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  • 1. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, s. 135-140Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels.

  • 2.
    Al-Zoubi, Noura
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Punkkinen, Marko Patrick John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Influence of Magnesium on hydrogenated ScAl1-xMgx alloys: a theoretical study2011Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, nr 10, s. 2848-2853Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.

  • 3.
    Al-Zoubi, Noura
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan..
    Schönecker, Stephan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Li, Xiaoqing
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Li, Wei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Elastic properties of 4d transition metal alloys: Values and trends2019Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, s. 273-280Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using the Exact Muffin-Tin Orbitals method within the Perdew-Burke-Ernzerhof exchange-correlation approximation for solids and solid surfaces (PBEso1), we study the single crystal elastic constants of 4d transition metals (atomic number Z between 39 and 47) and their binary alloys in the body centered cubic (bcc) and face centered cubic (fcc) structures. Alloys between the first neighbors Z(Z + 1) and between the second neighbors Z(Z + 2) are considered. The lattice constants, bulk moduli and elastic constants are found in good agreement with the available experimental and theoretical data. It is shown that the correlation between the relative tetragonal shear elastic constant C-fcc'-2C(bcc)' and the structural energy difference between the fcc and bcc lattices Delta E is superior to the previously considered models. For a given crystal structure, the equiatomic Z(Z + 2) alloys turn out to have similar structural and elastic properties as the pure elements with atomic number (Z + 1). Furthermore, alloys with composition Z(1-x)(Z + 2)(x) possess similar properties as Z(1-2x)(Z + 1)(2x). The present theoretical data on the structural and the elastic properties of 4d transition metal alloys provides consistent input for coarse scale modeling of material properties.

  • 4.
    Barkar, Thomas
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Effect of concentration dependent gradient energy coefficient on spinodal decomposition in the Fe-Cr system2018Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 143, s. 446-453Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Cahn–Hilliard equation is solved with thermodynamic and kinetic input, using the Thermo-Calc and DICTRA software packages rather than simpler models e.g. regular solution. A concentration dependent expression for the gradient energy coefficient is introduced and its effect on simulated decomposition is discussed. Simulations were carried out in 2D and 3D using the FiPy software package modified for non-linear problems.

  • 5. Bondarchuk, S. V.
    et al.
    Minaev, Boris F.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Two-dimensional honeycomb (A7) and zigzag sheet (ZS) type nitrogen monolayers. A first principles study of structural, electronic, spectral, and mechanical properties2017Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 133, s. 122-129Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two single-bonded 2D nitrogen allotropes of the honeycomb (A7) and zigzag sheet (ZS) topology have been calculated using density functional theory (DFT). The optical (vibrational, absorption, nuclear magnetic resonance), thermodynamic and elastic properties of the A7 and ZS sheets have been calculated for the first time. The band structure calculation have revealed a semiconducting nature of the ZS sheet with a direct gap of 1.246 eV, while the A7 monolayer behaves as an insulator with an indirect gap of 3.842 eV. Phonon dispersion calculations have justified these structures as vibrationally stable 2D materials. The IR spectroscopy completely failed in the characterization of the studied materials, while the Raman spectroscopy can be effectively applied for the experimental spectral identification. The absorption spectra demonstrate complete opacity of the A7 and ZS monolayers to the UV irradiation only above ca. 9 and 6 eV, respectively. Thus, the studied materials are expected to be transparent to the visible light. The electron arrangement of the nitrogen nuclei in the studied polynitrogen sheets is denser compared to the N2 molecule which follows from the calculation of the values of magnetic shielding tensors. The elastic constants reveal a robust mechanical stability of the studied 2D nitrogen allotropes. The Young moduli values are only twice as lower than that of the graphene molecule.

  • 6. Bonny, G.
    et al.
    Domain, C.
    Castin, N.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnenergiteknik.
    Malerba, L.
    The impact of alloying elements on the precipitation stability and kinetics in iron based alloys: An atomistic study2019Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 161, s. 309-320Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Iron based industrial steels typically contain a large number of alloying elements, even so-called low alloyed steels. Under irradiation, these alloying elements form clusters that have a detrimental impact of the mechanical properties of the steel. The stability and formation mechanisms of such clusters are presently not fully understood. Therefore, in this work, we study the thermal stability and formation kinetics of small solute clusters in the bcc Fe matrix. We use density functional theory (DFT) to characterize the binding energy of vacancy/solute clusters containing Cr, Mn, Ni, Cu, Si and P, thereby exploring >700 different configurations. The constructed DFT data base is used to fit a cluster expansion (CE) for the vacancy-FeCrMnNiCuSiP system. In turn, the obtained CE is applied in atomistic kinetic Monte Carlo simulations to study the effect of Mn, Ni, Cr, Si and P on the precipitation formation in the FeCu alloy. We conclude that the addition of Mn and Ni delay the precipitation kinetics by an order of magnitude. The additional alloying with traces of P/Si further delays the kinetics by an additional order of magnitude. We found that Si plays an essential role in the formation of spatially mixed MnNiCuSi cluster

  • 7.
    Borodulina, Svetlana
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Kulachenko, Artem
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Tjahjanto, Denny
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Constitutive modeling of a paper fiber in cyclic loading applications2015Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 110, s. 227-240Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The tensile response of dense fiber-based materials like paper or paperboard is mainly dependent of the properties of the fibers, which store most of the elastic energy. In this paper, we investigate the influence of geometrical and material parameters on the mechanical response of the pulp fibers used in paper manufacturing. We developed a three-dimensional finite element model of the fiber, which accounts for microfibril orientation of cellulose fibril, and the presence of lignin in the secondary cell wall. The results showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect, and is independent of the material properties of the fiber, as long as the deformations are elastic. Plastic strain accelerates the change in microfibril orientation and thus makes it material-dependent. The results also showed that the elastic modulus of the fiber has a non-linear dependency on microfibril angle, with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. Based on numerical results acquired from a 3D fiber model supported by available experimental evidence, we propose an anisotropic-kinematic hardening plasticity model for a fiber within a beam framework. The proposed fiber model is capable of reproducing the main features of the cyclic tensile response of a pulp fiber, such as stiffening due to changing microfibril angle. The constitutive model of the fiber was implemented in a finite-element model of the fiber network. By using the fiber network model, we estimated the level of strain that fiber segments accumulate before the typical failure strain of the entire network is reached.

