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  • 1.
    Krause, Andreas M.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Olsson, Pär A T
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Lund University.
    Music, Denis
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Interstitial diffusion of hydrogen in M7C3 (M=Cr,Mn,Fe)2023In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 218, article id 111940Article in journal (Refereed)
    Abstract [en]

    To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensitic steels, we have performed a density functional theory study on the solution energies and energy barriers for hydrogen diffusion in orthorhombic M7C3 (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice and cause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogen at different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03 eV/H-atom for Cr7C3, Mn7C3, and Fe7C3, respectively. Energy barriers for the diffusion of hydrogen atoms have been probed with the nudged elastic band method, which shows comparably low barriers for transport via interstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation between low solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that the presence of carbon tends to increase the energy barrier. Our results can explain previous experimental findings of hydrogen located in the bulk of CrC precipitates and provide a solid basis for future design efforts of steels with high strength and commensurable ductility.

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  • 2.
    Nigro, Claudio F.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). GKN Aerospace Sweden.
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Mellbin, Ylva
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Phase-field modelling: effect of an interface crack on precipitation kinetics in a multi-phase microstructure2021In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 227, p. 219-241Article in journal (Refereed)
    Abstract [en]

    Premature failures in metals can arise from the local reduction of the fracture toughness when brittle phases precipitate. Precipitation can be enhanced at the grain and phase boundaries and be promoted by stress concentration causing a shift of the terminal solid solubility. This paper provides the description of a model to predict stress-induced precipitation along phase interfaces in one-phase and two-phase metals. A phase-field approach is employed to describe the microstructural evolution. The combination between the system expansion caused by phase transformation, the stress field and the energy of the phase boundary is included in the model as the driving force for precipitate growth. In this study, the stress induced by an opening interface crack is modelled through the use of linear elastic fracture mechanics and the phase boundary energy by a single parameter in the Landau potential. The results of the simulations for a hydrogenated (α+β) titanium alloy display the formation of a precipitate, which overall decelerates in time. Outside the phase boundary, the precipitate mainly grows by following the isostress contours. In the phase boundary, the hydride grows faster and is elongated. Between the phase boundary and its surrounding, the matrix/hydride interface is smoothened. The present approach allows capturing crack-induced precipitation at phase interfaces with numerical efficiency by solving one equation only. The present model can be applied to other multi-phase metals and precipitates through the use of their physical properties and can also contribute to the efficiency of multi-scale crack propagation schemes.

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  • 3.
    Nigro, Claudio F.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Mellbin, Ylva
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Modeling of a Crack-induced Hydride Formation at a Grain Boundary in Metals2020In: International Journal of Offshore and Polar Engineering, ISSN 1053-5381, Vol. 30, no 4, p. 478-486, article id ISOPE-20-30-4-478Article in journal (Refereed)
    Abstract [en]

    Crack-induced hydride formation can occur in specific metallic structures, reducing their mechanical properties and facilitating failure. Grain boundaries are observed to be preferential sites for hydride formation. We present a phasefield approach describing the kinetics of crack-induced hydride formation at a grain boundary, by using the Allen-Cahn formulation and including the increase in grain boundary energy. Hydride development is found to occur at the crack tip and in the grain boundary. These regions seemingly evolve independently, except when the crack is very close to or lies in the grain boundary. 

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  • 4.
    Nigro, Claudio F.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Mellbin, Ylva
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Modelling of a Crack-Induced Hydride Formation near a Phase Boundary in Metals2019In: Proceedings of the Twenty-ninth (2019) International Ocean and Polar Engineering Conference / [ed] the International Society of Offshore and Polar Engineers (ISOPE), International Society of Offshore & Polar Engineers, 2019, p. 4186-4194Conference paper (Refereed)
    Abstract [en]

    In presence of hydrogen, crack-induced hydride formation can occur in specific metallic structures, reduce their mechanical properties and facilitate failure. Phase boundaries have been observed to be preferential site for hydride formation. The inevitable presence of both cracks and phase boundaries is consequently a threat for these metals. This paper presents a phase field approach describing hydride formation induced by a crack lying near a grain boundary, by using Allen-Cahn’s formulation associated with linear elastic fracture mechanics. The analysis of the results reveals an enhancement of crack-induced hydride development in the proximity of a grain boundary.

  • 5.
    Maimaitiyili, Tuerdi
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Photons for Engineering and Manufacturing Group, Paul Scherrer Institute, 5232 Villigen, Switzerland.
    Woracek, Robin
    European Spallation Source ERIC, 22100 Lund, Sweden; Nuclear Physics Institute of the CAS, 250 68 Husinec-Řež, Czech Republic.
    Neikter, Magnus
    Division of Materials Science, Luleå University of Technology, 971 81 Luleå, Sweden.
    Boin, Mirko
    Department of Microstructure and Residual Stress Analysis, Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany.
    Wimpory, Robert C
    Department of Microstructure and Residual Stress Analysis, Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany.
    Pederson, Robert
    Department of Engineering Science, University West, 46132 Trollhättan, Sweden.
    Strobl, Markus
    European Spallation Source ERIC, 22100 Lund, Sweden; Nuclear Physics Institute of the CAS, 250 68 Husinec-Řež, Czech Republic; Neutron Imaging and Applied Materials Group, Paul Scherrer Institute, 5232 Villigen, Switzerland.
    Drakopoulos, Michael
    Imaging and Microscopy Group, Diamond Light Source Ltd., Oxfordshire OX11 0DE, UK.
    Schäfer, Norbert
    Department of Nanoscale Structures and Microscopic Analysis, Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany.
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Residual Lattice Strain and Phase Distribution in Ti-6Al-4V Produced by Electron Beam Melting2019In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 4, article id 667Article in journal (Refereed)
    Abstract [en]

    Residual stress/strain and microstructure used in additively manufactured material are strongly dependent on process parameter combination. With the aim to better understand and correlate process parameters used in electron beam melting (EBM) of Ti-6Al-4V with resulting phase distributions and residual stress/strains, extensive experimental work has been performed. A large number of polycrystalline Ti-6Al-4V specimens were produced with different optimized EBM process parameter combinations. These specimens were post-sequentially studied by using high-energy X-ray and neutron diffraction. In addition, visible light microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) studies were performed and linked to the other findings. Results show that the influence of scan speed and offset focus on resulting residual strain in a fully dense sample was not significant. In contrast to some previous literature, a uniform α- and β-Ti phase distribution was found in all investigated specimens. Furthermore, no strong strain variations along the build direction with respect to the deposition were found. The magnitude of strain in α and β phase show some variations both in the build plane and along the build direction, which seemed to correlate with the size of the primary β grains. However, no relation was found between measured residual strains in α and β phase. Large primary β grains and texture appear to have a strong effect on X-ray based stress results with relatively small beam size, therefore it is suggested to use a large beam for representative bulk measurements and also to consider the prior β grain size in experimental planning, as well as for mathematical modelling.

