| 1. |
- Grechnev, G. E., et al.
(författare)
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Electronic structure and magnetic properties of RNi5-xCux alloys (R=Y, La, Ce)
- 2006
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 32:12, s. 1140-1146
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Tidskriftsartikel (refereegranskat)abstract
- A study is made of the electronic structure and magnetic properties of RN5-xCux alloys (R = Y, La, Ce). The influence of substitution of nickel by copper on the features of the band structure and behavior of the magnetic susceptibility of these alloys is investigated. An analysis is made of the electronic states and interactions responsible for the magnetic properties of RNi5-xCux alloys.
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| 2. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Constitutional and thermal point defects in B2 NiAl
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:9, s. 6003-6018
-
Tidskriftsartikel (refereegranskat)abstract
- The formation energies of point defects and the interaction energies of various defect pairs in NiAl are calculated from first principles within an order N, locally self-consistent Green's-function method in conjunction with multipole electrostatic corrections to the atomic sphere approximation. The theory correctly reproduces the ground state for the off-stoichiometric NiAl alloys. The constitutional defects (antisite Ni atoms and Ni vacancies in Ni-rich and Al-rich NiAl, respectively) are shown to form ordered structures in the ground state, in which they tend to avoid each other at the shortest distance on their sublattice. The dominant thermal defects in Ni-rich and stoichiometric NiAl are calculated to be triple defects. In Al-rich alloys another type of thermal defect dominates, where two Ni vacancies are replaced by one antisite Al atom. As a result, the vacancy concentration decreases with temperature in this region. The effective defect formation enthalpies for different concentration regions of NiAl are also obtained.
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| 3. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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Defect-induced magnetic structure in (Ga1-xMnx)As
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88:18, s. 187202-
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Tidskriftsartikel (refereegranskat)abstract
- We show that magnetic structures involving partial disorder of local magnetic moments on the Mn atoms in (Ga1-xMnx)As lower the total energy, compared to the case of perfect ferromagnetic ordering, when As defects on the Ga sublattice are present. Such magnetic structures are found to be stable for a range of concentrations of As antisites, and this result accounts for the observed magnetic moments and critical temperatures in (Ga1-xMnx)As . We propose an explanation for the stabilization of the partially disordered magnetic structures and conclude that the magnetization and critical temperatures should increase substantially by reducing the number of As antisite defects.
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| 4. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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First-principles calculations of the vacancy formation energy in transition and noble metals
- 1999
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 59:18, s. 11693-11703
-
Tidskriftsartikel (refereegranskat)abstract
- The vacancy formation energy and the vacancy formation volume of the 3d, 4d, and 5d transition and noble metals have been calculated within the local-density approximation. The calculations employ the order-N locally self-consistent Green’s-function method in conjunction with a supercell approach and include electrostatic multipole corrections to the atomic sphere approximation. The results are in excellent agreement with available full-potential calculations and with the vacancy formation energies obtained in positron annihilation measurements. The variation of the vacancy formation energy through a transition-metal series and the effects of crystal and magnetic structure are investigated and discussed.
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| 5. |
- Smirnova, E. A., et al.
(författare)
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Electronic topological transitions and phase stability in the fcc Al-Zn alloys
- 2002
-
Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 30:1, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed investigation of the phase equilibria and the Fermi surface in the Al-Zn system. Our calculation are based on the density functional theory and we use the linear muffin-tin orbital method and the Green's function technique. The calculated free energies of alloy formation exhibit the existence of a miscibility gap between the alloys containing approximately 10 and 55 at.% of Zn, in agreement with the phase diagram of the Al-Zn system. Seven electronic topological transitions (ETT) were found in Al-Zn system within the stability range of the fcc solid solution. A relation between these ETT and the phase stability of the fcc Al-Zn solid solutions is established. We show that extremum points on the concentration dependencies of the thermodynamic properties of Al-Zn alloys can be explained by band-filling effects.
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| 6. |
- Smirnova, E. A., et al.
(författare)
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Origin of the asymmetric spinodal decomposition in the Al-Zn system
- 2001
-
Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 64:2, s. 201011-201014
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Tidskriftsartikel (refereegranskat)abstract
- The Fermi surface and the phase equilibria in the Al-Zn system are studied theoretically in the framework of the density-functional theory, linear muffin-tin orbital method, and the Green's function technique. Analyzing the Fermi surfaces of the random alloys, we find a series of electronic topological transitions within the stability range of the fcc solid solution. A relation between these transitions and the phase stabilities of fee Al-Zn is established. We show that the asymmetry of the miscibility gap as well as of the mixing energy are due to the electronic topological transitions in the system.
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| 7. |
- Yang, Yi, et al.
(författare)
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Tuning Catalytic Activity of Ni-Co Nanoparticles Synthesized by Gamma-Radiolytic Reduction of Acetate Aqueous Solutions
- 2023
-
Ingår i: Advanced Materials Interfaces. - : Wiley-Blackwell. - 2196-7350. ; 10:17
-
Tidskriftsartikel (refereegranskat)abstract
- Transition metal-based catalysts show great potential to replace Pt-based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here, it is demonstrated that homogeneous alloy nanoparticles with varying Ni-to-Co ratio and controlled structure can be synthesized from aqueous Ni(Co) acetate solutions using a facile gamma-radiolytic reduction method. The obtained samples are found to possess defects that are ordered to form polytypes structures. The concentration of these defects depends on the Ni-to-Co ratio, as supported by the results of ab initio calculations. It is found that structural defects may influence the activity of catalysts toward the oxygen evolution reaction, while this effect is less pronounced with respect to the oxygen reduction reaction. At the same time, the activity of Ni-Co catalysts in the hydrogen evolution reaction is affected by formation of Ni-OH bonds on the surface rather than by the presence of structural defects. This study demonstrates that the composition of Ni-Co nanoparticles is an essential factor affecting their structure, and both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions.
