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Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis
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International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455 www.elsevier.com/locate/IJRMHM
Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis Karin Mannesson a, Mattias Elfwing b, Alexandra Kusoffsky b, Susanne Norgren b, ˚ gren a,* John A a
Division of Physical Metallurgy, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b Sandvik Tooling, R&D Materials and Processes, SE-126 80 Stockholm, Sweden Received 31 August 2007; accepted 18 October 2007
Abstract The WC carbide grain size is important for the technological properties of cemented carbide cutting tools. In the present work the WC carbide grain size distribution is determined after milling and sintering for 0.25, 1 and 8 h at 1430 °C. The WC grain size distribution, both for the powder after milling and the sintered specimens, is determined by two different methods, i.e. image analysis on scanning electron microscopy (SEM) images and electron backscatter diffraction (EBSD). It should be noted that in this work the 2D grain size distribution is considered. The EBSD analysis clearly shows that the special R2 boundaries are present in the powder and that their fraction decreases during sintering and particularly during the early stages. When the R2 boundaries are omitted in the EBSD analysis the results of the grain size measurements for the two methods agree quite well. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Carbides; Grain size; Image analysis; R2 boundaries
1. Introduction Cemented carbides are made by liquid-phase-sintering during which the grain size increases through coarsening. In so-called normal grain growth there is a gradual increase in grain size but no drastic change in the shape of the grain size distribution function except for its increase in average grain size. Abnormal grain growth, i.e. the excessive growth of some grains leading to a few very large grains, is often observed in cemented carbides. The abnormal grains have a negative influence on the properties and thus it is very important to learn how to avoid abnormal grain growth. It is also important to understand grain growth in general since the average grain size is important for the
*
Corresponding author. Tel.: +46 8 7909131; fax: +46 8 100411. ˚ gren). E-mail address: [email protected] (J. A
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.10.004
properties. Thus many studies have been performed on grain growth in cemented carbides. However, it is difficult to measure the grain size accurately and many different methods have been used. Often image analysis based on SEM images is used, but a drawback is that the resolution makes it hard to detect small grains and the method is very time-consuming. EBSD can be used to obtain better spatial resolution, since the images are constructed from crystallographic information. A problem with both methods is that abnormally large grains can be missed since the magnification is adjusted to see the most frequently occurring grain size. It should also be observed that the evaluations are performed on 2D cross-sections and in this work no attempts are done to transform the 2D grain size distribution to 3D. Kumar et al. [1] recently used EBSD to characterize grain boundaries during sintering of WC–Co cemented carbides. In particular they observed so-called R2 boundaries in WC. Such boundaries exhibit low interfacial energy and
450
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
they are usually not wetted by Co during sintering [2–4]. Lay and Loubradou [5] suggested that these grain boundaries were present already in the powder. Also Kim et al. [6] confirmed that the R2 grain boundaries present in liquid-phase-sintered WC–Co alloys originate from the powder. However, although they occur frequently in the powder directly after milling, they are largely annihilated during sintering [1]. At this point a word of caution is needed. The CSL (coincident-site-lattice) concept only refers to the relative orientation of two grains and does not take into account the orientation of the grain boundary itself. Usually the two special cases of twist and tilt boundary are considered but in general the grain boundary between two crystals of a given orientation relationship could have any orientation. One expects the low-energy only for some grain boundary orientations. For the case of R2 boundaries in WC the twist boundary represents the lowest energy [2]. Such a boundary is created by cutting the crystal along a ð1 0 1 0Þ plane and rotate one half 90° around its normal. The purpose of the present work is to investigate if EBSD is suitable for determination of the grain size distribution function during grain growth. The WC grain size distribution will be obtained after sintering using both image analysis based on SEM images and EBSD analysis. The similarities and differences between the methods are discussed. Especially the influence of the R2 boundaries are of interest since they are hard to observe using conventional image analysis. The present study is part of an attempt to model the WC carbide grain growth. The model [7] is based on 2D nucleation of growth and shrinkage ledges. The time evolution of the size distribution function is obtained by solving the non-steady-state Langer–Schwartz equation numerically. Also experiments are performed on a series of WC– 10 wt%Co powders with different grain size for a number of time and temperature sintering cycles. The comparison with experimental data is critical when validating the model and therefore it is important to have accurate experimental data. It is also essential to understand the differences between data obtained by the two techniques. 2. Experimental procedure 2.1. Materials A powder with a composition of WC–10 wt%Co was produced using raw material from H.C. Starck with a nominal WC grain size of 0.9 lm according to the Fisher sub sieve sizer (FSSS). The powder was milled for 40 h and heated with a rate of 10 °C/min to 1430 °C, sintered for a number of different holding times (15 min, 1 and 8 h) and furnace cooled. The grain size was determined both for the sintered specimens and for the powder before sintering after removal of pressing agent and Co. As shown by Wang et al. [8] and Kim et al. [9] the WC shape is strongly dependent on the C activity and therefore
Table 1 Physical properties of the specimens Holding time (h) 3
Density (g/cm ) HC a (kA/m) Sb a b
0.25
1
8
14.56 15.3 0.84
14.56 14.7 0.84
14.57 8.15 0.83
Coercivity. Relative magnetic saturation to Co (by weight).
