SwePub - sökning: WFRF:(Ruban Andrei. V.)

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1.	Serikov, V. V., et al. (författare) Formation of solid solutions of gallium in Fe-Cr and Fe-Co alloys : Mossbauer studies and first-principles calculations 2014 Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 614, s. 297-304 Tidskriftsartikel (refereegranskat)abstract Investigation of Ga influence on the structure of Fe-Cr and Fe-Co alloys was performed with the use of Mossbauer spectroscopy and X-ray diffraction methods. The experimental results are compared with results of first-principles calculations of the mixing and solubility energies for Ga in an Fe-X (X = Co, Cr) alloy both in ferromagnetic and paramagnetic states. It is shown that Ga mainly goes into the solid solutions of the base alloys. In the alloys of the Fe-Cr system, doping with Ga handicaps the decomposition of solid solutions, observed in the binary alloys, and increases its stability. In the alloys with Co, Ga also favors the uniformity of solid solutions. The results of the first-principles calculations testify in favor of a preferable dissolution of Ga in the FeCo regions of a multicomponent structure rather than FeCr regions, both types of regions being in the ferromagnetic state at the temperature of annealing. The analysis of Mossbauer experiments gives some grounds to conclude that if, owing to liquation, clusterization, or initial stages of phase separation, there exist regions enriched in iron, some amount of Ga atoms prefer to enter the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases).
2.	Glazyrin, K., et al. (författare) Importance of Correlation Effects in hcp Iron Revealed by a Pressure-Induced Electronic Topological Transition 2013 Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:11, s. 117206- Tidskriftsartikel (refereegranskat)abstract We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.
3.	Logunov, A. V., et al. (författare) Theoretical analysis of the alloying system and design of new nickel-base superalloys 2008 Ingår i: Doklady physics (Print). - : Pleiades Publishing Ltd. - 1028-3358 .- 1562-6903. ; 53:8, s. 438-441 Tidskriftsartikel (refereegranskat)
4.	Bogdanov, V. I., et al. (författare) Chemical and deformational interactions in solid solution of carbon in nickel 2012 Ingår i: Physics of metals and metallography. - 0031-918X .- 1555-6190. ; 113:9, s. 831-835 Tidskriftsartikel (refereegranskat)abstract A first-principles stuy of ordering phenomena in hcp interstitial solid solutions of oxygen and nitrogen in Ti, Zr and Hf has demonstrated that the dominant contributions to the interaction energy of interstitial atoms are of chemical nature; thus, it is necessary to modify the previously established concepts about the priority role of deformational interactions in interstitial solutions. We have continued studies of the role of chemical and deformational interactions of interstitial atoms by the example of solid solutions of carbon in nickel. The results obtained also confirm a significant role of chemical interactions between carbon atoms in these solid solutions. The results were compared with the experimental data on the enthalpy of carbon dissolution in nickel and on the coefficient of solutal expansion of the lattice.
5.	Bogdanov, V. I., et al. (författare) First-principles investigations of interatomic interactions in Ni3Al alloyed by interstitial and substitutional impurities 2013 Ingår i: Physics of metals and metallography. - 0031-918X .- 1555-6190. ; 114:3, s. 191-196 Tidskriftsartikel (refereegranskat)abstract First-principles calculations of the total energy of interstitial and substitutional solid solutions in intermetallic compound Ni3Al were performed based on methods using Vienna ab-initio simulation package (VASP). The results of the calculations for interstitial solutions of carbon in Ni3Al confirmed the priority role of chemical interactions over deformational ones for the nearest neighbors. We attempted to use first-principles methods of calculation of the deformation interaction and continuum approaching in the theory of solutions to calculate coefficients of the concentration changes of the lattice spacing. Comparison of the calculation results with experimental data of substitutional impurities in Ni3Al has shown that the proposed method can aid in the study of the distribution of impurity atoms on the sublattices of the ordered phases, intermetallic compounds. We have proposed a method of calculating the partial molar volume of impurity in interstitial solid solutions.
6.	Bogdanov, V. I., et al. (författare) Method for calculating coefficients of concentrational variations in lattice constants and the distribution of impurity atoms between sublattices in intermetallic compounds 2013 Ingår i: Bulletin of the Russian Academy of Sciences: Physics. - 1062-8738. ; 77:11, s. 1360-1362 Tidskriftsartikel (refereegranskat)abstract A method is proposed for calculating coefficients of concentrational variations in lattice constants in solid solutions, based on first-principles determination of the total energy of solid solutions and the continual approximation in the solutions theory, allowing for the deformation interaction of impurity atoms due to distortions of the solvent crystal lattice.
