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Thermodynamic modelling of carbides in multicomponent systems: theoretical and experimental approach
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns thermodynamic modeling of carbides in multicomponent systems. Focus has been made on systems interesting for cemented carbide production but the results are also useful for many other application were the material consist of different carbides, for example tool steels/high speed steels. The Co-W-C system forms the basis in cemented carbide production. An accurate thermodynamic description of this system is therefore crucial for extrapolation into higher order systems. New experimental results on the liquid+fcc+graphite+WC and liquid+fcc+WC+M6C equilibrium temperatures, that has recently been published, shows that these equilibrium temperatures are higher than the values used in the available assessment of Co-W-C. Since an accurate description of these equilibrium temperatures are very important for production of cemented carbides and when extrapolating into higher order systems a reassessment of the Co-W-C system is presented. Cr is sometimes deliberately added to lower the melting point, reduce grain growth and/or increase corrosion resistance in the production of cemented carbides. When adding chromium there is a risk of forming an unwanted M7C3 carbide. It is therefore of great interest to know the stability of this carbide. New experimental results on the maximum solubility of Co in the M7C3 is presented as well as a new thermodynamic description of the Co-Cr-C system which accurately describes the solubility of Co in the M7C3 carbide in the temperature range 1373- 1723 K. The assessment of a system, and the determination of Gibbs energy functions, is straightforward when reliable and consistent thermochemical and phase equilibrium information is available. However, reliable experimental information is often lacking or does not give a unique set of model parameters, and therefore different strategies to estimate information have been developed. In the present work the excess energies for A1-xBxC mixed carbides (where A and B are metals) have been calculated using ab-initio calculations, for 14 systems. A thorough comparison has been made with experimentally assessed excess energies. The comparison shows that ab-initio calculations can be used to predict the sign, magnitude and symmetry of the excess energy for A1-xBxC mixed carbides. The calculated excess energies have also successfully been used to describe several AC-BC systems where the experimental information does not give a unique determination of the excess energy in traditional CALPHAD modeling. Experimental work has also been done on the C-Co-Ti-V-W-Zr system in order to determine the extension of the miscibility gaps in TiC-ZrC and VC-ZrC into the (TiC or VC)-ZrC-WC system. Thermodynamic calculations were used to design samples that will form a miscibility gap in equilibrium with liquid, WC and graphite. Samples were produced from powder and sintered for 1 week in controlled atmosphere at 1300, 1410 and 1500 °C. From the microstructure it could be concluded that the samples form a miscibility gap in equilibrium with liquid, WC and graphite at all temperatures. The composition of the MCx carbides was measured using an analytic SEM. The new experimental information was used to assess the thermodynamic description for the TiC-ZrC system.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , v, 23 p.
Identifiers
URN: urn:nbn:se:kth:diva-10013ISBN: 978-91-7415-237-1 (print)OAI: oai:DiVA.org:kth-10013DiVA: diva2:201316
Presentation
2009-03-12, Sal B2, Materialvetenskap, KTH, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-03-04 Created: 2009-03-04 Last updated: 2010-10-19Bibliographically approved
List of papers
1. A revised thermodynamic description of the Co-W-C system
Open this publication in new window or tab >>A revised thermodynamic description of the Co-W-C system
2005 (English)In: Journal of Phase Equilibria and Diffusion, ISSN 1547-7037, Vol. 26, no 2, 152-160 p.Article in journal (Refereed) Published
Abstract [en]

 A thermodynamic reassessment of the Co-W-C system is presented. New information on the liquid (L)+face-centered cubic (fcc)+graphite+WC and L+fcc+WC+M6C equilibria has recently been published. Because these equilibria are very important for extrapolation to higher-order systems, the ternary system has been revised. A revision of the Co-W system also has been performed.