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| 8. |
- Andersson, David A., et al.
(författare)
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Thermodynamics of structural vacancies in titanium monoxide from first principles calculations
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:14, s. 144101-
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Tidskriftsartikel (refereegranskat)abstract
- The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.
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| 9. |
- Baykov, Vitaly, et al.
(författare)
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Magnetic properties of 3d impurities in GaAs
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier. - 0304-8853 .- 1873-4766. ; 310:2, s. 2120-2122
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Tidskriftsartikel (refereegranskat)abstract
- Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature TC in (GaMn) As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga(1-x-y)MnxCry) As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.
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| 10. |
- Bleskov, I. D., et al.
(författare)
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Ab initio calculations of elastic properties of Ru1-xNixAl superalloys
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:16
-
Tidskriftsartikel (refereegranskat)abstract
- Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C' and C11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density.
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| 11. |
- Gorbatov, O. I., et al.
(författare)
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Effective cluster interactions and pre-precipitate morphology in binary Al-based alloys
- 2019
-
Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 179, s. 70-84
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Tidskriftsartikel (refereegranskat)abstract
- The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al-Cu, Al-Mg, Al-Zn, and Al-Si alloys, we carry out cluster expansions of ab initio calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems. reserved.
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| 12. |
- Granas, O., et al.
(författare)
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Theoretical study of the Mo-Ru sigma phase
- 2008
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Ingår i: Calphad. - : Elsevier. - 0364-5916 .- 1873-2984. ; 32:1, s. 171-176
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Tidskriftsartikel (refereegranskat)abstract
- The thermodynamic properties of the Mo-Ru binary sigma-phase are investigated using a combination of ab initio calculations and CALPHAD modeling. Total energy calculations have been performed for the complete set of 32 end-member compounds of a 5-sublattice compound energy model. The internal crystallographic parameters for each end-member compound have been determined by minimising the total energy. A simpler, 3-sublattice model of the Mo-Ru a-phase is formulated on the basis of calculated total energies. The site occupancy is acquired by minimising the free energy given by the compound energy model. A strong preference of Mo and Ru towards high-coordination sites and icosahedral sites in the Mo-Ru a-phase is found and analysed in terms of the electronic structure.
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| 13. |
- Lousada, Claudio M., et al.
(författare)
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Adsorption of Hydrogen Sulfide, Hydrosulfide and Sulfide at Cu(110) - Polarizability and Cooperativity Effects. First Stages of Formation of a Sulfide Layer
- 2018
-
Ingår i: ChemPhysChem. - : Wiley-VCH Verlag. - 1439-4235 .- 1439-7641. ; 19:17, s. 2159-2168
-
Tidskriftsartikel (refereegranskat)abstract
- Understanding the surface site preference for single adsorbates, the interactions between adsorbates, how these interactions affect surface site specificity in adsorption and perturb the electronic states of surfaces is important for rationalizing the structure of interfaces and the growth of surface products. Herein, using density functional theory (DFT) calculations, we investigated the adsorption of H2S, HS and, S onto Cu(110). The surface site specificity observed for single adsorbates can be largely affected by the presence of other adsorbates, especially S that can affect the adsorption of other species even at distances of 13 Å. The large supercell employed with a surface periodicity of (6×6) allowed us to safely use the Helmholtz method for the determination of the dipole of the surface-adsorbate complex at low adsorbate coverages. We found that the surface perturbation induced by S can be explained by the charge transfer model, H2S leads to a perturbation of the surface that arises mostly from Pauli exclusion effects, whereas HS shows a mix of charge transfer and Pauli exclusion effects. These effects have a large contribution to the long range adsorbate-adsorbate interactions observed. Further support for the long range adsorbate-adsorbate interactions are the values of the adsorption energies of adsorbate pairs that are larger than the sum of the adsorption energies of the single adsorbates that constitute the pair. This happens even for large distances and thus goes beyond the H-bond contribution for the H-bond capable adsorbate pairs. Exploiting this knowledge we investigated two models for describing the first stages of growth of a layer of S-atoms at the surface: the formation of islands versus the formation of more homogeneous surface distributions of S-atoms. We found that for coverages lower than 0.5 ML the S-atoms prefer to cluster as islands that evolve to stripes along the [1 (Formula presented.) 0] direction with increasing coverage. At 0.5 ML a homogeneous distribution of S-atoms becomes more stable than the formation of stripes. For the coverage equivalent to 1 ML, the formation of two half-monolayers of S-atoms that disrupt the Cu−Cu bonds between the first and second layer is more favorable than the formation of 1 ML homogeneous coverage of S-atoms. Here the S−Cu bond distances and geometries are reminiscent of pyrite, covellite, and to some extent chalcocite. The small energy difference of ≈0.1 eV that exists between this structure and the formation of 1 ML suggests that in a real system at finite temperature both structures may coexist leading to a structure with even lower symmetry.
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| 14. |
- Stroev, A.Yu., et al.
