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  • 1. Cacucci, Arnaud
    et al.
    Tsiaoussis, Ioannis
    Potin, Valerie
    Imhoff, Luc
    Martin, Nicolas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    The interdependence of structural and electrical properties in TiO2/TiO/Ti periodic multilayers2013In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 11, p. 4215-4225Article in journal (Refereed)
    Abstract [en]

    Multilayered structures with 14-50 nm periods composed of titanium and two different titanium oxides, TiO and TiO2, were accurately produced by DC magnetron sputtering using the reactive gas pulsing process. The structure and composition of these periodic TiO2/TiO/Ti stacks were investigated by X-ray diffraction and transmission electronic microscopy techniques. Two crystalline phases, hexagonal close packed Ti and face centred cubic TiO, were identified in the metallic-rich sub-layers, whereas the oxygen-rich ones comprised a mixture of amorphous TiO2 and rutile phase. DC electrical resistivity rho measured for temperatures ranging from 300 to 500 K exhibited a metallic-like behaviour (rho(473K) = 1.05 x 10(-5) to 1.45 x 10(-6) Omega m) with a temperature coefficient of resistance ranging from 1.20 x 10(-3) K-1 for the highest period (Lambda = 50.0 nm) down to negative values close to -4.97 x 10(-4) K-1 for the smallest one (Lambda = 14.0 nm). A relationship between the dimensions of periodic layers and their collective electrical resistivity is proposed where the resistivity does not solely depend on the total thickness of the film, but also depends on the chemical composition and thickness of each sub-layer. Charge carrier mobility and concentration measured by the Hall effect were both influenced by the dimension of TiO2/TiO/Ti periods and the density of ionized scattering centres connected to the titanium concentration in the metallic sub-layers.

  • 2. Emmerlich, Jens
    et al.
    Music, Denis
    Eklund, Per
    Wilhelmsson, Ola
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Schneider, Jochen M.
    Högberg, Hans
    Hultman, Lars
    Thermal stability of Ti3SiC2 thin films2007In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 55, no 4, p. 1479-1488Article in journal (Refereed)
    Abstract [en]

    The thermal stability of Ti3SiC2(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to 1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti3SiC2 by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti3C2 slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti3C2 slabs into (1 1 1)-oriented TiC0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.

  • 3. Endrino, J L
    et al.
    Arhammar, C
    Gutierrez, A
    Gago, R
    Horwat, D
    Soriano, L
    Fox-Rabinovich, G
    Martin y Marero, D
    Guo, J
    Rubensson, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Andersson, Joakim
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Spectral evidence of spinodal decomposition, phase transformation and molecular nitrogen formation in supersaturated TiAlN films upon annealing2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 16, p. 6287-6296Article in journal (Refereed)
    Abstract [en]

    Thermal treatment of supersaturated Ti(1-x)Al(x)N films (x approximate to 0.67) with a dominant ternary cubic-phase were performed in the 700-1000 degrees C range. Grazing incidence X-ray diffraction (GIXRD) shows that, for annealing temperatures up to 800 degrees C, the film structure undergoes the formation of coherent cubic AlN (c-AlN) and TiN (c-TiN) nanocrystallites via spinodal decomposition and, at higher temperatures (>= 900 degrees C), GIXRD shows that the c-AlN phase transforms into the thermodynamically more stable hexagonal AIN (h-AlN). X-ray absorption near-edge structure (XANES) at the Ti K-edge is consistent with spinodal decomposition taking place at 800 degrees C, while Al K-edge and N K-edge XANES and X-ray emission data show the nucleation of the h-AlN phase at temperatures >800 degrees C, in agreement with the two-step decomposition process for rock-salt structured TiAlN, which was also supported by X-ray diffraction patterns and first-principle calculations. Further, the resonant inelastic X-ray scattering technique near the N K-edge revealed that N(2) is formed as a consequence of the phase transformation process.

  • 4.
    Furlan, Andrej
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Lu, Jun
    Hultman, Lars
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Control of crystallinity in sputtered Cr-Ti-C films2013In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 17, p. 6352-6361Article in journal (Refereed)
    Abstract [en]

    The influence of Ti content on crystallinity and bonding of Cr-Ti-C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr-C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrCx, and amorphous C (a-C). The addition of 10-20 at.% Ti leads to the crystallization of the amorphous CrCx and the formation of a metastable cubic (Cr1-xTix)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.

  • 5. Gebhardt, Thomas
    et al.
    Music, Denis
    Kossmann, Daniel
    Ekholm, Marcus
    Abrikosov, Igor A.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Schneider, Jochen M.
    Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 8, p. 3145-3155Article in journal (Refereed)
    Abstract [en]

    We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Young's modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Young's modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Young's modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Young's modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.

