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  • 1.
    Hedström, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Deformation and martensitic phase transformation in stainless steels2007Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The use of high-energy synchrotron x-rays which has enabled three- dimensional structural characterization from the nano- to the macroscopic scale, and now to the meso-scale, such as individual grains and dislocation structures, is a major scientific advance. This is the first technique with sufficient spatial resolution and penetration power to probe the local structure embedded deep within the material. This, together with good time resolution makes it suitable for investigations of e.g. phase transformations kinetics, stress-strain behaviour, and texture evolution in stainless steels. The micromechanical response of a metastable austenitic stainless steel and a duplex stainless steel during loading has been investigated by a series of high-energy x-ray diffraction experiments at the Advanced Photon Source (APS) in Argonne, IL, USA. Different measurement scales of the steels are tested, ranging from the behaviour of individual grains up to the macroscopic material behaviour. The experimental data is used as input to material models to validate and improve existing models for strain-induced martensite and mechanical properties. The x-ray investigations have revealed that autocatalytic Ü-martensite transformation is triggered by strains induced by the transformation itself in 301 and this was evidenced as bursts of Ü-martensite transformation during tensile loading. To the author's knowledge this behaviour has not been previously reported, and is of significant importance for the mechanical properties of the metastable stainless steels, since it provides strong local hardening and increases the time to neck formation. The å-martensite formation was investigated for 45 individual austenite bulk grains in 301 and the resolved shear stress was determined. Out of the 45 austenite grains probed one was observed to form å-martensite. The grain that formed å-martensite had the highest Schmid factor for the active slip system during fcc to hcp transformation. The behaviour of 301 during tensile loading at different strain rates was also investigated and it was concluded that even moderate strain rates produce adiabatic heating sufficient to suppress the martensite formation. The strain-induced martensitic transformation and the stress-strain behaviour was predicted by an extended Olson-Cohen model, finite element simulations for the temperature evolution and a radial return algorithm for the stress-strain behaviour. The measured and modelled results were in fair agreement. In addition, the phase specific stresses were measured during the experiments and these were in good agreement with the predicted results from the finite element model. Thus, it was concluded that the employed iso- work principle was a good assumption for the stress distribution between the phases. One way of tailoring the metastable austenitic stainless steels' microstructure with different phase fractions and deformation structures is by the reverse transformation from martensite to austenite. This was investigated for the cold rolled 301 steel, and the reverse transformation was observed to occur via two different mechanisms, one diffusion controlled and the other a diffusionless transformation. The onset of the diffusion reversion was about 450°C and the shear reversion became active at higher temperatures. The microstructure of shear reversed austenite consists of highly faulted austenite with an inherited lath like structure. The stress response of 15 individual austenite and ferrite grains deeply embedded in the bulk of a duplex stainless steel was measured during tensile loading. These results showed large intergranular stresses acting between grains due to grain interaction. The large intergranular stresses will have a significant effect on the two-phase behaviour during loading.

  • 2. Hedström, Peter
    Deformation induced martensitic transformation of metastable stainless steel AISI 3012005Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Metastable stainless steels are promising engineering materials demonstrating good corrosion resistance and mechanical properties. Their mechanical properties are however significantly affected by the deformation induced martensitic transformation. Hence, in order to use these steels to their full potential it is vital to have profound knowledge on this martensitic phase transformation. The aim of this thesis was therefore to investigate the evolution of phase fractions, texture, microstrains and microstructure to improve the current understanding of the deformation induced martensitic transformation in AISI 301. To investigate the deformation behavior of AISI 301, in-situ high-energy x- ray diffraction during tensile loading has been performed on samples suffering different cold rolling reduction. Ex-situ transmission electron microscopy, electron back-scattered diffraction and optical microscopy were also used to characterize the microstructure at different deformation levels. The results show that parts of the austenite transforms to both ά- martensite and ε-martensite during deformation of AISI 301. The transformation behavior of ά-martensite is however completely different from the transformation behavior of ε-martensite. ε-martensite forms in a parabolic behavior, while the ά-martensite transformation can be divided in three characteristic stages. The third transformation stage of ά-martensite has previously not been reported and it is characterized by a series of rapid transformations, each of which is followed by a period of yielding without any transformation. Moreover, the lattice strain evolution in the austenite at high plastic strains was found to be oscillatory, which is correlated with the stepwise transformation of ά-martensite as well as changes in x-ray peak broadening. This behavior was also coupled with the evolution of microstructure, where a distinct banded structure consisting of slip bands and Ü-martensite was observed at low plastic strains. This banded structure was however broken at high plastic strains when the ά-martensite grew larger and formed a block- shaped morphology. These findings lead to the conclusion that the three stages of ά- martensite transformation is due to different stages of nucleation and growth. The ά-martensite will first form as small nucleus, mainly at dislocation pile-ups along slip bands. The nucleuses will grow moderately in size and the structure will become saturated with nucleuses. Hence, the only way more ά-martensite can form is by growth of the existing nucleuses. This growth is very localized and seen as bursts in the transformation curve. The oscillatory behavior observed for the lattice strains during martensite formation possibly originate when semicoherent boundaries between austenite and ά-martensite become incoherent as the ά-martensite grow large.

