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WC-Co based cemented carbides with large Cr3C2 additions
International Journal of
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 417-422
WC-Co based cemented carbides with large Cr3C2 additions J. Zackrisson a*, B. Jansson b, G. S. Uphadyaya c, H.-O. Andr6n a aDepartment of Expertmental Physlcs, Chalmers Untversttyof Technology and Goteborg Umverszty, SE-412 9 Gotebopg, Sweden bSeco ToolsAB, SE-737 82 Fagersta, Sweden CDepartmentof Metallutgtcal Engmeenng, Indmn Institute of Technology, Kanpur208016, Indm
Received 9 March 1998, accepted 12 August 1998
Abstract
Five model alloys based on WC-16.6 vol% Co with different amounts of Cr3C2 (0-12 vol%) substituting WC were studied. Their microstructures were characterised with X-ray diffraction, scanning electron microscopy and transmission electron microscopy in combination with energy dispersive X-ray analysis. Thermodynamic calculations on the C - C o - C r - W system were carried out using the Thermo-Calc software. The mlcrostructures were related to previously pubhshed results on properties (magnetic coercivlty, hardness, transverse rupture strength and indentation fracture toughness) of these materials. The aim of this work was to investigate whether the posmve effect of small Cr3C2 additions on grain refinement and various propemes remains also for large Cr3C2 additions. For Cr3C2 addlhons larger than 2 vol%, the most obvious effect on the mlcrostructure was the formation of a chromium- and cobalt-rich M7C3 carbide. The Cr/Co ratio of this phase depends on the amount of Cr3C2 substituting WC. Some large WC grains were present in the WC-Co material, but not in the materials with Cr3C2additions. Apart from this, no major changes of the WC grain size with Cr3Ca content were observed. The partial replacement of tough binder by presumably brittle M7C3 carbide might explam the lower toughness of these materials. © 1998 Elsevier Science Ltd. All rights reserved. Keywords TEM/EDX, bander composition, M7C3formation; ThermoCalc, gram size
1. Introduction
If one process p a r a m e t e r in the fabrication of a material is changed, the properties of the final product will be influenced in one way or the other. Since the microstructure can be related both to the process parameters and to the properties, microstructural studies are a key to an improved understanding of the behaviour of materials. Optical microscopy and scanning electron microscopy are widely used for materials characterisation, but are often insufficient for more detailed microstructural studies and microanalysis. Therefore, there is a need for complementary techniques with better resolution, such as transmission electron microscopy including selected area electron diffraction and energy dispersive X-ray analysis. The microstructure of W C - C o based cemented carbides consists of hard WC grains surrounded by a tough cobalt-rich binder phase. Often, other carbides are added to the starting powder mixture to improve *Corresponding author. Jenm Zacknsson. Phone +46 31 772 3325, Fax. +46 31 772 3224, E-mall: jennl@fy chalmers se
the properties of the sintered product. The solubility of these carbides in WC is very low. However, both WC and the other carbides can, to some extent, be dissolved in the cobalt binder during sintering. The carbon content of the binder phase during sintering is of vital importance to the microstructure and microchemistry after sintering [1]. However, only small amounts of carbon remain in the binder after cooling. Some of the metal atoms stay dissolved in the cobalt, resulting in solid solution hardening of the binder phase. During sintering, a significant part of the dissolved atoms re-precipitates onto the undissolved carbide grains. This causes continuous grain growth. The presence of more than one metal carbide in the starting powder results in the formation of mixed carbide phases, either as homogeneous grains or as grains with a core-rim structure. Grain growth may also occur as a result of a coalescence process, involving grain boundary migration. In this case, the growth rate is much higher than in grain growth by re-precipitation. Consequently, it is desirable to try to limit coalescence, since abnormal grain growth is known to decrease the strength of materials.