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  • 6.
    Neikter, Magnus
    et al.
    Luleå University of Technology, Luleå, Sweden.
    Woracek, Robin
    European Spallation Source ERIC, Lund, Sweden; Nuclear Physics Institute of the CAZ, Czech Republic.
    Maimaitiyili, Tuerdi
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Paul Scherrer Institute, Villigen, Switzerland.
    Scheffzük, Ch
    Karlsruhe Institute of Technology, Karlsruhe, Germany; Frank Laboratory of Neutron Physics, Dubna, Russian Federation.
    Strobl, Markus
    Paul Scherrer Institute, Villigen, Switzerland; Niels Bohr Institute, Copenhagen, Denmark; Nuclear Physics Institute of the CAZ, Czech Republic.
    Antti, M-L
    Luleå University of Technology, Luleå, Sweden.
    Åkerfeldt, P
    Luleå University of Technology, Luleå, Sweden.
    Pederson, Robert
    University West, Trollhättan, Sweden.
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Alpha texture variations in additive manufactured Ti-6Al-4V investigated with neutron diffraction2018In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 23, p. 225-234Article in journal (Refereed)
    Abstract [en]

    Variation of texture in Ti-6Al-4V samples produced by three different additive manufacturing (AM) processes has been studied by neutron time-of-flight (TOF) diffraction. The investigated AM processes were electron beam melting (EBM), selective laser melting (SLM) and laser metal wire deposition (LMwD). Additionally, for the LMwD material separate measurements were done on samples from the top and bottom pieces in order to detect potential texture variations between areas close to and distant from the supporting substrate in the manufacturing process. Electron backscattered diffraction (EBSD) was also performed on material parallel and perpendicular to the build direction to characterize the microstructure. Understanding the context of texture for AM processes is of significant relevance as texture can be linked to anisotropic mechanical behavior. It was found that LMwD had the strongest texture while the two powder bed fusion (PBF) processes EBM and SLM displayed comparatively weaker texture. The texture of EBM and SLM was of the same order of magnitude. These results correlate well with previous microstructural studies. Additionally, texture variations were found in the LMwD sample, where the part closest to the substrate featured stronger texture than the corresponding top part. The crystal direction of the α phase with the strongest texture component was [112¯3].

  • 7.
    Ehrlin, Niklas
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Fisk, Martin
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Division of Solid Mechanics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden.
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Flow stress model for hydrogen degraded Inconel 7182018In: Mechanics of materials, ISSN 0167-6636, E-ISSN 1872-7743, Vol. 119, p. 56-64Article in journal (Refereed)
    Abstract [en]

    For life time estimation, it is desirable to capture the lowering of yield strength and premature failure that some alloys exhibits when subjected to hydrogen. For this, a mechanism based material model has been developed to simulate the hydrogen enhanced localized plasticity (HELP) for the superalloy IN718. The model accounts for the increase in mobility of moving dislocations during plastic deformation, whenever hydrogen is present in the material. Tensile tests performed at four different strain rates: 5 x 10(-5), 5 x 10(-4), 5 x 10(-3) and 5 x 10(-2) s(-1) show a difference in yield behaviour between hydrogen pre-charged and uncharged samples. No strain rate dependency of the hydrogen effect could be concluded. Two different hydrogen charging methods have been used: cathodic charging with molten salt as electrolyte, and high temperature gas charging. No differences in the tensile response could be seen between the two different charging methods. The proposed model was fitted against the experimental curves using a minimizing method and model parameters were obtained. Comprising iteratively updated parameters, the model is suited for implementation in finite element software.

  • 8.
    Nigro, Claudio F.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Olsson, Pär A T
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Phase structural ordering kinetics of second-phase formation in the vicinity of a crack2018In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 209, no 1-2, p. 91-107Article in journal (Refereed)
    Abstract [en]

    The formation of a second phase in presence of a crack in a crystalline material is modelled and studied for different prevailing conditions in order to predict and a posteriori prevent failure, e.g. by delayed hydride cracking. To this end, the phase field formulation of Ginzburg-Landau is selected to describe the phase transformation, and simulations using the finite volume method are performed for a wide range of material properties. A sixth order Landau potential for a single structural order parameter is adopted because it allows the modeling of both first and second order transitions and its corresponding phase diagram can be outlined analytically. The elastic stress field induced by the crack is found to cause a space-dependent shift in the transition temperature, which promotes a second-phase precipitation in vicinity of the crack tip. The spatio-temporal evolution during nucleation and growth is successfully monitored for different combinations of material properties and applied loads. Results for the second-phase shape and size evolution are presented and discussed for a number of selected characteristic cases. The numerical results at steady state are compared to mean-field equilibrium solutions and a good agreement is achieved. For materials applicable to the model, the results can be used to predict the evolution of an eventual second-phase formation through a dimensionless phase transformation in the crack-tip vicinity for given conditions.

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  • 9.
    Ehrlin, Niklas
    et al.
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Fisk, Martin
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Flow stress model for hydrogen degraded Inconel 7182017In: ICF 2017 - 14th International Conference on Fracture, Vol 1 / [ed] Emmanuel E. Gdoutos, International Conference on Fracture , 2017, p. 233-234Conference paper (Refereed)
    Abstract [en]

    A mechanism based material model has been developed to simulate hydrogen enhanced localized plasticity for superalloy IN718 in the present work. The model accounts for the increase in mobility of the moving dislocation during plastic deformation, whenever exposed for hydrogen. Two different hydrogen charging methods have been used: cathodic charging with molten salt as electrolyte, and high temperature gas chamber charging. Tensile tests performed at a strain rate of 5×10−5 s-1 show a clear difference between charged samples and uncharged samples. No or small differences could be seen between the two different charging methods. The material model was fitted against the experimental curves using a minimising method alternating fitting parameters. The agreement between the experimental value and the model was good.

  • 10.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Woracek, R.
    ESS AB, Lund, Sweden.
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Strobl, M.
    ESS AB, Lund, Sweden.
    Schäfer, N.
    Helmholtz-Zentrum Berlin, Germany.
    Fracture mechanical studies of additive manufactured Ti6Al4V by synchrotron X-ray diffraction2017In: ICF 2017 - 14th International Conference on Fracture, Vol 1 / [ed] Emmanuel E. Gdoutos, International Conference on Fracture , 2017, Vol. 1, p. 253-254Conference paper (Refereed)
    Abstract [en]

    Better understanding of the formation and distribution of hydrides, elastic and plastic strains in deformed polycrystalline, multiphase materials such as digitally manufactured Ti-6Al-4V is important for structural engineering. Polycrystalline Ti-6Al4V alloy samples produced by additive manufacturing techniques called electron beam melting (EBM) have been studied at third generation, high energy synchrotron X-Ray diffraction beam line with energy dispersive X-Ray diffraction setup. The elastic strain in the as built and hydrogenated material determined by extracting unit cell parameters of the existing phases in the system by using whole pattern analysis method Rietveld and Pawley. © 2017 ICF 2017 - 14th International Conference on Fracture. All rights reserved.

