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  • 51.
    Lousada, Claudio M.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Surface Chemistry of Oxygen on Aluminum-Performance of the Density Functionals: PBE, PBE0, M06, and M06-L2016In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 37, no 9, p. 787-794Article in journal (Refereed)
    Abstract [en]

    We investigated the performance of the density functional theory (DFT) functionals PBE, PBE0, M06, and M06-L for describing the molecular and dissociative adsorption of O2 onto pure and doped Al(111) surfaces. Adsorption of O2 was studied at the perfect Al(111) surface and compared with the case where an additional Al atom was present as an adatom. Additionally, we studied how these functionals perform when different dopants are present at the Al(111) surface in two distinct geometries: as an adatom or as a substitutional atom replacing an Al atom. The performance of the different functionals is greatly affected by the surface geometry. The inclusion of Hartree-Fock exchange in the functional leads to slight differences in adsorption energies for molecular adsorption of O2. These differences become very pronounced for dissociative adsorption, with the hybrids PBE0 and M06 predicting more exergonic adsorption than PBE and M06-L. Furthermore, PBE0 and M06 predicted trends in adsorption energies for defective and perfect surfaces which are in line with the experimental knowledge of the effects of surface defects in adsorption energies. The predictions of the non-hybrids PBE and M06-L point in the opposite direction. The analysis of the contributions of the van der Waals (vdW) forces to the adsorption energies reveals that the PBE and PBE0 functionals have similar difficulties in describing vdW interactions for molecular adsorption of O2 while the M06 functional can give a description of these forces with an accuracy which is at least similar to that of the correction of the D3 type.

  • 52.
    Lousada, Claudio M.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sophonrat, Nanta
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Weihong, Yang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology. KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
    Mechanisms of Formation of H, HO, and Water and of Water Desorption in the Early Stages of Cellulose Pyrolysis2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 23, p. 12168-12176Article in journal (Refereed)
    Abstract [en]

    Here, we report the results from a combined first-principles and experimental investigation of the initial stages of decomposition of cellulose during heating in pyrolysis. Density functional theory calculations with periodic boundary conditions were performed to investigate the formation of H and HO radicals and of the molecular products H2O, H-2, and H2O2 originating from their recombination. The stabilization that alcohol groups impart to adjacent C-radicals and the allylic recombination of unpaired electrons of neighboring C-radicals play decisive roles in the decomposition mechanism. This makes the simultaneous formation of H-center dot from C2 and HO center dot from C3 the most favorable process. The recombination of these radicals to form water leads to an additional stabilization of the reaction. The computed temperature-dependent reactions Gibbs' free energies reveal that desorption of H2O from intact cellulose occurs at T = 147 degrees C and that gas-phase water forms spontaneously from the decomposition of cellulose at T = 282 degrees C. These results are in excellent agreement with our experimental study of the pyrolysis done with pyrolysis gas chromatography/mass spectrometry at different temperatures. The experiments show that upon heating, a small amount of water is released from cellulose at 210 degrees C, and a considerably larger amount starts to be released at 280 degrees C.

  • 53.
    Lousada, Claudio Miguel
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Johansson, Adam Johannes
    Korzhavyi, Pavel A
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Thermodynamics of H2O Splitting and H-2 Formation at the Cu(110)-Water Interface2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 25, p. 14102-14113Article in journal (Refereed)
    Abstract [en]

    We used density functional theory to investigate the sequential oxidation of the (110) surface of fcc copper triggered by the dehydrogenation of molecularly adsorbed water the reactions studied did not involve any oxygen besides that present in the water molecules. According to the obtained Gibbs free energies, the formation of half a inonolayer of HO and the corresponding amount of hydrogen gas is spontaneous (Delta(r)G(0) < 0) starting from a monolayer of adsorbed water at Cu(110) The subsequent dehydrogenation steps necessary to ultimately form one monolayer of O atoms are nonspontaneous (Delta(r)G(o) > 0). We present a computationally efficient approach which shows good accuracy for determining the solvation energy of the Cu(110) surface, deviating only by 0.014 eV from literature data. The solvation effect imparts additional stabilization to several oxygen-containing species adsorbed at the Cu(110) surface. Additionally, we investigated the effect of an overlayer of water molecules at the surface where the dehydrogenation of H2O takes place. We found that even though the Gibbs free energy changes associated with the first steps of dehydrogenation of H2O at the Cu surface do not differ substantially from those without an additional water layer, subsequent dehydrogenation steps are favored by as much as 1.6 eV. In view of these results we discuss the importance of the hydrogen-bonding network formed when an overlayer of H2O is present in determining the reactivity of surface species. Additionally, we found a considerable effect of the second water layer on the surface relaxation, which differs significantly from the case where no second water layer is present. The hydrogen-bonding network has an important role in affecting the chemistry of the surface species but also in stabilizing the surface itself, which in turn affects the surface relaxation. These findings shed additional light on the modeling of surface processes in solution, which have implications for corrosion science and catalysis.

  • 54.
    Lousada, Claudio Miguel
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Soroka, Inna L.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Yagodzinskyy, Y.
    Tarakina, N. V.
    Todoshchenko, O.
    Hänninen, H.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jonsson, Mats
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Gamma radiation induces hydrogen absorption by copper in water2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 24234Article in journal (Refereed)
    Abstract [en]

    One of the most intricate issues of nuclear power is the long-term safety of repositories for radioactive waste. These repositories can have an impact on future generations for a period of time orders of magnitude longer than any known civilization. Several countries have considered copper as an outer corrosion barrier for canisters containing spent nuclear fuel. Among the many processes that must be considered in the safety assessments, radiation induced processes constitute a key-component. Here we show that copper metal immersed in water uptakes considerable amounts of hydrogen when exposed to γ-radiation. Additionally we show that the amount of hydrogen absorbed by copper depends on the total dose of radiation. At a dose of 69 kGy the uptake of hydrogen by metallic copper is 7 orders of magnitude higher than when the absorption is driven by H2(g) at a pressure of 1 atm in a non-irradiated dry system. Moreover, irradiation of copper in water causes corrosion of the metal and the formation of a variety of surface cavities, nanoparticle deposits, and islands of needle-shaped crystals. Hence, radiation enhanced uptake of hydrogen by spent nuclear fuel encapsulating materials should be taken into account in the safety assessments of nuclear waste repositories.

  • 55.
    Lousada, Cláudio M.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fernandes, Ricardo M. F.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Tarakina, Nadezda V.
    Soroka, Inna L.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Synthesis of copper hydride (CuH) from CuCO3·Cu(OH)2 – a path to electrically conductive thin films of Cu2017In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 46, no 20, p. 6533-6543Article in journal (Refereed)
    Abstract [en]

    The most common synthesis methods for copper hydride (CuH) employ hard ligands that lead to the formation of considerable amounts of metallic Cu as side-product. Here we explore a synthesis method for CuH(s) through the reaction of CuCO3 center dot Cu(OH)(2)(s) with hypophosphorous acid (H3PO2) in solution, via the formation of the intermediate Cu(H2PO2)(2)(aq) complex. The reaction products were characterized with XRD, FTIR and SEM at different reaction times, and the kinetics of the transformation of Cu(H2PO2)(2)(aq) to CuH(s) were followed with NMR and are discussed. We show that our synthesis method provides a simple way for obtaining large amounts of CuH(s) even when the synthesis is performed in air. Compared to the classic Wurtz method, where CuSO4 is used as an initial source of Cu2+, our synthesis produces CuH particles with less metallic Cu side-product. We attribute this to the fact that our reaction medium is free from the hard SO42- ligand that can disproportionate Cu(I). We discuss a mechanism for the reaction based on the known reactivity of the reagents and intermediates involved. We explored the possibility of using CuH(s) for making electrically conductive films. Tests that employed water-dispersed CuH particles show that this compound can be reduced with H3PO2 leading to electrically conductive thin films of Cu. These films were made on regular office paper and were found to be Ohmic conductors even after several weeks of exposure to ambient conditions. The fact that the synthesis reported here produces large amounts of CuH particles in aqueous media, with very little impurities, and the fact that these can then be converted to a stable electrically conductive film can open up new applications for CuH such as for printing electrically conductive films or manufacturing surface coatings.