  • 3. Hedström, Peter
    et al.
    Almer, Jon
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Lienert, Ulrich
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Evolution of residual strains in metastable austenitic stainless steels and the accompanying strain induced martensitic transformation2006In: Residual Stresses VII: ECRS 7 ; proceedings of the 7th European Conference on Residual Stresses / [ed] Walter Reimers; Sabine Quander, Uetikon-Zuerich: Trans Tech Publications Inc., 2006, p. 821-826Conference paper (Refereed)
    Abstract [en]

    The deformation behavior of metastable austenitic stainless steel AISI 301, suffering different initial cold rolling reduction, has been investigated during uniaxial tensile loading. In situ highenergy x-ray diffraction was employed to characterize the residual strain evolution and the strain induced martensitic transformation. Moreover, the 3DXRD technique was employed to characterize the deformation behavior of individual austenite grains during elastic and early plastic deformation. The cold rolling reduction was found to induce compressive residual strains in the austenite along rolling direction and balancing tensile residual strains in the -martensite. The opposite residual strain state was found in the transverse direction. The residual strain states of five individual austenite grains in the bulk of a sample suffering 2% cold rolling reduction was found to be divergent. The difference among the grains, considering both the residual strains and the evolution of these, could not be solely explained by elastic and plastic anisotropy. The strain states of the five austenite grains are also a consequence of the local neighborhood. Introduction The metastable austenitic stainless steels partially transform from the soft and ductile austenite to the hard and brittle martensite during deformation. This deformation induced martensitic transformation provides a compos...

  • 4.
    Hedström, Peter
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Han, Tong-Seok
    Yonsei University, Seoul.
    Lienert, Ulrich
    Argonne National Laboratory, Argonne, IL.
    Almer, Jonathan
    Argonne National Laboratory, Argonne, IL.
    Odén, Magnus
    Load partitioning between single bulk grains in a two-phase duplex stainless steel during tensile loading2010In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 58, no 2, p. 734-744Article in journal (Refereed)
    Abstract [en]

    The lattice strain tensor evolution for single bulk grains of austenite and ferrite in a duplex stainless steel during tensile loading to 0.02 applied strain has been investigated using in situ high-energy X-ray measurements and finite-element modeling. Single-grain X-ray diffraction lattice strain data for the eight austenite and seven ferrite grains measured show a large variation of residual lattice strains, which evolves upon deformation to the point where some grains with comparable crystallographic orientations have lattice strains different by 1.5 × 10-3, corresponding to a stress of ≈300 MPa. The finite-element simulations of the 15 measured grains in three different spatial arrangements confirmed the complex deformation constraint and importance of local grain environment.

  • 5. Hedström, Peter
    et al.
    Lienert, Ulrich
    Argonne National Laboratory, Argonne, IL.
    Almer, Jon
    Argonne National Laboratory, Argonne, IL.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Elastic strain evolution and ε-martensite formation in individual austenite grains during in situ loading of a metastable stainless steel2008In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 62, no 2, p. 338-340Article in journal (Refereed)
    Abstract [en]

    The (hcp) ε-martensite formation and the elastic strain evolution of individual (fcc) austenite grains in metastable austenitic stainless steel AISI 301 has been investigated during in situ tensile loading up to 5% applied strain. The experiment was conducted using high-energy X-rays and the 3DXRD technique, enabling studies of individual grains embedded in the bulk of the steel. Out of the 47 probed austenite grains, one could be coupled with the formation of ε-martensite, using the reported orientation relationship between the two phases. The formation of ε-martensite occurred in the austenite grain with the highest Schmid factor for the active {111}b12¯1N slip system.

  • 6.
    Hedström, Peter
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lienert, Ulrich
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Almer, Jon
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stepwise transformation behavior of the strain-induced martensitic transformation in a metastable stainless steel2007In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 56, no 3, p. 213-216Article in journal (Refereed)
    Abstract [en]

    In situ high-energy X-ray diffraction during tensile loading has been used to investigate the evolution of lattice strains and the accompanying strain-induced martensitic transformation in cold-rolled sheets of a metastable stainless steel. At high applied strains the transformation to α-martensite occurs in stepwise bursts. These stepwise transformation events are correlated with stepwise increased lattice strains and peak broadening in the austenite phase. The stepwise transformation arises from growth of α-martensite embryos by autocatalytic transformation.

