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  • 51.
    Hillert, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Centennial of the Diffusionless Paradigm of Bainite2012In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 12, p. 4487-4495Article in journal (Refereed)
    Abstract [en]

    The diffusionless growth model of bainite formation was included in an international consensus in 1912 that all transformation products of austenite form by an initial transformation to martensite. The introduction of isothermal treatment revealed that all the products form directly. However, for bainite, the idea of some relation to martensite survived and developed into the diffusionless paradigm. Zener introduced the T (o) concept for predicting the start temperature of lower bainite, but also described diffusional growth of upper bainite. The present description of the diffusionless growth model is now examined and criticized.

  • 52.
    Hillert, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Do bainitic and Widmanstatten ferrite grow with different mechanisms?2010In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 62, no 2, p. 75-77Article in journal (Refereed)
    Abstract [en]

    Caballero et al. recently presented new evidence for different growth mechanisms of Widmanstatten and bainitic ferrite. Their argument was based on Zener's hypothesis of diffusionless growth of bainitic ferrite. It is now demonstrated that Bhadeshia's model, based on Zener's hypothesis, predicts that some of the new measurements, claimed to fall above B-s and to be due to Widmanstatten ferrite, actually fall within the predicted temperature range of bainite, indicating that they cannot be used as new support for Bhadeshia's model.

  • 53.
    Hillert, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Diffusion in interstitial compounds with thermal and stoichiometric defects2005In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 98, no 5Article in journal (Refereed)
    Abstract [en]

    The ordinary flux equation for diffusion, which considers the composition gradient as the driving force, is seldom of much use in studying closely stoichiometric phases. Depending on the defect structure it would instead be profitable to use an appropriate function of the activity. Such functions will now be derived and it will be shown how the operating defect mechanism of diffusion can be identified from information on the variation of the activity inside a phase during diffusion. However, it is usually very difficult to measure the activity profile inside a phase. It will be shown how it can be obtained by combining results from several experiments. The method will be used to analyze experimental information on the formation of surface layers of Fe4N and Fe3C.

  • 54.
    Hillert, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Extremum principles for irreversible processes2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 8, p. 2063-2066Article in journal (Refereed)
    Abstract [en]

    Hamilton's extremum principle is a powerful mathematical tool in classical mechanics. Onsager's extremum principle may play a similar role in irreversible thermodynamics and may also become a valuable tool. His principle may formally be regarded as a principle of maximum rate of entropy production but does not have a clear physical interpretation. Prigogine's principle of minimum rate of entropy production has a physical interpretation when it applies, but is not strictly valid except for a very special case.

  • 55. Holm, T.a
    et al.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    The carbon potential during the heat treatment of steel2008In: The SGTE Casebook: Thermodynamics At Work, Elsevier Ltd , 2008, 2, p. 212-233Chapter in book (Other academic)
  • 56.
    Hou, Ziyong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    An experimental and theoretical study of precipitation during tempering of martensite in Fe-C-Cr alloys2018Doctoral thesis, comprehensive summary (Other academic)
  • 57.
    Hou, Ziyong
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Study of precipitation in martensitic Fe-C-Cr alloys during tempering: Experiments and modelling2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Understanding the precipitation reaction is very important since precipitation hardening is one of the most effective strengthening mechanisms in metallic alloys. In martensitic steels, a tempering heat treatment is often performed. During tempering various new phases are precipitated and the spatial and temporal evolution of these precipitates strongly influences the properties of the steel, such as strength/ductility, creep, fatigue and hot corrosion resistance. Therefore, the possibility of quantitative modelling of the precipitation process will provide many opportunities for advanced materials and process design and optimization as well as service life assessments. The Fe-C-Cr system forms the basis for tool steels and is consequently used in many applications such as e.g. metal forming operations. They are characterized by a high hardness and good toughness, even at elevated temperatures.In the present work, the as-quenched martensitic microstructures of four Fe-C-Cr alloys with varying Cr and C contents were characterized by Light Optical Microscopy (LOM) and Electron Microscopy. The effects of Cr and C on the morphology of martensite were investigated. It was found that Cr addition had a similar effect as C on the martensitic morphology and on the ratio of high-angle grain boundary (HAGB) to low-angle grain boundary (LAGB). However, the micro-hardness was unaffected by the Cr addition whilst it was strongly influenced by the C addition.In addition, a quantitative experimental characterization of the precipitates formed during tempering of the martensite was performed. The Langer-Schwartz theory combined with the Kampmann-Wagner-Numerical (KWN) method, as implemented in the software TC-PRISMA, was used to predict the precipitation of carbides after tempering in one of the model alloys: Fe-0.15C-4.0Cr (mass%). The microstructure characterization of the as-quenched material provided vital input parameters for the modelling work and a comparison was made between the modelling predictions and the experimental results. The effect of parameters such as dislocation density, grain size and interfacial energy on the precipitation of carbides was discussed.

