INFLUENCE OF GRAIN BOUNDARY SEGREGATION ON MECHANICAL PROPERTIES OF ACTIVATED SINTERED TUNGSTEN

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INFLUENCE OF GRAIN BOUNDARY

SEGREGATION ON MECHANICAL PROPERTIES

OF ACTIVATED SINTERED TUNGSTEN

S. Hofmann, H. Hofmann

To cite this version:

S. Hofmann, H. Hofmann. INFLUENCE OF GRAIN BOUNDARY SEGREGATION ON

MECHAN-ICAL PROPERTIES OF ACTIVATED SINTERED TUNGSTEN. Journal de Physique Colloques,

1985, 46 (C4), pp.C4-633-C4-640. �10.1051/jphyscol:1985468�. �jpa-00224723�

INFLUENCE OF GRAIN B O U N D A R Y S E G R E G A T I O N ON M E C H A N I C A L P R O P E R T I E S OF A C T I V A T E D S I N T E R E D T U N G S T E N

S. Hofmann and H. Hofmann

Max-Planck-Institut für Metallforschung , Institut für Werkstoffwissenschaften, Seestvasse 92, D-7000 Stuttgart 1, F.R.G.

Résumé - Le tungstène fritte entre 1300 et 1500°C, en présence de petites quantités de Ni et Fe, a une faible résistance mécanique. La résistance à la flexion est d'environ 400 MPa et la rupture est toujours intergranulaire. La cohésion intergranulaire est contrôlée par la teneur et la distribution d'impuretés comme 0, C, P et d'éléments d'addition comme Fe et Ni. Les mesures par S.E.A. montrent que la ségrégation de C augmente la cohésion des joints de grains dans le W, alors que la ségrégation de P la diminue. Abstract - Tungsten sintered with small Ni and Fe additions at temperatures ranging between 1300 and 1500°C, shows low mechanical strength. The bend strength is about 400 MPa and the samples show always an intercrystalline fracture mode. The strength of the interfaces is controlled by the amount and distribution of impurities such as 0, C, P and alloying elements such as Fe and Ni. AES measurements show, that segregation of carbon improves the cohesion of tungsten grain boundaries whereas P segregation decreased the cohesion.

INTRODUCTION

Small additions of Villa metals to refractory metals like W decrease the sintering temperatures below 1500°C. The mechanical strength of this so called activated sintered W is very poor /1,2/. All samples show an intercrystalline and therefore brittle fracture. It is well known that the segregation level at the grain boundary influences the cohesion. According to Seah /3,4/, the segregated atoms influence the force per area between the two parts of the solid when they are separated by a displacement normal to the fracture plane. Elements with a higher pair bonding energy compared to the matrix atoms increase the strength and vice versa. McMahon and Vitek /5/ proposed that segregated elements change the surface energy and therefore the work of fracture and the cohesion of the grain boundary. Additionally McMahon and Vitek also studied the influence of plastic work (e.g. crack blunting by dislocation emission) on the fracture energy and the relationship between the surface energy and the amount of plastic work. Small changes of the surface energy result in a pronounced change of the plastic work and also of the fracture energy. In binary alloys, only the interaction between the segregated elements and the host metal must be considered. In multicomponent systems such as activated sintered W, the interaction between the impurities and alloying elements among one another as well as with the host metal must be taken into account. Guttmann /6/ proposed a model of the influence of alloying elements on the segregation behavior of im-purities in ternary alloys. According to this model, alloying-elements can in-crease the segregation of impurities (cosegregation) when the interaction coeffi-cient between alloying element and impurity is stronger than the interaction coefficient between impurity and host metal. No model exists for the effect of the interaction between various segregated elements on the grain boundary cohesion. In the present paper, our aim is to show the influence of heat treatment on the segregation level at grain boundaries in activated sintered W. Additionally, the

C4-634 J O U R N A L D E PHYSIQUE

i n f l u e n c e o f the segregation l e v e l on the g r a i n boundary cohesion as w e l l as t h e e f f e c t of a l l o y i n g elements on t h e segregation o f i m p u r i t i e s w i l l be shown.

