Evolution of the WC grain shape in WC–Co alloys during sintering: Effect of C content

Int. Journal of Refractory Metals & Hard Materials 27 (2009) 140–148

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Evolution of the WC grain shape in WC–Co alloys during sintering: Effect of C content Aurélie Delanoë, Sabine Lay * Science et Ingénierie des Matériaux et Procédés, INPGrenoble-CNRS-UJF, BP75, 38402 St Martin-d’Hères Cedex, France

a r t i c l e

i n f o

Article history: Received 31 January 2008 Accepted 3 June 2008

Keywords: WC–Co Grain shape Effect of C and Co content

a b s t r a c t After liquid phase sintering in the Co binder WC grains are faceted. The common shape of WC is a prism based on a truncated triangle. The habit planes are two prismatic facets and the basal plane. In this work, the grain shape evolution is quantified at several stages of the sintering treatment using transmission electron microscopy. Two shape factors are used to measure the anisotropy between the two prismatic facets and between the prismatic and basal facets. The influence of the C content in the alloy is studied. The C/W ratio has a limited effect on the anisotropy between prismatic facets while a significant effect is recorded for the anisotropy between the basal and prismatic facets. The results are discussed as a function of the factors influencing the grain shape: grain growth, difference in energy between the facets and contacts with other WC grains. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction WC–Co cemented carbides are usually obtained by liquid phase sintering of a mixture of WC and Co powders. They consist in faceted WC grains embedded in a Co rich binder. The shape of WC grains is a truncated triangle prism terminated by two kinds of {1 0 1 0} prismatic facets and the (0 0 0 1) basal plane [1]. Wetting experiments and hardness measurements show the significant difference between the properties of the two sets of prismatic planes [2]. The WC grain shape is an important parameter to control during sintering since it influences the mechanical properties of the tools [3]. Depending on the application either very anisotropic WC grains [4] or on the contrary ‘‘rounded” grains [5] are required. This morphology likely results from the difference in energy [6–8] and in growth rate [9,10] between the facets. Grain boundaries are also expected to influence the grain shape since numerous contacts between WC grains occur at low Co content. They can modify the interface energy and hinder the shape evolution of the grains. Moreover the WC grain morphology was shown to depend on the C/W ratio of the alloy [11–13] and to be affected by the addition of inhibitors [14,15]. Very little experimental work has been devoted to the quantification of the grain shape [16] and to the effect of composition [9,13,17]. Any theoretical or experimental data should help in understanding the factors influencing the morphology during sintering. In this work, the morphology of WC grains is characterised in the powder and at several stages of the sintering

* Corresponding author. Tel.: +33 (0) 476826628; fax: +33 (0) 476826744. E-mail addresses: [email protected] (S. Lay), [email protected] (S. Lay). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.06.001

treatment using transmission electron microscopy (TEM). The effect of the C/W ratio is studied with different carbon contents. Two shape factors are used to measure the anisotropy between prismatic planes and the one between prismatic and basal planes. In order to evaluate the effect of grain boundaries on the final shape, the results are compared with alloys with a larger amount of Co binder. 2. Materials and method 2.1. Alloy preparation Two compositions lying in the three phase fields {WC + Co rich binder + Cg} and {WC + Co rich binder + M6C (M = W, Co)} were selected to study the effect of the C content [18]. In what follows the M6C phase is referred to as g. The Co content was chosen equal to 13 at% that is a usual value for the manufacture of tools. The C/W ratios were equal to 1.04 for the WC–13Co,C alloy and to 0.96 for WC–13Co,W (Table 1). The alloys were prepared from WC and Co powders with 0.9 lm and 1.5 lm in size respectively and graphite or W powders were added to adjust the C content. The powder mixtures were ball milled for 5 h. The compacts were heated in a graphite furnace for various times and temperatures using the processing conditions described in detail in [11]. The sintering treatment was interrupted in order to study the evolution of the WC grain shape after 1 h at 1200 °C, during the heating stage at 1350 °C, at 1450 °C and finally after 2 h at 1450 °C. The melting temperature of the binder was 1290 °C for WC–13Co,C and 1355 °C for WC–13Co,W [20]. The comparison between the C/W ratio before and after sintering shows that the alloys remain in

A. Delanoë, S. Lay / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 140–148 Table 1 Compositions of alloys and volume fraction of liquid at 1450 °C calculated from [19] Starting composition (at %)