  • 11.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Nelson Mandela Metropolitan University, Gardham Avenue, Port Elizabeth, 6031, South Africa.
    Wang, Z.
    School of Materials Science and Engineering, UNSW Australia, Sydney, 2052, New South Wales, Australia; Australian Synchrotron, 800 Blackburn Road, Clayton, 3168, Victoria, Australia.
    Daniels, J.
    School of Materials Science and Engineering, UNSW Australia, Sydney, 2052, New South Wales, Australia.
    Andrieux, J.
    European Synchrotron Radiation Facility, 6 rue J Horowitz, Grenoble, 38043, France.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, O.
    PIMM, ENSAM CNAM CNRS, 151 Boulevard d l'Hopital, Paris, 75013, France.
    In situ observation of gamma-ZrH formation by X-ray diffraction2017In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 695, p. 3124-3130Article in journal (Refereed)
    Abstract [en]

    We report on the measurement of the formation of gamma-ZrH during in situ gaseous charging. The measurements were undertaken using high-energy synchrotron X-ray diffraction. Experimental observation shows that gamma-ZrH can form at 180 degrees C from a mixture of alpha+delta while dehydrogenating at slow cooling rates. The observation is further supported by ex situ laboratory X-ray diffraction on deuterated Zr powder that has undergone a similar heat-treatment cycle. The crystal structure of gamma-ZrH refinement agrees with the reported P4(2)/n structure found in the literature. (C) 2016 Elsevier B.V. All rights reserved.

  • 12.
    Nigro, Claudio F.
    et al.
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Olsson, Pär A T
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Kinetics of Crack-Induced Hydride Formation in Hexagonal Close-Packed Materials2017In: International Hydrogen Conference (IHC 2016): Materials Performance in Hydrogen Environments / [ed] B. P. Somerday, P. Sofronis, ASME International , 2017Conference paper (Refereed)
    Abstract [en]

    The formation of a second phase, such as a brittle hydride, in presence of a crack is modeled for hexagonal close-packed metals. To this end, the Ginzburg-Landau phase-field formulation which relies on a sixth-order Landau potential is adopted. The crack-induced stress is implicitly included in the phase-field equation through the mechanical energy so only a single equation is solved providing computational efficiency. The second-phase precipitation evolution is studied for different crack inclinations in titanium and zirconium by using the finite volume method. The crack-induced stresses may lead to the phase transition temperature increasing above the system temperature in the crack-tip proximity. The resulting quenching effect induces a second-phase nucleation. The temporal evolution reveals that the transformation eventually reaches a steady state and the resulting precipitate geometry is dependent of the material parameters and conditions.

  • 13.
    Bjerkén, Christina
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Massih, Ali
    Malmö högskola, Faculty of Technology and Society (TS).
    Oriented ordering near line defects in crystals2017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 27, p. 2437-2467Article in journal (Refereed)
    Abstract [en]

    General properties of directed ordering near line defects, in particular an edge dislocation, in elastic crystals undergoing phase transition are studied using the two-component time-dependent Ginzburg-Landau equation in two dimensions or 2D-XY model. The associated Landau potential comprises a sixth-order term, cubic anisotropy terms and the field of the dislocation. In thermodynamic equilibrium, the phase diagram for the model is delineated. Upon quenching the system below its transition point, the temporal evolution of the order parameter components in the vicinity of the defect is numerically evaluated. The development of vortices, emanated from the model, is explored and their interaction with the dislocation is examined. The dislocation produced a vortex free circular region whose diameter grew almost linearly with time. The time-dependence of vortex density for various settings of the Landau potential coefficients are evaluated. The vortex density ( in 2D) decreased inversely with time, albeit faster in the absence of dislocation. By computing the two-point correlation function, we established that the dynamic scaling law is satisfied for the considered model if the distance is scaled by L = t(1/2) or by its half-width L-1/2 for a dislocation free crystal. Finally, phase transitions in improper ferroelectrics in the context of the model are discussed.

  • 14.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Nelson Mandela Metropolitan University, Gardham Avenue, Port Elizabeth, 6031, South Africa.
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Hoelzel, M.
    Forschungsneutronenquelle Heinz-Maier-Leibnitz (FRM II), Technische Universität Muünchen, Lichtenbergstr. 1, D-85747, Garching, Germany.
    Ion, John Christopher
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, O.
    PIMM, Ensam - Cnam - CNRS, 151 Boulevard de l'Hôpital, 75013, Paris, France.
    The preparation of Zr-deuteride and phase stability studies of the Zr-D system2017In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 485, p. 243-252Article in journal (Refereed)
    Abstract [en]

    Deuteride phases in the zirconium-deuterium system in the temperature range 25-286 degrees C have been studied in-situ by high resolution neutron diffraction. The study primarily focused on observations of delta ->gamma transformation at 180 degrees C, and the peritectoid reaction alpha + delta <-> gamma at 255 degrees C in commetcial grade Zr powder that was deuterated to a deuterium/Zr ratio of one to one. A detailed description of the zirconium deuteride preparation route by high temperature gas loading is also described. The lattice parameters of alpha-Zr, delta-ZrDx and epsilon-ZrDx were determined by whole pattern crystal structure analysis, using Rietveld and Pawley refinements, and are in good agreement with values reported in the literature. The controversial gamma-hydride phase was observed both in-situ and ex-situ in deuterated Zr powder after a heat treatment at 286 degrees C and slow cooling. (C) 2017 Elsevier B.V. All rights reserved.

  • 15.
    Ehrlin, Niklas
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Fisk, Martin
    Malmö högskola, Faculty of Technology and Society (TS).
    Cathodic hydrogen charging of Inconel 7182016In: AIMS Materials Science, ISSN 2372-0484, Vol. 3, no 4, p. 1350-1364Article in journal (Refereed)
    Abstract [en]

    The nickel based superalloy IN718 is known to be prone to hydrogen sensitivity, causing degradation of its mechanical properties. Therefore, during mechanical testing of hydrogen charged samples, a well-defined hydrogen distribution is essential to better understand the influence of hydrogen on dislocation movement and plasticity behavior. The possibility of charging cylindrical specimens of IN718 with hydrogen using cathodic charging is investigated. The method is based on an electro-chemical process using a molten salt electrolyte. The resulting hydrogen concentration is measured for various radii, and it is shown that the anisotropic diffusion coefficient resulting from electromigration, inherent in the charging method, must be taken into account as it has a major impact on the charging parameters of IN718. Also, no evidence of degassing during storage is found. Further, changes in surface roughness were examined by SEM, and only limited surface degradation is observed, which is not considered to significantly affect the results.

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  • 16.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Nelson Mandela Metropolitan University, Gardham Avenue, Port Elizabeth, 6031, South Africa.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Blackmur, M. S.
    Materials Performance Centre, School of Materials, Manchester University, Manchester, M1 7HS, United Kingdom.
    Zanellato, O.
    PIMM, Ensam - Cnam - CNRS, 151 Boulevard de l'Hôpital, Paris, 75013, France.
    Andrieux, J.
    European Synchrotron Radiation Facility, 6 rue J Horowitz, Grenoble, 38043, France.
    Ribeiro, F.
    Institut de Radioprotection et Sûreté Nucléaire, IRSN, Saint-Paul Lez Durance, BP3 13115, France.
    Observation of the δ to ε Zr‐hydride transition by in‐situ synchrotron X‐ray diffraction2016In: Crystal research and technology (1981), ISSN 0232-1300, E-ISSN 1521-4079, Vol. 51, no 11, p. 663-670Article in journal (Refereed)
    Abstract [en]

    We investigate the formation and dissolution of hydrides in commercially pure zirconium powder in-situ using highenergy synchrotron X-ray radiation. Experimental results showed a continuous phase transition between the delta and epsilon zirconium hydride phases with indication of a second order phase transformation.