EXPERIMENTAL PROCEDURE

Commercially pure tungsten powder c o n t a i n i n g 47 ppm P, 31 ppm Si, 27 ppm As, 20 ppm C and 550 ppm Mo as the main i m p u r i t i e s was used. The powder was doped w i t h 0.05 wt.% Ni and 0.17 wt.% Fe. This composition corresponds t o t h e s o l u - b i l i t y l i m i t o f Ni and Fe i n W 171. The mixed powder was c o l d i s o s t a t i c a l l y pressed a t 635 MPa. F i v e d i f f e r e n t s i n t e r i n g procedures were used (Table I): The compacts were heated up a t 3 K/min t o 1470°C i n f l o w i n g Hz atmosphere and subsequently quenched o r cooled a t 3 Klmin t o 1000°C and quenched i n He. A l t e r n a t i v e l y t h e compacts were preannealed i n Ar which contained 3 vol.% CH f o r 15 o r 30 min a t 1000°C and subsequently heated up a t 3 Klmin t o 1470°C on 82. These samples were cooled a t 3 Klmin t o 1000°C i n H2 and quenched o r were quenched and subsequently annealed a t 700°C f o r 4 h i n Hz.

The f o u r p o i n t bend strength, the d e n s i t y and g r a i n s i z e were measured as w e l l as t h e composition o f the f r a c t u r e surface u s i n g AES. For t h e AES measurements, f r e s h f r a c t u r e surfaces were obtained by impacting the specimens i n u l t r a h i g h vacuum (< 10-7 Pa) a t room temperature i n t h e f r a c t u r e stage o f a commercial Auger e l e c t r o n spectrometer ( t h i n f i l m analyzer, Phys. E l e c t r . Ind.). Auger spectra o f t h e

f r a c t u r e surfaces were taken about 1-2 min a f t e r f r a c t u r e and t h e r e f o r e contamina- t i o n (C,O) f r o m t h e r e s i d u a l gas atmosphere was minimized. Q u a n t i f i c a t i o n o f t h e AES r e s u l t s i s discussed i n Ref. /to/.

RESULTS

A l l specimens show a b r i t t l e i n t e r c r y s t a l l i n e f a i l u r e (Fig. 1). The bend strength, t h e o r e t i c a l d e n s i t y , g r a i n s i z e and t h e g r a i n boundary composition are l i s t e d i n Table 2. The d e n s i t y o f t h e samples was > 98.3 % o f t h e t h e o r e t i c a l density.

Fig. 1

-

T y p i c a l f r a c t u r e surface o f a c t i v a t e d s i n t e r e d W.

The bend s t r e n g t h decreases w i t h decreasing c o o l i n g r a t e . Annealing a t 700°C a l s o decreases t h e s t r e n g t h o f the s i n t e r e d a l l o y . S i m i l a r l y t h e g r a i n s i z e a l s o v a r i e s w i t h t h e c o o l i n g c o n d i t i o n s . Quenched samples show a g r a i n s i z e o f 28 pm whereas

Nr. preannealing c o o l i n g c o n d i t i o n s no no 15 min 15 min 30 min quenched

3 K/min t o 1000°C and quenched 3 K/min t o 1000°C and quenched quenched and annealed a t 700°C f o r 4 h

quenched and annealed a t 700°C f o r 4 h

Table 2 - Bend strength, g r a i n s i z e and g r a i n boundary compositions.

Sample Bend T h e o r e t i c a l Grain Grain Boundary

Strength Density Size Composition

t h e g r a i n s i z e o f s l o w l y cooled o r annealed samples i s 40 ym and 47 ym. S i m i l a r l y t h e g r a i n boundary composition a1 t e r s w i t h the d i f f e r e n t c o o l i n g c o n d i t i o n s . Slowly cooled o r annealed samples show n e a r l y t h e same g r a i n boundary composition, whereas i n quenched samples lower values o f Ni, Fe and

P

b u t h i g h e r values o f 0 and C are detected. The specimen preannealed i n CH / A r and a f t e r s i n t e r i n g cooled a t 3 K/min (sample No. 3) shows a very h i g h C c o n t e l t and a low P content a t the g r a i n boundary.

