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  • 101. Peil, Oleg E.
    et al.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu-Mn systems2007In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 310, no 2, p. 1561-1563Article in journal (Refereed)
  • 102. Ponomareva, A. V.
    et al.
    Ruban, A. V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vekilova, O. Yu.
    Simak, S. I.
    Abrikosov, I. A.
    Effect of pressure on phase stability in Fe-Cr alloys2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, p. 094422-Article in journal (Refereed)
    Abstract [en]

    The effect of hydrostatic pressure on the phase stability of Fe-Cr alloys has been studied using ab initio methods. We show that while pressure decreases the tendency toward the phase separation in the paramagnetic state of bcc alloys, in the ferromagnetic state it reduces the alloy stability at low Cr concentration and vice versa, makes the solid solution more stable at higher concentrations. This behavior of the phase stability can be predicted from the deviation of the lattice parameter from Vegard's law in bcc Fe-Cr alloys. On the atomic level, the pressure effect can be explained by the suppression of the local magnetic moments on Cr atoms, which gives rise to a decrease of the Fe-Cr magnetic exchange interaction at the first coordination shell and, as a result, to the observed variation of the ordering tendency between the Fe and Cr atoms.

  • 103. Ponomareva, A. V.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dubrovinskaia, N.
    Dubrovinsky, L.
    Abrikosov, I. A.
    Influence of global magnetic state on chemical interactions in high-pressure high-temperature synthesis of B2 Fe2Si2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 18Article in journal (Refereed)
    Abstract [en]

    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.

  • 104. Ponomareva, A. V.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Leoben, Austria.
    Mukhamedov, B. O.
    Abrikosov, I. A.
    Effect of multicomponent alloying with Ni, Mn and Mo on phase stability of bcc Fe-Cr alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 150, p. 117-129Article in journal (Refereed)
    Abstract [en]

    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.

  • 105. Pourovskii, L. V.
    et al.
    Miyake, T.
    Simak, S. I.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dubrovinsky, L.
    Abrikosov, I. A.
    Electronic properties and magnetism of iron at the Earth's inner core conditions2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 11, p. 115130-Article in journal (Refereed)
    Abstract [en]

    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.

  • 106. Pourovskii, L. V.
    et al.
    Ruban, Andrei V.
    Abrikosov, I. A.
    Vekilov, Y. K.
    Johansson, Börje
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering.
    Bulk ordering and surface segregation in Ni50Pt502001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6403, no 3Article in journal (Refereed)
    Abstract [en]

    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.

  • 107. Pourovskii, L. V.
    et al.
    Ruban, Andrei V.
    Johansson, Börje
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering.
    Abrikosov, I. A.
    Antisite-defect-induced surface segregation in ordered NiPt alloy2003In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 90, no 2Article in journal (Refereed)
    Abstract [en]

    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.

  • 108. Pourovskii, L. V.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ebert, H.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Fully relativistic spin-polarized exact muffin-tin-orbital method2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 9Article in journal (Refereed)
    Abstract [en]

    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.

  • 109. Poyurovskii, L. V.
    et al.
    Ruban, Andrei V.
    Abrikosov, I. A.
    Vekilov, Y. K.
    Johansson, Börje
    Application of the Monte Carlo method to the problem of surface segregation simulation2001In: JETP Letters: Journal of Experimental And Theoretical Physics Letters, ISSN 0021-3640, E-ISSN 1090-6487, Vol. 73, no 8, p. 415-419Article in journal (Refereed)
    Abstract [en]

    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.

  • 110. Qiu, R.
    et al.
    Forslund, Axel
    KTH, School of Industrial Engineering and Management (ITM).
    Bäcke, O.
    Iyer, A. H. S.
    Sattari, M.
    Janssen, W.
    Manns, T.
    Kümmel, J.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Stiens, D.
    Andrén, H. -O
    Halvarsson, M.
    Effects of gas flow on detailed microstructure inhomogeneities in LPCVD TiAlN nanolamella coatings2020In: Materialia, E-ISSN 2589-1529, Vol. 9, article id 100546Article in journal (Refereed)
    Abstract [en]