  • 17.
    Olsson, Pär
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Massih, Ali
    Malmö högskola, Faculty of Technology and Society (TS).
    Ab initio thermodynamics investigation of titanium hydrides2015In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 97, p. 263-275Article in journal (Refereed)
    Abstract [en]

    We report the results of an ab initio -based density functional theory study of the thermodynamic and structural properties of titanium hydrides. The thermodynamic modelling contains contributions from both vibrational and electronic excitations to the free energy and is conducted using the quasi-harmonic approximation (QHA). The enthalpy, entropy and heat capacity are computed over a wide range of temperature (0≲T⩽1000 K) and found to concur well with available experimental data. The simulations show that the Debye temperature varies significantly with temperature below about 50 K demonstrating that the Debye model is too simplistic for thermodynamic modelling throughout the entire temperature spectrum. Comparing the Debye temperature from QHA with that calculated from low temperature elastic constants reveals limited agreement, which suggests that the actual frequency distribution characteristics do not comply with that of the Debye model. The calculated thermodynamic properties are found to show many similarities to those of zirconium hydrides, which are discussed.

  • 18.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, Axel
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, Olivier
    Blackmur, Matthew
    Andrieux, Jerome
    Ribeiro, Fabienne
    In Situ Hydrogen Loading on Zirconium Powder2015In: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 22, no 4, p. 995-1000Article in journal (Refereed)
    Abstract [en]

    For the first time, various hydride phases in a zirconium-hydrogen system have been prepared in a high-energy synchrotron X-ray radiation beamline and their transformation behaviour has been studied in situ. First, the formation and dissolution of hydrides in commercially pure zirconium powder were monitored in real time during hydrogenation and dehydrogenation, then whole pattern crystal structure analysis such as Rietveld and Pawley refinements were performed. All commonly reported low-pressure phases presented in the Zr-H phase diagram are obtained from a single experimental arrangement.

  • 19.
    Bjerkén, Christina
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Massih, Ali
    Malmö högskola, Faculty of Technology and Society (TS).
    Kinetics of orientational phase ordering near line defects in crystals2015In: International Conference on Solid-Solid Phase Transformation in Inorganic Materials (PTM 2015), TMS PTM-2015: A combination print-CD volume , 2015, p. 36-36Conference paper (Other academic)
    Abstract [en]

    General properties of directed ordering near line defects in elastic crystals undergoing phase transition are studied using the two-component time-dependent Ginzburg-Landau equation. Upon quenching the system below its transition point, the temporal evolution of the order parameter in the vicinity of the defect is evaluated. The development of vortices is explored and their interaction with the structural defect is examined. Finally, phase transitions in improper ferroelectrics in the context of the model are discussed.

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  • 20. Wang, Zhiyang
    et al.
    Steuwer, Axel
    Liu, Nanxi
    Maimaitiyili, Tuerdi
    Malmö högskola, Faculty of Technology and Society (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Avdeev, Maxim
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Curfs, Caroline
    Kimpton, Justin A.
    Daniels, John E.
    Observations of temperature stability of γ-zirconium hydride by high-resolution neutron powder diffraction2015In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, no 661, p. 55-61Article in journal (Refereed)
    Abstract [en]

    The phase evolution in a zirconium–50 deuterium (Zr–50D, at.%) alloy system during thermal cycling has been investigated using in situ high-resolution neutron powder diffraction. The results showed that the peritectoid reaction α-Zr + δ-ZrD → γ-ZrD previously suggested to occur at high temperatures does not take place in the system. Slow cooling, from high temperatures (≥520 K) to room temperature at a rate of 5 K min–1, promoted the γ-hydride formation rather than fast cooling as reported earlier. In contrast to the observation that the δ-hydride present in the system remained at temperatures up to 740 K, the produced γ phase transformed to δ-hydride in the temperature range of 370 K to 559 K, with the transformation completing at approximately 559 K. It is confirmed that the formation of the γ-hydride was reproducible with slow cooling, and a diffusion-controlled sluggish δ-to γ-hydride transformation is suggested to be responsible for the favorable development of γ-hydride during slow cooling.

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  • 21.
    Olsson, Pär
    et al.
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS).
    Alvarez Holston, Anna-Maria
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Ab initio thermodynamics of zirconium hydrides and deuterides2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 86, p. 211-222Article in journal (Refereed)
    Abstract [en]

    We report the results of a systematic ab initio study of the elastic and thermodynamic properties of γ-ZrH, δ-View the MathML source-ZrD, and δ-ZrD1.5. In addition, pure α-Zr as well as the ε-ZrH2 and ε-ZrD2 phases are evaluated for reference. The calculations are performed using quantum mechanical density functional theory (DFT) with the frozen core projector augmented wave (PAW) approach and a generalised gradient approximated (GGA) exchange–correlation functional. To capture the variations of the thermodynamic quantities over a wide range of temperatures View the MathML source, the quasi-harmonic approximation approach is adopted where the influence of the vibrational and electronic free energies are included by means of the phonon and electron densities of state. This allows for quantifying the contributions of the electron density of states, which were not accounted for in the previous studies. All the pertinent elastic constants and phonon properties for the considered hydride/deuteride phases are calculated and compared with experimental data; which were not done before. We have further computed the entropy, heat capacity and enthalpy as well as low temperature thermodynamic properties such as the Debye temperature and the electronic heat capacity constant for all the hydride and deuteride phases. The results of our computations concur well with the corresponding data obtained by measurements that are reported in the literature and offer the necessary data and basis for multiscale modelling of zirconium alloys.

  • 22.
    Nigro, Claudio F.
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Olsson, Pär
    Malmö högskola, School of Technology (TS).
    Numerical simulations of the influence of cracks on the formation of hydrides in a metallic structure2014In: Proceedings of NSCM-27: the 27th Nordic Seminar on Computational Mechanics, Department of Mechanics, Royal Institute of Technology , 2014, p. 132-135Conference paper (Other academic)
    Abstract [en]

    The formation of hydrides at a crack tip is studied by using a numerical model based on the Ginzburg-Landau phase field formulation.

