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  • 1. Abu-Odeh, A.
    et al.
    Galvan, E.
    Kirk, T.
    Mao, Huahai
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Chen, Q.
    Mason, P.
    Malak, R.
    Arróyave, R.
    Efficient exploration of the High Entropy Alloy composition-phase space2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 152, p. 41-57Article in journal (Refereed)
    Abstract [en]

    High Entropy Alloys (HEAs), Multi-principal Component Alloys (MCA), or Compositionally Complex Alloys (CCAs) are alloys that contain multiple principal alloying elements. While many HEAs have been shown to have unique properties, their discovery has been largely done through costly and time-consuming trial-and-error approaches, with only an infinitesimally small fraction of the entire possible composition space having been explored. In this work, the exploration of the HEA composition space is framed as a Continuous Constraint Satisfaction Problem (CCSP) and solved using a novel Constraint Satisfaction Algorithm (CSA) for the rapid and robust exploration of alloy thermodynamic spaces. The algorithm is used to discover regions in the HEA Composition-Temperature space that satisfy desired phase constitution requirements. The algorithm is demonstrated against a new (TCHEA1) CALPHAD HEA thermodynamic database. The database is first validated by comparing phase stability predictions against experiments and then the CSA is deployed and tested against design tasks consisting of identifying not only single phase solid solution regions in ternary, quaternary and quinary composition spaces but also the identification of regions that are likely to yield precipitation-strengthened HEAs.

  • 2.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Stalmans, Rudy
    Ågren, John
    KTH, Superseded Departments, Materials Science and Engineering.
    Unified thermodynamic analysis of the stress-assisted γ→ε martensitic transformation in Fe–Mn–Si alloys1998In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 46, no 11, p. 3883-3891Article in journal (Refereed)
    Abstract [en]

    A unified thermodynamic analysis has been developed which allows the combined complex effects of applied stresses and alloy composition on the martensite start temperature, Ms(σ), to be explained and predicted. In this unified analysis, a thermodynamic analysis of the effect of applied stress on the relative stability of the high and low temperature phase is combined with existing thermodynamic descriptions of the alloy system. The calculated results are compared with experimental results for the γ martensitic transformation in Fe–Mn–Si shape memory alloys. For single crystals, the comparison shows that the influence of applied stresses can be directly predicted with a reasonable accuracy for a wide composition range. By integrating a polycrystallinity parameter in the analysis it is shown that the calculations are also valid for polycrystalline alloys. Application of this analysis to experimental values of Ms(σ) can also generate useful data for future optimizations of the thermodynamic description of several alloy systems.

  • 3.
    Asp Grönhagen, Klara
    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.
    Phase-field simulation of sintering and related phenomena: A vacancy diffusion approach2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, p. 1241-1248Article in journal (Refereed)
    Abstract [en]

    A phase-field model of sintering and related phenomena in a two-phase system and in a multi-phase system is presented. We consider diffusion of vacancies as the atomic mechanism for redistribution of material and we will use the familiar model of thermal vacancies in a crystal as our energy formulation. The solid material will thus be characterized by a low vacancy content and the surroundings by a very high vacancy content and a very low content of atoms. The surface of the solid body will be characterized by a continuous variation in vacancy content. The temporal development of particles during solid state sintering with effects such as wetting is shown in various simulations.

  • 4.
    Bonvalet, Manon
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sauvage, Xavier
    Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, 76000, Rouen, France.
    Blavette, Didier
    Normandie Univ, UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, 76000, Rouen, France.
    Intragranular nucleation of tetrahedral precipitates and discontinuous precipitation in Cu-5wt%Ag2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 164, p. 454-463Article in journal (Refereed)
  • 5.
    Bonvalet-Rolland, Manon
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH Royal Inst Technol, Dept Mat Sci & Engn, Stockholm, Sweden..
    Philippe, Thomas
    CNRS, Ecole Polytech, Phys Matiere Condensee, F-91120 Palaiseau, France..
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kinetic theory of nucleation in multicomponent systems: An application of the thermodynamic extremum principle2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 171, p. 1-7Article in journal (Refereed)
    Abstract [en]

    Nucleation kinetics in a multicomponent supersaturated solid solution is examined. Attachment rate of atoms to a nucleus of a size close to the critical one is determined combining a thermodynamic extremum principle and the Fokker-Planck equation. Two limiting cases are examined; when bulk diffusion controls the nucleation kinetics and when the process is limited by the interfacial mobility. The mixed regime is also treated. Moreover, the growth law in multicomponent alloys is derived in the general case, when both mechanisms are considered. Additionally, the attachment rate is derived, in the classical framework, from a new macroscopic growth equations and the fundamental role of the interfacial mobility is examined. These new general expressions, for the attachment rates and the growth laws, determined either applying the thermodynamic extremum principle or derived from the classical formalism are found to be consistent.

  • 6.
    Borgenstam, Annika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Edmonds, D.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Eutectoids with cementite as the major constituent in Fe-C-M alloys2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 103, p. 280-289Article in journal (Refereed)
    Abstract [en]

    The addition of a third element to hypereutectoid Fe-C alloys may cause inclusions of a minor constituent in the precipitate of cementite from austenite even if the initial austenite is supersaturated only with cementite. Cementite will often become the major constituent of this kind of microstructure. For Fe-C-Cu alloys this has been explained as the result of precipitation from supersaturated cementite. An alternative mechanism could be that the mixture of cementite and a minor constituent forms by simultaneous and cooperative growth of the two phases, i.e., by a reaction that may be regarded as eutectoid. This mechanism has already been applied to explain the occurrence of eutectoid colonies with cementite as the major constituent and a minor constituent for which there was no supersaturation initially. This phenomenon has been observed in hypereutectoid ternary Fe-C alloys with Al, Mn or Si. The necessary requirements on the ordinary isothermal phase diagram are now examined with a graphical method based on the slopes of tie-lines. It predicts the phenomenon in all cases where it has been observed, including Fe-C-Cu and not in the Fe-C-Ni and Fe-C-Cr systems where it has not been observed. The requirements become more evident when the calculated phase equilibria are plotted as an isothermal phase diagram with the alloy content as a function of the carbon activity instead of carbon content. Finally, a comparison is made with bainite in Fe-C alloys where ferrite is the major and cementite the minor constituent. The same two explanations have been proposed for that case.

