International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

www.elsevier.com/locate/ijrmhm

Effect of TaC on plastic deformation ofWC–Co and Ti(C,N)–WC–Co

Gustaf Ostberg a,*, Katharina Buss b, Mikael Christensen a, Susanne Norgren c,Hans-Olof Andren a, Daniele Mari b, Goran Wahnstrom a, Ingrid Reineck c

a Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goteborg, Swedenb Ecole Polytechnique Federale de Lausanne, Institut de Physique de la Matiere Complexe, CH-1015 Lausanne, Switzerland

c R&D Materials and Processes, AB Sandvik Coromant, SE-126 80 Stockholm, Sweden

Received 11 November 2004; accepted 12 April 2005

Abstract

The plastic deformation resistance of metal cutting inserts made from a WC–Co cemented carbide, a Ti(C, N)–WC–Co cermet

and corresponding materials with additions of TaC has been studied. The cermets were produced with both high and low carbon

activity, resulting in a total of six materials. Ab initio calculations of some WC/WC grain boundary geometries suggest that both Co

and Ta segregate substitutionally to the boundary and improve the grain boundary strength when substituting carbon. However,

only Co segregation was found experimentally, probably due to (Ta, W)C formation. Plastic deformation tests were performed with

a turning operation under controlled conditions. For the WC–Co, the addition of Ta had a positive effect for lower cutting speeds

but at higher speeds the effect was negative. Three-point bending tests indicated a beneficial effect of Ta in WC–Co, which was also

confirmed by internal friction (IF) measurements. However, after thermal cycling, the effect of Ta could be smaller, or even negative.

The Ta cermet produced with low carbon activity exhibited a better plastic deformation resistance during cutting but no apparent

effects of Ta could be seen either in IF measurements or in three-point bending tests of the cermets. However, a correlation was

found between plastic deformation during turning and IF spectra. In the cermet materials, binder phase lamella formation promotes

grain boundary sliding at high temperatures.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Cemented carbides; Characterisation; Plastic deformation; DFT; Internal friction

1. Introduction

Cemented carbides are composite materials with a

hard carbide or carbonitride skeleton embedded in a

tough binder metal and are commonly used in metal cut-

ting since they have an outstanding ability to resist high

temperatures and loads.

In metal cutting industry, the largest economical sav-

ings are usually made by increasing the number of pro-

0263-4368/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2005.04.010

* Corresponding author. Tel.: +46 31 772 3325; fax: +46 31 772 3224.

E-mail address: gusto@fy.chalmers.se (G. Ostberg).

duced parts per time unit. This means that cutting

speeds have to be increased, resulting in high tool loadsand working temperatures, which can be as high as

1000 �C. Due to the excellent resistance to abrasive wear

of modern wear resistant coated tools, these severe cut-

ting conditions can cause the material to deform plasti-

cally before any significant wear by other mechanisms

has occurred. Thus, it is often the deformation of the

tool, rather than the abrasive wear at its surface, that

determines its lifetime.Another study of plastic deformation of cemented

carbide cutting tools [1] has shown that the hard phase

skeleton is broken up during deformation and films of

146 G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

binder phase form between the grains and facilitate

grain boundary sliding.

Over the years different measures have been taken to

improve the deformation resistance of cemented car-

bides and cermets. One example is to reduce the grain

size, and thereby increase the hardness, by adding tran-sition metal carbides, such as VC and Cr3C2 [2–4]. In

cermets, Mo2C has been commonly used due to its

favourable effect on the wetting between the ceramic

skeleton and the binder phase which improves the

mechanical properties of the material [5]. Different

studies of mechanical properties have also been per-

formed on materials where Mo2C was added [6–10].

Other cubic carbides like TiC and TaC have been usedto increase the hardness of WC–Co since they have a

higher hardness than WC [11]. The advantageous effect

of TaC on cermets has also been reported by Rolander

et al. [12] who found that tantalum increased the plas-

tic deformation (PD) resistance during metal cutting.

