[Show abstract][Hide abstract] ABSTRACT: "ZenGen" is a script-tool which helps us to automatically generate first-principles input files of all the ordered compounds of a given crystal structure in a given system. The complete set of heats of formation of each end-members can then easily be used in the thermodynamic phase modeling. "ZenGen" is a free and open source code, which can be downloaded from http://zengen.cnrs.fr. In order to test its applicability, we have chosen the quaternary system, Cr-Mo-Ni-Re as a case study to be investigated. The binary solid solution parameters have been estimated from special quasirandom structures (SQS) calculations. The σ-phase has been fully described without any adaptation to its crystal structure, i.e. with a 5-sublattice model, through first-principles calculation of the 45=1024 different ordered quaternary configurations. Several tentative ab initio phase diagrams are presented.
[Show abstract][Hide abstract] ABSTRACT: "ZenGen" is a tool which aims to automatically generate first principles input files of all the ordered configurations of a given crystal structure generated by assigning each element to each crystal site. The heat of formation of every end-member can be easily used in the thermodynamic modeling of the considered phase. "ZenGen" is a free and open source licence (Cecill-B type), which could be downloaded from this site http://zengen.cnrs.fr/
[Show abstract][Hide abstract] ABSTRACT: In order to clarify controversial reports on the Fe–Re phase diagram, a new experimental investigation has been carried out. Three intermetallic phases have been evidenced, including the new report of the P phase found for the first time in a binary system. The phase relations involving the σ phase were established. In parallel, a first-principles study has been performed which provided the heat of formation of every ordered configuration for four intermetallic phases (D8b, A12, A13 and P). The mixing energy of solid solutions (fcc, bcc, hcp) was calculated using the special quasi-random structure method. Calculations were performed with the help of the density functional theory, with and without spin polarization. From these results, in the frame of the Compound Energy Formalism using the Bragg–Williams approximation, the Fe–Re phase diagram has been computed without the use of adjustable parameters. Different thermodynamic parameters obtained experimentally and theoretically, as the site occupancies, are compared. The computed phase diagram presents several differences with the experimental one. To understand these differences, the influence of several parameters on the phase stability, such as the magnetic contribution has been evaluated.
Full-text · Article · Nov 2014 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: Rhenium (Re) is used as an alloying element in Ni based superalloys but promotes the formation of fragile intermetallic compounds in many systems. The thermodynamic databases that allow the modeling of the properties of Ni based alloys are still incomplete and further data has to be integrated. In this context, we propose a new characterization of the system Mn–Re for temperatures below 1150 °C. We evidenced the existence of σ and χ (αMn) Frank–Kasper phases with very large solubility domains. We took a peculiar attention to the atom distribution in both phases and structural analyses (Rietveld refinement) reveal that Re has a strong preference for atomic sites with high CN. In addition, σ phase molar enthalpy of formation was measured by drop calorimetry at 1150 °C (40 and 55 at.% Re).
No preview · Article · Mar 2014 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: The structural stability of topologically close-packed phases in binary transition metal alloys is investigated with a combination of first-principles calculations based on density-functional theory and the Bragg–Williams–Gorsky approximation for the description of the configurational entropy. For a variety of different (i) exchange–correlation functionals, (ii) pseudopotentials, and (iii) relaxation schemes, for the relevant phases in Re–X (X = Ta, V, W) binary systems, we compare the energy of formation at T = 0 K, as well as the phase diagrams and site occupancies at finite temperatures. We confirm previous findings that the configurational entropy plays a stabilising role for complex phases in these systems at elevated temperatures. Small differences in the calculated energy of formation for different exchange–correlation functionals, pseudopotentials and relaxation schemes are expected, but give rise to qualitatively different phase diagrams. We employ these differences in order to estimate the order of magnitude of the standard deviation necessary in the qualitatively-reliable calculation of phase diagrams and site occupancies. In an attempt to determine the accuracy that is required to assure a qualitatively correct prediction of phase diagrams, we modify our first-principles results numerically by random variations with the determined standard deviation as maximum amplitude. Taking the order of site occupancies and the set of stable phases as simple criteria for a qualitatively correct prediction, we find that the accuracy required for the energy of formation of the individual configurations in these systems is approximately 5 meV/atom (≈0.5 kJ/mol at).