  • 7.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Hillert, M.
    Massive transformation in the Fe-Ni system2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 11, p. 2765-2775Article in journal (Refereed)
    Abstract [en]

    The critical limit for the massive gamma-->alpha transformation in the Fe-Ni system has been measured by isothermal heat treatment of diffusion couples. The position of the alpha/gamma interface at the end of the treatment could be identified but some growth occurred during the quench. Growth is probably hindered at the beginning of the quench by a redistribution of Fe and Ni at the interface during the isothermal treatment. During the quench it often happens that the alpha/gamma interfaces develop jagged shapes and even very fine plates although there should usually be no orientation relationship. The critical limit agrees fairly well with classical results for the formation of equiaxed ferrite from continuous cooling of homogeneous specimens. At 1023 K it coincides with the alpha/alpha + gamma phase boundary. At lower temperatures it moves inside the alpha + gamma two-phase field. The driving force for the massive transformation increases at decreasing temperature and the limiting composition never approaches the T-0 line.

  • 8.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Hillert, M.
    Nucleation of isothermal martensite2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 11, p. 2777-2785Article in journal (Refereed)
    Abstract [en]

    Two methods of evaluating an activation energy for the nucleation of martensite from kinetic information on isothermal martensite are discussed. In both methods the nucleation is described as thermally activated growth of an embryo up to a size where rapid growth can get started. The first method is based on the absolute Value of the rate of reaction at each separate temperature and depends on the neglect of the reverse reaction. The second method is based on the temperature dependence of the rate at low temperatures and includes the reverse reaction and it typically yields a lower activation energy. It can be used to define the region of low net driving force where the reverse reaction, which is neglected in the first method, is important and that method should not be used. A unified description of the nucleation of athermal and isothermal martensite is also presented.

  • 9.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Activation energy for isothermal martensite in ferrous alloys1997In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 45, no 2, p. 651-662Article in journal (Refereed)
    Abstract [en]

    The experimental information on isothermal α martensite in ferrous alloys is reviewed. From the kinetics one can clearly distinguish between three groups of alloys yielding isothermal martensite. The first group contains high alloy steels with a low Ms temperature. They form isothermal martensite with a temperature dependence corresponding to a very low activation energy, possibly 7 kJ/mol. The second group contains steels with an appreciable amount of carbon. Its rate of formation of isothermal martensite can be explained by assuming that it is triggered by submicroscopic plates of bainite formed with a rate controlled by carbon diffusion. The third group contains Fe---Ni alloys with up to about 25% Ni. There the temperature dependence corresponds to an activation energy of about 80 kJ/mol. It is proposed that their transformation is related to the transformation causing plateau II in experiments with very rapid cooling, a transformation which has previously been proposed to be related to the formation of bainite.

  • 10.
    Borgenstam, Annika
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Driving force for f.c.c.→b.c.c. martensites in Fe-X alloys1997In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 45, no 5, p. 2079-2091Article in journal (Refereed)
    Abstract [en]

    Information on Ms, the starting temperature for formation of martensite, is reviewed and one Ms line each for lath and plate martensite are drawn in a number of Fe-X phase diagrams. A reasonable interpretation of the data indicates the possibility that the distance between the two lines may vary linearly with temperature and be independent of the choice of alloying element. Using thermodynamic descriptions of the binary systems, the driving force for the start of the formation of the two kinds of martensite is calculated from the same interpretation of data. When plotted against temperature the results indicate that the driving force for martensite may not be much affected by solution hardening but may mainly be a function of temperature. For plate martensite it may have a fairly constant value of about 2100 J/mol. For lath martensite it may vary linearly, possibly from 500 J/mol at 800°C to 2100 J/mol at 250°C.

  • 11.
    Borgenstam, Annika
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hillert, Mats
    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.
    Metallographic evidence of carbon diffusion in the growth of bainite2009In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 57, no 11, p. 3242-3252Article in journal (Refereed)
    Abstract [en]

    There are two paradigms regarding the formation of bainite. One is based on the first stage being rapid, diffusionless growth or acicular ferrite and the subsequent formation of carbide occurring by precipitation from the supersaturated ferrite. All assumption that the first stage occurs as a series of subsequent rapid steps resulting in sub-units plays an important role as an explanation of the not so rapid growth observed macroscopically. The other paradigm is based on the first stage being the formation of acicular ferrite under carbon diffusion and on the subsequent growth of carbide and ferrite side by side. Metallographic observations are presented that support the second paradigm. It is difficult to see how they can be accounted for by the first paradigm, in particular the observation of the shapes of sub-units.

  • 12.
    Borgh, Ida
    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.
    Blomqvist, Andreas
    Ågren, John
    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.
    Synthesis and phase separation of (Ti,Zr)C2014In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 66, p. 209-218Article in journal (Refereed)
    Abstract [en]

    Synthesis and phase separation of (Ti,Zr)C were investigated in the present work. The (Ti,Zr)C phase was synthesized at 2200 C and subsequently aged at 1300 C for different times. The microstructure was investigated using X-ray diffraction and electron microscopy, and supplemented by first-principles calculations. The (Ti,Zr)C phase separates into a lamellar nanostructure with alternating Ti- and Zr-rich face-centered cubic domains as well as non-stoichiometric TiC and ZrC. The lamellar structure is a consequence of phase separation within the miscibility gap that is directionally constrained by high coherency stresses, as indicated by the first-principles calculations. Moreover, the increased hardness due to the phase separation suggests that the mixed carbide could be used as a strengthening constituent in, for example, cemented carbides.

