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Sökning: swepub > Johansson Börje > Refereegranskat > Punkkinen M. P. J.

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1.
  • Pitkänen, H., et al. (författare)
  • Ab initio study of the phase stability in paramagnetic duplex steel alloys
  • 2009
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - APS. - 1098-0121. ; 79:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Duplex stainless steels have many superior properties compared to conventional steels, this being mainly due to their microstructure containing approximately equal amount of ferrite and austenite phases formed by iron, chromium (or Cr equivalent), and nickel (or Ni equivalent). Using computational methods based on first-principles theories, the phase stability of paramagnetic Fe1-c-nCrcNin alloys (0.12 <= c <= 0.32 and 0.04 <= n <= 0.32) at high temperatures (greater than or similar to 1000 K) is addressed. It is shown that the stabilization of the ferrite-austenite two-phase field in duplex steels is a result of complex interplay of several competing phenomena. Taking into account only the formation energies yields a complete phase separation, strongly overestimating the two-phase region. The formation energies are calculated to be lower for the austenite than for the ferrite, meaning that the configurational entropy has a more significant impact on the stability field of the austenitic phase. The magnetic and vibrational free energies have opposite effects on the phase stability. Namely, the magnetic entropy favors the ferrite phase, whereas the vibrational free energy stabilizes the austenite phase. Combining the formation energies with the magnetic, vibrational, and configurational free energies, a region of coexistence between the two phases is obtained, in line with former thermodynamic assessments as well as with experimental observations.
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2.
  • Airiskallio, E., et al. (författare)
  • High temperature oxidation of Fe-Al and Fe-Cr-Al alloys : The role of Cr as a chemically active element
  • 2010
  • Ingår i: Corrosion Science. - 0010-938X. ; 52:10, s. 3394-3404
  • Tidskriftsartikel (refereegranskat)abstract
    • Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the alpha-Al2O3 film which is formed on the surface provided temperature is above 900 degrees C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales. (C) 2010 Elsevier Ltd. All rights reserved.
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3.
  • Airiskallio, E., et al. (författare)
  • Third element effect in the surface zone of Fe-Cr-Al alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.
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4.
  • Delczeg-Czirjak, Erna Krisztina, 1978-, et al. (författare)
  • Ab initio study of the elastic anomalies in Pd-Ag alloys
  • 2009
  • Ingår i: Physical Review B Condensed Matter. - 0163-1829. ; 79
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio total-energy calculations, based on the exact muffin-tin orbital method, are used to determine the elastic properties of Pd1−xAgx random alloys in the face-centered-cubic crystallographic phase. The compositional disorder is treated within the coherent-potential approximation. The single crystal and polycrystalline elastic constants and the Debye temperature are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation in the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from a simple linear or parabolic trend. The complex electronic origin of these anomalies is demonstrated. 
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5.
  • Kokko, K., et al. (författare)
  • Surface core-level shift of Pd at the AgcPd1-c(111) surface : Nonlinear subsurface effects
  • 2007
  • Ingår i: Surface Science. - 0039-6028. ; 601:23, s. 5419-5423
  • Tidskriftsartikel (refereegranskat)abstract
    • The surface core-level binding-energy shift (SCLS) of Pd at the AgcPd1-c(111) surface is calculated as a function of bulk concentration of the alloy. The equilibrium volume and the surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are evaluated within the Z + 1 approximation. The results are analysed using the mixing enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS to the bulk concentration is considered. This relation is shown to be mediated by the surface concentration profile which induces the observed nonlinear behaviour. The results are interpreted using a simple model for the alloy electronic structure.
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6.
  • Kuronen, A., et al. (författare)
  • Segregation, precipitation, and alpha-alpha ' phase separation in Fe-Cr alloys
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 92:21
  • Tidskriftsartikel (refereegranskat)abstract
    • Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analyzing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature, and time the present investigation combines Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods have to be used. Using the exact muffin-tin orbitals method with the coherent potential approximation (CPA-EMTO) the effective chemical potential as a function of Cr content (0-15 at. % Cr) is calculated for a surface, second atomic layer, and bulk. At similar to 10 at. % Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr-containing surfaces are expected when the Cr content exceeds similar to 10 at. %. The second atomic layer forms about a 0.3 eV barrier for the migration of Cr atoms between the bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. However, for Cr concentration regions less than 10 at. %, the ab initio (CPA-EMTO) result of the important role of the second atomic layer to the surface is not reproducible from the large-scale Monte Carlo molecular dynamics (MCMD) simulation. On the other hand, for the nominal concentration of Cr larger than 10 at. % the MCMD simulations show the precipitation of Cr into isolated pockets in bulk Fe-Cr and the existence of the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer systems. For high Cr concentration alloys the performed spectroscopic measurements support the MCMD simulations. Hard x-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in the Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10(-8) Torr pressure of the spectrometers showing intense Cr2O3 signal. Cr segregation and the formation of Cr-rich precipitates were traced by analyzing the experimental atomic concentrations and chemical shifts with respect to annealing time, Cr content, and kinetic energy of the exited electron.
