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Abnormal grain growth in cemented carbides - Experiments and simulations
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
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: 0000-0002-7656-9733
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0002-4521-6089
2011 (English)In: International journal of refractory metals & hard materials, ISSN 0958-0611, E-ISSN 2213-3917, Vol. 29, no 4, 488-494 p.Article in journal (Refereed) Published
Abstract [en]

The grain size and the grain size distribution are two of the most important factors when tailoring the mechanical properties of cemented carbides. In the present work the effect on the growth behavior when adding some abnormal grains in an initial fine grained powder is studied. It is clearly seen that abnormal grains in a fine grained matrix lead to faster grain growth and a higher average grain size.

Place, publisher, year, edition, pages
2011. Vol. 29, no 4, 488-494 p.
Keyword [en]
Cemented carbide, grain growth, abnormal grain growth, bimodal, sintering, modeling
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-32427DOI: 10.1016/j.ijrmhm.2011.02.008ISI: 000291138600013Scopus ID: 2-s2.0-79955530750OAI: oai:DiVA.org:kth-32427DiVA: diva2:410617
Note

QC 20110701

Available from: 2011-04-14 Created: 2011-04-14 Last updated: 2017-12-11Bibliographically approved
In thesis
1. WC grain growth during sintering of cemented carbides: Experiments and simulations
Open this publication in new window or tab >>WC grain growth during sintering of cemented carbides: Experiments and simulations
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cemented carbides are composite materials consisting of a hard carbide and a ductile binder. They are powdermetallurgically manufactured, where liquid-phase sintering is one of the main steps. The most common cemented carbide consists of WC and Co and it is widely used for cutting tools. Two of the most important factors controlling the mechanical properties are the WC grain size and the grain size distribution and thus it is of great interest to understand the grain growth behavior.

In this thesis the grain growth during sintering at 1430 °C is studied both experimentally and through computer simulations. The grain growth behavior in cemented carbides cannot be explained from the classical LSW-theory. The WC grains have a faceted shape necessitating growth by 2-D nucleation of new atomic layers or surface defects. A new model based on 2-D nucleation, long-range diffusion and interface friction is formulated.

Three powders having different average sizes are studied and both experiments and simulations show that a fine-grained powder may grow past a coarse-grained powder, indicating that abnormal grain growth has taken place in the fine-grained powder. Fine-grained powders with various fractions of large grains are also studied and it is seen that a faster growth is obtained with increasing fraction of large grains and that an initially slightly bimodal powder can approach the logaritmic normal distribution after long sintering times.

The grain size measurements are performed on 2-D sections using image analysis on SEM images or EBSD analysis. Since the growth model is based on 3-D size distributions the 2-D size distributions have to be transformed to 3-D, and a new method, Inverse Saltykov, is proposed. The 2-D size distribution is first represented with kernel estimators and the 3-D size distribution is optimized in an iterative manner. In this way both negative values in the 3-D size distribution and modifications of the raw data are avoided.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. vii, 42 p.
Keyword
Cemented carbide, Grain growth, Abnormal grain growth, Image analysis, Modeling, 3-D grain size distribution, Inverse Saltykov
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-32430 (URN)978-91-7415-915-8 (ISBN)
Public defence
2011-05-16, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110426Available from: 2011-04-26 Created: 2011-04-14 Last updated: 2011-04-26Bibliographically approved
2. 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)
Opponent
Supervisors
Note

QC 20131118

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

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Borgh, IdaBorgenstam, Annika

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