    Depositing homogeneous TiAlN coatings with a high Al content on cutting tool inserts is a challenging task. In this work, high-Al cubic Ti1- xAlxN coatings (average x = 0.8) with periodic Ti(Al)N (x = 0.5) and Al(Ti)N (x = 0.9) nanolamellae structure were synthesized by low pressure chemical vapour deposition (LPCVD) with different gas flow velocities, and the detailed microstructure was investigated by electron microscopy and simulations. Using a high gas flow rate, the columnar TiAlN grains with regular periodic nanolamella structures disappeared, the coating became enriched in Ti and hexagonal AlN (h-AlN) formed in the coating. The high Ti content is suggested to be caused by the high gas flow rate that increases the mass transport of the reactants. However, this does not influence the Al-deposition much as it is mainly limited by the surface kinetics due to the relatively low deposition temperature. Density functional theory (DFT) modelling and electron microscopy showed that h-AlN tends to form on the Ti(Al)N phase with a specific crystallographic orientation relationship. The Ti enrichment due to high gas flow rate promotes the formation of h-AlN, which therefore deteriorates the nanolamella structure and causes the disappearance of the columnar TiAlN grains. Thus, by designing the CVD process conditions to avoid too high gas flow rates, homogenous TiAlN coatings with high Al content and nanolamella structures can be deposited, which should yield superior cutting performance.

  • 111.
    Rahaman, Moshiour
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Finite tempreture elastic properties of B2-FeCo alloy: a first-principles studyManuscript (preprint) (Other academic)
  • 112.
    Rahaman, Moshiour
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    First-principles study of atomic ordering in fcc Ni-Cr alloys2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 6, p. 064103-Article in journal (Refereed)
    Abstract [en]

    We investigate atomic ordering in fcc Ni-rich Ni-Cr alloys using first-principles techniques and statistical mechanics simulations based on the Ising Hamiltonian with effective cluster interactions computed by the screened generalized perturbation method (SGPM) and projector augmented wave (PAW) method. We demonstrate that effective chemical interactions in this system are quite sensitive to alloy composition and in fact to the specific configurational state. The chemical interactions for the high-temperature random state produce the atomic short-range order (SRO) with intensity maximum close to the (2/3 2/3 0) point of the reciprocal space in agreement with the previous first-principles investigation. A consistent with diffuse neutron scattering data maximum at the (1 1/2 0) position is obtained onlywhenwe take into consideration relatively small strain-induced interactions, which solves a long-standing inconsistency between theory and experiment in this system. The calculated transition temperature of order-disorder transition of Ni2Cr alloy, 880 K, is in good agreement with the experimental value of 863 K.

  • 113.
    Rahaman, Moshiour
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Razumovskiy, Vsevolod
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Temperature dependence of stacking-fault and anti-phase boundary energies in Al3Sc from ab initio calculations2013In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 93, no 25, p. 3423-3441Article in journal (Refereed)
    Abstract [en]

    Temperature dependence of intrinsic stacking-fault energies (SFE) and anti-phase boundary energies (APBE) of AlSc is investigated in first-principles calculations using the axial Ising model and supercell approach. The temperature effect has been taken into consideration by including the one-electron thermal excitations in the electronic structure calculations, and vibrational free energy in the harmonic approximation as well as by using temperature dependent lattice constant. The latter has been determined within the Debye-Gruneisen model, which reproduces well the experimental data. The APBE and SFE are found to be reduced by about 10% in the temperature interval from 0 to 1000 K. It is shown that the inclusion of the free energy of lattice vibrations in the harmonic approximation increases the SFE further by about 4%. We also find a substantial contribution from local lattice relaxations in the case of APBE for the (111) plane and SFE leading to their reduction by about 30%.

  • 114.
    Rahaman, Moshiour
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mookerjee, Abhijit
    Johansson, Börje
    Magnetic state effect upon the order-disorder phase transition in Fe-Co alloys: A first-principles study2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 5, p. 054202-Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of global magnetization on the effective cluster interactions and order-disorder phase transition in FexCo1-x alloys. The effective cluster interactions are obtained by the screened generalized perturbation method as it is implemented in the exact muffin-tin orbitals formalism within the coherent potential approximation. The ordering transition from the high-temperature disordered body-centered cubic alloy to the ordered B2 phase is determined by Monte Carlo simulations. The calculated transition temperatures are in good agreement with the available experimental data for the effective interactions, which correspond to the experimentally observed magnetization at the order-disorder phase transition.

  • 115. Razumovskii, I. M.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Razumovskiy, Vsevolod I.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Logunov, A. V.
    Larionov, V. N.
    Ospennikova, O. G.
    Poklad, V. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    New generation of Ni-based superalloys designed on the basis of first-principles calculations2008In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 497, no 1-2, p. 18-24Article in journal (Refereed)
    Abstract [en]

    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.