(författare)
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Ab-initio based modeling of precipitation in Al–(Sc,Zr) alloy. Formation and stability of a core–shell structure
- 2023
-
Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 218
-
Tidskriftsartikel (refereegranskat)abstract
- Statistical alloy theory based on the Master Equation approach with ab initio calculated interatomic interactions is employed to investigate the growth of precipitates at the early stages of solid solution decomposition, as well as the dissolution of small precipitates during the coarsening stage, upon simulated annealing of ternary Al–Sc–Zr alloys. We show, in agreement with previous studies, that the Zr alloying to Al–Sc alloys promotes the formation of core–shell nanoparticles whose structure is found to be very sensitive to the parameters characterizing the solute diffusion rates in the alloy. We demonstrate that the core–shell structure of precipitates slows down the dissolution of small particles, thus hampering the microstructure coarsening at elevated temperatures.
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| 15. |
- Stroev, A. Yu., et al.
(författare)
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Solid solution decomposition and Guinier-Preston zone formation in Al-Cu alloys : A kinetic theory with anisotropic interactions
- 2018
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 2:3
-
Tidskriftsartikel (refereegranskat)abstract
- Using methods of statistical kinetic theory parametrized with first-principles interatomic interactions that include chemical and strain contributions, we investigated the kinetics of decomposition and microstructure formation in Al-Cu alloys as a function of temperature and alloy concentration. We show that the decomposition of the solid solution forming platelets of copper, known as Guinier-Preston (GP) zones, includes several stages and that the transition from GP1 to GP2 zones is determined mainly by kinetic factors. With increasing temperature, the model predicts a gradual transition from plateletlike precipitates to equiaxial ones and at intermediate temperatures both precipitate morphologies may coexist.
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| 16. |
- Walbrühl, Martin, et al.
(författare)
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Surface gradients in cemented carbides from first-principles-based multiscale modeling : Atomic diffusion in liquid Co
- 2017
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier. - 0263-4368. ; 66, s. 174-179
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Tidskriftsartikel (refereegranskat)abstract
- The kinetic modeling of cemented carbides, where Co is used as binder element, requires a detailed diffusion description. Up to now, no experimental self- or impurity diffusion data for the liquid Co system have been available. Here we use the fundamental approach based on ab initio molecular dynamics simulations to assess diffusion coefficients for the liquid Co system, including six solute elements. Our calculated Co self-diffusion coefficients show good agreement with the estimates that have been obtained using scaling laws from the available literature data. To validate the modeling method, we performed one set of calculations for liquid Ni self-diffusion, where experimental data are available, showing good agreement between theory and experiments. The computed diffusion data were used in subsequent DICTRA simulations to model the gradient formation in cemented carbide systems. The results based on the new diffusion data allows for correct predictions of the gradient thickness.
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| 17. |
- Xia, S., et al.
(författare)
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Nonlinear oxidation behavior in pure Ni and Ni-containing entropic alloys
- 2018
-
Ingår i: Frontiers in Materials. - : Frontiers Media S.A.. - 2296-8016. ; 5
-
Tidskriftsartikel (refereegranskat)abstract
- We performed a combined experimental and theoretical investigation of the oxidation behavior of pure Ni and of the following multi-component Ni-containing alloys with nearly equiatomic compositions: FeNi, CoFeNi, CoCrFeNi, and CoCrFeMnNi. The materials were exposed to air at ambient pressure and at a temperature of 800°C for 150 min, their weight-gain due to oxidation was continuously monitored and the products of oxidation were subsequently characterized by XRD. The most common oxides formed have spinel or halite structure and the materials resistance to oxidation increases as: FeNi < CoFeNi < Ni < CoCrFeMnNi < CoCrFeNi. We found further that the oxidation-resistance of the materials does not correlate linearly with the number of elements present, instead the type of elements impacts significantly the materials susceptibility to oxidative damage. Cr is the element that imparted higher resistance to oxidation while Mn and Fe worsened the materials performance. In order to better understand the mechanisms of oxidation we employed thermodynamic equilibrium calculations and predicted the phase stability of oxides of the elements that are present in the materials, in different ranges of temperature, composition and oxygen activity. Additionally, we determined the phase compositions for the thermodynamically stable oxides at 800°C. The results from the thermodynamic modeling are in good agreement with the experimental finds. The alloys with low resistance to oxidation such as CoFeNi and FeNi, form the Fe 3 O 4 spinel phase which tends to dominate the phase diagram for these materials. The presence of Cr increases the resistance to atomic rearrangement due to slow diffusion in the complex structure of Cr containing spinel phases. This causes the extremely high resistance to oxidation of the CoCrFeNi alloy. The presence of Mn in CoCrFeNi stabilizes the Mn 3 O 4 spinel, which reduces the oxidation-resistance of the alloys due to the high mobility of Mn.
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| 18. |
- Yang, Xiaoyong, et al.
(författare)
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Structural, electronic, mechanical and thermodynamic properties of U-Si intermetallic compounds : A comprehensive first principles calculations
- 2022
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Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 148, s. 104229-
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Tidskriftsartikel (refereegranskat)abstract
- Uranium silicides are proposed as the prominent accident tolerant fuels for the light water reactors (LWR) due to their high metal density and high thermal conductivity. Among the U-Si alloys, the alloy with high U:Si ratio is more favorable for nuclear fuel application due to the higher uranium density. Thus, the crystal structure, mechanical property, electronic structure, phonon band structure and thermodynamic property of U3Si, U3Si2 and USi compounds, along with the thermodynamic reaction between U-Si intermetallic compounds are systematically studied in our work. The optimized structural parameters of these U-Si alloys are comparable with previous results. Besides, all of them are metallic in nature. Since the calculated elastic constants satisfy the Born stability criteria, one can know U-Si alloys are mechanically stable. The phonon dispersion curves are obtained based on the density functional perturbation theory (DFPT). Accordingly, various thermophysical properties, such as Helmholtz free energy, heat capacity, internal energy and entropy are calculated. Furthermore, the reaction energies related to the formation of U3Si, U3Si2 and USi as well as transformation between them are calculated. It is revealed at the same chemical environment the reaction to form USi occurs more easily, whereas the high temperature and sufficient uranium environment are more in favor for fully silicification of uranium metal into U3Si. Theoretical investigation of this work is expected to provide some new insights for the application of uranium silicides as nuclear fuels and future exploration on the design and synthesis of new-type uranium silicides.