  • 8. Breidi, A.
    et al.
    Fries, S. G.
    Palumbo, M.
    Ruban, Andrei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Materials Center Leoben Forschung GmbH, Leoben, Austria.
    First-principles modeling of energetic and mechanical properties of Ni-Cr, Ni-Re and Cr-Re random alloys2016Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 117, s. 45-53Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni-Re, Ni-Cr and Cr-Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C' in fcc Ni-Cr phase indicating mechanical instability in Cr-rich Ni-Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni-Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young's modulus vs. the intrinsic ductility curve in Ni-rich Ni-Re fcc phase.

  • 9.
    Castin, N.
    et al.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Pascuet, M. I.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina..
    Messina, Luca
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Domain, C.
    EDF R&D, Dept Mat & Mecan Composants, F-77250 Moret Sur Loing, France..
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Pasianot, R. C.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina.;Comis Nacl Energia Atom CNEA, Gcia Mat, Av Gral Paz 1499, RA-1650 San Martin, Argentina..
    Malerba, L.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Advanced atomistic models for radiation damage in Fe-based alloys: Contributions and future perspectives from artificial neural networks2018Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 148, s. 116-130Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Machine learning, and more specifically artificial neural networks (ANN), are powerful and flexible numerical tools that can lead to significant improvements in many materials modelling techniques. This paper provides a review of the efforts made so far to describe the effects of irradiation in Fe-based and W-based alloys, in a multiscale modelling framework. ANN were successfully used as innovative parametrization tools in these models, thereby greatly enhancing their physical accuracy and capability to accomplish increasingly challenging goals. In the provided examples, the main goal of ANN is to predict how the chemical complexity of local atomic configurations, and/or specific strain fields, influence the activation energy of selected thermally-activated events. This is most often a more efficient approach with respect to previous computationally heavy methods. In a future perspective, similar schemes can be potentially used to calculate other quantities than activation energies. They can thus transfer atomic-scale properties to higher-scale simulations, providing a proper bridging across scales, and hence contributing to the achievement of accurate and reliable multiscale models.

  • 10. Cimpoesu, Fanica
    et al.
    Frecus, Bogdan
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Oprea, Corneliu I.
    Panait, Petre
    Girtu, Mihai A.
    Disorder, exchange and magnetic anisotropy in the room-temperature molecular magnet V[TCNE](x) - A theoretical study2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, s. 320-328Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE](x) (TCNE = tetracyanoethylene, x similar to 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra- or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V[TCNE](x). The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D-c = -0.03 K) of the bulk as well as a sizeable random anisotropy (D-r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.

  • 11. Jha, Prakash Chandra
    et al.
    Seal, Prasenjit
    Sen, Sabyasachi
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi.
    Chakrabarti, Swapan
    Static and dynamic polarizabilities of (CdSe)(n) (n=1-16) clusters2008Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, nr 2, s. 728-732Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Density functional theoretical calculations have been performed on small and medium-sized (CdSe)(n) (n = 1-16) clusters in order to evaluate the variation in electric polarizability and anisotropy in polarizability with increase in the cluster size at both static and Nd:YAG laser frequencies. The electric polarizability values at static frequency show a rapid decrease up to heptamer only with the exception observed for (CdSe)(5) clusters where a sudden increase in the polarizability value is observed. An even-odd oscillating behavior is observed in the anisotropy values between the dimer and heptamer CdSe clusters. Apart from polarizability and anisotropy calculations, the variation in the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital and second difference of energies with the cluster size have also been calculated. just like the anisotropy values, an even-odd oscillating behavior is also observed in the second difference value for the smaller CdSe clusters.

  • 12.
    Jin, Lai -Zhe
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Sandström, Rolf
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Non-stationary creep simulation with a modified Armstrong-Frederick relation applied to copper2009Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 46, nr 2, s. 339-346Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A previously formulated model using back stress to handle non-stationary creep during power-law breakdown is further developed. In particular, the way to integrate the back stress is modified. Usually the Armstrong-Frederick relation has been applied, but it can give unphysical results in the sense that the back stress exceeds the tensile strength of the material. Such a problem can be solved by replacing the back stress term in this relation with the back stress deviator. The creep model is applied to copper canister in waste packages intended for encapsulating spent nuclear fuel. These waste packages will be placed in the bedrock at a depth of about 500 m as a final stage of disposal. During storage, radioactivity-induced thermal evolution raises temperature in repositories and water-saturation generates pressure directly on the copper canister. The thermally activated creep in copper canister occurs readily. To estimate the amount of creep deformation, a finite element model is set up to compute the evolution of creep deformation in copper canister. The creep model takes both stationary and non-stationary creep into account The computed maximum creep strain is shown to be 7.8% over 10 years, which should not cause failure since measured creep elongations are in the range of 15-40%.

