The relationship between thermodynamic properties and local atomic structure of Al-TM (TM = Mn,Fe, Co,Ni, Cu) melts

The relationship between structure and thermodynamic properties of Al- transition metal (TM) (TM = Mn, Fe, Co, Ni, Cu) melts had been studied by using the results of X-ray diffraction experiments, reverse Monte Carlo simulations and the model of ideal associated solution (MIAS). It was considered that local atomic structure of melts is determined by dense non-crystalline atomic packing and energy of interatomic interactions. This allowed to choose the types of associates for the MIAS. It is shown that the prominent role of the atomic packing factor in the Al-Mn, Al-Fe melts leads to correlation between the local atomic ordering of melts and the one of relevant polytetrahedral phases and defines a set of associates. Strengthening of heteroatomic interactions in the Al-Co, Al-Ni melts leads to an increase of the energy factor role in determining their structure and properties that reduces the number of associates from three for the Al-Mn melts to one for the Al-Ni.     

Thermal stability of metastable nano-composites in planar flow cast Ti–Zr–Ni alloys

The influence of the solidification rate, the thermal stability, and devitrification process of the rapidly solidified Ti₄₅Zr₃₈Ni₁₇ alloy have been examined on ribbons prepared by the planar flow casting method. Differential scanning calorimetry in the continuous heating mode, X-ray diffraction, and electron microscopy techniques were applied. Comparison of the so-obtained microstructures revealed the competition between icosahedral quasi-crystalline and β-Ti(Zr) phases, both dispersed in an amorphous matrix. It has been found that the decomposition process of rapidly quenched ribbons consists of the sequence of several independent exothermic and endothermic reactions involving the additional precipitation of quasi-crystalline nanoparticles and both irreversible and reversible changes between the unstable high-temperature β and stable low-temperature α phases. The formation of the intermetallics as well as the transformation of quasi-crystals into Laves phase has been observed at higher temperatures in all ribbons.     

Structure and electronic structure of quasicrystal and approximant surfaces: a photoemission study

The structure and electronic structure of different high-symmetry surfaces of either quasicrystalline or approximant Al–Pd–Mn were studied by means of photoemission-based techniques such as X-ray photoelectron diffraction (XPD) and ultraviolet photoelectron spectroscopy. We find that the twofold (2f), 3f and 5f surfaces of icosahedral Al–Pd–Mn exhibit all the symmetry elements of the icosahedral non-crystallographic group. These XPD experiments can be modeled by single-scattering cluster calculations.

The bulk-terminated icosahedral or approximant surfaces are recovered after ion sputtering followed by annealing at T≈500–600 °C. A wealth of ordered surface phases (with different compositions) are found after sputtering and depending on the annealing temperature as, for example, a crystalline bcc multitwinned phase (for T<400 °C) or a stable decagonal quasicrystalline surface (for T>650 °C).

The icosahedral surfaces are characterised by a lowering of the density of states close to the Fermi edge, compatible with the opening of a pseudogap, as expected for a quasicrystal. The crystalline overlayers are characterised by a sharp Fermi edge, while the approximant and decagonal quasicrystalline surfaces also have a lowered density of states.     

Thermodynamic analysis of (Ni,Fe)3Al formation by mechanical alloying

(Ni, Fe)₃Al intermetallic compound was synthesized by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures of composition Ni₅₀Fe₂₅Al₂₅. Phase transformation and microstructure characteristics of the alloy powders were investigated by X-ray diffraction (XRD). The results show that mechanical alloying resulted in a Ni (Al, Fe) solid solution. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound. A thermodynamic model developed on the basis of extended theory of Miedema is used to calculate the Gibbs free-energy changes. Final product of MA is a phase having minimal Gibbs free energy compared with other competing phases in Ni–Fe–Al system. However in Ni–Fe–Al system, the most stable phase at all compositions is intermetallic compound (not amorphous phase or solid solution). The results of MA were compared with thermodynamic analysis and revealed the leading role of thermodynamic on the formation of MA product prediction.     

Thermodynamic evaluation and optimization of the (NaCl + KCl + AlCl3) system

All available phase equilibrium and thermodynamic data for the (NaCl + KCl + AlCl₃) system were collected and critically evaluated. An optimization was performed to obtain the parameters of one set of model equations for each phase (solids, liquid, gas) in order to best reproduce all the data simultaneously. In this way the data are rendered self-consistent, discrepancies among the data are identified, and extrapolations and interpolations can be performed. For the molten phase the Modified Quasichemical Model for short-range ordering was used, with monomeric Al³⁺ ions (corresponding to AlCl₄⁻ complexes in earlier models) predominating in alkali-rich melts, and dimeric aluminum species (corresponding to Al₂Cl₇⁻ complexes in previous models) predominating in AlCl₃-rich melts. No ternary model parameters were required for the liquid phase; the binary parameters suffice. The models can be used with Gibbs free energy minimization software to calculate phase diagram sections, vapor pressures, and all thermodynamic properties at all compositions and over extended ranges of temperature and pressure.     

