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Kinetics of Formation of Graded Layers on Cemented Carbides Experimenta Investigations and DICTRA Si

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Kinetics of Formation of Graded Layers on Cemented Carbides: Experimental Investigations and DICTRA Simulations Jos? e Garcia, Greta Lindwall, Orlando Prat, Karin Frisk PII: DOI: Reference: To appear in: Received date: Accepted date: S0263-4368(10)00172-1 doi: 10.1016/j.ijrmhm.2010.11.003 RMHM 3188 International Journal of Refractory Metals and Hard Materials 6 August 2010 6 November 2010

Please cite this article as: Garcia Jos? e, Lindwall Greta, Prat Orlando, Frisk Karin, Kinetics of Formation of Graded Layers on Cemented Carbides: Experimental Investigations and DICTRA Simulations, International Journal of Refractory Metals and Hard Materials (2010), doi: 10.1016/j.ijrmhm.2010.11.003

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Kinetics of Formation of Graded Layers on Cemented Carbides:

José Garcia 1,*, Greta Lindwall 2, Orlando Prat 3, Karin Frisk 2

2 3

Swerea KIMAB AB, P.O.Box 55970, SE-10206 Stockholm, Sweden

Max Planck Institute für Eisenforschung GmbH, Max Planck Str. 1, D-40237 Düsseldorf, Germany

Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides was investigated by experimental methods and DICTRA simulations. The layer formation obeys a parabolic law, indicating a diffusion-controlled process. For DICTRA

phase at the sintering temperature was investigated. Best agreement between experimental and simulations was obtained considering that the mobility of all metallic elements is two times slower compared with the mobility of the non-metallic elements.

Keywords:

Kinetics, diffusion, DICTRA modeling, cemented carbide, graded layer

* corresponding author: jose.garcia@helmholtz-berlin.de

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simulations, the influence of the mobilities for all diffusing elements in the liquid binder

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Abstract

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Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany

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Experimental Investigations and DICTRA Simulations

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1. Introduction The production of tough graded surface layers in cemented carbides is a key technology

thermal cracks originate in the coatings due to thermal expansion mismatches between

surface layers minimize the propagation of such thermal cracks into the cemented carbide.

reducing chipping of the coating during cutting operations. The mechanism of formation of tough graded layers has been previously investigated by experimental methods, diffusion models and computer simulations. Suzuki et al. [1]

atmospheres to produce tough WC-Co rich surface layers, which are free of cubic carbide phases (TiC, TaC or NbC); the so-called fcc-free layers. Suzuki et al. found that the fcc-

process. They postulated that the outward diffusion of nitrogen, due to dissociation of nitrogen containing components -such as TiN- during vacuum sintering, was the ratecontrolling process for fcc-free layer formation. Later, Schwarzkopf et al. [ 2 ] demonstrated that the denitridation effect was not strong enough to drive the formation of fcc-free layers. They postulated that the gradient formation was controlled by inward diffusion of Ti in the liquid binder; which was driven by the outward gradient of nitrogen. They presented a phenomenological model (parabolic rate equation) describing the effect of all important processing variables semi-quantitatively, which was in agreement with their experimental results. In Ref. [2] it was observed that the thickness of the fcc-free

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free layer formation showed a parabolic growth law, indicating a diffusion-controlled

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sintered nitrogen-containing cemented carbides at liquid phase temperatures in vacuum

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They also increase the surface toughness of the cemented carbide-coating composite

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the coating (~ 8 – 10 ? 10-6/K) and the cemented carbide (~ 5 – 6 ? 10-6/K). Tough graded

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to improve the cutting performance of coated cutting tools. In coated cemented carbides

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layers increased by increasing the nitrogen content, the sintering temperature and the sintering time.

some kinetic considerations. Their kinetic model considered the different nitrogen and titanium activity gradients on the bulk and surface of the cemented carbide at the sintering temperature. The predicted activity gradients yielded a nitrogen (and carbon)

kinetic model showed good agreement with their experimental results as well as with previous investigations on fcc-free layer formation reported in [1,2]. The first published DICTRA [4] simulations on fcc-free layer formation were reported by

matrix binder phase. A thermodynamic database for cemented carbides containing carbon and nitrogen was assessed and used for both Thermocalc calculations and DICTRA

mobilities of the elements in the liquid, was considered. Ekroth et al. assumed that all interacting elements (Co, Ti, W, C and N) had the same mobility in the liquid. The experimental and simulated data by Ekroth et al. showed good agreement, indicating that the diffusion and thermodynamic data are the two major factors controlling the gradient formation. Since the diffusion occurs only in the liquid binder, the hard phases are considered obstacles, the presence of which reduces the effective diffusion paths. To simulate this effect, a so-called labyrinth factor, λ(f), can be defined which reduces the diffusion coefficient matrix [7];

