TEM INVESTIGATIONS OF WC-Co ALLOYS AFTER CREEP EXPERIMENTS

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TEM INVESTIGATIONS OF WC-Co ALLOYS AFTER CREEP EXPERIMENTS

S. Lay, F. Osterstock, J. Vicens

To cite this version:

S. Lay, F. Osterstock, J. Vicens. TEM INVESTIGATIONS OF WC-Co ALLOYS AFTER CREEP EXPERIMENTS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-685-C1-689.

�10.1051/jphyscol:19861105�. �jpa-00225500�

Colloque Cl, supplement au n 0 2 , Tome 47, fevrier 1986 page cl-685

TEM INVESTIGATIONS OF WC-CO ALLOYS AFTER CREEP EXPERIMENTS

S. LAY, F. OSTERSTOCK and J. VICENS

Equipe MatBriaux-Microstructure du Laboratoire de

Cristallographie, Chimie et Physique des Solides, U.A. 251, ISMRa,UniversitB, F-14032 Caen Cedex, Prance

R6sumC - Des alliages carbure de tungstkne-cobalt ont CtG dCformCs par com- pression ou par flexion trois points dans un domaine de temp6rature 1000- 1350°C et dans un domaine de contrainte 30-1000 MPa. Dans ces conditions, l'exposant des contraintes n des alliages WC-CO est fonction uniquement du pourcentage volumique en cobalt et tend vers n

l pour Le carbure pur.

Lt6nergie dlactivation apparente est 550 kj-mole-1. Des observations en M.E.T. sur des 6chantillons de carbure pur deform6 H 1450°C montrent une dCformation intragranulaire importante. L1analyse de ces defauts a Ctk r6ali- see. De plus, une Ctude prCcise a montre un comportement different entre les joints de grains en coYncidence

2, 4 , 13a) et les joints de grains generaux. Ces analyses

i llaide des donn6es macroscopiques permettent de proposer un modPle pour la dkformation des composites carbure de tungstkne- cobalt.

Abstract - Carbide tungsten cobalt alloys were deformed in compression or in three point bending in a temperature range 1000-1350°C and in a stress domain 30-1000MPa. In these conditions, the stress exponent n of WC-CO alloys is a function of only the cobalt volurnic ratio and tends tow rds n B

1 for pure carbide. The apparent activation energy is 550 kj mole- . ^T.E.M. investiga- tions on pure carbide deformed at 1450°C show an extensive intragranular deformation. Analysis of these defects have been performed. Moreover an accurate study has shown a different behaviour between coincidence grain boundaries (1

2, 4 , 13a) and general grain boundaries. These data and macroscopic results allow to propose a model for the deformation of carbide

tungsten - cobalt composites.

I - INTRODUCTION

Up to recently the high temperature deformation behaviour has been mainly investi- gated at temperatures less than 1000°C. Only Gottschall, Williams and Ward /l/ made measurements at temperature up to 1400°C. A systematic study of the creep of WC-CO in the temperature range 1000-1350°C has been started in our laboratory 1 2 1 . Thus the general creep behaviour of the composite and the influence of the microstructure on its parameters have been determined. Due to the lack of informations on the dislocation structures after deformation, it has not been possible to propose a creep mechanism. The reason is the complexity of the defect structure of the carbide in the as-sintered state /1,3/.

The aim of this work is to get accurate informations on the dislocation structures after high temperature deformation. To do this we have previously annealed as- sintered samples at 1700°C

the defect structure thus disappears 141. These samples will be denoted as pure carbide in the remaining of this paper. Thin slices of these samples deformed at high temperature have been observed in T.E.M.. The expected results are correlated with macroscopic data in order to propose a microstructural creep model of the WC-CO composites.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861105

C l - 6 8 6 JOURNAL DE PHYSIQUE

I1 - EXPERIMENTAL PROCEDURE

Samples w i t h d i f f e r e n t g r a n u l o m e t r y (gWC

0 . 7 , 1.1, 2.2 pm) and d i f f e r e n t c o b a l t volumic r a t i o (V (CO) v