Thermodynamics of Zr<Subscript>52.5</Subscript>Cu<Subscript>17.9</Subscript>Ni<Subscript>14.6</Subscript>Al<Subscript>10</Subscript>Ti<Subscript>5</Subscript> bulk metallic glass forming alloy

Recently, multicomponent glass forming alloys have been found which exhibit extraordinary glass forming ability and cooling rates of less than 100 K/s are sufficient to suppress nucleation of crystalline phases and consequently bulk metallic glass (BMG) is formed. The undercooled melts of BMG systems have high thermal stability in the undercooled region. Therefore, it is interesting to study the thermodynamics of such materials. This article investigates the thermodynamic behavior of a BMG system namely Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ by estimating the Gibbs free energy difference ΔG, entropy difference ΔS, enthalpy difference ΔH between the undercooled liquid and corresponding equilibrium crystalline solid phase, in the entire temperature range from T _m to T _K. Glass forming ability (GFA) of this system has been investigated through various GFA parameters indicating the degree of ease of glass formation.     

Thermodynamic properties of melts based on the Al-Sm system

The equilibrium composition and thermodynamic characteristics of melts based on the Al-Sm system were studied by thermodynamic modeling using the ideal solution model of reaction products over the whole range of compositions 0 ?? x _Al ?? 1 in argon at a total pressure of 1 atm and temperatures from 1873 to 2100 K.     

Calorimetric Data and Phase Equilibria Assessment in Silicate Systems Optimization of Mixing Properties of Silicate Melts

The thermodynamic data are assessed by using molecular solution model with excess Gibbs energy of mixing expressed by Redlich-Kister equation with temperature dependent parameters. The optimized data involve phase equilibria, enthalpy and entropy of formation of crystalline phases, heat capacity (C_p) data of solid and liquid pure components, enthalpy of mixing of liquid pure components, enthalpy and entropy of fusion of solid phases. Thermodynamic quantities consistent with available experimental phase equilibria and calorimetric measurements are established for solid phases and liquids in the system CaO·SiO₂ (CS)-CaO·Al₂O₃·2SiO₂ (CAS₂)-2CaO·Al₂O₃·SiO₂ (C₂AS). This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental study and modeling of the thermodynamic properties of magnesium silicates

The thermodynamic properties of all MgO-SiO₂ system phases were studied by Knudsen mass spectrometry over a wide temperature range (1571–1873 K) and the whole range of compositions. An approach based on the generation of volatile interaction products formed in the reduction of oxide components was used. The reducing agents were Nb, Ta, and Mo. The observed ion current intensity ratios I(Mg⁺)/I(SiO⁺) were used to calculate the activities and partial thermodynamic functions of the components in liquid and crystalline MgO-SiO₂ mixtures and the integral thermodynamic functions of formation of magnesium ortho-and metasilicates. For the first time, direct and reliable information about the thermodynamic properties of all system phases at high temperatures was obtained. These results in combination with all the available data on the thermodynamic properties and phase equilibria in the MgO-SiO₂ system were used to develop a statistical-thermodynamic model of liquid magnesium silicates based on treating them as associated liquids. Simultaneously, the problem of obtaining self-consistent data on the thermodynamic functions of all phases and conditions of equilibria between them was solved. In addition to polymeric silicon-oxygen structures of arbitrary sizes and spatial configurations, heteromolecular complexes such as MS, M₂S, and M₃S (S=SiO₂ and M=MgO) were found to exist in liquid MgO-SiO₂ mixtures. The correctness of the results obtained was substantiated by the virtually complete coincidence of the calculated thermodynamic properties and phase equilibrium conditions with experimental data and their conformity to the general patterns characteristic of binary silicate systems.     

Synthesis of quasi-crystalline phases in the Al–Cu–Fe–Cr system

Phase equilibria in the Al–Cu–Fe system alloyed with 5% Cr were studied. Based on the data of X-ray powder diffraction analysis, electron microscopy, and differential thermal analysis, the effect of temperature on i ? d phase transitions in alloys Al₆₅Cu₂₅Fe₅Cr₅ and Al₇₀Cu₂₀Fe₅Cr₅. In the Al–Cu–Fe–Cr system, multiphase structures were detected; these structures are mixtures of quasi-crystalline and approximant phases, the contents and morphologies of which depend on the composition of the initial mixture and the crystallization rate.     

Coexistence of Immiscible Liquid Phases in the LaPO<Subscript>4</Subscript>–SiO<Subscript>2</Subscript>–NaF–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> System

It was determined that the system LaPO₄–SiO₂–NaF–Nb₂O₅ within the temperature range 850–1200°C has regions of immiscibility of liquid phases (silicate and phosphate–salt melts). The coexisting melts have contrast chemical and phase compositions and structural-textural features, because of which the methods for extracting rare-earth elements and niobium from these melts differ. The silicate melts form glass, whereas the phosphate–salt melts have high crystallization ability. The mutual solubility of the liquid phases does not exceed 5%. The components of the system are contrastively distributed between the silicate and phosphate–salt melts. A fraction of 95–97% of niobium is extracted into the silicate melt, and 93–95% of La and P is extracted into the phosphate–salt melt.     

