution ratio was no less than 25 as long as the copper content of the flux was less than 10 mass pct. The sulfur content in the ferrous alloy in equilibrium with the Al2S3-FeS flux was higher than that obtained by using Na2S-FeS flux, and it was concluded that the high copper distribution ratio of the Al2S3-FeS flux was brought about by the high activity coefficient of FeS in the flux. In experiments for recovering copper from the flux, copper in Al2S3-FeS-Cu2S flux was reduced by metallic aluminum at 1473 K. The FeS in the flux was primarily reduced and, after that, the copper was recovered in the form of Cu-Al-Fe alloy. The residual copper content in the flux could be decreased to less than 1 mass pct when the aluminum content in the alloy was higher than 40 mass pct. A process for copper removal from molten iron is proposed, which uses successive contacts between the Al2S3-FeS flux and molten iron at several stages with counterflow operation. It is suggested that 1 mass pct Cu in molten iron can be reduced to approximately 0.1 mass pct Cu using 100 kg flux/ton iron; the amount of aluminum required for the iterative use of the flux is about 10 kg/ton iron. By the recycling use of this Al2S3-FeS flux, it is suggested that copper removal from molten iron using the sulfide flux can be more effective.

SURFACE ALLOY FORMATION IN THE Cu–Pd(111) SYSTEM: A KTBIM APPROACH

We study the kinetics of formation and dissolution of surface alloy obtained by depositing one monolayer of Pd on Cu(111) at room temperature. This is performed within the kinetic tight-binding Ising model. We also present the equilibrium segregation isotherm for Pd c Cu 1-c(111) and show the relation between kinetics (surface alloy) and equilibrium (alloy surface) via the local equilibrium concept.

Growth and structure of thin Pt films on Cu(111) studied by low energy Li + ion scattering

The growth and structure of thin Pt films on Cu(111) have been studied by low energy Li + ion scattering. The results are compared to those obtained from a Cu 3Pt(111) single crystal alloy. Deposition of Pt on Cu(111) at coverages up to a few monolayers at room temperature (≤ 50°C) and then annealing to 300°C result in the formation of a metastable surface alloy. This surface alloy has been examined to consist of at least three layers of Cu 3Pt by analyzing the incident angle dependence associated with shadowing effects. The experimental results also show that the surface composition depends strongly on annealing temperature. The formation of a relatively stable surface alloy in local equilibrium over the few topmost layers is discussed on the basis of the segregation and ordering behaviour. No surface chemical ordering of the surface alloy has been evidenced in this study.

An electrochemical investigation of the thermodynamic properties of cerium stannide (CeSn 3) and liquid tincerium alloys

The thermodynamic properties of the intermetallic compound CeSn 3 and the tin-rich liquid tincerium alloys in equilibrium with CeSn 3 were studied by a high-temperature galvanic cell technique using the reversible concentration cell Ta, Ce∥CeCl 3, LiClKCl (eutectic)∥CeSn (two-phase alloy), Ta in the temperature range 723–873 K. The standard Gibbs energy of formation of CeSn 3 from pure solid cerium and pure liquid tin varies with temperature according to the equation ΔG f(CeSn 3) 0 = −260.08 + 0.0476 T (±2) kJ mol −1 The activity coefficients, excess partial molar free energies, enthalpies and entropies of cerium in the saturated liquid alloys have been computed from the e.m.f. data obtained in this study and the data for the solubility of cerium in liquid tin. The activity coefficients of cerium in liquid tin with reference to solid cerium can be expressed as G Ce xs = RT ln τ Ce = −249 + 0.059 T (±2) kJ mol −1

The solubility of Au-Ag alloy + AgCl in HCl/NaCl solutions at 300°C: New data on the stability of Au (1) chloride complexes in hydrothermal fluids