  • 56.
    Lousada, Cláudio M.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Johansson, Adam Johannes
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Molecular and dissociative adsorption of water and hydrogen sulfide at perfect and defective Cu(110) surfaces2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, p. 8111-8120Article in journal (Refereed)
    Abstract [en]

    We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of H2O and H2S at perfect and defective Cu(110) surfaces described using supercells with c(6 × 6) periodicity. The defective surface consists of a terrace surrounded by pits. We found considerable differences in adsorption modes and energies for H2O and H2S. At the defective Cu(110) surface, monomers of H2O and H2S preferentially adsorb at the terrace site and molecular adsorption of H2O is significantly more favorable than that of H2S. For dissociative adsorption however, the sulfur species are considerably more stable than the oxygen species. For monolayer (ML) coverages, there are small differences in the molecular adsorption energies for H2O and H2S. However, for the formation of 1 ML of HO and 1 ML of HS from 1 ML of H2O and 1 ML of H2S, respectively, with the release of H2(g), the differences are very large. The formation of 1 ML HO at the perfect Cu(110) surface is endoergic, while at the defective Cu(110) surface it is exoergic by −0.6 eV. For high coverages, H2S forms stacked half-monolayers that interact with each other via a complex hydrogen bond network with a strength per H2S molecule of −0.140 eV per H2S and −0.120 eV per H2S for H2S located in the underlayer and overlayer, respectively. The large distances between hydrogen bonded H2S molecules explain the preference for the formation of the two stacked half-monolayers of H2S instead of a single monolayer as it happens with H2O. Additionally, the formation of 1 ML of HS does not occur because of the spontaneous splitting of some H–S bonds resulting in surface bound HS and S and H2S molecules. Extensive surface reconstruction and relaxation accompanies adsorption of the sulfur adsorbates. Such reconstructions with outwards pull of Cu atoms can be at the origin of the weak adhesion of sulfide films that explains the release of CuS particles from copper sulfide films at copper surfaces. Overall, the surface defects here investigated induce non-linear effects in the molecular and dissociative adsorption energies of different O and S adsorbates.

  • 57.
    Lousada, Cláudio M.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Johansson, Adam Johannes
    Korzhavyi, Pavel A.
    Molecular and dissociative adsorption of water at a defective Cu(110) surface2017In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 658, p. 1-8Article in journal (Refereed)
    Abstract [en]

    We performed a density functional theory (DFT) investigation of the molecular and dissociative adsorption of water at the perfect Cu(110) and at a defective Cu surface that provides a broad spectrum of adsorption sites in terms of coordination of Cu atoms, with the aim of understanding the role of surface defects in the dissociation of water molecules. The molecular adsorption of water is spontaneous at both surfaces but at the defective surface we found two stable molecular adsorption structures that differ slightly in the disposition of the O-atoms of H2O on the surface plane but differ considerably on the orientation of their H-atoms. Additionally we studied the dissociative adsorption of water accompanied with formation of H2(g). At the defective surface, starting from 1 ML of molecularly adsorbed H2O, the dissociation of 0.22 ML of H2O leading to 0.22 ML of HO, 0.78 ML H2O and H2(g) is exergonic but the dissociation of 0.44 ML or more H2O molecules is endergonic. These findings are discussed in terms of the two main factors that affect the adsorption energies: the existence of exposed adatoms provides an environment that facilitates the interaction with small adsorbates leading to stronger bonds between the surface and such adsorbates; and at the same time, the limited polarizability of the defect sites causes high coverages of adsorbates that drag electron density from the surface to be unfavorable. The overall effect is that at the defective Cu(110) formation of low coverages of HO groups is more favorable than at the perfect Cu(110) while forming coverages higher than 0.44 ML of HO is less favorable than at the perfect Cu(110). These effects have their origin in the extent of the polarization of the Cu–O bonds.

  • 58.
    Magnusson, Hans
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Creep modelling of particle strengthened steels2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Materials used in thermal power plants have to resist creep deformation for time periods up to 30 years. Material evaluation is typically based on creep testing with a maximum duration of a few years. This information is used as input when empirically deriving models for creep. These kinds of models are of limited use when considering service conditions or compositions different from those in the experiments. In order to provide a more general model for creep, the mechanisms that give creep strength have to be identified and fundamentally described. By combining tools for thermodynamic modelling and modern dislocation theory the microstructure evolution during creep can be predicted and used as input in creep rate modelling. The model for creep has been utilised to clarify the influence of aluminium on creep strength as a part of the European COST538 action. The results show how AlN is formed at the expense of MX carbonitrides. The role of heat treatment during welding has been analysed. It has been shown that particles start to dissolve already at 800ºC, which is believed to be the main cause of Type IV cracking in commercial alloys.

    The creep strength of these steels relies on minor additions of alloying elements. Precipitates such as M23C6 carbides and MX carbonitrides give rise to the main strengthening, and remaining elements produce solid solution hardening. Particle growth, coarsening and dissolution have been evaluated. By considering dislocation climb it is possible to determine particle strengthening at high temperatures and long-term service. Transient creep is predicted by considering different types of dislocations. Through the generation and recovery of dislocation densities an increase in work hardening during primary creep is achieved. The role of substructure is included through the composite model. Cavity nucleation and growth are analysed in order to explain the intergranular fracture and to estimate the ductility.

  • 59.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Creep strain modelling of 9-12 Pct Cr steels based on microstructure evolution2007In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 38, no 9, p. 2033-Article in journal (Refereed)
    Abstract [en]

    Creep deformation is simulated for 9 pct Cr steels by using the Norton equation with the addition of back stresses from dislocations and precipitates. The composite model is used to represent the heterogeneous dislocation structure found in 9 to 12 pct Cr steels. Dislocation evolution is modeled by taking capturing and annihilation of free dislocations into account. Recovery of immobile dislocations is derived from the ability of dislocation climb. In spite of the fact that the initial dislocation density is high and is reduced during creep, primary creep is successfully modeled for a P92 steel. Subgrain growth is evaluated using a model by Sandström (1977). The long time subgrain size corresponds well to a frequently used empirical relation, with subgrain size inversely proportional to the applied stress.

  • 60.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Growth of Creep Cavities in 12% Cr Steels2009In: Creep & Fracture in High Temperature Components – Design & Life Assessment, 2nd International ECCC Conference, Empa, Dübendorf, Switzerland, 21-23 April, 2009 / [ed] I A Shibli, S R Holdsworth, LANCASTER, PA: DESTECH PUBLICATIONS, INC , 2009, p. 950-963Conference paper (Refereed)
    Abstract [en]

    The nucleation and growth of creep cavities will eventually occupy a considerable fraction ofthe grain boundary. This will lead to microcracks and intergranular fracture thus controllingthe ductility of the component. The traditional approach to predicting this type of failure is tosimulate cavities with only one size. Assumptions of an instant nucleation with symmetricallyplaced cavities make all cavities equally sized. It has been observed, in 12% Cr steels as wellas in other commercial alloys that cavities nucleate during all stages of creep. Creep cavitiesget randomly placed mostly at grain boundaries directed transverse to the loading direction.With continuous nucleation a size distribution of cavities appears, which is compared toobserved average cavity size. Constraints on cavity growth are introduced, which reduces thegrowth rate. This is needed in order to explain the cavity growth of 12% Cr steels.Furthermore, creep rupture will be derived based on the area fraction of cavities, thus explaining the intergranular failure.