  • 13.
    Borgh, Ida
    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.
    Odqvist, Joakim
    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.
    Gholinia, Ali
    Winiarski, Bartlomiej
    Withers, Philip J.
    Thompson, George E.
    Mingard, Ken
    Gee, Mark G.
    On the three-dimensional structure of WC grains in cemented carbides2013In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 13, p. 4726-4733Article in journal (Refereed)
    Abstract [en]

    In the present work, the size distribution and shape of WC grains in cemented carbides (WC-Co), with different Co contents, have been investigated in three dimensions. Direct three-dimensional (3-D) measurements, using focused ion beam serial sectioning and electron backscattered diffraction (EBSD), were performed and a 3-D microstructure was reconstructed. These measurements were supplemented by two-dimensional (2-D) EBSD and scanning electron microscopy on extracted WC grains. The data from 2-D EBSD collected on planar sections were transformed to three dimensions using a recently developed statistical method based on an iterative inverse Saltykov procedure. This stereological analysis revealed that the assumed spherical shape of WC grains during the Saltykov method is reasonable and the estimated 3-D size distribution is qualitatively in good agreement with the actual distribution measured from 3-D EBSD. Although the spherical assumption is generally fair, the WC grains have both faceted and rounded surfaces. This is a consequence of the relatively low amount of liquid phase during sintering, which makes impingements significant. Furthermore, the observed terraced surface structure of some WC grains suggests that 2-D nucleation is the chief coarsening mechanism to consider.

  • 14. Carlsson, S.
    et al.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    On the determination of residual stress and strain fields by sharp indentation testing. Part I: Theoretical and numerical analysis2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 12, p. 2179-2191Article in journal (Refereed)
    Abstract [en]

    Sharp indentation tests, presently represented by cone and Vickers indenters, are analysed theoretically and numerically in order to explore how equi-biaxial residual stress and strain fields can be determined from the global properties, i.e. the size of the contact area between indenter and material and the hardness. given by such tests. It is shown that the residual strain fields can be accurately correlated with the hardness value while residual stresses are related to the size of the contact area. The latter feature is explained by the Fact that the size of the: contact area is sensitive to elastic effects. The results are summarized in simple closed form relations. well-suited to be used in an experimental situation, and the range of validity for the resulting formulae is discussed. The predictions are compared with corresponding results taken from the literature and good agreement is found. An experimental scheme for determination of residual fields by indentation is also suggested.

  • 15. Carlsson, S.
    et al.
    Larsson, Per-Lennart
    KTH, Superseded Departments, Solid Mechanics.
    On the determination of residual stress and strain fields by sharp indentation testing. Part II: Experimental investigation2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 12, p. 2193-2203Article in journal (Refereed)
    Abstract [en]

    An experimental investigation has been carried out in order to study how residual stress and strain fields can be determined by sharp indentation testing. Aiming at a thorough understanding of the influence from general residual surface stress: and strain fields on global indentation parameters, i.e. hardness and contact area, Vickers indentation tests have been performed on specimens first exposed to standard four-point bend (4PB) or single edge notch bend (SEN(B)) loading. The experimental results have been evaluated based on the findings in a parallel theoretical/numerical investigation and are compared with finite element simulations as well as with corresponding results taken from the literature. Good agreement between experiments and numerical results has been found, particularly in a situation with no or little plastic deformation due to preloading.

  • 16.
    Chandrasekaran, Dilip
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Nygårds, Mikael
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A study of the surface deformation behaviour at grain boundaries in an ultra-low-carbon steel2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 18, p. 5375-5384Article in journal (Refereed)
  • 17. Chen, Hao
    et al.
    Borgenstam, Annika
    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.
    Zuazo, Ian
    Goune, Mohamed
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    van der Zwaag, Sybrand
    Application of interrupted cooling experiments to study the mechanism of bainitic ferrite formation in steels2013In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 12, p. 4512-4523Article in journal (Refereed)
    Abstract [en]

    New interrupted cooling experiments have been designed to study the kinetics of bainitic ferrite formation starting from a mixture of austenite and bainitic ferrite. It is found that the kinetics of bainitic ferrite formation during the cooling stage is determined by the isothermal holding time. The formation rate of bainitic ferrite at the beginning of the cooling decreases with increasing prior isothermal holding time. An unexpected stagnant stage during the cooling stage appears when the isothermal holding time increases to a critical point. There are two reasons for the occurrence of the stagnant stage: (i) a solute spike in front of the interface; and (ii) kinetic transition. A so-called Gibbs energy balance approach, in which the dissipation of Gibbs energy due to diffusion inside the interface and interface friction is assumed to be equal to the available chemical driving force, is applied to theoretically explain the stagnant stage. A kinetics transition from a fast growth mode without diffusion of Mn and Si inside the austenite-bainitic ferrite interfaces to a slow growth mode with diffusion inside the interface is predicted. The stagnant stage is caused by the transition to a slow growth mode. The Gibbs energy balance approach describes the experimental observations very well.

