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  • 1.
    Hallström, Samuel
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    On high temperature oxidation resistance: Towards the materials genome of high temperature alloys2014Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The efficiency of a heat engine increases with increasing service temperature. This leads to a demand for material that can withstand aggressive environments and sometimes also high load at higher and higher temperature. That is the rationale for the work of this thesis. It is divided into two parts, both addressing model alloy systems for components used in oxidizing high temperature environments. In the first part the phase equilibria and phase diagrams of Ni-Ru and Al-Ni-Ru are investigated from a thermodynamic point of view using the well known Calphad method. In particular, the debated existance of a miscibility gap between the aluminides NiAl and RuAl is considered. This led to a combined ab initio/Calphad approach and it is suggested that there is a miscibility gapat low temperature. In the second part, first of its kind diffusion simulations in oxides are performed in the technologically important Fe-O and Cr-O systems. In the simulations, the moving phase boundary problem is solved with use of temperature and composition dependent diffusion coefficients, that are evaluated for complex oxide phases modeled with up to four sublattices. This type of simulations attracts a lot of interest and it looks very promising for future extension to higher order systems.

    Fulltekst (pdf)
    hallstrom-phd-thesis
  • 2.
    Hallström, Samuel
    KTH, Tidigare Institutioner, Metallurgi.
    Thermodynamic assessment of the Ni-Ru system2004Inngår i: Journal of phase equilibria (Print), ISSN 1054-9714, E-ISSN 1544-1032, Vol. 25, nr 3, s. 252-254Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Ni-Ru system is assessed using the CALPHAD technique. The only available experimental information for this system concerns phase equilibria, whereas no thermochemical information has been found. The THERMO-CALC software is applied to obtain Gibbs energy (G) functions for the individual phases. The phase diagram calculated from the new G functions is in excellent agreement with the experimental diagram, and all the thermodynamic parameters are given in the Appendix. In addition, tentative activity diagrams and the metastable miscibility gaps in fcc and hcp phases are calculated from the new thermodynamic description.

  • 3.
    Hallström, Samuel
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Andersson, D.
    Ruban, A.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Thermodynamic reassessment of the Ni-Ru system and assessment of the Al-Ni-Ru system at 1273-1523 K using ab initio calculations2008Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, nr 15, s. 4062-4069Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Ni-Ru and Al-Ni-Ru systems are assessed with a combined CALPHAD and ab initio approach. Particular attention is paid to the possible existence of a miscibility gap in the B2 phase. Both face-centered cubic and body-centered cubic ordering are analyzed within the compound energy formalism. Ab initio calculations for the B2 phase show a similar trend as calorimetric measurements but the magnitude is much smaller. It is found that the calorimetric measurements cannot be reconciled with any reasonable phase diagram, whereas the ab initio results can. From the parameters obtained, isothermal sections in reasonable agreement with experimental phase diagrams are calculated. We have concluded that there is no miscibility gap in the B2 phase at 1273 K and higher temperatures.

  • 4. Hallström, Samuel
    et al.
    Halvarsson, Mats
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Jonsson, Torbjorn
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    High temperature oxidation of chromium: Kinetic modeling and microstructural investigation2013Inngår i: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 240, s. 41-50Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using CALPHAD methods, Cr2O3 growth on pure Cr is modeled using DICTRA and the vacancy model for diffusion. The results are compared with thermobalance and TEM experiments at 625 and 700 degrees C in O-2. The experimental scatter is significant, leading to a compromise suggestion. With the experimental conditions from the furnace exposures in this work, optimized mobilities are validated with a series of oxidation simulations. Despite the complex microstructure and initial growth rate variations, it is possible to reproduce the experimental oxide thicknesses with good accuracy, allowing for extension to multicomponent systems.

  • 5.
    Hallström, Samuel
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Modeling of diffusion in wustite and simulation of oxidation of iron at 600°C2008Inngår i: 6th European Stainless Steel Conference / [ed] Pentti Karjalainen and Staffan Hertzman, Helsinki: Jernkontoret , 2008, s. 273-278Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A long missing feature in the diffusion simulation software DICTRA has been the diffusion in oxides. Recently, this capability was implemented and in this report we present some results of the current achievements.DICTRA can now treat diffusion in basically any oxide, provided that there is diffusion data available and that the necessary mobilities have been assessed. The first system to be addressed was the important Fe-O system. It contains three different oxides, from the simple wustite to the complex magnetite, which has the spinel structure. The current  approach was successful and the work was continued with the Cr-O system and the technically important Fe-Cr-O. We will present the diffusion models and the results of the assessment, as well as successful simulations of oxidation where layers of oxides grow on top of an Fe or Fe-Cr substrate.

  • 6.
    Hallström, Samuel
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Höglund, Lars
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Modeling of iron diffusion in the iron oxides magnetite and hematite with variable stoichiometry2011Inngår i: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, nr 1, s. 53-60Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The vacancy model of diffusion is applied to magnetite and hematite, and mathematical expressions for the iron flux in the lattice-fixed frame of reference, as a function of the defect structure, are presented The defect structures, i e the vacancy content on the different type of sites, and the thermodynamic factors are calculated from the available Calphad type of thermodynamic descriptions for the oxides Expressions for Fe tracer diffusion coefficients are derived and the relations between mobility and tracer diffusivity are given The mobilities are fitted by a least-squares optimization to experimental data on tracer diffusion from the literature For magnetite, an excellent representation of the experimental tracer data is achieved together with a satisfactory description of the sparse chemical diffusion data available For hematite, the experimental scatter is very large and anomalous large frequency factors and activation energies have been reported In the present report a compromise is suggested.

  • 7. Jonsson, T.
    et al.
    Pujilaksono, B.
    Hallström, Samuel
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ågren, John
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Svensson, J. E.
    Johansson, L. G.
    Halvarsson, M.
    An ESEM in situ investigation of the influence of H2O on iron oxidation at 500 degrees C2009Inngår i: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 51, nr 9, s. 1914-1924Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The initial oxidation of iron and the effect of different O-2/H2O mixtures have been investigated through ESEM in situ exposure and analysis at 500 degrees C. In dry and wet air a two-layered magnetite forms beneath a thin, fine-grained hematite layer. while only a two-layered magnetite forms in H2O. The two-layered magnetite is separated by a straight interface (in all environments), which is suggested to be the original metal surface. The presence of low levels of water vapour (similar to 1% H2O) produces a thicker hematite layer (compared to dry air), exhibiting a large number of whiskers. At least four different factors are suggested to influence the local growth rate: the surface of the metal grain, the thickness of the hematite layer, the oxide grain size and the exposure environment. The ESEM in situ exposures have in addition been shown to have high quality and reproducibility.

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