ELASTIC AND ANELASTIC EFFECTS ASSOCIATED WITH PRECIPITATION PHENOMENA IN NON STOICHIOMETRIC SPINELS

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ELASTIC AND ANELASTIC EFFECTS ASSOCIATED

WITH PRECIPITATION PHENOMENA IN NON

STOICHIOMETRIC SPINELS

D. Fargeot, C. Gault, F. Platon, P. Boch

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque C5, suppZe'ment au nOIO, Tome 42, octobre 1981 page C5-899

ELASTIC AND ANELASTIC EFFECTS ASSOCIATED WITH PRECIPITATION PHENOMENA

IN NON STOICHIOMETRIC SPINELS

D. F a r g e o t , C. Gault, F. P l a t o n and P . Boch

Laboratoire de Mate'riaux Ce'rmiques,

ER4

CNRS N0539, E N S C I Limoges, France

A b s t r a c t .

-

(A1203),Mg0 compounds with n % 3

-

3.5 have two phases a t e q u i l i -

brium :

a

corundum p r e c i p i t a t e s i n a

y

s p i n e l matrix. The s p l a t c o o l i n g due t o plasma s p r a y i n g ( p o l y c r y s t a l s ) and V e r n e u i l ' s method ( s i n g l e c r y s t a l s ) lead to a m e t a s t a b l e y c u b i c s t a t e . Subsequent a n n e a l i n g gives t h e s t a b l e s t a t e .

The e v o l u t i o n s a r e r e l a t e d t o a s t r o n g i n c r e a s e of e l a s t i c moduli, and a d e c r e a s e of t h e damping. This was o b t a i n e d through t h r e e s t e p s : t h e f i r s t

two due t o p r e p r e c i p i t a t i o n and p r e c i p i t a t i o n , the t h i r d one to s i n t e r i n g .

1 . I n t r o d u c t i o n .

-

MgA1204 compound h a s a cubic s t r u c t u r e (y s p i n e l ) a t a l l tempera- t u r e s below t h e m e l t i n g p o i n t , and A1203 alumina has a t r i g o n a l s t r u c t u r e ( a comuhmX "non s t o i c h i o m e t r i c " s p i n e l s (A1 0 _{2 3 n}) MgO (with n

>

1, n = 1 corresponding t o MgA1204) a r e i n y phase a t high temperatures b u t a r e a mixture of

( a

+

y) phases a t room tem- p e r a t u r e ( t h e

a

phase may be c o n s i d e r e d a s p r e c i p i t a t e s i n a y s p i n e l m a t r i x ) : a l l

t h e s e p o i n t s may be s e e n on t h e diagram of Fig. 1.

The plasma s p r a y i n g of molten powders onto a cooled t a r g e t provokes a very f a s t s p l a t quenching, which f r e q u e n t l y l e a d s t o m e t a s t a b l e s t a t e s , with unusual c r y s t a l s t r u c t u r e s . Thus, plasma sprayed pure alumina i s i n a c u b i c

y

s t a t e , with s t r u c t u r a l vacancies r e l a t e d t o t h e f a c t t h a t A1203 h a s n o t t h e s t o i c h i o m e t r i c r a t i o of t h e s p i n e l s t r u c t u r e (1) ; (A1 0 ) MgO compounds a r e a l s o i n

y

phase a t room

2 3 n temperature a f t e r plasma s p r a y i n g .

The a n n e a l i n g o f such o u t of e q u i l i b r i u m s t a t e s p r o g r e s s i v e l y l e a d s t o t h e qui- l i b r i u m s t r u c t u r e : f o r i n s t a n c e plasma sprayed yalumina transforms to corundum a f t e r i n t e r m e d i a t e s t e p s r e l a t e d t o an o r d e r i n g of t h e s t r u c t u r a l vacancies ( 2 ) . I t i s worth n o t i n g t h a t t h e s e t r a n s f o r m a t i o n s a r e accompanied by s t r o n g v a r i a t i o n s of e l a s t i c and a n e l a s t i c p r o p e r t i e s : the e l a s t i c moduli i n c r e a s e , and the damping c a p a c i t y d e c r e a s e s ( 3 ) . Therefore, t h e measurements of t h e s e q u a n t i t i e s allow t o s t u d y t h e s t r u c t u r a l t r a n s f o r m a t i o n s : t h i s paper i s devoted t o t h e m o d i f i c a t i o n s of e l a s t i c cons t a n k and i n t e r n a l f r i c t i o n r e l a t e d t o t h e p r e c i p i t a t i o n phenomena i n non s t o i c h i o m e t r i c s p i n e l s , plasma sprayed p o l y c r y s t a l s and s i n g l e c r y s t a l s , of compositions : (A1203) 3Mg0 and (A1203) 3 . 5Mg0