It was suggested that tantalum should influence the

interfacial energies, resulting in a stronger hard phase

skeleton.The purpose of this study, which can be considered as

a continuation of the work of Rolander et al., is to get a

fundamental understanding of how Ta affects the micro-

structure, and thereby the PD resistance, of cemented

carbides and cermets. Plastic deformation of cutting in-

serts by turning tests under realistic conditions are com-

pared to internal friction (IF) and three-point bending

measurements under controlled conditions. The micro-structure is characterised with scanning electron micros-

copy (SEM) and transmission electron microscopy

(TEM) and the chemistry is analysed with energy dis-

persive X-ray analysis (EDX). Furthermore, first princi-

ple ab initio calculations are made on the WC–Co

system to predict segregation tendencies and interfacial

energies.

2. Materials and experimental procedures

Six model alloys were studied in this work and their

compositions after sintering are listed in Table 1. The

cermets were produced in two versions; one with high

(HC) and one with a lower carbon (LC) activity. All

Table 1

Composition as determined by chemical analysis after sintering (at%) of the

Alloy Co Ta

WC–Co 9.71 0.00

WC–Co–Ta 9.79 1.06

Ti(C, N)–WC–Co–HC 9.13 0.00

Ta–Ti(C, N)–WC–Co–HC 8.80 2.10

Ti(C, N)–WC–Co–LC 9.27 0.00

Ta–Ti(C, N)–WC–Co–LC 9.23 2.02

materials were designed to have the same binder volume

fraction of 10.2% at sintering temperature and the W-

solution in the Co-binder was to be comparable for

the cemented carbides and the HC cermets, resulting

in the unusually high carbon additions for a cermet.

Ta was added in amounts above the solubility limit inorder to avoid coarse sluggish precipitation of Ta into

the cubic (Ta, W)C phase in the cemented carbide. In-

stead, the nuclei of the cubic carbide can be retained

in the liquid at sintering temperature in order to precip-

itate the cubic phase formed during cooling on an al-

ready present phase. Moreover, the cermets were made

with as similar Ti/W (�10) and N/(C + N) (�0.37) ratios

as possible. The alloys were produced by powder metal-lurgical methods including mixing by milling, spray-dry-

ing, pressing and sintering. The materials were pressed

as triangular cutting inserts for the deformation testing

and as bars for the preparation of IF and three-point

bending specimens. Sintering temperatures were

1410 �C for the cemented carbides and 1480 �C for the

cermets. After sintering, the cutting inserts were coated

with a 5 lm TiC/Ti(C, N) layer to minimize abrasivewear. Some basic properties of the sintered materials

are shown in Table 2.

2.1. Turning tests

Plastic deformation (PD) of the materials was mea-

sured as the depression of the rake face of the insert after

a radial turning (facing) operation under controlled con-ditions. A cylindrical workpiece made from SS 2541

(0.34 wt% C, 1–1.5 wt% Cr, 3 wt% Ni, Mo) steel was

cut with a depth of cut of 1 mm and a feed of 0.3 mm/

rev. The cutting speed was kept constant during each

test and the cutting time was 30 s. Tests were performed

at cutting speeds between 300 and 475 m/min and for

every cutting speed a new cutting insert was used. Two

of such test sequences were performed for all six materi-als. In Fig. 1, the average plastic deformation of the two

test sequences are plotted for each material.

2.2. Electron microscopy

All scanning electron microscopy was performed with

a Leo Ultra55 FEG-SEM equipped with an Oxford

six model alloys studied in this work

W Ti N C

45.49 0.00 0.00 44.80

44.25 0.00 0.00 44.90

3.92 42.08 16.41 28.46

3.85 40.04 14.71 30.49

3.83 42.33 16.72 27.86

3.93 40.13 16.34 28.35

Table 2

General properties of the six alloys studied

Alloy Density

(g/cm3)

Porosity Grain sizea

(lm)

HV3 Co-magnetic W in ss

(at%)bCoercivity

(lT/cm3)

WC–Co 14.97 A02B00C00 2.10 1493 5.10 3.12 13.12

WC–Co–Ta 14.76 A06B08C00 2.59 1405 5.18 2.85 13.31

Ti(C, N)–WC–Co–HC 6.34 A04B02C00 2.05 1594 11.63 4.34 13.96

Ta–Ti(C, N)–WC–Co–HC 6.64 A02B02C00 2.52 1473 11.47 3.50 12.03

Ti(C, N)–WC–Co–LC 6.3 A04B04C00 Not measured 1468 11.34 4.77 13.92

Ta–Ti(C, N)–WC–Co–LC 6.67 A04B02C00 Not measured 1478 9.46 6.62 13.82

a Measured by mean linear intercept method.b Calculated from the Co-magnetic measurements.