  • 116. Razumovskiy, V. I.
    et al.
    Reyes-Huamantinco, A.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Materialc Center Leoben Forschung GmbH, Austria.
    Spin Wave method for the total energy of the paramagnetic state: Practical applications2015In: Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015 , 2015, p. 757-764Conference paper (Refereed)
    Abstract [en]

    The Spin Wave method for the total energy of paramagnetic state represents an alternative to the existing methods for modeling magnetic disorder in Density Functional Theory calculations. One of the main advantages of the method is its applicability to defect calculations of pure metals and alloys. A combination of the SW-method and the supercell approach provides one with a convenient way of ab initio calculations of a number of thermodynamic and kinetic properties using methods based on Hamiltonian formalism like the PAW method as implemented in VASP. The Hamiltonian-based VASP-PAW-SW and Green’s function-based EMTO-DLM methods have been used to calculate basic thermodynamic properties of paramagnetic iron (including thermal lattice expansion, bulk modulus and stacking fault energy). The accuracy and efficiency of both methods has been assessed by comparing the obtained results to available experimental and theoretical data.

  • 117.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Isaev, E. I.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio calculations of elastic properties of Pt-Sc alloys2008In: Intermetallics (Barking), ISSN 0966-9795, E-ISSN 1879-0216, Vol. 16, no 8, p. 982-986Article in journal (Refereed)
    Abstract [en]

    Elastic properties of substitutionally disordered Pt-rich Pt-Sc alloys and L1(2)-ordered Pt3Sc compound are derived from the first-principles calculations based on the exact muffin-tin orbitals (EMTO) method. We demonstrate that these alloys should exhibit a ductile behavior, which combined with relatively high melting temperature and strong cohesive properties make them a very promising candidate for high-temperature applications.

  • 118.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Isaev, Eyvaz I.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    New Pt-Based Superalloy System Designed from First Principles2009In: Advanced Intermetallic-Based Alloys For Extreme Environment And Energy Applications / [ed] Palm M; Bewlay BP; He YH; Takeyama M; Wiezorek JMK, 2009, Vol. 1128, p. 233-238Conference paper (Refereed)
    Abstract [en]

    Pt-Sc alloys with the gamma-gamma' microstructure are proposed as a basis for a new generation of Pt-based superalloys for ultrahigh-temperature applications. This alloy system was identified on the basis of first-principles calculations. Here we discuss the prospects of the Pt-Sc alloy system on the basis of calculated elastic properties, phonon spectra, and defect formation energies.

  • 119.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab-initio calculations of kinetic properties in ZrC and TiC carbides2011Conference paper (Refereed)
    Abstract [en]

    Self-diffusion of the metal and carbon atoms in TiC and ZrC carbides is studied by first principles methods. Our calculations yield point defects energies, vacancy jump barriers and diffusion pre-factors in TiC and ZrC. The results are in reasonable agreement with the available experimental data and suggest that the self-diffusion mechanism for metal atoms in these carbides may involve nearest-neighbor vacancy pairs (one metal and one carbon vacancy).

  • 120.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Lozovoi, A. Yu.
    Razumovskii, I. M.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Analysis of the Alloying System in Ni-Base Superalloys Based on Ab Initio Study of Impurity Segregation to Ni Grain Boundary2011In: EURO SUPERALLOYS 2010 / [ed] Heilmaier, M, Trans Tech Publications Inc., 2011, Vol. 278, p. 192-197Conference paper (Refereed)
    Abstract [en]

    A new approach to the design of Ni-based polycrystalline superalloys is proposed. It is based on a concept that under given structural conditions, the performance of superalloys is determined by the strength of interatomic bonding both in the bulk and at grain boundaries of material. We characterize the former by the cohesive energy of the bulk alloy, whereas for the latter we employ the work of separation of a representative high angle grain boundary. On the basis of our first principle calculations we suggest Hf and Zr as “minor alloying additions” to Ni-based alloys. Re, on the other hand, appears to be of little importance in polycrystalline alloys.

  • 121.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. National University of Science and Technology MISIS, Moscow, Russian Federation .
    Razumovskii, I. M.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Butrim, V. N.
    Vekilov, Yu. Kh.
    The influence of alloying elements on grain boundary and bulk cohesion in aluminum all: Ab initio study2012In: Thermec 2011 Supplement, Trans Tech Publications Inc., 2012, Vol. 409, p. 417-422Conference paper (Refereed)
    Abstract [en]

    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.

  • 122. Razumovskiy, Vsevolod I.
    et al.
    Reyes-Huamantinco, Andrei
    Puschnig, Peter
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Mat Ctr Leoben Forsch GmbH, Austria.
    Effect of thermal lattice expansion on the stacking fault energies of fcc Fe and Fe75Mn25 alloy2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 5, article id 054111Article in journal (Refereed)
    Abstract [en]

    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.