  • 58.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Babu, Prasath
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Coarsening of cementite during tempering of a martensitic steelManuscript (preprint) (Other academic)
    Abstract [en]

    Coarsening of cementite (M3C) in a martensitic steel alloy Fe–1C–1Cr (wt. %) during tempering at 700 °C was investigated by electron microscopy and kinetic modelling. It is shown that the large M3C carbides are mostly located at high-angle grain boundaries in the coarsening stage and simple kinetic simulations predict the experimentally observed mean size evolution well when grain boundary diffusion of Cr is taken into account. However, the particle size distribution of M3C maintain a log-normal distribution throughout the whole extended tempering process (5000 h at 700 °C), which indicates that a modified LSW distribution , as predicted by classical steady-state coarsening theory , is not fully adequate for practical purposes in tempering of martensitic steels.

  • 59.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Babu, Prasath
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Early stages of cementite precipitation during tempering of 1C-1Cr martensitic steelManuscript (preprint) (Other academic)
    Abstract [en]

    The precipitation of cementite (M3C) from as-quenched martensite during tempering at 500 and 700 °C was investigated in a Fe–1C–1Cr (wt. %) alloy. Tempering for a short duration at 700 °C results in a Cr/Fe ratio in the core region of M3C precipitates which is equal to the bulk alloy composition, while a shell on the surface of the precipitates exhibit a higher Cr concentration. With a prolonged tempering up to 5 hours, the shell concentration gradually increases towards the equilibrium value but the core region has not yet reached the equilibrium value. After tempering for 5 seconds at 500 °C, there is no Cr enrichment found at the M3C/matrix interface, while a transition to partitioning of Cr is found during the first 5 minutes of tempering at 500 °C. These experimental results indicate that M3C grows without significant partitioning of substitutional elements at both temperatures initially, i.e. growth is carbon diffusion controlled. This stage is, however, very short, and soon after 5 seconds at 700 °C and 5 min at 500 °C, Cr diffusion becomes important. Calculations using the diffusion simulation software DICTRA and precipitation simulation software TC-PRISMA were performed. The diffusion simulations using the local equilibrium interface condition show excellent agreement with experiments concerning Cr enrichment of the particles, but the size evolution is overestimated. On the other hand, the precipitation simulations underestimate the size evolution. It is suggested that a major improvement in the precipitation model could be achieved by implementing a modified nucleation model that considers nucleation far from the equilibrium composition.

  • 60.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Northeastern University, China.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Q.
    Xu, Y.
    Wu, D.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Quantitative modeling and experimental verification of carbide precipitation in a martensitic Fe-0.16 wt%C-4.0 wt%Cr alloy2016In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 53, p. 39-48Article in journal (Refereed)
    Abstract [en]

    Precipitation of carbides during tempering of a martensitic Fe-0.16 wt% C-4.0 wt% Cr alloy has been investigated by experimental analysis and quantitative modeling. It is found that both M7C3 and M23C6 form, at low- and high-angle grain boundaries in the martensite, as well as, at dislocations inside individual laths of martensite, during tempering at 700 °C. The applied Kampmann-Wagner numerical (KWN) modeling, utilizing CALPHAD thermodynamic and kinetic databases together with an assumption of local equilibrium and a constant tie-line, captures the main features of the precipitation, with a transient formation of metastable M23C6, and with M7C3 as the stable carbide. The predicted volume fraction and size are in reasonable agreement with extraction experiments for M7C3. However, for the metastable minority carbide M23C6, the modeling underestimates the size and overestimates the volume fraction within the transient time. With sound thermodynamic databases and physical parameter input, the adopted simplified modeling scheme is a valuable tool for materials design and optimization. Furthermore, by treating conditions at the phase interface more rigorously it is possible to account for different mechanisms of precipitation, such as e.g., non-partitioning local equilibrium, which could be important in systems where interstitial elements diffuse much faster than the substitutional ones.

  • 61.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Northeastern University, China .
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Chen, Qing
    Xu, Yunbo
    Di, Wu
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. AB Sandvik Materials Technology, RandD Centre, Sweden .
    Quantitative modeling and experimental verification of carbide precipitation in a martesnsitic Fe-0,16 wt.%C-4.0 wt.%Cr alloyManuscript (preprint) (Other academic)
  • 62.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Northeastern University, China .
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Xu, Yunbo
    Di, Wu
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. AB Sandvik Materials Technology, RandD Centre, Sweden .
    Microstructure of Martensite in Fe-C-Cr and its Implications for Modelling of Carbide Precipitation during Tempering2014In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 54, no 11, p. 2649-2656Article in journal (Refereed)
    Abstract [en]