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simulations [ 6 ]. For the DICTRA simulations a kinetic description, expressed in

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Ekroth et al. [5]. Their kinetic model assumed that all diffusion occurs in the liquid

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flux towards the surface and a titanium flux in the opposite direction. Results of the

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Gustafson and ?stlund [3] presented a model based on thermodynamic calculations and

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n n Dkjeff = λ ( f ) ? Dkj

(1)

group, Frykholm et al. [7] found that by using a labyrinth factor λ= f instead of λ = f?, a

In this work experimental investigations and DICTRA simulations on kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides are carried out. For the phase equilibria calculations, the thermodynamic description presented by Frisk et al. in [8] is used. The diffusion model is treated as in previous investigations [5,7], but the mobilities of the different elements in the cobalt binder phase at the sintering temperature are optimized to fit with the experimental results (thickness of fcc-free layers

2. Experimental

Cemented carbides samples from a mixture of WC, Co, TiN and (Ta,Nb)C were prepared by standard powder metallurgy following the method described in [9]. Raw powders provided by the company H.C. Starck and Umicore were used for the production of the samples. The description of the powders was as follows: WC with a Ctotal of 6.13 ± 0.05, TiN with a nitrogen content of 21.2 ± 0.1 wt% and (Ta,Nb)C 50/50 with a Ctotal of 8.75 ± 0.15 wt%. The Co was an Umicore extra fine powder (1.2-1.5 ?m) quality containing 0.2% impurities. The powder mixture of the cemented carbide is given in Tab.1. Vacuum sintering was carried out at 1450°C for 2, 3 and 5 h. After sintering the samples were cut, embedded in resin and polished. Fcc-free layer thicknesses were measured on scanning

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and phase distributions).

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better correspondence between experiments and DICTRA simulations was achieved.

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factor λ= f?, where f is the volume fraction of the matrix. In a later work of the same

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where Dnkj eff is the effective diffusion coefficient matrix. Ekroth et al. used the labyrinth

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electron microscopy (SEM) images of cross sectioned samples. Measurements of phase fraction distributions on the near-surface area were performed on SEM images using the

3. DICTRA modeling

For the kinetic simulations, a model for long range diffusion occurring in a continuous

assumed that all diffusion occurs in the Co matrix. This assumption is based on the fact that the carbide matrix in cemented carbides is continuous and that diffusivities in the liquid are much higher than in the solid phase. To account for the influence of the

For the description of the flux of species driven by a concentration gradient the multicomponent diffusion theory was applied [11]. The nitrogen out-diffusion is driven by the

so that the fluxes of elements are coupled due to their thermodynamic interaction. The law relating flux and concentration gradient is given by the multi-component extension of Fick’s first law:

n J k = ? ∑ Dkj j =1

n ?1

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titanium in-diffusion but also by the presence of other elements in the liquid cobalt phase,

?c j ?z

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dispersed hard phases the labyrinth factor λ(f)= f was considered, as suggested in [7].

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matrix with dispersed phases available in the DICTRA software was used. It was

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where Jk is the flux of species k in the direction of the z-axis and ?cj/?z is the concentration gradient of species. The diffusion coefficient matrix Dkjn, is a product of

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software analysis 5.0 [10] following the method described by Ekroth et al. in [5].

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two matrices; one consisting only of pure thermodynamic information and one consisting of the so-called mobilities, and can be expressed as follows:

indicating that the diffusion of one element depends on the concentration gradient of other elements. Here, δik is the Kronecker delta; i.e. δik = 1 when j = k and δik = 0

element i, and n is an arbitrary chosen reference element. By absolute-reaction theory argumentation, the mobility parameter, Mk, for an element k in a given phase can be divided into a frequency factor Mko and an activation energy factor Qk as reported in [12]:

Mk =

Mk ? ? Qk ? exp? ? RT ? RT ?

0

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otherwise [12]. xk is the mole fraction of element k and ?i is the chemical potential of the

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constant; hence their values depend on temperature, pressure and composition. In the kinetic database utilized by DICTRA, parameters for the atomic mobilities of the elements in a given phase are stored rather than diffusivities. Ideally, these parameters are assessed in respect to available diffusion coefficient data for the system of question. However, for many systems, like the liquid binder system, the availability of such data is limited and assumptions are needed. Both Ekroth et al. [5] and Frykholm et al. [7] assumed the same mobility for all diffusing elements in the liquid, and considered an activation energy, Q, of 65,000 J/mol. The frequency factor, Mk0, was given the value of 9.24·10-7 m2/s, which approximately reproduced their experimental results. Consequently, the value of the frequency factor will depend on the thermodynamic description utilized as well as the choice of labyrinth factor, λ(f), see Eq. (1). In this study, the same approach

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where R is the gas constant and T is the absolute temperature. Mko and Qk are not

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? ?? ?? n Dkj = ∑ (δ ik ? x k ) xi M i ? i ? i ? ?x i ? j ?x n

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(3) (4)

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as described above is applied; i.e. the mobilities of the diffusing elements in the liquid are varied until the agreement with the experimental results is satisfactory.