  • 123.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Effect of temperature on the elastic anisotropy of pure Fe and Fe0.9Cr0.1 random alloy2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 20, article id 205504Article in journal (Refereed)
    Abstract [en]

    The elastic properties of pure iron and substitutionally disordered 10 at. % Cr Fe-Cr alloy areinvestigated as a function of temperature using first-principles electronic-structure calculations bythe exact muffin-tin orbitals method. The temperature effects on the elastic properties are includedvia the electronic, magnetic, and lattice expansion contributions. We show that the degree ofmagnetic order in both pure iron and Fe90Cr10 alloy mainly determines the dramatic change of theelastic anisotropy of these materials at elevated temperatures. The effect of lattice expansion isfound to be secondary but also very important for quantitative modeling.

  • 124.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    First-principles study of elastic properties of Cr- and Fe-rich Fe-Cr alloys2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, article id 024106Article in journal (Refereed)
    Abstract [en]

    Elastic properties of substitutionally disordered Cr- and Fe-rich Fe-Cr alloys are derived from first-principles calculations using the exact muffin-tin orbitals method and the coherent potential approximation. A peculiarity in the concentration dependence of elastic constants in Fe-rich alloys is demonstrated and related to a change in the Fermi surface topology. Our calculations predict high values for the elastic constants of Cr-rich Fe-Cr alloys, but at the same time show that these alloys could be rather brittle according to the Pugh criterion (the ratio between shear and bulk moduli is calculated to be greater than 0.5).

  • 125.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Vacancy-cluster mechanism of metal-atom diffusion in substoichiometric carbides2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 5, p. 054203-Article in journal (Refereed)
    Abstract [en]

    We find, using ab initio atomistic simulations of vacancy-mediated diffusion processes in TiC and ZrC, that a multivacancy self-diffusion mechanism is operative for metal-atom diffusion in substoichiometric carbides. It involves a special type of a stable point defect, a metal vacancy "dressed" in a shell of carbon vacancies. We show that this vacancy cluster is strongly bound and can propagate through the lattice without dissociating.

  • 126.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Razumovskii, I. M.
    Lozovoi, A. Y.
    Butrim, V. N.
    Vekilov, Yu. Kh.
    The effect of alloying elements on grain boundary and bulk cohesion in aluminum alloys: An ab initio study2011In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 65, no 10, p. 926-929Article in journal (Refereed)
    Abstract [en]

    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.

  • 127.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vekilov, Yu. Kh.
    Razumovskii, I. M.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Butrim, V. N.
    Mironenko, V. N.
    Effect of Alloying Elements and Impurities on Interface properties in Aluminum Alloys2011In: Physics of the solid state, ISSN 1063-7834, E-ISSN 1090-6460, Vol. 53, no 11, p. 2189-2193Article in journal (Refereed)
    Abstract [en]

    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.

  • 128.
    Razumovskiy, Vsevolod
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. Mat Ctr Leoben Forsch GmbH, A-8700 Leoben, Austria.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Dilner, David
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Korzhavyi, Pavel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Effect of carbon vacancies on thermodynamic properties of TiC-ZrC mixed carbides2014In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 46, p. 87-91Article in journal (Refereed)
    Abstract [en]

    Thermodynamic properties of a TiZrC mixed carbide system are investigated by first-principles methods within density functional theory. Carbon vacancies are found to have a significant contribution to the thermodynamics of TiZrC mixed carbides. The temperature effect on the thermodynamic properties of the system is calculated taking into consideration the corresponding electronic and vibrational thermal excitations.

  • 129. Reddy, P. V. Sreenivasa
    et al.
    Kanchana, V.
    Vaitheeswaran, G.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Materials Center Leoben Forschung GmbH, Austria.
    Christensen, N. E.
    Evidence for the antiferromagnetic ground state of Zr2TiAl: a first-principles study2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 26, article id 265801Article in journal (Refereed)
    Abstract [en]

    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.