  • 13.
    Jin, Lai-Zhe
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Sandström, Rolf
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Creep of copper canisters in power-law breakdown2008Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 43, nr 3, s. 403-416Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    According to the Swedish KBS-3 concept the spent nuclear fuel will be placed in copper canisters 500 m down in the bedrock. In thestorage, the canister will creep under conditions that are well inside the power-law breakdown regime. To prevent creep rupture fromoccurring that could cause leakage of nuclides, finite element models are set up to study the evolution of creep deformation in the coppercanisters. In this paper, two finite element models for the secondary creep are formulated. The first one is based on a fundamental climb–glide creep law valid over a wide range of temperatures. The second one is on the basis of a generalised Norton equation fitted to secondarycreep data of phosphorus doped pure copper. The creep deformation is shown to be much larger in the lid and the bottom of the canistersthan in the cylindrical wall. In the latter a stationary creep state is reached only after very long time (30000 years). Since the deformation inthe copper canister is restricted by a cast iron insert and stress concentrations are reduced with time, the total creep strain is limited.

  • 14. Kaewmaraya, T.
    et al.
    Ramzan, M.
    Sun, WeiWei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Sagynbaeva, M.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Atomistic study of promising catalyst and electrode material for memory capacitors: Platinum oxides2013Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 79, s. 804-810Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Platinum oxides have the technological importance as evidenced by numerous studies concentrating on their crystal structures to attain the clear atomistic understanding but the controversy exists between the experimental and theoretical studies. In our present study, we report the electronic and optical properties of crystalline PtO and PtO2 on the basis of Heyd-Scuseria-Ernzerhof (HSE06) functional within the framework of the density functional theory (DFT). We present the structural parameters, electronic and optical properties of several proposed structures of PtO and PtO2. We find that PtS-type structure of PtO and CaCl2-type structure of PtO2 are the most stable structures of these materials on the basis of hybrid functional and they appear to be semiconductors with band gap values of 0.87 eV and 1.85 eV, respectively. The mechanical stability of these structures is also confirmed by calculating the phonon band structures. The corresponding structural parameters are found in good agreement with experimental values. Furthermore, we present the bader charge analysis and optical properties of these phases.

  • 15. Kaewmaraya, Thanayut
    et al.
    De Sarkar, Abir
    Sa, Baisheng
    Sun, Z.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Department of Physics and Astronomy, Uppsala University.
    Strain-induced tunability of optical and photocatalytic properties of ZnO mono-layer nanosheet2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, s. 38-42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Strain-induced tunability of several properties of ZnO monolayer nanosheet has been systematically studied using density functional theory. The band gap of the sheet varies almost linearly with uniaxial strain, while it shows a parabola-like behavior under homogeneous biaxial strain. Tensile strain reduces ionicity of Zn-O bonds, while compressive strain increases it. This provides ample implications for the photocatalytic dissociation of water molecules and the scission of polar molecules on ZnO nanosheet. The dynamical stability of the sheet assessed by the calculation of its vibrational frequencies has shown the sheet to be unstable for 10% and 7.5% compressive biaxial homogeneous strain.

  • 16. Karkina, L. E.
    et al.
    Karkin, I. N.
    Kuznetsov, A. R.
    Razumov, I. K.
    Korzhavyi, Pavel A.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. RAS, Inst Met Phys, Russia.
    Gornostyrev, Yu. N.
    Solute-grain boundary interaction and segregation formation in Al: First principles calculations and molecular dynamics modeling2016Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 112, s. 18-26Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interaction between solute atoms (Mg, Si, Ti) and grain boundaries (GBs) of different types in Al are investigated using two approaches: first principles total energy calculations and large scale atomistic simulations. We have found that both deformation (size effect) and electronic (charge transfer) mechanisms play an important role in solute-GB interaction. The deformation and electronic contributions to GB segregation energy for the considered solutes have been analyzed in dependence on the impurity and the GB type. Mg and Si atoms are calculated to segregate to GBs, while Ti atoms to repel from, GBs in Al. For the case of a symmetric special-type GB the interaction is found to be short-ranged. For a general-type GB the range of GB-solute interaction is found to be considerably longer. A method to estimate the segregation capacity of a GB has been proposed, which takes into account the solute-solute interactions, and shown to be able to correctly describe the GB enrichment in alloying elements. The features of the segregation formation in fine-grained materials produced by severe plastic deformation are discussed.

  • 17. Kissavos, A. E.
    et al.
    Simak, S. I.
    Olsson, Pär
    Department of Neutron Research, Ångström Laboratory, Uppsala University.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Abrikosov, I. A.
    Total energy calculations for systems with magnetic and chemical disorder2006Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 35, nr 1, s. 1-5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The accuracy of the exact muffin-tin orbitals method combined with the coherent potential approximation (EMTO-CPA) for total energy calculations for systems with magnetic and chemical disorder, which is present simultaneously, is investigated. The mixing enthalpy of ordered, as well as disordered FeCo, FeNi, and FeCu equiatomic ferromagnetic alloys is calculated with the EMTO-CPA method and with the full-potential projector augmented wave (PAW) method. The results are compared and found to be in excellent agreement with each other. The EMTO-CPA method, in combination with disordered local moment model, is then applied to calculate the mixing enthalpy of the random paramagnetic face-centered cubic (fcc) FeCo alloy, as well as body-centered cubic (bcc) FeCr and FeV alloys over the whole concentration range. The results are compared with experimental data and a very good agreement is found again.

  • 18. Koci, L.
    et al.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Belonoshko, Anatoly B.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure2008Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, nr 2, s. 605-610Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    There are two major ways to perform molecular dynamics (MD) calculations, namely classical and ab initio MD. As ab initio techniques require considerably longer calculation times, it is of interest to compare the results of the two methods. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the calculations, instead of only a solid structure (one-phase), can have a substantial impact on the results obtained. In this work, comparisons have been made between classical and ab initio methods applied to one- and two-phase systems for the melting of Ne at high pressure. The temperatures needed to melt the classical one-phase system are somewhat higher compared to the two-phase temperatures, evaluated at the same pressure. Furthermore, there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions. At 150 GPa, the calculations in this work show a melting temperature approximately 1000 K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This suggests a possible agreement between a two-phase ab initio and classical two-phase melting curve.

