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  • 1.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab-initio description of mono-vacancies in metals and alloys2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method.

    First we investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three closepacked metals. Besides the local density approximation (LDA), we consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, we select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure.

    This investigation is followed by a performance comparison of the three common gradientlevel exchange-correlation functionals for metallic bulk, surface and vacancy systems. We find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.

    Last we show the vacancy formation energies of paramagnetic Fe-Cr-Ni alloys as a function of chemical composition. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest-neighbors are in line with the experimental observation. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system.

  • 2.
    Delczeg, Lorand
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Erna
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Density functional study of vacancies and surfaces in metals2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 4, p. 045006-Article in journal (Refereed)
    Abstract [en]

    We compare the performances of three common gradient-level exchange-correlation functionals for metallic bulk, surface and vacancy systems. We find that approximations which, by construction, give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities are shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.

  • 3.
    Delczeg, Lorand
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg-Czirjak, Erna
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Assessing common density functional approximations for the ab initio description of monovacancies in metals2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 20Article in journal (Refereed)
    Abstract [en]

    Using the exact muffin-tin orbitals method, we investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of monovacancies in three close-packed metals. Besides the local density approximation (LDA), we consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, we select aluminum, nickel, and copper, all of them adopting the face centered cubic crystallographic structure. Our results show that, compared to the recommended experimental values, LDA is be the most reliable approximation for the vacancy formation energies in these metals. However, taking into account also the performances of the functionals for the equation of state changes the final verdict in favor of the generalized gradient approximations.

  • 4.
    Delczeg, Lorand
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg-Czirjak, Erna
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tian, Fuyang
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Density functional theory of light actinides with substitutional point defects inface centered and body centered cubic descriptionsManuscript (preprint) (Other academic)
  • 5.
    Delczeg, Lorand
    et al.
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio description of mono-vacancies in austenitic stainless steelsManuscript (preprint) (Other academic)
    Abstract [en]

    Using a first-principles computational method, we have calculated the vacancy formation energies of paramagnetic Fe-Cr-Ni alloys as a function of chemical composition. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest-neighbors are in line with the experimental observation. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system.

  • 6.
    Delczeg, Lorand
    et al.
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio description of monovacancies in paramagnetic austenitic Fe-Cr-Ni alloys2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, p. 174101-Article in journal (Refereed)
    Abstract [en]

    Using first-principles alloy theory, we calculate the vacancy formation energies of paramagnetic face-centered-cubic (fcc) Fe-Cr-Ni alloys as a function of chemical composition. These alloys are well-known model systems for low carbon austenitic stainless steels. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest-neighbor lattice sites are in line with the experimental observations. In particular, Ni is found to decrease and Cr to increase the vacancy formation energy of the ternary system. The results are interpreted in terms of effective chemical potentials. The impact of vacancy on the local magnetic properties of austenitic steel alloys is also investigated.

  • 7. Delczeg-Czirjak, E. K.
    et al.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O.
    Monovacancy formation energies and Fermi surface topological transitions in Pd-Ag alloys2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 22, article id 224107Article in journal (Refereed)
    Abstract [en]

    Using first-principles mean-field alloy theory, we calculate the vacancy formation energies of the face-centered-cubic Pd-Ag alloys as a function of chemical composition. The effect of Fermi surface topological transition on the composition dependence of the vacancy formation energies is detectable and is consistent with what has previously been shown for the bulk properties of Pd1-xAgx.

  • 8.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Bergqvist, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Eriksson, O.
    Gercsi, Z.
    Nordblad, P.
    Szunyogh, L.
    Johansson, Börje
    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.
    Microscopic theory of magnetism in the magnetocaloric material Fe2P1-xTx (T = B and Si)2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 4, p. 045126-Article in journal (Refereed)
    Abstract [en]

    Landau phenomenological theory in combination with first-principles calculations was used to reveal the origin of the metamagnetic nature and the unusually strong dependence of the ordering temperature with doping of the Fe2P compound. We show that the magnetism of the two sublattices occupied by Fe atoms has an entwined codependency, which is strongly influenced by alloying. We furthermore demonstrate that a constrained disordered local moment approach combined with Monte Carlo simulations can only reproduce the experimental ordering temperatures in these technologically important prototype alloys for magnetocaloric refrigeration.

  • 9.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    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.
    Eriksson, O.
    Division for Materials Theory, Division of Physics and Materials Science, Uppsala University.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio study of structural and magnetic properties of Si-doped Fe(2)P2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 8, p. 085103-Article in journal (Refereed)
    Abstract [en]

    Ab initio electronic-structure methods are used to study the properties of Fe(2)P(1-x)Si(x) in ferromagnetic and paramagnetic states. The site preference and lattice relaxation are calculated with the projector augmented wave method as implemented in the Vienna ab initio simulation package. The paramagnetic state is modeled by the disordered local magnetic moment scheme, and the chemical and magnetic disorder is treated using the coherent potential approximation in combination with the exact muffin-tin orbital formalism. The calculated lattice parameters, atomic positions, and magnetic properties are in good agreement with the experimental and other theoretical results. In contrast to the observation, for the ferromagnetic state the body centered ortho-rhombic structure (bco, space group I (mm2) under bar) is predicted to have lower energy than the hexagonal structure (hex, space group P (6) over bar 2m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is calculated to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The phonon vibrational free energy, estimated from the theoretical equation of state, turns out to stabilize the hexagonal phase, whereas the electronic and magnetic entropies favor the low symmetry orthorhombic structure.