  • 130.
    Renk, O.
    et al.
    Austrian Acad Sci, Erich Schmid Inst Mat Sci, Jahnstr 12, A-8700 Leoben, Austria..
    Enzinger, R.
    Graz Univ Technol, Inst Mat Phys, Petersgasse 16, A-8010 Graz, Austria..
    Gammer, C.
    Austrian Acad Sci, Erich Schmid Inst Mat Sci, Jahnstr 12, A-8700 Leoben, Austria..
    Scheiber, D.
    Mat Ctr Leoben Forsch GmbH, Roseggerstr 12, A-8700 Leoben, Austria..
    Oberdorfer, B.
    Graz Univ Technol, Inst Mat Phys, Petersgasse 16, A-8010 Graz, Austria.;Austrian Foundry Res Inst, Parkstr 21, A-8700 Leoben, Austria..
    Tkadletz, M.
    Univ Leoben, Chair Funct Mat & Mat Syst, Dept Mat Sci, Roseggerstr 12, A-8700 Leoben, Austria..
    Stark, A.
    Helmholtz Zentrum Geesthacht, Inst Mat Res, D-21502 Geesthacht, Germany..
    Sprengel, W.
    Graz Univ Technol, Inst Mat Phys, Petersgasse 16, A-8010 Graz, Austria..
    Pippan, R.
    Austrian Acad Sci, Erich Schmid Inst Mat Sci, Jahnstr 12, A-8700 Leoben, Austria..
    Eckert, J.
    Austrian Acad Sci, Erich Schmid Inst Mat Sci, Jahnstr 12, A-8700 Leoben, Austria.;Univ Leoben, Dept Mat Sci, Chair Mat Phys, Jahnstr 12, A-8700 Leoben, Austria..
    Romaner, L.
    Univ Leoben, Dept Mat Sci, Chair Phys Met & Metall Mat, Computat Mat Sci, Roseggerstr 12, A-8700 Leoben, Austria..
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures. Mat Ctr Leoben Forsch GmbH, Roseggerstr 12, A-8700 Leoben, Austria..
    Stainless steel reveals an anomaly in thermal expansion behavior of severely deformed materials2021In: Physical Review Materials, E-ISSN 2475-9953, Vol. 5, no 11, article id 113609Article in journal (Refereed)
    Abstract [en]

    Thermal expansion of materials is of fundamental practical relevance and arises from an interplay of several material properties. For nanocrystalline materials, accurate measurements of thermal expansion based on highprecision reference dilatometry allow inferring phenomena taking place at internal interfaces such as vacancy annihilation at grain boundaries. Here we report on measurements obtained for a severely deformed 316L austenitic steel, showing an anomaly in difference dilatometry curves which we attribute to the exceptionally high density of stacking faults. On the basis of ab intio simulations we report evidence that the peculiar magnetic state of the 316L austenitic steel causes stacking faults to expand more than the matrix. So far, the effect has only been observed for this particular austenitic steel but we expect that other magnetic materials could exhibit an even more pronounced anomaly.

  • 131. Reyes-Huamantinco, Andrei
    et al.
    Puschnig, Peter
    Ambrosch-Draxl, Claudia
    Peil, Oleg E.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Stacking-fault energy and anti-Invar effect in Fe-Mn alloy from first principles2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 6, p. 060201-Article in journal (Refereed)
    Abstract [en]

    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.

  • 132.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Structures.
    On segregation in multicomponent alloys: Surface segregation in austenite and FeCrCoNiMn alloys2021In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 187, article id 110080Article in journal (Refereed)
    Abstract [en]

    Segregation modelling in multicomponent random alloys within the single-site mean-field approximation is considered. As an example, the surface segregation in austenite Fe70Cr20Ni10 and equimolar Fe20Mn20Co20Cr20Ni20 random fcc alloy towards the (111) facet is calculated using ab initiomultiple scattering technique in the coherent potential approximation (CPA). The results show a very similar trend in both alloys: relatively strong surface segregation of Ni and strong anti-segregation of Cr. However, in the case of Fe70Cr20Ni10, the reversal of the surface enrichment from Ni to Fe is observed at 1750 K, while the surface of FeMnCoCrNi is Ni-rich up to 2500 K.

  • 133.
    Ruban, Andrei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Baykov, Vitaly
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dmitriev, V.V.
    Blanter, Mihail
    Ab initio based investigation of the oxygen and nitrogen interstitial ordering in hcp Hf, Zr, and Ti: An ab initio study2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 13, p. 134110-Article in journal (Refereed)
    Abstract [en]

    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.

  • 134.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Austria.
    First-principles modeling of the Invar effect in Fe65Ni35 by the spin-wave method2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 17, article id 174432Article in journal (Refereed)
    Abstract [en]

    Thermal lattice expansion of the Invar Fe0.65Ni0.35 alloy is investigated in first-principles calculations using the spin-wave method, which is generalized here for the ferromagnetic state with short-range order. It is shown that magnetic short-range order effects make a substantial contribution to the equilibrium lattice constant and cannot be neglected in the accurate ab initio modeling of the thermal expansion in Fe-Ni alloys. We also demonstrate that at high temperatures, close to and above the magnetic transition, magnetic entropy associated with transverse and longitudinal spin fluctuations yields a noticeable contribution to the equilibrium lattice constant. The obtained theoretical results for the temperature dependent lattice constant are in semiquantitative agreement with the experimental data apart from the region close the magnetic transition.