DISCUSSION

The bend s t r e n g t h o f a c t i v a t e d s i n t e r e d W i s dependent on t h e d e n s i t y /I/, g r a i n s i z e /I/, g r a i n shape /2/ and g r a i n boundary cohesion. For t h e study o f the i n f l u - ence o f t h e segregation l e v e l on t h e g r a i n boundary cohesion, t h e i n f l u e n c e o f g r a i n s i z e and d e n s i t y on t h e bend s t r e n g t h must be considered. I n r e c e n t works /1,7/ i t was found t h a t the s t r e n g t h o f a c t i v a t e d s i n t e r e d W v a r i e s w i t h the g r a i n s i z e d ( i n ym) and p o r o s i t y E as f o l l o w s :

C4-636 J O U R N A L D E PHYSIQUE

Ref. /7/

u

= 192

+

1266 d - l I 2 ( f o r sample w i t h 98,9 % d e n s i t y ) . (2) According t o a model o f McLean /8/ t h e bend s t r e n g t h o f W which shows i n t e r - c r y s t a l l i n e f r a c t u r e depends on the g r a i n s i z e as f o l l o w s /7/:

The model o f McLean does n o t t a k e i n t o account t h e i n f l u e n c e o f g r a i n j u n c t i o n s on the f r a c t u r e energy /9/. The d e v i a t i o n o f t h e crack from i t s i n i t i a l d i r e c - t i o n r e q u i r e s a d d i t i o n a l energy. Therefore t h e decrease o f bend s t r e n g t h w i t h i n - c r e a s i n g g r a i n s i z e i s s m a l l e r than i n eq. (3).

The measured bend s t r e n g t h e x t r a p o l a t e d t o 100 % t h e o r e t i c a l d e n s i t y ( w i t h eq. ( 1 ) ) as w e l l as the c a l c u l a t e d bend s t r e n g t h according t o eqs. ( I ) , (2) and (3) are l i s t e d i n Table 3. The values o f /I/ and /7/ show the same i n f l u e n c e o f g r a i n s i z e on the bend strength. Therefore, we conclude t h a t i t i s t h e d i f f e r e n c e i n g r a i n boundary cohesion which causes t h e d e v i a t i o n o f t h e bend s t r e n g t h o f the d i f f e r e n t t r e a t e d W samples from these values.

Table 3

-

I n f l u e n c e o f t h e g r a i n s i z e on t h e bend s t r e n g t h o f W (assumption: ~ n t e r c r y s t a l l i n e f r a c t u r e ) .

Grain a ( p = 100 % T.D.)

u

u

u

Size t h i s work /I/ /8,7/ /7/

(pm) (MPal (MPa) (MPa) (MPa)

Accordi t o the model o f Seah /3,4/ a g r a i n boundary cohesion parameter

XAFE(Xg6! can be c a l c u l a t e d . This parameter considers t h e f r a c t u r e energy change (AFE) t o r each segregated element i as a f u n c t i o n o f a c o n c e n t r a t i o n i n t h e boundary

xyb.

Assuming a l i n e a r s u p e r p o s i t i o n o f AFE o f a l l elements a t t h e boundary the f o l l o w i n g r e l a t i o n i s obtained f o r t h e combined e f f e c t o f t h e de- termined elements Ni, Fe, C ,

0,

P

/ l o / :