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| 19. |
- Ambrosch-Draxl, C., et al.
(författare)
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First-principles study of oxygen ordering in YBa2Cu3O7-x
- 2000
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Ingår i: Physica. C, Superconductivity. - 0921-4534 .- 1873-2143. ; 341, s. 1997-1998
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Tidskriftsartikel (refereegranskat)abstract
- We have performed first-principles calculations considering different patterns of oxygen/vacancy ordering in the Cu-O chains of YBa2Cu3O7-x. From the total-energy values we determine the effective interactions of the oxygen atoms within the basal plane by using the cluster expansion technique. Furthermore, we discuss the effect of ordering to the density of states at the Fermi level.
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| 20. |
- Baykov, Vitaly, et al.
(författare)
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Diffusion of Interstitial Mn in the Dilute Magnetic Semiconductor (Ga,Mn)As : The Effect of a Charge State
- 2008
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:17, s. 177204-
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Tidskriftsartikel (refereegranskat)abstract
- Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.
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| 21. |
- Baykov, Vitaly, et al.
(författare)
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Structural stability of intermetallic phases in the Zr-Sn system
- 2006
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 55:5, s. 485-488
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Tidskriftsartikel (refereegranskat)abstract
- A thermodynamic description of the intermetallic compounds in the Zr-Sn binary system has been obtained using total energy calculations by means of the Vienna ab initio simulation package. Our calculations show that hexagonal compounds Zr5Sn4 and Zr5Sn3 are the most stable phases in the Zr-Sn binary system. Their high stability is found to be due to hybridization of the Sn 5p with Zr 4d electronic states. Based on the calculated energies, the conclusion is made that Zr substitution on the Sri sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound.
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| 22. |
- Chen, Kaixuan, et al.
(författare)
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Morphological instability of iron-rich precipitates in Cu-Fe-Co alloys
- 2019
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Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 163, s. 55-67
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical properties of metallic materials are determined by their microstructure, and in particular, the different morphologies of precipitates lead to distinct strengthening effects. Usually, the shape of precipitates changes during growth and coarsening regimes, leading to modification of the macroscopic properties of the materials. Thus, understanding of this phenomenon is key to tailoring the precipitate strengthening of industrial alloys. In this article, a general approach to explain the shape instability of iron-rich nanoparticles in Cu-Fe-Co alloys during casting and ageing processes is proposed. The evolution of particle shape from sphere to cuboid to petal and finally splitting into eight subnanoparticles is observed using transmission electron microscopy. Phase-field modelling and thermodynamic calculations are combined into a general model that describes and elucidates the morphological evolution of precipitates in alloys in terms of particle size, interfacial and elastic strain energy, and chemical driving force. These findings have the potential to promote new microstructural design approaches for a wide range of materials.
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| 23. |
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| 24. |
- Delandar, Arash Hosseinzadeh, et al.
(författare)
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Ab-initio based search for late blooming phase compositions in iron alloys
- 2018
-
Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 509, s. 225-236
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Tidskriftsartikel (refereegranskat)abstract
- We present a systematic analysis, based on ab initio calculations, of concentrated solute arrangements and precipitate phases in Fe-based alloys. The input data for our analysis are the calculated formation and interaction energies of point defects in the iron matrix, as well as the energies of ordered compounds that represent end-members in the 4-sublattice compound energy model of a multicomponent solid solution of Mg, Al, Si, P, S, Mn, Ni, and Cu elements and also vacancies in bcc Fe. The list of compounds also includes crystal structures obtained by geometric relaxation of the end-member compounds that in the cubic structure show weak mechanical instabilities (negative elastic constants) and also the G-phase Mn-6(Ni,Fe)(16)(Si,P)(7) having a complex cubic structure. A database of calculated thermodynamic properties (crystal structure, molar volume, enthalpy of formation, and elastic constants) of the most stable late-blooming-phase candidates is thus obtained. The results of this ab initio based theoretical analysis compare well with the recent experimental observations and predictions of thermodynamic calculations employing Calphad methodology.
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| 25. |
- Delandar, Arash Hosseinzadeh, et al.
(författare)
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End-member compounds of a 4-sublattice model of multicomponent BCC solid solutions
- 2018
-
Ingår i: Data in Brief. - : Elsevier. - 2352-3409. ; 20, s. 1018-1022
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Tidskriftsartikel (refereegranskat)abstract
- The article presents ab initio calculated properties (total energies, lattice parameters, and elastic properties) for the complete set of 1540 end-member compounds within a 4-sublattice model of Fe-based solid solutions. The compounds are symmetry-distinct cases of integral site occupancy for superstructure Y (space group #227, type LiMgPdSn) chosen to represent the ordered arrangements of solvent atoms (Fe), solute atoms (Fe, Mg, Al, Si, P, S, Mn, Ni, Cu), and vacancies (Va) on the sites of a body-centered cubic lattice. The model is employed in the research article “Ab-initio based search for late blooming phase compositions in iron alloys” (Hosseinzadeh et al., 2018) [1].