  • 7. Hedström, Peter
    et al.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Almer, J.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Lienert, U.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Bernier, J.
    Lawrence Livermore National Laboratory.
    Terner, Mark
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Load partitioning and strain-induced martensite formation during tensile loading of a metastable austenitic stainless steel2009In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 40, no 5, p. 1039-1048Article in journal (Refereed)
    Abstract [en]

    In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and α′ martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower α′ martensite fraction observed at fracture for a strain rate of 10-2 s-1, as compared to 10-3 s-1. This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson-Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The α′ martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to α′ martensite during loading.

  • 8. Hedström, Peter
    et al.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Stress state and strain rate dependence of the strain-induced martensitic transformation in a metastable austenitic stainless steel2007In: Fundamentals of Martensite and Bainite towards Future Steels with High Performance: Challenges for Strength, Toughness, and Ductility : The 1st International Symposium on Steel Science, Kyoto, May 16 - 19, 2007 / [ed] Tadashi Furuhara; Kaneaki Tsuzaki, Tokyo: The Iron and Steel Institute of Japan , 2007, p. 171-174Conference paper (Refereed)
    Abstract [en]

    The strain-induced martensitic transformation in a metastable austenitic stainless steel is investigated by high-energy x-ray diffraction and material modeling. Two different deformation modes are used (cold rolling and uniaxial tensile loading) and the effect on the strain-induced martensitic transformation behavior is investigated. Moreover, three different strain rates during the uniaxial tensile loading are evaluated. The results show a sigmoidal transformation behavior of the strain-induced martensite in respect to true strain, for tensile loading. The effect of different strain rates is also clearly seen and it alters both the amount of transformed martensite and the transformation behavior. The martensite transformation is drastically decreased already at moderate strain rates such as 10-2 s-1, due to adiabatic heating of the sample. The material model used gives an accurate prediction of the strain-induced martensitic transformation behavior during tensile loading. This is valuable for further implementation of the current material model in industrial forming mulations of real components.

  • 9. Hedström, Peter
    et al.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The use of high energy SAXS/WAXS for structural characterization of stainless steels2005In: Stainless steel world 2005: [Stainless Steel World 2005 Conference & Expo, Maastricht, the Netherlands, 8 - 10 November 2005] ; conference papers, Zutphen: KCI Publ., , 2005Conference paper (Refereed)
  • 10.
    Knutsson, Axel
    et al.
    Luleå tekniska universitet.
    Hedström, Peter
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Reverse martensitic transformation and resulting microstructure in a cold rolled metastable austenitic stainless steel2008In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 79, no 6, p. 433-439Article in journal (Refereed)
    Abstract [en]

    The reverse martensitic transformation in cold-rolled metastable austenitic stainless steel has been investigated via heat treatments performed for various temperatures and times. The microstructural evolution was evaluated by differential scanning calorimetry, X-ray diffraction and microscopy. Upon heat treatment, both diffusionless and diffusion-controlled mechanisms determine the final microstructure. The diffusion reversion from α′-martensite to austenite was found to be activated at about 450°C and the shear reversion is activated at higher temperatures with Af′ ∼600°C. The resulting microstructure for isothermal heat treatment at 650°C was austenitic, which inherits the α′-martensite lath morphology and is highly faulted. For isothermal heat treatments at temperatures above 700°C the faulted austenite was able to recrystallize and new austenite grains with a low defect density were formed. In addition, carbo-nitride precipitation was observed for samples heat treated at these temperatures, which leads to an increasing Ms-temperature and new α′-martensite formation upon cooling.

  • 11.
    Mangalaraja, R.V.
    et al.
    University of Concepción.
    Mouzon, Johanne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Hedström, Peter
    Camurri, C.P.
    University of Concepción.
    Ananthakumar, S.
    Université Montpellier.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microwave assisted combustion synthesis of nanocrystalline yttria and its powder characteristics2009In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 191, no 3, p. 309-314Article in journal (Refereed)
    Abstract [en]

    Microwave assisted combustion synthesis is used for fast and controlled processing of advanced ceramics. Single phase and sinter active nanocrystalline cubic yttria powders were successfully synthesized by microwave assisted combustion using the organic fuels urea, citric acid and glycine as reducing agents. The precursor powders were investigated by thermogravimetry (TG) and differential scanning colorimetry (DSC) analyses. The as-prepared precursors and the resulting oxide powders calcined at 1100 °C in oxygen atmosphere were characterized for their structure, particle size and morphology. The thermal analyses (TG/DSC), X-ray diffraction (XRD) and Fourier transform infra red (FT-IR) results demonstrate the effectiveness of the microwave assisted combustion synthesis. The scanning electron microscopy (SEM) observations show the different morphologies of as-prepared powders and transmission electron microscopy (TEM) shows the particle sizes in the range of 30-100 nm for calcined powders for different fuels. The results confirm that the homogeneous, nano scale yttria powders derived by microwave assisted combustion have high crystalline quality and the morphology of the as-prepared precursor powders depends on the nature of organic fuel used.