WC–13Co,C* WC–13Co,W* WC–13CoC(10 h) WC–13Co,W(10 h) WC–34Co,C(10 h) WC–34Co,W(10 h)

Co

W

C

13 13 12.6 12.6 34.5 34.5

42.55 44.4 42.4 45.0 29.3 36.2

44.45 42.8 45.0 42.4 36.2 29.3

Fraction of liquid (vol %)

0.19 0.18 0.18 0.15 0.47 0.35

The star indicates that the composition was measured after 2 h at 1450 °C while for the other alloys the starting composition is given.

the same three phase field despite a small C loss during sintering. The effect of a long liquid phase sintering time was studied using a treatment of 10 h at 1450 °C. In order to limit the variation of the C/ W ratio, two other compositions called WC–13Co,C(10 h) and WC– 13Co,W(10 h) were used (Table 1). These alloys also lie in the three phase fields with the same Co content but the C/W ratios are equal to 1.2 and 0.8. Moreover the effect of Co content on the grain shape after sintering for 10 h at 1450 °C was studied using the WC– 34Co,C(10 h) and WC–34Co,W(10 h) alloys where the Co content is 34.5 at% and the C/W ratios are also equal to 1.2 and 0.8 respectively. The alloys sintered for 10 h at 1450 °C were prepared from WC powders with 0.85 lm as medium size. The powder mixtures were gentle milled for 30 min to limit any contamination and the holding time at 1200 °C was also longer (12 h). 2.2. Shape factors and experimental procedure 2.2.1. The truncation factor r WC has a hexagonal unit cell (P-6m2) with W atoms at (0, 0, 0) and C atoms at (1/3, 2/3, 1/2) positions (a = 0.2906 nm and c = 0.2837 nm). WC has no symmetry centre so the {1 0 1 0} and {1 0 1 0} facets are not equivalent. All observations report that one set of prismatic facets is significantly more developed than the other one [1] in agreement with interface energy calculations [8]. In this study, the truncation factor r quantifies the ratio between the two sets of observed prismatic facets. When WC grains are observed along h0 0 0 1i their projection is a truncated triangle (Fig. 1). r = Rashort/Ralong is the ratio between the length of the two types of facets: it measures the truncation of the triangle and expresses the anisotropy between the two sets of prismatic facets. In the assumption of an equilibrium shape the r factor depends on the energy of the two types of facets. The broadest facet is called Plong and the shortest Pshort. The broadest facets are related to the smallest energy cPlong and the shortest ones to the highest energy cPshort (1) 2cPlong

r¼

cPshort

1

Plong ð2  ccPshort Þ

141

2.2.2. The elongation factor k When WC grains are observed along the h1 1 2 0i direction their projection is a rectangle. k = t/h is the ratio of the thickness of the prism over the height of the truncated triangle (Fig. 1). The factor k that measures the elongation of the prism expresses the anisotropy between the prismatic facets and the basal facet called B facet. In the assumption of an equilibrium shape, the factor k is related to the cPlong and cB energies of the Plong and B facets by Eq. (2) that takes into account the truncation factor [13]

k¼

2cB ð2r þ 1Þ 3cPlong ðr þ 1Þ

ð2Þ

TEM was used to quantify the grain shape because it permits to precisely orientate the grains along special directions. Thin slices about 30 lm thick were prepared by mechanical thinning. They were then milled by argon ions impinging at an angle of 10°. Observations were performed using a 3010 JEOL microscope. The limitation of TEM is to observe areas less than about 1 lm thick. However, the size of the studied grains is usually larger than 1 lm. The measurement of the r values is valuable whatever the WC grain size because the truncated triangle has the same projection along the [0 0 0 1] direction whatever the specimen thickness. On the contrary if the middle of the WC grain is not contained in the TEM specimen the height h is underestimated and the k factor is overestimated. In order to avoid this problem only the largest grains with the largest height values were considered. This choice induces that only the k values of the largest grains were retained. 2.2.3. Dispersion of the results and precision of the shape factors At least 20 grains were studied for the determination of each shape factor. The values obtained far from the mean value were not considered. For the other measurements a mean value x was determined (3) as well as the standard deviation r to evaluate the scattering of the distribution (4):

1X xi nsffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ðx1  xÞ2 r¼ nðn  1Þ x¼

ð3Þ ð4Þ

where n is the number of measurements. For a distribution with a limited number of values (n < 30) the Student law can be used in statistics to determine a reliable interval for the mean value. In this work the interval x  1.7r/ p p n < x < x + 1.7r/ n was chosen. It corresponds to the probability of 90% that x lie in this interval. In what follows, the mean factors determined in this work will be given according to the relation p x ± 1.7r/ n.