5-37 %) were deformed i n c o m p r e s s i o n o r i n t h r e e p o i n t b e n d i n g u n d e r vacuum. The c r e e p e x p e r i m e n t s were performed i n t h e t e m p e r a t u r e r a n g e 1000-1350°C and i n a s t r e s s domain 30-1000 MPa. For e l e c t r o n m i c r o s c o p y i n v e s t i g a - t i o n s , specimens ( 5

5

5 m 3 ) were s p a r k - c u t from s i n t e r e d WC-CO a l l o y s ( 3 wt

CO). The s a m p l e s had been a n n e a l e d a t 1700°C f o r f o u r h o u r s . During t h i s t r e a t m e n t , t h e c o b a l t p h a s e e v a p o r a t e d . These samples h a v e been deformed i n c o m p r e s s i o n a t 1450°C and c o o l e d u n d e r l o a d w i t h a s t r a i n v a l u e o f 1 5 %. Thin s l i c e s were s p a r k machined down t o a t h i c k n e s s of 80pm and 3 m i n d i a m e t e r . These d i s c s were t h e n i o n m i l l e d f o r e l e c t r o n m i c r o s c o p e i n v e s t i g a t i o n s . J e o l lOOCX and 200CX m i c r o s c o p e s were used.

I11 - CREEP BEHAVIOUR OF WC-CO MATERIALS

R e s u l t s o f t h e c r e e p e x p e r i m e n t s h a v e b e e n p l o t t e d i n a log;-log

diagram. The r e s u l t i n g c u r v e s have a s i g m o i d a l s h a p e w i t h t h r e e s t a g e s p o s s e s s i n g a d i f f e r e n t s t r e s s e x p o n e n t . The i n t e r m e d i a t e s t a g e ( s t a g e 1 1 ) h a s b e e n p a r t i c u l a r l y s t u d i e d . I n s t a g e 11, t h e s t r e s s e x p o n e n t , n , i s shown t o depend o n l y on t h e c o b a l t v o l u m i c r a t i o , t e n d i n g t o

v a l u e o f 1 a s t h e c o b a l t c o n t e n t t e n d s t o z e r o i n d e p e n d e n t l y w i t h t h e c a r b i d e g r a i n s i z e ( F i g a 0 l ) . The i n f l u e n c e of t h e l a t t e r a p p e a r s however i n - -2 t h e s t r a i n r a t e e x p r e s s i o n a s

B 0

bWC , where EwC i s t h e mean c a r b i d e g r a i n s i z e . The v a l u e o f t h e a p p a r e n t a c t i v a t i o n e n e r g y QA i n s t a g e I1 i s 550 k j mole-> ( F i g . 2 ) . A same v a l u e f o r QA h a s been found f o r a l l t h e b a t c h e s u s e d i n t h i s s t u d y . T h i s v a l u e i s h i g h when compared w i t h s e l f - d i f f u s i o n e n e r g y of t h e c o b a l t . On t h e o t h e r hand t h e d e f o r m a t i o n o f t h e m a t e r i a l i s o b t a i n e d a t h i g h s t r e s s l e v e l s . It a p p e a r s

t h u s t h a t t h e d e f o r m a t i o n of t h e c o m p o s i t e is c o n t r o l l e d by t h e c a r b i d e phase.

O b s e r v a t i o n s o f deformed m a t e r i a l s had been made u s i n g a s c a n n i n g e l e c t r o n m i c r o s c o p e . The specimen had been p o l i s h e d , t h e n s c r a t c h e d and f i n a l l y deformed t o v a r i o u s l e v e l s o f d e f o r m a t i o n . G r a i n boundary s l i d i n g o f t h e c a r b i d e g r a i n s had b e e n o b s e r v e d e i t h e r a s i n d i v i d u a l c r y s t a l s o r a s g r o u p s of c r y s t a l s 1 2 1 .

I V - TEM OBSERVATIONS AND ANALYSIS I N PURE W C DEFORMED AT 1450°C

TEM o b s e r v a t i o n s of t h e c a r b i d e p h a s e a f t e r h i g h t e m p e r a t u r e d e f o r m a t i o n r e v e a l a n e x t e n s i v e i n t r a g r a n u l a r d e f o r m a t i o n . D i s l o c a t i o n t a n g l e s , e x t e n d e d d i s l o c a t i o n s and

A 2 0 2 5 compression

0 A103

ow m 0 H 0 3 1

0 .

-

.