  • 19.
    Korzhavyi, Pavel A.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
    Sandström, Rolf
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
    Monovacancy in copper: Trapping efficiency for hydrogen and oxygen impurities2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 84, s. 122-128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The structure and binding energy of vacancy-impurity complexes in copper are studied using first-principles calculations based on density functional theory. A single vacancy is found to be able to trap up to six hydrogen atoms which tend to be situated inside the vacancy at off-center positions (related to the octahedral interstitial positions of the ideal fcc lattice). The binding energy of an H atom dissolved in the Cu lattice (octahedral interstitial position) to a vacancy is calculated to be about 0.24 eV, practically independent of the number of H atoms already trapped by the vacancy, up to the saturation with 6 hydrogens. For an oxygen impurity in Cu, a monovacancy is shown to be a deep trap (with a binding energy of 0.95 eV). The position of a trapped O atom inside a vacancy is off-center, almost a half-way from the nearest octahedral interstitial to the vacancy center. Such a vacancy-O cluster is shown to be a deep trap for dissolved hydrogen (the calculated binding energy is 1.23 eV). The trapping results in the formation of an OH-group, where the H atom is situated near the vacancy center, and the O atom is displaced from the center along a 〈100〉 direction towards a nearby octahedral interstitial position. Further hydrogenation of the monovacancy-OH cluster is calculated to be energetically unfavourable. McNabb-Forster's equations are generalised to describe the competition between a deep hydrogen trap and a shallow one. It is demonstrated that the deep trap is almost fully filled, which explains why some of hydrogen is strongly bound and cannot be removed without vacuum treatment at elevated temperatures.

  • 20.
    Larsson, Henrik
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Combined probability distributions of random-walks: A new method to simulate diffusion processes2005Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 34, nr 3, s. 254-263Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two related methods to simulate diffusion processes are presented. Both are based on conceiving diffusion as a random-walk process. Several example simulations are presented: single phase diffusion couples, diffusion controlled growth and prediction of Kirkendall porosity. Comparisons with experimental results and simulation software based on established technique (DICTRA) show good agreement with results obtained from the presented models.

  • 21.
    Levämäki, Henrik
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Tian, Ye
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Ropo, M.
    An automated algorithm for reliable equation of state fitting of magnetic systems2019Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 156, s. 121-128Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In computational physics and materials science ground-state properties are often extracted from an equation of state fit to energy-volume data. Magnetic systems often have multiple magnetic phases present in the energy-volume data, which poses a challenge for the fitting approach because the results are sensitive to the selection of included fitting points. This is because practically all popular equation of state fitting functions, such as Murnaghan and Birch-Murnaghan, assume just one phase and therefore cannot correctly fit magnetic energy-volume data that contains multiple phases. When fitting magnetic energy-volume data it is therefore important to select the range of fitting points in such a way that only points from the one relevant phase are included. We present a simple algorithm that makes the point selection automatically. Selecting fitting points automatically removes human bias and should also be useful for large-scale projects where selecting all fitting points by hand is not feasible.

  • 22.
    Li, Ruihuan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Dalian University of Technology, China.
    Zhang, Pengbo
    Li, Xiaojie
    Ding, Jianhua
    Wang, Yuanyuan
    Zhao, Jijun
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Uppsala University, Sweden.
    Effects of Cr and W additions on the stability and migration of He in bcc Fe: A first-principles study2016Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 123, s. 85-92Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Density functional theory calculations have been performed to study the effects of alloying Cr and W on the stability and diffusivity of interstitial He impurity in body-centered cubic (bcc) Fe host lattice. The interaction between two close Cr/W atoms is repulsive. The relative stable position for an interstitial He remains the tetrahedral interstitial site in the presence Cr. Energetically, He prefers to locate far away from W inside Fe host lattice due to the strong repulsive interaction between He and W. On the other hand, the He migration barrier becomes lower in the presence of Cr (0.026 eV) and W(0.049 eV), as compared to 0.059 eV for pure Fe. Addition of Cr is benefit for He self-trapping, while W is against. The effective diffusivity of He decreases with increasing Cr and W concentrations. Moreover, the additions of Cr and W slightly hinder He being trapped by monovacancy.

  • 23.
    Li, Wei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Lu, Song
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Hu, Qing-Miao
    Mao, Huahai
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Termodynamisk modellering.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    The effect of Al on the 475 degrees C embrittlement of Fe-Cr alloys2013Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 74, s. 101-106Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aluminum addition to ferritic stainless steels was found to effectively suppress the deleterious 475 degrees C embrittlement resulting from the phase decomposition in concentrated Fe-Cr alloys. With the aim of revealing the mechanism behind this prosperous phenomenon, here we investigate the effect of Al on the interfacial energy and formation energy of Fe-Cr solid solutions. The interface between the decomposed Fe-rich alpha and Cr-rich alpha' phases carries a positive excess energy, which is of significant importance on determining the process of phase separation. Using ab initio alloy theory, we show that for the alpha-Fe70Cr20Al10/alpha'-Fe100-x-yCryAlx (0 <= x <= 10, 55 <= y <= 80) interface, the Al content (x) barely changes the interfacial energy. However, for the alpha-Fe100-x-yCryAlx/alpha'-Fe10Cr90 (0 <= x <= 10, 0 <= y <= 25) interface, the interfacial energy increases with Al content due to the variation of the formation energies of the Fe-Cr alloys upon Al alloying. Our ab initio results are supported by CALPHAD calculations, and suggest that the beneficial effect of Al on ferritic steels is mainly due to its thermodynamical effect on the alpha' phase.

  • 24. Li, Xiaoqing
    et al.
    Zhang, Chong
    Zhao, Jijun
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Mechanical properties and defective effects of bcc V-4Cr-4Ti and V-5Cr-5Ti alloys by first-principles simulations2011Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, nr 9, s. 2727-2731Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    V-(4-5) wt.% Cr-(4-5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V-4Cr-4Ti and the V-5Cr-5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.