ð1Þ

A truncation factor equal to 1 corresponds to hexagonal grains and expresses the equality between the interfaces energies. On the contrary, a triangular shape is related to a difference in energy.

Fig. 1. Morphology of WC grains in WC–Co alloys showing the (0 0 0 1) basal and {1 0 1 0} prismatic facets.

3. Results 3.1. Comparison of the alloys microstructure sintered at 1450 °C The microstructure of the WC–13Co,C and WC–13Co,W alloys was observed by scanning electron microscopy (SEM) for sintering treatments from 1200 °C to 1450 °C for 2 h in [11]. Especially the grain size evolution at 1450 °C was quantified. Two populations of WC grains were observed: the smallest ones that are the most numerous called the matrix grains and large grains being in the tail of the intercept distributions. The WC grain size was shown to increase more heavily in the C rich alloy but more homogeneously than in the W rich alloy. From the measurement of a limited number of intercepts, at least 600, the grain size distribution was determined in the alloys sintered for 10 h. The microstructure of

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WC–13Co,C sintered for 2 h and WC–13Co,C(10 h) were compared (Fig. 2). In both alloys, the two populations of grains are observed (Fig. 3). In WC–13Co,C(2 h), the matrix grains are in the range 0.5– 2 lm and large grains between 2.5 and 7 lm. In WC–13Co,C(10 h) matrix grains range between 0.5 and 3 lm and large ones between 6 and 15 lm. The mean grain size was determined equal to 1.6 lm in this alloy. When the Co content is increased the range size of the two populations is unchanged but the amount of large grains is increased and the mean grain size increases up to 1.8 lm. In the W rich alloys, the two sets of grains are also present. After 2 h at 1450 °C, the matrix grains range between 0.5 and 1.5 lm and the large ones between 2 and 6 lm. In WC–13Co,W(10 h) the matrix grains are slightly smaller than in WC–13Co,W(2 h), they range between 0.3 and 1.2 lm while the largest ones are smaller close to 4 lm. In WC–34Co,W(10 h) nearly no difference compared to WC–13Co,W(10 h) is detected. The mean intercept value is unchanged and equal to 0.7 lm in WC–13Co,W(10 h) and in WC– 34Co,W(10 h). 3.2. Study of the WC grain truncation The mixtures of WC and Co powders were first examined in order to determine the starting morphology of WC grains. SEM and TEM show that few isolated WC grains are present in the powder, mainly WC grains agglomerates are observed. Moreover, the grains do not have a prism shape. They are rounded, neither prismatic nor basal facets are observed at the surface of WC grains. A significant part of grain boundaries in the agglomerates are R = 2 grain boundaries in agreement with previous studies [21,22]. These special grain boundaries can be described by a rotation of 90° of one crystal around a h1 0 1 0i axis [23] that gives rise to a very good matching at the boundary [24] (Fig. 4). After the heating stage at 1200 °C for 1 h a significant change of the WC grain shape has taken place whatever the alloy. The majority of WC grains are faceted according to the prismatic and basal facets (Fig. 5). The r values are given as a function of the size of the WC grains that is defined as the side of a triangle with a surface equal to that of the studied truncated triangle. The measurements in WC–Co,C detect a mean value of 0.32 ± 0.05 and in WC–Co,W 0.29 ± 0.06 (Fig. 6) (Table 2). Although a significant scattering is observed the truncation factor is similar