  • 6.
    Huang, He
    et al.
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden;Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China.
    Li, Xiaoqing
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Dong, Zhihua
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Li, Wei
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Huang, Shuo
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Meng, Daqiao
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China.
    Lai, Xinchun
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China.
    Liu, Tianwei
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China.
    Zhu, Shengfa
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden; Wigner Res Ctr Phys, Inst Solid State Phys & Opt, H-1525 Budapest, Hungary.
    Critical stress for twinning nucleation in CrCoNi-based medium and high entropy alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 149, p. 388-396Article in journal (Refereed)
    Abstract [en]

    The CrCoNi-based medium and high entropy alloys (MHEAs) have drawn much attention due to their exceptional mechanical properties at cryogenic temperatures. The twinning critical resolved shear stress (CRSS) is a fundamental parameter for evaluating the strength-ductility properties of MHEAs. Here we construct and apply an extended twinning nucleation Peierls-Nabarro (P-N) model to predict the twinning CRSSes of face-centered cubic (FCC) CrCoNi-based MHEAs. The order of the twinning CRSSes of the selected alloys is CrCoNi > CrCoNiMn > CrCoNiFe > CrCoNiFeMn and the values are 291, 277, 274 and 236 MPa, respectively. These theoretical predictions agree very well with the experimental twinning CRSSes of CrCoNi and CrCoNiFeMn accounting for 260 +/- 30 and 235 +/- 10 MPa, respectively and are perfectly consistent with the strength-ductility properties including yield stress, ultimate tensile stress and uniform elongation for fracture of the FCC CrCoNi-based MHEAs obtained at cryogenic temperatures. The present method offers a first-principle quantum-mechanical tool for optimizing and designing new MHEAs with exceptional mechanical properties.

  • 7.
    Isaeva, Leyla
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sundberg, Jill
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Mukherjee, Soham
    Indian Institute of Science.
    Pelliccione, Christopher J.
    Illinois Institute of Technology.
    Lindblad, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Segre, Carlo U.
    Illinois Institute of Technology.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sarma, D.D.
    Indian Institute of Science.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Amorphous W-S-N thin films: The atomic structure behind ultra-low friction2015In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 82, p. 84-93Article in journal (Refereed)
  • 8.
    Isaeva, Leyla
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sundberg, Jill
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Mukherjee, Soham
    Indian Institute of Science.
    Pelliccione, Christopher J.
    Illinois Institute of Technology.
    Lindblad, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Segre, Carlos U
    Illinois Institute of Technology.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sarma, D. D.
    Indian Institute of Science.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Amorphous W-S-N thin films: the atomic structure behind ultra-low friction2015In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 82, p. 84-93Article in journal (Refereed)
    Abstract [en]

    Amorphous W–S–N in the form of thin films has been identified experimentally as an ultra-low friction material, enabling easy sliding by the formation of a WS2 tribofilm. However, the atomic-level structure and bonding arrangements in amorphous W–S–N, which give such optimum conditions for WS2 formation and ultra-low friction, are not known. In this study, amorphous thin films with up to 37 at.% N are deposited, and experimental as well as state-of-the-art ab initio techniques are employed to reveal the complex structure of W–S–N at the atomic level. Excellent agreement between experimental and calculated coordination numbers and bond distances is demonstrated. Furthermore, the simulated structures are found to contain N bonded in molecular form, i.e. N2, which is experimentally confirmed by near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy analysis. Such N2 units are located in cages in the material, where they are coordinated mainly by S atoms. Thus this ultra-low friction material is shown to be a complex amorphous network of W, S and N atoms, with easy access to W and S for continuous formation of WS2 in the contact region, and with the possibility of swift removal of excess nitrogen present as N2 molecules.

  • 9.
    Ji, Zong-Wei
    et al.
    Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, 72 Wenhua Rd, Shenyang 110016, Liaoning, Peoples R China.;Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Lu, Song
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Hu, Qing-miao
    Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, 72 Wenhua Rd, Shenyang 110016, Liaoning, Peoples R China..
    Kim, Dongyoo
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Yang, Rui
    Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, 72 Wenhua Rd, Shenyang 110016, Liaoning, Peoples R China..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Mapping deformation mechanisms in lamellar titanium aluminide2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 144, p. 835-843Article in journal (Refereed)
    Abstract [en]

    Breakdown of Schmid's law is a long-standing problem for exploring the orientation-dependent deformation mechanism in intermetallics. The lack of atomic-level understanding of the selection rules for the plastic deformation modes has seriously limited designing strong and ductile intermetallics for high-temperature applications. Here we put forward a transparent model solely based on first principles simulations for mapping the deformation modes in gamma-TiAl polysynthetic twinned alloys. The model bridges intrinsic energy barriers and different deformation mechanisms and beautifully resolves the complexity of the observed orientation-dependent deformation mechanisms. Using the model, one can elegantly reveal the atomic-level mechanisms behind the unique channeled flow phenomenon in lamellar TiAl alloys.