Plastic deformation

0

0.2

0.4

0.6

0.8

1

1.2

300 350 400 450 475Cutting speed (m/min)

PD (m

m)

WC-Co Hi C cermet Lo C cermetWC-Co + Ta Hi C cermet + Ta Lo C cermet + Ta

Fig. 1. Results from plastic deformation tests. Every point in each

curve is an average of two measurements.

G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154 147

INCA system for EDX analysis. TEM analyses were

performed in a Philips CM200 FEG-TEM equipped

with a Link Isis� EDX system.

SEM samples were prepared by cutting cross-

sections through the deformed cutting edge as is shown

in Fig. 2. Further details on the procedures for material

Fig. 2. SEM micrograph showing a top-view of the deformed cutting

edge of an insert. The highlighted area marks the material removed

when making the cross-section.

analysis and sample preparation are described elsewhere

[1].

2.3. Three-point bending

Three-point bending tests were performed at a con-

stant strain rate of _e ¼ 1.5� 10�5 s�1. Carbide skeleton

samples were produced by etching the cobalt. Bending

tests were performed on complete and skeleton samples,

with size 35 · 7 · 3.5 mm3, at different temperatures be-

tween 800 and 1200 �C. Details about the procedure aregiven elsewhere [1].

2.4. Mechanical spectroscopy

Mechanical spectroscopy is used to determine the IF

which is a measure of the dissipative processes, e.g., dif-

fusion and dislocation movements, acting at different

temperatures. The IF was measured both at constantfrequency, as a function of temperature and at constant

temperature as a function of frequency on bars,

35 · 4 · 1 mm3 in size. A more thorough description of

the method can be found elsewhere [1].

3. Ab initio calculations—method

Ab initio calculations were performed for ten differ-

ent translation states in the WC(0001)/WC(1�210)asymmetric tilt boundary, shown in Fig. 3. In five of

the translational states, the close packed (0001) plane

interface is metal terminated, and in the other five it is

carbon terminated.

In the calculations of the energetics, only the internal

energy given by the total energies obtained from densityfunctional theory (DFT) is taken into account. This is the

dominating contribution to the interface energetics, and

all other terms are neglected based on the assumption

of them being small [13] together with a tendency for can-

cellation of temperature-dependent terms for differences

in free energy between the considered structures [14].

Further details about the calculations are given else-

where [1,15].

Fig. 3. Relative position of interface atoms at all studied grain

boundary geometries containing segregated (Co, Ta) metal atoms.

Segregated atoms are white, C atoms black, and W atoms grey. The

atoms in two close packed (0001) layers, and one (1�210) layer are

displayed. Atoms in the same layer are connected.

148 G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

4. Results

4.1. Turning tests

From Fig. 1, it can be seen that the PD resistance of

WC–Co is unaffected or slightly improved by the Ta

addition at cutting speeds up to 450 m/min but at higher

speeds the Ta containing material undergoes larger

deformation.

For the cermets the behaviour is very different

between the high and low carbon materials. The PDresistance for the HC cermet is strongly impaired by

the addition of Ta whereas for the low carbon material

it is clearly improved over the whole range of measure-

ment.