care has been taken to produce alloys with the same C activity at the different sintering times. In Table 1 some physical properties of the specimens are given. Since the relative magnetic saturation does not vary the C activity is the same in all specimens. 2.2. Image analysis The samples were mounted in bakelite and polished using diamond suspension down to 1 lm and thereafter polished with colloidal silica as a final step to obtain a smooth surface. To reveal the grain boundaries the polished samples were etched in 20% Murakami solution for 1 min. To analyze the powder after milling, before sintering, first the polymeric binder was removed and secondly the Co was removed using hydrochloric acid. After removing the polymeric binder and the Co-content, the powder was mounted in Cu-powder and heated at 1100 °C for 10 min and thereafter the same preparation technique as for the sintered samples was used. The image analysis was performed on 10 randomly set fields per sample each field containing 300–600 particles. The images were obtained using backscatter electron mode in the SEM and the magnification was adjusted for every sample to obtain a relevant number and size of the grains to be analyzed. Equivalent circle diameter was used for grain size determination and the grain boundaries were marked by hand before using the image analysis program for grain detection and size determination. 2.3. EBSD analysis For the EBSD analysis the samples were prepared using the same polishing method as for the image analysis but subsequently ion etched using Ar+ ions. This etching method is used to ensure that a large area for high-quality electron backscatter patterns is obtained. The microscope was operated at 20 kV accelerating voltage and the sample was tilted 70° towards the EBSD detector. The step size used was 30 nm for the powder, 50 nm, 50 nm and 130 nm for sintering during 15 min, 1 h and 8 h, respectively. The pixel size is quadratic, i.e step size times step size. We have chosen to define 1 pixel as the smallest grain size, which seems reasonable since such a grain is easy to detect in the SEM. The grain boundaries are defined as an angle deviation of 2° or more in the crystal orientation between two adjacent grid points. Special grain boundaries, e.g. twin bound-
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
aries and coincidence site lattice boundaries, can be detected and classified using EBSD. In WC, R2 grain boundaries are commonly observed. However, their technological importance is still not clear. Using EBSD, it is possible to perform WC grain size measurements by treating the R2 grain boundaries as ordinary WC/WC boundaries (two WC crystals separated by a R2 boundary will be treated and measured as two individual grains), or by leaving the R2 grain boundaries out of the analysis (two WC crystals separated by a R2 boundary will be treated and measured as one grain). In the present work both approaches are considered and compared to image analysis to evaluate the importance of the R2 boundaries on the grain growth modeling. 3. Results and discussion Figs. 1–3 show the evolution of the microstructure during sintering by means of optical microscope and EBSD images. The results of the grain size measurements from the image and EBSD analysis are shown in Tables 2 and 3 and are also presented as cumulative particle size distributions, shown in Figs. 4–7. The EBSD results are presented both as ‘‘EBSD incl. R2’’, where the R2 boundaries are treated as ordinary grain boundaries and ‘‘EBSD excl. R2’’, where the R2 boundaries were omitted from the analysis. When analyzing the powder after milling before sintering it becomes clear that the R2 grain boundaries are not revealed when imaging with SEM backscatter mode. It is therefore
451
necessary to use the EBSD technique, that is based on crystallographic information, to reveal the R2 boundaries. The measured grain size of the powder using image analysis differs significantly from the FSSS grain size. This is partly due to agglomerates, that are seen as one grain when using FSSS and partly due to the fact that the image analysis is performed on a section and thus gives a smaller average grain size than FSSS. The average grain size is smaller when the R2 boundaries are included in the EBSD analysis because such grain boundaries divide large grains into smaller subgrains. The exclusion of the R2 boundaries does not always give a better agreement with the image analysis data. However when looking at the full grain size distribution the correspondence between EBSD and image analysis is better when the R2 boundaries are excluded. In the distribution curve for the powder, Fig. 4, a flat slope is obtained which reveals a very broad particle size distribution with a large fraction of fine grains. The agreement between the image analysis and EBSD analysis, without R2 boundaries, is satisfactory, although the resolution of the image analysis limits the detected number of very small grains. The EBSD curve, when including the R2 boundaries, is slightly shifted towards a smaller grain size as a result of the large number of very small grains, which origin from the small grains connected to a larger grain with a R2 boundary. After sintering for 15 min a much steeper curve is obtained which indicates that many of the fine grains have
Fig. 1. The microstructure using optical microscope. (a) Powder after milling, (b) sintered for 15 min, (c) sintered for 1 h and (d) sintered for 8 h.