    The microstructure of as-quenched martensite in four Fe-C-Cr alloys (0.15C-1Cr, 0.15C-4Cr, 1C-1Cr, 1C-4Cr, mass%) has been investigated. Moreover, the microstructures served as input for setting up modeling of carbide precipitation during tempering of martensite. The modelling was conducted using the Langer-Schwartz approach and the software TC-PRISMA, which retrieves thermodynamic data from the Thermo-Calc databank. It was found that the martensite in the low carbon steels is predominantly lath martensite with units arranged parallel to each other. On the other hand, the plate martensite dominates the microstructure in the high carbon steels. The ratio of high-angle to low-angle grain boundaries was found to increase with increasing Cr in the low carbon steels, which indicates that Cr has a similar effect as C on the lath martensite microstructure, however, the micro-hardness remained unaffected by the addition of Cr. Finally, the precipitation modeling clearly demonstrates the importance of proper definition of the initial microstructure for predictive modelling. Parameters such as dislocation density and frequency of high-angle grain boundaries have a drastic effect on e.g. the mean size of carbides.

  • 63.
    Hou, Ziyong
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Linder, David
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ström, Valter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Holmström, E.
    Sandvik Coromant R&D, SE 126 80 Stockholm, Sweden.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Evaluating magnetic properties of composites from model alloys – Application to alternative binder cemented carbides2019In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 168, p. 96-99Article in journal (Refereed)
    Abstract [en]

    The magnetic properties of 85Ni-15Fe model alloys containing Co, W and C have been studied with the intent to isolate the influence of alloy chemistry on quality control measurements of alternative binder cemented carbides. The results show a strong influence of dissolved W on the Curie temperature and the saturation magnetization. The amount of dissolved C, and the presence of WC precipitates, on the other hand, is shown to have negligible effect. Furthermore, the magnetic coercivity is indicated to be entirely dominated by the microstructural features and quite insensitive to composition.

  • 64.
    Huyan, Fei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    A study of "475°C embrittlement" in Fe-20Cr and Fe-20Cr-X (X=Ni, Cu, Mn) alloys2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

  • 65.
    Huyan, Fei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    A study of "475°C embrittlement" in Fe-20Cr and Fe-20Cr-X (X=Ni, Cu, Mn) alloys2012Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

  • 66.
    Huyan, Fei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Computational Materials Design of Medium Mn Steels2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Medium Mn steels (~ 3–10 mass % Mn), a new category of advanced high strength steels, attracted worldwide research interests recent years due to their excellent mechanical properties and low cost. These steels have fine microstructures and contain large fraction of metastable retained austenite (~ 30 volume %), therefore exhibit excellent strength and elongation. The fine microstructure is mainly introduced by an intercritical annealing process.

    To accelerate the design of such steels, materials design is applied. The materials design concept is a systematic method. Contrary to conventional methods largely based on trial and error, it is based on the classical processing–structure–properties relationships and a quantitative knowledge of each relation represented by a mathematical model, so-called linkage model. Such models are thus an essential part in materials design.

    The present thesis aims to develop a framework used for materials design of medium Mn steel. The development of models which serve as linkage tools is thus the focus. Tensile properties, i.e. strength and elongation, are set as the design objectives driven by the industrial application.

    The major part is concentrated on the linkage tools of processing–structure, i.e. models and simulations to predict the microstructure evolution associated with processing. These linkage tools are based on thermodynamic calculations and kinetic simulations using the commercially available Thermo-Calc and DICTRA software. To be specific, the processing involves austenitization and quenching as well as intercritical annealing and quenching; while the associated structure involves transformation of austenite to martensite and reversion of martensite to austenite. Therefore the following aspects have been studied:

    1. martensite fraction with undercooling;
    2. austenite reversion during intercritical annealing;
    3. influence of austenite grain size on martensite start temperature;
    4. mechanical stability of retained austenite.

    Besides these, prediction of tensile properties is studied in the last part, which serves as an example of a linkage tool of structure–properties.

    Via integrating these models, to achieve certain tensile properties, the required microstructure and the associated processing can be traced back.

  • 67.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Larker, Richard
    Rubin, Per
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Effect of Solute Silicon on the Lattice Parameter of Ferrite in Ductile Irons2014In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 54, no 1, p. 248-250Article in journal (Refereed)
    Abstract [en]

    The effect of solute silicon on the ferrite lattice parameter has been investigated using X-ray diffraction in cast ductile irons (DI) with nominal Si contents between 2.50 and 4.56 wt%. It was found that silicon changes the ferrite lattice parameter by –0.00185 Å per wt% Si. This contraction coefficient is three times larger than the most commonly used Si coefficient in the literature. Since substitutional solution by silicon contracts the ferrite lattice while the interstitial solution by carbon expands the lattice, the Si contraction coefficient found will have a significant effect on subsequent evaluation of the carbon content in austempered Si-alloyed ductile irons and steels.