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  • 23.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Phase ordering kinetics of second-phase formation near an edge dislocation2014In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 94, no 6, p. 569-593Article in journal (Refereed)
    Abstract [en]

    The time-dependent Ginzburg-Landau (TDGL) equation for a single component non-conservative structural order parameter is used to study the spatio-temporal evolution of a second phase in the vicinity of an edge dislocation in an elastic crystalline solid. A symmetric Landau potential of sixth-order is employed. Dislocation field and elasticity modify the second-order and fourth-order coefficients of the Landau polynomial, respectively, where the former makes the coefficient singular at the origin. The TDGL equation is solved numerically using a finite volume method, where a wide range of parameter sets is explored. Computations are made for temperatures both above and below the transition temperature of a defect-free crystal Tc0. In both cases, the effects of the elastic properties of the solid and the strength of interaction between the order parameter and the displacement field are examined. If the system is quenched below Tc0, a steady state is first reached on the compressive side of the dislocation. On the tensile side, the growth is held back. The effect of thermal noise term in the TDGL equation is studied. We find that if the dislocation is introduced above Tc0, thermal noise supports the nucleation of the second phase, and a steady state will be attained earlier than if the thermal noise was absent. For a dislocation-free solid, we have compared our numerical computations for a mean-field (spatially averaged) order parameter versus time with the late time growth of the ensemble-averaged order parameter, calculated analytically, and find that both results follow upper asymptotes of sigmoid curves.

  • 24.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blackmur, Matthew
    Zanellato, Oliver
    Andrieux, Jerome
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Fabienne, Ribeiro
    In-situ hydrogen charging of zirconium powder to study isothermal precipitation of hydrides and determination of Zr-hydride crystal structure2013Conference paper (Refereed)
    Abstract [en]

    Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen which leads to hydrogen concentration build-up over time. It is well known that the formation of hydrides will degrade the material and leads to, for example, delayed hydride cracking during high burn up. Even though zirconium hydrides have been studied for several decades, there still remain some controversies regarding the formation mechanisms, exact crystal structure, and stability of various hydride phases. This study uses high resolution synchrotron radiation as a probing tool to observe the precipitation and dissolution of hydrides in highly pure zirconium powder during in-situ hydrogen charging. The experiment enabled the direct observation of the hydride formation and phase transformations. It, also, provided high quality data for crystal structure determination.

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  • 25.
    Olsson, Pär
    et al.
    Malmö högskola, School of Technology (TS).
    Blomqvist, Jacob
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Phase stability off ZrH1.5: a DFT study2013In: Svenska Mekanikdagar 2013, 2013, p. 46-46Conference paper (Other academic)
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  • 26.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Malmö högskola, School of Technology (TS).
    Blackmur, Matthew
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Adnan, Safdar
    Malmö högskola, School of Technology (TS).
    Residual stress and hydrogen effect on Ti-6Al-4V alloys produced by Electron Beam Melting2013Conference paper (Other academic)
    Abstract [en]

    There is an internal, self-balanced stress known as residual stress (RS) that exists in all alloy systems without any external applied forces. Depending on the compressive or tensile nature and magnitude of the RS, it significantly affects the mechanical properties of the materials. Therefore, it is crucial to know the nature and magnitude of RS in material for safe and economical operation. In this work, we used unique, multipurpose, high energy (50-150 KeV) beamline I12-JEEP (Joint Engineering, Environment and Processing) at Diamond Light Source in UK with Energy Dispersive X-Ray Diffraction (EDXRD) setup to map up the RS states in one of the most popular titanium alloys with a code name Ti6Al4V. This type of titanium alloy is widely used in biomedical and aerospace industry because of their excellent combination of a high strength/weight ratio and good corrosion resistance. The Ti6Al4V which we were investigated are produced using electron beam melting (EBM) technique as a function of EBM processing parameters. In addition to relation between RS and processing parameters of EBM, the hydride formation versus processing parameters and as well as the effect of residual stress to the hydride precipitation in EBM built Ti6Al4V were investigated. To find out the effect of EBM processing parameters to the residual stress development, various samples produced with different beam size, scanning speed and different building thickness were investigated. From each type of the sample four specimens were prepared and three of them loaded with hydrogen in different concentrations, i.e. the hydrogen concentration of the various samples are 262, 772, 951 and 1410 ppm. Other than these alloy samples we also measured clean and hydrided original powder samples which are used for make these solid samples in our studies. After data collection, the whole pattern fitting method Rietveld and Pawely were performed with structure analysis software package Topas-Academic and GSAS. .

  • 27.
    Blomqvist, Jakob
    et al.
    Malmö högskola, School of Technology (TS).
    Olsson, Pär
    Malmö högskola, School of Technology (TS).
    Steuwer, Axel
    Maimaitiyili, Tuerdi
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Towards a multiscale understanding of hydride induced embrittlement in zirconium and titanium: theory and experiments2013In: Thermec' 2013 Rio Hotel Las Vegas USA, Abstract book, 2013, article id 140Conference paper (Other academic)
  • 28.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Zanellato, Oliver
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Hölzel, M
    Exact crystal structure of the zirconium hydride determined by neutron powder diffraction2012Conference paper (Other academic)
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  • 29.
    Blomqvist, Jakob
    et al.
    Malmö högskola, School of Technology (TS).
    Johansson, Johan
    Materials Technology, Studsvik Nuclear AB; Nyköping, SE-611 82, SWEDEN.
    Alvarez, Anna-Maria
    Materials Technology, Studsvik Nuclear AB; Nyköping, SE-611 82, SWEDEN.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Structure and Thermodynamical Properties of Zirconium Hydrides from First-Principle2012In: Proceedings of the 15th international conference on environmental degradation of materials in nuclear power systems-water reactors / [ed] Jeremy T. Busby; Gabriel Ilevbare; Peter L. Andresen, John Wiley & Sons, 2012, p. 671-679Conference paper (Other academic)
    Abstract [en]

    Zirconium alloys are used as nuclear fuel cladding material due to their mechanical and corrosion resistant properties together with their favorable cross-section for neutron scattering. At running conditions, however, there will be an increase of hydrogen in the vicinity of the cladding surface at the water side of the fuel.The hydrogen will diffuse into the cladding material and at certain conditions, such as lower temperatures and external load, hydrides will precipitate out in the material and cause well known embrittlement, blistering and other unwanted effects. Using phase-field methods it is now possible to model precipitation build-up in metals, for example as a function of hydrogen concentration, temperature and external load, but the technique relies on input of parameters, such as the formation energy of the hydrides and matrix. To that end, we have computed, using the density functional theory (DFT) code GPAW, the latent heat of fusion as well as solved the crystal structure for three zirconium hydride polymorphs: delta-ZrH1.6, gamma-ZrH, and epsilon-ZrH2.

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  • 30.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Dissolution driven crack branching in polycarbonate2011In: Fatigue and Fracture of Engineering Materials and Structures, Vol. 34, no 4, p. 227-239Article in journal (Refereed)
    Abstract [en]

    Stress corrosion, in the form of chemically assisted crack growth, in polycarbonate is examined with focus on crack branching characteristics. Cracks with finite width are observed; this is to be expected for dissolution driven cracking. The cracks branched repeatedly and crack widths before and after branching are measured. Both symmetric and asymmetric branching is found. The dissolution rate is assumed to be a linear function of the strain along the crack surface. In the literature, it is proposed that the crack width is proportional to the square of the mode I stress intensity factor. Energy considerations lead to that the sum of branch widths must equal the width of the unbranched crack. The results from the present study correspond fairly well with this assumption. The branching angle is found to be 32º ± 12º, which is in line with results for sharp cracks reported in the literature. The mean growth direction of the branches is found to deviate slightly from the expected straight. No significant correlation between angles and crack widths is found. The scatter in results is mainly addressed to the inherent perturbation sensitivity of stress corrosion cracking. Also numerically simulations of crack branching is performed. These results show promising agreement with the experiments.

