NATURE OF THE BOUNDARY PLANES IN TUNGSTEN CARBIDE-COBALT COMPOSITES

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NATURE OF THE BOUNDARY PLANES IN TUNGSTEN CARBIDE-COBALT COMPOSITES

J. Vicens, S. Lay, M. Benjdir, G. Nouet

To cite this version:

J. Vicens, S. Lay, M. Benjdir, G. Nouet. NATURE OF THE BOUNDARY PLANES IN TUNGSTEN CARBIDE-COBALT COMPOSITES. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-353-C1- 358. �10.1051/jphyscol:1990156�. �jpa-00230317�

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COLLOQUE DE PHYSIQUE

Colloque Cl, suppl6ment au nOl, Tome 51, janvier 1990

NATURE OF THE BOUNDARY PLANES IN TUNGSTEN CARBIDE-COBALT COMPOSITES

J. VICENS, S. LAY, M. BENJDIR and G. NOUET

Laboratoire d'Etudes et de Recherches sur les Materiaux, CNRS U R A 1317, Institut des Sciences de la Matiere et du Rayonnement, Boulevard du MarBchal Juin, F-14032 Caen Cedex, France

Resume

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Une etude systematique des joints de grains asymetriques correspondant a une rotation autour de la direction dense

<2ii0> a ete realisee dans le carbure de tungstene ou cette orientation est observee frequenunent. Les orientations donnant naissance a une coincidence tridimensionnelle sont comparees a celles pouvant se decrire par un parallelisme entre plans denses a lfinterface. I1 a ete ainsi montre que les deux descriptions sont theoriquement tres proches lfune de l'autre pour un grand nombre dforientations. Des observations de joints asymetriques sont presentees.

Abstract- A systematic study of asymmetrical grain boundaries corresponding to rotations around the close packed <2110>

direction has been realized in tungsten carbide where these orientations are often found. Orientations giving rise to a tridimensional coincidence have been compared to those described by a parallelism of dense planes of the two crystals at the interface. So it has been shown that the two descriptions are theoretically very close to each other for a large number of orientations. Experimental observations of asymmetrical grain boundaries are presented.

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INTRODUCTION

Many experiments have shown that tridimensional coincidence orientations lead to a low energy boundary formation when the grain boundary plane is parallel to a dense plane of the CSL (Coincidence Site Lattice) /l/.

Parallelism of close packed planes and directions across the interface has often been observed to be the common characteristic of favoured orientations. Many examples can be found in precipitation phenomena /2/ as well as in heterophase structures /3, 4, 5/. Obser- vations have already demonstrated that parallelism of close packed planes and directions can take place instead of orientations giving rise to a high tridimensional coincidence.

The aim of this work is to examine the two approaches in tungsten carbide grain boundaries of WC-CO composites where coincidence orientations are often found for carbide grain boundaries /6/. A large number of asymmetrical tilt grain boundaries have been exami- ned. The two descriptions are given and compared to experimental results.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990156

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COLLOQUE DE PHYSIQUE

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^MATERIAL

The tungsten carbide-cobalt materials are prepared by a liquid phase sintering process where cobalt is mainly used as the binder phase.

At the sintering temperature (W 1400°C) the liquid cobalt phase can dissolve some amount of tungsten carbide crystals. After saturation of the liquid cobalt phase and during the cooling process, precipi- tation, redeposition and growth of tungsten carbide crystals on preexisting carbide crystals occur.

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STRUCTURAL CONSIDERATION

Tungsten carbide has a single hexagonal structure with two atoms by unit cell (Fig. 1). The lattice parameters are a = 0.2906 nm and c=0.2837 nm with a (c/a) ratio close to 1 (0.976). The equilibrium shape of tungsten carbide crystals consists of triangular base crys- tals with facets parallel to (10i0) planes limited in the perpendi- cular direction by the basal plane.