  • 61.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Influence of aluminium on creep strength of 9–12% Cr steels2009In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 527, p. 118-125Article in journal (Refereed)
    Abstract [en]

    The influence of aluminium on creep strength of 9% Cr steels is predicted by a fundamental model forcreep. Through thermodynamic modelling the particle structure is determined for a temperature andcomposition range. This shows how AlN is formed at the expense of MX carbonitrides of VN characterwhen the aluminium content is increased. The remaining MX particles are of NbC type and have approximatelyone fifth of the original phase fraction. The evolution in microstructure such as particle coarseningis included in the model as well as the recovery. Rupture time is predicted using a modified Norton equationincluding back-stresses calculated from microstructure. The predictions show correspondence tosome of the creep data for the steel P91 over a temperature and stress range. Furthermore, simulationwith high Al content verifies the observed early failure of Al rich components. Overall, the simulationsshow a decrease in rupture time by a factor 6 due to Al additions of up to 0.2%.

  • 62.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels2010In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 41A, no 13, p. 3340-3347Article in journal (Refereed)
    Abstract [en]

    The influence of weld simulated heat treatments of 9-12% steels is evaluated by a fundamental model for creep. The heat affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multi-component system, showing significant M23C6 coarsening in the BCC matrix. Dissolution simulations of MX and M23C6 are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modelling is performed based on the heat affected microstructure, showing strength reduction of weld simulated material by 12% at 850ºC and 26% at 900ºC. The main cause of this degradation is believed to be the loss of the smallest carbonitrides.

  • 63.
    Magnusson, Hans
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Modelling of the influence of Laves phase on the creep properties in 9% Cr steels2007In: PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE / [ed] Jaske, CE; Jaske, CE, 2007, p. 519-526Conference paper (Refereed)
    Abstract [en]

    Nucleation and growth of Laves phase are calculated for a multi-component system. Coarsening Of MX, M23C6 and Laves are also determined. The influence on creep strength is discussed by analysing particle hardening and solid solution strengthening. A model for particle size distribution is presented in order to determine the amount of dislocations that can climb across particles or generate Orowan loops.

    The model for solid solution hardening is based on a solution of Fick's second law with a moving frame of reference for the concentration profiles around a climbing dislocation. This is done in order to determine the slowdown in dislocations velocity due to solute drag. The results show a loss in creep strength as the Laves phase grows.

  • 64.
    Mahadeva, Sreekanth
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Magnetism in Band Gap Engineered Sputtered MgxZn(1-x)O Thin Films2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This dissertation presents a comprehensive study of the intrinsic room temperature ferromagnetism, RTFM, in technologically important thin films of ZnO, MgO, Mg@ZnO, the so-called d0–magnets that do not contain any intrinsic magnetic elements. We also present the first report on magnetism in Mn doped MgO films fabricated by dc magnetron sputtering. We have just published (April 2013 ‘on-line’) a state of the art review entitled ‘p-type ZnO Theory, growth, properties, and devices’ in the prestigious journal ‘Progress in Materials Science’, summarizing the recent advances of the studies on p-type ZnO thin films and pointing out the major challenges that remain in the field. The experimental work then focuses on the magnetic properties of band gap engineered Mg@ZnO films exploiting the fact that by substitutional doping of Mg for Zn in ZnO it is possible to tailor new materials with bandgap energy in the range 3.3 eV to 7.2 eV, thus extending the possibilities for new magnetic and optical device applications. In addition, we show that by doping Mn in MgO its magnetic properties can be enhanced to saturation values as high as 38.5 emu/cm3 in a 92 nm thick film. These studies involve extensive characterization of the high quality films in the thickness range of nanometers, using SQUID magnetometer for magnetic properties, XRD for structural analysis, Dual beam HRSEM/FIB and AFM for accurate film cross-sectioning and surface morphology, EDXS for elemental analysis, UV-VIS NIR for measuring the band gap of MgxZn(1-x)O films, Mg K-edge NEXAFS experiment in order to understand electronic structure of specific cations present in the thin films The origin of the observed room temperature feerromaganetism is attributed to cation vacancies and its consequences on the polarization about these vacancies in the oxides...

    ZnO films are promising materials for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p–n junction are the key steps to realize these applications. However, reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (VO and Zni) and/or hydrogen incorporation. Considerable efforts to obtain p-type ZnO by doping different elements with various techniques have resulted in remarkable progress in the field both from theoretical and experimental point of view. In our paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, VZn and Oi act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band gap engineering and room temperature ferro magnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.

    In presenting the current studies, we first focus on the sputtering process in order to produce high quality films. From a comparative study of RTFM, in MgO films deposited by sputtering from 99.999% pure metallic Mg, Vs MgO targets respectively on glass/Si substrates under identical ambience during deposition it is found that the metallic targets give the best magnetic properties (e.g: with maximum Ms values of ~13.75 emu/g vs ~ 4.2 emu/g respectively on Si substrates.(supplement 2). Furthermore, the Ms values are strongly film thickness dependent with Mg target while it is weakly so for films from MgO target. Also, the as deposited MgO films using metallic Mg target are found to be amorphous, while it is nanocrystalline when the films are sputtered off MgO targets. The overall Ms values are found to be dependent on the oxygen content in the atmosphere during deposition, increasing to 2.69 emu/g at a oxygen partial pressure of 40% of the total working gas pressure. On annealing to nanocrystallize these films in the temperature range 600 to 8000C strong XRD peaks corresponding to (200) orientation are observed, and Ms values decrease proportionately. (supplement 3).

    With the above information on studies for optimizing the effect of sputtering gas, film thickness, and oxygen partial pressure, PO2, comprehensive investigations on band gap engineering and magnetism in MgxZn(1-x)O co-sputtered thin films from Mg and Zn targets are then closely examined. The optical band gap calculated from absorption spectra shows that the band gaps of Mg-doped ZnO thin films increased linearly from 3.33 to 4.074 eV. Our studies indicate that both the magnetic properties and the band gap of the film can be tailored by tuning the film thickness and PO2 in the working gas.

    In summary, RTFM ordering in the thin films originates from cation vacancies which couple ferromagnetically and establish long range magnetic order. The ferromagnetic ordering in these materials is shown to arise from defects situated at the cation sites. Electronic structure studies of some selected films disclose that the unoccupied O 2p states exist and this unoccupied state results from cation vacancy (VMg).

  • 65.
    Mahadeva, Sreekanth K.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Fan, Jun C.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Biswas, Anis
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Sreelatha, K. S.
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Magnetism of amorphous and nanocrystalized dc-sputter-deposited MgO Thin Films2013In: NANOMATERIALS, ISSN 2079-4991, Vol. 3, no 3, p. 486-497Article in journal (Refereed)
    Abstract [en]

    We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O-2 containing working gas atmosphere of (N-2 + O-2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (P-O2) similar to 10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (MS) value of 2.68 emu/cm(3) obtained for the MgO film deposited in P-O2 of 10% increases to 9.62 emu/cm3 for film deposited at P-O2 of 40%. However, the MS values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 degrees C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, MS decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

  • 66.
    Mahadeva, Sreekanth K.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Quan, Zhi-Yong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fan, Jun C.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Sreelatha, K. S.
    Belova, Ljubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Puzniak, Roman
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Suppression of Ferromagnetic Ordering in thicker co-sputtered Mn doped MgO Films2013In: Nanostructured metal oxides for advanced applications: symposium held April 1-5, 2013, San Francisco, California, U.S.A, Materials Research Society, 2013, Vol. 1552, p. 83-88Conference paper (Refereed)
    Abstract [en]

    We report on preliminary studies of low (14 at.%) and high (53at.%) concentration Mn doped MgO films deposited by co-sputtering from metallic Mn and Mg targets. The structural, surface morphologies and magnetic properties of the films of different thickness were studied. All the as grown films are found to be amorphous and film surfaces are found to be flawless and homogeneous. We observe at room temperature robust ferromagnetic loops with a saturation magnetization value that is a function of film thickness reaching a maximum of ~38.5 emu/cm3 in the Mn0.53Mg0.47O film at a thickness of ~92 nm. In thicker films room-temperature ferromagnetic ordering is suppressed and eventually at a thickness around 120nm the expected diamagnetism of the bulk appears. The origin of ferromagnetism may be attributed to cation defects at the Mg-site.