  • 31.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Nucleation of second-phase near elastic defects in crystalline solids2011In: Proceedings of MS&T'11, TMS , 2011Conference paper (Other academic)
    Abstract [en]

    The problem of heterogeneous nucleation of second-phase in alloys in the vicinity of elastic defects is considered. The defect can be a dislocation line or a crack tip residing in a crystalline solid. We use a compressible Ising-like model, represented by the Ginzburg-Landau equation, to describe the spatiotemporal evolution of the order parameter in the environs of the defect. The model accounts for the elasticity of the solid and the interaction of order parameter field with the elastic field of the defect. A finite volume numerical method is used to solve the governing partial differential equation for the order parameter. We examine the nature of the phase transition and discuss the phase diagram topology near and away from the defect. We discuss our calculations in light of observations of formation of hydrides in zirconium and titanium alloys.

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  • 32. Steuwer, Axel
    et al.
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS).
    Maimaitiyili, Tuerdi
    Malmö högskola, School of Technology (TS).
    Zanellato, Olivier
    Curfs, Caroline
    du Plessis, H.E
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Structure of the Zirconium Hydride Polymorphs Determined by Neutron and Synchrotron X-ray Powder Diffraction2011Conference paper (Other academic)
    Abstract [en]

    Zirconium alloys, widely used in the nuclear industry, Fig 1, have a strong affinity for hydrogen that leads to hydrogen pick up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are presumed to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature, exact structure and stability of the γ-ZrH phase, which is closely related to the δ-ZrH phase through ordering of the hydrogen on tetrahedral sites on alternating 110 planes.

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  • 33. Blomqvist, Jacob
    et al.
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    A First Principle Atomistic Pre-study of a Multiscale Model of Hydride Induced Embrittlement in Zirconium2010Conference paper (Other academic)
    Abstract [en]

    Hydrogen induced embrittlement and associated cracking effects in Zirconium alloys impose a serious problem, for example, within the nuclear industry where these materials are used in a range of applications, including fuel claddings. To get a fuller understanding of the hydrogen diffusion, hydride formation and phase transformations taking place at conditions of external stress and at elevated temperature it is crucial to augment experimental measurements with multi-level modelling. It is, for example, possible to simulate hydride phase-transformation trough phase-field calculations[1]. In order to get accurate results from these calculations, however, it is important to have basic physical information, such as interfacial energies and elastic constants. The latter were obtained completely from first principle calculations of density functional theory. In pure zirconium, at low hydrogen concentrations and at low temperatures, hydrogen atoms are dissolved into the matrix and preferentially occupy tetrahedral sites within the thermodynamically stable aZr hexagonal closed packed unit cell. At higher concentrations the various face centred cubic and tetragonal hydride phases start to form. Using the plane-wave code DACAPO, within the generalized gradient approximation, the lattice parameters and the ionic positions of bulk aZr, dZrHx and gZrHx were relaxed with respect to total energies. The formation energies as well as bulk moduli were then obtained by fitting total energies as a function of lattice parameters with a 3rd or 4th order polynomial. By comparing the present results with previous calculations and experiments it was possible to validate the methods for further use. The results, thus, comprise a foundation and serves to verify methods and codes. The next step is to compute more complex properties such as surface free energies, or interfacial energies, of bi-phase Zr-H systems, so that the full multiscale model can be implemented. 1. V. Vaithyanathan et al., Acta Materialia 52 (2004) p. 2973

  • 34.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Environmentally assisted initiation and growth of multiple surface cracks2010In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 47, no 14-15, p. 1838-1846Article in journal (Refereed)
    Abstract [en]

    The initial stages of stress corrosion on an amorphous polymer is investigated. This is done by exposing stressed specimens of polycarbonate to an acetone and water solution. The surface develops two distinct features of degradation that appear on different length scales when subjected to tensile stress. Small pits form on the surface and make it rough. These pits are in the order of micrometers, and are found to be randomly distributed. They occur even without load and seem to slightly increase in number with increasing stress. In the millimeter domain, visible to the bare eye, surface cracks are formed transverse to the direction of loading. The occurrence of cracks is seen to have a positive stress threshold value, exceeding which, a linear increase of number of cracks with stress is found. The manners in which the cracks grow and coalesce on the surface are examined. It is seen that they do not meet crack tip to crack tip. Instead, they avoid each other initially and coalesce crack tip to crack side. The results are discussed in the light of mechanical considerations. A stress analysis for a few configurations of meeting cracks supports the experimental observations. With assumptions of stress corrosion crack growth and coalescence, a simulation of cracks growing from randomly distributed initiation sites is performed. Similar crack patterns as obtained in the experiments are found.

  • 35.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ortiz, Michael
    Evolution of anodic stress corrosion cracking in a coated material2010In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 165, no 2, p. 211-221Article in journal (Refereed)
    Abstract [en]

    In the present paper, we investigate the influence of corrosion driving forces and interfacial toughness for a coated material subjected to mechanical loading. If the protective coating is cracked, the substrate material may become exposed to a corrosive media. For a stress corrosion sensitive substrate material, this may lead to detrimental crack growth. A crack is assumed to grow by anodic dissolution, inherently leading to a blunt crack tip. The evolution of the crack surface is modelled as a moving boundary problem using an adaptive finite element method. The rate of dissolution along the crack surface in the substrate is assumed to be proportional to the chemical potential, which is function of the local surface energy density and elastic strain energy density. The surface energy tends to flatten the surface, whereas the strain energy due to stress concentration promotes material dissolution. The influence of the interface energy density parameter for the solid-fluid combination, interface corrosion resistance and stiffness ratios between coating and substrate is investigated. Three characteristic crack shapes are obtained; deepening and narrowing single cracks, branched cracks and sharp interface cracks. The crack shapes obtained by our simulations are similar to real sub-coating cracks reported in the literature.

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  • 36.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Blomqvist, Jacob
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Exact Crystal Structure of the γ-ZrD/δ-ZrD system2010Report (Other academic)
    Abstract [en]

    Zirconium alloys have a strong affinity for hydrogen which leads to hydrogen pick-up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are known to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature and stability of the γ-ZrH phase, which is closely related to the δ-ZrH phase through ordering of the hydrogen on tetrahedral sites in the matrix. In this report, we give short introduction to our experimental work at FRM II in September 2010. In this work, we use high-resolution neutron powder diffraction on deuterated samples to re-determine and verify the reported structures which essentially date back to the 1960s [1].