  • 68.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Yan, Jiayi
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Experimental Determination and Thermodynamic Modeling of Msσ for a Medium Mn SteelManuscript (preprint) (Other academic)
    Abstract [en]

    Medium Mn steels have attracted worldwide interests recent years due to their excellent mechanical properties and low cost. These steels contain large fraction (~30%) of metastable retained austenite and exhibit good elongation due to transformation-induced plasticity (TRIP). In order to obtain the highest elongation, the mechanical stability of austenite, quantified using Msσ, needs to be optimized. Msσ is defined as the highest temperature at which martensite can form under stress without austenite yielding by slip. The present work aims to formulate a model of Msσ which can be used to design medium Mn steels with optimized elongation. In the present work, an Fe–0.18C–5.08Mn (mass %) steel was intercritically annealed at 650 °C. Based on tensile tests at different temperatures using a single specimen method, the Msσ temperatures were experimentally determined to about 0 °C regardless of intercritical annealing time between 15 min and 3 h. Microstructure observations showed that large austenite grains with a globular shape are more transformed than thin-film ones, and thus the former probably governs the determined Msσ. Msσ was further predicted at the crossing point of yielding by martensite formation and by austenite slip; the former was modeled by expanding an existing model of martensite start temperature and the latter by a constitutive model. The predicted Msσ showed reasonable agreement with the determined values. The model also indicated that a large and a small austenite grain have similar Msσ, which could partly explain why the determined Msσ is rather constant regardless of IA time.

  • 69.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Yan, Jiayi
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Influence of Sub-micron Austenite Grain Size on Martensitic Transformation in a Medium Mn SteelManuscript (preprint) (Other academic)
    Abstract [en]

    Many previous studies suggest that austenite stability increases with decreasing grain size, i.e. the martensite start temperature, Ms, decreases. Therefore, the influence of small austenite grain size would be high for many modern steels containing fine austenite. Models from literature deviate severely from each other for grain sizes below 1 μm where there is a lack of experimental data. Besides, the experimental data are mostly obtained from a fully austenitic microstructure with equiaxed grains. This raises concerns about the applicability of these models to design medium Mn steels, where the austenite stability is essential for elongation via transformation-induced plasticity. The present work concerns the influence of grain size on martensitic transformation for thin-film austenite in a medium Mn steel. After IA, austenite grains exhibit two morphologies, thin-film like and globular, while the former is dominant. The globular austenite is less stable and responsible for the Ms measured by dilatometry. Similar to particles, the austenite grains are isolated and dispersed, and autocatalysis from surrounding austenite is minimized. Therefore in this work an approach to describe the transformation in small particles has been adopted, where the number fraction of partly or fully transformed austenite grains (F) is phenomenologically formulated as a function of temperature and grain size. Experimental data from the present work and from literature have been used to derive a model of Ms and grain size for thin-film austenite in medium Mn steels, using the cross-sectional area of austenite grain as a variable. The current model provides a practical and useful way of determining austenite stability from 2-dimensional micrograph, and can be used in designing medium Mn steels with optimized austenite stability.

  • 70.
    Huyan, Fei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Yan, Jiayi
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    On the tuning of austenite stability in a medium mn trip steel2015In: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, International Conference on Solid-Solid Phase Transformations in Inorganic Materials , 2015, p. 933-934Conference paper (Refereed)
  • 71. Hättestrand, Mats
    et al.
    Larsson, Petter
    Chai, Guocai
    Nilsson, Jan-Olof
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Study of decomposition of ferrite in a duplex stainless steel cold worked and aged at 450-500°C2009In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 499, p. 489-492Article in journal (Refereed)
    Abstract [en]

    The influence of cold-deformation on ferrite decomposition in duplex stainless steel during heat treatment at 450-500 °C was investigated using micro-hardness measurements and transmission electron microscopy. It was found that cold-deformation can change the mechanism of the α → α + α′ phase separation in the ferrite from nucleation and growth to spinodal decomposition. This finding is discussed in terms of the influence of an increased dislocation density on coherency strains

  • 72.
    Höglund, Lars
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Simulation of Carbon Diffusion in Steel Driven by a Temperature Gradient2010In: Journal of Phase Equilibria and Diffusion, ISSN 1547-7037, Vol. 31, no 3, p. 212-215Article in journal (Refereed)
    Abstract [en]

    The basis of thermomigration in multicomponent alloys is summarized, and the general equations are given and implemented in the DICTRA software. Experimental information from Okafor et al. is analyzed with the new simulations and it is concluded that steady- state conditions was not established during their experiment. A heat of transport Q(C)* = -44000 J/mol, almost four times larger than the value given by Okafor et al., was found to give a satisfactory representation of the experimental information.

  • 73.
    Jeppsson, Johan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Simulation of Phase Transformations and coarsening: Computational tools for alloy development2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The final properties of an alloy are highly interlaced with its microstructure. It is therefore essential to control the evolution of the microstructure of the material during the fabrication process. Nowadays, materials design involves an increasing part of computational design to complement the traditional experimental trial and error approach. Such simulations of the process can decrease the number of material prototypes and shorten the development time for new alloys.

