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Simulation of Phase Transformations and coarsening: Computational tools for alloy development
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
2011 (English)Doctoral 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , iv, 40 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-31454ISBN: 978-91-7415-891-5OAI: oai:DiVA.org:kth-31454DiVA: diva2:404154
Public defence
2011-03-25, F3, Lindstedtsvägen 28, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
QC 20110316Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-03-16Bibliographically approved
List of papers
1. Analytical treatment of diffusion during precipitate growth in multicomponent systems
Open this publication in new window or tab >>Analytical treatment of diffusion during precipitate growth in multicomponent systems
2008 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 8, 1890-1896 p.Article in journal (Refereed) Published
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.

Keyword
precipitation, diffusion, kinetics, thermodynamics, NPLE, alloys, transformations, kinetics
Identifiers
urn:nbn:se:kth:diva-17541 (URN)10.1016/j.actamat.2007.12.037 (DOI)000255993800024 ()2-s2.0-41849090445 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-03-16Bibliographically approved
2. Modified mean field models of normal grain growth
Open this publication in new window or tab >>Modified mean field models of normal grain growth
2008 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 56, no 18, 5188-5201 p.Article in journal (Refereed) Published
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.

Keyword
Grain growth, Simulation, Thermodynamics, Onsager's principle, size distribution, 2 dimensions, state, law
Identifiers
urn:nbn:se:kth:diva-17948 (URN)10.1016/j.actamat.2008.06.034 (DOI)000260704100025 ()2-s2.0-53049088002 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-03-16Bibliographically approved
3. Simulation of the Soaking and Gas Jet Cooling in a Continuous Annealing Line using Dilatometry
Open this publication in new window or tab >>Simulation of the Soaking and Gas Jet Cooling in a Continuous Annealing Line using Dilatometry
2010 (English)In: Steel Research International, ISSN 1611-3683, Vol. 81, no 2, 158-167 p.Article in journal (Refereed) Published
Abstract [en]

The present study concerns the simulation of a continuous annealing line (CAL), using dilatometry. Simulations of CAL have been performed on four commercial steel grades with different chemical compositions in order to investigate how the alloying elements C, Mn, Si and B affect the microstructure and hardness of dual phase (DP) and martensitic steels. Three annealing cycles corresponding to those used in a CAL have been applied. When annealing intercritically, as is the case in DP-steel production, the materials do not reach equilibrium during soaking. Mn and C increase the austenite content and consequently the hardness of the materials. Higher levels of Si (0.4 wt %) are required to retard the formation of new ferrite during cooling in the gas jet section, prior to quenching. 6 increases hardenability effectively when annealing in the austenite region but is not as efficient during intercritical annealing, which implies that boron restrains ferrite nucleation rather than impeding ferrite growth. Results from DICTRA calculations show that it is possible to simulate the phase transformations during soaking, gasjet cooling and quenching.

Keyword
dilatometry, simulation, continuous annealing, high strength steels
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-24971 (URN)10.1002/srin.200900097 (DOI)000275142400009 ()2-s2.0-77949483612 (ScopusID)
Note
QC 2010104Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2011-03-16Bibliographically approved
4. Inverse Saltykov analysis for particle-size distributions and their time evolution
Open this publication in new window or tab >>Inverse Saltykov analysis for particle-size distributions and their time evolution
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 3, 874-882 p.Article in journal (Refereed) Published
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.

Keyword
Grain size, Image analysis, 3-D distribution, Carbides
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-30977 (URN)10.1016/j.actamat.2010.09.046 (DOI)000286690100003 ()2-s2.0-78650680735 (ScopusID)
Note
QC 20110310Available from: 2011-03-10 Created: 2011-03-07 Last updated: 2011-04-26Bibliographically approved
5. Carbide grain growth in cemented carbides
Open this publication in new window or tab >>Carbide grain growth in cemented carbides
2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 5, 1912-1923 p.Article in journal (Refereed) Published
Abstract [en]

Abnormal grain growth is often observed in cemented carbides during sintering, but cannot be understood in terms of the classical LSW theory. In this work the grain growth behavior during sintering at 1430 °C is studied both experimentally and by means of computer simulations. A model based on several processes—2-D nucleation of growth ledges, mass transfer across the interface and long-range diffusion coupled in series—is formulated and the equations are solved numerically. Both computer simulations and experimental studies reveal that the grain growth behavior is strongly influenced by the initial size distribution.

Keyword
Abnormal grain growth, Carbides, Grain growth, Modeling, Sintering, 2-d nucleations, Cemented carbides, Experimental studies, Grain growth behavior, Long-range diffusion, LSW theory, Model-based, Carbide tools, Computational methods, Computer simulation, Grain size and shape
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-31450 (URN)10.1016/j.actamat.2010.11.056 (DOI)000287775400006 ()2-s2.0-79551497846 (ScopusID)
Note
QC 20110316Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2011-04-26Bibliographically approved
6. Modeling of dispersoid precipitation in multicomponent alloys
Open this publication in new window or tab >>Modeling of dispersoid precipitation in multicomponent alloys
(English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453Article in journal (Other academic) Submitted
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.

Keyword
Phase transformations, simulation, nucleation and growth, coarsening, multicomponent
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-31453 (URN)
Note
QS 20120327Available from: 2011-03-16 Created: 2011-03-16 Last updated: 2012-03-27Bibliographically approved

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