  • 18.
    Chen, K. X.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Univ Sci & Technol Beijing, Sch Mat Sci & Engn, Beijing 100083, Peoples R China..
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Natl Univ Sci & Technol MISiS, Mat Modelling & Dev Lab, Moscow 119049, Russia..
    Demange, G.
    Univ Rouen Normandy, GPM, CNRS UMR6634, F-76801 St Etienne Du Rouvray, France..
    Zapolsky, H.
    Univ Rouen Normandy, GPM, CNRS UMR6634, F-76801 St Etienne Du Rouvray, France..
    Patte, R.
    Univ Rouen Normandy, GPM, CNRS UMR6634, F-76801 St Etienne Du Rouvray, France..
    Boisse, J.
    Lorraine Univ, LEMTA, CNRS UMR 7563, F-54500 Vandoeuvre Les Nancy, France..
    Wang, Z. D.
    Univ Sci & Technol Beijing, Sch Mat Sci & Engn, Beijing 100083, Peoples R China..
    Morphological instability of iron-rich precipitates in Cu-Fe-Co alloys2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 163, p. 55-67Article in journal (Refereed)
    Abstract [en]

    The mechanical properties of metallic materials are determined by their microstructure, and in particular, the different morphologies of precipitates lead to distinct strengthening effects. Usually, the shape of precipitates changes during growth and coarsening regimes, leading to modification of the macroscopic properties of the materials. Thus, understanding of this phenomenon is key to tailoring the precipitate strengthening of industrial alloys. In this article, a general approach to explain the shape instability of iron-rich nanoparticles in Cu-Fe-Co alloys during casting and ageing processes is proposed. The evolution of particle shape from sphere to cuboid to petal and finally splitting into eight subnanoparticles is observed using transmission electron microscopy. Phase-field modelling and thermodynamic calculations are combined into a general model that describes and elucidates the morphological evolution of precipitates in alloys in terms of particle size, interfacial and elastic strain energy, and chemical driving force. These findings have the potential to promote new microstructural design approaches for a wide range of materials.

  • 19. Chen, Q.
    et al.
    Jeppsson, Johan
    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.
    Analytical treatment of diffusion during precipitate growth in multicomponent systems2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 8, p. 1890-1896Article in journal (Refereed)
    Abstract [en]

    We propose an approximate growth rate equation that takes into account both cross-diffusion and high supersaturations for modeling precipitation in multicomponent systems. We then apply it to an Fe-alloy in which interstitial C atoms diffuse much faster than substitutional solutes, and predict a spontaneous transition from slow growth under ortho-equilibrium to fast growth under the non-partitioning local equilibrium condition. The transition is caused by the decrease in the Gibbs-Thomson effect as the growing particle becomes larger. The results agree with DICTRA simulations where full diffusion fields are calculated.

  • 20. Chen, Q.
    et al.
    Sundman, Bo
    KTH, Superseded Departments, Materials Science and Engineering.
    Calculation of Debye temperature for crystalline structures - A case study on Ti, Zr, and Hf2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 6, p. 947-961Article in journal (Refereed)
    Abstract [en]

    The methods to calculate the Debye temperature from elastic moduli have been reviewed. The approximation approach due to Moruzzi e al was critically examined by considering experimental elastic constant data for all the cubic elements. It was found that many cubic elements are exceptions with regard to the assumed constant scaling factor for the expression of the average sound velocity in terms of the bulk modulus, and consequently the Debye temperature of a cubic element must be calculated from the knowledge of all the elastic constants of the system. On the other hand, a fairly constant scaling factor has been found to exist fur the hexagonal elements. Through the study of experimental data, some empirical relationships have been observed between the high temperature entropy-Debye temperature theta (D)(0) and the low temperature limit of the Debye temperature theta (D)(-3). For those structures that are dynamically unstable at low temperatures, we proposed a way to obtain their theta (D)(0) from the calculated isotropic bulk moduli. The methods have been applied to calculate the Debye temperatures of hcp, bcc, and fee Ti, Zr, and Hf from their elastic moduli derived from ah initio calculations. The calculated results agree very well with the experimental data.

  • 21. Ekroth, M.
    et al.
    Frykholm, R.
    Lindholm, M.
    Andren, H. O.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Gradient zones in WC-Ti(C,N)-Co-based cemented carbides: Experimental study and computer simulations2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 9, p. 2177-2185Article in journal (Refereed)
    Abstract [en]

    The formation of surface zones with a composition gradient during sintering of WC-Ti(C,N)-Co cemented carbides has been studied experimentally and by computer simulations. The microstructure has been investigated with SEM and EPMA. The simulations are based on a solution of the multicomponent diffusion equations, coupled with thermodynamic calculations using thermodynamic descriptions of the individual phases. The results from the simulations are in good agreement with the experimental results, indicating that diffusion and the thermodynamic properties are the two major factors that control the gradient structure formation.

  • 22. Erneman, J.
    et al.
    Schwind, M.
    Andren, H. O.
    Nilsson, J. O.
    Wilson, A.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    The evolution of primary and secondary niobium carbonitrides in AISI 347 stainless steel during manufacturing and long-term ageing2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 1, p. 67-76Article in journal (Refereed)
    Abstract [en]

    Nb(C,N) precipitates were studied in a niobium-stabilised stainless steel (AISI 347) statically aged at 700 degrees C. Scanning electron microscopy and energy filtered transmission electron microscopy were used to determine the volume fraction and precipitate size of primary and secondary Nb(C,N) after ageing times between 0 and 70,000 h. The experimental data were correlated with simulations of Nb(C,N) formation based on the assumption that the process is controlled by diffusion. These simulations provide a rationale for the existence of two sets of mobium carbonitrides in commercial tubes of AISI 347. Growth of primary Nb(C,N) occurred essentially during manufacturing, with no significant growth at 700 degrees C. Rapid dissolution and re-precipitation of secondary Nb(C,N) occurred during manufacturing. Coarsening at 700 degrees C of secondary particles was modelled using the Lifshitz-Slyozov-Wagner theory, which overestimated the coarsening rate. These problems were overcome with a model developed by the authors. This model takes both growth and coarsening into account.