SO KJ MOLE-' -6

1

-

30 _ g l480 l l p 0 128p1230 11.80 1!30 1080 -'OC

0 10 20

v,(Co)%- 5 . 5 6 7 8

a4-

O K

F i g . 1 - S t r e s s e x p o n e n t a s a f u n c t i o n F i g . 2 - Creep a c t i v a t i o n e n e r g i e s f o r of c o b a l t volume r a t i o . v a r i o u s m a t e r i a l s

V (CO)

5 % A103 (DW

2.2 pm), H03T

(8

0.8 pm), ~ $ 0 0 3 (DwC

0.7 pm) (&Fee p o i n t b e n d i n g )

V v (CO)

37 % A2025

0.7 pm) (corn-

p r e s s i o n ) .

ved i n WC a r e due t o t h e h i g h t e m p e r a t u r e d e f o r m a t i o n t r e a t m e n t . D i f f e r e n t t y p e s o f s u b g r a i n s have been found ; t h e y a r e m a i n l y t i l t s u b g r a i n b o u n d a r i e s formed by a s i n g l e f a m i l y o f p e r f e c t d i s l o c a t i o n s 113 <1120>(1), < 0 0 0 1 > ( I I ) and 113 <1123>(111).

They can r e s u l t from d i s l o c a t i o n s o u r c e s i n t h e p l a n e s (0001) o r (1010) f o r t y p e s ( I ) and ( 1 1 ) o r p y r a m i d a l (1121) p l a n e s f o r t y p e (111) and c l i m b p r o c e s s e s .

As a c o m p a r i s o n , d e f e c t s r e s u l t i n g from p l a s t i c d e f o r m a t i o n a t room t e m p e r a t u r e a r e p r e d o m i n a n t l y e x t e n d e d d i s l o c a t i o n s 116 <1123> which a r e g l i s s i l e c o n f i g u r a t i o n s - i n t h e ( l i 0 0 ) p l a n e s / 3 , 7 , 8 / . Some i s o l a t e d and d i s s o c i a t e d d i s l o c a t i o n s 116 <1123>

h a v e b e e n a l s o found a f t e r h i g h t e m p e r a t u r e d e f o r m a t i o n . They c l e a r l y show a recom- b i n a t i o n o v e r a p a r t of t h e i r l i n e due t o cllrnb p r o c e s s e s ; A d e t a i l e d a n a l y s i s o f s u c h a d i s l o c a t i o n i s p r e s e n t e d o n F i g . 3. F i g . 3 a ) i s a weak beam image o b t a i n e d w i t h t h e 0111 r e f l e c t i o n . The d i s l o c a t i o n is d i s s o c i a t e d i n t h e p r i s m a t i c ( 1 0 i 0 ) p l a n e (12.5 !m) i n t h e p a r t d e n o t e d L1 and n o t d i s s o c i a t e d i n t h e p a r t L2. I n F i g . 3 b ) t h e (1010) d i s s o c i a t i o n p l a n e is n e a r l y p a r a l l e l t o t h e e l e c t r o n beam

t h e r e - f o r e t h e two d i s l o c a t i o n l i n e s a r e s u p e r i m p o s e d i n t h e p a r t L2. F i g . 3 c ) is t h e s t e r e o g r a p h i c p r o j e c t i o n of t h e p r e c e d e n t c a s e . The-Burgers v e c t o r , t h e d i s l o c a t i o n l i n e d i r e c t i o n s L and L and t h e d i s s o c i a t i o n ( 1 0 1 0 ) p l a n e a r e i n d i c a t e d on t h e

drawing. 1 2

S y s t e m a t i c o b s e r v a t i o n s o f g r a i n b o u n d a r i e s h a v e b e e n performed a f t e r h i g h tempera- t u r e d e f o r m a t i o n on c o i n c i d e n c e and g e n e r a l g r a l n b o u n d a r i e s . I t a p p e a r s t h a t many c o i n c i d e n c e g r a i n b o u n d a r i e s f r e q u e n t l y o b s e r v e d i n t h e m a t e r i a l ( 3 ) (C

2 , 4 , 1 3 a ) r e v e a l t h e p r e s e n c e o f a number o f d i s l o c a t i o n s i n t h e i r boundary p l a n e . On t h e c o n t r a r y t h e y a r e n o t o b s e r v e d i n g e n e r a l g r a i n b o u n d a r i e s . A d e t a i l e d a n a l y s i s of d i s l o c a t i o n s i n a