0263-4368/98/$ - - see front matter © 1998 Elsevier Science Ltd. All rights reserved PII' S0263-4368(98)00048-1
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J Zacknsson et aL/InternattonalJournal of RefractoryMetals & Hard Materials 16 (1998) 417-422
Small additions of Cr3C2 are often used in the production of cemented carbides to improve corrosion resistance and to restrict grain growth of WC. The latter is believed to be due to slower grain boundary migration (and hence limited coalescence of WC grains) caused by Cr segregation to the WC grain boundaries [2]. Since Cr3C2 has been found to have a positive influence on the properties of WC-Co based materials, it is of interest to more thoroughly investigate the effect of larger Cr3C2 additions on the microstructure and properties of cemented carbides. This paper reports results from a microstructural study of five model W C - C o alloys, in which the WC has been partly substituted by Cr3C2. The microstructures have been characterised by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis and scanning electron microscopy. The Thermo-Calc software (provided by ThermoCalc AB, Stockholm) was used for thermodynamic calculations on the C - C o - C r - W system. 2. Experimental details 2.1. Materials
Five alloys based on WC-16.57 vol% Co with additions of 0-12 vol% Cr3C 2 partly replacing the WC have been studied. Alloy compositions are given in Table 1 (in wt%). The materials were sintered in a hydrogen atmosphere at 1420°C for 1 hour. Sintered porosity, magnetic coercivity, transverse rupture strength, hardness, indentation fracture toughness and corrosion resistance of these alloys have been measured and reported previously [3]. 2.2. Microstructural characterisation
The microstructure of the five experimental alloys was investigated with X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM). XRD was used to determine the presence of phases. Grain size and grain shape were studied in SEM using Table 1 Composmonof the fivealloysbefore smterlng (in wt%) Alloy
0 vol% Cr3C2 2 vol% Cr3C2 4 vol% Cr3C4 6 vol% Cr3C4 12 vol% Cr3C2
Powder mixture WC
Cr3C2
Co
90.0 89.0 87.9 86 7 83 2
0 09 1.9 29 60
10 0 10 1 10 2 10.4 10.8
backscattered electron imaging, which gives an atomic number contrast suitable to distinguish the hard phase from the binder phase. Quantitative EDX microanalysis of the different phases was performed in combination with TEM. TEM was also used to study the microstructure in more detail, and to give complementary phase information from selected area electron diffraction (SAED). The instruments used for the microstructural characterisation were a Jeol 2000FX TEM eqmpped with a Link AN10000 system for EDX analysis and a CamScan $4-80DV SEM with a Link eXL EDX system. The specimens for the TEM work were prepared by polishing, dimpling and ion-milling. 2.3. Thermodynamic calculations
The so called Calphad technique is widely used to describe thermodynamic properties of materials. The method aims at constructing a thermodynamic description of the alloy system of interest based on modelling the Gibbs energy of each phase in the system as a function of temperature, pressure and constitution. All available thermodynamic information, experimental data or information from more fundamental physical modelling, is used to get optimum values of model parameters. Once the Gibbs energy functions are available, phase diagrams and other thermodynamic properties of interest can be calculated by applying standard thermodynamic relations. Thermodynamic calculations have been carried out utilising the Thermo-Calc software [4]. A thermodynamic description of the C - C o - C r - W system has been obtained by combining available model parameters in the SGTE solution databank from ThermoCalc and a recent thermodynamic evaluation of the C o - C r - C system [5].
3. Results
It was found that the Cr3C2 addition affected both the presence and composition of the different phases. 3.1. WC and Cr3C2
SEM backscattered electron imaging revealed that a few WC grains substantially larger than the others were observed in the material without Cr3C2 addition. No such large WC grains were found in the materials formed with Cr3C2 additions. Large Cr3C2 additions did not seem to have any major effect on the WC grain size, see Fig. 1. The TEM/EDX investigations showed that the WC grains did not dissolve any chromium. No remaining Cr3C2 was observed and, hence, all of the Cr3C2 was found to be dissolved during sintering.
J Zacknsson et al/Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 417-422 3.2. Binder phase T h e results f r o m T E M / E D X m e a s u r e m e n t s of t h e m e t a l c o n t e n t o f the b i n d e r p h a s e is p r e s e n t e d in T a b l e 2. It was f o u n d t h a t the Cr3C2 a d d i t i o n s c a u s e d b e t w e e n 4 a n d 6 a t % c h r o m i u m to b e dissolved in the cobalt binder, whereas the tungsten content of the
419
b i n d e r r e m a i n e d at a p p r o x i m a t e l y 1 a t % a n d was not affected by t h e Cr3Ca additions. 3.3. Chromium-rich M7C3 A c c o r d i n g to the T E M / E D X analyses (see T a b l e 2) a c h r o m m m - r i c h p h a s e was p r e s e n t in the alloys with
Fig. 1 SEM mlcrographs that provide an overview ot the mIcrostructures of the five model alloys. Imaging was performed in the backscattered electron mode, which results m atomic number contrast Hence, tungsten carbide appears bright and the cobalt binder dark (a) 0 vol% Cr3C2, (b) 2 vol% Cr3C2, (c) 4 vol% Cr3C2. (d) 6 VOI% Cr3C2 and (e) 12 vol% Cr3C2.Apart from the few large WC grams present m the 0 vol% Cr3C2 material, no major changes m WC gram sxze as a function of Cr3C2addition were observed
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J Zacknsson et aL/Internattonal Journal of Refractory Metals & Hard Materials l O (1998) 417-422
Table 2 Metal content of the binder phase of the five alloys determined by TEM/EDX (in at%). For the alloyswith large additions of Cr3C4(4, 6 and 12 vol%) a chromium-rich M7C3 phase was found to be present. For those alloys, the metal content of thin phase is also given The mean values of several EDX analysesare reported Alloy
0 vol% Cr~C2 2 vol% Cr3C2 4vo1% Cr3C2 6vo1% Cr3C2 12 vol% Cr3Cz
Binder p h a s e
Chromium-rmh M7C3
Co
Cr
W
Co
Cr
W
99_+1 93 _+1 95+1 95_+1 93 +_1
0 6 _+1 4+1 4+1 6 -+1
1_+1 1_+1 1+1 1_+1 1i 1
--41+1 43-+1 33 +_1
--58+1 55-+1 64 -+1
--1_+1 2+1 3 _+i
large Cr3C2 additions ( > 4 vol%). The existence of this phase could not be verified with XRD, since no other peaks than those arising from tungsten carbide and the cobalt binder phase was found in the diffractograms. However, large (100 #m) areas of a chromium-rich phase was also observed with SEM, see Fig. 2. Selected area electron diffraction in the T E M was used to determine the structure of the chromium-rich phase. It was found to be an orthorhombic pseudohexagonal M7C3 carbide. T E M / E D X analysis revealed that its chromium and cobalt contents depended on the amount of Cr3C2 added to the starting powder. The metal conent of this phase in the different alloys is given in Table 2. TEM bright field imaging of this phase revealed that its grain shape was similar to the grain shape of the binder phase, filling out space between the hard grains, see Fig. 3. It seemed to contain a high density of planar faults, visible due to diffraction contrast. This was also indicated by streaking in the selected area diffraction patterns.