  • 23. Erneman, J.
    et al.
    Schwind, Martin
    KTH, Superseded Departments, Materials Science and Engineering.
    Liu, P.
    Nilsson, J. O.
    Andren, H. O.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Precipitation reactions caused by nitrogen uptake during service at high temperatures of a niobium stabilised austenitic steel2004In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 52, no 14, p. 4337-4350Article in journal (Refereed)
    Abstract [en]

    Precipitation phenomena in type 347 austenitic stainless steels have been investigated after long-term heat treatment and creep in air at 700 and 800 degreesC. Nitrogen uptake was observed during long-term creep testing at 800 degreesC. No such effect was observed at 700 degreesC although times up to about 70,000 h were used. The major phases precipitated after long time exposure at 800 degreesC were primary Nb(C,N), Z-phase, Cr2N and M23C6, while primary Nb(C,N), secondary Nb(C,N) and sigma-phase were the major phases at 700 degreesC. Z-phase precipitated in both intragranular and intergranular form at 800 degreesC. Large precipitates exhibiting a core/rim structure showed a rim of Z-phase surrounding undissolved primary Nb(C,N). The microstructural evolution during creep deformation in air at 800 degreesC was modelled thermodynamically. The model satisfactorily predicts nitrogen uptake and the essential features of the evolution of the microstructure with time. The precipitation sequence could be qualitatively described, although it was not possible to model the formation of all precipitates.

  • 24.
    Francis, E.
    et al.
    Univ Manchester, Sch Mat, MSS Tower, Manchester M1 3BB, Lancs, England..
    Babu, Prasath
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Harte, A.
    Univ Manchester, Sch Mat, MSS Tower, Manchester M1 3BB, Lancs, England..
    Martin, T. L.
    Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England.;Univ Bristol, Tyndall Ave, Bristol, Avon, England..
    Frankel, R.
    Univ Manchester, Sch Mat, MSS Tower, Manchester M1 3BB, Lancs, England..
    Jadernas, D.
    Studsvik Nucl AB, Nykoping, Sweden.;Idaho Natl Lab, POB 1625, Idaho Falls, ID USA..
    Romero, J.
    Westinghouse Elect Sweden AB, Vasteras, Sweden..
    Halistadius, L.
    Westinghouse Elect Co, Columbia, SC USA..
    Bagot, P. A. J.
    Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England..
    Moody, M. P.
    Univ Oxford, Dept Mat, Parks Rd, Oxford OX1 3PH, England..
    Preuss, M.
    Univ Manchester, Sch Mat, MSS Tower, Manchester M1 3BB, Lancs, England..
    Effect of Nb and Fe on damage evolution in a Zr-alloy during proton and neutron irradiation2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 165, p. 603-614Article in journal (Refereed)
    Abstract [en]

    Detailed analysis was carried out on proton and a neutron irradiated Nb-containing Zr-alloy to study the evolution of dislocation loop size and densities as well as the formation and evolution of irradiation induced precipitation/clustering. The results obtained here have been contrasted against previously published work on a Nb-free Zr-alloy [1, 21 to investigate the mechanistic reason for the improved resistance to irradiation-induced growth of Nb-containing Zr alloys. The combined use of bright field scanning transmission electron microscopy, ultra-high-resolution energy dispersive spectroscopy and atom probe tomography analysis provides evidence of evenly distributed radiation-induced Nb dusters that have formed during the early stage of proton irradiation and Fe-rich nano-rods near Fe-containing second phase particles. The former seems to have a profound effect on <a> loop and subsequent <c> loop formation, keeping <a> loop size small but number density high while <c> loops seem to initially form at similar dose levels compared to a Nb-free alloy but <c> loop line density does not increase during further irradiation. It is hypothesized that the formation of the Nb nano-precipitates/clusters significantly hinders mobility and growth of <a> loops, resulting in a small size, high number density and limited ability of <a> loops to arrange along basal traces compared to Nb-free Zr-alloys. It is suggested that it is the limited <a> loop arrangement that slows down <c> loop formation and the root cause for the high resistance of Nb-containing Zr-alloys to irradiation-induced growth. 

  • 25. Frykholm, R.
    et al.
    Ekroth, M.
    Jansson, B.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Andren, H. O.
    A new labyrinth factor for modelling the effect of binder volume fraction on gradient sintering of cemented carbides2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 4, p. 1115-1121Article in journal (Refereed)
    Abstract [en]

    To increase the cutting performance of WC-MC-Co cemented carbide tools, it is common to use a high temperature CVD process to coat them with thin wear resistant layers. During the process cracks are unavoidably introduced in the coating. To prevent crack propagation it is of interest to create a tough surface zone in the substrate, enriched in WC and binder phase. A way to create such a zone is to sinter a nitrogen-containing cemented carbide in a nitrogen free atmosphere. This formation of gradient structures has been extensively studied using microscopy and simulations, and it has been shown that the process is driven by diffusion in the binder phase. However, the diffusion paths are partly blocked by the dispersed particles. This effect can be formally handled by considering effective diffusivities by introducing a so-called labyrinth factor, lambda. In prior work it has been assumed that lambda =f(2) where f is the volume fraction of the binder. The validity of this assumption has been studied by simulations and experimental analysis of gradient sintered WC-Ti(C,N)-Co cemented carbides containing 5.0, 6.7, 10.0 and 20.0 vol% binder phase. It was found that by using the labyrinth factor lambda = f(2) instead of a better correspondence between experiments and simulations can be achieved.

  • 26. Gebhardt, Thomas
    et al.
    Music, Denis
    Kossmann, Daniel
    Ekholm, Marcus
    Abrikosov, Igor A.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schneider, Jochen M.
    Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 8, p. 3145-3155Article in journal (Refereed)
    Abstract [en]

    We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Young's modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Young's modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Young's modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Young's modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.