7.	Gorbatov, O. I., et al. (författare) Role of Magnetism in the Formation of a Short-Range Order in Iron-Silicon Alloys 2011 Ingår i: Journal of Experimental and Theoretical Physics. - 1063-7761 .- 1090-6509. ; 112:5, s. 848-859 Tidskriftsartikel (refereegranskat)abstract The formation of a short-range order in soft magnetic Fe-Si alloys depending on the annealing temperature has been investigated theoretically and experimentally. The B2-type short-range order has been observed in samples quenched from temperatures T > T(C) (where T(C) is the Curie temperature) with the content c(Si) close to the boundary of the two-phase region. Annealing at temperatures T < T(C) for the content c(Si) >= 0.08 leads to an increase in the fraction of regions with the D0(3)-type short-range order. The mechanism of the formation of the short-range order in Fe-Si solid solutions has been analyzed by the Monte Carlo simulation with the ab initio calculated interatomic interaction parameters. It has been shown that the energy of the effective Si-Si interaction in bcc iron strongly depends on the magnetic state of the matrix. As a result, the B2-type short-range order is formed at T > T(C) and is fixed at quenching, whereas the D0(3)-type short-range order is equilibrium in the ferromagnetic state. The results reveal the decisive role of magnetism in the formation of the short-range order in Fe-Si alloys and allow the explanation of the observed structural features of the alloys depending on the composition and temperature.
8.	Razumovskii, I. M., et al. (författare) New generation of Ni-based superalloys designed on the basis of first-principles calculations 2008 Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 497:1-2, s. 18-24 Tidskriftsartikel (refereegranskat)abstract A new approach to the design of Ni-based single crystal superalloys is proposed. It is based on a concept that under given structural conditions, the creep-rupture characteristics of superalloys are mainly determined by interatomic bonding given by the cohesive energy. In order to characterize the individual contribution of each alloying element to the strength properties at high temperature, we introduce a parameter, X, which is the partial molar cohesive energy of an alloy component. This parameter is then obtained in the total energy first-principles calculations for a usual set of alloying elements. We demonstrate that creep-rupture characteristics of alloys indeed correlate with the total gain partial molar cohesive energy due to alloying and find that W, Ta, and Re have the highest values of X, and should therefore play the major role in providing high-temperature strength of superalloys. Based on this finding, we design three new superalloys with a high content of W and show that they have superior creep-rupture properties compared not only with their counterparts with the lower content of W, but also with the best Ru-bearing Ni-based superalloys.
9.	Ruban, Andrei V., et al. (författare) First-principles study of point defects in Ni3Al 2014 Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 94:1, s. 20-34 Tidskriftsartikel (refereegranskat)abstract The energetics and structural properties of native, substitutional and interstitial defects in Ni3Al have been investigated by first-principles methods. In particular, we have determined the formation energies of composition conserving defects and established that the so-called penta defect, which consists of four vacancies on Ni sublattice and Ni antisite on the Al sublattice, is the main source of vacancies in Ni3Al. We show that this is due to the strong Ni-site preference of vacancies in Ni3Al. We have also calculated the site substitution behaviour of Cu, Pd, Pt, Si, Ti, Cr, V, Nb, Ta and Mo and their effect on the concentration expansion coefficient. We show the latter information can used for an indirect estimate of the site substitution behaviour of the alloying elements. The solution energy of carbon and its effect on the lattice constant of Ni3Al have been obtained in the dilute limit in the first-principles calculations. We have also determined the chemical and strain-induced carbon-carbon interactions in the interstitial positions of Ni3Al. These interactions have been subsequently used in the statistical thermodynamic simulations of carbon ordering in Ni3Al.
10.	Blanter, M. S., et al. (författare) Ab initio Based O-O Investigation and the Snoek Relaxation in Nb-O 2012 Ingår i: Diffusion and Defect Data Pt.B. - : Trans Tech Publications Inc.. - 9783037853382 ; , s. 63-68 Konferensbidrag (refereegranskat)abstract It is common knowledge that interstitial-interstitial interaction influence on the Snoek relaxation. We used a computer simulation of this effect in the Nb-O alloy to test the adequacy of various models of the O-O interaction and clarify the mechanism of this effect The energy calculations in the first twelve coordination shells have been performed by the projector augmentedwave (PAW) method as implemented in the Vienna ab initio simulation package (VASP). The energies have been calculated in different ways which vary in the method of determination the energy of non-interacting O-O pairs. The energies calculated on the various variants are similar but in one case there is O-O repulsion in all first twelve coordination shells, whereas in another one can see attraction in four of twelve shells. Internal friction Q -1 was calculated as a sum of the contributions from individual interstitial atoms in different environments, each of which being assumed to be the Debye function. It is assumed that long-range interaction of oxygen atoms affects the distribution of these atoms and the energy of each interstitial atom in the octahedral interstices before a jump and after a jump. The Monte Carlo method is used for simulating short-range order of interstitial atoms and for calculating values of energy changes. Comparison of the calculated temperature and concentration dependence of the Snoek peak with the published data showed that the PAW supercell calculation of the O-O interactions in Nb describes the behavior of the interstitial solid solution adequately. It proves also that the impact of interstitial atom concentration on the Snoek relaxation is connected to the mutual attraction of these atoms.
