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An insight into using DFT data for Calphad modelling of solid phases in the third generation of Calphad databases, case study for Al
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
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2017 (English)Manuscript (preprint) (Other academic)
Abstract [en]

 In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. Harmonic vibrations of atoms are modelled using the Einstein temperature; anharmonic vibrations, electronic and magnetic contributions to the solid phases are represented by specific terms. The two-state model is used for the liquid phase. To continue the development, a new description for unary aluminum is presented in this work. In particular, finite temperature density-functional-theory (DFT) results are used to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point. 

Place, publisher, year, edition, pages
2017.
Keywords [en]
Al, Calphad modelling, ab-initio, DFT.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-215210OAI: oai:DiVA.org:kth-215210DiVA, id: diva2:1146996
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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