Thermodynamic properties of silicate glasses and melts: VII. System MgO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>

Processes of evaporation and thermodynamic properties of glasses and melts of the system MgO-B₂O₃-SiO₂ in the range of 1550–1800 K were studied for the first time by high-temperature mass-spectrometry. The resulting activities and Gibbs energies of components and also corresponding excess values point to negative deviations from ideal behavior and to the absence of immiscibility fields in glasses and melts of the system under study in the studied temperatures and concentration intervals.     

Electrical and thermal transport properties of icosahedral and decagonal quasicrystals

Electrical and thermal transport properties of quasicrystals are reviewed on the examples of i-Ag-In-Yb and i-Al-Cu-Fe icosahedral phases and d-Al-Co-Ni decagonal phase. Using samples of single-grain morphology and high structural quality, and performing the measurements along well-defined crystallographic directions, the following basic questions in the context of physical properties of quasicrystals are addressed, both experimentally and theoretically: (1) are the unusual transport properties of quasicrystals introduced by the quasiperiodicity of the structure or are they a consequence of complex local atomic order with no direct relationship to the quasiperiodicity; (2) what is the role of the electronic structure of quasicrystals in their electronic transport properties, especially the pseudogap in the electronic density of states in the vicinity of the Fermi energy; (3) what is the anisotropy of the transport coefficients along different crystallographic directions for icosahedral and decagonal quasicrystals and (4) what are the true intrinsic properties of quasicrystalline phases?     

Thermodynamic Properties and Transport Coefficients of CO<Subscript>2</Subscript>–Cu Thermal Plasmas

This paper presents the calculated values of equilibrium compositions, thermodynamic properties and transport coefficients (viscosity, electrical conductivity and thermal conductivity) for CO₂–Cu thermal plasmas. With several copper mass proportions, the calculation is performed at temperatures 2000–30,000 K and various pressures 0.1–16 bar. Gibbs free energy minimization is used to determine species compositions and thermodynamic properties and the well-known Chapman–Enskog method is applied to calculating transport properties. Furthermore, great attention is paid to cope with the interactions between all the particles in the determination of collision integrals. The results are illustrated indicating the effect of the copper proportions and pressure on the fundamental properties of CO₂–Cu thermal plasmas. It can be found that a small quantity of copper (less than 10 %) can significantly modify the charged species densities and electrical conductivity especially at low temperature. While for other properties, the influences can be noticeable only when the copper proportion is above 10 %.     

Application of Model of Ideal Solution of Interaction Products and Thermodynamic Modeling

The essence of the ideal solution of interaction products (ISIP) model for systems involving strong interactions of the components is described. With the help of this model and thermodynamic simulation methods, the activities of the components and partial and integral thermodynamic characteristics of Fe-Si solutions at 1873–2003 K in an initial Ar atmosphere and at a common pressure of 1 atm were calculated. The compositions and structures of Fe-Si solutions according to the ISIP model are determined as statistical mixtures of [Fe], [Si] and the clusters [FeSi], [FeSi₂], [Fe₃Si] and [Fe₅Si₃]. The calculated values of activities, thermodynamic characteristics, compositions and structures of Fe-Si solutions agree satisfactorily with reliable experimental data. The thermochemical and thermodynamic properties of the compounds FeSi, FeSi₂, Fe₃Si and Fe₅Si₃ were revised. The temperature dependences of the activities of the components, and also the influence of temperature elevation on the composition of Fe-Si melts are discussed.     

Refining the thermodynamic functions of scandium triflouride ScF<Subscript>3</Subscript> in the condensed state

Refined thermodynamic functions (entropy, enthalpy increments, and reduced Gibbs energy) of scandium trifluoride ScF₃ in the crystalline and liquid states in the temperature range 5–2500 K are presented.     

Thermodynamic properties of Al-Y system melts

The thermochemical properties of Al-Y melts were determined by isoperibolic calorimetry. It was established that the minimal value of integral mixing enthalpies is equal to −40.8 + 0.4 kJ/mol at 1770 K and x _Y = 0.41. The thermodynamic properties of liquid alloys were modeled by the developed procedure using the coordinates of a liquidus line in the phase diagram of the Al-Y system, and by the theory of ideal associated solutions. The component activities exhibit high negative deviations from the Raoult law. The Gibbs energies of mixing of Al-Y melts have a minimum of −30.6 kJ/mol at x _Y = 0.48.     

Vapor formation and thermodynamic properties of the gallium-lead system melts

A vapor composition and thermodynamic properties of melts of the gallium-lead system were studied by high-temperature mass-spectrometry. It was shown on the basis of thermodynamic calculations that Ga_1?x Pb _x melts are characterized by an appreciable positive deviation from the ideal behavior in the temperature range 780–1170 K.