4.2. Characterisation

In Fig. 4, the microstructures of the undeformedmaterials are compared. At a first glance, the morphol-

ogies of the cemented carbides with and without Ta look

quite similar. However, except for the presence of cubic

(W, Ta)C grains the WC grains (a-phase) are also

slightly larger and have a less faceted grain shape in

the Ta material. Some porosity and a less homogeneous

binder phase distribution can also be seen when Ta is

added.When comparing the morphologies of the HC cer-

mets, quite large differences can be seen between the

Ta and no-Ta material. The fraction of rim area is high-

er and the fraction of cores is lower in the hard phase

grains (c-phase) of the Ta material. Furthermore, the

grains are more rounded and the grain size is signifi-

cantly larger. To explain this, thermodynamic calcula-

tions of the effect of Ta on the melting point of thebinder phase were made. In Fig. 5, the melting temper-

ature is plotted as a function of carbon activity during

sintering and it can be seen that Ta lowers the melting

temperature overall and also makes the melting point

more sensitive to variations in carbon activity. Obvi-

ously, the liquid state sintering has been taken to a later

stage due to the lowered melting point of the binder

phase, which explains the morphology of the HC Ta cer-met. Thus, a direct comparison should not be done be-

tween the turning test results of the high carbon

cermets, since the two materials represent completely

different microstructures.

In the LC cermets, there are small differences between

the morphologies of the Ta and no-Ta materials (Fig.

4(e) and (f)). The most obvious difference is that the rims

of the hard phase grains in the Ta cermet have higherbrightness, indicating a higher solution of heavy

elements.

TEM-EDX linescans of grain and phase boundaries

in the Ta materials are shown in Fig. 6. The EDX probe

size is 8 nm at FWHM and, if considering beam broad-

ening effects in the specimen, the total probe size should

be slightly larger than 10 nm at FWHM. Hence, in Fig.

6(a) and (d) there are small overlaps of the spots. A clearindication of segregation of Co can be seen both in the

a/a boundaries and the c/c boundaries. However, nei-

ther in the grain boundaries, nor in the Co/WC phase

boundaries, any signs of Ta segregation or enrichment

could be seen. It should be noted that the presence of

Co in the hard phase grains is caused by background

noise and is only apparent. The Ta content in WC is also

just apparent due to the overlap between the Ta and Wpeaks in the EDX spectrum. Hence, what is seen as Ta

in the WC is just the tail of the W peak.

In Fig. 7 the six materials are shown after deforma-

tion by turning testing. All materials exhibit a partly

broken up hard phase skeleton where binder phase has

infiltrated some grain boundaries and formed lamellae

between the grains. This process has been described pre-

viously [1].In Fig. 8, the deformed region of the Ta cemented

carbide has been outlined. Apparently, the porosity

which can be seen in the undeformed bulk is absent.

A noticeable difference seen in the TEM between the

Ta and no-Ta HC cermet is that there appears to be a

larger misfit between the cores and the rims in the hard

phase grains (see Fig. 9). The deformed HC Ta cermet

also seems to have wider and more frequently occurringbinder lamellae than the no-Ta material. However, this

is most likely just a sign of the higher degree of deforma-

tion rather than a direct effect of the Ta addition.

4.3. Three-point bending

Stress–strain curves at different temperatures for the

cemented carbide and the cermets are shown in Fig.10(a) and (b), respectively, for comparison between the

no-Ta and Ta materials. For the WC–Co, the addition

Fig. 4. SEM micrographs of the undeformed materials. (a) WC–Co, (b) WC–Co with Ta, (c) high carbon cermet, (d) high carbon cermet with Ta,

(e) low carbon cermet and (f) low carbon cermet with Ta. The somewhat uneven contrast of the binder phase is due to topographic effects.

G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154 149

of Ta leads to an increase of the flow stress at 900 and

1000 �C. As expected, the fracture strain decreases

whereas the flow stress increases. However, at 1200 �C,the flow stress of the material with Ta is equivalent tothat of the WC–Co without Ta.

The cermets show surprising results in three-point

bending. Despite the significant differences that they dis-

play in the turning test results, all four cermets perform

very similarly at all temperatures in three-point bending.

Apparently, three-point bending is not sensitive enough

for the mechanisms that lead to the deformation of the

cutting edge or the test conditions are too different toallow a comparison.

4.4. Mechanical spectroscopy

In general, the WC–TaC–Co shows a lower IF than

WC–Co and the IF is clearly reduced all along the tem-

perature spectrum (Fig. 11). However, at high tempera-

ture, the structure with tantalum seems more unstable.