  • 27. Gong, Y.
    et al.
    Young, D. J.
    Kontis, P.
    Chiu, Y. L.
    Larsson, H.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Thermo-Calc Software AB, Råsundavägen 18, SE-169 67 Solna, Sweden.
    Shin, A.
    Pearson, J. M.
    Moody, M. P.
    Reed, R. C.
    On the breakaway oxidation of Fe9Cr1Mo steel in high pressure CO22017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 130, p. 361-374Article in journal (Refereed)
    Abstract [en]

    Fundamental insights into the mechanism of breakaway oxidation in Fe9Cr1Mo steel are deduced, through advanced characterisation and modelling. Degradation at 600 °C/∼42 bar CO2 for ∼20,000 h is emphasised: conditions relevant to components such as the finned superheater tubes used for advanced gas-cooled nuclear reactors. It is shown that such conditions are sufficient to cause carbon saturation of the metallic substrate, as confirmed by direct observation of extensive carbide precipitation but also numerical analysis of the carbon transport. Thus the observation of graphite precipitation close to the scale/metal interface is rationalised. Nonetheless, the activity of carbon at the scale/metal interface does not reach unity – with respect to graphite – at time zero. A modelling method is proposed which accounts for this kinetic retardation of the attack; this can be used to interpolate across the regimes within which breakaway oxidation is prevalent. It is a plausible model for extrapolation to the lower temperatures relevant to service conditions and is suitable for lifetime estimation – so-called ‘remnant life analysis’ – of such safety-critical components when prone to this form of attack.

  • 28.
    Gorbatov, O. I.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Lulea Univ Technol, Appl Phys, Div Mat Sci, Dept Engn Sci & Math, S-97187 Lulea, Sweden.; Nosov Magnitogorsk State Tech Univ, Lab Mech Gradient Nanomat, Magnitogorsk 455000, Russia..
    Stroev, A. Yu
    Natl Res Ctr Kurchatov Inst, Moscow 123182, Russia.;Moscow Inst Phys & Technol, Dolgoprudnyi 141700, Moscow Region, Russia..
    Gornostyrev, Yu. N.
    RAS, Ural Div, Inst Met Phys, Ekaterinburg 620219, Russia.;Inst Quantum Mat Sci, Ekaterinburg 620107, Russia..
    Korzhavyi, P. A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. RAS, Ural Div, Inst Met Phys, Ekaterinburg 620219, Russia..
    Effective cluster interactions and pre-precipitate morphology in binary Al-based alloys2019In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 179, p. 70-84Article in journal (Refereed)
    Abstract [en]

    The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al-Cu, Al-Mg, Al-Zn, and Al-Si alloys, we carry out cluster expansions of ab initio calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems. reserved.

  • 29.
    Grönhagen, Klara
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, Johan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Grain-boundary segregation and dynamic solute drag theory: A phase-field approach2007In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 55, p. 955-960Article in journal (Refereed)
    Abstract [en]

    We propose a model based on a phase-field approach to study grain-boundary segregation and solute drag. We will show that it is possible to model the dynamics of grain-boundary segregation to a stationary boundary as well as solute drag on a moving boundary with the same phase-field model. We shall achieve this by introducing a concentration dependency in the height of the double-well potential in the Gibbs-energy expression. As the model then will be able to treat the build-up of a concentration spike in the boundary as well as its disappearance we shall term this treatment dynamic solute-drag theory.

  • 30.
    Göthelid, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Haglund, S.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Influence of O and Co on the early stages of sintering of Wc-Co: A surface study by AES and STM2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 17, p. 4357-4362Article in journal (Refereed)
    Abstract [en]

    The influence of oxygen on the sintering behavior of WC-Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650 degreesC results in a 2x2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM rip. Annealing at 850 degreesC removes excess Co and leaves only the 2x2 surface. Oxygen exposure of the 2x2 surface results in a clustered cobalt oxide overlayer which on annealing at 750 degreesC breaks up and restores the 2x2 structure as the metallic Co wets the surface.

  • 31.
    Hallström, Samuel
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Andersson, D.
    Ruban, A.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Thermodynamic reassessment of the Ni-Ru system and assessment of the Al-Ni-Ru system at 1273-1523 K using ab initio calculations2008In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 15, p. 4062-4069Article in journal (Refereed)
    Abstract [en]

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

  • 32.
    Hallström, Samuel
    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.
    Modeling of iron diffusion in the iron oxides magnetite and hematite with variable stoichiometry2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 1, p. 53-60Article in journal (Refereed)
    Abstract [en]

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

  • 33. Hedström, Peter
    et al.
    Han, Tong-Seok
    Lienert, Ulrich
    Almer, Jonathan
    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 x 10(-3), corresponding to a stress of similar to 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.

  • 34. Hillert, M.
    et al.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Effect of surface free energy and surface stress on phase equilibria2002In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 50, no 9, p. 2429-2441Article in journal (Refereed)
    Abstract [en]

    Due to a call for modification of the graphical method of describing phase equilibria for systems with a second-phase inclusion using a common tangent, as previously presented, the thermodynamic basis for such diagrams has now been examined in detail but only for isotropic surface proper-ties. For an incompressible second phase with composition independent surface properties there seems to be no need of modification. For compressible liquids and solids under constant pressure the common tangent construction can still be used but the evaluation of how much the Gibbs energy curve for the second phase should be lifted is more complicated. For spherical inclusions of compressible solids there will be an additional factor containing the compressibility. The equilibrium condition is modified for all cases if the surface properties change with composition and that will cause severe complications for the common tangent construction. There may be a particular composition dependence of the specific surface free energy for an interstitial phase but it can be treated as any other phase. Although the partial volumes appear in the equilibrium conditions for each component, they play only a minor role for the two-phase equilibrium in binary systems. An exception is the equilibrium between a particle and its own vapor.

