Thermodynamic analysis of stable and metastable carbides in the Mn-V-C system and predicted phase diagram

The thermodynamic properties of the Mn-V-C system are not known from experiments, and there is a need for information on the stability of the various phases and the Mn-V-C phase diagram. This kind of information has been obtained by us. Our approach combines the methods of phenomenological modeling and extrapolation from the lower-order systems, which have been developed in the so-called CALPHAD (i.e., CALculation of PHAse Diagrams) work, with predictions of unknown thermodynamic quantities. The predictions are based on regularities in bonding properties and vibrational entropy of 3d-transition metal carbides. By using predicted Gibbs energy functions as input information in the Hillert-Staffansson two-sublattice phenomenological model, we take into account the substitution of Mn for V in all carbide phases. Our results are summarized in tables of thermodynamic parameters, calculated isothermal sections of the phase diagram, and the liquidus surface.     

Predictive approach to thermodynamic properties of the metastable Cr3C carbide

In thermodynamic modeling of phase diagrams it is often necessary to deal with the properties of metastable compounds, which are not known from experiments. As an illustrative example, we choose the Cr₃C(oP16) carbide, which is involved in the modeling of the Me₃C(oP16) (cementite) structure of the Fe-Cr-C system but is metastable in the Cr-C system. We discuss in detail the estimation of its thermodynamic properties, relying on regularities in bonding properties of 3d-transition metal carbides, and an account of the vibrational entropy through the so-called entropy Debye temperature. Our predictions are compared with values derived in thermodynamic modeling of the Fe-Cr-C phase diagram. Relying on the present results, we perform calculations of metastable phase equilibria in the Cr-C system and use them in analyzing information about Cr₃C from splat-quenching experiments.     

Thermodynamic properties of nickel

This work reviews and discusses the data and information on the thermodynamic properties of nickel available through May 1984. These properties include heat capacity, enthalpy, enthalpy of transition and melting, vapor pressure, and enthalpy of vaporization. The recommended values for heat capacity cover the temperature range from 1 to 3200 K. The recommended values for enthalpy, entropy, Gibbs energy function, and vapor pressure cover the temperature range from 298.15 to 3200 K.     

Synthesis,electron transport properties of transition metal nitrides and applications

To understand the electron transport properties of transition metal nitrides (MN), electronic structure relationship between metal and corresponding nitrides is important. In binary nitrides, when nitrogen atoms occupy interstitial sites of metal lattice, volume expansion started initially without changing structure of metal lattice. Above certain concentration of nitrogen into interstitial sites of lattice, the system starts stabilizing its energy to minimum that in turn changes to another crystal structure. The chemical bonding in MN is due to the mixing of d-orbitals of M and p-orbitals of N. This is confirmed theoretically and experimentally such as X-ray photoelectron spectroscopy. The Fermi energy is generally lowered by the introduction of vacancies. However, reports on the particle size effect in the electrical resistivity of nitrides are scanty. One reason is that the role of the particle size in resistivity is difficult to determine because there is a need to understand N concentration. It poses a challenge to the synthesis of nanostructured transition metal nitrides. The transition metal binary nitrides show unusual electron transport, optical and magnetic properties as compared to their metal counterparts. Electronic properties of all transition metal nitrides known till date are discussed. Different ways of synthesis of nitrides and their applications are mentioned.     

Structure and thermal stability of nanocrystalline materials

Nanocrystalline materials, which are expected to play a key role in the next generation of human civilization, are assembled with nanometre-sized “building blocks” consisting of the crystalline and large volume fractions of intercrystalline components. In order to predict the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to understand precisely how the structures of crystalline and intercrystalline regions vary with decrease in crystallite size. In addition, study of the thermal stability of nanocrystalline materials against significant grain growth is both scientific and technological interest. A sharp increase in grain size (to micron levels) during consolidation of nanocrystalline powders to obtain fully dense materials may consequently result in the loss of some unique properties of nanocrystalline materials. Therefore, extensive interest has been generated in exploring the size effects on the structure of crystalline and intercrystalline region of nanocrystalline materials, and the thermal stability of nanocrystalline materials against significant grain growth. The present article is aimed at understanding the structure and stability of nanocrystalline materials.     

Thermodynamic properties of aluminum

This work reviews and discusses the data on the thermodynamic properties of aluminum available through May 1984. However, two papers dated 1985 which are useful to this work are also included. These properties include heat capacity, enthalpy, enthalpy of transition and melting, vapor pressure, and enthalpy of vaporization. The recommended values for heat capacity cover the temperature range from 0.1 to 2800 K. The recommended values for enthalpy, entropy, Gibbs energy function, and vapor pressure cover the temperature range from 298.15 to 2800 K.     