Whereas the IF of WC–Co without Ta is very stable,

i.e., heating and cooling are very similar, the IF of

WC–TaC–Co increases at high temperature and a hys-

teresis is formed above 1125 �C. Moreover, it can be ob-served that the IF continues to increase at high

temperature upon repeated thermal cycling and above

1180 �C the IF of WC–TaC–Co becomes even higher

than that of WC–Co. In fact, the increase of IF at high

temperature in the Ta containing sample is due to the

presence of a peak named PW4, which cannot be seen

in the temperature spectra at 1 Hz [1]. As can be seen

in Fig. 12 the maximum of this peak is located at1125 �C at 10�2 Hz and PW4 is enhanced by the presence

of Ta.

The general temperature spectrum of the cermets

resembles very much the one formerly presented [16].

However, at the maximum temperature of the tempera-

ture scans, the IF increases during a 2 h annealing. This

increase of IF is different for the four cermets. A cycle

composed of a heating and cooling scan (Fig. 13) leads

Fig. 5. Thermodynamical calculations showing the effect of Ta content

on the melting temperature of the binder phase as a function of carbon

concentration.

150 G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

to the formation of a hysteresis, the area of which is very

different depending on the type of cermet.

WC/WC boundary in WC-TaC-Co

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100 110

Distance (nm)

Distance (nm)

Com

posi

tion

(at%

)

Atomic% Co KAtomic% Ta L

Com

posi

tion

(at%

)

Com

posi

tion

(at%

)

(Ti,Ta,W)(C,N)/Co boundary in HC Ta cermet

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80

Atomic% Co KAtomic% Ta LAtomic% W LAtomic% Ti K

a b

dc

Fig. 6. TEM-EDX linescans of (a) a WC/WC boundary in WC–TaC–Co,

boundary in the HC Ta cermet and (d) a (Ti, W, Ta)(C, N)/Ti(C, N) bounda

grains is an artifact due to background noise. In (d) Ti is used as balance.

4.5. Ab initio calculations

The heat of segregation to WC/WC grain boundary

has been calculated for Co and Ta. In each case, the seg-

regant is replacing a tungsten (carbon) atom in the

(1�210) interface plane in tungsten (carbon) terminatedgrain boundaries (Fig. 3). The results for segregation

to all studied grain boundary geometries as well as seg-

regation to free WC surfaces are given in Table 3. Posi-

tive values indicate situations where segregation is

energetically favourable.

It can be seen that segregation to free WC surfaces is

favourable for Co, but not for Ta. The propensity for

Co to adsorb on the WC surface can be related to thegood wetting of Co on WC.

For segregation to WC/WC grain boundaries, it is

found that Ta will not segregate to any tungsten termi-

nated boundaries, while it might be favourable for Co to

segregate. Segregation to carbon terminated boundaries

is highly favourable for both Co and Ta, although the

tendency for segregation is larger for Co.

The effect of segregated intergranular atoms on thegrain boundary strength has also been investigated.

The strength is taken to be the ideal work of separation,

Wsep, calculated as the difference between the sum of

Distance (nm)

Distance (nm)

WC/Co boundary in WC-TaC-Co

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140

Com

posi

tion

(at%

) Atomic% Co KAtomic% Ta LAtomic% W L

(Ti,Ta,W)(C,N)/Ti(C,N) boundary in HC Ta cermet

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120 140

Atomic% Co KAtomic% Ta LAtomic% W L

(b) a Co/WC boundary in WC–TaC–Co, (c) a Co/(Ti, Ta, W)(C, N)

ry in the HC Ta cermet. Note: The presence of Co and Ta in the WC

Fig. 7. SEM micrographs of the plastically deformed materials. (a) WC–Co deformed at 475 m/min, (b) WC–Co with Ta deformed at 475 m/min, (c)

high carbon cermet deformed at 500 m/min, (d) high carbon cermet with Ta deformed at 450 m/min, (e) low carbon cermet deformed at 400 m/min

and (f) low carbon cermet with Ta deformed at 450 m/min. Some examples of binder phase lamellae have been marked by white arrows. The direction

of the total macroscopic force is marked with black arrows. Note that the somewhat uneven contrast of the binder phase is due to topographic

contrast.