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| 26. |
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| 27. |
- Delandar, Arash Hosseinzadeh, et al.
(författare)
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The Role of Glide during Creep of Copper at Low Temperatures
- 2018
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Ingår i: Metals. - : MDPI. - 2075-4701. ; 8:10
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Tidskriftsartikel (refereegranskat)abstract
- Copper canister will be used in Scandinavia for final storage of spent nuclear fuel. The copper will be exposed to temperatures of up to 100 degrees C. The creep mechanism at near ambient temperatures has been assumed to be glide of dislocations, but this has never been verified for copper or other materials. In particular, no feasible mechanism for glide based static recovery has been proposed. To attack this classical problem, a glide mobility based on the assumption that it is controlled by the climb of the jogs on the dislocations is derived and shown that it is in agreement with observations. With dislocation dynamics (DD) simulations taking glide but not climb into account, it is demonstrated that creep based on glide alone can reach a quasi-stationary condition. This verifies that static recovery can occur just by glide. The DD simulations also show that the internal stress during creep in the loading direction is almost identical to the applied stress also directly after a load drop, which resolves further classical issues.
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| 28. |
- Ehteshami, Hossein
(författare)
-
Finite temperature properties of elements and alloy phases from first principles
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- First principles calculations are usually concerned with properties calculated at temperature 0 K. However, the industrially important materials are functioning at finite temperatures. To fill such a gap a first-principles based modeling of free energy has been developed in this thesis and finite temperature properties of different phases of Fe and Mn have been calculated and contrasted with available experimental data.In particular, using partitioning of the Helmholtz free energy, thermophysical properties of paramagnetic Fe have been reported. The heat capacity, lattice constant, thermal expansion and elastic moduli of γ- and δ-Fe show a good agreement with available experimental data. In the case of α-Fe, we observe a good agreement for elastic moduli and thermal expansion with experiments but the heat capacity is not well-reproduced in the calculations because of the large contribution of magnetic short-range which our models are not capable of capturing.α- and β-Mn theoretically pose a challenge for direct simulations of thermodynamic properties because of the complexity of magnetic and crystal structure. The partitioning of free energy has been used and thermodynamics of these phases have been derived. The obtained results show a good agreement with experimental data suggesting that, despite the complexities of these phases, a rather simple approach can well describe their finite temperature properties. High temperature phases of Mn, γ and δ, are also theoretically challenging problems. Employing a similar approach to Fe, thermophysical properties of these high symmetry phases of Mn have been reported which also show good agreement with available experimental data.The point defect and metal-self diffusion in titanium carbide (TiC), a refractory material, have been investigated in the present work. The common picture of metal-vacancy exchange mechanism for metal self-diffusion was shown to be unable to explain the experimentally observed values of activation energy. Several new clusters of point defects such as vacancies and interstitials have been found and reported which are energetically lower that a single metal vacancy. In a subsequent study, we showed that some of these clusters can be considered as mediators of metal self-diffusion in TiC.Evaluation of structural properties of Ti(O,C), a solid solution of TiC and β-TiO, from supercell approach is an extremely difficult task. For a dilute concentration of O, we show the complexity of describing an impurity of O in TiC using supercell approach. A single-site method such as the exact muffin-tin orbital method in the coherent potential approximation (EMTO-CPA) is a good alternative to supercell modeling of Ti(O,C). However, a study of Ti(O,C) using EMTO-CPA requires a further development of the technique regarding the partitioning of space. The shape module of EMTO has been modified for this purpose. With the help of the modified module, Ti(O,C) have been studied using EMTO-CPA. The results for the divacancy concentration and corresponding lattice parameter variations show good agreement with experimental data.
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| 29. |
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| 30. |
- Ehteshami, Hossein, et al.
(författare)
-
Thermodynamic properties of paramagnetic α- and β−Mn from first principles: The effect of transverse spin fluctuations
- 2017
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 1:073803
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles-based thermodynamic modeling of cubic alpha and beta phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Gruneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in alpha-Mn and site I in beta-Mn on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in alpha-Mn is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in beta-Mn should be treated as localized.
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| 31. |
- Ehteshami, Hossein, et al.
(författare)
-
Thermophysical properties of paramagnetic Fe from first principles
- 2017
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:22
-
Tidskriftsartikel (refereegranskat)abstract
- A computationally efficient, yet general, free-energy modeling scheme is developed based on first-principles calculations. Finite-temperature disorder associated with the fast (electronic and magnetic) degrees of freedom is directly included in the electronic structure calculations, whereas the vibrational free energy is evaluated by a proposed model that uses elastic constants to calculate average sound velocity of the quasiharmonic Debye model. The proposed scheme is tested by calculating the lattice parameter, heat capacity, and single-crystal elastic constants of alpha-, gamma-, and delta-iron as functions of temperature in the range 1000-1800 K. The calculations accurately reproduce the well-established experimental data on thermal expansion and heat capacity of gamma- and delta-iron. Electronic and magnetic excitations are shown to account for about 20% of the heat capacity for the two phases. Nonphonon contributions to thermal expansion are 12% and 10% for alpha- and delta-Fe and about 30% for gamma-Fe. The elastic properties predicted by the model are in good agreement with those obtained in previous theoretical treatments of paramagnetic phases of iron, as well as with the bulk moduli derived from isothermal compressibility measurements [N. Tsujino et al., Earth Planet. Sci. Lett. 375, 244 (2013)]. Less agreement is found between theoretically calculated and experimentally derived single-crystal elastic constants of gamma- and delta-iron.