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  • 37.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Mean-field strain in zirconium hydride2010Report (Other academic)
    Abstract [en]

    The formation of hydrides in zirconium alloys will cause degradation of the materials. Despite extensive study, the mechanisms and effects of hydrogen embrittlement have remained unclear. In this report, we had presented the calculation results of mean-field strain by different hydride phases in zirconium alloys, like δ-hydride and γ-hydride. The results of δ-hydride are compared with experimental data obtained by X-ray diffraction method by Bai et al.\cite{Baib} and note \cite{Ali} of Massih. The result of δ-hydride is identical with Massih but have some differences with Bai et al. We had also compared our calculated mean-field strain by γ-hydride with δ-hydride. The γ-hydride showed lower value than δ.

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  • 38.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    The influence of biaxial loading on branching of a dissolution driven stress corrosion crack2010In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 77, no 11, p. 1989-1997Article in journal (Refereed)
    Abstract [en]

    Stress corrosion cracking occurs due to the synergistic interaction between mechanical load and corrosion reactions. In this study, branching during anodic dissolution driven crack growth is studied using an adaptive FE procedure. The crack has an inherent blunt tip due to the dissolution, and the growth is treated as a moving boundary problem with a strain-assisted evolution law. Simulations are performed with different degrees of load biaxiality. It is found that increasing biaxiality promotes branching. No conditions for when branching takes place are found. Crack growth rates for branches are investigated, and it is found that, after an initial acceleration, constant growth rates can be reached, as well as decreasing speed and eventual arrest. The influence of T-stresses and perturbations sensitivity are discussed.

  • 39.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Branching of a dissolution driven stress corrosion crack2009In: Proceedings of the International Conference on Crack Paths 2009, European Structural Integrity Society , 2009Conference paper (Other academic)
    Abstract [en]

    Stress corrosion cracking occurs due to the synergistic interaction between mechanical load and corrosion reactions. Some types of stress corrosion crack branch heavily. Here, branching during dissolution driven crack growth is studied using an adaptive FE method. A strain-assisted evolution law is used for the inherently blunted crack. No criterion for crack growth is needed as for a sharp crack, neither for the growth direction. Several simulations are performed with different degrees of load biaxiality. It is found that large biaxiality promotes branching, but no conditions for when branching takes place is found. Instead, branching seems to occur rather randomly due to the perturbation sensitivity of a dissolution driven crack. Also crack growth rates for branched cracks are investigated, and it is found that both constant growth rates can be reached, as well as decreasing rates and eventual arrest. The cracks follow a mode I crack path, however local changes may occur due to the perturbation sensitivity.

  • 40.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Melin, Solveig
    Growth of a short fatigue crack - A long term simulation using a dislocation technique2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 5, p. 1196-1204Article in journal (Refereed)
    Abstract [en]

    In this study, the growth of a short edge crack during more than 14 000 cycles of fatigue loading is investigated in detail. An edge crack, in a semi-infinite body with no pre-existing obstacles present, is modelled in a boundary element approach by a distribution of dislocation dipoles. The fatigue cycles are fully reversed (R = -1), and the load range is well below the threshold for long fatigue cracks. The developing local plasticity consists of discrete edge dislocations that are emitted from the crack tip. The movements of discrete dislocations are restricted to slip along preferred slip planes. The present model is restricted to a 2D plane strain problem with a through-thickness crack, assuming no 3D irregularities. A remote load is applied perpendicular to the crack extension line, and the material parameters are those of a BCC crystal structure. The competition between influence of the global loading on and local shielding of the crack tip governs the crack growth. The growth rate increases in discrete steps with short periods of retardation, from approximately the size of Burgers vector, b, up to 25 b per cycle as the length of the crack is tripled. The plastic zone changes from having an elongated, slender form to include a low angle grain boundary, which, eventually, divides into two parts. The crack growth is found to change from constant acceleration to constant growth rate as the event of the low-angle grain boundary split is approached. ; The results are compared to long crack characteristics, for which linear elastic fracture mechanics and Paris law can be used to predict fatigue crack growth. The exponent in Paris law varies between 1 and 0 in the present study, i.e. smaller than typical values for ductile BCC materials. The ratio between static and cyclic plastic zone sizes is found to increase during crack growth, and the angle of the general plastic zone direction increases, showing a tendency towards long crack values. The characteristics of the simulated crack growth, found in the present study, are typical for below-threshold growth, with slow acceleration, constant growth rate. and, eventually, either arrest or transition to long crack growth behaviour, as reported in the literature.

  • 41.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Rimoli, Julian
    Ortiz, Michael
    Stress corrosion crack growth beneath a stiff coating: influence of chemical potential and interface toughness2009In: Proceedings 12th International Conference on Fracture, Ottawa 2009, ICF12 , 2009Conference paper (Other academic)
    Abstract [en]

    In the present paper we investigate the influence of corrosion driving forces and interfacial toughness for a coated material subjected to mechanical loading. If the protecting coating is cracked, the substrate material may become exposed to a corrosive media. For a stress corrosion sensitive substrate material, this may lead to detrimental crack growth. A crack is assumed to grow by anodic dissolution, inherently leading to a blunted crack tip. The rate of dissolution along the crack surface is assumed to be proportional to the chemical potential, which is function of the local surface energy density and the elastic strain energy density. The surface energy tends to flatten the surface, whereas the strain energy due to stress concentration promotes material dissolution. The evolution of the crack surface is modelled as a moving boundary problem using an adaptive finite element method. The crack shapes obtained by our simulations are remarkably similar to real stress corrosion cracks reported in the literature.

  • 42.
    Masic, Alma
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Overgaard, Niels
    Malmö högskola, School of Technology (TS).
    Heyden, Anders
    Malmö högskola, School of Technology (TS).
    A Combined Model for Biofilm Development and Biocorrosion2008Conference paper (Other academic)
    Abstract [en]

    We propose a combined model investigating the interplay between a growing biofilm on a metallic surface and environmental suported stress corrosion. The biofilm model incorporates both substrate diffusion, bacteria metabolism and biofilm development, making it possible to estimate the pH at the metallic surface. The biocorrosion model is based on strain driven dissolution. The final model iterates between biofilm development that changes the pH at the surface and biocorrosion that changes the geometry of the substratum.

  • 43.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Melin, Solveig
    Influence of low-angle grain boundaries on short fatigue crack growth studied by a discrete dislocation method2008In: Proceedings 17th European Conference on Fracture, Brno 2008, VUTIUM Brno, Czech Republic , 2008Conference paper (Other academic)
    Abstract [en]

    The growth of short fatigue cracks in or below the threshold regime are highly influenced by the local microstructure. In the present study, the influence of low-angle boundaries on the growth and arrest of microstructurally short edge cracks in crystalline material is investigated using a discrete dislocation method. The crack is modelled by boundary elements consisting of dislocation dipoles. An external load that is fully reversed during the fatigue cycle is applied to obtain a crack growth rate of a few Burgers vector per cycle. The crack growth and the developing plasticity are given by the emission, and eventual annihilation, of dislocations at the crack tip. The low-angle grain boundary is modelled as an array of aligned dislocations. It is found that the developing local plasticity, is crucial for the growth rates. Different stress ranges are used and both positive and negative low-angle grain boundaries are considered. It is found that depending on sign of the of the grain boundary, different growth behaviour are found. Generally, a positive low-angle grain boundary gives a low, rather stress range independent growth rate. A negative grain boundary results in increasing crack growth when the stress range is increased. The distance between the crack tip and the low-angle boundary is found not to significantly influence the crack growth characteristics, for the geometries and load ranges considered in this study.