    In this thesis several microstructure models, aimed for process design, have been suggested. The ambition has been to develop physically based models that are capable to represent the evolution of hundreds of grain or particle sizes, where the models should be possible to run on a standard computer with simulation times less than one day. To achieve this goal, simplified approaches have been suggested, which are accurate enough for the growth rate of grains and particles. The microstructure models have all in common that size distributions of grains or particles are simulated with mean-field approaches. Several of the models also utilize composition and temperature dependent thermodynamic and kinetic properties continually throughout the simulations. These properties have been calculated with programming interfaces to Thermo-Calc and DICTRA together with appropriate thermodynamic and kinetic databases. The materials that have been considered in the present thesis are low alloyed steels, aluminium alloys and cemented carbides. The models are however generic in the sense that all materials can be handled if appropriate thermodynamic, kinetic and property databases exist for the alloy.

  • 74.
    Jeppsson, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Mannesson, Karin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Inverse Saltykov analysis for particle-size distributions and their time evolution2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 3, p. 874-882Article in journal (Refereed)
    Abstract [en]

    In this work a new method for transforming 2-D to 3-D size distributions is proposed. A representation of the 2-D size distributions is constructed from the data of measured radii with a statistical method called the kernel density estimator. The method yields a smooth density estimation that is more accurate than the classic histogram. The 3-D distribution is optimized from the 2-D density estimate in an iterative manner. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 75.
    Jeppsson, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Modeling of dispersoid precipitation in multicomponent alloysIn: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453Article in journal (Other academic)
    Abstract [en]

    A model for nucleation, growth and coarsening of precipitates in multicomponent, multiphase systems is presented. High supersaturation and volume fraction of theprecipitate phase are considered. Deviation from local equilibrium at the phaseinterface is treated by means of a model based on trans-interface diffusion. Anexample simulation predict a sudden transition from diffusion-controlled to massivegrowth during continues cooling of an Fe-5%Ni alloy. The precipitation model iscompared with experiments in the Al-Sc-Mg system. To calculate the equilibriumphases, the chemical driving forces, equilibrium concentrations and diffusivities, thecommercial softwares Thermo-Calc and Dictra were used. The main advantage ofthis strategy is that there is nearly no restriction on a special alloy system.

  • 76.
    Jeppsson, Johan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Modified mean field models of normal grain growth2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 18, p. 5188-5201Article in journal (Refereed)
    Abstract [en]

    Models of normal grain growth can either start from a postulated kinetic law for individual grains and yield a distribution of grain sizes or they can start from a postulated distribution and the kinetic law may be derived. Both methods are studied and a whole family of distributions based on new kinetic laws are derived using the first method. Both methods have recently been applied using Onsager's extremum principle but it is now shown that more classical procedures are sufficient. Kinetic laws give an indication of what physical factors govern the growth or shrinkage of individual grains. A Rayleigh's distribution seems to indicate that large grains are surrounded by grains smaller than the critical size and small grains are surrounded by grains larger than the critical size. The effects of the new family of kinetic laws on the development of grain size distributions are studied by numerical simulations.

  • 77.
    Johansson, Andreas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Influence of microstructure on tool lifespan during circular sawing at Ovako AB2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The influence of microstructural variation on tool lifespan during circular sawing process at Ovako AB has been investigated for spheroidized SAE52100 steel. Several factors have been established as potential sources of variation on the lifespan of the saw blade, beside the microstructure, based on a theoretical background and on an evaluation of the production route. Much emphasis has been put on finding and controlling these factors in order to isolate the microstructural influence.

    Initially data from the production was analysed followed by a controlled experiment. During the analysis of retroperspective data it was clear that data collected from production contained too much variation in order for a relationship between the microstructure and the tool lifespan to be distinguishable.

    During the experiment, alterations in microstructure were achieved by varying the heat treating furnaces and the heat cycles during the heat treating process. The trials performed indicate that the amount of lamellar pearlite at the surface is influencing the saw-blade lifespan to a greater extent compared to the hardness in the middle of the tube wall. It is possible to heat treat the tubes in various way in order to increase the saw-blade lifespan during circular sawing. The observed variations in saw-blade lifespan, for the same orders, in the retroperspective analysis can to a large extent be explained with difficulties to asses when to replace the saw-blades.

  • 78. Jonsson, T.
    et al.
    Pujilaksono, B.
    Hallström, Samuel
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Svensson, J. E.
    Johansson, L. G.
    Halvarsson, M.
    An ESEM in situ investigation of the influence of H2O on iron oxidation at 500 degrees C2009In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 51, no 9, p. 1914-1924Article in journal (Refereed)
    Abstract [en]

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

  • 79. Kaufman, Larry
    et al.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    CALPHAD, first and second generation - Birth of the materials genome2014In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 70, no 1, p. 3-6Article in journal (Refereed)
    Abstract [en]

    The "materials genome" was given a rather vague definition in US President Obama's announcement in 2011. We argue that the materials genome, analogously with biological genomes, should be defined as a set of information (databases) allowing prediction of a material's structure, as well as its response to processing and usage conditions. The materials genome is thus encoded in the language of CALPHAD thermodynamics and kinetics, as such databases are major parts of integrated computational materials engineering.