2. V a r i a t i o n s of Young's modulus d u r i n g a n n e a l i n g of plasma sprayed d e p o s i t s Young's modulus E h a s been measured d u r i n g a r e g u l a r i n c r e a s e of temperature a t a r a t e of l0C/mn i n plasma sprayed (A1 0 _{2 3 3}) MgO d e p o s i t s . The method of measurement

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C5-900 JOURNAL DE PHYSIQUE

i s a high temperature p u l s e echo u l t r a s o n i c technique which has been d e s c r i b e d elsewhere ( 4 ) . It u s e s a m a g n e t o s t r i c t i v e t r a n s d u c e r , which works i n t h e 100-300 kHz

frequency range, w i t h a composite waveguide ( t u n g s t e n and s i n t e r e d alumina) between the t r a n s d u c e r and t h e sample. Such a sample i s a s q u a r e beam (2.5 x 2.5

x

50 mm),

machined o u t of the raw plasma sprayed d e p o s i t s . The measurement temperature range v a r i e s from 20°C t o 1800°C.

M~AI,O,~ ~AI,O,

50 75 100

mole % Al,O,

Fig. 1

-

Phase diagram MgA1204-A1203 F i g . 2

-

V a r i a t i o n s of (Eo i s Young's En "

modulus a t 2oec of the as-sprayed material) u l t r a s o n i c a l l y measured a t 200 KHz. Tem- p e r a t u r e i n c r e a s i n g r a t e : 60°c/h.

E

Fig. 2 shows the v a r i a t i o n s of t h e

-

r a t i o (E i s t h e modulus a t t h e temperature T ,

Eo

Eo i s t h e room temperature v a l u e ) w i t h i n c r e a s i n g temperature up t o 1 6 5 0 ' ~ . A s i t has been p r e v i o u s l y n o t i c e d (3) t h e d e c r e a s e of Young's modulus e x h i b i t s modulus d e f e c t s , which i n d i c a t e s u c c e s s i v e s t e p s o f s t r u c t u r a l t r a n s f o r m a t i o n s .

A t the beginning, t h e d e c r e a s e o f E i s q u i t e l i n e a r up t o 500°C, with a s l o p e 1

-

aE/aT =-3.8 10-4'~-1, Eo b e i n g 10.5 10l0 Pa. This s l o p e v a l u e i s c l o s e t o t h a t

E 0 1

found f o r plasma sprayed pure alumina (5) : aE/aT =-4.5 I O - ~ ~ C - ' . A t SOO'C, t h e r a t e of d e c r e a s e of E becomes slower and a f i r s f p o s i t i v e modulus anomaly a p p e a r s ,

AE

which corresponds t o a reinforcement (-) of a b o u t 7 % a t 700°C. Beyond 7 0 0 ' ~ t h e Eo 1

rat;? of decrease of E a c c e l e r a t e s and then slows down ( a t about 1000°C) : t h i s AE

second n e g a t i v e modulus anomaly (-) i s a b o u t

-

8 % when compared w i t h t h e e x t r a - Eo 2

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The room temperature v a l u e of E i s E l = 13.1 lo10 Pa ( t h i s low v a l u e i s due to p o r o s i t y ) . 3. E v o l u t i o n s a t high temperature ( T > 1 0 0 0 ~ ~ ) I n t h i s temperature range t h e p r e c i p i t a t i o n of the e q u i l i b r i u m

a

p r e c i p i t a t e s i s expec- t e d ( 6 ) : t h i s i s i l l u s t r a t e d by X r a y d i f - f r a c t i o n diagrams performed on a plasma sprayed sample a f t e r a n n e a l i n g a t l l 0 0 ~ C (Fig.3). Only y phase appears i n t h e case of the raw d e p o s i t (Fig. 3a) whereas i n - c r e a s i n g times of a n n e a l i n g a t IIOO°C l e a d to two m o d i f i c a t i o n s ( F i g . 3b and 3c) :

t h e

y

peaks move toward t h e low a n g l e s and t h e i r i n t e n s i t y d e c r e a s e s ; t h e

a

peaks develop. The f i r s t e f f e c t i s due to t h e y

m a t r i x which i s ~ 1 2 0 3 pooring ; t h e second e f f e c t i s r e l a t e d t o t h e A1203 p r e c i p i t a t e s . Therefore, t h e i ~ c r e a s e of Young's modulus between 1000 and 1300°C can be a t t r i b u t e d

to t h e p r e c i p i t a t i o n of t h e a phase. Other measurements a f t e r i s o t h e r m a l a n n e a l i n g i n t h i s temperature range have always shown (3) t h a t

a

p r e c i p i t a t i o n g i v e s a s t r o n g increase of E and a d e c r e a s e of i n t e r n a l f r i c t i o n .