Fig. 8. SEM micrograph of the deformed WC–TaC–Co. The image

shows an overview of a cutting edge cross-section. The porosity (black

spots) seen in the bulk region is absent in the deformed region.

G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154 151

energies of the free cleavage surfaces and the interface

energy of the intact boundary.

The results for the change in Wsep due to the presence

of segregants are given in Table 4. It is found that both

elements probably have a detrimental effect on the

boundary strength when present in tungsten terminatedboundaries. In contrast, both Co and Ta have a very

large strengthening effect in carbon terminated bound-

aries, where the effect is somewhat larger for Ta.

5. Discussion

5.1. WC–Co

In line with the results from the cutting tests, the

three-point bending measurements show an improved

resistance to plastic deformation of the WC–Co

Fig. 9. TEM micrographs of typical hard phase grains in the undeformed high carbon cermet without Ta (a) and with Ta (b). The material with Ta

apparently has more misfit dislocations in the interface between the core and the rim (marked with arrows).

Fig. 10. The effect of Ta on three-point bending deformation of (a) the

WC–Co and (b) the cermets at different high temperatures.

Fig. 11. IF heating–cooling cycle between RT and 1480 K of WC–Co

and WC–TaC–Co (at 1 Hz). Heating and cooling are indicated by

arrows.

Fig. 12. Frequency scans of WC–Co and WC–TaC–Co showing the

increase of PW4 upon the Ta addition.

152 G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

with Ta at intermediate temperatures (up to 1000 �C).However, this positive effect has disappeared at

1200 �C. Also in IF measurements, the tantalum seems

to have different effects on WC–Co at low and high

Fig. 13. IF heating–cooling cycle between RT and 1207 �C of all four

cermet grades (at 1 Hz). Heating is the lower and cooling the upper

curve of each hysteresis. During isothermal measurements at 1207 �Cthe IF continues to increase.

Table 3

Heat of segregation to free WC surfaces for Co and Ta

Surface Esegr (eV/atom)

Co Ta

WC(1210)-SubW 0.42 �0.09

WC(1210)-SubC 0.04 �1.07

Grain boundary

W-term (a) �0.06 �0.05

W-term (b) 0.26 �0.53

W-term (c) �0.22 �0.28

W-term (d) 0.60 �0.79

W-term (e) �0.52 �0.43

C-term (a) �0.52 �1.79

C-term (b) 2.38 2.36

C-term (c) 1.31 0.69

C-term (d) 2.03 1.52

C-term (e) 1.35 0.17

Table 4

Change in Wsep due to the presence of segregants

Grain boundary DWsep (J/m2)

Co Ta

W-term (a) �0.53 0.05

W-term (b) �0.18 (�0.48)

W-term (c) (�0.71) (�0.20)

W-term (d) 0.19 (�0.77)

W-term (e) �0.27 (�0.37)

C-term (a) (�0.62) (�0.79)

C-term (b) 2.58 3.79

C-term (c) 1.40 1.94

C-term (d) 2.20 2.86

C-term (e) 1.44 1.37

Figures in parenthesis mean that segregation does not occur for any

value of the carbon potential.

G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154 153

temperatures. It has been shown that the PW4 peak is re-

lated to the infiltration of WC grain boundaries by the

cobalt, which thereby enhances grain boundary sliding

[1]. PW4, and possibly boundary infiltration, are en-

hanced by the presence of Ta.

Although predicted by the ab initio calculations, the

Ta does not segregate to the grain boundaries which

may be explained by that the driving force for formingcubic (Ta, W)C grains exceeds that of segregation. Thus,

the resistance to grain boundary infiltration, as calcu-

lated by the ab initio simulations, is governed by just

the Co segregation and the explanation of the positive

effect of Ta has to be found somewhere else than in

the a/a grain boundaries.

The initial, slightly positive, effect of Ta may be due to

the higher hardness of the cubic carbide grains whichmakes them less likely to deform and, thus, the deforma-

tion of the skeleton is hindered at first. However, at high-

er cutting speeds (temperatures) the effect of Ta is instead

negative, indicating that the c/a grain boundaries slide,

or get infiltrated by Co, easier than the a/a boundaries.