  • 10.
    Kadas, Krisztina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Andersson, Matilda
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Holmström, Erik
    Wende, Heiko
    Karis, Olof
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Surface and Interface Science.
    Urbonaite, Sigita
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Butorin, Sergei M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Soft X-Ray Physics.
    Nikitenko, Sergey
    Kvashnina, Kristina O.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Structural properties of amorphous metal carbides: Theory and experiment2012In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 12, p. 4720-4728Article in journal (Refereed)
    Abstract [en]

    By means of theoretical modeling and experimental synthesis and characterization, we investigate the structural properties of amorphous Zr-Si-C. Two chemical compositions are selected: Zr0.31Si0.29C0.40 and Zr0.60Si0.33C0.07. Amorphous structures are generated in the theoretical part of our work by the stochastic quenching (SQ) method, and detailed comparison is made regarding the structure and density of the experimentally synthesized films. These films are analyzed experimentally using X-ray absorption spectroscopy, transmission electron microscopy and X-ray diffraction. Our results demonstrate a remarkable agreement between theory and experiment concerning bond distances and atomic coordination of this complex amorphous metal carbide. The demonstrated power of the SQ method opens up avenues for theoretical predictions of amorphous materials in general.

  • 11. Lauridsen, J.
    et al.
    Eklund, P.
    Jensen, J.
    Ljungcrantz, H.
    Öberg, A.
    Lewin, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Flink, A.
    Högberg, H.
    Hultman, L.
    Microstructure evolution of Ti-Si-C-Ag nanocomposite coatings deposited by DC magnetron sputtering2010In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 58, no 20, p. 6592-6599Article in journal (Refereed)
    Abstract [en]

    Nanocomposite coatings consisting of Ag and TiCx (x < 1) crystallites in a matrix of amorphous SiC were deposited by high-rate magnetron sputtering from Ti-Si-C-Ag compound targets. Different target compositions were used to achieve coatings with a Si content of similar to 13 at.%, while varying the C/Ti ratio and Ag content. Electron microscopy, helium ion microscopy, X-ray photoelectron spectroscopy and X-ray diffraction were employed to trace Ag segregation during deposition and possible decomposition of amorphous SiC. Eutectic interaction between Ag and Si is observed, and the Ag forms threading grains which coarsen with increased coating thickness. The coatings can be tailored for conductivity horizontally or vertically by controlling the shape and distribution of the Ag precipitates. Coatings were fabricated with hardness in the range 10-18 GPa and resistivity in the range 77-142 mu Omega cm.

  • 12.
    Lee, Ja Kyung
    et al.
    Sungkyunkwan Univ, Inst Basic Sci, IBS Ctr Integrated Nanostruct Phys CINAP, Suwon 16419, South Korea.;Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea..
    Park, Bumsu
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea..
    Song, Kyung
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea.;KIMS, Mat Modeling & Characterizat Dept, Chang Won 51508, South Korea..
    Jung, Woo Young
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea..
    Tyutyunnikov, Dmitry
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Yang, Tiannan
    Penn State Univ, Dept Mat Sci & Engn, University Pk, PA 16802 USA..
    Koch, Christoph T.
    Humboldt Univ, Dept Phys, D-12489 Berlin, Germany..
    Park, Chan Gyung
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea..
    van Aken, Peter A.
    Max Planck Inst Solid State Res, Stuttgart Ctr Elect Microscopy, Heisenbergstr 1, D-70569 Stuttgart, Germany..
    Kim, Young-Min
    Sungkyunkwan Univ, Inst Basic Sci, IBS Ctr Integrated Nanostruct Phys CINAP, Suwon 16419, South Korea.;Sungkyunkwan Univ, Dept Energy Sci, Suwon 16419, South Korea..
    Kim, Jong Kyu
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea..
    Bang, Junhyeok
    KBSI, Spin Engn Phys Team, Daejeon 305806, South Korea..
    Chen, Long-Qing
    Penn State Univ, Dept Mat Sci & Engn, University Pk, PA 16802 USA..
    Oh, Sang Ho
    Pohang Univ Sci & Technol POSTECH, Dept Mat Sci & Engn, Pohang 37673, South Korea.;Sungkyunkwan Univ, Dept Energy Sci, Suwon 16419, South Korea..
    Strain-induced indium clustering in non-polar a-plane InGaN quantum wells2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 145, p. 109-122Article in journal (Refereed)
    Abstract [en]

    In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic, clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.