in the two alloys. This result shows that the Plong/Pshort anisotropy occurs at the onset of the sintering stage since r is very different from 1. Moreover it does not depend on the C content at this stage. Some measurements were performed at 1350 °C to check the evolution of the WC grain shape. In WC–13Co,C no significant difference is found after liquid formation while for WC,13Co,W, r is decreased to 0.20 ± 0.03. To make a measurement at the beginning of the liquid formation for WC–13Co,W, r was also measured at 1450 °C–0 h. A close value of 0.24 ± 0.03 was found. After 2 h at 1450 °C, the r values are influenced by the grain size in WC–Co,W (Figs. 7 and 8). In this alloy a large scattering of the measurements and a mean value of 0.35 ± 0.12 is obtained for the grains smaller than about 1.5 lm. When the grain size increases, a decrease of the scattering and an increase of the mean value is recorded with r = 0.48 ± 0.06. This observation likely features the two populations of WC grains detected from Fig. 3b, the smallest grains that are dissolving and the largest that are growing [13]. This effect is not visible in WC–Co,C where a mean value of 0.32 ± 0.04 is measured. In the alloys sintered for 10 h at 1450 °C the difference between the r values is decreased but the truncation value is smaller in the W rich alloy where it is equal to 0.26 ± 0.04 compared to 0.35 ± 0.06 in WC–13Co,C(10 h) (Fig. 9). The effect of the Co content on the truncation factor at the end of the sintering cycle was evaluated after 10 h at 1450 °C. In the W rich alloy the truncation factor is unchanged. A strong effect is observed in WC–34Co,C(10 h) where a smaller scattering occurs and the mean r value decreases from 0.35 to 0.14 [13]. 3.3. Study of the WC grain elongation After heating for 1 h at 1200 °C, WC grains show smooth basal facets whatever the C content (Fig. 10). The distribution of k values is given for both alloys as a function of the grain size (Fig. 11). This grain size is defined as the diameter of a sphere with the same volume as the studied grain. A large scattering is observed for the small sizes. It can be related to the measurement error as discussed in Section 2.2. After a limit size the values are less dispersed. Only the k values measured for the largest grains are considered. The elongation factor has the same value in both alloys, 0.76 ± 0.10 for WC–13Co,C and 0.73 ± 0.08 for WC–13Co,W (Table 3). When

Fig. 2. SEM images of the alloys sintered at 1450 °C (a) WC–13Co,C sintered for 2 h, (b) WC–13Co,C(10 h), (c) WC–34Co,C(10 h), (d) WC–13Co,W sintered for 2 h, (e) WC– 13Co,W(10 h), (f) WC–34Co,W(10 h).

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50

25 WC-34Co,C(10h) WC-13Co,C(10h) WC-13Co,C(2h)

20

WC-34Co,W(10h) WC-13Co,W(10h) WC-13Co,W(2h)

40

f(%)

f(%)

15

30

10

20

5

10

0

0 0

5

10

15

20

0

d (μm)

1

2

3

4

5

6

d (μm)

Fig. 3. (a) Comparison of the intercept distributions in the C rich alloys sintered for 2 h and 10 h at 1450 °C. (b) Comparison of the intercept distributions in the W rich alloys sintered for 2 h and 10 h at 1450 °C.

Fig. 4. (a) SEM micrograph of the WC–13Co,C powder mixture. TEM images of the powder: (b) WC grain observed along h1 1 2 0i, (c) WC grain observed along h0 0 0 1i, (d) WC agglomerate showing R = 2 grain boundaries. The diffraction patterns show the orientation of the crystals.

Fig. 5. Examples of WC grains observed at 1200 °C (a) in WC–13Co,C (r = 0.55) (b) in WC–13Co,W (r = 0.13).

the temperature increases to 1350 °C no significant change is recorded. The composition does not influence the elongation factor at this temperature. After sintering for 2 h at 1450 °C, the elongation is smaller in WC–13Co,C, (0.40 ± 0.03) than in WC–13Co,W (0.59 ± 0.04) (Fig. 12). Grains are therefore flatter in the C rich alloy (Fig. 13). After 10 h at 1450 °C, the difference between the alloys is increased. WC grains are flatter in WC–13Co,C(10 h) (0.28 ± 0.02) and more elongated in WC–13Co,W(10 h) (0.90 ± 0.10) (Fig. 14). The W rich alloy shows the largest scattering. This trend is confirmed in the al-

loys containing a larger amount of Co for which no significant change is measured.

4. Discussion The literature on WC–Co alloys emphasises the effect of the initial powder grain shape and size, the composition and the Co amount on the morphology of WC grains. These parameters modify the interface energies and the WC grain growth that both influence

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1.0 1200ºC-1h

0.9

WC-13Co,W WC-13Co,C

0.8 0.7 0.6

r 0.5 0.4 0.3

WC-13Co,C 0.32

WC-Co,W 0.29

0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

a (µm) Fig. 6. r values recorded after heating at 1200 °C for 1 h versus the grain size in WC–13Co,C and WC–13Co,W.

