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Developing the third generation of Calphad databases: what can ab-initio contribute?
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
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: urn:nbn:se:kth:diva-215214ISBN: 978-91-7729-553-2 (print)OAI: oai:DiVA.org:kth-215214DiVA, id: diva2:1147006
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
List of papers
1. On the third-generation Calphad databases: An updated description of Mn
Open this publication in new window or tab >>On the third-generation Calphad databases: An updated description of Mn
2015 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 252, no 10, p. 2199-2208Article in journal (Refereed) Published
Abstract [en]

Aiming for better extrapolations and predictabilities of thermodynamic properties of materials, new thermodynamic models are implemented in the third-generation Calphad databases. In these models, each term contributing to the Gibbs energy has an explicit physical meaning. Furthermore, descriptions of thermodynamic properties of materials are valid from 0K up to high temperatures far above the melting point. As a starting point for the development of large self-consistent third-generation database, the new models in the present work are applied to the unary manganese system. Taking into account both the calculated first principles results and experimental data, thermodynamic model parameters are evaluated. Thermodynamic properties predicted using this description, agree very well with available data. The calculated properties vary smoothly in the whole temperature range, which is another important improvement compared to the second-generation databases.

Place, publisher, year, edition, pages
[Bigdeli, Sedigheh; Mao, Huahai; Selleby, Malin] KTH Royal Inst Technol, Dept Mat Sci & Engn, S-10044 Stockholm, Sweden.: , 2015
Keywords
Calphad approach, manganese, thermodynamic assessment, thermodynamic database
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-176358 (URN)10.1002/pssb.201552203 (DOI)000362722300008 ()2-s2.0-84943197727 (Scopus ID)
Note

QC 20151104

Available from: 2015-11-04 Created: 2015-11-03 Last updated: 2017-12-01Bibliographically approved
2. New description of metastable hcp phase for unaries Fe and Mn: Coupling between first-principles calculations and CALPHAD modeling
Open this publication in new window or tab >>New description of metastable hcp phase for unaries Fe and Mn: Coupling between first-principles calculations and CALPHAD modeling
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2016 (English)In: Physica Status Solidi B, ISSN 1521-3951, no 9, p. 1830-1836Article in journal (Refereed) Published
Abstract [en]

The main focus in developing the third generation of CALPHADdatabases is to model thermodynamic properties of materialsby using models which are more physically based andvalid down to 0K. First-principles calculations are helpful tochoose and validate those models. Reliable calculation results,for example, at very low temperatures or on metastable systemsreveal physical facts which might be inaccessible by experiments.Following our earlierwork for modeling thermodynamicproperties of pure elements (i.e., Fe and Mn) in third-generationCALPHAD databases, the (hcp) phase was modeled as ametastable phase in the present work. Although hcp phase isjust observed in these two elements under ultra-high pressure, inthe binary Fe–Mn this phase is metastable at ambient temperaturesand pressures. Therefore, it should be properly modeled inunaries for later optimization of binary systems. Based on densityfunctional theory (DFT) calculations, the magnetic groundstate and the magnetic properties of -Fe, -Mn, and their binarysolution phase were calculated. It was found that -Fe is antiferromagnetic(type II) while -Mn has a paramagnetic groundstate. Accordingly, magnetic contributions to thermodynamicproperties were accurately modeled. Moreover, by means ofthe extrapolation of experimental data for the thermodynamicproperties of binary systems and high-pressure data for unaries,the metastable hcp phases at ambient pressure were modeledfor the third-generation CALPHAD database, consistently withother stable phases in the elements Fe and Mn.

Place, publisher, year, edition, pages
John Wiley & Sons, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-197630 (URN)10.1002/pssb.201600096 (DOI)000383605400020 ()2-s2.0-84971233904 (Scopus ID)
Note

QC 20161207

Available from: 2016-12-05 Created: 2016-12-05 Last updated: 2017-10-06Bibliographically approved
3. A new description for pure C; in developing the third generation of Calphad databases.
Open this publication in new window or tab >>A new description for pure C; in developing the third generation of Calphad databases.
(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
Carbon, Calphad, Einstein model, DFT, Modelling
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-215207 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20171010

Available from: 2017-10-04 Created: 2017-10-04 Last updated: 2017-10-10Bibliographically approved
4. An insight into using DFT data for Calphad modelling of solid phases in the third generation of Calphad databases, case study for Al
Open this publication in new window or tab >>An insight into using DFT data for Calphad modelling of solid phases in the third generation of Calphad databases, case study for Al
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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. 

Keywords
Al, Calphad modelling, ab-initio, DFT.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-215210 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20171010

Available from: 2017-10-04 Created: 2017-10-04 Last updated: 2017-10-10Bibliographically approved
5. A thermodynamic assessment of the binary Fe-Mn system for the third generation of Calphad databases
Open this publication in new window or tab >>A thermodynamic assessment of the binary Fe-Mn system for the third generation of Calphad databases
2017 (English)Manuscript (preprint) (Other academic)
Abstract [en]

In developing the third generation of Calphad databases, after having proper models for the unaries, the next step is to assess higher-order systems, i.e. binaries, ternaries etc. A new description for the Fe-Mn system is presented in this work, based on the Calphad approach. New models with a stronger physical basis are used to model Gibbs energy of the phases. For this purpose, the revised magnetic model is used to fit the magnetic properties versus the most recent experimental and DFT data. An acceptable magnetic phase diagram is reproduced, which results in a more reasonable fit for the phase diagram and will prevent possible artefacts in higher-order systems. The descriptions are valid down to 0 K, which make them very useful as an input for modelling phase transformations occurring at low temperatures.  

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-215212 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20171010

Available from: 2017-10-04 Created: 2017-10-04 Last updated: 2017-11-13Bibliographically approved
6. DFT based calculations of thermodynamic properties for paramagnetic bcc-Mn at high temperatures
Open this publication in new window or tab >>DFT based calculations of thermodynamic properties for paramagnetic bcc-Mn at high temperatures
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In developing the third generation of Calphad databases, density-functional-theory methods are particularly important for validating physical models and for providing input data where experiment is unavailable. Since Cal-phad modelling needs data at finite temperatures additional sampling tech-niques are required, and the recently developed TU-TILD method is one such technique that allows for an efficient and accurate calculation of free energies. It is used in the present work for the complicated paramagnetic bcc phase of Mn. Our results show a good agreement with experimental data, validating that this method is well suited for calculating thermody-namic properties of magnetically complex and dynamically unstable phases as needed for Calphad modelling. It is also shown that a transition from bcc to the ω phase occurs below 1200 K.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-215213 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20171010

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

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