11.	Blanter, M. S., et al. (författare) Interaction of interstitial atoms and configurational contribution to their thermodynamic activity in V, Nb, and Ta 2017 Ingår i: Physics of metals and metallography. - : Maik Nauka/Interperiodica. - 0031-918X .- 1555-6190. ; 118:2, s. 105-112 Tidskriftsartikel (refereegranskat)abstract The pairwise interaction energies of O–O and N–N in bcc metals of group VB, which were calculated earlier using first-principles methods, have been employed to analyze the effect of the interatomic interactions on the configurational contribution to the thermodynamic activity. The strong effect of interstitial- interstitial interaction has been shown. The configurational contribution grows in the row (Nb–N) → (V–N) → (Ta–N) → (Nb–O) → (V–O) → (Ta–O), which is caused by a weakening of the mutual attraction of interstitial atoms in these solid solutions. The strong repulsion that characterizes the majority of coordination shells only weakly affects the thermodynamic activity. The character of the temperature dependence of the configurational contribution is defined by the strength of the mutual attraction of the interstitial atoms, i.e., upon strong attraction, the contribution increases with increasing temperature (Nb–N, V–N, Ta–N, and Nb–O) and, upon weak attraction, it decreases (V–O and Ta–O).
12.	Blanter, M. S., et al. (författare) Interstitial-interstitial interactions in bcc VB group metals : Ab initio calculations 2013 Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 74:5, s. 716-722 Tidskriftsartikel (refereegranskat)abstract Chemical and strain-induced interactions of oxygen and nitrogen interstitials in bcc V and Ta have been obtained for the first 12 coordination shells in the supercell first-principles calculations by the Viena ab initio simulation package (VASP). It is shown that the chemical interactions are dominating at the first several coordination shells while the strain-induced interactions become important at more distant coordination shells. The latter are in reasonable agreement with the earlier results of a microscopic phenomenological Krivoglaz-Kanzaki-Khachaturyan model (KKKM). The obtained interstitial-interstitial interactions have been used in calculations of the concentration dependence of the oxygen Snoek peak, which is in good agreement with the existing experimental data.
13.	Blanter, M. S., et al. (författare) Ordering in V-O and V-N solid solutions : Computer simulation 2014 Ingår i: Bulletin of the Russian Academy of Sciences: Physics. - 1062-8738. ; 78:10, s. 1030-1034 Tidskriftsartikel (refereegranskat)abstract The atomic structures of interstitial solid solutions O and N in V at relatively low concentrations O(N)/V = 1/16 or 1/8 are calculated using the Monte Carlo method. A combined model of long-range interaction between interstitial atoms is employed. The first 12 shells contain ab initio energies and the energies in shells 13–18 are calculated on the basis of a phenomenological model of deformation interaction. The ordered solid solutions are long-period structures with body-centered tetragonal crystal lattices and tetragonality c/a < 1.
14.	Blanter, M. S., et al. (författare) Strain-induced interaction of interstitials in IVA group hcp metals 2010 Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 71:10, s. 1416-1420 Tidskriftsartikel (refereegranskat)abstract Strain-induced (elastic) interactions of oxygen, nitrogen and carbon atoms in IVA group metals, alpha-Ti, Zr, and -Hf, are calculated in the framework of the microscopic Krivoglaz-Kanzaki-Khachaturyan theory. The experimental elastic constants, lattice spacing of the host metal, and concentration expansion coefficients are used as the input numerical parameters. The resulting interactions are stronger in a-Ti than in alpha-Zr and alpha-Hf. A comparative analysis of interactions in the hcp IVA group metals with those in bcc and fcc solid solutions reveals the crystal structure effect. In general, the strain-induced interactions of O, N, and C in hcp IVA group metals are weaker than in bcc solid solutions and are stronger than in fcc solid solutions.
15.	Dmitriev, V. V., et al. (författare) Ab initio based investigation of interstitial interactions and Snoek relaxation in Nb-O 2012 Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 73:2, s. 182-187 Tidskriftsartikel (refereegranskat)abstract Chemical and strain-induced effective pair interactions of interstitial oxygen atoms in bcc Nb have been determined in supercell first-principles calculations using Vienna ab initio simulation package (VASP). The strain-induced interactions are in reasonable agreement with those obtained earlier within a phenomenological microscopic Krivoglaz-Kanzaki-Khachaturyan model (KKKM). At the same time, the chemical interactions, which have been considered to be small in earlier theoretical considerations, turned out to be dominating at the first several coordination shells. The obtained interactions have been used in calculations of the concentration- and temperature-dependence of the internal friction Snoek peak. The theoretical results are found to be in good agreement with the existing experimental data.
16.	Dmitriev, V. V., et al. (författare) Interaction of interstitial nitrogen atoms in Nb : Ab initio calculations 2012 Ingår i: Bulletin of the Russian Academy of Sciences: Physics. - 1062-8738. ; 76:1, s. 1-6 Tidskriftsartikel (refereegranskat)abstract Ab initio calculations of pair nitrogen interstitials interaction in the first 12 coordination shells of a Nb crystal lattice are performed using the Vienna ab initio simulation package (VASP), and chemical and strain-induced contributions are analyzed. It is shown that rapidly decreasing chemical repulsion prevails in the nearest coordination shells, whereas strain-induced (elastic) interaction makes the main contribution in more distant shells.