F i g u r e 2 shows the e x p e r i m e n t a l l y determined f r a c t u r e s t r e n g t h ( e x t r a p o l a t e d t o I 0 vm w i t h eq. ( 2 ) ) versus t h e g r a i n boundary cohesion parameter. With t h e

xgb

values o f Table 2 , a good c o r r e l a t i o n i s obtained, i n d i c a t i n g t h a t t h e o b s e d e d behavior can a t l e a s t s e m i q u a n t i t a t i v e l y be i n t e r p r e t e d by t h e model o f Seah. The e v a l u a t i o n o f data necessary t o q u a n t i f y t h e model o f McMahon / 5 / i s v e r y d i f f i c u l t . I n p a r t i c u l a r , the i n f l u e n c e o f segregated elements on t h e s u r f a c e energy (Y) and t h e r e f o r e a l s o on the p l a s t i c work yp o f W i s unknown. When i n t e r - c r y s t a l l i n e f r a c t u r e occurs, y i s v e r y low /9,1 I/. Therefore, t h e bend s t r e n g t h of a c t i v a t e d s i n t e r e d W i s o n l b dependent on y. A t room temperature, y o f W ranges between 4,4 and 6,3 J/m2 /12/ and o n l y small changes w i t h segregation

occur /5/. Therefore, t h e bend s t r e n g t h o f a c t i v a t e d s i n t e r e d W a l s o v a r i e s o n l y s l i g h t l y w i t h the segregation l e v e l and t h e r e f o r e w i t h the c o o l i n g c o n d i t i o n s . This i s i n agreement w i t h t h e measurements (Table 2). The bend s t r e n g t h o f samples w i t h a h i g h segregation l e v e l o f C (37 at.% C) i s o n l y 19 % h i g h e r than t h e s t r e n g t h o f samples which -show a h i g h P s e g r e t a t i o n l e v e l (30 at.%

P).

From Table 2 i t i s e v i d e n t t h a t an i n v e r s e c o r r e l a t i o n between t h e C and P con- t e n t a t t h e g r a i n boundary e x i s t s ( F i g . 3 ) . This f i g u r e a l s o shows the c o r r e l a - t i o n between the Ni, Fe and

0

c o n c e n t r a t i o n a t the g r a i n boundary and t h e segre- g a t i o n l e v e l of P. The concentrations o f C as w e l l as 0 decrease whereas those o f N i and Fe increase w i t h i n c r e a s i n g P content. According t o Guttmann /6/ cosegre- g a t i o n i s p o s s i b l e when the i n t e r a c t i o n c o e f f i c i e n t a between t h e a l l o y i n g element and W as w e l l as between the i m p u r i t y and W i s lower than the i n t e r a c t i o n c o e f f i - c i e n t between t h e a l l o y i n g element and t h e i m p u r i t y . The r e l a t i v e i n t e r a c t i o n coef- f i c i e n t a ' I = CXMI

-

a w ~

:

(I = i m p u r i t y , M = a l l o y i n g element) f o r t h e i n - vestigate! systems are 11s% i n Table 4. and akgp a r e p o s i t i v e , whereas a l l o t h e r combination o f segregated element show a a t i v e i n t e r a c t i o n c o e f f i - c i e n t . A p o s i t i v e value o f a ' expresses t h e f a c t t h a t the balance between a l l t h e i n t e r a c t i o n s between atoms r e s u l t s i n a r e l a t i v e a t t r a c t i o n between t h e s o l u t e s w i t h r e s p e c t t o W. This i s i n agreement w i t h t h e observed segregation behavior ( F i g . 3). The contents o f N i and Fe a t t h e g r a i n boundary increase w i t h i n c r e a s i n g P content. A d d i t i o n a l l y , t e r n a r y W-M-I combinations w i t h a negative a ' value a l s o show a negative c o r r e l a t i o n i n t h e segregation behavior. With i n - c!&asing P c o n t e n t (and t h e r e f o r e i n c r e a s i n g N i and Fe c o n t e n t ) a t the g r a i n boundary t h e C as w e l l as t h e 0 l e v e l decrease (Fig. 3 ) . I t i s i n t e r e s t i n g t o note, t h a t the 0 l e v e l i s very low (4-10 at.%). The reason f o r t h i s low 0 l e v e l i s t h e v e r y s t r o n g i n t e r a c t i o n between W and 0 (1203 k J / m l ) , i.e., the forma- t i o n o f oxides i n the b u l k i s preferred.

The amount o f segregated P i s governed by the segregation f r e e energy o f P, AG

.