Full-text · Article · Jan 2014 · Computational Materials Science
[Show abstract][Hide abstract] ABSTRACT: The phase equilibria and thermodynamic properties of the Mo–Re system are studied by combining first-principle and CALPHAD approach. The mixing enthalpies in the bcc and hcp solution phases are estimated by first-principle calculations using the special quasirandom structures. The liquid, bcc and hcp phases are described by a substitutional solution model. The intermetallic phases, σ and χ, are described with the compound energy formalism with, respectively, 5 and 4 sublattices (SL) using the formation enthalpies of all the end-members directly from ab initio calculations. A phase diagram in agreement with the available experimental knowledge is obtained thanks to a least square procedure involving a limited number of parameters. Introducing all the elements in all the sublattices of the structure allows a proper description of the configuration of the intermetallic phases. Different simplifications of the description of the σ phase are considered. The ideal 4SL simplification is equivalent to the full description. The 3SL and 2SL models require excess parameters in order to fit reasonably the experimental phase diagram. Among these, only the (Mo,Re)10(Mo,Re)12(Mo,Re)8 model allows to closely approximate the low temperature thermodynamic properties of the full description.
[Show abstract][Hide abstract] ABSTRACT: In the purpose of increasing cycle length and/or fuel burn up, several theoretical and experimental studies have been performed at CEA. Among them, prospective neutronic calculations have shown that addition of a few weight percents of erbium into the cladding materials could be a promising alternative to the introduction of the neutronic poison directly into the nuclear fuel pellets. Thus, fabrication of homogeneous Zr-Er alloys has been assessed, at least up to 10 wt.% of erbium and, based on the as-received mechanical properties, an optimum erbium concentration ranging from 3 to 6 wt.% has been derived. However, due to the high oxygen thermodynamic affinity of erbium, thermal treatments have to be controlled during the fabrication route in order to limit Er2O3 precipitation and coarsening which may have detrimental effect on the ductility/toughness of Zr-Er alloys. In parallel, in order to get more fundamental insights into the underlying phase diagrams, thermodynamic studies have been devoted to experimental assessment and modeling of the Zr-Er-(H-O) system. Due to detrimental influence of erbium on the corrosion resistance, a 3-layer « sandwich » clad prototype has been developed with use of corrosion resistant inner/outer Zr-1Nb layers to protect the intermediate Zr-Er layer from direct water exposure.
Compared to a reference Zr-1Nb(O) alloy that has been subjected to the same fabrication route, the three-layers clad prototype shows limited decrease in ductility due to pre-hydriding or after high temperature steam oxidation (e.g. in the case of LOCA). Moreover, the studies performed so far have shown a spectacular hydride trapping capacity of the intermediate Zr-Er layer both for hydrogen coming from nominal outer corrosion or due to massive secondary hydriding in case of direct access of water to the Zr-Er intermediate layer. Using µ-ERDA measurements, detailed studies of the hydrogen spatial redistribution upon thermal cycling have been done. A simple model has been successfully used to characterize the cooling rate influence on the through-wall clad thickness partitioning of hydrogen/hydrides between the three layers, after cooling from a temperature corresponding to full dissolution of hydrides.
Corresponding author: firstname.lastname@example.org
[Show abstract][Hide abstract] ABSTRACT: One of the many singularities of Frank-Kasper phases is their ability to accommodate extremely large composition ranges by atom mixing on the different sites of the crystal structures. This phenomenon will be reviewed in the present paper with special emphasis on the experimental demonstration of this phenomenon, the theoretical calculation of disordered structures and the modeling of these phases.
[Show abstract][Hide abstract] ABSTRACT: A review of the metal–hydrogen systems modeled or investigated with the CALPHAD method is presented. The specific features of metal–hydrogen systems in relation with the CALPHAD modeling are detailed, and the problems and needs related to the description of such systems are highlighted.
No preview · Article · Dec 2012 · JOM: the journal of the Minerals, Metals & Materials Society
[Show abstract][Hide abstract] ABSTRACT: The Er-O-Zr ternary system has been investigated experimentally along two isothermal sections at 800 degrees C and 1100 degrees C. In order to obtain pure and homogeneous samples, powder metallurgy has been used. The samples have been synthesized using pure Er and Zr powder obtained by the hydride route. The study has been focused on the Zr rich corner and the results allow defining the co-solubility domains at both temperatures and the nature of the phases in equilibrium with alpha(zr) and beta(zr).