17.	Razumovskiy, Vsevolod I., 1985-, et al. (författare) Effect of Alloying Elements and Impurities on Interface properties in Aluminum Alloys 2011 Ingår i: Physics of the solid state. - 1063-7834 .- 1090-6460. ; 53:11, s. 2189-2193 Tidskriftsartikel (refereegranskat)abstract The segregation energies of B, Si, P, Cr, Ni, Zr, and Mg on the special grain boundary (GB) Σ5 (210)[100] and on the open (210) surface of aluminum have been determined and the GB splitting energy has been calculated by the density functional theory methods. It has been shown that all elements listed above enrich the GB; for B, Si, P, Cr, Ni and Zr, Mg, interstitial and substitutional sites are preferred, respectively. The effect of alloying elements on the GB binding has been estimated using the parameter η equal to the change in the fracture work of the aluminum GB when adding alloying element atoms. From the viewpoint of strengthening the GB binding forces, Zr, Cr, Ni, and Mg are efficient, Si and B are neutral and phosphorus weakens GBs.
18.	Reddy, P. V. Sreenivasa, et al. (författare) Evidence for the antiferromagnetic ground state of Zr2TiAl : a first-principles study 2017 Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 29:26 Tidskriftsartikel (refereegranskat)abstract A detailed study on the ternary Zr-based intermetallic compound Zr2TiAl has been carried out using first-principles electronic structure calculations. From the total energy calculations, we find an antiferromagnetic L1(1)-like (AFM) phase with alternating ( 1 1 1) spin-up and spin-down layers to be a stable phase among some others with magnetic moment on Ti being 1.22 mu B. The calculated magnetic exchange interaction parameters of the Heisenberg Hamiltonian and subsequent Heisenberg Monte Carlo simulations confirm that this phase is the magnetic ground structure with Neel temperature between 30 and 100 K. The phonon dispersion relations further confirm the stability of the magnetic phase while the non-magnetic phase is found to have imaginary phonon modes and the same is also found from the calculated elastic constants. The magnetic moment of Ti is found to decrease under pressure eventually driving the system to the non-magnetic phase at around 46 GPa, where the phonon modes are found to be positive indicating stability of the non-magnetic phase. A continuous change in the band structure under compression leads to the corresponding change of the Fermi surface topology and electronic topological transitions (ETT) in both majority and minority spin cases, which are also evident from the calculated elastic constants and density of state calculations for the material under compression.
19.	Abrikosov, I. A., et al. (författare) Multiscale approach to theoretical simulations of materials for nuclear energy applications : Fe-Cr and Zr-based alloys 2013 Ingår i: Advances in materials for nuclear energy. - : Materials Research Society. - 9781605114910 ; , s. 3-14 Konferensbidrag (refereegranskat)abstract We review basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: Fe-Cr and Zr-based alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of Fe-Cr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For Zr-Nb alloys, we demonstrate how microscopic parameters calculated from first-principles can be used in higher-level models.
20.	Forslund, Axel, et al. (författare) Ab initio simulations of the surface free energy of TiN(001) 2021 Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:19 Tidskriftsartikel (refereegranskat)abstract The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around 78 meV angstrom(-2) at 0 K to 73 meV angstrom(-2) at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.
21.	Gorbatov, Oleg I., et al. (författare) Effect of composition on antiphase boundary energy in Ni3Al based alloys : Ab initio calculations 2016 Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 93:22 Tidskriftsartikel (refereegranskat)abstract The effect of composition on the antiphase boundary (APB) energy of Ni-based L1(2)-ordered alloys is investigated by ab initio calculations employing the coherent potential approximation. The calculated APB energies for the {111} and {001} planes reproduce experimental values of the APB energy. The APB energies for the nonstoichiometric gamma' phase increase with Al concentration and are in line with the experiment. The magnitude of the alloying effect on the APB energy correlates with the variation of the ordering energy of the alloy according to the alloying element's position in the 3d row. The elements from the left side of the 3d row increase the APB energy of the Ni-based L1(2)-ordered alloys, while the elements from the right side slightly affect it except Ni. The way to predict the effect of an addition on the {111} APB energy in a multicomponent alloy is discussed.
22.	Gorbatov, Oleg I., et al. (författare) Effect of magnetism on the solubility of 3d elements in BCC iron : Results of first-principle investigations 2013 Ingår i: Physics of metals and metallography. - 0031-918X .- 1555-6190. ; 114:8, s. 642-653 Tidskriftsartikel (refereegranskat)abstract The methods of quantum-mechanical simulation have been used to study alloys of bcc iron with 3d transition metals in the ferromagnetic and paramagnetic states. It has been shown that the main factor that determines the solubility of the 3d elements is their electronic structure. The energy of the solution, mixing, and effective interatomic interactions vary regularly depending on the position of the element in the Periodic Table and on the magnetic state of the matrix. In some cases, depending on the magnetic state, changes in these quantities lead to the violation of the Hume-Rothery rules that determine the solubility of substitutional elements in alloys. The results obtained help us to understand the microscopic mechanisms that determine the solubility of alloying elements and their effect on the phase stability and structural state of steels.
23.	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.