  • 135.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Self-trapping of carbon atoms in alpha '-Fe during the martensitic transformation: A qualitative picture from ab initio calculations2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 14, p. 144106-Article in journal (Refereed)
    Abstract [en]

    Strain-induced and chemical interactions of interstitial carbon atoms in bcc or alpha-Fe are obtained in first-principles calculations. Subsequent Monte Carlo simulations show that at low temperatures, carbon atoms prefer to occupy at least two different octahedral sublattices, which is due to quite strong attractive interactions of carbon atoms at the corresponding coordination shells. The direct total-energy calculations of one of the obtained ordered structures with composition Fe16C2, show that it is more stable than the predicted earlier structure with the same composition but carbon atoms occupying only one octahedral sublattice. This indicates that the long-existing thermodynamic mean-field theory of ordering of carbon in alpha-Fe assuming strong preference of carbon atoms to occupy only one octahedral sublattice is deficient. It is shown that the presence of carbon atoms only at one octahedral sublattice in the experimentally observed martensitic phase, alpha'-Fe, is a self-trapping effect. It occurs during a displacive martensitic transformation from gamma- to alpha-Fe, which kinematically transfers the carbon atoms from a single fcc octahedral sublattice to one of three octahedral sublattices, where they appear to be locked by a consequent tetragonal distortion minimizing elastic energy of the phase. The latter creates a strong preference for carbon atoms to be only at one already occupied octahedral sublattice preventing them from further distribution over the other sublattices.

  • 136.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Single-site mean-field approach to thermal defects in binary ordered alloys2014In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 94, no 11, p. 1192-1201Article in journal (Refereed)
    Abstract [en]

    A single-site mean-field approach for the concentration of thermal defects in a binary intermetallic AB compound is proposed, which is a modification of previously existing Wagner-Schottky-type models. A numerical investigation of the model is done for the case of thermal defects in NiAl.

  • 137. Ruban, Andrei V.
    Surface composition of ordered alloys: An off-stoichiometric effect2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 17Article in journal (Refereed)
    Abstract [en]

    I predict the existence of an off-stoichiometric effect in ordered alloys in the form of a distinct transition in the surface segregation behavior of alloy components near the bulk stoichiometric composition. It is caused by the discontinuity in the effective chemical potential at the stoichiometric composition. The effect is predicted to occur at the (111) surface of ordered Ni3Al and Pt3Fe alloys.

  • 138.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Austria.
    Thermal vacancies in random alloys in the single-site mean-field approximation2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 13, article id 134115Article in journal (Refereed)
    Abstract [en]

    A formalism for the vacancy formation energies in random alloys within the single-site mean-filed approximation, where vacancy-vacancy interaction is neglected, is outlined. It is shown that the alloy configurational entropy can substantially reduce the concentration of vacancies at high temperatures. The energetics of vacancies in random Cu0.5Ni0.5 alloy is considered as a numerical example illustrating the developed formalism. It is shown that the effective formation energy increases with temperature, however, in this particular system it is still below the mean value of the vacancy formation energy, which would correspond to the vacancy formation energy in a homogeneous model of a random alloy, such as given by the coherent potential approximation.

  • 139.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Configurational thermodynamics of alloys from first principles: effective cluster interactions2008In: Reports on progress in physics (Print), ISSN 0034-4885, E-ISSN 1361-6633, Vol. 71, no 4, p. 046501-Article, review/survey (Refereed)
    Abstract [en]