2 g r a i n boundary is p r e s e n t e d on F i g . 4. Four g r a i n s ( d e n o t e d

C

F i g . 3 - Climb o f a d i s s o c i a t e d d i s l o c a - t i o n

a ) weak beam image o b t a i n e d w i t h g

=

0111. The p a r t d e n o t e d LI of t h e d i s l o -

c a t i o n l i n e i s d i s s o c i a t e d

A recombina-

t i o n of t h e d i s l o c a t i o n l i n e

o b s e r v e d

i n t h e p a r t L The d i s l o c a t i o n i s l o c a l l y

d i s s o c i a t e d ?A ^C.

B r i g h t f i e l d image

o b t a i n e d w i t h g

1011. The two p a r t i a l

d i s l o c a t i o n s a r e superimposed. c ) S t e r e o -

g r a p h i c p r o j e c t i o n . The d i s l o c a t i o n l i n e

d i r e c t i o n s Ll.and L 2 , t h e B u r g e r s v e c t o r

and t h e dissociation p l a n e a r e i n d i c a t e d .

C l - 6 8 8 JOURNAL DE PHYSIQUE

1, 2 , 3 and 4 ) a r e imaged i n F i g . 4.a. A c h a r a c t e r i z a t i o n o f t h e i r o r i e n t a t i o n h a s been performed. X

2 g r a i n b o u n d a r i e s h a v e been found between 1 - 2 and 2 - 3 g r a i n s and g r a i n b o u n d a r i e s between 1 - 4 and 3 - 4 a r e i n a random o r i e n t a t i o n . I n t e r a c t i o n s between s u b g r a i n s ( d e n o t e d S - J on F i g . 4 . a ) and g r a i n b o u n d a r i e s can be a l s o s e e n . Only d i s l o c a t i o n s i n t h e C

2 g r a i n boundary 1 - 2 have been ana- l y z e d . A m a g n i f i e d image of t h e s e d i s l o c a t i o n s i s p r e s e n t e d on F i g . 4.b. The s t e r e o - g r a p h i c p r o j e c t i o n i s p r e s e n t e d o n F i g . 4.c. The B u r g e r s v e c t o r of t h e s e d i s l o c a - t i o n s 1 1 3 [ i 2 i 3 ] h a s a component o u t o f t h e (7100) boundary p l a n e ( F i g . 4 . c ) . T h e r e f o r e t h e movement of t h e s e g r a i n boundary s t r u c t u r a l d i s l o c a t i o n s (G.B.S.D.) i n v o l v e s g l i d e and c l i m b p r o c e s s e s .

These o b s e r v a t i o n s c a n b e r e l a t e d t o p r e v i o u s s t u d i e s ( 9 , 1 0 , 1 1 ) c o n c e r n i n g t h e g r a i n boundary s l i d i n g b e h a v i o u r and d i s l o c a t i o n s t r u c t u r e s

h a s been proved t h a t s l i d i n g of c o i n c i d e n c e g r a i n b o u n d a r i e s i s d i f f i c u l t and s l i d e h a r d e n i n g oc- c u r s . D i s l o c a t i o n s a r e c o n s e q u e n t l y o b s e r v e d . These o b s e r v a t i o n s a r e q u a l i t a t i v e l y i n a g r e e m e n t w i t h t h e p r e s e n t i n v e s t i g a t i o n . They c a n b e e x p l a i n e d by a d i s l o c a t i o n model o f s l i d i n g ( 1 2 ) where a b s o r p t i o n of l a t t i c e d i s l o c a t i o n s i n c o i n c i d e n c e g r a i n b o u n d a r i e s i s more d i f f i c u l t t h a n i n g e n e r a l g r a i n b o u n d a r i e s . I f a c l i m b p r o c e s s i s i n v o l v e d f o r t h e movement of G.B.S.D., t h e amount o f s l i d i n g depends on t h e g r a i n boundary s t r u c t u r e . The g r a i n boundary d i f f u s i o n and i n consequence t h e c l i m b r a t e a r e more d i f f i c u l t i n c o i n c i d e n c e g r a i n b o u n d a r i e s t h a n i n g e n e r a l g r a i n b o u n d a r i e s .