24.	Ponomareva, A. V., et al. (författare) Effect of multicomponent alloying with Ni, Mn and Mo on phase stability of bcc Fe-Cr alloys 2018 Ingår i: Acta Materialia. - : Acta Materialia Inc. - 1359-6454 .- 1873-2453. ; 150, s. 117-129 Tidskriftsartikel (refereegranskat)abstract Fe-Cr system attracts lot of attention in condensed matter physics due to its technological importance and extraordinary physics related to a non-trivial interplay between magnetic and chemical interactions. However, the effect of multicomponent alloying on the properties of Fe-Cr alloys are less studied. We have calculated the mixing enthalpy, magnetic moments, effective chemical, strain-induced and magnetic exchange interactions to investigate the alloying effect of Ni, Mn, Mo on the phase stability of the ferromagnetic bcc Fe−Cr system at zero K. We demonstrate that the alloying reduces the stability of Fe-Cr alloys and expands the region of spinodal decomposition. At the same time, the mixing enthalpy in ternary Fe100-≿-05Cr≿Ni05 alloys indicates a stability of solid solution phase up to 6 at. % Cr. In Fe100-≿-07Cr≿Ni05Mn01Mo01 alloys, we did not find any alloy composition that has negative enthalpy of formation. Analyzing magnetic and electronic properties of the alloys and investigating magnetic, chemical and strain-induced interactions in the studied systems, we provide physically transparent picture of the main factors leading to the destabilization of the Fe-Cr solid solutions by the multicomponent alloying with Ni, Mn, Mo.
25.	Ponomareva, A. V., et al. (författare) Influence of global magnetic state on chemical interactions in high-pressure high-temperature synthesis of B2 Fe2Si 2009 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:18 Tidskriftsartikel (refereegranskat)abstract We show that effective chemical interactions in an alloy can be tuned by its global magnetic state, which opens exciting possibilities for materials synthesis. Using first-principles theory we demonstrate that at pressure of 20 GPa and at high temperatures, the effective chemical interactions in paramagnetic Fe-Si system are strongly influenced by the magnetic disorder favoring a formation of cubic Fe2Si phase with B2 structure, which is not present in the alloy phase diagram. Our experiments confirm theoretical predictions, and the B2 Fe2Si alloy is synthesized from Fe-Si mixture using multianvil press.
26.	Pourovskii, L. V., et al. (författare) Antisite-defect-induced surface segregation in ordered NiPt alloy 2003 Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 90:2 Tidskriftsartikel (refereegranskat)abstract By means of first principles simulations we demonstrate that tiny deviations from stoichiometry in the bulk composition of the NiPt-L1(0) ordered alloy have a great impact on the atomic configuration of the (111) surface. We predict that at T=600 K the (111) surface of the Ni51Pt49 and Ni50Pt50 alloys corresponds to the (111) truncation of the bulk L1(0) ordered structure. However, the (111) surface of the nickel deficient Ni49Pt51 alloy is strongly enriched by Pt and should exhibit the pattern of the 2x2 structure. Such a drastic change in the segregation behavior is due to the presence of different antisite defects in the Ni- and Pt-rich alloys and is a manifestation of the so-called off-stoichiometric effect.
27.	Pourovskii, L. V., et al. (författare) Bulk ordering and surface segregation in Ni50Pt50 2001 Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 6403:3 Tidskriftsartikel (refereegranskat)abstract Interatomic interactions obtained from the effective screened generalized-perturbation method have been applied in Monte Carlo simulations to derive the bulk and surface-alloy configurations for Ni50Pt50 The calculated order-disorder transition temperature and short-range order parameters in the bulk compare well with experimental data. The surface-alloy compositions for the (111) and (110) facets above the ordering transition temperature are also found to be in a good agreement with experiments. It is demonstrated that the segregation profile at the (110) surface of NiPt is mainly caused by the unusually strong segregation of Pt into the second layer and the interlayer ordering due to large chemical nearest-neighbor interactions.
28.	Pourovskii, L. V., et al. (författare) Electronic properties and magnetism of iron at the Earth's inner core conditions 2013 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 87:11, s. 115130- Tidskriftsartikel (refereegranskat)abstract We employ state-of-the-art ab initio simulations within the dynamical mean-field theory to study three likely phases of iron (hcp, fcc, and bcc) at the Earth's core conditions. We demonstrate that the correction to the electronic free energy due to correlations can be significant for the relative stability of the phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi-liquid behavior, and where a Curie-Weiss behavior of the uniform susceptibility hints at a local magnetic moment still existing at 5800 K and 300 GPa. We predict that all three structures have sufficiently high magnetic susceptibility to stabilize the geodynamo.
29.	Pourovskii, L. V., et al. (författare) Fully relativistic spin-polarized exact muffin-tin-orbital method 2005 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:9 Tidskriftsartikel (refereegranskat)abstract The exact muffin-tin-orbital (EMTO) method is generalized for fully relativistic (FR) spin-polarized calculations. In the present implementation we solve self-consistently the four-component Dirac equation by using the Green's function formalism. Substitutional disorder is treated within the coherent potential approximation. To obtain accurate total energies we use the full-charge density technique. We apply the FR-EMTO Green's function method to calculate the ground-state properties of delta-Pu. We also calculate spin and orbital magnetic moments in random bcc, fcc, and hcp Fe-Co alloys, as well as in the B2 ordered and partially ordered phase.