4. Results and discussion 4.1. Microscopy

Light microscopy images of the cemented carbide microstructures after sintering in

consisting of WC-Co and depleted of cubic carbides phases formed at the outer surface area of all samples. In the bulk of the samples three phases can be observed, corresponding to WC (light gray phase), Co (white phase) and the mixed carbide fcc-

forms a core-rim type structure consisting of a (Ti, W, Ta, Nb)(C,N) phase [12]. The fccfree layer thicknesses are of 23 ±1, 32 ±1 and 45 ±1 ?m after 2, 3 and 5 h respectively.

process.

4.2. DICTRA simulations of fcc-free layer growth kinetics
A number of DICTRA calculations were performed in order to make conclusion about the kinetic description of the liquid necessary for the simulations. The composition used in the simulations was determined by chemical analysis of sintered samples (Tab. 2). The first calculation was performed defining the same mobility for all diffusing elements; i.e. 1·10-9/RT m2/s. At 1450°C and with an activation energy of 65,000 J/mol this yields a frequency factor of approximately Mk0 = 9.34·10-9 m2/s. Results of the DICTRA

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The fcc-free layer formation obeys a parabolic law, indicating a diffusion-controlled

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phase (dark gray phase). Considering the raw composition of the mixture, the fcc-phase

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vacuum conditions at 1450°C are shown in Fig. 1. As expected, a fcc-free layer

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simulation of fcc-free layer formation for this situation are shown in Fig. 2a. It can be observed that the phase distribution for WC, Co and the fcc phase fits reasonably well

too fast, reaching a fcc-free layer thickness of 33 ?m already after 4500 s (compare Fig. 1, 2h, fcc-free layer ~ 23 ?m). This result indicates that the assumption of the same mobility for all elements is inappropriate to simulate the diffusion-controlled process for the

For further calculations it seems reasonable to assume that the mobility of the light nonmetallic elements (C and N) is larger than for the heavier metallic elements (Ti, Ta, Nb, W and Co).

dividing the initial mobilities by 10 is considered. The mobility of C and N was not altered (?1.0·10-9/RT m?/s). It is expected that the kinetics of layer growth will be

validated this hypothesis. Nevertheless, the kinetics of layer formation was too slow. The experimentally observed layer thickness of ~ 23 ?m was not reached after 2 h at 1450°C. The Co phase fraction and WC profile showed incorrect distributions compared with the experimental results (see Fig. 2b); the Co out-diffusion increased uninterrupted towards the surface during the whole simulation leading to unrealistic high binder phase contents (up to 40 vol%) in the fcc-free layer. The WC phase showed an unusual decrease inside the fcc-free layer, probably driven by the increased Co content on the surface. In order to increase the kinetics of fcc-free layer formation, the mobility of all metallic elements was divided by 5. DICTRA simulations showed that this assumption slightly

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reduced due to the slower diffusivity of the metallic elements. The DICTRA simulations

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Hence, a reduction of the mobility of all metallic elements (Nb, Ta, Ti, W and Co) by

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conditions investigated, because it leads to an overestimated layer grow.

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with previous reported profiles [5]. However, the kinetics of fcc-free layer formation is

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increased the fcc-free layer thickness but yielded an incorrect distribution of the Co and WC phase, similar to the results obtained for the previous calculations (Fig. 2b). These

and the non-metallic elements should be reduced.

The next calculation was carried out by dividing the mobilities of the metallic elements (Nb, Ta, Ti, W and Co) by 2. The results of the DICTRA simulation after 2 h vacuum

shown in Fig. 3. In the SEM image, the light gray phase corresponds to WC, the dark grey to fcc-phase and the black to the cobalt binder phase. The contrast between phases of the SEM image is used for the determination of the phase fraction distribution as a

of phase fractions shows the formation of a fcc-free layer of 20 ?m, which has a slightly higher WC content and increased Co content as in the bulk. Beneath the fcc-free layer an

last case fits very well in both fcc-free layer thickness and phase distribution (Fig. 4). Using this last assumption the kinetics of fcc-free layer growth for 2, 3 and 5 h was simulated, showing a very good agreement with the experimental results (see Fig. 4).