  • 80.
    Kolmskog, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Does Bainite form with or without diffusion?: The experimental and theoretical evidence2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys.

  • 81.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel2011In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 42A, no 13, p. 3941-3951Article in journal (Refereed)
    Abstract [en]

    The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.

  • 82.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Babu, Sudarsanam Suresh
    Terasaki, Hidenori
    Komizo, Yu-Ichi
    Direct Observation that Bainite can Grow Below M-S2012In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 43A, no 13, p. 4984-4988Article in journal (Refereed)
    Abstract [en]

    In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

  • 83.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Thermodynamic Analysis of the Critical Condition for Acicular FerriteArticle in journal (Other academic)
  • 84.
    Kolmskog, Peter
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Kinetic Study of Transformations ofAustenite in a 4.12 mass% Cr 0.88 mass% C SteelManuscript (preprint) (Other academic)
  • 85.
    Larsson, Henrik
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Simulation of phase transformations using the lattice-fixed frame of reference2005In: Solid-Solid Phase Transformations in Inorganic Material 2005, Vol 2 / [ed] Howe, JM; Laughlin, DE; Lee, JK; Dahmen, U; Soffa, WA, 2005, p. 789-793Conference paper (Refereed)
    Abstract [en]

    A method for simulating diffusion processes directly in the lattice-fixed frame of reference is described. The method does not require any assumptions regarding the conditions at phase interfaces. An example simulation in the Au-Pt system is presented. A fair agreement with experimental data is obtained.

  • 86.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Trans-interface diffusivity in the Fe-Ni system2007In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 56, no 1, p. 61-64Article in journal (Refereed)
    Abstract [en]

    The trans-interface diffusivity in the Fe-Ni system has been estimated by fitting simulations to experimental data. For diffusion across the alpha/gamma phase boundary, the mobility of species divided by an interfacial thickness of 1 nm, M-k/delta, is approximately 5% of the phase interface mobility M-int at 873 K and 15% at 973 K.

  • 87.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Engstrom, A.
    A homogenization approach to diffusion simulations applied to alpha+gamma Fe-Cr-Ni diffusion couples2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 9, p. 2431-2439Article in journal (Refereed)
    Abstract [en]

    The diffusion of Fe, Cr and Ni in one dimension in and between one- and two-phase alpha/gamma regions was simulated using independently assessed thermodynamic and kinetic data. Simulation results compare favourably with experimental Tesults obtained previously by one of the present authors (A. Engstrom). For example, the formation of a gamma layer between an alpha and a gamma + alpha region is correctly predicted. Neither phase interfaces nor individual phases are explicitly considered; instead, locally averaged kinetic properties are used and locally minimized Gibbs energy is assumed.

  • 88.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Thermocalc Software, SE-11364 Stockholm, Sweden.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Thermocalc Software, SE-11364 Stockholm, Sweden.
    A scheme for more efficient usage of CALPHAD data in simulations2015In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 50, p. 1-5Article in journal (Refereed)
    Abstract [en]

    A method is suggested that allows thermodynamic data to be dynamically stored and retrieved. The purpose of the method is to reduce computer simulation time when Calphad type databases are being used. Some test simulations are presented and these indicate that simulations can be made to run 5-40 times faster without any significant loss of accuracy.

  • 89.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Multiphase diffusion simulations in 1D using the DICTRA homogenization model2009In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 33, no 3, p. 495-501Article in journal (Refereed)
    Abstract [en]

    A model for multiphase simulations in 1D previously reported in [H. Larsson, A. Engstrom, Acta Mater. 54 (2006) 2431] has been further developed and incorporated in the DICTRA software. The model is based on the assumption of local equilibrium and locally averaged kinetic properties, which computationally transforms the problem into a single-phase diffusion process. The applicability of the model is discussed and example simulations are presented.

  • 90.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy. Thermo-Calc Software, Stockholm, Sweden.
    Jansson, Magnus
    Rate of change at equilibrium2015In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 51, p. 220-223Article in journal (Refereed)
    Abstract [en]

    A general procedure to evaluate derivatives of thermodynamic quantities with respect to equilibrium conditions (external variables, parameters) at equilibrium is outlined. An example calculation is given.

