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Equilibrium aspects of Cr-alloyed cemented carbides
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.ORCID iD: 0000-0002-3283-0549
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: urn:nbn:se:kth:diva-175856ISBN: 978-91-7595-712-8 (print)OAI: oai:DiVA.org:kth-175856DiVA: diva2:862725
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
List of papers
1. 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
2. Thermodynamic analysis of the Co-Cr-C system
Open this publication in new window or tab >>Thermodynamic analysis of the Co-Cr-C system
Show others...
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
3. Thermodynamic analysis of the W-Co-Cr system supported by ab initio calculations and verified with quaternary data
Open this publication in new window or tab >>Thermodynamic analysis of the W-Co-Cr system supported by ab initio calculations and verified with quaternary data
2015 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 50, 59-67 p.Article in journal (Refereed) Published
Abstract [en]

The present work aims at assessing the W-Co-Cr system with focus on including the ternary R-phase in the thermodynamic description. Enthalpies of formation at 0 K of all considered R-phase end-members are calculated using density functional theory and used in the assessment due to the scarceness of the experimental information. The resulting assessment is verified by comparing with recent experimental data in the W-Co-Cr-C quaternary system.

Keyword
W-Co-Cr, W-Co-Cr-C, Phase diagrams, Thermodynamics, Ab initio, DFT
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-172665 (URN)10.1016/j.calphad.2015.04.012 (DOI)000358467900006 ()2-s2.0-84929323392 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150901

Available from: 2015-09-01 Created: 2015-08-27 Last updated: 2017-12-04Bibliographically approved
4. Thermodynamic calculations and experimental verification in the WC-Co-Cr cemented carbide system
Open this publication in new window or tab >>Thermodynamic calculations and experimental verification in the WC-Co-Cr cemented carbide system
2015 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 48, 257-262 p.Article in journal (Refereed) Published
Abstract [en]

Tungsten carbide and cobalt have always been, and still are, the foundation of cemented carbides. Modem grades include several other alloying elements, apart from just WC-Co, added for several different purposes. For example, by adding chromium it is possible to produce extremely fine grained grades compared to straight WC-Co grades and thus the freedom to tailor the properties of the material is increased. By applying thermodynamic calculations it is possible to design the material and also avoid some of the costly trial-and-error procedures. However, there is also a need for experimental verification in order to have confidence in the predictive calculations. The present work concerns the application of thermodynamic calculations to some relevant compositions together with experimental verification in the WC-Co-Cr system. Special focus is given to the limiting conditions for precipitation of unwanted phases with regards to the Cr-content in the binder, C-content and melting temperatures. No regard is taken to the presumable Cr-solubility in the WC phase itself.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
WC-Co-Cr, Phase diagrams, Cr7C3, Thermodynamics
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-159978 (URN)10.1016/j.ijrmhm.2014.09.016 (DOI)000347586400037 ()2-s2.0-84907495184 (Scopus ID)
Note

QC 20150304

Available from: 2015-03-04 Created: 2015-02-12 Last updated: 2017-10-31Bibliographically approved
5. Finite temperature ab initio calculated thermodynamic properties of orthorhombic Cr3C2
Open this publication in new window or tab >>Finite temperature ab initio calculated thermodynamic properties of orthorhombic Cr3C2
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-176045 (URN)
Note

QS 2015

Available from: 2015-10-29 Created: 2015-10-29 Last updated: 2015-10-29Bibliographically approved
6. Neutron and X-ray diffraction study of (Cr,Co)7C3
Open this publication in new window or tab >>Neutron and X-ray diffraction study of (Cr,Co)7C3
Show others...
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-176044 (URN)
Note

QS 2015

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

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