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Fig. 2. The microstructure using EBSD. The R2 boundaries are seen in white and the normal grain boundaries in black. The WC grains are red and the Co binder phase is blue. (a) Powder after milling, the yellow phase is Cu, (b) sintered for 15 min, (c) sintered for 1 h and (d) sintered for 8 h.
Fig. 3. Typical R2 boundaries after (a) 15 min and (b) 8 h revealing the typical straightening-up process.
dissolved, Fig. 5. There is still a satisfactory agreement between the image analysis and EBSD curve without R2 boundaries. Also here the EBSD curve including R2 boundaries is shifted to a smaller grain size. When comparing the cumulative curves after 0 h and 0.25 h it is clear that no matter which technique is used, EBSD or image analysis, the particle size distribution changes from poly-disperse towards Gaussian. After longer sintering times a better agreement between image analysis and EBSD with and without the R2 grain boundaries is obtained. This is mainly caused by the
decreased density of the R2 grain boundaries but also partly caused by the fact that the fine grains are more easy to detect in EBSD than in image analysis. At longer time the fraction of fine grains decreases. As can be seen in Fig. 2 there are many R2 grain boundaries already in the powder but their fraction decreases with increasing sintering time. These observations thus confirm the observations by Kumar et al. [1]. In particular it should be noted that the R2 boundaries are usually curved and consequently they are generally not of the low-energy twist type. There should thus be a
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
453
Table 2 Number of grains analyzed and analyzed area for the different techniques Holding time (h)
0
0.25
Analysis technique No. of grains Area (lm2)
Image analysis 4098 –
Analysis technique No. of grains Area (lm2)
Image analysis 3398 2890
EBSD excl R2 8136 –
EBSD incl R2 12,099 –
Image analysis 4237 2890
EBSD excl R2 4049 2480
EBSD incl R2 5074 2480
Image analysis 5882 29,700
1
EBSD excl R2 4816 2480
EBSD incl R2 6245 2480
EBSD excl R2 4297 16,300
EBSD incl R2 4951 16,300
EBSD excl R2 0.67 3.15 0.056
EBSD incl R2 0.58 3.08 0.056
EBSD excl R2 1.52 16.8 0.15
EBSD incl R2 1.40 16.8 0.15
8
Table 3 A compilation of the grain diameter of the samples measured using EBSD and image analysis Holding time (h)
0
0.25
Analysis technique Average (lm) Max (lm) Min (lm)
Image analysis 0.30 1.42 0.068
Analysis technique Average (lm) Max (lm) Min (lm)
Image analysis 0.68 5.10 0.091
EBSD excl R2 0.26 1.00 0.034
EBSD incl R2 0.21 0.96 0.034
Image analysis 0.61 3.34 0.091
EBSD excl R2 0.72 3.25 0.080
EBSD incl R2 0.64 3.16 0.056
Image analysis 1.37 19.9 0.28
1
8
100 90 80
100
incl Σ2 excl Σ2
90
image analysis
80
60 50 40
image analysis
60 50 40
30
30
20
20
10
10
0 −2 10
excl Σ2
70
Cumulative %
Cumulative %
70
incl Σ2
−1
10
0
Diameter [ μm]
10
1
10
0 −2 10
−1
10
0
Diameter [ μm]
10
1
10
Fig. 4. Grain size distribution, given in number of particles, for the powder before sintering.
Fig. 5. Grain size distribution, given in number of particles, after sintering for 15 min at 1430 °C.