  • 35.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    A model-based continuum treatment of ordering and spinodal decomposition2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 13, p. 2491-2497Article in journal (Refereed)
    Abstract [en]

    Using the absolute reaction rate theory, a continuum treatment of ordering and spinodal decomposition is derived from the original planar discrete lattice model which was based on a Bragg-Williams type of approach. For small amplitudes the result agrees with Cahn's linearized equation for spinodal decomposition. The kinetic equation for ordering has a factor not included in the Allen-Cahn equation which depends on the local degree of order and may have important consequences.

  • 36.
    Hillert, Mats
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    An application of irreversible thermodynamics to diffusional phase transformations2006In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 54, no 1, p. 99-104Article in journal (Refereed)
    Abstract [en]

    The method of transforming the set of flux equations for two simultaneous processes into a new set of processes is reviewed and applied to diffusion in one-phase materials where the two primary equations describe the individual diffusion of the components and the new ones describe the migration of Kirkendall markers and interdiffusion. It is then modified to apply to a two-phase material with a diffusional transformation and the new equations then describe the migration of the phase interface and interdiffusion across the interface. Comparison is made with previous attempts. In order to treat the transition from diffusion-controlled to partitionless growth it may be necessary to introduce a third primary flux equation. It is possible to incorporate this third flux within the other two, in which case it will contribute substantially to their cross-coefficients.

  • 37.
    Hillert, Mats
    KTH, Superseded Departments, Materials Science and Engineering.
    Solute drag in grain boundary migration and phase transformations2004In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 52, no 18, p. 5289-5293Article in journal (Refereed)
    Abstract [en]

    Cahn's equation for solute drag has recently been applied to phase transformations but the method of application has been disputed. A new equation for solute drag is now proposed, which is more general and applies to multicomponent systems. It yields the same result for the migration of grain boundaries but a different result for phase transformations. It has the advantage of being equivalent to the approach based on the dissipation of Gibbs energy. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 38.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Höglund, Lars
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Diffusion-controlled lengthening of Widmanstatten plates2003In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 51, no 7, p. 2089-2095Article in journal (Refereed)
    Abstract [en]

    Various treatments of the diffusion-controlled lengthening of plates during precipitation from a supersaturated solid solution are compared numerically. Serious doubts are raised about the validity of Trivedi's treatment which has been generally accepted. A recent treatment based directly on Ivantsov's solution gives results in between those of Trivedi's equation and Zener's equation, as modified by Hillert.

  • 39.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Schalin, M.
    Modeling of solute drag in the massive phase transformation2000In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 48, no 2, p. 461-468Article in journal (Refereed)
    Abstract [en]

    The role of solute drag in the massive phase transformation is evaluated through the dissipation of Gibbs energy by diffusion. As an introduction, the solute drag in grain boundary migration is first examined using the wedge-shaped energy function considered by Cahn and by Lucke and Stuwe. The effect of a diffusivity varying from a low value in the bulk to a high value in the center of the boundary is examined. It decreases the solute drag drastically. For the massive phase transformation it is demonstrated how the solute drag increases by the tendency for segregation and by a high diffusivity in the interface but it decreases if the diffusivity is lower than in the parent phase. The most important contribution to solute drag comes from the spike of solute atoms in the parent phase being pushed forward by the advancing interface. The spike is thus an obstacle for growth that must be broken through in order for diffusion-controlled growth to turn partitionless. The possibility of dynamic nucleation is also discussed.

  • 40.
    Hillert, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Schwind, M.
    Selleby, Malin
    KTH, Superseded Departments, Materials Science and Engineering.
    Trapping of vacancies by rapid solidification2002In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 50, no 12, p. 3283-3291Article in journal (Refereed)
    Abstract [en]

    A model has been developed for the process of trapping of vacancies in rapid solidification of pure metals, which includes the effect of solute drag where vacancies play the role of solute. Within a reasonable range of parameter values it predicts that substantial trapping cannot occur unless the solidification velocity is 1 m/s or higher. It is demonstrated that the intrinsic mobility of the liquid/solid interface should not be evaluated without considering the effects of vacancy trapping and solute drag caused by vacancies. The model is applied to copper and unknown parameters are evaluated from information on the solidification velocity as a function of undercooling.

  • 41. Hillert, Mats
    et al.
    Selleby, Malin
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Sundman, Bo
    An attempt to correct the quasichemical model2009In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 57, no 17, p. 5237-5244Article in journal (Refereed)
    Abstract [en]

    An attempt was made to correct the quasichemical model to avoid predictions of negative configurational entropy when long-range order is impossible as in liquids. The attempt was successful for negative interaction energies but for positive interaction energies the simplest possible variant of the model predicted two narrow miscibility gaps. It was not possible to improve this by adding regular solution parameters. However, using the next possible variant of the correction term these difficulties may be avoided.

  • 42.
    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.

  • 43. Hou, T. P.
    et al.
    Li, Y.
    Wu, K. M.
    Peet, Mathew James
    Hulme-Smith, Christopher
    Guo, L.
    Magnetic-field-induced magnetism and thermal stability of carbides Fe6-xMoxC in molybdenum-containing steels2016In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 102, p. 24-31Article in journal (Refereed)
    Abstract [en]

    A hybrid method combining first-principles calculations and Weiss molecular field theory with thermodynamic data has been implemented to explore the origin of magnetic-field-induced precipitation behaviors for alloy carbides. The paramagnetic Mo atom disturbed the order of magnetic moment and resulted in a decrease in the Curie temperature for alloy carbide Fe<sub><it>6-x</it></sub>Mo<sub><it>x</it></sub>C. The temperature dependence of magnetic moment and saturation magnetization of Fe atoms at different Wyckoff positions, as well as the saturation or induced magnetization of Fe<sub><it>6-x</it></sub>Mo<sub><it>x</it></sub>C, decreased with increasing temperature. The higher Fe content and external magnetic field greatly increased the magnetization of alloy carbides. Two kinds of stella quadrangula lattices were employed to account for the total magnetism which was derived from the contribution of different Wyckoff sites of Fe atoms and Fe-C distances. The calculated total free energy taking into account magnetic field, temperature and composition was sufficient to provide quantitative agreement with experiment. The investigation of the effects of external field on the carbide precipitation behaviors led to a better understanding of the magnetic-field-induced phase transformation mechanism in heat resistant steels.