  • 67.
    Mahdavi Shahri, Meysam
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue Assessment of Friction Stir Welded Joints in Aluminium Profiles2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Friction stir welding (FSW) is a low heat input solid state welding technology. It is often used for fabrication of aluminium alloys in transportation applications including railway, shipbuilding, bridge structures and automotive components. In these applications the material is frequently subject to varying load conditions and fatigue failure is a critical issue. In most cases standard codes and fatigue guidelines for aluminium welded joints address only welded structures with conventional welding methods but not those with FSW procedure. In the scope of this thesis fatigue life assessment of friction stir welded components was performed using theoretical approaches along with finite element method (FEM). The further aim of this study was to generate a basis for standardization of fatigue assessment of friction stir welded joints.

    Friction stir welded hollow aluminium panels of alloy 6005A are investigated. The panels are used for train wall sides, train floors, deck and bridges. Each panel is made of several profiles that are joined with the friction stir welding method. Fatigue bending tests were performed for profiles in these panels. Fatigue cracks and failure appeared at notches in the profiles. With FEM simulations critical positions for crack initiation and failure were identified. The method of critical distance was used to analyse and estimate the fatigue life. It was shown that the failure location and fatigue limit could be predicted for both base metal and weld location. Choice of welding procedure (clamping condition) can significantly influence the fatigue life. It was shown that for some panels the critical distance method was not able to explain the failure in the weld. In this case fracture mechanics together with residual stress analysis were used successfully to predict the failure.

    Assuming homogeneous material properties throughout the weld and the base material, FEM analysis for T and overlap joints as well can provide a reasonable fatigue prediction. This suggests that the same assumption can be extended to complex components for failure analysis of the friction stir welded joints when using the critical distance method.

    Fatigue assessment of friction stir welded joints was also performed using standard codes Eurocode 9 and IIW. Fatigue curves of traditional fusion welded joints were used. The results are in reasonable agreement with experimental data and FEM predictions.

  • 68.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Höglund, Torsten
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Eurocode 9 to estimate the fatigue life of friction stir welded aluminium panels2012In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 45, p. 307-313Article in journal (Refereed)
    Abstract [en]

    Eurocode 9 is a standard that covers the design of building and engineering structures made from wrought and cast aluminium alloys. A part of the Eurocode 9 handles the design of aluminium structures susceptible to fatigue. Eurocode 9 has data for aluminium alloys and welded structures for conventional welding methods (fusion welding) except for friction stir welding processes. The present study compares fatigue test results from friction stir welded joints with fatigue curves of traditional fusion welded joints which are presented in Eurocode 9. The results are in reasonable agreement with experimental data and FEM predictions. This suggests that Eurocode 9 can be used for estimating the fatigue strength of friction stir welded joints.

  • 69.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Effective notch stress and critical distance method to estimate the fatigue life of T and overlap friction stir welded joints2012In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 25, p. 250-260Article in journal (Refereed)
    Abstract [en]

    The interface notch is a general characteristic of friction stir welded joints and appears in many types of the welds. Indeed interface notch most likely is a location that fatigue failure can occur there. This paper is concerned with the fatigue of friction stir welded joints. In the present paper fatigue of T and over lap joints were modelled base on critical distance method. Two material properties Delta K-th, the fatigue threshold stress intensity factor and Delta sigma(0), the fatigue limit were used in order for estimating the fatigue life at interface location. This is shown that assuming homogeneous material throughout the weld and the base material in FE analysis still can provide a reasonable fatigue prediction for friction stir joints when using critical distance method. This suggests the same procedure can be used for complex component when failure appears in the interface notch. In order to verify it the critical distance prediction method was also used for actual component where the failure occurred in the weld. The predicted fatigue limit deviated only 15% from the experimental result.

  • 70.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of fabrication stresses on fatigue life of friction stir welded aluminium profiles2012In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 212, no 7, p. 1488-1494Article in journal (Refereed)
    Abstract [en]

    Influence of clamping and welding procedure on fatiguelife of a frictionstir (FS) weldedaluminiumprofile has been analysed. Two series of aluminiumprofiles (series A and B) have been fatigue tested. The different clamping conditions in series A and B gave rise to different failure positions that could not be explained neither by the effective notch stress method, nor by the critical distance methods. The clamping in series A introduced plastic deformation at the weld notch that gave rise to tensile residual stresses. The failure position could be explained from the value of the stress intensity factor provided that the residual stresses were considered.

  • 71.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Osikowicz, Wojciech
    Sapa Technology.
    Critical distance method to estimate the fatigue life time of friction stirwelded profiles2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 37, p. 60-68Article in journal (Refereed)
    Abstract [en]

    The fatigue failure of friction stir (FS) welded aluminium panels of alloy 6005A has been analysed. Thesepanels are produced with two main joint geometries: namely half overlap and hourglass. Presence ofcrack like notches (interface between the welded work pieces) and blunt notches (corners and bendsin the base metal) have been studied with finite element method (FEM) stress analysis combined withthe critical distance method. It was shown that the failure location and fatigue limit could be predictedfor three of the four types of profiles considered. Choice of the welding procedure (clamping condition),however, can significantly influence the fatigue life and prediction accuracy. When different welding procedurewas used the weld failure was not reproduced by model and failure location was not predictedcorrectly. In this case fracture mechanics approach along with residual stress analysis has been used toanalyse the failure. It is also shown that local plastic deformation on the crack tip is induced by clampingresulting in tensile residual stresses at the crack tip. Taking stress intensity factor of the residual stressinto account, the position of the failure can be explained

  • 72.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandstöm, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue Strength of Friction Stir WeldedAluminium Profile for Train Car Application2008Conference paper (Refereed)
    Abstract [en]

    Friction stir welded aluminium alloys are used for many applications in transportation. In theseapplications the material is frequently subject to varying load conditions, making fatigue failure acritical issue. In the scope of this paper, the fatigue performance of friction stir welded profiles ofAl-alloy 6005A has been investigated. A profile that is used for railway car wall side panels waschosen. The profiles were joined together with the friction stir welding method at both sides ofthe profile. 3-point fatigue bending tests were performed for the profiles. Stress ranges givingfailure after 105 to 106 cycles with the stress ratio R=0.1 were used.With FEM simulations critical positions for crack initiation and failure were identified in the parentmetal. These positions were all at narrow radii in the profiles. In fact, the fatigue failures alwaysoccurred at these positions. The maximum stress varied somewhat between the positions. Inspite of this, the cracking and failure took place at six different positions indicating that the profilehad a balanced design.The maximum von Mises stress at the failure positions as determined with FEM was about 50%higher than the corresponding uniaxial fatigue data for the same number of cycles. Thissuggests that the cracks initiate at places with high stresses but propagate into areas with lowerstresses.The FSW joint was a lap-butt joint with a sharp notch (interface between the work pieces) next tothe weld nugget. Since both sides of the profile were welded, such sharp notches appeared onboth sides. In most profiles cracks initiated and propagated form the notch where the centre loadwas placed. However, failure never took place there, i.e. complete fracture was not observed.The direction in which the cracks propagated was perpendicular to the plane of the profiles andperpendicular to the maximum stress direction. On the opposite side to the load position nocracking was observed