Table 2 Mean truncation values r, standard deviation of the distribution, number of measurements and reliable interval values recorded for the C rich and W rich alloys Truncation factor C rich

1200 °C–1 h 1350 °C 1450 °C–0 h 1450 °C–2 h (large) 1450 °C–2 h (matrix) 1450 °C–10 h (13Co) 1450 °C–10 h (34Co)

W rich

r

r

n

p 1.7r/ n

0.32 0.35

0.14 0.19

20 31

0.05 0.06

0.32

0.10

19

0.04

0.35 0.14

0.17 0.06

22 11

0.06 0.03

r

r

n

p 1.7r/ n

0.29 0.20 0.24 0.48 0.35 0.26 0.25

0.15 0.09 0.09 0.17 0.27 0.10 0.04

21 20 21 22 14 21 8

0.06 0.03 0.03 0.06 0.12 0.04 0.02

the grain shape. In particular the WC grain growth is enhanced by the use of submicronic WC powders and the presence of a C excess [25,26]. In the C rich alloys, the shape factors are believed to be influenced by the growth of the facets [13] since a rather large growth rate is recorded even after a long sintering time [27]. On the contrary in WC–34Co,W(10 h) the shape is assumed to be at the equilibrium since the growth rate is very limited in W rich alloys [27] and the measured shape factors agree with calculated interface energies [13]. The comparison of WC–13Co,W(2 h) and

WC–13Co,W10 h) alloys shows the presence of larger grains in the alloy sintered for the smallest time. The main difference between the two alloys is the W content that is larger in WC,13Co, W(10 h). The limitation of grain growth in WC,13Co, W(10 h) and also in WC,34Co,W(10 h) could be due to a better distribution of W that would limit the local variations of C potential in the alloy. Moreover in W rich alloys sintered for 10 h, the mean shape factors are very close for matrix and large grains. The shape quantified in WC–13Co,W(2 h) is therefore considered to be influenced by grain growth and the one in WC–34Co,W(10 h) to be at the equilibrium. The comparison of the alloys shows that the WC mean intercept value is slightly increased in WC–34Co,C(10 h) compared to WC– 13Co,C(10 h) alloys while it is equal in WC–13Co,W(10 h) and WC–34Co,W(10 h). In these alloys the volume fraction of liquid is different (Table 1). Up to now few studies have been devoted to the effect of Co content on grain growth. The results obtained in the W rich alloy are similar to those obtained in alloys lying in the two phase field domain {WC + Co rich binder} where no effect of Co content was recorded [28]. On the other hand the intercept increase in the C rich alloy agrees with the work on C rich alloys [29] showing that grain growth is enhanced when the Co amount is increased. The evolution of the two shape factors along the sintering stage summarized below is interpreted within this framework. 4.1. Scattering of the shape factors The scattering of the r and k values can be due to the influence of contacts with other WC grains as will be discussed by comparing the alloys with different amounts of Co. It can also be related to the destruction of the agglomerates present in the powder. The WC powder mainly consists of agglomerates of WC grains related by random or special grain boundaries. A large part of these latter are R = 2 grain boundaries that can be used to study the evolution of grain boundaries during sintering. During liquid phase sintering, the content of R = 2 grain boundaries was shown to decrease [30] what suggests that Co infiltrates WC agglomerates and destroys some WC/WC grain boundaries. Some TEM observations at 1200 °C in the C rich alloy and at 1200 °C and 1350 °C in the W rich alloy show the presence of small holes or Co pools in the grain boundaries (Fig. 15). These latter are already present in the mixture of WC and Co powders after milling. Their presence in grain boundaries was checked only at the solid state and at this stage

Fig. 7. Examples of large WC grains observed at 1450 °C–2 h (a) in WC–13Co,C (r = 0.20), (b) in WC–13Co,W (r = 0.68).

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2.4

1.0

1450ºC-2h

0.9

1200ºC-1h

WC13-Co,W WC-13Co,C

0.8

WC-13Co,C

2.0

WC-13Co,W 1.6

0.7 0.6

r 0.5 0.4

WC-13Co,C 0.7 6

k 1.2

WC-13Co,W 0.48

0.8

0.35

0.3

0.4

WC-13Co,C 0.32

0.2

0.0 0.0

0.1

WC-13Co,W 0.73 0.3

0.5

0.8

1.0

1.3

1.5

1.8

d (µm)

0.0 0

1

2

3

4

6

5

a (µm) Fig. 8. r values recorded after heating at 1450 °C for 2 h versus the grain size in WC–13Co,C and WC–13Co,W.