  • 44.
    Huang, He
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dong, Zhihua
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Huang, Shuo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Meng, Daqiao
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Lai, Xinchun
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Liu, Tianwei
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Zhu, Shengfa
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Dept Phys & Astron, Div Mat Theory, SE-75120 Uppsala, Sweden.;Wigner Res Ctr Phys, Inst Solid State Phys & Opt, H-1525 Budapest, Hungary..
    Critical stress for twinning nucleation in CrCoNi-based medium and high entropy alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 149, p. 388-396Article in journal (Refereed)
    Abstract [en]

    The CrCoNi-based medium and high entropy alloys (MHEAs) have drawn much attention due to their exceptional mechanical properties at cryogenic temperatures. The twinning critical resolved shear stress (CRSS) is a fundamental parameter for evaluating the strength-ductility properties of MHEAs. Here we construct and apply an extended twinning nucleation Peierls-Nabarro (P-N) model to predict the twinning CRSSes of face-centered cubic (FCC) CrCoNi-based MHEAs. The order of the twinning CRSSes of the selected alloys is CrCoNi > CrCoNiMn > CrCoNiFe > CrCoNiFeMn and the values are 291, 277, 274 and 236 MPa, respectively. These theoretical predictions agree very well with the experimental twinning CRSSes of CrCoNi and CrCoNiFeMn accounting for 260 +/- 30 and 235 +/- 10 MPa, respectively and are perfectly consistent with the strength-ductility properties including yield stress, ultimate tensile stress and uniform elongation for fracture of the FCC CrCoNi-based MHEAs obtained at cryogenic temperatures. The present method offers a first-principle quantum-mechanical tool for optimizing and designing new MHEAs with exceptional mechanical properties. 

  • 45.
    Höglund, Lars
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Ågren, John
    KTH, Superseded Departments, Metallurgy.
    Analysis of the Kirkendall effect, marker migration and pore formation2001In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 49, no 8, p. 1311-1317Article in journal (Refereed)
    Abstract [en]

    Some aspects of Kirkendall and lattice plane migration are studied by means of numerical simulations. The simulations are performed using individual mobilities assessed in this work and thermodynamic data available in literature. The simulations show that the position and stability of the Kirkendall plane as well as a number of other interesting features can be predicted using numerical simulation software in combination with assessed thermodynamic and kinetic data.

  • 46.
    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.

  • 47.
    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.

  • 48.
    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.

  • 49.
    Ji, Zong-Wei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Chinese Academy of Sciences, China.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hu, Qing-miao
    Kim, Dongyoo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Yang, Rui
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Mapping deformation mechanisms in lamellar titanium aluminide2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 144, p. 835-843Article in journal (Refereed)
    Abstract [en]

    Breakdown of Schmid's law is a long-standing problem for exploring the orientation-dependent deformation mechanism in intermetallics. The lack of atomic-level understanding of the selection rules for the plastic deformation modes has seriously limited designing strong and ductile intermetallics for high-temperature applications. Here we put forward a transparent model solely based on first principles simulations for mapping the deformation modes in gamma-TiAl polysynthetic twinned alloys. The model bridges intrinsic energy barriers and different deformation mechanisms and beautifully resolves the complexity of the observed orientation-dependent deformation mechanisms. Using the model, one can elegantly reveal the atomic-level mechanisms behind the unique channeled flow phenomenon in lamellar TiAl alloys.

  • 50. Khanna, R.
    et al.
    Sahajwalla, V.
    Simento, N.
    Seetharaman, Seshadri
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Atomistic Monte Carlo simulations on the influence of sulphur during high-temperature decarburization of molten iron-carbon alloys2010In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 58, no 6, p. 2225-2236Article in journal (Refereed)
    Abstract [en]

    We report a Monte Carlo simulation study of the molten Fe-C-S system with the aim of developing a theoretical understanding of the influence of sulphur during decarburization reactions in Fe-C alloys. Focussing specifically on the role played by free surfaces, computer simulations were based on the hexagonal atomistic model of Fe-C-S system using isotropic atomic interaction parameters; free surfaces were characterized by a missing layer of atoms. Three geometrical configurations, namely a liquid bath, a prismatic block and a spherical droplet, were investigated. Simulations were carried out as a function of melt carbon and sulphur concentration, temperatures and surface/volume ratios of the simulation cell. Sulphur atoms were found to preferentially concentrate in the top few layers, with the second layer showing the highest amounts of sulphur; very little sulphur was observed in the bulk liquid. This trend was observed in all three simulation configurations over a wide carbon/sulphur concentration range and temperatures. Significant levels of iron were observed in the top surface layer. The influence of free surfaces on atomic concentration profiles was found to be a strong function of the surface/volume ratio. The surface segregation of S was more pronounced for small exposed surfaces and was much smaller for liquids with large exposed surfaces. The presence of surface-active sulphur resulted in a major re-distribution of carbon. Carbon tended to concentrate deeper in the bulk, with the surface region being severely depleted of carbon. In addition to several new findings and a better understanding of liquid surfaces, these simulations have helped overcome major limitations of Sain and Belton's model. Key experimental results on decarburization have been explained within the framework of our simulations. These simulation results have significant implications for surface decarburization reactions and carbon-boil phenomena in smelting technologies.

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