  • 73. Martinsson, Åsa
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Hydrogen depth profile in phosphorus-doped, oxygen-free copper after cathodic charging2012In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 47, no 19, p. 6768-6776Article in journal (Refereed)
    Abstract [en]

    Spent nuclear fuel, in Sweden, is planned to be put in 50-mm thick copper canisters and placed in 500-m depth in the bedrock. Depending on the conditions in the repository, an uptake of hydrogen in the copper may occur. It is therefore necessary to establish how a hydrogen uptake affects the microstructure in both the surface and the bulk. Phosphorus-doped, oxygen-free copper has been cathodically charged with hydrogen for up to 3 weeks. The amount of hydrogen as a function of the distance from the surface was measured by two methods: glow discharge optical emission spectrometry and melt extraction. The penetration of the increased hydrogen content was about 50 mu m. Extensive bubble formation took place during the charging. A model has been formulated for the diffusion of hydrogen into the copper, the bubble formation and growth. The model can describe the total amount of hydrogen, the number of bubbles and their sizes as a function of the distance from the surface. Bubbles close to the surface caused the surface to bulge due to the high hydrogen pressure. From the shape of the deformed surface, the maximum hydrogen pressure could be estimated with the help of stress analysis. The maximum pressure was found to be about 400 MPa, which is almost an order of magnitude larger than previously recorded values for electroless deposited copper.

  • 74. Masood Hafez Haghighat, Seyed
    et al.
    Martin-Bragado, Ignacio
    Lousada, Cláudio M.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Korzhavyi, Pavel A.
    Discrete Models: Down to Atoms and Electrons2016In: Handbook of Software Solutions for ICME, Weinheim, Germany: Wiley-VCH Verlagsgesellschaft, 2016, p. 385-431Chapter in book (Other academic)
    Abstract [en]

    This chapter reviews some computational methods and software tools available for modeling materials at small scales where the matter exhibits its discrete nature. Discrete models operate on different levels, ranging from the electronic structure and atomistic level to the mesoscopic level at which the discrete objects are molecules or crystal defects rather than the individual atoms. The chapter considers the main methods of atomistic modeling, molecular dynamics (MD) and Monte Carlo (MC), and quantum mechanics (QM) electronic structure methods. It discusses the effective potentials, interatomic interactions, and force fields (FFs), which serve the purpose of passing the physical information between the different levels of modeling. Two atomistic methods are also reviewed. Kinetic MC (KMC) is first introduced as a technique able to link the nanoscale and mesoscale domains but relying heavily on atomistic parameterizations. Then, the powerful MD technique is explained, and some links between both techniques are also hinted.

  • 75. Miranda Pérez, A. F.
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Calliari, I.
    Reyes Valdés, F. A.
    Sigma phase precipitation on welded SAF 2205 Duplex Stainless Steels after isothermal heat treatment2014In: Materials Research Society Symposium Proceedings, 2014, p. 177-182Conference paper (Refereed)
    Abstract [en]

    Duplex stainless steels are commonly used for various applications owing to their superior corrosion resistance and/or strength. They have ferromagnetic behavior together with a good thermal conductivity and a lower thermal expansion as a result of higher ferrite content than austenitic steels. Their ferrite matrix suffers a decomposition process during aging in the temperature range 650-950°C producing precipitation of austenite, σ and x, carbides and nitrides. These intermetallic phases are known to be deleterious for corrosion resistance and mechanical properties. In this work the effect of aging time during isothermal treatment at 850°C and 900°C on the microstructure of SAF 2205 Duplex Stainless Steels welded plates has been investigated. The aim of this work is to determine the morphology of σ phase, and perform a quantitative analysis of the precipitation process Submerged Arc Welding is used for processing. It produces a high content of δ ferrite in the heat affected zone and low content of austenite in the weld. Microstructural examination shows that the σ phase precipitates at δ ferrite/γ interphases. Longer aging treatments give rise to an increase of volume fraction together with a coarser morphology.

  • 76. Moosapour, Mina
    et al.
    Hajabasi, Mohammad Ali
    Ehteshami, Hossein
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Thermoelastic damping effect analysis in micro flexural resonator of atomic force microscopy2014In: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 38, no 11-12, p. 2716-2733Article in journal (Refereed)
    Abstract [en]

    In the design of high-Q micro/nano-resonators, dissipation mechanisms may have damaging effects on the quality factor (Q). One of the major dissipation mechanisms is thermoelastic damping (TED) that needs an accurate consideration for prediction. Aim of this paper is to evaluate the effect of TED on the vibrations of thin beam resonators. In particular, we will focus on cantilever beam resonator used in atomic force microscopy (AFM). AFM resonator is actually a cantilever with a spring attached to its free end. The end spring is considered to capture the effect of surface stiffness between tip and sample surface. The coupled governing equations of motion of thin beam with consideration of TED effects are derived. In general, there are four elastic equations that are coupled with thermal conduction equation. Based on accurate assumptions, these equations are simplified and the various boundary conditions have been used in order to validate the computational procedure. In order to accurately determine TED effects, the coupled thermal conduction equation is solved for the temperature field by considering three-dimensional (3-D) heat conduction along the length, width and thickness of the beam. Weighted residual Galerkin technique is used to obtain frequency shift and the quality factor of the thin beam resonator. The obtained results for quality factor, frequency shift and sensitivity change due to thermo-elastic coupling are presented graphically. Furthermore, the effects of beam aspect ratio, stress-free temperature on the quality factor and the influence of the surface stiffness on the frequencies and modal sensitivity of the AFM cantilever with and without considering thermo-elastic damping effects are discussed.

  • 77.
    Pilhagen, Johan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    The fracture mechanisms in duplex stainless steels at sub-zero temperatures2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The aim of the thesis was to study the susceptibility for brittle failures and the fracture process of duplex stainless steels at sub-zero temperatures (°C). In the first part of the thesis plates of hot-rolled duplex stainless steel with various thicknesses were used to study the influence of delamination (also known as splits) on the fracture toughness. The methods used were impact and fracture toughness testing. Light optical microscopy and scanning electron microscopy were used to investigate the microstructure and fracture surfaces. It was concluded that the delaminations caused a loss of constraint along the crack front which resulted in a stable fracture process despite the presence of cleavage cracks. These delaminations occurred when cleavage cracks are constrained by the elongated austenite lamellae. The pop-in phenomenon which is frequently observed in duplex stainless steels during fracture toughness testing was shown to occur due to these delaminations. The susceptibility for pop-in behaviour during testing increased with decreasing plate thickness. The toughness anisotropy was also explained by the delamination phenomenon.In the second part of the thesis duplex stainless steel weld metals from lean duplex and super duplex were investigated. For the lean duplex weldments with different nickel contents, tensile, impact and fracture toughness testing were conducted from room temperature to sub-zero temperatures. The result showed that increased nickel content decreased the susceptibility for critical cleavage initiation at sub-zero temperatures. The super duplex stainless steel weldment was post weld heat treated. The fracture sequence at low temperature was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture. Energy-dispersive X-ray spectroscopy investigation of the weld metals showed that substitutional element partitioning is small in the weld metal. However, for the post weld heat treated weldments element partitioning occurred which resulted in decreased nickel content in the ferrite.

  • 78.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Delaminations by Cleavage Cracking in Duplex Stainless Steels at Sub-zero Temperatures2014In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45A, no 3, p. 1327-1337Article in journal (Refereed)
    Abstract [en]

    Impact toughness testing was conducted on 10 and 30 mm plates of 2205 together with a 30 mm plate of LDX 2101(A (R)) duplex stainless steel (DSS). The testing temperatures were between 153 K (-120 A degrees C) and room temperature. Interrupted fracture toughness tests of the 10 mm plate and a 50 mm plate of 2205 were also performed. The conclusion from the fractographic investigation was that the delaminations that occur in hot-rolled DSSs were cleavage fractures. The toughness anisotropy can be explained by the cleavage fracture and the appearance of the microstructure. The result from the interrupted fracture toughness test revealed that the delaminations initiated prior to the maximum force plateau and propagated ahead of the stable crack growth during testing. Estimated upper limit for the fracture delamination initiation toughness at sub-zero temperatures for the 2205 base metal according to the crack-tip opening displacement method was 28 to 61 mu m for the 10 mm plate, 70 to 106 mu m for the 30 mm plate and below 100 mu m for the 50 mm plate.