3.4. Thermodynamics
A calculated phase diagram can be found in Fig. 4. The phase diagram is calculated using the system components Co, WC, Cr3C2, and C at constant amount of Co (10 wt%) and of C (0 wt%). Thus the system is closed and WC is replaced by the same mass of Cr3C2 along the horizontal axis. At the sintering temperature, 1420°C, all Cr3Cz will be dissolved in the liquid phase. However, during cooling after sintering there will be a primary precipitation of M7C3 phase from the liquid if the Cr3C2 content is within approximately 3.5 to 7 wt%. This is the case for the 12 vol% (6 wt%) Cr3C2 material. At higher Cr3C2 contents there will also be a precipitation of M3C2 phase. If the Cr3C2 content is lower than 3.5 wt% solid binder will precipitate first, but some M7C3 can still be precipitated from the liquid for Cr3C2 contents higher than 1 wt%. This is expected to occur in the 4 and 6 vol% alloys (1.9 and 2.9 wt%, respectively). When all the binder has solidified there is a driving force for replacing the M7C3 phase with the M3C2 phase. However, this transformation will be very sluggish due to the coarse areas of M7C3 carbide formed due to precipitation from the liquid phase. The metallic mol fractions of the M7C3 phase at the solidus temperature is according to the calculations approximately 0.23 Co, 0.01 W and 0.76 Cr.
4. Discussion In this discussion, we relate our new microstructural and thermodynamic results to some of the properties of these alloys measured and reported earlier [3]. Apart from the chromium-rich M7C3 phase, the most obvious microstructural difference between the materials is that the WC grain size distribution in the materials with Cr3C2 additions is fairly homogeneous, whereas the W C - C o material contains some WC grains significantly larger than the average grain size. This is believed to be due to abnormal grain growth in the absence of Cr3C2 additions. When Cr3C2 is present, chromium segregates to the WC grain boundaries. Thereby, the grain boundary migration required for coalescence and abnormal grain growth to occur is slowed down [2]. 4.1. 0 - 2 v o l % CoC2
Fig 2. SEM mmrographofthe 12 vol% Cr3C2material, showing the chrommm-nch M7C3phase (marked)
For small additions of Cr3C2 the magnetic coercivity increases, i.e. a higher reverse field is required to reduce the magnetization from saturation to zero in such a cemented carbide than in a W C - C o without carbide addition. The coercivity increases with the
J Zacknsson et al/Internattonal Journal of Refractory Metals & Hard Matertals 16 (1998) 417-422
WC/Co interphase area, since these phase boundaries pin magnetic domain walls. There is an inverse relation between the WC grain size and the WC/Co interphase area, which suggests that the WC grain size should decrease for small Cr3C2 additions. This is consistent with the somewhat smaller WC grain size in the 2 vol% Cr3C2 material measured and reported by Banerjee et
al. [31. Vickers hardness was not affected by Cr3C2 additions smaller than 2 vol%, whereas both transverse rupture strength and indentation fracture toughness increased slightly with increasing Cr3C2 content. This might be explained by the slight decrease in WC grain size introduced by the small Cr3C2 additions, together with the absence of very large WC grains.