V - MICROSTRUCTURAL MODEL OF THE WC-CO COMPOSITES

The c o r r e l a t i o n between t h e p r e v i o u s r e s u l t s and t h e M.E.B. and T.E.M. o b s e r v a t i o n s of deformed s a m p l e s a l l o w s o n e t o p r o p o s e two s i m u l t a n e o u s mechanisms a c t i n g i n t h e c a r b i d e p h a s e d u r i n g d e f o r m a t i o n

- a g r a i n boundary s l i d i n g o f " i s o l a t e d " c r y s t a l s whose g r a i n b o u n d a r i e s a r e i n t e r p h a s e b o u n d a r i e s o r random g r a i n b o u n d a r i e s , g i v i n g a s t r e s s e x p o n e n t of n

1.

F i g . 4 - A n a l y s i s o f d i s l o c a t i o n s i n a X

2 g r a i n boundary

a ) g r a i n b o u n d a r i e s between 1 - 2 and 2 - 3 g r a i n s a r e X

2 g r a i n b o u n d a r i e s . D i s l o c a t i o n s a r e imaged i n t h e i r boundary p l a n e s . G r a i n b o u n d a r i e s between 1 - 4 and 3 - 4 g r a i n s a r e g e n e r a l g r a i n b o u n d a r i e s . F r e q u e n t i n t e r a c t i o n s between s u b g r a i n s ( d e n o t e d S - ^J) and X = 2 g r a i n b o u n d a r i e s a r e o b s e r v e d .

b) M a g n i f i c a t i o n of t h e g r a i n boundary 1 - 2.

c ) S t e r e o g r a p h i c p r o j e c t i o n c o r r e s p o n d i n g t o t h e g r a i n boundary 1 - 2.

boundaries. This mechanism is described by a power law creep (n > ^1). The observed value of n is a function of the relative number of these grain boundaries. Their ratio depends on the cobalt volumic ratio and seems to be a consequence of the sintering process.

A schematic representation of the microstructural model is presented in figure 5 where the three elements of the microstructure are indicated

obalt

chains of carbi e crystals linked by coincidence grain boundaries& and

6

carbide grains . Sliding of "isolated" crystals induces the intragranular defor- mation of the a jacent crystal chains. Both mechanisms are accomodated by intragra- nular dislocation movement (glide and climb). This explains the constancy of the apparent activation energy over the whole domain of cobalt volumic ratio.

Fig. 5 - Schematic representation of the microstructure of WC-CO composites.

V1 - CONCLUSION

Detailed microscopic investigations correlated with high temperature creep results show that WC-CO composites have a very specific microstructure which influences strongly their mechanical behaviour.

REFERENCES

/l/ Gottschall, R.J., Williams W.S. and Ward, T.D., Phil. Mag. A, 41, ^{(1980) 1.}

/2/ Osterstock, F., thesis, university of CAEN, France (1980).

/3/ Hagkge, S., thesis, University of CAEN, France (1979).

/h/ Lay, S., Delavignette, P. and Vicens, J., Phys. Stat. sol. (a) - ^{89 (1985)} ....

151 Lay, S., thesis, University of CAEN, France (1985).

161 Lay, S., Chermant, J.L. and Vicens, J., Proc. Internat. Conf. Deformation of Ceramic Materials. Ed. Tressler R.E. and Bradt R.C., Vol. 18, Plenum Press, New York (1984) 463.

171 Hagege, S., Vicens, J., Nouet, G. and Delavignette, P - , Phys. Stat. Sol. (a), 61 (1980) 675.

181 K b b s , M.K. and Sinclair, R., Acta Metall., 29 (1981) 1645.

191 Watanabe, T., Yamada, M - , Shima, S. and ~arashima, S., Phil. Mag., A40 ⁽¹⁹⁷⁹⁾

667.

/10/ Biscondi, M., J. Phys., f t 2 2 (1982) 293.

1111 Kokawa, H., Watanabe, T. and Karashima, S., Phil. Mag., 44A (1981) 1239.

/12/ Horton, C.A.P., Silcock, J.M. and Kegg, G.R., Phys. Stat. Sol. (a) 6 ⁽¹⁹⁷⁴⁾

215.