driving force for rearranging the R2 boundaries, i.e. to straighten them up along the ð1 0 1 0Þ planes. This is clearly seen in Fig. 3. To be able to analyze the occurrence of R2 boundaries a new measure is introduced, the density of R2 boundaries measured as the absolute length of R2 boundaries per area unit of WC in the analyzed area. This information is given in Table 4, together with the fraction of all WC/WC boundaries that are R2. It is clearly seen that the density of R2 boundaries decreases during sintering, most rapidly during the early stages. The difference between the EBSD
curves with and without the R2 boundaries decreases dramatically after sintering for 15 min, Fig. 5. This is in agreement with previous observations by Kumar et al. who also observed that the fraction of R2 boundaries decreased during sintering. They suggested that this decrease may promote to the rapid grain growth in the early stages. To the present authors the role of the R2 boundaries seems less clear. As already pointed out they are typically curved after milling and there is a driving force for straighten them up to become parallel with the ð1 0 1 0Þ planes. It should not be ruled out that some portions of
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K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455 100
80
incl Σ2
excl Σ2
image analysis
image analysis
70
Cumulative %
excl Σ2
1.4
Average grain size [μm]
90
1.6 incl Σ2
60 50 40 30
1.2
1
0.8
0.6
20
0.4 10 0 −2 10
−1
10
0
Diameter [ μm]
1
10
10
incl Σ2 excl Σ2 image analysis
70
Cumulative %
1
2
3
4
5
6
7
8
Fig. 8. Average grain size versus time.
R2 boundaries and the WC coarsening as two parallel and independent processes. One may speculate over the origin of the R2 boundaries. It seems reasonable to believe that they appear during the growth of WC and then forms as straight boundaries with the low-energy twist boundary orientation. This would thus be similar to the formation of annealing twins during recrystallization of a deformed metal. Upon the severe plastic deformation during milling the R2 boundaries curve and during sintering they tend to their energetically most favorable form.
100
80
0
Time [h]
Fig. 6. Grain size distribution, given in number of particles, after sintering for 1 h at 1430 °C.
90
0.2
60 50 40 30
4. Conclusions
20 10 0 −2 10
−1
10
0
Diameter [ μm]
1
10
10
Fig. 7. Grain size distribution, given in number of particles, after sintering for 8 h at 1430 °C.
Table 4 R2 fraction of all WC/WC boundaries in the analyzed area and the density of R2 boundaries (measured as absolute length of R2 boundaries per area unit of WC in the analyzed area) Holding time (h)
0a
0.25
1
8
Fraction R2 (%) Length R2/area unit WC (lm1)
– 1.5
12.5 0.38
11.2 0.31
6.7 0.05
a
A good agreement between image analysis and EBSD for WC grain size measurements is obtained if the R2 grain boundaries are considered as special boundaries which are omitted from the grain detection routine. An interesting observation for both techniques is that the WC grain size distribution is poly-disperse in the powder before milling and Gaussian after sintering. Despite the fact that the amount of R2 boundaries decrease rapidly during the initial stages of sintering we believe that they have no direct effect on the WC grain growth, Fig. 8. We thus suggest that the presence of R2 boundaries do not need to be included in modeling of the grain growth, since the image analysis gives enough information to describe the grain size evolution during sintering.
Heating for 10 min at 1100 °C.
Acknowledgements such curved boundaries may even be wetted by Co. Anyhow, the curved parts of the R2 boundaries should be similar to incoherent grain boundaries and have a considerable mobility. In general the straightening-up process would lead to a lower content of R2 boundaries. It seems reasonable, as a first approximation, to consider the migration of
This is a part of a project financed by the Brinell Centre Inorganic Interfacial Engineering (BRIIE), supported by the Swedish Agency for Innovation Systems (VINNOVA), AB Sandvik Coromant, Seco Tools AB and Atlas Copco Secoroc AB. The authors would like to thank professor G. Wahnstro¨m for stimulating discussions. Dr. Bo Jansson
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
at Seco Tools AB and Dr. Go¨ran Stenberg at Atlas Copco Secoroc AB are also acknowledged. References [1] Kumar V, Fang ZZ, Wright SI, Nowell MM. An analysis of grain boundaries and grain growth in cemented carbide using orientation imaging microscopy. Metall Mater Trans A 2006;37A:599–607. [2] Christensen M, Wahnstro¨m G. Effects of cobalt intergranular segregation and interface energetics in WC–Co. Acta Mater 2004;52(8): 2199–207. ¨ stberg G, Farooq MU, Christensen M, Andre´n H-O, Klement U, [3] O Wahnstro¨m G. Effect of R2 grain boundaries on plastic deformation of WC–Co cemented carbides. Mater Sci Eng A 2006;416(1):119–25.
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[4] Kim C-S, Rohrer GS. Geometric and crystallographic characterization of WC surfaces and grain boundaries in WC–Co composites. Interf Sci 2004;12:19–27. [5] Lay S, Loubradou M. Characteristics and origin of clusters in submicron WC–Co cermets. Philos Mag 2003;83:2669–79. [6] Kim J-D, Kang S-J, Lee J-W. Formation of grain boundaries in liquidphase-sintered WC–Co alloys. J Am Ceram Soc 2005;88:500–3. ˚ gren J [in press]. [7] Mannesson K, A [8] Wang Y, Heusch M, Lay S, Allibert CH. Microstructure evolution in the cemented carbides WC–Co I. Effect of the C/W ratio on the morphology and defects of the WC grains. Phys Stat Sol 2002;193:271–83. [9] Kim S, Han S-H, Park J-K, Kim H-E. Variation of WC grain shape with carbon content in the WC–Co alloys during liquid-phase sintering. Scripta Mater 2003;48:635–9.