  • 25.
    Li, Yunguo
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Sweden .
    Li, Yan-Ling
    Sun, Weiwei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Sweden .
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Sweden .
    Dynamic stability of the single-layer transition metal dichalcogenides2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, s. 206-212Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the quest for advanced semi-conductors, we have expanded our knowledge on a series of single-layer TMDs by calculating the electronic structure and lattice dynamic stability based on the first-principles density functional theory. The single layers of Mo and W dichalcogenides are found to be stable with P-6m2 symmetry. The reduction of dimension opens up and increases the bandgap. The charge transfer is found to decrease from sulfide to selenide and to telluride due to the decrease of electronegativity of chalcogen, which also induces the reduction of bandgap. The TA mode softening is found along Gamma-K direction and becomes more significant from sulfide to selenide and to telluride in the single-layer TMDs of Mo and W, which corresponds to the vibration of transition metal cations along y-axis. The single layers of Nb dichalcogenides are found to be instable with P-6m2 symmetry but stable with P-3m1 symmetry. It is also speculated that the interactions of cations mediated by electron-phonon coupling are accountable for the dynamic instability of the single-layer TMDs of Nb with P-6m2 symmetry. The unstable P-6m2 single-layer Nb dichalcogenides can transform to the stable P-3m1 structure during the exfoliation from the bulk, via the displacement of two anion layers of the sandwich structure.

  • 26.
    Liu, Peng
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    De Sarkar, Abir
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Shear strain induced indirect to direct transition in band gap in AlN monolayer nanosheet2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 86, s. 206-210Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic band structure of aluminum nitride (AlN) nanosheets under different kinds of strains has been investigated. Our first principles based studies show that the homogeneous biaxial strain is most effective as compared to uniaxial and shear strain in tuning the band gap. Large shear strain (>= 7.5%) and 10% uniaxial strain along the zig-zag direction induces an indirect-to-direct transition in band gap. The response of these 2 dimensional AlN nanosheets to strain is different from that of its 3 dimensional bulk phase counterparts to pressure. Quantum confinement effects govern the behavior of the former. The dynamical stability of AlN nanosheets have also been assessed under different modes of strains through phonon dispersion calculations. The AlN nanosheets are found to be dynamically stable under tensile strain and shear strain along zig-zag directions, while the sheets are found to be unstable under compressive strain.

  • 27. Loyola, C.
    et al.
    Davis, Sergio
    Universidad de Chile, Chile.
    Peralta, J.
    Gutiérrez, G.
    Onset of failure in argon by the effect of a shockwave: A molecular dynamics study2010Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 49, nr 3, s. 582-587Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Molecular dynamic simulations of shockwaves in solid argon were performed. The simulation cell contains 51,840 atoms at 5 K interacting by means of a pairwise potential. The shockwave itself was introduced explicitly in the simulation by a piston hitting the sample from one side of the simulation box, at speeds ranging from 1.2 to 1.3 times the speed of sound in solid argon at the chosen density. In order to characterize the sample in terms of both structural and dynamic properties, we determine the density and temperature profiles according the advance of the shockwave, evaluating, for different slabs, the pair-distribution function, coordination number as well as performing a common neighbor analysis for the atoms. Our simulations reproduce the experimental Hugoniot curve and show how the material is break due to rarefaction waves. The picture that emerges is that when the shockwave starts, a local melting is produced in a region of the sample. Then, as the shockwave travels through the sample, a high density disordered phase is identified. When the piston stops, a rarefaction wave develops, producing a large tensile stress, which finally causes the failure of the sample.

  • 28. Lu, S.
    et al.
    Hu, Qing-Miao
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Yang, R.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Rare earth elements in alpha-Ti: A first-principles investigation2009Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 46, nr 4, s. 1187-1191Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interaction energies between substitutional rare earth (RE) atoms, between RE and interstitial C, N, O, H atoms, as well as between RE and vacancies in alpha-Ti are calculated via first-principles density-functional theory with projector augmented-wave (PAW) pseudopotentials. The results show that the RE-vacancy and RE-RE interactions are attractive due to the weaker RE-Ti bond than the host Ti-Ti bond. All of the RE atoms investigated in this paper are repulsive to C and N, but attractive to H. RE-O interactions are repulsive for the light RE atoms, though the interactions are very weak for the heavy RE atoms. The mechanism underlying the interactions and their possible influence on the properties of Ti alloys are discussed.

  • 29.
    Lukinov, Tymofiy
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Rosengren, Anders
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Belonoshko, Anatoly B.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Impact of crystal lattice defects on crystal melting: A molecular dynamics study2013Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 79, s. 95-98Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An ideal infinite lattice, when subjected to heating does not melt at the thermodynamic melting temperature. Instead, it remains solid metastably up to considerably higher temperatures. This effect is called superheating. We performed a molecular dynamics simulation of Xenon using Lennard-Jonesium potential with several types of defects. We observed a superheating effect on the chosen model for several pressures and found that the presence of grain inclusion or grain interface eliminates the superheating effect.

  • 30.
    Lukinov, Tymofiy
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Rosengren, Anders
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Martoňák, Roman
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori. Comenius University in Bratislava, Slovakia .
    Belonoshko, Anatoly B.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    A metadynamics study of the fcc-bcc phase transition in Xenon at high pressure and temperature2015Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 107, s. 66-71, artikel-id 6523Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Abstract To theoretically find a stable solid phase is not a trivial task even at 0 K. The difficulties multiply at high temperature (T) because even more elaborate crystal structure prediction methods fail in the vicinity of the melting transition. Moreover, if the submelting phase is dynamically unstable at low T some methods cannot be applied at all. The method of metadynamics allows finding local minima of Gibbs free energy without additional simplifications. However, so far this method has been mainly used for study of pressure-induced solid-solid phase transitions and not in searching for T-induced ones. Here we study the applicability of the technique to the latter class of problems as well as to the approximate determination of the transition temperature. We apply the metadynamics method to study the solid-solid phase transition in Xe described by the Buckingham potential at high temperature and observe the fcc-bcc phase transition in a pressure-temperature range consistent with earlier results.