1 1450ºC-10 h

0.9

WC-13Co,W(10h)

0.8

Fig. 11. k factor versus the grain size in WC–13Co,C and WC–13Co,W after heating at 1200 °C for 1 h.

in the boundaries at the solid state should favour the infiltration of Co in the grain boundaries during liquid phase sintering and cause the destruction of some grain boundaries. This process should lead to the formation of WC grains with an irregular shape. 4.2. Effect of C/W ratio in the WC–13Co alloys

WC-13Co,C(10h)

0.7 0.6

r 0.5 0.4

WC-13Co,C(10h) 0.35

0.3 0.2

WC-13Co,W(10h) 0.26

0.1 0 0

2

4

6

8

10

12

14

a (μm) Fig. 9. r values recorded at 1450 °C–10 h versus the grain size in WC–13Co,C(10 h) and WC–13Co,W(10 h).

the grain boundaries are rather rounded. After liquid phase sintering the grain boundaries become straighter and Co pools or holes are no more detected in these latter. The presence of these defects

At 1200 °C, the same truncation and elongation values are recorded for the C rich and W rich alloys (Figs. 16 and 17). This result agrees with the fact that sintering occurs at the solid state in both alloys and so the shape evolution is expected to be limited at this stage. When the sintering temperature and time are increased, the truncation factor is rather constant in the C rich alloy while it fluctuates in the W rich alloy. Except after 2 h at 1450 °C, it is observed that an excess of C slightly increases the truncation factor. The examination of the k factors recorded along the liquid phase sintering shows a significant decrease in the C rich alloy with a very small scattering while the elongation values fluctuate and show a larger scattering in the W rich alloy except after 2 h at 1450 °C. For this latter as well as for C rich alloys sintered at the liquid state, it is interesting to note that the k value corresponding to a growth shape is associated with a small scattering. The comparison of the values shows that an excess of C heavily decreases the elongation factor.

Fig. 10. Examples of WC grains observed for alloys sintered for 1 h at 1200 °C (a) in WC–13Co,C (k = 0.70), (b) in WC–13Co,W (k = 0.85). The labels B and P correspond to the basal and prismatic facets.

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Table 3 Mean elongation values k, standard deviation of the distribution, number of measurements and reliable interval values recorded for the C rich and W rich alloys

1.8

C rich

1200 °C–1 h 1350 °C 1450 °C–2 h 1450 °C–10 h (13Co) 1450 °C–10 h (34Co)

0.76 0.83 0.4 0.28 0.25

1.2

W rich

r 0.19 0.18 0.05 0.03 0.06

p 1.7 r/ n

n 18 14 12 23 26

WC-13Co,W(10h) WC-13Co,C(10h)

1.4

Elongation factor

k

1450ºC-10h

1.6

0.10 0.10 0.03 0.02 0.02

k 0.73 0.77 0.59 0.9 0.82

r

p 1.7 r/ n

n

0.21 0.21 0.09 0.29 0.21

18 14 12 23 26

0.08 0.10 0.04 0.10 0.07

Only the data for the largest grains are used for the determination of the k factor.

k

1

0.90

0.8 0.6

0.2

0.4

8

0.2 0 0

2

4

6

8

d (μm)

Fig. 14. k factor in WC–13Co,C(10 h) and WC–13Co,W(10 h) sintered for 10 h at 1450 °C versus the grain size.

2.5

1450ºC- 2h

WC-13Co,W

2.0

WC-13Co,C

1.5

k 1.0

0.59 0.5

0.40 0.0 0

2

4

6

d (µm) Fig. 12. k factor in WC–13Co,C and WC–13Co,W sintered for 2 h at 1450 °C versus the grain size.