4.2. 4-12 vol% Cr3C2 For large Cr3C2 additions, we observed no major changes in WC grain size with backscattered electron imaging in the scanning electron microscope. In these materials, the grain growth of WC is also likely to be influenced by the formation of a third phase (chromium-rich M7C3).Hence, it is difficult to separate the effect of the Cr3C2 content. Previous measurements
421
show slight variations but no systematic dependence of WC grain size on the Cr3C2addition [3]. The most pronounced effect of larger Cr3Cz additions on the microstructure of these materials is the formation of a chromium-rich M7C3 carbide. This new information can be used to explain some of the property changes that were previously not fully understood. The magnetic coercivity was almost constant for 2-6 vol% Cr3C2 additions, but drastically increased in the 12 vol% Cr3C2 material. This was suggested to be due to precipitation of a finely dispersed non-magnetic phase within the binder, such as Co3W [3]. However, no such phase was observed and, hence, we believe that the formation of the coarse grained chromium-rich M7C3carbide can be responsible for the changes in the magnetic coercivity measurements. This phase was present in the three alloys with 4-12 vol% Cr3C2 additions, but its volume fraction in the 12 vol% Cr3C2 material was significantly higher than in the others, which might explain the substantial increase in coercivity. Hardness seemed to be unaffected by the Cr3C2 substitution also for larger Cr3C2 additions. This is reasonable since the WC grain sizes were similar in
Fig. 3 (a) TEM bright field image of the binder phase (marked) (b) TEM bright field image of the chrommm-nch M7C3 (marked and the corresponding (112) selected area diffraction pattern. The gram shape of the binder phase and the chrommm-rlch MvC3 xs similar, filhng out regions between the hard WC grams, but the contrast of the two phases is completely different.
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J Zackrtsson et al./Internanonal Journal of Refractory Metals & Hard Materials 16 (1998) 417-422
14 0 0
m.n
hq+WC
1350 \
the (presumably rather brittle) chromium-rich M7C3 carbide.
t
_ _ i _ _ i _ _ i
//~
hq+WC
5. C o n c l u s i o n s
-J 1 3 0 0 b] g I
ta -1
\ liq~ \ +WC \
rr 1 2 5 0
/ /
liq+WC
/!iq +wC / +M7C3
bJ 1 2 0 0
a.E ILl
fcc ~ +WC/
~- 1 1 5 0
~
I
1100
fcc+WC+M3C2
~ 0
fcc+liq+WC+M3C2
2
I 4
WEIGHT_PERCENT
T 6
T 8
10
CR3C2
Fig 4. A calculated phase diagram using the system components Co, WC, Cr3C2, and C at constant amount of Co (10 wt%) and of C (0 wt%). Thus the system is closed and WC is replaced by the same mass of CrsC2along the horizontal axis these materials. Transverse rupture strength and indentation fracture toughness decreased with increasing Cr3C2 content. The process previously suggested to explain this behaviour was, that since all Cr3C2 has to be dissolved in the binder due to its low solubility in WC, and that the solubility of carbides in the binder phase is limited, tungsten is removed from the binder. In this way, the binder was believed to lose strength because of the higher solid solution hardening effect of tungsten in cobalt, than of chromium in cobalt. However, T E M / E D X analysis of the binder phase in these materials after sintering shows that the amount of tungsten dissolved in the binder is low and independent of the Cr3C2 addition. Hence, the loss of solid solution hardening of the binder cannot be responsible for the loss of strength than comes with larger CrsC2 additions. Instead, we find it likely that this is caused by the gradual replacement of tough binder phase by
• The average WC grain size was not much affected by the Cr3C2 additions. However, in the material without CrsC2 addition, some substantially larger WC grains were observed. No such grains were found in the materials with Cr3C2 additions. • No Cr3C2 was found to be undissolved after sintering. • For larger Cr3C2 additions (4, 6 and 12 vol%) a chromium- and cobalt-rich MvCs carbide was formed. This is supported by thermodynamic calculations. The Cr/Co ratio varied with the amount of Cr3C2 added. • The low toughness of the materials with larger Cr3C2 additions (4, 6 and 12 vol%) can be explained by the gradual replacement of binder by the chromiumand cobalt-rich M7C3 carbide. • Large additions of Cr3C2 ( > 2 vol%) do not result in cemented carbides with properties that can be extrapolated from those with less than 2 vol% Cr3C2.