In the light of the above discussion, it can be expected

that slightly higher TaC contents will increase the

amount of the cubic carbides. Though this may improvethe hardness it will also increase the brittleness which re-

sults in earlier fracture of the inserts. If the TaC content

is much higher there will be formation of grains with a

core/rim structure and a direct comparison with the

materials studied here should not be made.

5.2. Cermets

The presence of Ta combined with the high carbon

content (largest IF hysteresis) seems to lead to an in-

creased sensitivity to cobalt infiltration of the grain

boundaries, which is seen in the SEM by the wider

and more frequent occurring lamellae.

None of the cermets show any significant differences

in bending, which is surprising in view of the differences

displayed by the turning tests. However, a correlationcan be found between the area of the hysteresis in the

IF temperature spectra (Fig. 13) and the wear of the

tools. From the IF point of view, the hysteresis can be

associated with a changing structure. The IF even

changes under isothermal conditions at 1207 �C (Fig.

11). Just as for the WC–Co, the IF increase at high tem-

perature is correlated to the presence of a peak PT4, due

to the Co infiltration of grain boundaries. The isother-mal increase of IF leading to the hysteresis is then re-

lated to a progressive infiltration of grain boundaries.

When the hysteresis is large, the high temperature infil-

trated state tends to maintain upon cooling.

It should be noted that at 1200 �C considerable solid

state sintering occurs. Microstructural coarsening by

dissolution/reprecipitation is therefore expected. Possi-

bly this is also an additional mechanism of plastic defor-mation, although the time available during a turning test

is much shorter.

154 G. Ostberg et al. / International Journal of Refractory Metals & Hard Materials 24 (2006) 145–154

The plastic deformation measured after turning at

different cutting speeds, where the cutting speed can

be, even if only qualitatively, related to the temperature

at the cutting edge, seems correlated with IF spectra ob-

tained as a function of temperature. In contrast to the

three-point bending, where the experiments are carriedout after a time of temperature stabilization of about

45 min and which shows similar results for all cermet

grades, the turning application involves changing tem-

perature conditions. The differences observed for the

four cermets could then be related to these conditions

in correspondence with the IF results.

A positive influence of the Ta on the deformation

behaviour can then only be achieved when the carboncontent is carefully adjusted. With too high carbon con-

tent, the Ta does not have a positive effect.

The compositions of the grades studied here were

chosen to be comparable as regards solid solution of

W, grain size etc and have not been optimised as regards

mechanical properties. Thus, an increase in the TaC may

lead to an increase of the plastic deformation resistance

if the sintering temperature is adjusted accordingly, tocompensate for the lowered melting point of the binder

phase. However, for much higher contents of TaC, hea-

vy cores will start to form which may increase toughness

and tool life [17], but not necessarily resistance to plastic

deformation.

6. Conclusions

• Ab initio calculations predict that Ta and Co segre-

gate substitutionally to WC/WC grain boundaries

and thereby increase the work of separation for the

boundaries.

• Co segregation was found by TEM-EDX at the grain

boundaries but no signs of Ta segregations could be

seen, neither in the WC–TaC–Co nor in the cermets,but in the WC–TaC–Co cubic (W, Ta)C grains are

formed.

• In the WC–Co, Ta has different effects at different

temperatures. The initial slight positive effect is

explained by the strengthening effect of harder

(Ta, W)C grains on the carbide skeleton. The nega-

tive effect at higher temperatures is explained by slid-

ing or infiltration of c/a grain boundaries.• In the cermet materials, the presence of Ta in the bin-

der phase makes the its melting point very sensitive to

the C activity during sintering. Hence, the same sin-

tering cycle gives different microstructure for the

HC and LC cermets.

• No difference in deformation resistance is seen by

three-point bending for the four cermets, whereas

the deformation resistance of the inserts during turn-

ing show substantial differences. IF heating curves for

each of the cermets exhibit a hysteresis which can be

attributed to binder phase infiltration of grain bound-

aries. The area of the hysteresis can be correlated to

the deformation achieved by the turning tests.

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