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On the three-dimensional structure of WC grains in cemented carbides
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-4556-032X
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-1102-4342
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0003-3598-2465
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0002-7656-9733
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2013 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 61, no 13, 4726-4733 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, the size distribution and shape of WC grains in cemented carbides (WC-Co), with different Co contents, have been investigated in three dimensions. Direct three-dimensional (3-D) measurements, using focused ion beam serial sectioning and electron backscattered diffraction (EBSD), were performed and a 3-D microstructure was reconstructed. These measurements were supplemented by two-dimensional (2-D) EBSD and scanning electron microscopy on extracted WC grains. The data from 2-D EBSD collected on planar sections were transformed to three dimensions using a recently developed statistical method based on an iterative inverse Saltykov procedure. This stereological analysis revealed that the assumed spherical shape of WC grains during the Saltykov method is reasonable and the estimated 3-D size distribution is qualitatively in good agreement with the actual distribution measured from 3-D EBSD. Although the spherical assumption is generally fair, the WC grains have both faceted and rounded surfaces. This is a consequence of the relatively low amount of liquid phase during sintering, which makes impingements significant. Furthermore, the observed terraced surface structure of some WC grains suggests that 2-D nucleation is the chief coarsening mechanism to consider.

Place, publisher, year, edition, pages
2013. Vol. 61, no 13, 4726-4733 p.
Keyword [en]
Three-dimensional EBSD, Three-dimensional characterisation, Carbides, Coarsening, Inverse Saltykov
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-125544DOI: 10.1016/j.actamat.2013.05.008ISI: 000321601400003Scopus ID: 2-s2.0-84879422588OAI: oai:DiVA.org:kth-125544DiVA: diva2:640271
Funder
Vinnova
Note

QC 20130813

Available from: 2013-08-13 Created: 2013-08-09 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Aspects of Structural Evolution in Cemented Carbide – Carbide Size, Shape and Stability
Open this publication in new window or tab >>Aspects of Structural Evolution in Cemented Carbide – Carbide Size, Shape and Stability
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cemented carbide is a composite material used in applications like cutting tools and rock drilling inserts. The material commonly consists of WC grains embedded in a Co-rich binder phase and the material properties strongly depend on the WC grain size. Hence, to tailor the properties it is important to understand the fundamental mechanisms of grain coarsening. At the same time, the higher demands on material properties today also require new solutions. In the present work, some different aspects of structural evolutions in cemented carbides have been investigated.

The first part of the work considers WC grain coarsening by means of size, size distribution and shape. Some efforts of the work have been to evaluate the effects of C-activity and initial WC powder size and distribution on the coarsening behavior in the material using different characterization techniques, e.g. scanning electron microscopy, and electron backscattered diffraction. Additionally, two earlier developed models are used and evaluated with the experimental data. The results indicate that the C-activity will affect size, size distribution and shape of the WC grains. It was also observed that the initial WC powder size and size distribution will have a large influence on the WC grain coarsening. The statistical shape was found to fit a spherical approximation but for individual grains both faceted and non-faceted shapes was observed. Steps and planar defects were observed supporting that the nucleation of new atomic layers is the main rate limiting mechanism for grain coarsening.

The second part of this work considers the carbide phase stability in the (Ti,Zr)C system. The phase stability was investigated after synthesizing and aging a mixed (Ti,Zr)C using X-ray diffraction and different types of electron microscopy techniques. A decomposed lamellar structure was found with a composition variation of approximately 10% between the 50-75 nm thick lamellas. The experimental investigations were supported by computational work and the results were in good agreement. Additionally, two cemented carbide related systems were studied. A miscibility gap was found in the two investigated systems, (Ti,Zr,W)(C,N)-Co or Fe-graphite, and the effect of N2-gas pressure was investigated suggesting a critical N2-gas pressure below 0.1 bar.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xii, 72 p.
Keyword
Cemented carbide, (Ti, Zr)C, Coarsening, Phase separation, C-activity, initial WC powder size, electron backscattered diffraction, microscopy, X-ray diffraction
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-133933 (URN)978-91-7501-944-4 (ISBN)
Public defence
2013-12-13, F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
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Note

QC 20131118

Available from: 2013-11-18 Created: 2013-11-13 Last updated: 2013-11-18Bibliographically approved

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Borgh, IdaOdqvist, JoakimBorgenstam, Annika

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