  • 10.
    Delczeg-Czirjak, Erna Krisztina
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ropo, Matti
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Department of Physics and Astronomy, University of Turku, Finland.
    Punkkinen, M.P.J
    Department of Physics and Astrononmy, University of Turku, Finland.
    Johansson, Börje
    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.
    Ab initio study of the elastic anomalies in Pd-Ag alloys2009In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 79Article in journal (Refereed)
    Abstract [en]

    Ab initio total-energy calculations, based on the exact muffin-tin orbital method, are used to determine the elastic properties of Pd1−xAgx random alloys in the face-centered-cubic crystallographic phase. The compositional disorder is treated within the coherent-potential approximation. The single crystal and polycrystalline elastic constants and the Debye temperature are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation in the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from a simple linear or parabolic trend. The complex electronic origin of these anomalies is demonstrated.

     

  • 11.
    Lorand, Delczeg
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Density functional study of mono-vancacies in metals and austenitic steel alloys2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method.

    First I investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three close  packed metals. Besides the local density approximation (LDA), I consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, I select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure.

    This investigation is followed by a performance comparison of the three common gradient level exchange-correlation functionals for metallic bulk, surface and vacancy systems. I find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.

    I also calculate the vacancy formation energies of paramagnetic face centered cubic (fcc) Fe-Cr-Ni alloys as a function of chemical composition. These alloys are well known model systems for low carbon austenitic stainless steels. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest neighbor lattice sites are in line with the experimental observations. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system. The results are interpreted in terms of effective chemical potentials. The impact of vacancy on the local magnetic properties of austenitic steel alloys is also investigated.

    I made a performance comparison of local density and generalized gradient level approach on substitutional defects in five light actinides. This is a complex test for high density calculations to check the weaknesses of the local density approximation against gradient level ones. I believe the existing other gradient level approaches fit our error bar in the obtained data and shows similar trends against the very limited number of experimental data. Based on our ab initio results, I predict that vacancies are more easily formed (more stable) in the fcc(bcc) lattice for U, Np and Pu and in the bcc(fcc) lattice for Th and Pa.

  • 12.
    Punkkinen, Marko Patrick John
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kokko, K.
    Levämäki, H.
    Ropo, M.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg-Czirjak, Erna Krisztina
    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.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Adhesion of the iron-chromium oxide interface from first-principles theory2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 49, p. 495501-Article in journal (Refereed)
    Abstract [en]

    We determine the interface energy and the work of separation of the Fe/Cr2O3 interface using first-principles density functional theory. Starting from different structures, we put forward a realistic interface model that is suitable to study the complex metal-oxide interaction. This model has the lowest formation energy and corresponds to an interface between Fe and oxygen terminated Cr2O3. The work of separation is calculated to be smaller than the intrinsic adhesion energy of pure Fe or Cr2O3, suggesting that stainless steel surfaces should preferentially break along the metal-oxide interface. The relative stabilities and magnetic interactions of the different interfaces are discussed. Next we introduce Cr atoms into the Fe matrix at different positions relative to the interface. We find that metallic Cr segregates very strongly to the (FeCr)/Cr2O3 interface, and increases the separation energy of the interface, making the adhesion of the oxide scale mechanically more stable. The Cr segregation is explained by the enthalpy of formation.

  • 13.
    Tian, Fu-Yang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Chen, Nan-Xian
    Delczeg, Lorand
    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.
    Interlayer potentials for fcc (111) planes of Pd-Ag random alloys2012In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 63, p. 20-27Article in journal (Refereed)
    Abstract [en]

    Using ab initio alloy theory in combination with the Chen-Mobius inversion method, we obtain the interaction potentials between layers of the face centered cubic (fcc) (111) plane family for elemental Pd and Ag metals and for Pd0.5Ag0.5 random alloy. Based on the interlayer potentials, we investigate properties related to the fcc (111) planes, such as the stacking fault energy, shear elastic constant, surface energy, surface relaxations, and phonon dispersion along the fcc [111] direction. The results agree well with those obtained by fully self-consistent ab initio calculations and also with the available experimental data, indicating that the properties related to the close-packed planes are well described by the interlayer potentials.

  • 14.
    Tian, Fuyang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg, Lorand
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Chen, Nanxian
    Varga, Lajos Karoly
    Shen, Jiang
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Structural stability of NiCoFeCrAlx high-entropy alloy from ab initio theory2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 8, p. 085128-Article in journal (Refereed)
    Abstract [en]

    First-principles alloy theory predicts that at room temperature the paramagnetic NiCoFeCrAlx high entropy alloys adopt the face centered cubic (fcc) structure for x less than or similar to 0.60 and the body centered cubic (bcc) structure for x greater than or similar to 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.

  • 15.
    Tian, Fuyang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Varga, Lajos Karoly
    Chen, Nanxian
    Delczeg, Lorand
    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.
    Ab initio investigation of high-entropy alloys of 3d elements2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 7, p. 075144-Article in journal (Refereed)
    Abstract [en]

    Single-phase high-entropy alloys are investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). Choosing the paramagnetic face-centered-cubic NiCoFeCr alloy as an example, we compare the CPA results with those obtained using the supercell (SC) method. For the equilibrium Wigner-Seitz radius and elastic properties, the single-site mean-field approximation turns out to yield consistent results with the SC approach. Next, we employ the EMTO-CPA method to study the bulk properties of CuNiCoFeCrTix (x = 0.0-0.5,1.0) and NiCoFeCrTi high-entropy alloys. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Theory predicts NiCoFeCr and CuNiCoFeCr to be more isotropic and less ductile than the Ti-containing single-phase alloys (CuNiCoFeCrTix with x greater than or similar to 0.4 and NiCoFeCrTi).

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