    Phase equilibria in alloys to a great extent are governed by the ordering behavior of alloy species. One of the important goals of alloy theory is therefore to be able to simulate these kinds of phenomena on the basis of first principles. Unfortunately, it is impossible, even with present day total energy software, to calculate entirely from first principles the changes in the internal energy caused by changes of the atomic configurations in systems with several thousand atoms at the rate required by statistical thermodynamics simulations. The time-honored solution to this problem that we shall review in this paper is to obtain the configurational energy needed in the simulations from an Ising-type Hamiltonian with so-called effective cluster interactions associated with specific changes in the local atomic configuration. Finding accurate and reliable effective cluster interactions, which take into consideration all relevant thermal excitations, on the basis of first-principles methods is a formidable task. However, it pays off by opening new exciting perspectives and possibilities for materials science as well as for physics itself. In this paper we outline the basic principles and methods for calculating effective cluster interactions in metallic alloys. Special attention is paid to the source of errors in different computational schemes. We briefly review first-principles methods concentrating on approximations used in density functional theory calculations, Green's function method and methods for random alloys based on the coherent potential approximation. We formulate criteria for the validity of the supercell approach in the calculations of properties of random alloys. The generalized perturbation method, which is an effective and accurate tool for obtaining cluster interactions, is described in more detail. Concentrating mostly on the methodological side we give only a few examples of applications to the real systems. In particular, we show that the ground state structure of Au3Pd alloys should be a complex long-period superstructure, which is neither DO22 nor DO23 as has been recently predicted.

  • 140.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Impact of magnetism on Fe under Earth's core conditions2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 1, p. 014405-Article in journal (Refereed)
    Abstract [en]

    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.

  • 141.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM).
    Dehghani, M.
    Atomic configuration and properties of austenitic steels at finite temperature: Effect of longitudinal spin fluctuations2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, article id 104111Article in journal (Refereed)
    Abstract [en]

    High-temperature atomic configurations of fcc Fe-Cr-Ni alloys with alloy composition close to austenitic steel are studied in statistical thermodynamic simulations with effective interactions obtained in ab initio calculations. The latter are done taking longitudinal spin fluctuations (LSF) into consideration within a quasiclassical phenomenological model. It is demonstrated that the magnetic state affects greatly the alloy properties, and in particular, it is shown that the LSF substantially modify the bonding and interatomic interactions of fcc Fe-Cr-Ni alloys even at ambient conditions. The calculated atomic short-range order is in reasonable agreement with existing experimental data for Fe0.56Cr0.21Ni0.23, which has strong preference for the (001)-type ordering between Ni and Cr atoms. A similar ordering tendency is found for the Fe0.75Cr0.17Ni0.08 alloy composition, which approximately corresponds to the widely used 304 and 316 austenitic steel grades.

  • 142.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Katsnelson, M. I.
    Olovsson, W.
    Simak, S. I.
    Abrikosov, I. A.
    Origin of magnetic frustrations in Fe-Ni Invar alloys2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 5Article in journal (Refereed)
    Abstract [en]

    Pair exchange parameters J(ij) of the classical Heisenberg Hamiltonian for magnetic interactions in the archetypical Invar system, face centered cubic (fcc) Fe-Ni alloys, are calculated from the first principles. The magnetic structure of Fe-Ni alloys in the region of volumes and electron concentrations related to the Invar effect is highly frustrated. However, the origin of such a frustration in concentrated alloys and in the pure fcc Fe are different. While in Fe it is due to the long-range oscillating J(ij), in alloys with high Ni concentration it is mainly the consequence of a huge dispersion of the nearest-neighbor exchange parameters, caused by the local environment effects.

  • 143.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Khmelevskyi, S.
    Mohn, P.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Temperature-induced longitudinal spin fluctuations in Fe and Ni2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 5Article in journal (Refereed)
    Abstract [en]

    We have developed an ab initio framework for calculating parameters of a high-temperature magnetic Hamiltonian. In the adiabatic approximation, this includes transverse and longitudinal magnetic excitation spectra on equal footing. The exchange interaction parameters of the Hamiltonian for bcc Fe and fcc Ni are determined from constrained local spin-density approximation calculations. Finite temperature magnetic properties of the resulting model Hamiltonian are then investigated by a Monte Carlo simulation technique. The calculated Curie temperatures and paramagnetic susceptibilities are in good agreement with experimental data for both metals. We demonstrate that the temperature-induced longitudinal spin fluctuations are important for high temperature properties such as susceptibility and magnetic specific heat.

  • 144.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Khmelevskyi, Sergii
    Mohn, Peter
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Magnetic state, magnetovolume effects, and atomic order in Fe65Ni35 Invar alloy: A first principles study2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 1Article in journal (Refereed)
    Abstract [en]

    We employ the locally self-consistent Green's function technique and exact muffin-tin orbital method to investigate magnetic state and ground state properties of Invar Fe65Ni35 alloy. We show that it is in a chemically disordered state, characterized by a relatively small amount of atomic short-range order, above the magnetic ordering temperature. We speculate that it should remain in this state below the Curie temperature upon applying usual heat treatment for the Invar alloys. The magnetic state at the experimental lattice spacing is shown to be sensitive to the type of approximation for the exchange-correlation functional: While the magnetic ground state is purely ferromagnetic in the generalized gradient approximation, there is a small amount of Fe atoms with magnetic moment antiferromagnetically aligned relative to the global magnetization in the local density approximations. The local spin-density approximation, however, fails to yield correctly the equilibrium lattice spacing, whereas the generalized gradient approximation reproduces it reasonably well. The anomalous spontaneous volume magnetostriction leading to the Invar effect is found to be approximate to 3%, in fair agreement with the experimental estimate of approximate to 2.2%.