  • 13.
    Li, Xiaoqing
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Schönecker, Stephan
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    First-principles prediction of the stacking fault energy of gold at finite temperature2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 135, p. 88-95Article in journal (Refereed)
    Abstract [en]

    The intrinsic stacking fault energy (ISFE) gamma is a material parameter fundamental to the discussion of plastic deformation mechanisms in metals. Here, we scrutinize the temperature dependence of the ISFE of Au through accurate first-principles derived Helmholtz free energies employing both the super cell approach and the axial Ising model (AIM). A significant decrease of the ISFE with temperature, -(36-39) % from 0 to 890 K depending on the treatment of thermal expansion, is revealed, which matches the estimate based on the experimental temperature coefficient d gamma/dT closely. We make evident that this decrease predominantly originates from the excess vibrational entropy at the stacking fault layer, although the contribution arising from the static lattice expansion compensates it by approximately 60%. Electronic excitations are found to be of minor importance for the ISFE change with temperature. We show that the Debye model in combination with the AIM captures the correct sign but significantly underestimates the magnitude of the vibrational contribution to gamma(T). The hexagonal close-packed (hcp) and double hcp structures are established as metastable phases of Au. Our results demonstrate that quantitative agreement with experiments can be obtained if all relevant temperature-induced excitations are considered in first-principles modeling and that the temperature dependence of the ISFE is substantial enough to be taken into account in crystal plasticity modeling. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 14. Lu, Song
    et al.
    Hu, Qing-Miao
    Delczeg-Czirjak, Erna Krisztina
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Determining the minimum grain size in severe plastic deformation process via first-principles calculations2012In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 11, p. 4506-4513Article in journal (Refereed)
    Abstract [en]

    Although the stacking fault energy (SFE) is a fundamental variable determining the minimum grain size (d(min)) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd-Ag, Pd-Cu, Pt-Cu and Ni-Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd-Ag, Pd-Cu and Ni-Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt-Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and d(min), in SPD experiments and show that the d(min) values can be evaluated from first-principles calculations.

  • 15. Lu, Song
    et al.
    Hu, Qing-Miao
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stacking fault energies of Mn, Co and Nb alloyed austenitic stainless steels2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 14, p. 5728-5734Article in journal (Refereed)
    Abstract [en]

    The alloying effects of Mn, Co and Nb on the stacking fault energy (SFE) of austenitic stainless steels, Fe-Cr-Ni with various Ni contents, are investigated via quantum-mechanical first-principles calculations. In the composition range (c(Cr) = 20%, 8 <= c(Ni) <= 20%, 0 <= c(Mn), c(Co), c(Nb) <= 8%, balance Fe) studied here, it is found that Mn always decreases the SFE at 0 K but increases it at room temperature in high-Ni (c(Ni) greater than or similar to 16%) alloys. The SFE always decreases with increasing Co content. Niobium increases the SFE significantly in low-Ni alloys; however, this effect is strongly diminished in high-Ni alloys. The SFE-enhancing effect of Ni usually observed in Fe-Cr-Ni alloys is inverted to an SFE-decreasing effect by Nb for c(Nb) greater than or similar to 3%. The revealed nonlinear composition dependencies are explained in terms of the peculiar magnetic contributions to the total SFE.

  • 16.
    Lu, Song
    et al.
    Univ Turku, Dept Phys & Astron, FI-20014 Turku, Finland.;Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Li, Ruihuan
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.;Dalian Univ Technol, Sch Phys & Optoelect Technol, Dalian 116024, Peoples R China.;Dalian Univ Technol, Coll Adv Sci & Technol, Dalian 116024, Peoples R China..
    Kadas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Zhang, Hualei
    Xi An Jiao Tong Univ, Frontier Inst Sci & Technol, Ctr Microstruct Sci, State Key Lab Mech Behav Mat, Xian 710049, Peoples R China..
    Tian, Yanzhong
    Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China..
    Kwon, Se Kyun
    Pohang Univ Sci & Technol, Grad Inst Ferrous Technol, Pohang 37673, South Korea..
    Kokko, Kalevi
    Univ Turku, Dept Phys & Astron, FI-20014 Turku, Finland..
    Hu, Qing-Miao
    Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China..
    Hertzman, Staffan
    Avesta Res Ctr, Outokumpu Stainless Res Fdn, SE-77422 Avesta, Sweden..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary.;Avesta Res Ctr, Outokumpu Stainless Res Fdn, SE-77422 Avesta, Sweden..
    Stacking fault energy of C-alloyed steels: The effect of magnetism2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 122, p. 72-81Article in journal (Refereed)
    Abstract [en]

    First-principles calculations have been performed to study the effect of C on the stacking fault energy (SFE) of paramagnetic gamma-Fe and Fe-Cr-Ni austenitic steel. In these systems, the local magnetic structure is very sensitive to the volume in both fcc and hcp structures, which emphasizes the importance of the magnetovolume coupling effect on the SFE. The presence of C atom suppresses the local magnetic moments of Fe atoms in the first coordination shell of C. Compared to the hypothetical nonmagnetic case, paramagnetism significantly reduces the effect of C on the SFE. In the scenario of C being depleted from the stacking fault structure or twin boundaries, e.g., due to elevated temperature, where the chemical effect of C is dissipated, we calculate the C-induced volume expansion effect on the SFE. The volume induced change in the SFE corresponds to more than 50% of the total C effect on the SFE obtained assuming uniform C distribution.