  • 44.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Investigation of Branching in Polycarbonate Due to Stress Corrosion2008In: Proceedings 17th European Conference on Fracture, Brno 2008, VUTIUM Brno, Czech Republic , 2008Conference paper (Other academic)
    Abstract [en]

    In this study, experiments have been made to investigate how the width of stress corrosion cracks change when branching occurs. The amount of dissolved material is investigated using and optical microscope. Crack width before and after branching and characteristic crack angels are measured. It was found that the total width of the branches is preserved after branching and a connection to the stress intensity factor at the crack tips is suggested. It is assumed that quasistatic calculations for branched cracks, found in literature, can be applied to these cracks. Experiments have been performed on polycarbonate with acetone as a dissolving agent. Experimental results are compared with numerically simulated results as well as with data found in literature.

  • 45.
    Masic, Alma
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Overgaard, Niels
    Malmö högskola, School of Technology (TS).
    Heyden, Anders
    Malmö högskola, School of Technology (TS).
    Investigation of the Interaction between Biocorrosion and Biofilm Development based on the pH Profile2008Conference paper (Other academic)
    Abstract [en]

    We propose a combined model for investigating the interplay between a growing biofilm on a metallic surface and stress corrosion. The biofilm model incorporates both substrate dissolution, bacteria metabolism and biofilm development, making it possible to estimate the pH at the metallic surface. The biofilm model is base on the well-known Wanner-Gujer model, cf. [1] for biofilm development and applied to a metabolism involving pH, cf. [7,8,9]. Including pH in the metabolism makes it possible to estimate the pH at the substratum as a function of the biofilm thickness, which then can be provided as input to the biocorrosion model.

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  • 46.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Simulation of corrosion fatigue crack growth under mixed-mode loading2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 3-4, p. 440-451Article in journal (Refereed)
    Abstract [en]

    The kinking of a corrosion crack due to mixed-mode fatigue loading is studied using an adaptive finite element procedure. The rate of material dissolution is assumed to be proportional to the stretching of the corroding surface. The dissolution of material is governed by a corrosion law, where no criterion is needed for neither crack growth nor growth direction. The problem is treated as a general moving boundary problem. The kink angles are found to be in very good agreement with results for sharp cracks using criteria reported in the literature.

  • 47.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Experiment and simulation of a crack growing by material dissolution2007In: Abstracts 1st Bertram Broberg symposium on mechanics of materials;1, 2007Conference paper (Other academic)
    Abstract [en]

    Crack growth and crack path under a fracture mechanical experiment in polycarbonate is studied. The material, a polycarbonate, is photo elastic witch makes it possible to do an optical stress field analysis. The crack path and crack branching is also predicted by an adaptive finite element analysis. The crack growth is a result of stress induced, chemically activated material dissolution that completely determines the geometry of the specimen, including the shape and length of the crack. In this case, the mechanisms and processes leading to crack growth are assumed to be incorporated in a general material dissolution process. As a consequence of this assumption there is no need for a crack criterion or crack path hypothesis. The lack of crack path criterion imposes an arbitrary length scale where the crack width scales with the square of the applied load but the crack growth rate is independent of the load. Numerical calculations suggest that the crack is capable of branching and that the branches will continue to grow at the same rate as the original crack. The numerical results are verified by the photo elastic experiments and a reasonable agreement is established.

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    FULLTEXT01
  • 48.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Experiment och simulering av materialupplösningsspricka2007In: Abstracts Svenska mekanikdagarna, Luleå, Sweden, Luleå tekniska universitet, 2007Conference paper (Other academic)
    Abstract [sv]

    Sprickväg och sprickgrening studeras under brottmekaniska experiment i polykarbonat. Detta material är spänningsoptiskt och fotoelastiska analyser utförs. Sprickväg och sprickgrening beräknas också med hjälp av en adaptiv finit element-metod. Spricktillväxten är ett resultat av spänningsunderstödd, kemiskt eller biologiskt aktiverad, materialupplösning, som helt bestämmer provkroppens geometri inklusive sprickans form och längd. På så sätt är de processer som leder till sprickpropagering inkluderade i en allmän materialupplösningsprocess. Som en konsekvens av detta behövs varken brottkriterium eller sprickvägshypotes. Den fotoelastiska undersökningen jämförs med de numeriska resultaten. Avsaknaden av brottkriterium gör att sprickpropageringsprocessen saknar längdskala. Därmed följer att sprickans bredd skalar med kvadraten på den pålagda lasten medan spricktillväxthastigheten blir oberoende av pålagd last. Förhållandet gör vidare att sprickan förgrenas och att alla grenar fortsätter att växa utan att förlora fart.

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    FULLTEXT01
  • 49. Singh, Ram N
    et al.
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Kishore, Ramaswamy
    Banjeree, Srikumar
    Hydride induced embrittlement of zirconium alloy pressure tubes2007In: Proceedings of the 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, 2007Conference paper (Other academic)
    Abstract [en]

    Zirconium alloy tubes act as miniature pressure vessels in Pressurized Heavy Water Reactors (PHWR) and are subjected to aqueous corrosion resulting in hydrogen pick up. Hydrogen exceeding solid solubility precipitates out as brittle hydride phase and may cause embrittlement of the host matrix. Two forms of embrittlement have been recognized for dilute zirconium alloy pressure tubes – gross and localized. Gross embrittlement is caused due to uniformly distributed precipitate, whereas localized embrittlement is caused by hydrogen migration leading to damage accumulation within a small region over a period of time before catastrophic failure of the component may occur. The parameters like fracture toughness, threshold stress for reorientation of hydrides, threshold hydrogen concentration for blister formation, delayed hydride cracking (DHC) velocity, threshold stress intensity factor for DHC initiation are used as fitness criteria for service assessment of the pressure tubes. In this paper, some of the results generated using Indian pressure tube material will be discussed.

  • 50.
    Ståhle, Per
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Jivkov, Andrey
    On dissolution driven crack growth2007In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 6, p. 1880-1890Article in journal (Refereed)
    Abstract [en]

    The formation and growth of a crack in a body subjected to stress driven material dissolution is studied. The rate of material dissolution is proportional to strain energy and curvature of the body surface. The formation of a crack from a plane surface is preceded by an evolving surface roughness. The continued dissolution enhances roughness amplitude resulting in pit formation. As the pit grows deeper into the material, it assumes the shape of a crack. The sharpness of the crack reaches its maximum during this transition from a pit to a crack. As the crack grows, a self-similar state is gradually assumed. During this phase characteristic lengths of the crack shape scale with the crack length. In line with this the crack progressively becomes blunt. The widest part of the crack when unloaded is in the vicinity of the crack tip. A consequence of the model is that no criterion is needed for crack growth. Neither is a criterion needed for determination of the crack path. It also follows that the crack growth rate is almost independent of the remote load. Further, spontaneous crack branching is anticipated. A motivation for this is given.

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