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| 32. |
- Eloirdi, R., et al.
(författare)
-
Dilution effect on the U-5f states : U in an Ag matrix
- 2005
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 386:1-2, s. 70-74
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of U-diluted in an Ag matrix has been studied in situ by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). UxAg100-x thin films (x = 0-100 at.%) were produced by sputter co-deposition in an Ar atmosphere. UPS spectra of the Ag-4d line indicate formation of a homogeneous mixture despite the fact that U and Ag do not form stable alloys. A major goal of this work was to find out whether the dilution of U atoms in an inert matrix with no bonding states induces the localisation of the U-5f states. Both U-4f core level spectra and the U-5f spectra indicate U-5f delocalisation, down to 5 at.% of uranium in UxAg100-x films.
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| 33. |
- Eriksson, Olle, et al.
(författare)
-
Electronic structure and magnetism of diluted magnetic semiconductors
- 2004
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 16:48, s. S5481-S5489
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and magnetism of selected diluted magnetic semiconductors (DMS) is reviewed. It is argued that the effect of antisite defects plays an important role in the magnetism of DMS materials and that these defects lower the saturation moment and ordering temperature. We also show that the interatomic exchange of these materials is short ranged. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we show that-the observed critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, are reproduced with good accuracy. We show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned on the Ga (or Zn) sites. This suggests that the ordering of DMS materials is heavily influenced by magnetic percolation and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.
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| 34. |
- Ghadami Yazdi, Milad, et al.
(författare)
-
Structure dependent effect of silicon on the oxidation of Al(111) and Al(100)
- 2019
-
Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 684, s. 1-11
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy. Upon O 2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al 2 O 3 , and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied. Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness.
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| 35. |
- Gorbatov, O. I., et al.
(författare)
-
Dependence of vacancy-solute interactions on magnetic state in dilute iron-based alloys
- 2011
-
Ingår i: Solid-Solid Phase Transformations in Inorganic Materials. - Zurich-Durnten, Switzerland : TRANS TECH PUBLICATIONS. ; , s. 979-984
-
Konferensbidrag (refereegranskat)abstract
- Vacancy-solute interactions play a crucial role in diffusion-controlled phase transformations, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. The knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) under irradiation, as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on density functional theory and employing the locally self-consistent Green’s function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Ti – Cu), and 4d (Nb – Ag) elements. Interactions within the first two coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a very strong effect on the vacancy-solute interactions.
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| 36. |
- Gorbatov, O. I., et al.
(författare)
-
Vacancy-solute interactions in ferromagnetic and paramagnetic bcc iron : Ab initio calculations
- 2011
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 419:1-3, s. 248-255
-
Tidskriftsartikel (refereegranskat)abstract
- Vacancy-solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc-Cu), and 4d (Y-Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy-solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed.
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| 37. |
- Gorbatov, Oleg, et al.
(författare)
-
Many-body interactions in the Al–Cu system
- 2017
-
Ingår i: Data in Brief. - : Elsevier Inc.. - 2352-3409. ; 15, s. 770-774
-
Tidskriftsartikel (refereegranskat)abstract
- The article contains computational data of many-body interactions in Al–Cu alloys, obtained using PAW-VASP calculations. Pairwise, three-site, and four-site interactions are presented. Mentioned data are relevant to the research article “Many-body mechanism of Guinier-Preston zones stabilization in Al–Cu alloys”.
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| 38. |
- Hugosson, Håkan Wilhelm, et al.
(författare)
-
Phase stabilities and structural relaxations in substoichiometric TiC1-x
- 2001
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 6316:16, s. 165116-
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations have been used to study the effect of vacancies and relaxation around the vacancy sites in substoichiometric TiC1-x. The effect of relaxation on phase stabilities, equilibrium volumes. and electronic structure of the substoichiometric phases was studied using a combined approach of pseudopotential plane wave and full-potential linear muffin-tin orbital methods. A relaxation away from the vacancies was found for the titanium atoms, the magnitude of which increased with vacancy concentration and the inclusion of nearest-neighbor carbon atom relaxation. The inclusion of local relaxations was found to correctly predict the off-stoichiometric equilibrium composition of titanium carbide. The anomalous volume behavior of TiC at small vacancy concentration is explained as an effect of the local relaxation of the atoms surrounding the vacancy sites, but we do not find that the lattice parameter of any of the studied stoichiometries is larger than that of ideal stoichiometric TiC.
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| 39. |
- Hutchinson, Bevis, et al.
(författare)
-
Elasticity and wave velocity in fcc iron (austenite) at elevated temperatures – Experimental verification of ab-initio calculations
- 2018
-
Ingår i: Ultrasonics. - : Elsevier. - 0041-624X .- 1874-9968. ; 87, s. 44-47
-
Tidskriftsartikel (refereegranskat)abstract
- High temperature crystal elasticity constants for face centred cubic austenite are important for interpreting the ultrasonic properties of iron and steels but cannot be determined by normal single crystal methods. Values of these constants have recently been calculated using an ab-initio approach and the present work was carried out to test their applicability using laser-ultrasonic measurements. Steel samples having a known texture were examined at temperatures between 800 °C and 1100 °C to measure the velocity of longitudinal P-waves which were found to be in good agreement with modelled values.
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| 40. |
- Isaev, Eyvaz, et al.