  • 79.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of lattice orientation on the fracture toughness of duplex stainless steel 22052010In: Duplex World 2010, Beaune, France, 13-15 October 2010, 2010Conference paper (Other academic)
    Abstract [en]

    Fracture toughness measurements on single-edge bend T-L specimens at low temperatures forduplex stainless steels show that 10 mm plate is prone to exhibit pop-in failure. These pop-in have been relatedto the initiation of splits that is commonly seen after fracture toughness testing of this type of material.Microstructural and texture characterisations have been conducted with light optical microscope and electronbackscatter diffraction to evaluate the cause behind split initiation and why these splits cause pop-in behaviourin thinner plates. The results are that 10 mm plates have a strong cube-on-edge texture in the ferritic phase whilethe 50 mm plate has a weak texture. This indicates that the initiation of splits may be due to the interactionbetween ferrite and austenite crystallographic orientation and on the stress and strain constraints enforced by thelamellar microstructure. This uniform lattice orientation for the ferritic phase in the 10 mm plate can explainwhy thinner plates of 2205 are prone to exhibit pop-in behaviour during fracture toughness testing.

  • 80.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of nickel on the toughness of lean duplex stainless steel welds2014In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 602, p. 49-57Article in journal (Refereed)
    Abstract [en]

    Three weldments with the nickel contents 13, 4.9 and 6.0 wt% were made from 30 mm LDX 2101((R)) plates. The weldments were subjected to tensile, impact and fracture toughness testing. The aim was to evaluate the susceptibility for brittle failure in the weld metal at sub-zero temperatures (degrees C). The amount of ferrite was higher for the 1.3 wt% nickel weldment compared to the other two which had similar phase composition and mean free ferrite distance. The result from the tensile testing showed that for the weldment with the highest nickel content the ductility remained unchanged with decreasing temperature while the other two weldments became less ductile with decreasing temperature. J-integral based fracture toughness testing showed a significant difference in the susceptibility for brittle failure with higher values for the weldment with 6 wt% nickel than for the others with lower nickel content.

  • 81.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Loss of constraint during fracture toughness testing of duplex stainless steels2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 99, p. 239-250Article in journal (Refereed)
    Abstract [en]

    Delamination of the fracture surfaces, so called splits, is an important phenomenon that occurs at sub-zero temperature for hot-rolled duplex stainless steels during impact and fracture toughness testing. To evaluate how the splits influence the fracture toughness, sub-zero temperature fracture toughness testing of 50, 30 and 10 mm thick plates of hot rolled 2205 duplex stainless steel was performed. The results show that the splits cause loss of constraint along the crack front. This can be observed as local difference in crack growth in the specimen. The initiation fracture toughness is not influenced by the specimen thickness. Furthermore, due to the delamination the material exhibits a stable fracture process despite the presence of cleavage fracture. This is interfering with the master curve method so for evaluating the fracture toughness at sub-zero temperatures an assessment of the fracture resistance curve is instead suggested. For assessing the brittle crack behaviour at sub-zero temperatures it is proposed to use the split initiation as a "failure" criteria. The splits are also the cause of the pop-in behaviour observed for the duplex stainless steels. The susceptibility for pop-in is influenced by the microstructure.

  • 82.
    Pilhagen, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sieurin, Henrik
    Scania CV AB, Södertälje, Sweden.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fracture toughness of a welded super duplex stainless steel2014In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 606, p. 40-45Article in journal (Refereed)
    Abstract [en]

    Fracture toughness testing was conducted on standard single-edge notched bend bar specimens of base and weld metal. The material was the SAF 2906 super duplex stainless steel. The aim was to evaluate the susceptibility for brittle failure at sub-zero temperatures for the base and weld metal. The base metal was tested between -103 and -60. °C and was evaluated according to the crack-tip opening displacement method. The fracture event at and below -80. °C can be described as ductile until critical cleavage initiation occurs, which caused unstable failure of the specimen. The welding method used was submerged arc welding with a 7. wt% nickel filler metal. The welded specimens were post-weld heat treated (PWHT) at 1100. °C for 20. min and then quenched. Energy-dispersive X-ray spectroscopy analysis showed that during PWHT substitutional element partitioning occurred which resulted in decreased nickel content in the ferrite. The PWHT weld metal specimens were tested at -72. °C. The fracture sequence was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture.

  • 83.
    Pirouznia, Pouyan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    High cycle fatigue properties of stainless martensitic chromium steel springs2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    For many materials and components like in high speed trains and airplanes fatigue failures occur in the range of over 107 load cycles which is called the high cycle fatigue range. A modern version of the springs was invented which are applied in a certain application.

    Ultrasonic fatigue testing (20 kHz machine) was conducted for evaluating the steel of the springs. This research explores the fundamental understanding of high cycle fatigue testing of strip steel and assesses a stainless martensitic chromium steel at the high cycle fatigue range. Finite element modeling was conducted to gain knowledge about the effect of various parameters. Significant attention was devoted to the fatigue failure initiations by SEM/EDS.

    The work demonstrated that the method of investigation for high cycle fatigue test is reliable. Fatigue failure at this range was initiated by internal defects which all included non-metallic inclusion. A critical distance was defined Within the strip fatigue specimen where all the fatigue failure initiated. The 3D stress field in the specimen was determined by FEM modeling and the local applied stress at the whole of the flat part of specimen and critical distance was estimated. FEM was also employed to give additional information about the effect of parameters. It was established that damping had the largest influence. The local applied stress of the fatigue test was calculated by means of FEM and SEM analysis. It was used to adjust the S-N curve which resulted in 15% lower values than the nominal applied stress.

  • 84. Pérez, A. F. M.
    et al.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Calliari, I.
    Valdés, F. A. R.
    Sigma phase precipitation on welded SAF 2205 Duplex Stainless Steels after isothermal heat treatment2014In: Journal of the Society for American Music, ISSN 1752-1963, Vol. 1611, no 2Article in journal (Refereed)
    Abstract [en]

    Duplex stainless steels are commonly used for various applications owing to their superior corrosion resistance and/or strength. They have ferromagnetic behavior together with a good thermal conductivity and a lower thermal expansion as a result of higher ferrite content than austenitic steels. Their ferrite matrix suffers a decomposition process during aging in the temperature range 650-950° C producing precipitation of austenite, σ and χ, carbides and nitrides. These intermetallic phases are known to be deleterious for corrosion resistance and mechanical properties. In this work the effect of aging time during isothermal treatment at 850°C and 900°C on the microstructure of SAF 2205 Duplex Stainless Steels welded plates has been investigated. The aim of this work is to determine the morphology of σ phase, and perform a quantitative analysis of the precipitation process. Submerged Arc Welding is used for processing. It produces a high content of δ ferrite in the heat affected zone and low content of austenite in the weld. Microstructural examination shows that the σ phase precipitates at δ ferrite/γ interphases. Longer aging treatments give rise to an increase of volume fraction together with a coarser morphology.