Full-text · Article · Aug 2012 · Journal of Nuclear Materials
[Show abstract][Hide abstract] ABSTRACT: The solid solution based on Nb5Si3 (Cr5B3 structure type, D8l, tI32, I4/mcm, No140, a=6.5767 Å, c=11.8967 Å) in the Nb–Si–B system was studied from the structural and thermodynamic point of view both experimentally and by ab initio calculations. Rietveld refinement of powder X-ray synchrotron data allowed to determine the boron to silicon substitution mechanism and the structural parameters. Ab initio calculations of different ordered compounds and selected disordered alloys allowed to obtain in addition to the enthalpy of formation of the solution, substitution mechanism and structural parameters which are in excellent agreement with the experimental data. The stability of the phase is discussed.
Full-text · Article · Jun 2012 · Journal of Solid State Chemistry
[Show abstract][Hide abstract] ABSTRACT: The reaction between hydrogen and erbium has been studied by DFT calculations. The enthalpies of formation of the compounds Er, ErH, ErH2 and ErH3 generated by the ordered insertion of hydrogen atoms in the tetrahedral and octahedral (or triangular) sites of the fcc and hcp structures have been calculated. In addition, ErH3 has been calculated with its reported trigonal superstructure. The results of the calculations are consistent with the experimental observation of the sequence Er (hcp) → ErH2 (fcc) → ErH3 (trigonal), qualitatively, and with the measured enthalpies of formation, quantitatively. Additionally, the densities of states of all the compounds are studied in detail.
Full-text · Article · Mar 2012 · International Journal of Hydrogen Energy
[Show abstract][Hide abstract] ABSTRACT: The hydrogenation properties of the LaNi5−xMx (M = Rh, Ir, Au) compounds have been studied. The Ni substitution has several consequences: pressure plateau splitting and increase of plateau pressure. This latter observation disagrees with the general rule that a cell volume increase of the alloy should result in a plateau pressure lowering. In order to elucidate the origin of this anomalous behaviour, DFT calculations have been performed on both LaNi5−xRhx and LaNi5−xPtx intermetallic compounds, which, according to the present and previous experimental work, present a similar anomaly. We discuss our results in light of the models proposed in the literature. We conclude that, in the case of a Ni substitution by 4d or 5d elements, the size effect alone fails in predicting the hydrogen absorption properties while the rule of reverse stability is obeyed.
No preview · Article · Jan 2012 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: The phase diagrams of the eight ternary systems Ag–La–Ni, Au–La–Ni, Ir–La–Ni, La–Ni–Os, La–Ni–Pd, La–Ni–Re, La–Ni–Rh and La–Ni–Ru have been investigated experimentally in the La-poor region. In each system, the ternary extension of the CaCu5 phase based on LaNi5 has been particularly well studied in order to measure the solubility limit of the third element. In most systems, the nature of the phases in equilibrium with the CaCu5 phase has been established and a partial phase diagram could be drawn. The structural properties (lattice parameters and substitution site(s) of the third element) of the ternary CaCu5 phase have been studied in detail.Graphical abstractResearch highlights► Partial determination of 8 ternary phase diagrams. ► Study of the ternary extension of the CaCu5-type phase in these systems. ► Site occupancies and lattice parameters of the CaCu5-type phase.
[Show abstract][Hide abstract] ABSTRACT: The quinary system D–H–Pd–Rh–T has been described thermodynamically by the CALPHAD approach. Previous descriptions of the binary subsystems have been used. To model the high pressure data an equation of state for the gases D2 and T2 compatible with the CALPHAD approach has been obtained similar to that previously used for H2. A complete literature search has been undertaken for the three ternary systems H–Pd–Rh, D–Pd–Rh and Pd–Rh–T and the most significant experimental data have been selected for a thermodynamic assessment of these systems. In order to complement the available data, pressure–composition curves have been measured at different temperatures for the two last systems in the present work. Calculations and optimization of the system under para-equilibrium conditions, i.e. in pseudo-binary systems (Pd,Rh)–H, (Pd,Rh)–D or (Pd,Rh)–T, have been achieved using a pseudo-atom describing the Pd–Rh solid solution. This special method allows the presence of a miscibility gap in the binary metallic system to be dealt with. We show that a simple combination of the binary systems alone is unable to properly describe these ternary systems and that ternary interaction parameters have to be introduced. The binary and ternary systems may then be combined to perform calculations in the quinary D–H–Pd–Rh–T system. It is believed that extrapolation in systems containing different isotopes are fairly accurate provided that the so-called Toop model is used.