(författare)
-
Intrinsic defects and transition metal impurities in GaAs
- 2004
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 272:3, s. 1961-1962
-
Tidskriftsartikel (refereegranskat)abstract
- Thermodynamics of intrinsic point defects, 3d-transition metal (TM) impurities, and various defect pairs in GaAs have been studied by means of the locally self-consistent Green's function method. Antisite defects, As-Ga, are found to be the most energetically favorable defects in the As-rich GaAs. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The magnetic moments of TM impurities in the GaAs host are calculated under the assumption that orbital moments of TM are quenched and the total magnetization is due to spill ordering of electrons in the crystal. Thermodynamic possibility of formation of complexes between TM-atoms is investigated.
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| 41. |
- Karkin, I. N., et al.
(författare)
-
Effect of Ni and Al on the Decomposition Kinetics and Stability of Cu-Enriched Precipitates in Fe-Cu-Ni-Al Alloys : Results of MD plus MC Simulation
- 2021
-
Ingår i: Physics of metals and metallography. - : MAIK NAUKA/INTERPERIODICA/SPRINGER. - 0031-918X .- 1555-6190. ; 122:5, s. 498-503
-
Tidskriftsartikel (refereegranskat)abstract
- To understand the effects of doping elements on the formation and structure of Cu-enriched precipitates, different stages of Fe-Cu-Ni-Al alloy decomposition are studied using the combined MC + MD approach, which includes the Monte Carlo (MC) and molecular dynamics (MD) simulations. It is shown that the surface of the precipitates enriches in the doping elements at early stages of the decomposition, which is significant for the structure stability against the bcc -> 9R transition of the copper enriched precipitates.
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| 42. |
- Kar'kin, I. N., et al.
(författare)
-
Kinetics of Early Decomposition Stages in Diluted bcc Fe-Cu-Ni-Al Alloy : MC plus MD Simulation
- 2019
-
Ingår i: Physics of the solid state. - : PLEIADES PUBLISHING INC. - 1063-7834 .- 1090-6460. ; 61:4, s. 601-608
-
Tidskriftsartikel (refereegranskat)abstract
- A combined approach including the Monte Carlo and molecular-dynamics simulation, decomposition kinetics and segregation formation in the multicomponent low-alloy Fe-1.5Cu-2Ni-1.5Al (at %) alloy is studied. It is shown that the formation of Cu nanoparticles surface-enriched with Ni and Al (coprecipitation mode) includes several stages: (i) the formation of clusters consisting of several Cu atoms, (ii) their enrichment with Ni and Al atoms, and (iii) redistribution of Ni and Al atoms with the formation of a surface layer providing stabilization of Cu nanoparticles. Observed structural features of segregations and their stability in Fe-Cu-Ni-Al alloys is discussed.
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| 43. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Ab Initio Study of Phase Equilibria in TiCx
- 2002
-
Ingår i: Physical Review Letters. - : The American Physical Society. - 0031-9007 .- 1079-7114. ; 88:1, s. 015505-1-01505-4
-
Tidskriftsartikel (refereegranskat)abstract
- The phase diagram for the vacancy-ordered structures in the substoichiometric TiCx (x=0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti2C, Ti3C2, and Ti6C5) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.
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| 44. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Atomic, electronic, and magnetic structure of iron-based sigma-phases
- 2005
-
Ingår i: Integrative and Interdisciplinary Aspects of Intermetallics. - WARRENDALE : MATERIALS RESEARCH SOCIETY. - 1558997903 ; , s. 517-522
-
Konferensbidrag (refereegranskat)abstract
- A combination of ab initio total energy calculations with Calphad approach is applied to model the site occupancy and thermodynamic properties of the Fe-Cr, Co-Cr, Fe-V, and Fe-Mo binary sigma-phases as a function of composition and temperature. For each binary sigma-phase the parameters of the model are the ab initio calculated total energies of so-called end-member compounds, which represent all the 2(5)=32 variants of complete occupancy of each of the five crystallographically inequivalent sites by one or the other alloy component, The paramagnetic state of the sigma-phases has been taken into account within the disordered local moment approach. The Fe and Co atoms are found to retain high spin moments when they occupy high-coordination-number sites in the structure. Using our model we were able to reproduce the experimentally observed site occupancy in the FeCr sigma-phase. The calculated site occupancies in the Co-Cr, Fe-V, and Fe-Mo sigma-phases are also presented and discussed.
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| 45. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Electronic structure and effective chemical and magnetic exchange interactions in bcc Fe-Cr alloys
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:5, s. 054202-
-
Forskningsöversikt (refereegranskat)abstract
- Electronic structure calculations are employed in order to investigate the cohesive properties (lattice parameter, enthalpy of formation, and bulk modulus) of random Fe-Cr alloys as a function of composition and magnetic state, as well as to derive the chemical and magnetic exchange interactions of the constituent atoms. The calculations predict certain anomalies in the cohesive properties of ferromagnetic alloys at a concentration of about 7 at % Cr; these anomalies may be related to the changes in Fermi-surface topology that occur with composition in this alloy system. The obtained interatomic interactions are used as parameters in the configurational (Ising) and magnetic (Heisenberg) Hamiltonians for modeling finite-temperature thermodynamic properties of the alloys. We discuss the approximations and limitations of similar modeling approaches, investigate the origin of existing difficulties, and analyze possible ways of extending the theoretical models in order to capture the essential physics of interatomic interactions in the Fe-Cr or similar alloys where magnetism plays a crucial role in the phase stability.