International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455 www.elsevier.com/locate/IJRMHM
Analysis of WC grain growth during sintering using electron backscatter diffraction and image analysis Karin Mannesson a, Mattias Elfwing b, Alexandra Kusoffsky b, Susanne Norgren b, ˚ gren a,* John A a
Division of Physical Metallurgy, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden b Sandvik Tooling, R&D Materials and Processes, SE-126 80 Stockholm, Sweden Received 31 August 2007; accepted 18 October 2007
Abstract The WC carbide grain size is important for the technological properties of cemented carbide cutting tools. In the present work the WC carbide grain size distribution is determined after milling and sintering for 0.25, 1 and 8 h at 1430 °C. The WC grain size distribution, both for the powder after milling and the sintered specimens, is determined by two different methods, i.e. image analysis on scanning electron microscopy (SEM) images and electron backscatter diffraction (EBSD). It should be noted that in this work the 2D grain size distribution is considered. The EBSD analysis clearly shows that the special R2 boundaries are present in the powder and that their fraction decreases during sintering and particularly during the early stages. When the R2 boundaries are omitted in the EBSD analysis the results of the grain size measurements for the two methods agree quite well. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Carbides; Grain size; Image analysis; R2 boundaries
1. Introduction Cemented carbides are made by liquid-phase-sintering during which the grain size increases through coarsening. In so-called normal grain growth there is a gradual increase in grain size but no drastic change in the shape of the grain size distribution function except for its increase in average grain size. Abnormal grain growth, i.e. the excessive growth of some grains leading to a few very large grains, is often observed in cemented carbides. The abnormal grains have a negative influence on the properties and thus it is very important to learn how to avoid abnormal grain growth. It is also important to understand grain growth in general since the average grain size is important for the
*
Corresponding author. Tel.: +46 8 7909131; fax: +46 8 100411. ˚ gren). E-mail address: [email protected] (J. A
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.10.004
properties. Thus many studies have been performed on grain growth in cemented carbides. However, it is difficult to measure the grain size accurately and many different methods have been used. Often image analysis based on SEM images is used, but a drawback is that the resolution makes it hard to detect small grains and the method is very time-consuming. EBSD can be used to obtain better spatial resolution, since the images are constructed from crystallographic information. A problem with both methods is that abnormally large grains can be missed since the magnification is adjusted to see the most frequently occurring grain size. It should also be observed that the evaluations are performed on 2D cross-sections and in this work no attempts are done to transform the 2D grain size distribution to 3D. Kumar et al. [1] recently used EBSD to characterize grain boundaries during sintering of WC–Co cemented carbides. In particular they observed so-called R2 boundaries in WC. Such boundaries exhibit low interfacial energy and
450
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
they are usually not wetted by Co during sintering [2–4]. Lay and Loubradou [5] suggested that these grain boundaries were present already in the powder. Also Kim et al. [6] confirmed that the R2 grain boundaries present in liquid-phase-sintered WC–Co alloys originate from the powder. However, although they occur frequently in the powder directly after milling, they are largely annihilated during sintering [1]. At this point a word of caution is needed. The CSL (coincident-site-lattice) concept only refers to the relative orientation of two grains and does not take into account the orientation of the grain boundary itself. Usually the two special cases of twist and tilt boundary are considered but in general the grain boundary between two crystals of a given orientation relationship could have any orientation. One expects the low-energy only for some grain boundary orientations. For the case of R2 boundaries in WC the twist boundary represents the lowest energy [2]. Such a boundary is created by cutting the crystal along a ð1 0 1 0Þ plane and rotate one half 90° around its normal. The purpose of the present work is to investigate if EBSD is suitable for determination of the grain size distribution function during grain growth. The WC grain size distribution will be obtained after sintering using both image analysis based on SEM images and EBSD analysis. The similarities and differences between the methods are discussed. Especially the influence of the R2 boundaries are of interest since they are hard to observe using conventional image analysis. The present study is part of an attempt to model the WC carbide grain growth. The model [7] is based on 2D nucleation of growth and shrinkage ledges. The time evolution of the size distribution function is obtained by solving the non-steady-state Langer–Schwartz equation numerically. Also experiments are performed on a series of WC– 10 wt%Co powders with different grain size for a number of time and temperature sintering cycles. The comparison with experimental data is critical when validating the model and therefore it is important to have accurate experimental data. It is also essential to understand the differences between data obtained by the two techniques. 2. Experimental procedure 2.1. Materials A powder with a composition of WC–10 wt%Co was produced using raw material from H.C. Starck with a nominal WC grain size of 0.9 lm according to the Fisher sub sieve sizer (FSSS). The powder was milled for 40 h and heated with a rate of 10 °C/min to 1430 °C, sintered for a number of different holding times (15 min, 1 and 8 h) and furnace cooled. The grain size was determined both for the sintered specimens and for the powder before sintering after removal of pressing agent and Co. As shown by Wang et al. [8] and Kim et al. [9] the WC shape is strongly dependent on the C activity and therefore
Table 1 Physical properties of the specimens Holding time (h) 3
Density (g/cm ) HC a (kA/m) Sb a b
0.25
1
8
14.56 15.3 0.84
14.56 14.7 0.84
14.57 8.15 0.83
Coercivity. Relative magnetic saturation to Co (by weight).