4.3. Effect of Co content in the alloys sintered for 10 h When the Co content is increased, the r factor significantly decreases in the C rich alloy while it remains constant in the W rich alloy and in both types of alloys, the scattering is reduced. The elongation is unchanged in both alloys. Moreover the comparison of the microstructures shows that the increase of Co does not change much the size range of WC grains whatever the C content. The literature data report that a larger Co amount induces a smaller contiguity of the WC phase [31]. The observations therefore indicate that the contacts with other

WC grains have little influence on the grain shape in the W rich alloy unlike the C rich alloy. This result can be related to the energies of the facets determined in the W rich alloy where the shape is assumed to be at the equilibrium. Ab-initio calculations evaluate the general grain boundary energy at prismatic facets close to 2.7 J/m2 [32] and the average interface energy at prismatic facets in the range 0.7–2.3 J/m2 [17]. The value 0.7 J/m2 corresponds to a completely coherent interface and 2.3 J/m2 to an incoherent interface. To our knowledge no special orientation occurs between WC and Co at prismatic facets. It is therefore likely that the interface energy corresponds to the incoherent state and so is close to that of grain boundary energy. The fact that these energies are close in the W rich alloy agrees with the small influence of contacts on the truncation factor. The results of this work also show that the WC contacts at basal facets have no influence on the shape of WC grains in the W rich alloys. They suggest that the interface energy at basal facets would be of the same order than the one of grain boundaries. This assumption is checked by the experiment. In WC–34Co,W(10 h) r was found equal to 0.25 and k to 0.82. The ratio between the cB and cPlong interface energies can be calculated from (2) and is found very close to 1. This result indicates similar energies for the basal and the prismatic facets and thus for basal facets and grain boundaries [17,32]. The rather large scattering of k values in WC– 34Co,W(10 h) could be related to this equality. This result is in agreement with observed microstructures showing grains with

Fig. 13. Large WC grains observed for alloys sintered for 2 h at 1450 °C (a) in WC–13Co,C (k = 0.33), (b) in WC–13Co,W (k = 0.69).

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147

Fig. 15. R = 2 grain boundaries showing Co pools: (a) WC–13Co,W sintered at 1200 °C for 1 h and (b) WC–13Co,W heated up to 1350 °C.

r

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1200ºC 1350ºC 1450ºC-0h 1450ºC-2h 1450ºC-10h 1450ºC-10h-34Co

Small grains

Large grains

0.35 0.25

0.14 WC-Co,C

WC-Co,W

Fig. 16. Summary of the r values measured in C rich and W rich alloys versus the temperature.

1.6 1200ºC 1350ºC 1450ºC-2h 1450ºC-10h 1450ºC-10h-34Co

1.4 1.2 1

k

0.82

0.8 0.6

of the C and Co content. The observation of the WC grain shape in the powder and during heating shows that the faceting of WC grains occurs in the solid state. At the onset of the sintering process in the solid state the C content has no effect on the WC grain shape. During liquid phase sintering, the morphology evolves as a function of the composition. For the low Co content, the grains are a little less triangular on the C rich side than on the W rich side. They become much less elongated in the C rich alloys when the sintering time increases while a limited evolution is observed in the W rich alloys. For a larger Co content the grains become more triangular for the C rich alloy. The large scattering of the shape factors is partly due to the contact with other WC grains and decreases when the Co amount increases. It can also be due to the disintegration of grain boundaries during liquid phase sintering. In the C rich alloy, the shape is believed to be influenced by the grain growth. This effect is likely enhanced by the use of submicron WC powder for the preparation of the alloys. In the W rich alloy where the shape is assumed to be at the equilibrium for the longest heating time and the largest Co content, the results indicate that the starting shape observed at the solid state is nearly at the equilibrium. This result could explain the limited driving force for grain growth observed in W rich alloys. Acknowledgements

0.4

0.25

0.2 0 WC-Co,C

WC-Co,W

Fig. 17. Summary of k values measured in C rich and W rich alloys versus the temperature.

This work was supported financially by Sandvik Hard Materials and by the French National Association for Technical Research (ANRT). The authors are grateful to Dr C.H. Allibert, INPGrenoble, and E. Pauty, Sandvik Hard Materials, for fruitful discussion and advice. References

partly rounded habit planes due to the contact with other WC grains in W rich alloys [33]. It is noticeable that at 1200 °C the shape corresponds nearly to the equilibrium shape in the W rich alloys. This observation and the close values found for the grain boundary and interface energy could explain the smaller grain growth recorded in these alloys compared to C rich alloys. 5. Summary In this work, an experimental approach is used to quantify the WC grain shape during the sintering of WC–Co alloys as a function

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