  • 91.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jonsson, T.
    Naraghi, R.
    Gong, Y.
    Reed, R. C.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Oxidation of iron at 600 degrees C - experiments and simulations2016In: Materials and corrosion - Werkstoffe und Korrosion, ISSN 0947-5117, E-ISSN 1521-4176Article in journal (Refereed)
    Abstract [en]

    Pure iron has been oxidized at 600°C and 1bar in dry O2 (oxygen partial pressure 0.05, bal. N2) and the mass gain as well as the thicknesses of the individual oxide phases have been measured. The oxidation process has been simulated using a modified version of the homogenization model as implemented in Dictra; this has helped to rationalize the kinetics of oxide scale formation and in particular the evolution of the hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO) which form. Independently assessed thermodynamic and kinetic Calphad databases are needed for the calculations; details of these are given. Reasonable agreement between simulation results and experimental data is obtained, though it is concluded that the large influence of grain boundary diffusion on the oxidation rate needs further consideration.

  • 92.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Reed, R. C.
    On the numerical simulation of diffusion-controlled reactions under local equilibrium conditions2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 15, p. 3754-3760Article in journal (Refereed)
    Abstract [en]

    To estimate the interfacial velocity in a multicomponent moving boundary problem, existing procedures require an interative technique too be used if a sharp interface methods is employed. The need to use an interactive technique can cause convergence problems; these become more frequent as more components are added. In this paper, two new methods are suggested which allow the velocity of a phase interface to be evaluated directly from the fluxes of the components Wider the assumption of local equilibrium. The methods assume that the interface has a fixed width in which the incoming and outgoing components are distributed in a way that moves the interface while maintaining local equilibrium. Example simulations are presented and results are ill good agreement with established front-tracking simulation software (DICTRA) moreover. the superior convergence of the new methods is demonstrated .

  • 93.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Strandlund, H.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Unified treatment of Kirkendall shift and migration of phase interfaces2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 4, p. 945-951Article in journal (Refereed)
    Abstract [en]

    A recent treatment of diffusion in a lattice-fixed frame of reference, which can predict the Kirkendall shift, is modified to be applicable also to the migration of phase interfaces. The net flux across a reference point in the lattice, yielding the Kirkendall shift, as well as the net flux across a phase interface, yielding its migration, are obtained from the same flux equation. Numerical calculations are presented and demonstrate that the method gives expected results. The strength of the method is that it is not based on the assumption of local equilibrium and it is capable of automatically yielding close to local equilibrium conditions except at high supersaturations. There it describes an increasing deviation from local equilibrium as the initial alloy composition is moved inside the one-phase field for the new phase. The interaction of a migrating phase interface and Kirkendall markers is also illustrated.

  • 94.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Combined probability distributions of random-walks: A new method to simulate diffusion processes2005In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 34, no 3, p. 254-263Article in journal (Refereed)
    Abstract [en]

    Two related methods to simulate diffusion processes are presented. Both are based on conceiving diffusion as a random-walk process. Several example simulations are presented: single phase diffusion couples, diffusion controlled growth and prediction of Kirkendall porosity. Comparisons with experimental results and simulation software based on established technique (DICTRA) show good agreement with results obtained from the presented models.

  • 95. Li, Yan
    et al.
    Huyan, Fei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ding, Wei
    Microstructure and tensile properties of a 0.20C-4.86Mn steel after short intercritical-annealing times2019In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 35, no 2, p. 220-230Article in journal (Refereed)
    Abstract [en]

    This work studies the microstructure and tensile properties of a cold-rolled Fe-0.20C-4.86Mn (mass %) steel after short intercritical annealing (IA) times using scanning and transmission electron microscopy, and uniaxial tensile tests. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The experimental results show that IA temperature has a strong influence on the final microstructure and tensile properties while IA time has less. The fractions of retained austenite are much higher after IA at 650 and 675 degrees C than the other IA temperatures, and thus improving elongation. Simulations using the DICTRA software and constitutive modelling are further performed to assist the understanding of the microstructure evolution and stress-strain curves.

  • 96. Li, Yan
    et al.
    Huyan, Fei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Yan, Jiayi
    Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Ding, Wei
    Bayan Obo multi-metallic resource comprehensive utilization Key lab, Inner Mongolia University of Science and Technology.
    Influence of Short Intercritical Annealing Times on the Microstructure and Tensile Properties of a Cold-rolled Medium Mn SteelManuscript (preprint) (Other academic)
    Abstract [en]

    This work studies the microstructure and tensile properties of a cold-rolled Fe–0.204C–4.86Mn (mass %) steel after short intercritical annealing (IA) times, i.e. 3 and 10 min. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The microstructure evolution is studied using scanning and transmission electron microscopy, and the tensile properties are obtained using uniaxial tensile tests. The experimental results show that IA temperature (600–700 °C) has strong, while IA time has less, influence on the final microstructure and tensile properties. The fractions of retained austenite are much higher after IA at 650 and 675 °C (~ 10 vol. %) than the other IA temperatures, and thus result in improved elongation (~ 20–30 %). Simulations using the DICTRA software and constitutive modeling are further performed to assist the understanding of the microstructure evolution and stress-strain curves.