  • 31. Magyari-Kope, B.
    et al.
    Vitos, Levente
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Johansson, Börje
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Kollar, J.
    Model structure of perovskites: cubic-orthorhombic phase transition2002Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 25, nr 4, s. 615-621Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We extend the revised global parametrization method (revGPM) of orthorhombic perovskites with Pbnm symmetry to describe the changes in the lattice parameters and internal atomic coordinates within a broad range of structural distortions, including the vicinity of the orthorhombic-cubic phase transition. The applicability of the revGPM in the prediction of the evolution of the structural parameters under increasing hydrostatic pressure is demonstrated through test calculations performed for geophysically important perovskites. The present results are compared with the available theoretical and experimental data.

  • 32. Mahjoub, R.
    et al.
    Cao, Weimin
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialens processvetenskap.
    Khanna, R.
    Sahajwalla, V.
    An atomistic Monte Carlo investigation on the Solid-Liquid phasetransition in BCC iron: The role of boundary conditionsIngår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801Artikel i tidskrift (Övrigt vetenskapligt)
  • 33.
    Nygårds, Mikael
    et al.
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Gudmundson, Peter
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Three-dimensional periodic Voronoi grain models and micromechanical FE-simulations of a two-phase steel2002Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 24, nr 4, s. 513-519Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A three-dimensional model is proposed for modeling of microstructures. The model is based on the finite element method with periodic boundary conditions. The Voronoi algorithm is used to generate the geometrical model, which has a periodic grain structure that follows the original boundaries of the Voronoi cells. As an application, the model is used to model a two-phase ferrite/pearlite steel. It is shown that periodic cells with only five grains generate representative stress-strain curves.

  • 34. Ramzan, M.
    et al.
    Li, Yunguo
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Chimata, R.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Electronic, mechanical and optical properties of Y2O3 with hybrid density functional (HSE06)2013Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 71, s. 19-24Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, we have investigated the electronic, optical and mechanical properties of the Y2O3 crystal by first-principle calculations based on the density-functional theory. The generalized gradient approximation (GGA-PBE) and hybrid exchange-correlation functional (HSE06) are both used for comparative study. It is found that, the band gap of Y2O3 calculated by HSE06 method (6.0 eV) is in good agreement with the experimental band gap data (5.5 eV), and HSE06 gives better electronic structure description close to experiments. Then we calculate the elastic constants, and derive the corresponding properties i.e.; bulk, shear and Young's moduli, and Poisson's ratio. Our calculated elastic and mechanical properties correspond well with experimental data. Besides, we also obtain the equilibrium lattice and bulk modulus of yttria by fitting the Birch-Murnaghan equation of state. It is found that, the HSE06 well reproduce the experimental lattice parameters, equilibrium volume and bulk modulus of Y2O3. Based on the accurate described crystal and electronic structure and mechanical properties by HSE06 method, the optical properties of this material are also analyzed.

  • 35. Razumovskiy, V. I.
    et al.
    Popov, M. N.
    Ding, H.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Formation and interaction of point defects in group IVb transition metal carbides and nitrides2015Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 104, s. 147-154Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Point defects in the group IVb transition metal carbides and nitrides are investigated by means of density functional theory calculations. We focus on a description of a complex vacancy behavior of the sub-stoichiometric carbides and nitrides and find a strong tendency toward vacancy clustering in the carbides. Our results demonstrate that a special type of a stable point defect, a metal vacancy "dressed'' in a shell of six carbon vacancies can be a dominant type of metal-vacancy-containing defect in the carbon-deficient sub-stoichiometric carbides, whereas the simplest point defects appear to be dominant in the nitrogen-deficient sub-stoichiometric nitrides. We also show that such clusters are strongly bound in carbides and that temperature has a relatively small effect on the overall defect stability of group IVb transition metal carbides and nitrides.

  • 36.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Helium induced void and bubble formation in MgO2012Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 60, s. 53-58Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Trapping of helium in magnesium oxide vacancies and vacancy clusters is studied using empirical potential calculations. Based on the results, a rate-theory model is formulated in order to simulate the kinetics and long time dynamics of helium trapping. The simulations are in good agreement with helium annealing experiments in the literature. In particular, the tendency for helium to stabilise vacancy clusters is found to be strongly dependent on He concentration and temperature.

  • 37.
    Sa, Baisheng
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Zhou, Jian
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Sun, Zhimei
    First-principles investigations of electronic and mechanical properties for stable Ge2Sb2Te5 with van der Waals corrections2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 82, s. 66-69Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Te-Te weak van der Waals-type bonding plays an important role in Ge2Sb2Te5, a widely investigated phase-change material and a potential topological insulator. In this work, we have studied the electronic and mechanical properties of stable Ge2Sb2Te5 using ab initio calculations with the van der Waals corrections. The results show that the van der Waals corrections combined with hybrid functions improve the descriptions of the electronic structure of stable Ge2Sb2Te5. The band gap of similar to 0.5 eV in very good agreement with the experimental value for stable Ge2Sb2Te5 has been successfully reproduced. Furthermore, we have predicted the elastic constants and mechanical properties of stable trigonal Ge2Sb2Te5.