Thermodynamic properties of titanium

This work reviews and discusses the data and information on the thermodynamic properties of titanium available through May 1984. These properties include heat capacity, enthalpy, enthalpy of transition and melting, vapor pressure, and enthalpy of vaporization. The recommended values for heat capacity cover the temperature range from 1 to 3800 K. The recommended values for enthalpy, entropy, Gibbs energy function, and vapor pressure cover the temperature range from 298.15 to 3800 K.     

X-ray crystallographic and thermal studies on the hydrides of magnesium and its intermetallics

Magnesium nickel hydride, Mg₂NiH₄, exists in two crystallographic modifications, the low temperature phase crystallizing in monoclinic structure and the high temperature phase having a cubic structure. The phase transition (∼510 K) was accompanied by a small composition change. The enthalpies and entropies of formation of these hydrides were calculated from the DTA data and compared with the values obtained by other methods.     

On the stability of intermetallic phases

A general methodology using atomic clusters is applied to three problems connected to the study of alloy phase stability. The cluster method proposed by Allen and Cahn is applied to non-ideal hcp structures under tetrahedral approximation using multiatom interactions. The possible ground-state structures which are stable at absolute zero temperature are obtained. A geometrical representation in 4D parameter space of the possible strengths of multiatom interactions permitted for these structures is illustrated in terms of a 2D analogue. Extending these ideas, the cluster variation method (CVM) proposed by Kikuchi is applied to fcc structures under tetrahedral approximation to find the effect of multiatom interactions on the topology of the coherent phase diagrams in which all the phases present are derivable by mere rearrangement of atoms on the parent disordered structure. In addition, the possible invariant reactions are identified in such coherent phase diagrams. Finally the CVM is applied for calculating a model incoherent phase diagram, that of Ti-Zr system, where disordered hcp and bcc phases are present. The free energies of hcp and bcc phases are formulated using CVM procedures respectively under tetrahedral-octahedral and tetrahedral approximations. The CVM is shown to be in better agreement with the thermodynamic data and to be able to reproduce the correct value of measured enthalpy of transformation compared to that given by the regular solution model, which significantly overestimates the same.     

Thermodynamic properties of alloys and phase equilibria in the In-Sn-Sb system

We report the thermodynamic properties of alloys and phase equilibria in the In-Sn-Sb system studied by emf measurements at temperatures from 600 to 830 K. Different types of exchange reactions in the electrochemical cell (?) W|In|In⁺ in electrolyte |In _x Sb _y Sn _z | W (+) are considered, and the ways of reducing their influence on the accuracy of emf measurements are analyzed. A number of T?x sections of the In-Sn-Sb phase diagram are refined.     

The electronic origins of alloy phase stability

The program entitled Accelerated Supercomputer Initiative (ASCI) aims at the utilization of state-of-the-art computational architecture for the study and simulation of materials properties relevant to the enduring stockpile. In order to carry out these calculations, it is necessary to have in hand a well-developed theoretical understanding of the corresponding computational algorithms. In this paper, we review some central elements of such theoretical developments as they relate to the study of phase stability of metallic alloys. These developments are based on the determination of the electronic structure of the alloys and related properties, such as total energies at zero temperature (ground-state energies). Parameters extracted from such calculations are then used to carry out the study of thermodynamic quantities such as ordering tendencies of the alloys and concentration–temperature–pressure phase diagrams. The results of numerical calculations carried out with respect to alloys of the transition metals are used to illustrate the methodology described here. When the methodology is extended to the materials of relevance to the stockpile it becomes evident that the difficulties of its numerical implementation increase rapidly, soon demanding the capabilities of massively parallel architectures and large memory capacity, thus being ideally suited for the framework of ASCI. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase transformations and phase stability in nanocrystalline materials

The current state of the researches on diffusionless phase transformations, including allotropic, polymorphous and martensitic transformations, and phase stability are reviewed. The behaviors of phase transformation and phase stability in nanocrystalline materials are markedly affected by the non-equilibrium conditions involved in their preparation, as a result, in this review an ideal demonstrating method of critical size for the stability of a high-temperature phase at low-temperatures is suggested, and the intrinsic conditions of the phase stability are clarified. Our recent experiments exhibit that the reversal transformation temperatures of low-temperature phases in nanocrystalline Co bulk metal and Fe–30Ni wt% alloy are significantly raised up over 800 °C when their grain sizes are smaller than about 15 nm, while in the reported experiments of nanocrystalline particles or films the reversal transformation temperature lowers with decreasing grain size or is independent of grain size. Therefore, the author suggests that more experiments and theories for phase stability in reversal transformation should be performed. The study of grain growth kinetics of nanocrystalline materials, as a basic of investigating phase stability, is another attention aspect.     