5. Conclusions
Summarizing, tough fcc-free graded layers were produced by liquid phase sintering of TiN-containing cemented carbides in vacuum atmospheres at 1450°C. The kinetics of formation of graded layers was simulated by DICTRA. The influence of the mobility of all diffusing elements in the liquid cobalt binder phase at the sintering temperature was

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increment of cubic carbide phases is observed. The DICTRA simulation results for this

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function of depth by image analysis as described in [5,7]. The quantitative determination

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sintering and the corresponding scanning electron microscopy (SEM) sample image are

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observations led to the conclusion that the difference in mobility between the metallic

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investigated. Best agreement between experimental and simulations were obtained by considering that the mobility of all metallic elements (W, Co, Ti, Ta and Nb) is two times

obtained mobility values can only be regarded as fitting parameters for the specific experimental situation. Their values will depend on many factors such as the applied thermodynamic description as well as the choice of labyrinth factor. By accounting for

influence of composition and process parameters in the production of though graded surface layer on cemented carbides.

Dr. José Garcia thanks the financial support of the joint research group Microstructural Analysis (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH / Ruhr

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Universit?t Bochum).

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Acknowledgement

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this aspect, DICTRA calculations is an important tool for simulation and prediction of the

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slower compared with the mobility of C and N. However, it should be noted that the

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Fig. 1: Tough fcc-free layer (WC-Co) formation as a function of time (light microscopy).
Fcc-free layer thicknesses (d) are of 23 ±1, 32 ±1 and 45 ±1 ?m after 2, 3 and 5 h

Fig. 2: Dictra simulation of fcc-free layer formation at 1450°C and 2 h vacuum sintering:
same mobility for all elements (a); mobilities of metallic elements 10 times slower than

Fig. 3: SEM image and corresponding Dictra simulation of cemented carbide after 2 h
vacuum sintering at 1450°C. The metallic element mobilities are 2 times slower than that

observed.

(left). Dictra simulations of fcc-free layer formation after 5 h vacuum sintering at 1450°C (right).

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Fig. 4: Kinetics of fcc-free layer growth: Dictra calculations and experimental results

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of C and N. Good fitting between Dictra simulations and experimental results are

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that of C and N (b).

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respectively.

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Table 1: Powder mixture composition (in wt.-%). WC
84

TiN0.94
3

(Ta0.5Nb0.5)C
5

Co

W
balance

Co
8.0

Ti
2.70

Ta
2.34

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Nb

Table 2: Element composition used for simulations (in wt.-%).

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C
5.59

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N
0.38

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2.21

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FIGURE 1

FIGURE 2

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FIGURE 3

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FIGURE 4

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Ms. Ref. No.: IJRMHM-D-10-00115 Title: Kinetics of Formation of Graded Layers on Cemented Carbides: Experimental Investigations and DICTRA Simulations Research highlights

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Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides.

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Investigations by experimental methods and DICTRA simulations. Fcc-free layer growth obeys parabolic law indicating a diffusion-controlled

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Study of influence of mobilities for all diffusing elements in DICTRA simulations.

N.

References

[1] Suzuki H, Hayashi K, Taniguchi Y. The beta-free layer formed near the surface of vacuum-sintered WC–beta–Co alloys containing nitrogen. Trans Jap Inst Metals 1981;22:758-64.

[2] Schwarzkopf M, Exner HE, Fischmeister HF, Schintlmeister W. Kinetics of compositional modification of (W, Ti)C-WC-Co alloy surfaces. Mat Sci Eng A 1988;105/106:225-31. [3] Gustafson P, ?stlund ?. Binder-phase enrichment by dissolution of cubic carbides. Int J Refract Met Hard Mater 1994;12 3:129-36. [4] Thermocalc and Dictra software, http://www.thermocalc.com.

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Best fitting if mobilities of W Ti Ta and Nb is two times slower than that of C and

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process.

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[5] Ekroth M, Frykholm R, Lindholm M, Andrén HO, ?gren J. Gradient zones in WC–

Acta Mat 2000;48:2177-85.

[6] Thermodynamic carbonitride database CN1b, CAMPADA, Royal Institute of Technology Sweden 1999.

[7] Frykholm R, Ekroth M, Jansson B, ?gren J, Andren HO. A new labyrinth factor for modelling the effect of binder volume fraction on gradient sintering of cemented carbides.

[8 ] Frisk K, Dumitrescu L, Ekroth M, Jansson B, Kruse O, Sundman B. Development of a database for cemented carbides: Thermodynamic modeling and experiments. J Phase Equil 2001; 22 6:645-55

EP1880031;16.11.2006.

[10 ] Software AnalySIS 5.0: Olympus Soft Imaging System 2008. [11 ] Borgenstam A, Engstrom A, Hoglund L, ?gren J. DICTRA, a tool for simulation of diffusional transformations in alloys. J Phase Equil 2000; 21 3:269-80. [12] Andrén HO. Microstructures of cemented carbides. Mat. Design 2001;22:491-98.

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[9 ] Garcia J, Kipperer K. Hard metal body with tough surface region. European Patent

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Acta Mat 2003; 51:1115-21.

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Ti(C,N)–Co-based cemented carbides: experimental study and computer simulations.




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