[Show abstract][Hide abstract] ABSTRACT: Bcc Ti24.5V59.3Fe16.2 alloys containing 10 and 30% of C14 Laves phase inclusions were prepared by induction melting followed by annealing at 1000°C. X-ray powder diffraction and BSE microscopy confirmed the presence of the C14 Laves phase (average composition Ti35.4V32.3Fe32.3) embedded in the bcc matrix. The two end members of the series, the C14 Laves phase and the bcc Ti24.5V59.3Fe16.2 alloy, have very different hydrogenation behaviors. The C14 Laves phase does not absorb as much hydrogen as does the bcc phase. No equilibrium plateau and little hysteresis between absorption and desorption were observed at 25°C for the C14 Laves on the PCI curves whereas those of the bcc sample present one equilibrium plateau and significant hysteresis between absorption and desorption. As a result, the absorption capacity and the length of the equilibrium plateau of the multiphase alloys decrease with the C14 Laves phase content. The hydrogenation properties of an as-cast bcc Ti24.5V59.3Fe16.2 sample were also investigated: the kinetics of the first hydrogenation is found to be slower and the plateau pressures higher for the as-cast alloy than for the annealed sample.
No preview · Article · Feb 2011 · Solid State Ionics
[Show abstract][Hide abstract] ABSTRACT: The hydrogenation properties of the LaNi5 (CaCu5 type, hP6, P6/mmm) and Pd substituted derivatives LaNi5−xPdx compounds have been studied in the whole homogeneity range of the solid solution (0.25≤x≤1.5). The pressure versus hydrogen content isotherms show several plateaus and an increase of the plateau pressure as a function of palladium concentration. The volume increase of the Pd substituted alloys should have resulted in a lowering, and not an increase, of the plateau pressure, according to the conventional models based on the size effect. In order to elucidate the origin of this anomalous behavior, both an experimental and a theoretical ab initio electronic structure investigation have been carried out. Experimentally, the nature and the structural properties of the hydrides have been studied by both in situ and ex situ neutron diffraction. The crystal structures of the three hydride phases are reported (LaNi3.5Pd1.5D1.96, filled-up CaCu5 type, P6/mmm; LaNi4PdD2.72, LaNi2(Ni0.75Pt0.25)3H2.61 type, oI128, Ibam; LaNi4.75Pd0.25D5.67, partly ordered CaCu5 type, P6mm). In addition, the phase diagram of LaNi5−xPdx-H system has been investigated. The electronic and thermodynamic properties of both intermetallic compounds and the hydrides have been studied by ab initio electronic structure calculations. The theoretical results are in good agreement with our experimental data, and show that electronic interactions play a major role in the hydrogenation behavior of these Pd substituted intermetallic compounds, and that these effects cannot be accounted for by a simplistic model based on the size effect alone.
No preview · Article · Jan 2011 · Journal of Solid State Chemistry
[Show abstract][Hide abstract] ABSTRACT: The hydrogenation properties of Fe–Ti–V bcc alloys have been studied by the Sieverts method. Starting from a fully determined ternary phase diagram, a large number of compositions have been synthesized and pressure–composition–isotherms have been measured at different temperatures both in the low and high hydrogen concentration regions. The enthalpies of absorption and desorption have been found to vary linearly as a function of composition in the ternary domain and depend on the ones of the pure elements. The absorption capacity is mainly dependent on the iron concentration. Additionally, the crystal structure of the hydrides has been studied. For the first time a progressive and continuous distortion from body centered cubic (bcc) to body centered tetragonal (bct) structure has been evidenced.
No preview · Article · Jan 2011 · Journal of Alloys and Compounds