30.	Poyurovskii, L. V., et al. (författare) Application of the Monte Carlo method to the problem of surface segregation simulation 2001 Ingår i: JETP Letters. - : Pleiades Publishing Ltd. - 0021-3640 .- 1090-6487. ; 73:8, s. 415-419 Tidskriftsartikel (refereegranskat)abstract A generalization of the Monte Carlo method to the case of grand canonical ensemble allowing the elimination of the problem of determination of the chemical potential of alloy components was proposed. The method is particularly convenient for the calculations of surface segregations because it excludes time-consuming calculation of the temperature-dependent bulk chemical potential mu (T). The new method was used for calculating segregations at the (100), (110), and (111) surfaces of the Ni50Pd50 alloy using the Ising model with ab initio effective interatomic interaction potentials.
31.	Razumovskiy, Vsevolod I., et al. (författare) Effect of thermal lattice expansion on the stacking fault energies of fcc Fe and Fe75Mn25 alloy 2016 Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 93:5 Tidskriftsartikel (refereegranskat)abstract Temperature dependent stacking fault energies in fcc Fe and the Fe75Mn25 random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave ( SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the Fe75Mn25 alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.
32.	Razumovskiy, Vsevolod I., et al. (författare) The effect of alloying elements on grain boundary and bulk cohesion in aluminum alloys : An ab initio study 2011 Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 65:10, s. 926-929 Tidskriftsartikel (refereegranskat)abstract The effect of B, Si, P, Cr, Ni, Zr and Mg impurities on cohesive properties of Al and its special grain boundary (GB) Sigma 5 (2 1 0) [1 0 0], as well as their segregation behavior at the GB and (2 1 0) surface are studied from first principles. Our analysis determines Ni to be the best and P the worst alloying elements in regard to the overall resistance to decohesion of Al alloys.
33.	Razumovskiy, Vsevolod I., 1985-, et al. (författare) The influence of alloying elements on grain boundary and bulk cohesion in aluminum all : Ab initio study 2012 Ingår i: Thermec 2011 Supplement. - : Trans Tech Publications Inc.. - 9783037853047 ; , s. 417-422 Konferensbidrag (refereegranskat)abstract The effect of B, Si, P, Cr, Ni, Zr and Mg on cohesive properties of Al and the specialgrain boundary (GB) Σ5 (210)[100], as well as their segregation behavior at the GB and the (210)surface are studied by first principles method. The analysis of these parameters allows us to singleout Ni as the best and phosphorus as the worst interatomic bond strengthening alloying elements.
34.	Reyes-Huamantinco, Andrei, et al. (författare) Stacking-fault energy and anti-Invar effect in Fe-Mn alloy from first principles 2012 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:6, s. 060201- Tidskriftsartikel (refereegranskat)abstract Based on state-of-the-art density-functional-theory methods we calculate the stacking-fault energy of the prototypical high-Mn steel Fe-22.5 at% Mn between 300 and 800 K. We estimate magnetic thermal excitations by considering longitudinal spin fluctuations. Our results demonstrate that the interplay between the magnetic excitations and the thermal lattice expansion is the main factor determining the anti-Invar effect, the hcp-fcc transformation temperature, and the stacking-fault energy, all of which are in good agreement with measurements.
35.	Ruban, Andrei, et al. (författare) Ab initio based investigation of the oxygen and nitrogen interstitial ordering in hcp Hf, Zr, and Ti : An ab initio study 2010 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:13, s. 134110- Tidskriftsartikel (refereegranskat)abstract We investigate the ordering of oxygen and nitrogen interstitials in hcp Zr, Hf, and Ti using the corresponding oxygen-oxygen and nitrogen-nitrogen interactions obtained in the state-of-the-art first-principles calculations. Two main contributions, chemical and strain induced, to the interstitial-interstitial interactions are obtained by different techniques. We find that there is the strong repulsion between interstitial atoms at the nearest-and next-nearest-neighbor coordination shells, which is solely determined by the chemical interaction determined on a fixed ideal lattice, while both contributions are important for more distant coordination shells. The Monte Carlo simulations reveal the existence of three stoichiometric compositions, MeI1/6, MeI1/3, and MeI1/2, for the ground-state structures of interstitials, having different ordering types. Our results for the structures of oxygen interstitials are in good agreement with existing experimental data for the Ti and Hf alloys. In the case of Zr-O interstitial alloys, we correctly predict the general type of ordering, although the detailed structure is at variance the experimental observations. The ordering transition temperatures in some cases are overestimated by a factor of 2. We also predict the ordering type of nitrogen interstitials in hcp Ti, Zr, and Hf, which are similar to those in the case of oxygen interstitials.
36.	Ruban, Andrei V., et al. (författare) Impact of magnetism on Fe under Earth's core conditions 2013 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:1, s. 014405- Tidskriftsartikel (refereegranskat)abstract Using a microscopic phenomenological model for longitudinal spin fluctuations (LSFs) based on density functional theory calculations, we demonstrate that under the Earth's core conditions (P approximate to 360 GPa, T approximate to 6000 K), Fe acquires substantial local magnetic moment, up to 1.3 mu(B), for different crystal structure modifications. We demonstrate that the LSFs produce a substantial effect on the magnetic and thermodynamic properties of iron, in particular, its equilibrium volume under solid Earth's core conditions.