  • 17. Luo, Hu-Bin
    et al.
    Li, C. M.
    Hu, Q. M.
    Kulkova, S. E.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Yang, R.
    First-principles investigations of the five-layer modulated martensitic structure in Ni(2)Mn(Al(x)Ga(1-x)) alloys2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 15, p. 5938-5945Article in journal (Refereed)
    Abstract [en]

    In this paper, the five-layer modulated (5M) martensitic structures of Ni(2)Mn(Al(x)Ga(1-x)), with x = 0, 0.1 and 0.2, are investigated by the use of the exact muffin-tin orbital method in combination with the coherent potential approximation. The 5M martensite is modeled by varying c/a (shear) and wave-like displacements of the atoms on (1 1 0) plane (shuffle) scaled by eta according to Martynov and Kokorin (J. Phys. III 2, 739 (1992)). For Ni(2)MnGa, we obtain 5M martensite with equilibrium c/a of 0.92 and eta of 0.08, in reasonable agreement with the experiment results (0.94 and 0.06, respectively). c/a and eta are linearly coupled to each other. Al-doping increases c/a and decreases eta, but the linear c/a similar to eta coupling remains. Comparing the total energies of the 5M martensite and L2(1) austenite, we find that the martensite is more stable than the austenite. Al-doping increases the relative stability of the austenite and finally becomes energetically degenerated with the 5M martensite at an Al atomic fraction (x) of about 0.26. The relative phase stability is analyzed based on the calculated density of states. The calculated total magnetic moments mu(0) as a function of c/a exhibit a maximum around the equilibrium c/a. Al-doping reduces mu(0)

  • 18. Luo, Hu-Bin
    et al.
    Li, C. M.
    Hu, Q. M.
    Yang, R.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Theoretical investigation of the effects of composition and atomic disordering on the properties of Ni2Mn(Al1-xGax) alloy2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 3, p. 971-980Article in journal (Refereed)
    Abstract [en]

    The influences of composition and Mn-Ga/Al disordering on the magnetic and elastic properties of the Ni2Mn(Al1-xGax) Hensler alloy are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The transition temperature from the completely disordered B2 phase to the ordered L2(1) phase, estimated by the use of Bragg-Williams-Gorsky approximation, is in excellent agreement with experiments. The site-occupancy in the partially disordered alloy is determined by comparing the free energies of the alloys with different site-occupation configurations. It is found that Mn atoms generally prefer to exchange with Al atoms for the alloy with low degree of disorder and low Ga concentration. With increasing degree of disorder and Ga concentration, Mn may exchange with Ga as well. The total magnetic moment of the completely ordered alloy exhibits a minimum around x = 0.7 due to the competition between the increasing magnetic moments of Mn atoms and the decreasing moments of Ni atoms with increasing x. For the partially disordered alloy, both total and local magnetic moments of Mn and Ni decrease linearly with increasing degree of disorder, and Ga atoms show significant magnetism. The shear moduli C of the alloys are calculated with respect to both x and the degree of disorder. The results show that C decreases with increasing x and degree of disorder. The dependence on composition and on the degree of disorder of the martensitic transition temperatures is discussed in terms of the calculated C.

  • 19.
    Shi, Songxin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Beihang Univ, Sch Mat Sci & Engn, Beijing 100191, Peoples R China.;Beihang Univ, Int Res Inst Multidisciplinary Sci, Ctr Integrated Computat Mat Engn, Beijing 100191, Peoples R China..
    Zhu, Linggang
    Beihang Univ, Sch Mat Sci & Engn, Beijing 100191, Peoples R China.;Beihang Univ, Int Res Inst Multidisciplinary Sci, Ctr Integrated Computat Mat Engn, Beijing 100191, Peoples R China..
    Zhang, Hu
    Beihang Univ, Sch Mat Sci & Engn, Beijing 100191, Peoples R China..
    Sun, Zhimei
    Beihang Univ, Sch Mat Sci & Engn, Beijing 100191, Peoples R China.;Beihang Univ, Int Res Inst Multidisciplinary Sci, Ctr Integrated Computat Mat Engn, Beijing 100191, Peoples R China..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mapping the relationship among composition, stacking fault energy and ductility in Nb alloys: A first-principles study2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 144, p. 853-861Article in journal (Refereed)
    Abstract [en]

    Transition metals (TMs) are extensively used to improve the mechanical properties of niobium based alloy, one of the most promising high-temperature materials. Yet the microscopic mechanism of the alloying effects of these transition metals on the mechanical properties is unclear. In this study, we have mapped out the composition-SFE-ductility relationship for TM-alloyed Nb systems by comprehensively investigating the unstable stacking fault energies (SFEs), gamma(us), and the ductility in binary and ternary Nb alloys using the first-principles calculations. It is found that the valence electron concentration can be used as the key descriptor to evaluate the SFE of Nb matrix, which is applicable to both binary and ternary alloys. The microscopic mechanism arises from the electron redistribution in the local stacking fault area. Moreover, for ternary Nb-Ti based alloys, the interaction between Ti and the third alloying elements has negligible effect on the SFE of the systems, and the valence-electron rule still dominates. The alloying effects on the ductility are further illustrated based on the ratio between surface energies and SFEs. The composition-SFE-ductility map obtained by our theoretical calculations is calibrated by available experimental data.