References
[1] Exner HE. Physical and chemical nature of cemented carbides International Metals Reviews 1979,4 149-73 [2] Henjered A, Hellsmg M, Andr6n H-O, Nordfn H Quantltatwe mlcroanalysls of carbide-carbide interfaces in WC-Co-based cemented carbides. Mater Sci. Technol 1986;2'847-55 [3] Banerjee D, Lal GK, Uphadyaya GS. Effect of binder-phase modification and Cr3Cz addition on properties of WC-10Co cemented carbides. Journal of Materials Engineering and Performance 1995;4'563-72 [4] Sundman B, Jansson B, Andersson JO. CALPHAD 1984,9.153-90 [5] KusoffskyA, Jansson B. CALPHAD 1997,21(3)321-33
REFRACTORYMETALS & HARDMATERIALS ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 417-422
WC-Co based cemented carbides with large Cr3C2 additions J. Zackrisson a*, B. Jansson b, G. S. Uphadyaya c, H.-O. Andr6n a aDepartment of Expertmental Physlcs, Chalmers Untversttyof Technology and Goteborg Umverszty, SE-412 9 Gotebopg, Sweden bSeco ToolsAB, SE-737 82 Fagersta, Sweden CDepartmentof Metallutgtcal Engmeenng, Indmn Institute of Technology, Kanpur208016, Indm
Received 9 March 1998, accepted 12 August 1998
Abstract
Five model alloys based on WC-16.6 vol% Co with different amounts of Cr3C2 (0-12 vol%) substituting WC were studied. Their microstructures were characterised with X-ray diffraction, scanning electron microscopy and transmission electron microscopy in combination with energy dispersive X-ray analysis. Thermodynamic calculations on the C - C o - C r - W system were carried out using the Thermo-Calc software. The mlcrostructures were related to previously pubhshed results on properties (magnetic coercivlty, hardness, transverse rupture strength and indentation fracture toughness) of these materials. The aim of this work was to investigate whether the posmve effect of small Cr3C2 additions on grain refinement and various propemes remains also for large Cr3C2 additions. For Cr3C2 addlhons larger than 2 vol%, the most obvious effect on the mlcrostructure was the formation of a chromium- and cobalt-rich M7C3 carbide. The Cr/Co ratio of this phase depends on the amount of Cr3C2 substituting WC. Some large WC grains were present in the WC-Co material, but not in the materials with Cr3C2additions. Apart from this, no major changes of the WC grain size with Cr3Ca content were observed. The partial replacement of tough binder by presumably brittle M7C3 carbide might explam the lower toughness of these materials. © 1998 Elsevier Science Ltd. All rights reserved. Keywords TEM/EDX, bander composition, M7C3formation; ThermoCalc, gram size
1. Introduction
If one process p a r a m e t e r in the fabrication of a material is changed, the properties of the final product will be influenced in one way or the other. Since the microstructure can be related both to the process parameters and to the properties, microstructural studies are a key to an improved understanding of the behaviour of materials. Optical microscopy and scanning electron microscopy are widely used for materials characterisation, but are often insufficient for more detailed microstructural studies and microanalysis. Therefore, there is a need for complementary techniques with better resolution, such as transmission electron microscopy including selected area electron diffraction and energy dispersive X-ray analysis. The microstructure of W C - C o based cemented carbides consists of hard WC grains surrounded by a tough cobalt-rich binder phase. Often, other carbides are added to the starting powder mixture to improve *Corresponding author. Jenm Zacknsson. Phone +46 31 772 3325, Fax. +46 31 772 3224, E-mall: jennl@fy chalmers se
the properties of the sintered product. The solubility of these carbides in WC is very low. However, both WC and the other carbides can, to some extent, be dissolved in the cobalt binder during sintering. The carbon content of the binder phase during sintering is of vital importance to the microstructure and microchemistry after sintering [1]. However, only small amounts of carbon remain in the binder after cooling. Some of the metal atoms stay dissolved in the cobalt, resulting in solid solution hardening of the binder phase. During sintering, a significant part of the dissolved atoms re-precipitates onto the undissolved carbide grains. This causes continuous grain growth. The presence of more than one metal carbide in the starting powder results in the formation of mixed carbide phases, either as homogeneous grains or as grains with a core-rim structure. Grain growth may also occur as a result of a coalescence process, involving grain boundary migration. In this case, the growth rate is much higher than in grain growth by re-precipitation. Consequently, it is desirable to try to limit coalescence, since abnormal grain growth is known to decrease the strength of materials.