37.	Shallcross, Sam, et al. (författare) An ab initio effective Hamiltonian for magnetism including longitudinal spin fluctuations 2005 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:10 Tidskriftsartikel (refereegranskat)abstract We discuss the use of the magnetic force theorem using different reference states upon which the perturbative approach is based. Using a fixed spin disordered local moment state one finds good Curie (or Neel) temperatures, and good energetics for planar spin spirals in the 3d magnets Fe, fcc Co, Ni, Mn, and Cr, though worse agreement for small theta spin spirals. On the other hand, the ferromagnetic reference state provides excellent energetics for small theta spin spirals in Fe, fcc Co, and Ni, and by extension magnon energies under the assumption of adiabacity. However, planar spin spiral energetics and transition temperatures show worse agreement. The reasons for this, and for the case of fcc Co where both approaches work very well, are discussed. We further provide an extension of the mapping of the quantum problem to include longitudinal fluctuations within force theorem based approaches, and discuss the role they will play in magnetic phase transitions. This construction is tested using planar spin spirals where q is fixed but the moment is allowed to relax. It is demonstrated that results from this approach and directly calculated ab initio values agree very well.
38.	Žguns, Pjotrs A., et al. (författare) Configurational energy landscape of oxygen vacancy migration at coherent (111) CeO2/C-type Gd2O3 interface: DFT and cluster expansion study Annan publikation (övrigt vetenskapligt/konstnärligt)
39.	Žguns, Pjotrs A. (författare) Configurational Thermodynamics of the CeO2-Gd2O3 System : A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces 2018 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract In this thesis, we study the configurational thermodynamics of Ce1-xGdxO2-x/2 x ≤ 1 or CGO. We apply a combined Density Functional Theory (DFT), cluster expansion and Monte Carlo (MC) approach in which the configurational energy of CGO is described by means of the Ising-type Hamiltonian. The interactions are determined by the cluster expansion of total energies calculated with DFT for a set of various cation–anion configurations. This allows one to perform on- the-fly calculations of the configurational energies in MC simulations of cation and anion ordering.The cluster interactions are essentially electrostatic and long-range, and describe the configurational energetics in the entire range of concentrations rather well.The phase diagram obtained in the MC simulations allows one to rationalise existing experimental data and is largely in agreement with that. We observe the phase separation into pure oxides, in equilibrium, below ca. 1000 K for x ≤ 1 (however it is kinetically hindered as diffusion of cations below ca. 1500 K is slow). We also observe the C-type oxygen–vacancy ordering in the x = 0.3–1 range below ca. 1200–3300 K (C phase), and a largely disordered, fluorite (F) phase in the x ≤ 0.3 range and above the ordering temperature.The DFT supercell calculations of the F and C phase configurations obtained in MC simulations allow us to study the effect of concentration and ordering on the lattice relaxations, lattice parameter and elastic moduli, providing insights into relation between preparation conditions, structure and properties.The bulk cluster interactions appear to be applicable also to coherent CeO2/C- type Gd2O3 interfaces, hence we examine a configurational energy landscape of the oxygen vacancy migration therein.This combined approach can be applied to study configurational thermodynamics of similar materials as well as the influence of configurational state on ionic conductivity and other properties.
40.	Zguns, Pjotrs A., et al. (författare) Phase diagram and oxygen-vacancy ordering in the CeO2-Gd2O3 system : a theoretical study 2018 Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 20:17, s. 11805-11818 Tidskriftsartikel (refereegranskat)abstract We present the phase diagram of Ce1-xGdxO2-x/2 (CGO), calculated by means of a combined Density Functional Theory (DFT), cluster expansion and lattice Monte Carlo approach. We show that this methodology gives reliable results for the whole range of concentrations (x x(Gd) <= 1). In the thermodynamic equilibrium, we observe two transitions: the onset of oxygen-vacancy (O-Va) ordering at ca. 1200-3300 K for concentrations x(Gd) = 0.3-1, and a phase separation into CeO2 and C-type Gd2O3 occurring below ca. 1000 K for all concentrations. We also model 'quenched' systems, with cations immobile below 1500 K, and observe that the presence of random-like cation configurations does not prevent C-type vacancy ordering. The obtained transition temperatures for Va ordering agree rather well with existing experimental data. We analyse the effect of vacancy ordering and composition on the lattice parameters and relaxation pattern of cations.
41.	Abdoshahi, Neda, et al. (författare) On the energetics of the cubic-to-hexagonal transformations in TiAl plus Mo alloys 2022 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 240 Tidskriftsartikel (refereegranskat)abstract Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transforma-tion energetics between bcc and hcp ordered and disordered phases in the TiAl+Mo model alloy system. By employing two complementary techniques I. VASP-SQS, and II. EMTO-CPA, we can show that chemical disorder flattens the energy landscape but may introduce a small barrier. Unlike that, the energetics of ordered phases are barrier-less and hence would suggest a spontaneous transformation. Finally, we show that Mo stabilizes the bcc phases, leading to a barrier-less transformation hcp -+ bcc for both ordered and disordered states when Mo content exceeds ti 12 at.%.
42.	Abdoshahi, Neda, et al. (författare) Structural stability and mechanical properties of TiAl plus Mo alloys : A comprehensive ab initio study 2021 Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221, s. 117427- Tidskriftsartikel (refereegranskat)abstract Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc /3o phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the /3o and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.