  • 38.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Slabanja, M.
    Holmestad, Randi
    Ab initio simulations of clustering and precipitation in Al-Mg-Si alloys2007Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 40, nr 3, s. 309-318Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A class of proposed coherent precipitate structures (Guinier-Preston zones) in the Al-Mg-Si alloy are investigated using first-prin- ciples density functional theory methods. The cluster expansion method is used to extract effective interaction parameters, providing the means for large scale energy calculations of alloy structures. The Mg1Si1 L1(0) structure and structures related to the Mg5Si6 beta '' phase are studied in more detail, and e.g., precipitate/matrix interface energies are presented. Using direct first-principles calculations we show that the former phase is dynamically unstable and thus must be stabilized by the surrounding Al matrix. Monte Carlo simulations and free-energy techniques are used to study the Al rich side of the phase diagram with the current CE parameters, and kinetic Monte Carlo simulations are used to study clustering in the disordered phase. The implications of our findings are discussed in the framework of classical nucleation theories, and we outline possible nucleation mechanisms.

  • 39.
    Skrinjar, Olle
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Larsson, Per-Lennart
    KTH, Tidigare Institutioner, Hållfasthetslära.
    On discrete element modelling of compaction of powders with size ratio2004Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 31, s. 131-146Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A numerical procedure, based on a discrete element method (DEM), for analysing cold compaction of spherical powders, is presented. In the numerical model, packing followed by compaction of up to 10,000 powder particles is simulated. Perfectly plastic material behaviour is assumed for convenience, but not for necessity, and as a result, local contacts between frictionless particles are described by a linear force–displacement relation. The numerical model is described in detail and its applicability to compaction problems of different complexity is discussed. Explicit results are presented for the case of isostatic compaction of spherical powders with size ratio and include applied pressure as function of densification as well as the evolution of contacts for individual particles. The present results are compared in detail with corresponding results from previous theoretical, experimental and numerical studies and the validity of fundamental assumptions made in previous theoretical analyses are discussed.

  • 40.
    Strandlund, Henrik
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    High-speed thermodynamic calculations for kinetic simulations2004Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Computational materials science, Vol. 29, nr 2, s. 187-194Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Simulation of phase transformations may be divided into two parts: the simulation of kinetics and the calculation of thermodynamic quantities. A number of softwares for thermodynamic calculations are available and it is thus convenient to use such software to obtain an accurate description of thermodynamic properties and couple them to a software simulating the kinetics. A major problem then is that the computational work to evaluate the thermodynamic quantities is often too heavy unless very simplified thermodynamic models are used. That problem may be solved by calculating the thermodynamic quantities in selected points and time steps only and apply an artificial neural network to obtain the values in all other points. This approach has successfully been applied to diffusion in alloys and has enabled us to incorporate thermodynamic data from the Thermo-Calc software package into simulations of diffusion in a very efficient way.

  • 41. Strandlund, Henrik
    et al.
    Ödqvist, Joakim
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    An effective mobility approach to solute drag in computer simulations of migrating grain boundaries2008Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, nr 2, s. 265-273Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An approach for taking solute drag into account in simulations of grain boundary migration in multicomponent systems in two or three dimensions has been developed. The new method is based on an effective mobility which is a function of the thermodynamic properties in the bulk and the grain boundary and the total driving force. Examples from phase-field simulations in Fe-Ni and Ni-Cr-Fe are presented.

  • 42.
    Sun, Weiwei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Hussain, Tanveer
    De Sarkar, Abir
    Maark, Tuhina Adit
    Luo, Wei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Improvement in the desorption of H-2 from the MgH2 (110) surface by means of doping and mechanical strain2014Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 86, s. 165-169Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this letter, density functional theory has been employed to investigate the release or desorption of hydrogen from the MgH2 (1 1 0) surface. To improve upon the energetics for hydrogen desorption from this system, the effects of strain and doping by Al, Si, Ti have been explored. Both of these two effects have been found to be effective. The strain applied along the X direction induces more prominent effects than along the Y direction. Regarding the doping, the system doped with Al gives the most noticeable effect. The Si doped system shows the least improvement while the Ti doped system lies in between as compared to the other two. The combination of doping and strain effects is found to be more efficacious.

  • 43. Terentyev, D.
    et al.
    Olsson, Pär
    Department Matériaux et Méchanique des Composants, EFD-R and D, Les Renardieres, France.
    Klaver, T. P. C.
    Malerba, L.
    On the migration and trapping of single self-interstitial atoms in dilute and concentrated Fe-Cr alloys: Atomistic study and comparison with resistivity recovery experiments2008Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 43, nr 4, s. 1183-1192Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Atomistic simulations have been used to characterize the mobility of single self interstitial atoms (SIAs) in Fe-Cr alloys of different compositions. Density functional theory (DFT) results concerning the interaction energies between an SIA and Cr atoms in different configurations and relative positions have been extended to concentrated alloys by using an empirical potential (EP). This EP, fitted to a set of DFT data so as to provide a correct heat of mixing and point defect features, has been further validated. Static calculations using the EP allowed the existence of configuration traps for SIAs to be identified and their strength and concentration to be assessed. Dynamic simulations were used to estimate the diffusion coefficient of the SIA, as well as to characterize the primary damage state after low-temperature electron irradiation (1-5 MeV), in Fe-Cr alloys of different Cr content. The results correlate with available experimental data and provide a qualitative and partially also quantitative explanation for the observed differences in the resistivity recovery stages in diluted and concentrated Fe-Cr alloys of different composition.

  • 44.
    Tian, Fu-Yang
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Chen, Nan-Xian
    Delczeg, Lorand
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Interlayer potentials for fcc (111) planes of Pd-Ag random alloys2012Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 63, s. 20-27Artikel i tidskrift (Refereegranskat)
    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.