  • 97.
    Li, Zhou
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Mao, Huahai
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Selleby, Malin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Thermodynamic Modeling of Pure Co Accounting Two Magnetic States for the Fcc Phase2018In: JOURNAL OF PHASE EQUILIBRIA AND DIFFUSION, ISSN 1547-7037, Vol. 39, no 5, p. 502-509Article in journal (Refereed)
    Abstract [en]

    The thermodynamic functions of the pure Co were assessed using CALPAHD method for the third generation thermodynamic databases. To model the magnetic properties of the cobalt, a two-state magnetic model was accounted for the fcc phase. Calculated results were compared with the experimental information and a good fit to the experimental data was achieved.

  • 98.
    Lin, Sen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Deformation-Induced Martensitic Transformation and Mechanical Properties of Duplex and Austenitic Stainless Steels: A Synchrotron X-Ray Diffraction Study2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Metastable austenitic and duplex stainless steels are widely used materials in industrial anddomestic applications, owing to their attractive characteristics such as good corrosion resistanceand favorable mechanical properties. Both types of steel experience enhanced mechanicalproperties during plastic deformation due to the formation of the martensite phase from theparent austenite phase, this is called deformation-induced martensitic transformation (DIMT).It is therefore of technical interest to study the transformation mechanism and its impact onmechanical properties for a better understanding and ultimately for developing new materialswith improved performance in certain applications.

    In the present thesis, two austenitic stainless steels (201Cu, HyTens® 301) and two duplexstainless steels (FDX25®, FDX27®) were investigated. Samples were tensile tested during insitusynchrotron radiation experiments performed at the Cornell High Energy SynchrotronSource (CHESS), Ithaca, USA. Tests were performed at both room temperature and at elevatedtemperatures. The collected diffraction data were then processed by software such as Fit2D andMATLAB. Quantitative phase fraction analysis based on the direct comparison method wasperformed successfully. Microstructural analysis of samples before deformation and after thefull tensile testing was also performed using electron microscopy.

    The deformation induced martensitic transformation took place in HyTens 301, FDX25 andFDX27, but in 201Cu the austenite was stable during the tensile tests conducted here. The a’-martensite formed in a significantly higher fraction than the ε-martensite in all alloys. At roomtemperature, the critical stress levels for martensitic transformation were 490 MPa, 700 MPaand 700MPa for HyTens 301, FDX25 and FDX27, respectively.

  • 99.
    Linder, David
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Walbrühl, Martin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Yan, Jia-Yi
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    An energy release rate approach to cemented carbide fracture toughness for computational materials designManuscript (preprint) (Other academic)
    Abstract [en]

    Integrated computational materials engineering and computational materials design have the potential to greatly accelerate materials development at reduced cost compared to conventional experimentally-based methods. These methodologies, however, require physically-based property models to be truly predictive. Fracture toughness is a critical material property of cemented carbides for high-performance mining and metal cutting tools. In the present work, a fracture toughness model framework based on the energy release rate formalism is presented and applied to conventional and alternative-binder cemented carbides. The framework is physically-based and designed to be modular, where each sub-model can be independently modified or replaced without disturbing the calculation-flow of the overall framework. In the presented examples, the sub-models are based on e.g. finite element simulations and atomistic calculations as well as limited calibration to experimental data. The model framework is intended for integration with previously developed computational tools and models, such as a composite hardness model and a grain growth model, for computational design of novel and improved cemented carbides with the aim to potentially substitute cobalt as the dominating binder phase in cemented carbides.

  • 100.
    Linder, David
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Yan, Jia-Yi
    Walbrühl, Martin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Modeling confined ductile fracture – a void-growth and coalescence approachManuscript (preprint) (Other academic)
    Abstract [en]

    In a composite material a soft, ductile matrix can be confined by a hard, brittle phase, altering its deformation and fracture behavior. Increasing confinement leads to embrittlement of the matrix and, in turn, also the composite. From a materials design perspective, it is usually desired to avoid brittle fracture without compromising the hardness of the material. Understanding confined ductile fracture is therefore critical for modeling the mechanical response of composite materials with fine microstructure. The present work is focused on confined ductile fracture of a thin ductile film, with elasto-plastic power-law hardening behavior, sandwiched between ideal linear elastic substrates. Fracture of the ductile layer is modeled by growth and coalescence of prescribed voids in 2D. Influences of material properties, initial void volume fraction, geometric constraints and elastic mismatch are investigated. The results show a loss of ductility with decreasing film thickness that is accompanied by a severe decrease in fracture initiation toughness as well as an increased stress at the interface. The influence of materials properties is significant in all cases while the effect of initial void volume fraction is comparatively less critical for highly confined materials than for bulk materials. Increasing confinement also results in increasing normal stress at the phase interface, promoting interface decohesion prior to ductile fracture of the film. The present approach and results are a step towards more detailed prediction of composite fracture toughness and crack-growth resistance.

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