  • 20.
    Sun, Xun
    et al.
    Xi An Jiao Tong Univ, State Key Lab Mech Behav Mat, Frontier Inst Sci & Technol, Ctr Microstruct Sci, Xian 710049, Shaanxi, Peoples R China..
    Zhang, Hualei
    Xi An Jiao Tong Univ, State Key Lab Mech Behav Mat, Frontier Inst Sci & Technol, Ctr Microstruct Sci, Xian 710049, Shaanxi, Peoples R China..
    Lu, Song
    Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Ding, Xiangdong
    Xi An Jiao Tong Univ, State Key Lab Mech Behav Mat, Frontier Inst Sci & Technol, Ctr Microstruct Sci, Xian 710049, Shaanxi, Peoples R China..
    Wang, Yunzhi
    Xi An Jiao Tong Univ, State Key Lab Mech Behav Mat, Frontier Inst Sci & Technol, Ctr Microstruct Sci, Xian 710049, Shaanxi, Peoples R China.;Ohio State Univ, Dept Mat Sci & Engn, 2041 Coll Rd, Columbus, OH 43210 USA..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 140, p. 366-374Article in journal (Refereed)
    Abstract [en]

    Using ab initio alloy theory, we investigate the lattice stability of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys considering the competing body-centered cubic (bcc) and face-centered cubic (fcc) crystal structures. The theoretical lattice constants increase with increasing x, in good agreement with experimental data. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with observations. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fee structure shows weak composition dependence. Within their own stability fields, both structures are predicted to be paramagnetic at ambient conditions. Bain path calculations support that within the duplex region both phases are dynamically stable. As compared to AlxCrFeCoNi, equiatomic Mn addition is found to shrink the stability range of the fcc phase and delay the appearance of the bcc phase in terms of Al content, thus favoring the duplex region in 3d-metals based high-entropy alloys.

  • 21.
    Tamm, Artur
    et al.
    Los Alamos Natl Lab, Mat Sci & Technol Div, Los Alamos, NM 87544 USA.;Univ Tartu, Inst Technol, IMS Lab, EE-50411 Tartu, Estonia..
    Aabloo, Alvo
    Univ Tartu, Inst Technol, IMS Lab, EE-50411 Tartu, Estonia..
    Klintenberg, Mattias
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stocks, Malcolm
    Oak Ridge Natl Lab, Oak Ridge, TN 54321 USA..
    Caro, Alfredo
    Los Alamos Natl Lab, Mat Sci & Technol Div, Los Alamos, NM 87544 USA..
    Atomic-scale properties of Ni-based FCC ternary, and quaternary alloys2015In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 99, p. 307-312Article in journal (Refereed)
    Abstract [en]

    The aim of this study is to characterize some atomic-scale properties of Ni-based FCC multicomponent alloys. For this purpose, we use Monte Carlo method combined with density functional theory calculations to study short-range order (SRO), atomic displacements, electronic density of states, and magnetic moments in equimolar ternary NiCrCo, and quaternary NiCrCoFe alloys. According to our study, the salient features for the ternary alloy are a negative SRO parameter between Ni-Cr and a positive between Cr-Cr pairs as well as a weakly magnetic state. For the quaternary alloy we predict negative SRO parameter for Ni-Cr and Ni-Fe pairs and positive for Cr-Cr and Fe-Fe pairs. Atomic displacements for both ternary and quaternary alloys are negligible. In contrast to the ternary, the quaternary alloy shows a complex magnetic structure. The electronic structure of the ternary and quaternary alloys shows differences near the Fermi energy between a random solid solution and the predicted structure with SRO. Despite that, the calculated EXAFS spectra does not show enough contrast to discriminate between random and ordered structures. The predicted SRO has an impact on point-defect energetics, electron-phonon coupling and thermodynamic functions and thus, SRO should not be neglected when studying properties of these two alloys.

  • 22.
    Tengdelius, Lina
    et al.
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Broitman, Esteban
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Lu, Jun
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Eriksson, Fredrik
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Birch, Jens
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Hultman, Lars
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Hogberg, Hans
    Linkoping Univ, Dept Phys Chem & Biol IFM, Thin Film Phys Div, SE-58183 Linkoping, Sweden..
    Hard and elastic epitaxial ZrB2 thin films on Al2O3(0001) substrates deposited by magnetron sputtering from a ZrB2 compound target2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 111, p. 166-172Article in journal (Refereed)
    Abstract [en]

    Zirconium diboride (ZrB2) exhibits high hardness and high melting point, which is beneficial for applications in for e.g. metal cutting. However, there is limited data on the mechanical properties of ZrB2 films and no data on epitaxial films. In this study, ZrB2(0001) thin films, with thicknesses up to 1.2 mu m, have been deposited on Al2O3(0001) substrates by direct current magnetron sputtering from a compound target. X-ray diffraction and transmission electron microscopy show that the films grow epitaxially with two domain types exhibiting different in-plane epitaxial relationships to the substrate. The out-of-plane epitaxial relationship was determined to ZrB2(0001)parallel to Al2O3(0001) and the in-plane relationships of the two domains to ZrB2[10 (1) over bar0]parallel to Al2O3[10 (1) over bar0] and ZrB2[11 (2) over bar0]parallel to Al2O3[10 (1) over bar0]. Mechanical properties of the films, evaluated by nanoindentation, showed that all films exhibit hardness values above 45 GPa, a reduced Young's modulus in the range 350-400 GPa, and a high elastic recovery of 70% at an applied load of 9000 mu N.