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| 46. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Exploring configurational degrees of freedom in disordered solids
- 2018
-
Ingår i: International Conference of Computational Methods in Sciences and Engineering 2018 (ICCMSE 2018). - : American Institute of Physics (AIP). - 9780735417663
-
Konferensbidrag (refereegranskat)abstract
- Recent computational modeling studies of configurational degrees of freedom in oxide-hydroxides of aluminum and copper are reviewed. Density functional theory calculations are employed to investigate the effect of hydrogen on the structural stability of gamma-alumina and to explore the configurational space of cuprous hydroxide CuOH. Free energy modeling, taking into account configurational and vibrational degrees of freedom, shows that the studied hydrogenated oxides of aluminum and copper are metastable compounds, whereas the respective oxides are thermodynamically stable phases. Selected structural and optical properties of the studied compounds, evaluated using a hybrid functional approach, are reported and compared with available experimental data.
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| 47. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Exploring monovalent copper compounds with oxygen and hydrogen
- 2012
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 109:3, s. 686-689
-
Tidskriftsartikel (refereegranskat)abstract
- New important applications of copper metal, e.g., in the areas of hydrogen production, fuel cell operation, and spent nuclear fuel disposal, require accurate knowledge of the physical and chemical properties of stable and metastable copper compounds. Among the copper(I) compounds with oxygen and hydrogen, cuprous oxide Cu(2)O is the only one stable and the best studied. Other such compounds are less known (CuH) or totally unknown (CuOH) due to their instability relative to the oxide. Here we combine quantum-mechanical calculations with experimental studies to search for possible compounds of monovalent copper. Cuprous hydride (CuH) and cuprous hydroxide (CuOH) are proved to exist in solid form. We establish the chemical and physical properties of these compounds, thereby filling the existing gaps in our understanding of hydrogen- and oxygen-related phenomena in Cu metal.
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| 48. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
First-principles calculations as a new tool in steel research
- 2008
-
Ingår i: 6th European Stainless Steel Conference, Science and Market Helsinki, Finland, June 10-13, 2008. - : Jernkontoret. - 9197413194 ; , s. 279-284
-
Konferensbidrag (refereegranskat)abstract
- Great scepticism over the earliest applications of the first-principles theory to the calculation of the ground state properties of real systems in the 70’s and 80’s has gradually turned into a widespread belief in its quantitative accuracy and predictive power, due to a great number of successful applications to very different systems and problems, so that several leading materials manufacturing companies have initiated first-principles based research programs aimed at design of new materials. However, the role of first-principles calculations in the design process, and, most importantly, their potential, remain quite obscure. We therefore try to give a “balanced” view on the possibilities of first-principles calculations, not only telling success stories, but also discussing their problems and limitations. Then, we demonstrate uniqueness of the information that can be obtained using first-principles methods. Thus we determine their place among the traditional research methods and tools in materials science. In order to show the capabilities of first-principles calculations, we present some recent applications of first-principles methods to investigation of the thermodynamic and kinetic properties of austenitic (including high-nitrogen) and ferritic steels, at the atomic level. Finally, we indicate problems, related to steel research and development, where first-principles methods (together with other experimental or theoretical techniques) can lead to substantial progress or even breakthroughs.
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| 49. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Free energy of metals from quasi-harmonic models of thermal disorder
- 2021
-
Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 11:2, s. 195-
-
Tidskriftsartikel (refereegranskat)abstract
- A simple modelling method to extend first-principles electronic structure calculations to finite temperatures is presented. The method is applicable to crystalline solids exhibiting complex thermal disorder and employs quasi-harmonic models to represent the vibrational and magnetic free energy contributions. The main outcome is the Helmholtz free energy, calculated as a function of volume and temperature, from which the other related thermophysical properties (such as temperature-dependent lattice and elastic constants) can be derived. Our test calculations for Fe, Ni, Ti, and W metals in the paramagnetic state at temperatures of up to 1600 K show that the predictive capability of the quasi-harmonic modelling approach is mainly limited by the electron density functional approximation used and, in the second place, by the neglect of higher-order anharmonic effects. The developed methodology is equally applicable to disordered alloys and ordered compounds and can therefore be useful in modelling realistically complex materials.
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| 50. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
-
Thermodynamic properties of copper compounds with oxygen and hydrogen from first principles
- 2010
-
Rapport (refereegranskat)abstract
- We employ quantum-mechanical calculations (based on density functional theory and linear response theory) in order to test the mechanical and chemical stability of several solid-state configurations of Cu1+, Cu2+, O2–, H1–, and H1+ ions. We begin our analysis with cuprous oxide (Cu2O, cuprite structure), cupric oxide (CuO, tenorite structure), and cuprous hydride (CuH, wurtzite and sphalerite structures) whose thermodynamic properties have been studied experimentally. In our calculations, all these compounds are found to be mechanically stable configurations. Their formation energies calculated at T = 0 K (including the energy of zero-point and thermal motion of the ions) and at room temperature are in good agreement with existing thermodynamic data. A search for other possible solid-state conformations of copper, hydrogen, and oxygen ions is then performed. Several candidate structures for solid phases of cuprous oxy-hydride (Cu4H2O) and cupric hydride (CuH2) have been considered but found to be dynamically unstable. Cuprous oxy-hydride is found to be energetically unstable with respect to decomposition onto cuprous oxide and cuprous hydride. Metastability of cuprous hydroxide (CuOH) is established in our calculations. The free energy of CuOH is calculated to be some 50 kJ/mol higher than the average of the free energies of Cu2O and water. Thus, cuprite Cu2O is the most stable of the examined Cu(I) compounds.
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