  • 45.
    Tian, Fuyang
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. University of Science and Technology Beijing, China.
    Varga, Lajos Karoly
    Shen, Jiang
    Vitos, Levente
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik. c Wigner Research Centre for Physics, Hungary; Uppsala University, Sweden.
    Calculating elastic constants in high-entropy alloys using the coherent potential approximation: Current issues and errors2016Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 111, s. 350-358Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The new class of high-entropy alloys (HEAs) materials have shown interesting properties, such as high strength and good ductility. However, HEAs present a great challenge for conventional ab initio calculations and the few available theoretical predictions involve a large degree of uncertainty. An often adopted theoretical tool to study HEAs from first-principles is based on the exact muffin-tin orbitals (EMTO) method in combination with the coherent potentials approximation (CPA), which can handle both chemical and magnetic disorders. Here we assess the performance of EMTO-CPA method in describing the elastic properties of HEAs based on Co, Cr, Fe, Mn, and Ni. We carefully scrutinize the effect of numerical parameters and the impact of various magnetic states on the calculated properties. The theoretical results for the elastic moduli are compared to the available experimental values.

  • 46.
    Villanueva, Walter
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftssäkerhet.
    Grönhagen, Klara
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Amberg, Gustav
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Fysiokemisk strömningsmekanik.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Multicomponent and multiphase simulation of liquid-phase sintering2009Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 47, nr 2, s. 512-520Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Numerical simulation of liquid-phase sintering using a multicomponent and multiphase model is presented. The model consists of convective concentration and phase-field equations coupled with the Navier-Stokes equations with surface tension forces. The governing equations are nondimensionalized and an adaptive finite element method is utilized. An idealized phase diagram, surface energies, and typical dimensionless parameters are some input into the model. Important dynamics in liquid-phase sintering such as rapid wetting and motion of particles due to capillary forces are studied. Some factors that are known to significantly affect the dynamics of the sintering process such as contact angles and volume ratios are also investigated. In addition, numerical results on the motion of particles due to capillary forces were compared with an existing analytical model. Good agreement between numerical and analytical results is obtained within the validity of the analytical model.

  • 47. Vitos, Levente
    et al.
    Johansson, Börje
    Skriver, H. L.
    Kollar, J.
    Stability of fcc (110) transition and noble metal surfaces2000Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 17, nr 04-feb, s. 156-159Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have used density functional theory in conjunction with the full charge density linear muffin-tin orbitals method and a cluster expansion based on interatomic pair potentials to derive the energetics of vicinal surfaces on transition and noble metals, and we apply the results in a study of the stability of the fee (1 1 0) facet of these metals against the formation of 'rippled' surfaces consisting of vicinal facets with Miller indices (2 lambda + 1, 2 lambda, + 1,1) and (2 lambda + 1, 2 lambda + 1, (1) over bar).

  • 48.
    Vitos, Levente
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Larsson, K.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Hanson, A.
    Hogmark, S.
    An atomistic approach to the initiation mechanism of galling2006Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 37, nr 3, s. 193-197Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sliding metallic contacts are accompanied by a severe wear phenomenon named galling. This is manifested in adhesion followed by a substantial increase of friction and subsequent deterioration of the contact surfaces. Experiments on titanium- and vanadium-nitride coated surfaces slid against steels indicate that VN exhibits low friction and improved galling characteristics, clearly outperforming the widely used TiN coating. Using first principles simulations, based on the exact muffin-tin orbital and pseudopotential methods, we present an insight to the atomic level processes initiating galling at Ti(V)N-Fe interfaces. It is found that the equilibrium atomic-scale contacts between nitrides and Fe are adhesive. We demonstrate that the close-packed (001) VN-Fe interface exhibits a lower atomic-level roughness and a reduced atomic-friction coefficient than that in the case of TiN. These theoretical results indicate that VN is less prone to galling compared to TiN.

  • 49. Vitos, Levente
    et al.
    Skriver, H. L.
    Johansson, Börje
    Kollar, J.
    Application of the exact muffin-tin orbitals theory: the spherical cell approximation2000Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 18, nr 1, s. 24-38Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a self-consistent electronic structure calculation method based on the exact muffin-tin orbitals (EMTO) theory developed by O.K. Andersen, O. Jepsen and G. Krier [in: V. Kumar, O.K. Andersen, A. Mookerjee (Eds.), Lectures on Methods of Electronic Structure Calculations, Word Scientific, Singapore, 1994, pp. 63-124] and O.K. Andersen, C. Arcangeli, R.W. Tank, T. Saha-Dasgupta, G. Krier, O. Jepsen and I. Dasgupta [Mater. Res. Sec. Symp. Proc. 491 (1998) 3-34]. The EMTO theory can be considered as an improved screened KKR (Korringa-Kohn-Rostoker) method which is able to treat large overlapping potential spheres. Within the present implementation of the EMTO theory the one-electron equations are solved exactly using Green's function formalism, and Poisson's equation is solved within the spherical cell approximation (SCA). To demonstrate the accuracy of the SCA-EMTO method, test calculations have been carried out.

  • 50.
    Wang, Cao
    et al.
    Department of Materials and Environmental Chemistry,Stockholm University.
    Cheng, Laifei
    Northwest Polytechnical University, China.
    Zhao, Zhe
    Department of Materials and Environmental Chemistry, Stockholm University.
    FEM analysis of the temperature and stress distribution in spark plasma sintering: Modelling and experimental validation2010Ingår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 49, nr 2, s. 351-362Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A fully thermal-electrical-mechanical coupled and dynamic finite element model (FEM) is implemented to analyze of the temperature and stress distribution in spark plasma sintering (SPS) process. The real densification behaviour is also integrated by the moving mesh technique. The simulation studies were conducted using COMSOL and a range of die sizes, heating rates and uniaxial stresses were considered. The further validation experiments are implemented to validate the simulation results. The detailed microstructure investigations generally demonstrate that the temperature and stress profile obtained in present model are correct. But further development of complicated models is still needed for more precise prediction of sintering condition and microstructure development in SPS.

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