care has been taken to produce alloys with the same C activity at the different sintering times. In Table 1 some physical properties of the specimens are given. Since the relative magnetic saturation does not vary the C activity is the same in all specimens. 2.2. Image analysis The samples were mounted in bakelite and polished using diamond suspension down to 1 lm and thereafter polished with colloidal silica as a final step to obtain a smooth surface. To reveal the grain boundaries the polished samples were etched in 20% Murakami solution for 1 min. To analyze the powder after milling, before sintering, first the polymeric binder was removed and secondly the Co was removed using hydrochloric acid. After removing the polymeric binder and the Co-content, the powder was mounted in Cu-powder and heated at 1100 °C for 10 min and thereafter the same preparation technique as for the sintered samples was used. The image analysis was performed on 10 randomly set fields per sample each field containing 300–600 particles. The images were obtained using backscatter electron mode in the SEM and the magnification was adjusted for every sample to obtain a relevant number and size of the grains to be analyzed. Equivalent circle diameter was used for grain size determination and the grain boundaries were marked by hand before using the image analysis program for grain detection and size determination. 2.3. EBSD analysis For the EBSD analysis the samples were prepared using the same polishing method as for the image analysis but subsequently ion etched using Ar+ ions. This etching method is used to ensure that a large area for high-quality electron backscatter patterns is obtained. The microscope was operated at 20 kV accelerating voltage and the sample was tilted 70° towards the EBSD detector. The step size used was 30 nm for the powder, 50 nm, 50 nm and 130 nm for sintering during 15 min, 1 h and 8 h, respectively. The pixel size is quadratic, i.e step size times step size. We have chosen to define 1 pixel as the smallest grain size, which seems reasonable since such a grain is easy to detect in the SEM. The grain boundaries are defined as an angle deviation of 2° or more in the crystal orientation between two adjacent grid points. Special grain boundaries, e.g. twin bound-
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
aries and coincidence site lattice boundaries, can be detected and classified using EBSD. In WC, R2 grain boundaries are commonly observed. However, their technological importance is still not clear. Using EBSD, it is possible to perform WC grain size measurements by treating the R2 grain boundaries as ordinary WC/WC boundaries (two WC crystals separated by a R2 boundary will be treated and measured as two individual grains), or by leaving the R2 grain boundaries out of the analysis (two WC crystals separated by a R2 boundary will be treated and measured as one grain). In the present work both approaches are considered and compared to image analysis to evaluate the importance of the R2 boundaries on the grain growth modeling. 3. Results and discussion Figs. 1–3 show the evolution of the microstructure during sintering by means of optical microscope and EBSD images. The results of the grain size measurements from the image and EBSD analysis are shown in Tables 2 and 3 and are also presented as cumulative particle size distributions, shown in Figs. 4–7. The EBSD results are presented both as ‘‘EBSD incl. R2’’, where the R2 boundaries are treated as ordinary grain boundaries and ‘‘EBSD excl. R2’’, where the R2 boundaries were omitted from the analysis. When analyzing the powder after milling before sintering it becomes clear that the R2 grain boundaries are not revealed when imaging with SEM backscatter mode. It is therefore
451
necessary to use the EBSD technique, that is based on crystallographic information, to reveal the R2 boundaries. The measured grain size of the powder using image analysis differs significantly from the FSSS grain size. This is partly due to agglomerates, that are seen as one grain when using FSSS and partly due to the fact that the image analysis is performed on a section and thus gives a smaller average grain size than FSSS. The average grain size is smaller when the R2 boundaries are included in the EBSD analysis because such grain boundaries divide large grains into smaller subgrains. The exclusion of the R2 boundaries does not always give a better agreement with the image analysis data. However when looking at the full grain size distribution the correspondence between EBSD and image analysis is better when the R2 boundaries are excluded. In the distribution curve for the powder, Fig. 4, a flat slope is obtained which reveals a very broad particle size distribution with a large fraction of fine grains. The agreement between the image analysis and EBSD analysis, without R2 boundaries, is satisfactory, although the resolution of the image analysis limits the detected number of very small grains. The EBSD curve, when including the R2 boundaries, is slightly shifted towards a smaller grain size as a result of the large number of very small grains, which origin from the small grains connected to a larger grain with a R2 boundary. After sintering for 15 min a much steeper curve is obtained which indicates that many of the fine grains have
Fig. 1. The microstructure using optical microscope. (a) Powder after milling, (b) sintered for 15 min, (c) sintered for 1 h and (d) sintered for 8 h.