7.
  • Lang, J. J. K., et al. (författare)
  • Ab initio and scanning tunneling microscopy study of an indium-terminated GaAs(100) surface : An indium-induced surface reconstruction change in the c(8x2) structure
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:24, s. 245305
  • Tidskriftsartikel (refereegranskat)abstract
    • Technologically useful indium- (In) terminated c(8 x 2)-reconstructed GaAs(100) substrate surface has been studied by first-principles calculations and scanning tunneling microscopy (STM) measurements. Our total-energy calculations demonstrate the stability of four different so-called zeta a structures with In monomer rows and In coverage between 0.5 and 2 monolayers on the GaAs(100) substrate. Thus, we introduce a surface system, which stabilizes the zeta a reconstruction. Furthermore, an interesting trend is found. Atomic structure of the c(8 x 2) reconstruction depends on the surface-layer cation and substrate volumes, which, in principle, allows to tune the surface structure by cation adsorption. This phenomenon is related to the peculiar c(8 x 2) atomic surface structure, which shows mixed surface layer, including both anions and cations, and uncommon metallic-type cations in the zeta a structure, which do not show covalent bonds. Our results predict a structural transition from the zeta structure to the zeta a structure as the surface cation size is increased at 0 K. The found transition is probably related to the disordered surface structures (consisting of zeta and zeta a building blocks) found experimentally by x-ray diffraction at room temperature. Comparison of the STM images, calculated for various c(8 x 2) models, with the former and present measured STM images of In/GaAs(100) c(8 x 2) supports the presence of stable zeta a reconstructions.
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8.
  • Punkkinen, M. P. J., et al. (författare)
  • Calculation of surface core-level shifts within complete screening : Problems with pseudohydrogenated slabs
  • 2008
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 77:24
  • Tidskriftsartikel (refereegranskat)abstract
    • By considering GaAs(110) and Si(100)(2x1) surfaces, it is shown that the use of the pseudohydrogen atoms to saturate the dangling bonds at one side of the slab modeling a free surface can lead to distorted surface core-level shifts within the complete screening picture. The effect is linked to the polarization of the slab and to the change in the bulklike electronic structure close to pseudohydrogenated part of the slab. It is demonstrated that these problems can be avoided if the pseudohydrogenated slab is large enough and the bulk reference layer is properly chosen. One easy way to control these errors is to monitor the planar-averaged potential energy curve of the slab.
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9.
  • Punkkinen, M. P. J., et al. (författare)
  • Magnetism of (FeCo)Si alloys : Extreme sensitivity on crystal structure
  • 2006
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 73:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Theoretical results for (FeCo)Si substitutional random alloys, obtained within the framework of the exact muffin-tin orbitals method in combination with the coherent potential approximation, are presented. For concentration range 0.1 < x < 0.9, the magnetic structure of Fe1-xCoxSi is found to be almost degenerate, which is reflected in extreme sensitivity of the local magnetic moments on the internal structure parameters and lattice constant. Results are analyzed and discussed using Stoner, covalent magnetism, and rigid band models. In Fe-rich and in Co-rich alloys, the composition dependence of the magnetic moments can be understood in terms of a simple model. It is shown that disorder has a significant effect on magnetism in (FeCo)Si alloys.
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10.
  • Punkkinen, M. P. J., et al. (författare)
  • Surface core-level shifts of GaAs(100)(2x4) from first principles
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 76:11
  • Tidskriftsartikel (refereegranskat)abstract
    • First-principles calculations show that measured surface core-level shifts (SCLSs) of the GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated As 3d and Ga 3d SCLSs for the beta 2 and alpha 2 reconstructions of the GaAs(100)(2x4) surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)(2x4) surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the GaAs(100)(2x4) surface is increased gradually, the area of the alpha 2 reconstruction increases on the surface, but the beta 2 phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.
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