The impact of Ti and temperature on the stability of Nb5Si3 phases: a first-principles study

Abstract

Nb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb₅Si₃ polymorphs are known, namely αNb₅Si₃ (tI32 Cr₅B₃-type, D8_l), βNb₅Si₃ (tI32 W₅Si₃-type, D8_m) and γNb₅Si₃ (hP16 Mn₅Si₃-type, D8₈). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb₅Si₃ depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb₅Si₃ silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb₅Si₃ was found to decrease significantly with increasing Ti content. The γNb₅Si₃ was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb₅Si₃ and γNb₅Si₃ and decreased for βNb₅Si₃. With the addition of Ti the αNb₅Si₃ and γNb₅Si₃ became less ductile, whereas the βNb₅Si₃ became more ductile. When Ti was added in the αNb₅Si₃ and βNb₅Si₃ the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly.     

Calculation of phase diagrams of binary fcc and ideal hcp alloys undergoing ordering transitions

The tetrahedron approximation of the cluster variation method (CVM) has been employed to investigate phase diagrams having fcc-based ordered and disordered phases. This approximation is also applicable to the binary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the number of configurations and hence entropy of a crystal lattice having definite distribution of clusters (points, pairs, triangles, tetrahedra, etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions denoted by α and β are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizing the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of α and β parameters on the topology of the resulting phase diagrams is observed by assigning several negative and positive values to them. The invariant reactions were also determined in each case. Finally a study was made on the Cd-Mg diagram.     

Quasi-Binary Transition Metal Dichalcogenide Alloys: Thermodynamic Stability Prediction,Scalable Synthesis,and Application

Transition metal dichalcogenide (TMDCs) alloys could have a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single-phase. Here, a theory-guided synthesis approach is reported to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature–composition phase diagrams using first-principles calculations are generated to identify the stability of 25 quasi-binary TMDC alloys, including some involving non-isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for the CO₂ reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, iii) excellent energy efficiency in a high rate Li–air battery, and iv) high break-down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.     

Measurement of high temperature phase stability and thermophysical properties of alloy 740

In the present study, the characterisation of phase stability and measurement of different thermophysical properties of alloy 740 has been carried out. The transformation temperatures including liquidus/solidus and corresponding enthalpy of transformation have been measured for different phase changes up to melting using dynamic calorimetry. Further, the enthalpy increment data have been measured in the temperature range of 473–1473?K to obtain the heat capacity using static calorimetry. The present calorimetric data have been analysed in corroboration with the results obtained using JMatPro and Thermo-Calc simulation. In addition, the temperature dependence of other thermophysical properties such as thermal expansivity, density, thermal diffusivity and thermal conductivity are also measured in the range of 300–1473?K using thermomechanical analyser and laser flash method.     

Thermodynamic analysis of growth of iron oxide films by MOCVD using tris(<Emphasis Type="Italic">t</Emphasis>-butyl-3-oxo-butanoato)iron(III) as precursor

Thermodynamic calculations, using the criterion of minimization of total Gibbs free energy of the system, have been carried out for the metalorganic chemical vapour deposition (MOCVD) process involving the β-ketoesterate complex of iron [tris(t-butyl-3-oxo-butanoato)iron(III) or Fe(tbob)₃] and molecular oxygen. The calculations predict, under different CVD conditions such as temperature and pressure, the deposition of carbon-free pure Fe₃O₄, mixtures of different proportions of Fe₃O₄ and Fe₂O₃, and pure Fe₂O₃. The regimes of these thermodynamic CVD parameters required for the deposition of these pure and mixed phases have been depicted in a ‘CVD phase stability diagram’. In attempts at verification of the thermodynamic calculations, it has been found by XRD and SEM analysis that, under different conditions, MOCVD results in the deposition of films comprising pure and mixed phases of iron oxide, with no carbonaceous impurities. This is consistent with the calculations.     

新型Ni基高温合金中析出相的热力学模拟计算

利用Thermo-Calc热力学计算软件与相应的Ni基高温合金数据库,计算了一种新型镍基高温合金中可能析出的平衡相,并研究了化学成分的变化对主要析出相的影响,分析了各相的析出规律,为进一步挖掘合金组织潜力和改善合金使用性能提供了理论依据.     

Microstructure,phase stability and properties of CoCr0.5CuxFeyMoNi compositionally complex alloys

Liquid-phase separation occurred in CoCrCu_xFeMoNi (x?≥?0.5) alloys when the mixing entropies are positive in our previous work. So in this work, CoCr_0.5Cu_xFe_yMoNi alloys are designed to investigate the microstructure, component phases and properties. FCC and BCC were detected in CoCr_0.5Cu_xFe_yMoNi alloys, accompanied with a topologically close-packed μ phase. A parameter, V_R, was defined, where V_R is the ratio of the volume fraction of BCC plus μ phases and that of FCC phase. The maximum strength and the ductility were obtained at the minimum V_R of the alloys, whereas the hardness increased with the increasing V_R. It can be presumed that the strange balance between strength, ductility and hardness is an indirect result of the degree of brittle failure.