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K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
Fig. 2. The microstructure using EBSD. The R2 boundaries are seen in white and the normal grain boundaries in black. The WC grains are red and the Co binder phase is blue. (a) Powder after milling, the yellow phase is Cu, (b) sintered for 15 min, (c) sintered for 1 h and (d) sintered for 8 h.
Fig. 3. Typical R2 boundaries after (a) 15 min and (b) 8 h revealing the typical straightening-up process.
dissolved, Fig. 5. There is still a satisfactory agreement between the image analysis and EBSD curve without R2 boundaries. Also here the EBSD curve including R2 boundaries is shifted to a smaller grain size. When comparing the cumulative curves after 0 h and 0.25 h it is clear that no matter which technique is used, EBSD or image analysis, the particle size distribution changes from poly-disperse towards Gaussian. After longer sintering times a better agreement between image analysis and EBSD with and without the R2 grain boundaries is obtained. This is mainly caused by the
decreased density of the R2 grain boundaries but also partly caused by the fact that the fine grains are more easy to detect in EBSD than in image analysis. At longer time the fraction of fine grains decreases. As can be seen in Fig. 2 there are many R2 grain boundaries already in the powder but their fraction decreases with increasing sintering time. These observations thus confirm the observations by Kumar et al. [1]. In particular it should be noted that the R2 boundaries are usually curved and consequently they are generally not of the low-energy twist type. There should thus be a
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
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Table 2 Number of grains analyzed and analyzed area for the different techniques Holding time (h)
0
0.25
Analysis technique No. of grains Area (lm2)
Image analysis 4098 –
Analysis technique No. of grains Area (lm2)
Image analysis 3398 2890
EBSD excl R2 8136 –
EBSD incl R2 12,099 –
Image analysis 4237 2890
EBSD excl R2 4049 2480
EBSD incl R2 5074 2480
Image analysis 5882 29,700
1
EBSD excl R2 4816 2480
EBSD incl R2 6245 2480
EBSD excl R2 4297 16,300
EBSD incl R2 4951 16,300
EBSD excl R2 0.67 3.15 0.056
EBSD incl R2 0.58 3.08 0.056
EBSD excl R2 1.52 16.8 0.15
EBSD incl R2 1.40 16.8 0.15
8
Table 3 A compilation of the grain diameter of the samples measured using EBSD and image analysis Holding time (h)
0
0.25
Analysis technique Average (lm) Max (lm) Min (lm)
Image analysis 0.30 1.42 0.068
Analysis technique Average (lm) Max (lm) Min (lm)
Image analysis 0.68 5.10 0.091
EBSD excl R2 0.26 1.00 0.034
EBSD incl R2 0.21 0.96 0.034
Image analysis 0.61 3.34 0.091
EBSD excl R2 0.72 3.25 0.080
EBSD incl R2 0.64 3.16 0.056
Image analysis 1.37 19.9 0.28
1
8
100 90 80
100
incl Σ2 excl Σ2
90
image analysis
80
60 50 40
image analysis
60 50 40
30
30
20
20
10
10
0 −2 10
excl Σ2
70
Cumulative %
Cumulative %
70
incl Σ2
−1
10
0
Diameter [ μm]
10
1
10
0 −2 10
−1
10
0
Diameter [ μm]
10
1
10
Fig. 4. Grain size distribution, given in number of particles, for the powder before sintering.
