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Experimental and theoretical study of carbides in the Co-Cr-C system: Structure, alloying and stability
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.ORCID iD: 0000-0002-3283-0549
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: urn:nbn:se:kth:diva-141129ISBN: 978-91-7595-008-2 (print)OAI: oai:DiVA.org:kth-141129DiVA: diva2:694776
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
List of papers
1. Thermodynamic analysis of the Co-Cr-C system
Open this publication in new window or tab >>Thermodynamic analysis of the Co-Cr-C system
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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.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-141157 (URN)10.1016/j.calphad.2014.05.003 (DOI)000340325700024 ()2-s2.0-84903144977 (Scopus ID)
Note

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

Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2017-12-06Bibliographically approved
2. Experimental Determination of the Solubility of Co in the Cr-Based Carbides Cr23C6, Cr7C3, and Cr3C2
Open this publication in new window or tab >>Experimental Determination of the Solubility of Co in the Cr-Based Carbides Cr23C6, Cr7C3, and Cr3C2
2014 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45, no 11, 4820-4828 p.Article in journal (Refereed) Published
Abstract [en]

Thermodynamic calculations based on the CALPHAD method are nowadays often applied in the design of new materials due to increasing demands on shorter lead times for development. However, such calculations rely heavily on the assessed thermodynamic descriptions, which in turn rely on the amount and quality of available experimental data, especially for binary and ternary sub-systems. The ternary Co-Cr-C system is an extremely important subsystem to, e.g., multi-component cemented carbide grades, such as W-Co-Cr-M-C (M = Ti,Ta,Nb,V,Zr,Hf), as well as Cr-containing Co-base alloys. In the case of the Co-Cr-C system, there is a lack of reliable data on the solubility of Co in Cr-carbides. Therefore, the present work concerns an experimental study of the solubility of Co in all three of the Cr-based carbides, i.e., Cr23C6, Cr7C3, and Cr3C2. This was done by synthesizing appropriate samples in the M7C3+M23C6+liquid and M7C3+M3C2+graphite three-phase fields. The results show that a recent thermodynamic description of the Co-Cr-C system is unable to reproduce the experimentally determined solubilities. Therefore, the present study provides important input for future alloy development and improvement of the thermodynamic description of the Co-Cr-C system.

Keyword
Alloy development, CALPHAD method, Cemented carbides, Experimental determination, Multicomponents, Solubility of CO, Thermodynamic calculations, Thermodynamic description
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-141171 (URN)10.1007/s11661-014-2441-8 (DOI)000341086700016 ()2-s2.0-84906779253 (Scopus ID)
Funder
Swedish Research Council
Note

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

Available from: 2014-02-11 Created: 2014-02-11 Last updated: 2017-12-06Bibliographically approved
3. Structural Determination of (Cr,Co)7C3
Open this publication in new window or tab >>Structural Determination of (Cr,Co)7C3
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2013 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Chromium is one of the most well-known WC grain growth inhibitors in cemented carbides. It is thus vital to understand and to be able to thermodynamically model the prevailing phase equilibria in the WC-Co-Cr system. To do this it is important that the lower order systems, such as the Co-Cr-C system, are correctly described. Previous investigations have shown that the M7C3 (M=Cr,Co,W) phase is the first carbide to form when Cr is added in excess to the WC+fcc-Co/liquid+graphite phase field. However, the exact structure of this phase has not been investigated and there are many proposed structures already for the binary Cr7C3 carbide, ranging from trigonal, via hexagonal to orthorhombic symmetry. Recent investigations show that the hexagonal structure belonging to the P63mc space group is the stable structure at 0 K. In the present study the binary Cr7C3 carbide and a mixed M7C3 carbide are investigated. The structures of both carbides and preferential positions for Co atoms in the mixed carbide are determined by XRD measurements in combination with ab initio calculations and Rietveld refinement.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-126797 (URN)
Conference
18th Plansee Seminar, 3 – 7 June, 2013 in Reutte, Austria
Note

QC 20130902

Available from: 2013-08-21 Created: 2013-08-21 Last updated: 2015-12-08Bibliographically approved

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