0263-4368/98/$ - - see front matter © 1998 Elsevier Science Ltd. All rights reserved PII' S0263-4368(98)00048-1
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J Zacknsson et aL/InternattonalJournal of RefractoryMetals & Hard Materials 16 (1998) 417-422
Small additions of Cr3C2 are often used in the production of cemented carbides to improve corrosion resistance and to restrict grain growth of WC. The latter is believed to be due to slower grain boundary migration (and hence limited coalescence of WC grains) caused by Cr segregation to the WC grain boundaries [2]. Since Cr3C2 has been found to have a positive influence on the properties of WC-Co based materials, it is of interest to more thoroughly investigate the effect of larger Cr3C2 additions on the microstructure and properties of cemented carbides. This paper reports results from a microstructural study of five model W C - C o alloys, in which the WC has been partly substituted by Cr3C2. The microstructures have been characterised by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis and scanning electron microscopy. The Thermo-Calc software (provided by ThermoCalc AB, Stockholm) was used for thermodynamic calculations on the C - C o - C r - W system. 2. Experimental details 2.1. Materials
Five alloys based on WC-16.57 vol% Co with additions of 0-12 vol% Cr3C 2 partly replacing the WC have been studied. Alloy compositions are given in Table 1 (in wt%). The materials were sintered in a hydrogen atmosphere at 1420°C for 1 hour. Sintered porosity, magnetic coercivity, transverse rupture strength, hardness, indentation fracture toughness and corrosion resistance of these alloys have been measured and reported previously [3]. 2.2. Microstructural characterisation
The microstructure of the five experimental alloys was investigated with X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM). XRD was used to determine the presence of phases. Grain size and grain shape were studied in SEM using Table 1 Composmonof the fivealloysbefore smterlng (in wt%) Alloy
0 vol% Cr3C2 2 vol% Cr3C2 4 vol% Cr3C4 6 vol% Cr3C4 12 vol% Cr3C2
Powder mixture WC
Cr3C2
Co
90.0 89.0 87.9 86 7 83 2
0 09 1.9 29 60
10 0 10 1 10 2 10.4 10.8
backscattered electron imaging, which gives an atomic number contrast suitable to distinguish the hard phase from the binder phase. Quantitative EDX microanalysis of the different phases was performed in combination with TEM. TEM was also used to study the microstructure in more detail, and to give complementary phase information from selected area electron diffraction (SAED). The instruments used for the microstructural characterisation were a Jeol 2000FX TEM eqmpped with a Link AN10000 system for EDX analysis and a CamScan $4-80DV SEM with a Link eXL EDX system. The specimens for the TEM work were prepared by polishing, dimpling and ion-milling. 2.3. Thermodynamic calculations
The so called Calphad technique is widely used to describe thermodynamic properties of materials. The method aims at constructing a thermodynamic description of the alloy system of interest based on modelling the Gibbs energy of each phase in the system as a function of temperature, pressure and constitution. All available thermodynamic information, experimental data or information from more fundamental physical modelling, is used to get optimum values of model parameters. Once the Gibbs energy functions are available, phase diagrams and other thermodynamic properties of interest can be calculated by applying standard thermodynamic relations. Thermodynamic calculations have been carried out utilising the Thermo-Calc software [4]. A thermodynamic description of the C - C o - C r - W system has been obtained by combining available model parameters in the SGTE solution databank from ThermoCalc and a recent thermodynamic evaluation of the C o - C r - C system [5].
3. Results
It was found that the Cr3C2 addition affected both the presence and composition of the different phases. 3.1. WC and Cr3C2
SEM backscattered electron imaging revealed that a few WC grains substantially larger than the others were observed in the material without Cr3C2 addition. No such large WC grains were found in the materials formed with Cr3C2 additions. Large Cr3C2 additions did not seem to have any major effect on the WC grain size, see Fig. 1. The TEM/EDX investigations showed that the WC grains did not dissolve any chromium. No remaining Cr3C2 was observed and, hence, all of the Cr3C2 was found to be dissolved during sintering.
J Zacknsson et al/Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 417-422 3.2. Binder phase T h e results f r o m T E M / E D X m e a s u r e m e n t s of t h e m e t a l c o n t e n t o f the b i n d e r p h a s e is p r e s e n t e d in T a b l e 2. It was f o u n d t h a t the Cr3C2 a d d i t i o n s c a u s e d b e t w e e n 4 a n d 6 a t % c h r o m i u m to b e dissolved in the cobalt binder, whereas the tungsten content of the
419
b i n d e r r e m a i n e d at a p p r o x i m a t e l y 1 a t % a n d was not affected by t h e Cr3Ca additions. 3.3. Chromium-rich M7C3 A c c o r d i n g to the T E M / E D X analyses (see T a b l e 2) a c h r o m m m - r i c h p h a s e was p r e s e n t in the alloys with
Fig. 1 SEM mlcrographs that provide an overview ot the mIcrostructures of the five model alloys. Imaging was performed in the backscattered electron mode, which results m atomic number contrast Hence, tungsten carbide appears bright and the cobalt binder dark (a) 0 vol% Cr3C2, (b) 2 vol% Cr3C2, (c) 4 vol% Cr3C2. (d) 6 VOI% Cr3C2 and (e) 12 vol% Cr3C2.Apart from the few large WC grams present m the 0 vol% Cr3C2 material, no major changes m WC gram sxze as a function of Cr3C2addition were observed
420
J Zacknsson et aL/Internattonal Journal of Refractory Metals & Hard Materials l O (1998) 417-422
Table 2 Metal content of the binder phase of the five alloys determined by TEM/EDX (in at%). For the alloyswith large additions of Cr3C4(4, 6 and 12 vol%) a chromium-rich M7C3 phase was found to be present. For those alloys, the metal content of thin phase is also given The mean values of several EDX analysesare reported Alloy
0 vol% Cr~C2 2 vol% Cr3C2 4vo1% Cr3C2 6vo1% Cr3C2 12 vol% Cr3Cz
Binder p h a s e
Chromium-rmh M7C3
Co
Cr
W
Co
Cr
W
99_+1 93 _+1 95+1 95_+1 93 +_1
0 6 _+1 4+1 4+1 6 -+1
1_+1 1_+1 1+1 1_+1 1i 1
--41+1 43-+1 33 +_1
--58+1 55-+1 64 -+1
--1_+1 2+1 3 _+i
large Cr3C2 additions ( > 4 vol%). The existence of this phase could not be verified with XRD, since no other peaks than those arising from tungsten carbide and the cobalt binder phase was found in the diffractograms. However, large (100 #m) areas of a chromium-rich phase was also observed with SEM, see Fig. 2. Selected area electron diffraction in the T E M was used to determine the structure of the chromium-rich phase. It was found to be an orthorhombic pseudohexagonal M7C3 carbide. T E M / E D X analysis revealed that its chromium and cobalt contents depended on the amount of Cr3C2 added to the starting powder. The metal conent of this phase in the different alloys is given in Table 2. TEM bright field imaging of this phase revealed that its grain shape was similar to the grain shape of the binder phase, filling out space between the hard grains, see Fig. 3. It seemed to contain a high density of planar faults, visible due to diffraction contrast. This was also indicated by streaking in the selected area diffraction patterns.
