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Thermodynamic analysis of the Co-Cr-C system
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics. Sandvik Coromant RandD, Sweden .
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics. Thermo-Calc Software AB, Sweden .
Sandvik Coromant R&D, Stockholm.
Sandvik Mining R&D, Stockholm.
Show others and affiliations
2014 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 46, 226-236 p.Article in journal (Refereed) Published
Abstract [en]

A reassessment of the thermodynamic description of the Co–Cr–C system was performed to take into account recent experimental information on the solubility of Co in Cr-based carbides. Density Functional Theory (DFT) calculations coupled with phonon calculations were performed to calculate the heat capacity and Gibbs energies of formation of stable and metastable carbides in the Co–Cr–C system as a function of temperature, within the limits of the Harmonic or the Quasi Harmonic Approximation. Resulting Gibbs energies were compared with earlier experimental studies and assessments, where calculated values for Cr23C6, Cr7C3 and Cr3C2 from the present work were seen to fall within the experimental scatter. The calculated heat capacity and Gibbs energy of formation as a function of temperature for the metastable Co3C2 compound together with recent experimental information was used in the evaluation of the thermodynamic parameters. As a result, the new and improved thermodynamic description accounts for the solubility of Co in M3C2 in contrast to previous descriptions, where this was neglected due to a complete lack of experimental information. Furthermore, a better representation of previously reported liquidus temperatures was achieved, without increasing the number of parameters in the liquid phase. Other relevant features of the phase diagram and thermochemical properties were also well represented.

Place, publisher, year, edition, pages
2014. Vol. 46, 226-236 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-141157DOI: 10.1016/j.calphad.2014.05.003ISI: 000340325700024Scopus ID: 2-s2.0-84903144977OAI: oai:DiVA.org:kth-141157DiVA: diva2:695175
Note

Updated from "Manuscript" to "Article". QC 20140909

Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Experimental and theoretical study of carbides in the Co-Cr-C system: Structure, alloying and stability
Open this publication in new window or tab >>Experimental and theoretical study of carbides in the Co-Cr-C system: Structure, alloying and stability
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The research presented in this thesis explores the structure, alloying and stability of the carbides in the Co-Cr-C system by using the CALPHAD approach supported by experimental measurements and ab initio calculations.

WC-Co based cemented carbide materials consist of hard WC grains and a ductile Co-based binder. Cr is usually added to cemented carbides for the purpose of reducing grain growth during sintering or to increase corrosion resistance. The Co-Cr-C system is highly relevant for Cr-containing cemented carbides. For example, it determines the allowable C-content interval in the quaternary W-Co-Cr-C system with respect to the formation of unwanted Cr-based carbides.

The CALPHAD method is based on critical assessment of model parameters constituting phenomenological expressions of the Gibbs energy and its derivatives, e.g. enthalpy, entropy and heat capacity. A thermodynamic assessment based on the CALPHAD method relies heavily on the available experimental or ab initio calculated information.

The crystal structures of Cr23C6 and Cr3C2 have previously been accurately determined. However, various authors reported different crystal structures for the Cr7C3 carbide. A combined experimental and ab initio investigation of the crystal structure of Cr7C3 and M7C3 (M=Co,Cr) was thus performed. Furthermore, the stabilities of all end-member compounds of the mixed carbide at 0 K were

determined by ab initio calculations.

Available experimental information on phase equilibria with carbides in the Co-Cr-C system was found to be insufficient to describe all the features in the phase diagram. Therefore, the liquid + M23C6 + M7C3 and M7C3 + M3C2 + graphite equilibria (M=Co,Cr) were investigated experimentally with focus on the solubility of Co in the M23C6, M7C3 and M3C2 carbides.

Finally, a reassessment of the thermodynamic description of the Co-Cr-C system was performed, which resulted in a set of thermodynamic models and parameters that accurately describe all of the considered information.

Applied experimental methods and theoretical models are explained throughout the thesis. Important examples from the appended papers are also given.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. viii, 39 p.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-141129 (URN)978-91-7595-008-2 (ISBN)
Presentation
2014-03-07, Sal Kuben N111, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Note

QC 20140212

Available from: 2014-02-12 Created: 2014-02-07 Last updated: 2014-02-12Bibliographically approved
2. Equilibrium aspects of Cr-alloyed cemented carbides
Open this publication in new window or tab >>Equilibrium aspects of Cr-alloyed cemented carbides
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In view of the extensive use of Cr as a grain growth inhibitor in WC-Co cemented carbides this thesis comprises a combined experimental and ab initio study of a number of critical issues pertaining to phase equilibria of the subsystems to the W-Co-Cr-C system.

To be able to predict which of the stable Cr-carbides forms above the solubility limits of Cr and C in the fcc phase the respective solubility of Co in Cr23C6, Cr7C3 and Cr3C2 are investigated experimentally. Furthermore, the site occupancies of Co in Cr7C3 are investigated by neutron diffraction as a first step towards a more realistic model for this phase.

The energetics of the ternary intermetallic R-phase and the unstable Co3C2 end-member compound are investigated by density functional theory. For Co3C2, the quasi-harmonic vibrational Gibbs energy is also calculated.

By subsequent CALPHAD assessment an improved thermodynamic description is developed and its agreement with experiment is investigated. The resulting thermodynamic description allows for improved accuracy in predicting the formation of carbide phases as a function of C content and Co/Cr ratio as well as liquidus and solidus temperatures.

Remaining issues may be attributed to the thermodynamic description of the W-Cr-C system and the stability of the Cr-based carbides in the Cr-C system. In the case of the Cr-based carbides, severe experimental scatter prevents an accurate determination of the stability of either of them. As a first attempt to resolve the situation, a state of the art ab initio approach is applied to calculate the finite temperature thermodynamic properties of Cr3C2, benchmarked with reported heat capacity and relative thermal expansion measurements.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. vii, 57 p.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-175856 (URN)978-91-7595-712-8 (ISBN)
Public defence
2015-11-25, sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2011-3569
Note

QC 20151029

Available from: 2015-10-29 Created: 2015-10-23 Last updated: 2015-10-29Bibliographically approved

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