  • 85.
    Randelius, Mats
    et al.
    Swerea Kimab, Sweden.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. Swerea Kimab, Sweden.
    Melander, Arne
    KTH, School of Industrial Engineering and Management (ITM), Production Engineering, Welding Technology. Swerea Kimab, Sweden.
    Fatigue Strength of Conventionally Cast Tool Steels and its Dependence of Carbide Microstructure2012In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 83, no 1, p. 83-90Article in journal (Refereed)
    Abstract [en]

    The axial fatigue strength at two million cycles was experimentally determined for two conventionally cast tool steels and successfully compared with results from a fatigue limit model. Specimens were tested both in the rolling and transverse direction and showed large differences in fatigue properties due to the segregated carbide microstructure. Rolling direction specimens experienced higher fatigue strength than the transverse direction specimens. This is due to smaller carbides present in the load affected cross section of the rolling direction fatigue test bars compared to the cross section of the transverse direction fatigue test bars. Fractographic analysis of failed specimens showed that large carbides had caused fatigue failure, which was also predicted by the model. Measured size distributions of carbides and inclusions were used as input data in the model. The probability that at least one particle will be present in the material volume having a size larger than the threshold value for crack propagation was calculated.

  • 86.
    Razumovskiy, Vsevolod
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, VinnExcellence Centre for Hierarch. Eng. of Industrial Materials, HERO-M.
    Thermodynamic and kinetic properties of Fe-Cr and TiC-ZrC alloys from Density Functional Theory2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The complete and accurate thermodynamic and kinetic description of any systemis crucialfor understanding and predicting its properties. A particular interest is in systemsthat are used for some practical applications and have to be constantly improved usingmodification of their composition and structure. This task can be quite accuratelysolved at a fundamental level by density functional theory methods. Thesemethods areapplied to two practically important systems Fe-Cr and TiC-ZrC.The elastic properties of pure iron and substitutionally disordered Fe-Cr alloy are investigatedas a function of temperature and concentration using first-principles electronicstructurecalculations by the exact muffin-tin orbitals method. The temperature effectson the elastic properties are included via the electronic, magnetic, and lattice expansioncontributions. It is shown that the degree of magnetic order in both pure iron andFe90Cr10 alloy mainly determines the dramatic change of the elastic anisotropy of thesematerials at elevated temperatures. A peculiarity in the concentration dependence ofthe elastic constants in Fe-rich alloys is demonstrated and related to a change in theFermi surface topology.A thermodynamic model for the magnetic alloys is developed from first principles andapplied to the calculation of bcc Fe-Cr phase diagram. Various contributions to the freeenergy (magnetic, electronic, and phonon) are estimated and included in the model. Inparticular, it is found that magnetic short range order effects are important just abovethe Curie temperature. The model is applied for calculating phase equilibria in disorderedbcc Fe-Cr alloys. Model calculations reproduce a feature known as a Nishizawahorn for the Fe-rich high-temperature part of the phase diagram.The investigation of the TiC-ZrC system includes a detailed study of the defect formationenergies and migration barriers of point defects and defect complexes involvedin the diffusion process. It is found, using ab initio atomistic simulations of vacancymediateddiffusion processes in TiC and ZrC, that a special self-diffusion mechanism isoperative for metal atom diffusion in sub-stoichiometric carbides. It involves a noveltype of a stable point defect, a metal vacancy ”dressed” in a shell of carbon vacancies.It is shown that this vacancy cluster is strongly bound and can propagate through thelattice without dissociating.

  • 87.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Effect of temperature on the elastic anisotropy of pure Fe and Fe0.9Cr0.1 random alloy2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 20, article id 205504Article in journal (Refereed)
    Abstract [en]

    The elastic properties of pure iron and substitutionally disordered 10 at. % Cr Fe-Cr alloy areinvestigated as a function of temperature using first-principles electronic-structure calculations bythe exact muffin-tin orbitals method. The temperature effects on the elastic properties are includedvia the electronic, magnetic, and lattice expansion contributions. We show that the degree ofmagnetic order in both pure iron and Fe90Cr10 alloy mainly determines the dramatic change of theelastic anisotropy of these materials at elevated temperatures. The effect of lattice expansion isfound to be secondary but also very important for quantitative modeling.

  • 88.
    Razumovskiy, Vsevolod I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Vacancy-cluster mechanism of metal-atom diffusion in substoichiometric carbides2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 5, p. 054203-Article in journal (Refereed)
    Abstract [en]

    We find, using ab initio atomistic simulations of vacancy-mediated diffusion processes in TiC and ZrC, that a multivacancy self-diffusion mechanism is operative for metal-atom diffusion in substoichiometric carbides. It involves a special type of a stable point defect, a metal vacancy "dressed" in a shell of carbon vacancies. We show that this vacancy cluster is strongly bound and can propagate through the lattice without dissociating.

  • 89.
    Razumovskiy, Vsevolod
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling. Mat Ctr Leoben Forsch GmbH, A-8700 Leoben, Austria.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Dilner, David
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Korzhavyi, Pavel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Effect of carbon vacancies on thermodynamic properties of TiC-ZrC mixed carbides2014In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 46, p. 87-91Article in journal (Refereed)
    Abstract [en]

    Thermodynamic properties of a TiZrC mixed carbide system are investigated by first-principles methods within density functional theory. Carbon vacancies are found to have a significant contribution to the thermodynamics of TiZrC mixed carbides. The temperature effect on the thermodynamic properties of the system is calculated taking into consideration the corresponding electronic and vibrational thermal excitations.

  • 90.
    Ruban, Andrei V.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Razumovskiy, Vsevolod I.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Spin-wave method for the total energy of paramagnetic state2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, p. 174407-Article in journal (Refereed)
    Abstract [en]

    Spin-wave formalism provides a convenient alternative way of modeling the high-temperature paramagnetic state for a certain type of magnets within the framework of Hamiltonian-type electronic-structure methods. For Heisenberg systems, it is formally equivalent to the so-called disordered local moment approach, which is usually used in the methods based on the coherent potential approximation within the Green's function or multiple-scattering techniques. In this paper, we demonstrate that the spin-wave method has certain advantages when it comes to the calculation of forces and relaxations. It also allows one to take magnetic short-range-order effects into consideration. As examples of the application of the spin-wave method, we calculate the energy of the paramagnetic state in fcc Co and bcc Fe, the vacancy formation energy, elastic constants, and phonon spectrum in bcc paramagnetic Fe. We demonstrate that magnetic short-range-order effects play a crucial role in the mechanical stabilization of the bcc Fe at high temperature in the paramagnetic state.

  • 91.
    Sandberg, Nils
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Chang, Zhongwen
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Korzhavyi, Pavel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modeling of the magnetic free energy of self-diusion in bcc FeManuscript (preprint) (Other academic)
  • 92.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Basic model for primary and secondary creep in copper2012In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 1, p. 314-322Article in journal (Refereed)
    Abstract [en]

    A model for primary and secondary creep is established. The starting point is a combination of basic models for tensile stress-strain curves and for secondary creep. The derived model can describe experimental creep strain curves for Cu 50 p.p.m. P in the temperature interval 75-250 degrees C with the same precision as the variation in the experimental creep strain curves for identical conditions. No fitting parameters are involved in this representation. The model has also been applied successfully to creep specimens with round notches, where multiaxial stress states are present. One main aim of deriving a basic creep model is to improve the accuracy of extrapolation. That this is possible has been demonstrated for creep tests with estimated secondary creep rates down to 5 x 10(-22) s(-1). In conventional creep rupture testing strain rates down to 1 x 10(-12) s(-1) can be recorded. As a background, a summary of empirical methods for the extrapolation of creep strain data is given.

  • 93.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Influence of phosphorus on the tensile stress strain curves in copper2016In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 470, p. 290-296Article in journal (Refereed)
    Abstract [en]

    Copper canisters are planned to be used for final disposal of spent nuclear fuel in Sweden. The canisters will be exposed to slow plastic straining over extensive periods of time. To be able to predict the mechanical properties a range of basic models have previously been developed for copper with and without phosphorus (Cu-OFP, Cu-OF). Already with the small amount of phosphorus added in the canisters (60 wt. ppm) dramatic improvements in the measured creep strength and the creep ductility are found. The basic models are further developed in the present paper. The influence of phosphorus on slow strain rate tests is analysed. It is shown that the main effect of phosphorus is that it prevents brittle rupture, which is modelled by taking creep cavitation into account.