3.4. Thermodynamics
A calculated phase diagram can be found in Fig. 4. The phase diagram is calculated using the system components Co, WC, Cr3C2, and C at constant amount of Co (10 wt%) and of C (0 wt%). Thus the system is closed and WC is replaced by the same mass of Cr3C2 along the horizontal axis. At the sintering temperature, 1420°C, all Cr3Cz will be dissolved in the liquid phase. However, during cooling after sintering there will be a primary precipitation of M7C3 phase from the liquid if the Cr3C2 content is within approximately 3.5 to 7 wt%. This is the case for the 12 vol% (6 wt%) Cr3C2 material. At higher Cr3C2 contents there will also be a precipitation of M3C2 phase. If the Cr3C2 content is lower than 3.5 wt% solid binder will precipitate first, but some M7C3 can still be precipitated from the liquid for Cr3C2 contents higher than 1 wt%. This is expected to occur in the 4 and 6 vol% alloys (1.9 and 2.9 wt%, respectively). When all the binder has solidified there is a driving force for replacing the M7C3 phase with the M3C2 phase. However, this transformation will be very sluggish due to the coarse areas of M7C3 carbide formed due to precipitation from the liquid phase. The metallic mol fractions of the M7C3 phase at the solidus temperature is according to the calculations approximately 0.23 Co, 0.01 W and 0.76 Cr.
4. Discussion In this discussion, we relate our new microstructural and thermodynamic results to some of the properties of these alloys measured and reported earlier [3]. Apart from the chromium-rich M7C3 phase, the most obvious microstructural difference between the materials is that the WC grain size distribution in the materials with Cr3C2 additions is fairly homogeneous, whereas the W C - C o material contains some WC grains significantly larger than the average grain size. This is believed to be due to abnormal grain growth in the absence of Cr3C2 additions. When Cr3C2 is present, chromium segregates to the WC grain boundaries. Thereby, the grain boundary migration required for coalescence and abnormal grain growth to occur is slowed down [2]. 4.1. 0 - 2 v o l % CoC2
Fig 2. SEM mmrographofthe 12 vol% Cr3C2material, showing the chrommm-nch M7C3phase (marked)
For small additions of Cr3C2 the magnetic coercivity increases, i.e. a higher reverse field is required to reduce the magnetization from saturation to zero in such a cemented carbide than in a W C - C o without carbide addition. The coercivity increases with the
J Zacknsson et al/Internattonal Journal of Refractory Metals & Hard Matertals 16 (1998) 417-422
WC/Co interphase area, since these phase boundaries pin magnetic domain walls. There is an inverse relation between the WC grain size and the WC/Co interphase area, which suggests that the WC grain size should decrease for small Cr3C2 additions. This is consistent with the somewhat smaller WC grain size in the 2 vol% Cr3C2 material measured and reported by Banerjee et
al. [31. Vickers hardness was not affected by Cr3C2 additions smaller than 2 vol%, whereas both transverse rupture strength and indentation fracture toughness increased slightly with increasing Cr3C2 content. This might be explained by the slight decrease in WC grain size introduced by the small Cr3C2 additions, together with the absence of very large WC grains.