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ASYMMETRIC TILT GRAIN BOUNDARIES IN WC

During the sintering process the tungsten carbide crystals develop well defined facets and tend to adopt the equilibrium shape. Two planes play an important role in the formation of tungsten carbide interfaces, the (10i0) prismatic plane and the (0001) basal plane often observed as main facets in WC crystals /7/.

The usual boundary- plane found between carbide grains in WC-CO cermets is the (1010) prismatic plane. Low energy twist boundaries admitting the prismatic plane as boundary plane are frequently observed in WC.

All these s m e t r i c a l boundaries are described by a rotation around a common <1010> axis. In contrast asymmetrical configurations are formed when the two crystals are rotated around the <2110> axis. In this case the usual boundary plane is parallel to the basal plane of one crystal. Consequently the indices of the boundary plane in the other crystal are high crystallographic indices for an exact coin- cidence orientation.

The list of -coincidence orientations corresponding to rotations around the <2110> axis is given in the table 1 up to 2=100 for the ratio (c/a12 = 1. The table indicates for each Z value the planes (crystal 2) nearly parallel to the basal plane and to the (0i10) prismatic plane (crystal 1) respectively. The angular deviation a

from these planes has also been denoted. Half cases have a low index plane parallel or very close to the basal plane for an exact coin- cidence orientation. For the other cases the deviation is quite low ( W ) except for some examples (27, Z31, Z61). It appears also that coincidence descriptions with high indices can also been described by a parallelism between the densest planes of WC : (0001)11/(1010)2

( Z 52b and 297).

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TABLE 1

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List of the coincidence orientations (X<100) corresponding to rotations by 8 angle around the < 2 i i 0 > common axis for the ratio ( c / a 1 2 = 1 . The (okie) planes ( I kl

+

^{l il}

+

^{l &} l 5 1 0 ) for the crystal 2 found parallel or close to the ( 0 0 0 1 ) and ( 0 1 1 0 ) planes of the crystal 1 have been determined for each Z coincidence orientation. The deviation angle a from these planes has been calculated with the real (c/a) ratio of WC.

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EXPERIMErJTAL OBSERVATIONS

Six grain boundaries related to rotations around the common < 2 i I 0 >

axis are presented in Fig. 2 to 7 with their corresponding diffraction pattern. Crystallographic data of these six cases have been indicated in table 2 which gives the smallest experimental rotation angle 8,, the closest Z value, the angular deviation A8 from the coincidence orientation and the indices of the grain boundary plane.

(okie )

,

4 ( 0 0 0 1 ) , ( a ) ( ' 1

0352 ( 0 . 6 )

o l i o

( 8 . 2 ) 0112 ( 2 . 8 ) 0 1 x 1 (1.6) 0 1 i 3 ( 1 . 2 ) 0223 ( 1 . 3 ) 0223 ( 5 . 2 ) 0 2 2 1 ( 3 . 3 ) 0 3 j 1 ( 1 . 6 ) 0 1 i 4 ( 0 . 7 )

o i i z

( 1 . 6 )

o l i o

( 2 . 2 ) 0li1 ( 4 . 2 ) 0225 ( 0 . 5 ) 0311 ( 1 . 9 ) 0 3 5 1 ( 0 . 4 ) 0 1 i 5 ( 0 . 5 ) 0225 (1.7)

o l i l

( 1 . 1 ) 0 2 2 1 ( 0 . 1 ) 0 l i 0 ( 0 . 6 )

e ( * )

6 0 8 1 . 7 9 3 2 . 2 0 4 6 . 8 3 21.79 3 8 . 2 1 42.10 6 9 . 4 3 7 5 . 1 8 1 6 - 4 3 2 7 . 8 0 8 7 . 8 0 5 2 . 6 6 24.42 71.64 7 3 . 1 7 1 3 . 1 7 2 6 . 0 1 4 9 . 5 8 6 6 . 0 1 8 9 . 4 1

.