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of Young's modulus (about 50 %) i n t h e case of a plasma sprayed pure alumina ( 2 ) . I n the p r e s e n t case of (A1203)3Mg0 non s t o i c h i o m e t r i c s p i n e l such a n e f f e c t e x p l a i n s t h e p l a t e a u between 1300°C and 1 6 0 0 ' ~ b u t i t appears t h a t t h e d e n s i f i c a t i o n i s lower than f o r pure alumina, t h a t has been confirmed by S E M o b s e r v a t i o n s and by d e n s i t y measurements.

4. E v o l u t i o n s a t medium temperatures (T < 1000')

No important e v o l u t i o n appears under 5 0 0 ~ ~ . Beyond t h i s temperature, the s t r u c t u r a l v a r i a t i o n s a r e expected i n two ways. F i r s t , by a rearrangement of quenching d e f e c t s : t h i s i s the c a s e even i n p u r e alumina, where t h e i n i t i a l s t a g e of t r a n s f o r m a t i o n of t h e m e t a s t a b l e

y

phase proceeds by an o r d e r i n g of t h e s t r u c t u r a l v a c a n c i e s , which l e a d s t o t h e

6

phase. Secondly, by t h e p r e c i p i t a t i o n of intermediate phases, a s p o i n t e d o u t by Heuer and a 1 (7) i n s i n g l e c r y s t a l s of a c l o s e concentra-

t i o n (A1203)3.5Mg0 a s t h a t of t h e p o l y c r y s t a l s s t u d i e d h e r e . These a u t h o r s r e p o r t t h a t p r e p r e c i p i t a t i o n phenomena o c c u r a t temperatures a s low a s 650°C, f o r l o n g ageing times ; a f t e r 10 mn a t 8 5 0 ' ~ c o h e r e n t p a r t i c l e s have been i d e n t i f i e d , b u t t h e i r temperature range of e x i s t e n c e h a s n o t been determined ; f o r l o n g e r times t h e p a r t i c l e s coarsen and i n t e r m e d i a t e p r e c i p i t a t e s appear, e s s e n t i a l l y i n t h e v i c i n i t y of subboundary and d i s l o c a t i o n s . During a l l t h e s e p r e c i p i t a t i o n s t a g e s mechanical p r o p e r t i e s vary : Knoop hardness and f o u r p o i n t bend s t r e n g t h reach two s u c c e s s i v e peaks a f t e r 0.25 h and 25 h . a t 850°C.

Plasma s p r a y i n g of non s t o i c h i o m e t r i c s p i n e l powders l e a d t o o u t of e q u i l i b r i u m s t a t e s , b u t t h e Verneuil method of p r e p a r a t i o n of s i n g l e c r y s t a l s a l s o l e a d s t o a m e t a s t a b l e y phase. Therefore, t h e measurements i n t h e plasma sprayed p o l y c r y s t a l s have been completed by e x p e r i m e n t a t i o n s i n V e r n e u i l ' s s i n g l e c r y s t a l s (8) of compo- s i t i o n (A1203) 3.5Mg0.

5. V a r i a t i o n s of e l a s t i c and a n e l a s t i c p r o p e r t i e s of s i n g l e c r y s t a l s

P u l s e echo techniques have been used i n t h e frequency range of 10 t o 20 MHz (a phase comparison method and a p u l s e echo o v e r l a p method). The measured q u a n t i t i e s were t h e u l t r a s o n i c v e l o c i t i e s V S 1 , VS2, V _Land u l t r a s o n i c a t t e n u a t i o n a _{S1' " ~ 2 '}_{0 1 ~}

f o r t h e s h e a r and l o n g i t u d i n a l waves p r o p a g a t i n g a l o n g t h e <110> a x i s . The samples were c u b i c shaped (10 x 10 x 10 mm) w i t h a s e t of f a c e s p a r a l l e l t o a (110) plane. The v a r i a t i o n s of the u l t r a s o n i c a t t e n u a t i o n s

a

and of t h e corresponding velo- c i t i e s a f t e r a n n e a l i n g up to 100 h a t 8 5 0 ' ~ a r e p l o t t e d i n F i g . 4 and show t h a t i n a l l c a s e s t h e f i r s t p e r i o d of a n n e a l i n g provokes a d e c r e a s e of a t t e n u a t i o n b u t only a s l i g h t v a r i a t i o n of t h e v e l o c i t i e s . The c a l c u l a t i o n of t h e Zener e l a s t i c c o n s t a n t s C , C' and B from t h e v a l u e s of V S l , VS2 and V shows t h a t t h e bulk modulus i s n o t