4.2. 4-12 vol% Cr3C2 For large Cr3C2 additions, we observed no major changes in WC grain size with backscattered electron imaging in the scanning electron microscope. In these materials, the grain growth of WC is also likely to be influenced by the formation of a third phase (chromium-rich M7C3).Hence, it is difficult to separate the effect of the Cr3C2 content. Previous measurements
421
show slight variations but no systematic dependence of WC grain size on the Cr3C2addition [3]. The most pronounced effect of larger Cr3Cz additions on the microstructure of these materials is the formation of a chromium-rich M7C3 carbide. This new information can be used to explain some of the property changes that were previously not fully understood. The magnetic coercivity was almost constant for 2-6 vol% Cr3C2 additions, but drastically increased in the 12 vol% Cr3C2 material. This was suggested to be due to precipitation of a finely dispersed non-magnetic phase within the binder, such as Co3W [3]. However, no such phase was observed and, hence, we believe that the formation of the coarse grained chromium-rich M7C3carbide can be responsible for the changes in the magnetic coercivity measurements. This phase was present in the three alloys with 4-12 vol% Cr3C2 additions, but its volume fraction in the 12 vol% Cr3C2 material was significantly higher than in the others, which might explain the substantial increase in coercivity. Hardness seemed to be unaffected by the Cr3C2 substitution also for larger Cr3C2 additions. This is reasonable since the WC grain sizes were similar in
Fig. 3 (a) TEM bright field image of the binder phase (marked) (b) TEM bright field image of the chrommm-nch M7C3 (marked and the corresponding (112) selected area diffraction pattern. The gram shape of the binder phase and the chrommm-rlch MvC3 xs similar, filhng out regions between the hard WC grams, but the contrast of the two phases is completely different.
422
J Zackrtsson et al./Internanonal Journal of Refractory Metals & Hard Materials 16 (1998) 417-422
14 0 0
m.n
hq+WC
1350 \
the (presumably rather brittle) chromium-rich M7C3 carbide.
t
_ _ i _ _ i _ _ i
//~
hq+WC
5. C o n c l u s i o n s
-J 1 3 0 0 b] g I
ta -1
\ liq~ \ +WC \
rr 1 2 5 0
/ /
liq+WC
/!iq +wC / +M7C3
bJ 1 2 0 0
a.E ILl
fcc ~ +WC/
~- 1 1 5 0
~
I
1100
fcc+WC+M3C2
~ 0
fcc+liq+WC+M3C2
2
I 4
WEIGHT_PERCENT
T 6
T 8
10
CR3C2
Fig 4. A calculated phase diagram using the system components Co, WC, Cr3C2, and C at constant amount of Co (10 wt%) and of C (0 wt%). Thus the system is closed and WC is replaced by the same mass of CrsC2along the horizontal axis these materials. Transverse rupture strength and indentation fracture toughness decreased with increasing Cr3C2 content. The process previously suggested to explain this behaviour was, that since all Cr3C2 has to be dissolved in the binder due to its low solubility in WC, and that the solubility of carbides in the binder phase is limited, tungsten is removed from the binder. In this way, the binder was believed to lose strength because of the higher solid solution hardening effect of tungsten in cobalt, than of chromium in cobalt. However, T E M / E D X analysis of the binder phase in these materials after sintering shows that the amount of tungsten dissolved in the binder is low and independent of the Cr3C2 addition. Hence, the loss of solid solution hardening of the binder cannot be responsible for the loss of strength than comes with larger CrsC2 additions. Instead, we find it likely that this is caused by the gradual replacement of tough binder phase by
• The average WC grain size was not much affected by the Cr3C2 additions. However, in the material without CrsC2 addition, some substantially larger WC grains were observed. No such grains were found in the materials with Cr3C2 additions. • No Cr3C2 was found to be undissolved after sintering. • For larger Cr3C2 additions (4, 6 and 12 vol%) a chromium- and cobalt-rich MvCs carbide was formed. This is supported by thermodynamic calculations. The Cr/Co ratio varied with the amount of Cr3C2 added. • The low toughness of the materials with larger Cr3C2 additions (4, 6 and 12 vol%) can be explained by the gradual replacement of binder by the chromiumand cobalt-rich M7C3 carbide. • Large additions of Cr3C2 ( > 2 vol%) do not result in cemented carbides with properties that can be extrapolated from those with less than 2 vol% Cr3C2.
References
[1] Exner HE. Physical and chemical nature of cemented carbides International Metals Reviews 1979,4 149-73 [2] Henjered A, Hellsmg M, Andr6n H-O, Nordfn H Quantltatwe mlcroanalysls of carbide-carbide interfaces in WC-Co-based cemented carbides. Mater Sci. Technol 1986;2'847-55 [3] Banerjee D, Lal GK, Uphadyaya GS. Effect of binder-phase modification and Cr3Cz addition on properties of WC-10Co cemented carbides. Journal of Materials Engineering and Performance 1995;4'563-72 [4] Sundman B, Jansson B, Andersson JO. CALPHAD 1984,9.153-90 [5] KusoffskyA, Jansson B. CALPHAD 1997,21(3)321-33