  • 94.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Andersson, H. C. M.
    Creep in phosphorus alloyed copper during power-law breakdown2008In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 372, no 1, p. 76-88Article in journal (Refereed)
    Abstract [en]

    During the first phase of storage, creep will take place in the copper canisters in the KBS-3 package for nuclear waste. The temperatures are below 100 degrees C, and the creep is well inside the power-law breakdown regime. Creep models for this situation have been developed. The analysed material is pure copper with about 50 ppm phosphorus. Constitutive equations for creep and other plastic deformation have been set up based on a generalised Norton expression and Kocks-Mecking's model for the back stress. A model for the minimum creep rate based on fundamental principles for climb and glide has been derived. This model gives the correct order of magnitude for the creep rate in the temperature range from 400 to 20 degrees C without the use of fitted parameters. The creep exponent varies from 5 to 105 in this interval. The constitutive equations have also been formulated for multiaxial stress states.

  • 95.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Andersson, H. C. M.
    The effect of phosphorus on creep in copper2008In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 372, no 1, p. 66-75Article in journal (Refereed)
    Abstract [en]

    Pure copper with an addition of about 50 ppm phosphorus is the planned material for the outer part of the waste package for spent nuclear fuel in Sweden. Phosphorus is added to improve the creep ductility but it also strongly increases the creep strength. In the present paper the influence of phosphorus on the strength properties of copper is analysed. Using the Labusch-Nabarro model it is demonstrated that 50 ppm has a negligible influence on the yield strength in accordance with observations. For slow moving dislocations, the interaction energy between the P-atoms and the dislocations gives rise to an agglomeration and a locking. The computed break away stresses are in agreement with the difference in creep stress of copper with and without P-additions.

  • 96.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Andersson, Henrik C. M.
    Creep during power-law breakdown in phosphorus alloyed copper2008In: PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE 2007, VOL 9 / [ed] Jaske, CE; Jaske, CE, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2008, p. 419-426Conference paper (Refereed)
    Abstract [en]

    Copper alloyed with 50 ppm phosphorus (Cu-OFP) is selected for canisters for nuclear waste packages to avoid a low creep ductility, which is sometimes present in pure copper. The operating temperatures of these canisters are in the range from 0 to 100 degrees C. Creep readily takes place in copper even at room temperature. At temperatures below 100 degrees C, creep is well inside the power-law breakdown regime. The creep exponent is in the range from 30 to 100. Since creep models for this situation are missing in the literature, a new model for the minimum creep rate based on fundamental principles for climb and glide has been derived. This model gives the correct order of magnitude for the creep rate in the temperature range from 20 to 400 degrees C without the use of fitted parameters. Design against creep can either be based on the total applied stress or the effective stress. In the first case the constitutive equations can be directly obtained from the minimum experimental creep rates. A new approach is proposed to handle the effective stress case, which is based on the initial creep rates. The phi-model is used to relate the initial creep rate to the minimum one. It is shown how the constitutive equations for the creep rate and the back stress can be transferred to multiaxial stress states for use in FE-modelling.

  • 97.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Farooq, Muhammad
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Ivarsson, Bo
    Influence of particle formation during long time ageing on mechanical properties in the austenitic stainless steel 3102012In: Materials at High Temperature, ISSN 0960-3409, E-ISSN 1878-6413, Vol. 29, no 1, p. 1-7Article in journal (Refereed)
    Abstract [en]

    Intense precipitation may occur in high-temperature materials during service. This precipitation significantly affects the mechanical properties. Coarse precipitates can reduce ductility and toughness dramatically. To investigate the phenomenon, precipitation in the austenitic stainless steel AISI 310 is quantified in a parallel paper. The samples were aged up to 5000 h at 800 degrees C. Especially, sigma-phase and M23C6-carbides were analysed. A model for the influence of coarse particles on ductility has been developed and successfully applied to the measured values. A model due to Chen and Mott has been used to compute the effect of particles on toughness. A satisfactory agreement with the observations was obtained.

  • 98.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Farooq, Muhammad
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Zurek, Joanna
    Basic creep models for 25Cr20NiNbN austenitic stainless steels2013In: Materials research innovations (Print), ISSN 1432-8917, E-ISSN 1433-075X, Vol. 17, no 5, p. 355-359Article in journal (Refereed)
    Abstract [en]

    Basic models for solid solution and precipitation hardening during creep are presented for the austenitic stainless steels 25Cr20NiNbN (TP310HNbN, HR3C, DMV310N). The solid solution hardening is a result of the formation of Cottrell clouds of solutes around the dislocations. In addition to slowing down the creep, the solutes increase the activation energy for creep. The increase in activation energy corresponds to the maximum binding energy between the solutes and the dislocations. The formation of fine niobium nitrides during service enhances the creep strength. It is found that the nitrides have an exponential size distribution. In the modelling the critical event is the time it takes for a dislocation to climb over a particle. The creep models can accurately describe the observed time and temperature dependence of the creep rupture strength.

  • 99.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Hallgren, Josefin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    The role of creep in stress strain curves for copper2012In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 422, no 1-3, p. 51-57Article in journal (Refereed)
    Abstract [en]

    A model for plastic deformation in pure copper taking work hardening, dynamic recovery and static recovery into account, has been formulated using basic dislocation mechanisms. The model is intended to be used in finite-element computations of the long term behaviour of structures in Cu-OFP for storage of nuclear waste. The relation between the strain rate and the maximum flow stress in the model has been demonstrated to correspond to strain rate versus stress in creep tests for oxygen free copper alloyed with phosphorus Cu-OFP. A further development of the model can also represent the primary and secondary stage of creep curves. The model is compared to stress strain curves in compression and tension for Cu-OFP. The compression tests were performed at room temperature for strain rates between 5 x 10(-5) and 5 x 10(-3) s(-1). The tests in tension covered the temperature range 20-175 degrees C for strain rates between 1 x 10(-7) and 1 x 10(-4) s(-1). Consequently, it is demonstrated that the model can represent mechanical test data that have been generated both at constant load and at constant strain rate without the use of any fitting parameters.

  • 100.
    Sandström, Rolf
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modified Armstrong-Frederick Relation for Handling Back Stresses in FEM Computations2009In: CREEP & FRACTURE IN HIGH TEMPERATURE COMPONENTS: DESIGN & LIFE ASSESSMENT ISSUES, PROCEEDINGS / [ed] Shibli IA, Holdsworth SR, LANCASTER: DESTECH PUBLICATIONS, INC , 2009, p. 836-847Conference paper (Refereed)
    Abstract [en]

    If transient phenomena are important in creep design, a non-stationary creep model must be applied. One way of achieving this is to apply a back stress. The back stress reflects the history of the creep deformation. During primary creep the back stress normally increases from zero to a stationary value representing secondary creep. A common way to derive the back stress in FEM-modelling is to use the Armstrong-Frederick relation. It is demonstrated that this can give rise to unexpected results in the sense that back stress components can be much larger than the corresponding applied stress components. Modification of the Armstrong-Frederick relation are proposed that avoid this problem. An alternative approach is also given. In this case the back stress is assumed to be a scalar. Then its value can be derived in the same way as in the uniaxial case, for example using the Bergstrom-Kocks-Mecking model for FCC alloys. In the Odqvist equation for the creep rate, the effective stress is reduced with the back stress, but the stress deviator is left unmodified. The models are applied to copper canisters for storage of nuclear waste. It is demonstrated that the models give virtually the same result.

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