Fig. 5. Grain size distribution, given in number of particles, after sintering for 15 min at 1430 °C.
driving force for rearranging the R2 boundaries, i.e. to straighten them up along the ð1 0 1 0Þ planes. This is clearly seen in Fig. 3. To be able to analyze the occurrence of R2 boundaries a new measure is introduced, the density of R2 boundaries measured as the absolute length of R2 boundaries per area unit of WC in the analyzed area. This information is given in Table 4, together with the fraction of all WC/WC boundaries that are R2. It is clearly seen that the density of R2 boundaries decreases during sintering, most rapidly during the early stages. The difference between the EBSD
curves with and without the R2 boundaries decreases dramatically after sintering for 15 min, Fig. 5. This is in agreement with previous observations by Kumar et al. who also observed that the fraction of R2 boundaries decreased during sintering. They suggested that this decrease may promote to the rapid grain growth in the early stages. To the present authors the role of the R2 boundaries seems less clear. As already pointed out they are typically curved after milling and there is a driving force for straighten them up to become parallel with the ð1 0 1 0Þ planes. It should not be ruled out that some portions of
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K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455 100
80
incl Σ2
excl Σ2
image analysis
image analysis
70
Cumulative %
excl Σ2
1.4
Average grain size [μm]
90
1.6 incl Σ2
60 50 40 30
1.2
1
0.8
0.6
20
0.4 10 0 −2 10
−1
10
0
Diameter [ μm]
1
10
10
incl Σ2 excl Σ2 image analysis
70
Cumulative %
1
2
3
4
5
6
7
8
Fig. 8. Average grain size versus time.
R2 boundaries and the WC coarsening as two parallel and independent processes. One may speculate over the origin of the R2 boundaries. It seems reasonable to believe that they appear during the growth of WC and then forms as straight boundaries with the low-energy twist boundary orientation. This would thus be similar to the formation of annealing twins during recrystallization of a deformed metal. Upon the severe plastic deformation during milling the R2 boundaries curve and during sintering they tend to their energetically most favorable form.
100
80
0
Time [h]
Fig. 6. Grain size distribution, given in number of particles, after sintering for 1 h at 1430 °C.
90
0.2
60 50 40 30
4. Conclusions
20 10 0 −2 10
−1
10
0
Diameter [ μm]
1
10
10
Fig. 7. Grain size distribution, given in number of particles, after sintering for 8 h at 1430 °C.
Table 4 R2 fraction of all WC/WC boundaries in the analyzed area and the density of R2 boundaries (measured as absolute length of R2 boundaries per area unit of WC in the analyzed area) Holding time (h)
0a
0.25
1
8
Fraction R2 (%) Length R2/area unit WC (lm1)
– 1.5
12.5 0.38
11.2 0.31
6.7 0.05
a
A good agreement between image analysis and EBSD for WC grain size measurements is obtained if the R2 grain boundaries are considered as special boundaries which are omitted from the grain detection routine. An interesting observation for both techniques is that the WC grain size distribution is poly-disperse in the powder before milling and Gaussian after sintering. Despite the fact that the amount of R2 boundaries decrease rapidly during the initial stages of sintering we believe that they have no direct effect on the WC grain growth, Fig. 8. We thus suggest that the presence of R2 boundaries do not need to be included in modeling of the grain growth, since the image analysis gives enough information to describe the grain size evolution during sintering.
Heating for 10 min at 1100 °C.
Acknowledgements such curved boundaries may even be wetted by Co. Anyhow, the curved parts of the R2 boundaries should be similar to incoherent grain boundaries and have a considerable mobility. In general the straightening-up process would lead to a lower content of R2 boundaries. It seems reasonable, as a first approximation, to consider the migration of
This is a part of a project financed by the Brinell Centre Inorganic Interfacial Engineering (BRIIE), supported by the Swedish Agency for Innovation Systems (VINNOVA), AB Sandvik Coromant, Seco Tools AB and Atlas Copco Secoroc AB. The authors would like to thank professor G. Wahnstro¨m for stimulating discussions. Dr. Bo Jansson
K. Mannesson et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 449–455
at Seco Tools AB and Dr. Go¨ran Stenberg at Atlas Copco Secoroc AB are also acknowledged. References [1] Kumar V, Fang ZZ, Wright SI, Nowell MM. An analysis of grain boundaries and grain growth in cemented carbide using orientation imaging microscopy. Metall Mater Trans A 2006;37A:599–607. [2] Christensen M, Wahnstro¨m G. Effects of cobalt intergranular segregation and interface energetics in WC–Co. Acta Mater 2004;52(8): 2199–207. ¨ stberg G, Farooq MU, Christensen M, Andre´n H-O, Klement U, [3] O Wahnstro¨m G. Effect of R2 grain boundaries on plastic deformation of WC–Co cemented carbides. Mater Sci Eng A 2006;416(1):119–25.
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