The solubility of Au-Ag alloy (electrum) + AgCl (chlorargyrite) was measured in aqueous HCl/NaCl solutions at 300°C. Equilibrium between the two solid phases provided a convenient and highly effective fO2 sensor. By varying the initial electrum composition (XAu = 0.3 to 1.0), total chloride concentration (0.1 to 5.0 molal), and pH (HCl/(NaCl + HCl) = 0.003 to 1.0), it was possible to survey a wide range in oxidation state (log fO2 = −13.3 to −35.1). High metal solubilities were measured under these conditions (0.3 to >3000 ppm Au; 1280 to >40,000 ppm Ag). All evidence indicates that Au dissolved as an aurous ( + 1) chloride complex at 300°C. This species subsequently disproportionated to a mixture of auric (+III) species and metallic Au upon cooling to room temperature.The solubility data were used to determine equilibrium constants for the following reactions: reactionlogK300°,Psat Au(s) + 2Cl− + H+ + 14O2(g) = AuCl2− + 12H2O +3.86 ± 0.39 (A1) Ag(s) + 2C1− + H+ + 14O2(g) = AgCl2− + 12H2O +9.23 ±0.15 (A2) Au(s) + AgCl2− = Ag(s) + AuCl2−−5.35 ± 0.41 (A3) The value of log K for (A2) is in excellent agreement with previous studies (Seward, 1976; Zotov et al., 1986a,b), whereas the value for (Al) is consistent with the data of Nikolaeva et al. (1972), Helgeson (1969), and Zotov et al. (1991), but not with the results of Henley (1973) and Wood et al. (1987). The log K for (A3) is the first direct determination of a distribution coefficient for exchange of Au and Ag between solid solution and aqueous solution.The above data were combined with results from previous experimental studies to compile a complete set of log K values for reactions (A1), (A2), and (A3) in the temperature range 25 to 350°C. Silver is far more soluble than Au as a chloride complex throughout this temperature range. Nonetheless, AuCl2− may be an important agent for Au transport at T > 300°C, or in highly oxidized (e.g., surface-derived) fluids. Because of the very strong thermodynamic force for Ag to partition into the aqueous phase (reaction A3), Au-Ag alloys in equilibrium with Cl-rich and S-poor fluids will tend to be Au-rich, especially at lower temperature.

Experimental tests of garnet peridotite oxygen barometry

We have performed experiments aimed at testing the calibration of oxygen barometers for the garnet peridotite [garnet (Gt)-olivine (Ol)-orthopyroxene (Opx)] phase assemblage. These involved equilibrating a thin layer of garnet sandwiched between layers of olivine and orthopyroxene at 1300°C and 23–35 kbar for 1–7 days. Oxygen fugacity was controlled (but not buffered) by using inner capsules of Fe−Pt alloy or graphitc or molybdenum sealed in welded Pt outer capsules. Post-experiment measurement of fO2 was made by determining the compositions of Pt-Fe alloy sensors at the interface between garnet and olivine + orthopyroxene layers. The composition of alloy in equilibrium with olivine + orthopyroxene was approached from Fe-oversaturated and Fe-undersaturated conditions in the same experiment with, in general, excellent convergence. Product phase compositions were determined by electron microprobe and a piece of the garnet layer saved for 57Fe Mossbauer spectroscopy. The latter gave the Fe3+ content of the garnet at the measured P-T-fO2 conditions. Approach to equilibrium was checked by observed shifts in Fe3+ content and by the approach of garnet-olivine Fe−Mg partitioning to the expected value. The compositions of the phases were combined with mixing properties and thermodynamic data to calculate an apparent fO2 from two possible garnet oxybarometers:- (1) $$\begin{gathered} 2Ca_3 Fe_2 Si_3 O_{12} + 2Mg_3 Al_2 Si_3 O_{12} + 4FeSiO_3 = 2Ca_3 Al_2 Si_3 O_{12} \hfill \\ Gt Gt Opx Gt \hfill \\ + 8FeSi_{0.5} O_2 + 6MgSiO_3 + O \hfill \\ Ol Opx \hfill \\ \end{gathered} $$ and (2) $$\begin{gathered} 2Fe_3 Fe_2 Si_3 O_{12} = 8FeSi_{0.5} O_2 + 2FeSi_3 O_2 \hfill \\ Gt Ol Opx \hfill \\ \end{gathered} $$ Comparison of calculated fO2s with those measured by the Pt-Fe sensors demonstrated that either barometer gives the correct answer within the expected uncertainty. Data from the first (Luth et al. 1990) has an uncertainty of about 1.6 logfO2 units, however, while that from equilibrium (2) (Woodland and O'Neill 1993) has an error of +/- 0.6 log units, comparable to that of the spinel peridotite oxybarometer. We therefore conclude that equilibrium (2) may be used to calculate the fO2 recorded by garnet peridotites with an uncertainty of about +/- 0.6 log units, providing the potential to probe the oxidation environment of the deep continental lithosphere. Preliminary application based on data from Luth et al. (1990) indicates that garnet peridotite xenoliths from Southern Africa record oxygen fugacities about 3.0 log units below the FMQ (fayalite-magnetite-quartz) buffer. These are substantially more reducing conditions than those recorded by continental spinel lherzolites which typically give oxygen fugacities close to FMQ (Wood et al. 1990).

Experimental determination of Mn-Mg mixing properties in garnet, olivine and oxide

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known f O 2. Assuming that the log f O 2 of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for γMn: logγMn = −5.25 + 3.67 XMn + 11.41X2 Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f O 2. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) WMn = 19.9kj/Mol; WMg = 13.7kj/Mol; (b) WMn = 19.9kj/Mol; WMg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: Wol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: Wgt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of WGt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive WOl and WGt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.