Keyword
Composition, Extrapolation, Optimization, Phase equilibria, Reliability, Solubility, Thermodynamics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-10008 (URN)10.1361/15477030522473 (DOI)000228076100006 ()2-s2.0-17444411529 (Scopus ID)
Note
QC 20101019Available from: 2009-03-03 Created: 2009-03-03 Last updated: 2010-10-19Bibliographically approved
2. Experimental and thermodynamic evaluation of the Co-Cr-C system
Open this publication in new window or tab >>Experimental and thermodynamic evaluation of the Co-Cr-C system
2006 (English)In: International Journal of Materials Research, ISSN 1862-5282, Vol. 97, no 9, 1243-1250 p.Article in journal (Refereed) Published
Abstract [en]

New experimental results on the liquid + graphite + M7C3 equilibrium are presented. The solubility of Co in the M7C3 carbide has been measured at 1523 and 1723K. These measurements together with recently published measurements on the fcc + graphite + M7C3 equilibrium show that the solubility of Co in the M7C3 is much higher than predicted from thermodynamic calculations. A new thermodynamic description of the Co–Cr –C system is presented which accurately describes the solubility of Co in the M7C3 carbide in the temperature range 1373 – 1723K.

Keyword
Co-Cr-C, calphad, thermodynamic evaluation, M7C3, solubility
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-10009 (URN)000241402700010 ()2-s2.0-33750075040 (Scopus ID)
Note
QC 20101019Available from: 2009-03-03 Created: 2009-03-03 Last updated: 2010-10-19Bibliographically approved
3. Combined ab-initio and experimental assessment of A1-xBxC mixed carbides
Open this publication in new window or tab >>Combined ab-initio and experimental assessment of A1-xBxC mixed carbides
2008 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 32, no 4, 615-623 p.Article in journal (Refereed) Published
Abstract [en]

The excess energies for A1 - x Bx C mixed carbides (where A and B are metals) have been calculated using ab-initio calculations, for 14 systems. A thorough comparison has been made with experimentally assessed excess energies. The comparison shows that conventional ab-initio calculations applied to rather simple structural models can be used to predict the sign, magnitude and symmetry of the excess energy for A1 - x Bx C mixed carbides. The calculated excess energies have also successfully been used to describe several AC-BC systems where the experimental information does not give a unique determination of the excess energy in traditional CALPHAD modelling. The systems that have been studied are CrC-TiC, HfC-NbC, HfC-TaC, HfC-TiC, HfC-VC, NbC-TaC, NbC-VC, NbC-ZrC, TaC-VC, TaC-ZrC, TiC-VC, TiC-ZrC and VC-ZrC.

Keyword
Ab-initio, Calphad, Excess energy, Mixed carbides, Thermodynamic modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-10010 (URN)10.1016/j.calphad.2008.07.014 (DOI)000261776200002 ()2-s2.0-55549099564 (Scopus ID)
Note
QC 20101019Available from: 2009-03-03 Created: 2009-03-03 Last updated: 2017-12-13Bibliographically approved
4. Experimental and thermodynamic evaluation of the miscibility gaps in MC carbides for the C-Co-Ti-V-W-Zr system
Open this publication in new window or tab >>Experimental and thermodynamic evaluation of the miscibility gaps in MC carbides for the C-Co-Ti-V-W-Zr system
2009 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 33, no 3, 530-538 p.Article in journal (Refereed) Published
Abstract [en]

Experimental work on the C-Co-Ti-V-W-Zr system in order to determine the extension of the miscibility gaps in TiC-ZrC and VC-ZrC is presented. Thermodynamic calculations were used to design samples that will form a miscibility gap in equilibrium with liquid, WC and graphite. Samples were produced from powder and sintered for 1 week in controlled atmosphere at 1300, 1410 and 1500 {ring operator}C. From the microstructure it could be concluded that the samples form a miscibility gap in equilibrium with liquid, WC and graphite at all temperatures. The composition of the MCx carbides was measured using an analytic SEM. The new experimental information was used to assess the thermodynamic description for the TiC-ZrC system. This work provides valuable information on the miscibility gap in the C-Co-Ti-V-W-Zr system not published previously.

Keyword
CALPHAD, Experimental, MC carbide, Miscibility gap, Thermodynamics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-10012 (URN)10.1016/j.calphad.2009.03.002 (DOI)000270346200012 ()
Note
QC 20101019. Uppdaterad från Accepted till Published (20101019).Available from: 2009-03-04 Created: 2009-03-04 Last updated: 2017-12-13Bibliographically approved

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