All the boundaries have an asymmetric character and have been found parallel to the densest planes of WC ( 0 0 0 1 ) (* 1 to 5 ) and ( 0 1 1 0 )

(= 3 and 6 ) with two facetted boundaries (a 3 and 6 ) . If a is nearly

equal to A8 and _a, low, it could be attributed to a parallelism between low index planes (* 2 and 5 ) . For the cases 1, 3 and 4 , where A0 is low and a, high, the orientation is better described b y a 3 dizensional coincidence orientation.

(okie)

,

4 ( 0 l i 0 ) , ( a ) ( ' 1

0 l i 2 ( 0 . 6 )

o l i S

( 4 . 5 ) 0352 ( 1 . 6 )

o l i i

( 5 . 3 ) 0 2 Z i ( 2 . 1 )

o l i i

( 3 . 4 )

o l i i

( 0 . 5 ) 0 l E ( 0 . 1 ) 0 l i Z ( 0 . 9 ) 0 3 3 i ( 0 . 1 ) 0332 ( 2 . 8 )

o o o i

( 2 . 2 ) 0225 ( 0 . 4 ) 0 2 2 i ( 0 . 5 )

o l i 5

( 2 . 3 ) 0 l i 4 (1.1) 0 4 S i ( 0 . 7 ) 025% ( 2 . 1 ) 0255 ( 3 . 5 ) 025% ( 0 . 3 ) 0 0 0 i ( 0 . 6 ) 4

7 1 3 1 9 2 8 a 2 8 b 3 1 3 7 4 3 4 9 5 2 a 5 2 b 6 1 67 7 3 7 6 a 7 6 b 7 9 9 1 a 9 1 b 9 7

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Cl-356 COLLOQUE DE PHYSIQUE

TABLE 2

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Experimental data. The low index planes (crystal 2) close to the basal or prismatic planes (crystal 1) have been denoted with the angular deviation a, from the exact parallelism. a is the angular deviation between these planes in the exact coincidence orientation.

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CONCLUSION

The experimental observations of asymmetrical grain boundaries in WC show no favoured tendency of an exact parallelism between low index planes. Only two cases have been found in agreement with this description. It is actually very difficult to differentiate the parallelism between low index planes from a deviation from an exact coincidence orientation. It is likely to be due to the WC structure where the two descriptions are very close to each other.

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REFERENCES

/l/ Bollmann, W., "Crystal Lattice, Interfaces, .Matrices1t, Boll- mann, Geneve, 1982.

/2/ Dahmen, U., Acta Metall. 30, (1982), 63.

/3/ Erb, U., Abel, W., Gleiter, H., Scripta Metall. U, (1982), 1317.

/4/ Gleiter, H., J. Physique 46, (1985), C4 393.

/5/ Fecht, H.J., Lojkowski, W., Gleiter, H., J. Physique 46, (1985), C4 107.

/6/ Vicens, J., Laurent-Pinson, E., Chermant, J.L., Nouet, G., J.

Physique 49, (1988), 271.

/7/ Vicens, J., Lay, S., Laurent-Pinson, E., Nouet, G., Summer School, "Surfaces and Interfaces of Ceramic Materialsw, (1988) NATO AS1 Oleron, France, to be published.

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Fig. 1 : WC structure.

Fig. 2 : Two prismatic planes rotated by 54'. The boundary plane has been found parallel to the (0001)1 plane.

Fig. 3 : Exact parallelism between the two (0001)1 and ( 0 3 5 1 ) ~ planes is observed in the boundary shown in (a) with its corresponding diffraction pattern (b).

Fig. 4 : Facetted boundary imaged in high resolution. The two facets are parallel to (0001) and (1010) (a)

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Corresponding diffraction pattern (b)

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Cl-358 COLLOQUE DE PHYSIQUE

O O C

Fig. 5 : Part of a boundary between two WC crystals found in a 267 coincidence orientation (a) with its corresponding diffraction pattern (b)

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Fig. 6 : Boundary (a) where an exact parallelism between the two densest planes (0001) and (01i0) has been found (b).

Flq. 7 _:Grain facetted along the (0001) and (0170) planes. A part of the boundary plane has been found parallel to the prismatic plane.