  • 23.
    Tian, Li-Yun
    et al.
    Royal Inst Technol KTH, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    Lizarraga, Raquel
    Royal Inst Technol KTH, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    Larsson, Henrik
    Royal Inst Technol KTH, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden..
    Holmstrom, Erik
    Sandvik Coromant R&D, SE-12680 Stockholm, Sweden..
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    A first principles study of the stacking fault energies for fcc Co-based binary alloys2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 136, p. 215-223Article in journal (Refereed)
    Abstract [en]

    The stacking fault energy is closely related to structural phase transformations and can help to understand plastic deformation mechanisms in materials. Here we perform first principles calculations of the stacking fault energy in the face centered cubic (fcc) Cobalt-based binary alloys Co1-x M-x, where M = Cr, Fe, Ni, Mo, Ru, Rh, Pd and W. We investigate the concentration range between 0 and 30 at.% of the alloying element. The results are discussed in connection to the phase transition between the low temperature hexagonal close packed (hcp) and the fcc structures observed in Co and its alloys. By analyzing the stacking fault energies, we show that alloying Co with Cr, Ru, and Rh promotes the hcp phase formation while Fe, Ni and Pd favor the fcc phase instead. The effect of Mo and W on the phase transition differs from the other elements, that is, for concentrations below 10% the intrinsic stacking fault energy is lower than that for pure fcc Co and the energy barrier is higher, whereas above 10% the situation reverses. We carry out also thermodynamic calculations using the ThermoCalc software. The trends of the ab initio stacking fault energy are found to agree well with those of the molar Gibbs energy differences and the phase transition temperature in the binary phase diagrams and give a solid support for the phase stability of these alloys.

  • 24. Uchida, H. T.
    et al.
    Wagner, S.
    Hamm, M.
    Kuersehner, J.
    Kirchheim, R.
    Hjörvarsson, Björgvin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Pundt, A.
    Absorption kinetics and hydride formation in magnesium films: Effect of driving force revisited2015In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 85, p. 279-289Article in journal (Refereed)
    Abstract [en]

    Electrochemical hydrogen permeation measurements and in situ gas-loading X-ray diffraction measurements were performed on polycrystalline Mg films. Hydrogen diffusion constants, the hydride volume content and the in-plane stress were determined for different values of driving forces at 300 K. For alpha-Mg-H, a hydrogen diffusion constant of D-H(Mg) = 7(+/- 2) . 10(-11) m(2) s(-1) was determined. For higher concentrations, different kinetic regimes with reduced apparent diffusion constants Vat were found, depending on the driving force, decreasing to about D-H(tot) = 10(-18) m(2) s(-1). This lowest measured diffusion constant is two orders of magnitude larger than that of bulk beta-MgH2, and the difference is ascribed to a contribution from a fast diffusion along grain boundaries. The different kinetics regimes are attributed to the spatial distribution of hydrides. A heterogeneous hydride nucleation and growth model is suggested that is based on hemispherical hydrides spatially distributed according to the nuclei densities expressed as a function of the driving force. The model allows us to qualitatively explain the complex stress development, the different diffusion regimes and the blocking-layer thickness. As the blocking-layer thickness inversely scales with the driving force, small driving forces allow the hydriding of large film volume fractions. Maximum stress situations occur for hydride distances reaching four times the hydride radius and for hydride distances equaling the film thickness. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 25.
    Vitos, Levente
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Nilsson, J-O
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Alloying effects on the stacking fault energy in austenitic stainless steels from first-principles theory2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 14, p. 3821-3826Article in journal (Refereed)
    Abstract [en]

    The stacking fault energy (SFE) of austenitic stainless steels has been determined using a quantum mechanical first-principles approach. We identify the electronic, magnetic and volume effects responsible for the compositional dependence of the SFE. We find that both the alloying element and the composition of the host material are important for understanding the alloying effects. Our results show that no simple and universally valid composition equations exist for the SEE.