43.	Abrikosov, Igor, et al. (författare) Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys 2007 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 76:1, s. 014434- Tidskriftsartikel (refereegranskat)abstract We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.
44.	Alling, Björn, 1980-, et al. (författare) A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN 2011 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203- Tidskriftsartikel (refereegranskat)abstract We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
45.	Alling, Björn, et al. (författare) Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys 2009 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:13 Tidskriftsartikel (refereegranskat)abstract We demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.
46.	Alling, Björn, 1980-, et al. (författare) Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations 2007 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:045123 Tidskriftsartikel (refereegranskat)abstract Wedescribe an efficient first-principles method that can be used tocalculate mixing enthalpies of transition metal nitrides with B1 structureand substitutional disorder at the metal sublattice. The technique isbased on the density functional theory. The independent sublattice modelis suggested for the treatment of disorder-induced local lattice relaxationeffects. It supplements the description of the substitutional disorder withinthe coherent potential approximation. We demonstrate the excellent accuracy ofthe method by comparison with calculations performed by means ofthe projector augumented wave method on supercells constructed as specialquasirandom structures. At the same time, the efficiency of thetechnique allows for total energy calculations on a very finemesh of concentrations which enables a reliable calculation of thesecond concentration derivative of the alloy total energy. This isa first step towards first-principles predictions of concentrations and temperatureintervals where the alloy decomposition proceeds via the spinodal mechanism.We thus calculate electronic structure, lattice parameter, and mixing enthalpiesof the quasibinary alloy c-Ti1−xAlxN. The lattice parameter follows Vegard'slaw at low fractions of AlN but deviates increasingly withincreasing Al content. We show that the asymmetry of themixing enthalpy and its second concentration derivative is associated withsubstantial variations of the electronic structure with alloy composition. Thephase diagram is constructed within the mean-field approximation.
47.	Bergman, Anders, et al. (författare) Magnetic properties of Fe/Co(001) superlattices from first-principles theory 2006 Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:17 Tidskriftsartikel (refereegranskat)abstract The magnetic properties of Fe/Co(001) superlattices have been studied using fully-relativistic first-principles theories. The average magnetic moment shows a behavior similar to bulk Fe-Co alloys, i.e., an enhanced magnetic moment for low Co concentrations, as described by the Slater-Pauling curve. The maximum of the magnetization curve, however, is lowered and shifted towards the Fe-rich compositions. The increased average magnetic moment for the Fe-rich superlattices, compared to bulk Fe, is due to an enhancement of the Fe spin moment close to the Fe-Co interface. The orbital moments were found to be of the same size as in bulk. The effect of interface roughness on the magnetic properties was investigated, and it was found that-despite local fluctuations due to the varying coordination-the average magnetic moment is only slightly affected. From a mapping of first-principles interactions onto the screened generalized perturbation method, we calculate the temperatures for when Fe/Co superlattices break up into an alloy configuration. Furthermore, the tetragonal distortion of the superlattice structure was found to only have a minor effect on the magnetic moments. Also, the calculated easy axis of magnetization is in the film plane for all compositions studied. It lies along the [100] direction for Fe-rich superlattices and along the [110] direction for Co-rich compositions. The transition of the easy axis occurs around a Co concentration of 50%.
48.	Bligaard, T., et al. (författare) Pareto-optimal alloys 2003 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 83:22, s. 4527-4529 Tidskriftsartikel (refereegranskat)abstract Large databases that can be used in the search for new materials with specific properties remain an elusive goal in materials science. The problem is complicated by the fact that the optimal material for a given application is usually a compromise between a number of materials properties and the cost. In this letter we present a database consisting of the lattice parameters, bulk moduli, and heats of formation for over 64 000 ordered metallic alloys, which has been established by direct first-principles density-functional-theory calculations. Furthermore, we use a concept from economic theory, the Pareto-optimal set, to determine optimal alloy solutions for the compromise between low compressibility, high stability, and cost.
49.	Bochkarev, A. S., et al. (författare) A single-volume approach for vacancy formation thermodynamics calculations 2016 Ingår i: Europhysics letters. - : EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY. - 0295-5075 .- 1286-4854. ; 116:1 Tidskriftsartikel (refereegranskat)abstract The vacancy formation Gibbs free energy, enthalpy and entropy in fcc Al, Ag, Pd, Cu, and bcc Mo are determined by first-principles calculations using the quasi-harmonic approximation to account for vibrational contributions. We show that the Gibbs free energy can be determined with sufficient accuracy in a single-volume approach using the fixed equilibrium volume of the defect-free supercell. Although the partial contributions to the Gibbs free energy, namely, the formation enthalpy and entropy exhibit substantial errors when obtained directly in this approach, they can be computed from the Gibbs free energy using the proper thermodynamic relations. Compared to experimental data, the temperature dependence of the vacancy formation Gibbs free energy is accounted for at low temperatures, while it overestimates the measurements at high temperature, which is attributed to the neglect of anharmonic effects.
50.	Bodlos, R., et al. (författare) Ab initio investigation of the atomic volume, thermal expansion, and formation energy of WTi solid solutions 2021 Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:4 Tidskriftsartikel (refereegranskat)abstract WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Gruneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at.% Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Gruneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.

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