  • 145.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles theory of magnetically driven anomalous ordering in bcc Fe-Cr alloys2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 9, p. 094436-Article in journal (Refereed)
    Abstract [en]

    Ab initio perturbation-theory techniques, such as the generalized perturbation method and magnetic force theorem, are used to determine the Heisenberg exchange interaction parameters and the effective cluster interactions in Fe-rich bcc Fe-Cr alloys in different magnetic states. We establish a direct connection between chemical and magnetic exchange interactions, as well as their dependence on the global magnetic state of the alloy. These findings have important implications for phase equilibria in magnetic alloys. In particular, we demonstrate that the experimentally reported concentration interval of anomalous ordering in Fe-Cr alloys is determined by the thermal history of the alloys through the value of global magnetization at the annealing temperature.

  • 146.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Materials Center Leoben Forschung GmbH, Leoben, Austria.
    Peil, O. E.
    Mat Ctr Leoben Forsch GmbH, A-8700 Leoben, Austria..
    Impact of thermal atomic displacements on the Curie temperature of 3d transition metals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 17, article id 174426Article in journal (Refereed)
    Abstract [en]

    It is demonstrated that thermally induced atomic displacements from ideal lattice positions can produce considerable effect on magnetic exchange interactions and, consequently, on the Curie temperature of Fe. Thermal lattice distortion should, therefore, be accounted for in quantitatively accurate theoretical modeling of the magnetic phase transition. At the same time, this effect seems to be not very important for magnetic exchange interactions and the Curie temperature of Co and Ni.

  • 147.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Popov, V. A.
    Portnoi, V. K.
    Bogdanov, V. I.
    First-principles study of point defects in Ni3Al2014In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 94, no 1, p. 20-34Article in journal (Refereed)
    Abstract [en]

    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.

  • 148.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Razumovskiy, Vsevolod I.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles based thermodynamic model of phase equilibria in bcc Fe-Cr alloys2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 17, p. 174111-Article in journal (Refereed)
    Abstract [en]

    A first-principles based thermodynamic model for magnetic alloys is applied to the calculation of the Fe-Cr phase diagram restricted by the bcc structure. The model includes magnetic, electronic, phonon, and local atomic relaxations contributions to the free-energy derived from ab initio calculations. Atomic short-range-order effects are found to be relatively small and they have been neglected in the calculations, assuming that alloys are in the completely random state. In contrast, we have taken into consideration magnetic short-range-order effects, which are found to be very important in particular above the Curie temperature. The calculated phase diagram is in reasonable agreement with the latest CALPHAD assessment. Our calculations reproduce a feature known as a Nishizawa horn for the Fe-rich high-temperature part of the phase diagram.

  • 149.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Razumovskiy, Vsevolod I.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Spin-wave method for the total energy of paramagnetic state2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, p. 174407-Article in journal (Refereed)
    Abstract [en]

    Spin-wave formalism provides a convenient alternative way of modeling the high-temperature paramagnetic state for a certain type of magnets within the framework of Hamiltonian-type electronic-structure methods. For Heisenberg systems, it is formally equivalent to the so-called disordered local moment approach, which is usually used in the methods based on the coherent potential approximation within the Green's function or multiple-scattering techniques. In this paper, we demonstrate that the spin-wave method has certain advantages when it comes to the calculation of forces and relaxations. It also allows one to take magnetic short-range-order effects into consideration. As examples of the application of the spin-wave method, we calculate the energy of the paramagnetic state in fcc Co and bcc Fe, the vacancy formation energy, elastic constants, and phonon spectrum in bcc paramagnetic Fe. We demonstrate that magnetic short-range-order effects play a crucial role in the mechanical stabilization of the bcc Fe at high temperature in the paramagnetic state.

  • 150.
    Ruban, Andrei V.
    et al.
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering.
    Shallcross, S.
    Simak, S. I.
    Skriver, H. L.
    Atomic and magnetic configurational energetics by the generalized perturbation method2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 12Article, review/survey (Refereed)
    Abstract [en]

    It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-Kohn-Rostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.

1234 101 - 150 of 183
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