  • 26.
    Wiklund, Urban
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Rubino, Stefano
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Kádas, Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skorodumova, Natalia V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hedberg, Stefan
    Outokumpu Stainless AB, Avesta, Sweden.
    Collin, Marianne
    AB Sandvik Tooling R&D, Stockholm, Sweden.
    Olsson, Annika
    Ångström Materials Academy, Uppsala University, Sweden.
    Leifer, Klaus
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
    Experimental and theoretical studies on stainless steel transfer onto a TiN-coated cutting tool2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 1, p. 68-74Article in journal (Refereed)
    Abstract [en]

    Stainless steel is a good example of a metal that is not easily machined. To explain such behavior an understanding of the fundamental adhesion between the workpiece and the tool is invaluable. It is a well-known fact that build-up layers form in the interface, but little attention has been given to the very first layer that adheres to the tool surface. Although this layer rapidly becomes covered by successive material transfer, this layer and its ability to stick to the tool surface control the successive material transfer and influence the cutting properties. In this work, a quick stop test is employed to interrupt the cutting of a 316L stainless steel using a TiN-coated cemented carbide cutting insert. Different analytical techniques, such as transmission electron microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, as well as theoretical atomistic modeling, were used to study the early adhesion.

  • 27.
    Wilhelmsson, Ola
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Eklund, P
    Högberg, H
    Hultman, L
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Structural, electrical and mechanical characterization of magnetron-sputtered2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 11, p. 2563-2569Article in journal (Refereed)
    Abstract [en]

    V2GeC MAX-phase thin films were deposited by DC magnetron sputter epitaxy in the temperature range 450–850 °C. The MAX-phase nucleates directly on (0 0 0 l)-oriented sapphire-wafer substrates without the need for a seed layer. The films contain, however, a small fraction of binary vanadium carbide (VCx) inclusions. X-ray diffraction analysis furthermore shows that these inclusions partly consist of the ordered superstructure V8C7. The amount of Ge in the films decreases at higher temperatures, which can be attributed to Ge evaporation. At temperatures below 450 °C the films consist of polycrystalline Ge and an X-ray amorphous carbide phase attributed to VCx or V2C. No MAX-phase was observed in this temperature region. The electrical and mechanical properties of the films were characterized.

  • 28.
    Yun, Younsuk
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Oppeneer, Peter M.
    Kim, Hanchul
    Park, Kwangheon
    Defect energetics and Xe diffusion in UO2 and ThO22009In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 57, no 5, p. 1655-1659Article in journal (Refereed)
    Abstract [en]

    We have performed ab initio total energy calculations to investigate the defect energetics and diffusion behavior of Xe in UO2 and ThO2 matrices. All calculations have been carried out Using density functional theory within the generalized gradient approximation and applying the projector-augmented-wave method. Our results Show that the formation and migration energies of vacancy defects are more than twice as high in ThO2 compared with UO2. Another notable difference between the two oxides is the role played by an oxygen vacancy ill the movement of a cation vacancy. An vacancy enhances the movement Of a uranium vacancy by lowering its migration energy by about 1 eV, but a similar effect is not observed in ThO2. The different behavior of cation vacancies in the two oxides strongly affects the mobility of fission gases and leads to differences in their respective diffusion behavior. We sui, est that the strong resistance against oxidation of ThO2 prevents the creation and migration of defects. and results ill a lower mobility of fission gases ill ThO2 as compared to UO2.

  • 29.
    Zlotea, Claudia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Andersson, Yvonne
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Microstructural modifications induced by hydrogen absorption in Mg5Ga2 and Mg6Pd2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 20, p. 5559-5564Article in journal (Refereed)
    Abstract [en]

    We have recently proposed a new method to design one-dimensional structures of MgH2 in the nano- and micrometer ranges by hydrogen-induced disproportionation of bulk Mg24Y5. The present study confirms the same behavior in hydrogenated Mg5Ga2 and Mg6Pd. Single-crystalline one-dimensional structures and microparticles of MgH2 are formed by hydrogen absorption and subsequent partial disproportionation of Mg5Ga2 and Mg6Pd. The MgH2 whiskers and particles grow with different morphologies for different alloying partners. Growth mechanisms are proposed in relation to the morphology and the chemical surface composition of original compounds.

  • 30. Zlotea, Claudia
    et al.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Özbilen, Sedat
    Moretto, Pietro
    Andersson, Yvonne
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry.
    Hydrogen desorption studies of the Mg24Y5–H system: Formation of Mg tubes, kinetics and cycling effects2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 11, p. 2421-2428Article in journal (Refereed)
    Abstract [en]

    The current study focuses on the hydrogen desorption properties of hydrogenated Mg24Y5. Recently, we have reported the formation of unidirectional MgH2 structures by hydrogen absorption and induced disproportionation of Mg24Y5. During hydrogen desorption, a complex voiding phenomenon produces Mg tubes and carved particles with nano-sized walls. The selected area electron diffraction patterns demonstrate that the Mg tubes are single crystals. A harmonized picture of the unidirectional growth based on different Mg vapor models is proposed. The kinetic properties of hydrogen desorption are improved as compared with commercial MgH2. Hydrogenation/dehydrogenation cycling lowers the thermal stability of the hydrogen desorption at the expense of the total desorbed hydrogen capacity. Both whiskers and microparticles are depleted into clusters of nanoparticles after extensive cyclin

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