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A new description for pure C; in developing the third generation of Calphad databases.
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, Computational Thermodynamics.ORCID iD: 0000-0001-5031-919X
(English)In: Journal of Phase Equilibria and DiffusionArticle in journal (Other academic) Submitted
Abstract [en]

In connection to developing the third generation of Calphad databases a new thermodynamic description is presented for unary carbon. Models used in this work have more physical basis and are valid down to 0 K. The anisotropy in graphite, caused by weak Van der Waals inter-plane forces makes it difficult to fit the heat capacity data by a single Einstein tempera-ture for modelling the harmonic vibration of the atoms. By using multiple Einstein temperatures this problem is solved and a good agreement with the experimental data at low temperatures is achieved. Diamond is mod-eled using new models too, and the two-state model is used for modelling the liquid phase.

Keywords [en]
Carbon, Calphad, Einstein model, DFT, Modelling
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-215207OAI: oai:DiVA.org:kth-215207DiVA, id: diva2:1146994
Funder
VINNOVA, 2012-02892
Note

QC 20171010

Available from: 2017-10-04 Created: 2017-10-04 Last updated: 2017-10-10Bibliographically approved
In thesis
1. Developing the third generation of Calphad databases: what can ab-initio contribute?
Open this publication in new window or tab >>Developing the third generation of Calphad databases: what can ab-initio contribute?
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Developing the third generation of Calphad databases with more physical basis valid within a wider temperature range is the aim of the present work. Atomistic scale (ab-initio) methods, particularly techniques based on DFT theory, are used for modelling dierent phenomena, so as to gauge the capacity for use in Calphad modelling. Several systems are investigated in this work for studying dierent phenomena, such as magnetism and vibration of atoms. In the case of pure elements (unaries), thermodynamic properties of Mn, Al and C are optimized in the whole temperature range by the help of new models. In addition, DFT results and specic characteristics of these elements are also used to develop models for describing magnetic properties and atomic vibrations. With regards to coupling between DFT and Calphad, the EMTO technique is used for determining the magnetic ground state of the metastable hcp phase in Fe and Mn, and the TU-TILD technique is used for modelling solid phases above the melting point. TU-TILD is also used for calculating thermodynamic properties of bcc Mn at nite temperatures. The same phenomena are investigated in higher-order systems, i.e. the binaries Fe-Mn and Mn-C. Thermodynamic properties and phase diagrams of these systems are assessed against experimental data. Moreover, the revised magnetic model is used for modelling magnetic properties in these systems.

It is shown through this investigation that although the DFT methods are powerful tools for model development and for resolving discrepancies between dierent experimental datasets, they should not be overly-trusted. Caution must be taken when using DFT results, since the approximations and assumptions for computational implementations may cause some errors in the results. Moreover, implementing them into Calphad software as a connected methodology is not currently accessible due to the computational limitations.

It is concluded that coupling between the DFT and Calphad approaches can currently be achieved by using DFT results as an input in Calphad modelling. This will help to improve them until they can be integrated into the Calphad approach by the progress of computational possibilities.

One of the advantages of developing the third generation Calphad databases is the possibility of using the 0 K DFT results in Calphad modelling, since the new databases are valid down to 0 K. This has not been possible in the past, and such potential opens a new door to bring more physics into the Calphad approach.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 49
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-215214 (URN)978-91-7729-553-2 (ISBN)
Public defence
2017-10-27, Q2, Osquldas väg 10, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Note

QC 20171006

Available from: 2017-10-06 Created: 2017-10-04 Last updated: 2017-10-06Bibliographically approved

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Bigdeli, SedighehSelleby, Malin

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