I n t h e s i x component system under study (W, Ni, Fe, 0, C, P) the chemical i n t e p - a c t i o n o f a l l the elements leads t o an a l t e r a t i o n o f the segregation f r e e energy, A G

,

which can be described by an expanded expression o f Guttmann's equa- ~ ~ ~ ~ t i 8 n 1151:

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DE

PHYSIQUE

Fig. 3

-

C o r r e l a t i o n between t h e P-content a t t h e g r a i n boundary and t h e segregation l e v e l o f

Ni,

Fe, C and 0.

50

Table 4

-

R e l a t i v e i n t e r a c t i o n f o e f f i c i e n t ? o f segregated elements i n W. F b M I

-

aWI

-

aWM; a = -AH../XiXj; AHij from /13/ and /14/.)

1 J

-

i

=

Ni, Fe,C,O

Segregated Elements Fe-P Fe-C Fe-0 Fe-Ni Ni-P N i - C N i - 0 P-0 P-C C-0

where A G : ~ ~ ~ = 2 aWp (x!

-

xkb) +

r

a j p

(xYb

-

x!) (5b) i

b

w i t h i = Fe, Ni,

0,

C , X and

x:~

are t h e b u l k and g r a i n boundary molar f r a c t i o n s o f element i

,

r e s p e c t i v e l y .

With eq. (5) t h e value o f AGp i n the W a l l o y cannot be c a l c u l a t e d because AG; i s unknown. With t h e data l i s t e d i n Table 2 i t i s p o s s i b l e t o make an e s t i m a t i o n o bthe i n f l u e n c e o f t h e a d d i t i o n a l segregated elements on Gp and t h e r e f o r

R

On

X i

.

TO s i m p l i f y eq. 5b, ~b was neglected because

X!

<< XyB. Values o f bGC em o f t e f i v e i n v e s t i g a t e d samples are l i s t e d i n Table 5 (aWp 151 k J / m l /14/). Al- though t h e d e s c r i p t i o n o f t h e s i x component system by a sum o f t h e d i f f e r e n c e s o f b i n a r y i n t e r a c t i o n terms i n eq. (5b) i s a r a t h e r s i m p l i f i e d approximation, a

good c o r r e l a t i o n between the observed P content a t the g r a i n boundary and A G ~ ~ ~ ~ i s obtained (Fig.

4).

High negative values o f AG;hem reduce AGp and t h e r e f o r e

t h e segregation l e v e l o f P i s low.

Fig. 4

-

Measured segregation l e v e l o f P versus A G : ~ ~ ~ c a l c u l a t e d w i t h eq. (5b).

Table

5

-

A G : ~ ~ ~ o f the i n v e s t i g a t e d samples.

Sample bG~hem P

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CONCLUSIONS

The observations described above can be summarized as f o l l o w s :

-

The c o o l i n g c o n d i t i o n s i n f l u e n c e the segregation l e v e l a t t h e g r a i n boundary.

-

_{C improves t h e g r a i n boundary s t r e n g t h whereas P and, l e s s e f f e c t i v e l y , 0, N i}

and Fe decrease t h e cohesion. This i s i n agreement w i t h t h e model p r e d i c t i o n s o f Seah /3,4/.

-

_{The segregation o f N i , Fe and P i s enhanced by each o t h e r .}

-

_{With i n c r e a s i n g N i , Fe and P c o n t e n t a t t h e g r a i n boundary, the C and 0 con-} t e n t decrease.

-

The l a t t e r r e s u l t s a r e i n agreement w i t h t h e multicomponent segregation t h e o r y o f Guttmann

/6/.

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,

C. and R.M. German, Met. Trans. A.,

x,

2031 (1983).

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457 (1981).

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955 (1981).

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2,

507 (1979). /6/ Guttmann, M., Surface Sci.

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Hofmann, H., Ph.D. Thesis, TU B e r l i n (1983).

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-

/ l o /

Hofmann, H. and S. Hofmann, S c r i p t a Met.

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42,

763 (1980).

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