  • 12.
    Mangalaraja, R.V.
    et al.
    University of Concepción.
    Mouzon, Johanne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Hedström, Peter
    Kero, Ida
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Ramam, K.V.S.
    University of Concepción.
    Camurri, C.P.
    University of Concepción.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Combustion synthesis of Y2O3 and Yb-Y2O3: Part 1: Nanopowders and their characterization2008In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 208, no 1-3, p. 415-422Article in journal (Refereed)
    Abstract [en]

    Nanosized yttrium oxide and ytterbium doped yttrium oxide powders were prepared by ceramic combustion techniques such as flash combustion, citrate gel decomposition and glycine combustion using urea, citric acid and glycine respectively as fuels. As synthesized precursors and calcined powders were characterized for their structural, particle size and morphology, and the optimization of calcination process by differential scanning calorimetry and thermal gravimetry. The thermal analyses together with XRD results demonstrate the effectiveness of the combustion process for the synthesis of pure phase nanocrystalline powders. Nanocrystalline pure yttria powders were obtained by the calcination of as-prepared precursors at 1100 °C for 4 h.

  • 13.
    Odén, Magnus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Rogström, L.
    Linköping University.
    Terner, Mark
    Hedström, Peter
    Almer, J.
    Argonne National Laboratory, Argonne, IL.
    llavsky, J.
    Argonne National Laboratory, Argonne, IL.
    In situ small-angle x-ray scattering study of nanostructure evolution during decomposition of arc evaporated TiAlN coatings2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 5, p. 53114-Article in journal (Refereed)
    Abstract [en]

    Small-angle x-ray scattering was used to study in situ decomposition of an arc evaporated TiAlN coating into cubic-TiN and cubic-AlN particles at elevated temperature. At the early stages of decomposition particles with ellipsoidal shape form, which grow and change shape to spherical particles at higher temperatures. The spherical particles grow at a rate of 0.18 Å/°C while coalescing

  • 14. Terner, Mark
    et al.
    Hedström, Peter
    Almer, Jon
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Ilavsky, J.
    Advanced Photon Source (APS), Argonne National Laboratory, Argonne.
    Odén, Magnus
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Residual stress evolution during decomposition of Ti(1-x)Al(x)N coatings using high-energy X-rays2006In: Residual stresses VII: ECRS 7 ; proceedings of the 7th European Conference on Residual Stresses, Trans Tech Publications Inc., 2006, p. 619-624Conference paper (Refereed)
    Abstract [en]

    Residual stresses and microstructural changes during phase separation in Ti33Al67N coatings were examined using microfocused high energy x-rays from a synchrotron source. The transmission geometry allowed simultaneous acquisition of x-ray diffraction data over 360° and revealed that the decomposition at elevated temperatures occurred anisotropically, initiating preferentially along the film plane. The as-deposited compressive residual stress in the film plane first relaxed with annealing, before dramatically increasing concurrently with the initial stage of phase separation where metastable, nm-scale c-AlN platelets precipitated along the film direction. These findings were further supported from SAXS analyses.

  • 15.
    Ukonsaari, Jan
    et al.
    Vattenfall Research & Development.
    McCarthy, Donald
    Prakash, Braham
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Hedström, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Tribological studies on an EAL lubricated bearing bronze - steel pair under reciprocating sliding conditions2008In: Tribologia : Finnish Journal of Tribology, ISSN 0780-2285, Vol. 27, no 1, p. 3-18Article in journal (Refereed)
    Abstract [en]

    Boundary lubricated journal bearings are found in various applications involving oscillatory sliding conditions. Environmental adaptation of hydraulic systems includes the introduction of synthetic esters. These new environmentally adapted lubricants (EALs) have shown very good boundary lubrication performance but also condition sensitivity. This study examines an oil lubricated bronze pin on hardened steel configuration in a reciprocating friction and wear test machine. Three synthetic esters were tested with a 1 mm stroke length. Results were compared with those for a mineral oil. The tribological performance with synthetic ester lubricant can, under certain conditions, be very good. SEM-EDS and XRD surface sensitive studies indicate the formation of a soft, copper enriched outer contact layer. The layer's nature and contact mechanisms clearly affect the performance of the different lubricants. (17 refs.)

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