L

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2 T ~ . 850% f ~ i o H z p r e s e n t l y s t u d i e d by t r a n s m i s s i o n e l e c t r o n

a,

1 A / - -

bw.1 microscopy.

Furthermore, t h e s e r e s u l t s may be compared w i t h those o b t a i n e d i n plasma sprayed samples a f t e r a n n e a l i n g a t t h e same temperature of 850°C ( F i g . 6 ) . Here t h e measurements were made by a resonance method of d i s c s of 2 cm diameter and 2 m

thickness, a t frequency of about 20 KHz.

A maximum i s observed f o r E a f t e r 0 . 4 h

01

I

VS? ~ - ) and t h e i n t e r n a l f r i c t i o n s t r o n g l y decreasss 6650

_.

d u r i n g t h e f i r s t hour of t h e a n n e a l i n g . 6600 M 1 1 10 m

t ~ h ) 6. Conclusion

-

This s t u d y confirms

F i e .

.,

4

-

U l t r a s o n i c a t t e n u a t i o n

a

and the s e n s i t i v i t v o f e l a s t i c and a n e l a s t i c u l t r a s o n i c v e l o c i t i e s V measured a t

20°C a f t e r cumulative a n n e a l i n g a t 8 5 0 ' ~ in a s i n g l e - c r y s t a l (Al203) 3.gMg0 Propagation a l o n g a <110> a x i s ( t h e l e t t e r L i s r e l a t e d t o t h e l o n g i t u d i n a l waves, S1 and S2 t o t h e s h e a r waves).

Tv =850eC C (10-'"1 1621 J I I 26.5 26 tlhl .01 1 1 10 100

Fig. 5- Evolution of the two s h e a r cons- t a n t s C and C' and of t h e b u l k modulus B

o b t a i n e d from t h e t h r e e v e l o c i t i e s

VL

,

VS1, VS2 r e p o r t e d i n f i g u r e 4.

For t h e (A1 0 ) MgO compound t h r e e

2 3 3

p r o p e r t y measurements t o s t r u c t u r a l changes i n ceramic m a t e r i a l s and p a r t i c u l a r l y to p r e c i p i t a t i o n phenomena, a s i t has been p r e v i o u s l y shown f o r m e t a l s .

Fig. 6- V a r i a t i o n of i n t e r n a l f r i c t i o n Q-I

and Young's modulus (Eo i s t h e i n i t i a l v a l u e a t room temperayure) measured by a resonnant d i s c method a t 2 0 ' ~ a f t e r cumu- l a t i v e a n n e a l i n g a t 8 5 0 " ~ .

main temperature ranges of v a r i a t i o n s of t h e e l a s t i c moduli have been observed :

-

500°C t o 1000°C, where t h e v a r i a t i o n s of Young's modulus and of i n t e r n a l f r i c t i o n i n plasma sprayed p o l y c r y s t a l s a r e analogous to t h e v a r i a t i o n s of e l a s t i c c o n s t a n t s and of u l t r a s o n i c a t t e n u a t i o n induced by t h e p r e c i p i t a t i o n of i n t e r m e d i a t e m e t a s t a b l e phases i n s i n g l e c r y s t a l s of a c l o s e composition.

-

1 0 0 0 ' ~ t o 1300°C, where t h e development of t h e

a

p r e c i p i t a t e s l e a d s t o a s t r o n g i n c r e a s e of Young's modulus (about 50 % ) .

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The c r y s t a l l o g r a p h i c s t u d y o f t h e p r e c i p i t a t i o n i n t h e low t e m p e r a t u r e r a n g e i s now t o be made by TEM and X r a y d i f f u s i o n which w i l l l e a d t o t h e i d e n t i f i c a t i o n of t h e p h a s e s and t o t h e knowledge o f t h e i r k i n e t i c s o f f o r m a t i o n and o f r e v e r s i o n . Such d a t a a r e i n d i s p e n s a b l e t o q u a n t i t a t i v e l y i n t e r p r e t e t h e e l a s t i c and a n e l a s t i c e f f e c t s .

7. R e f e r e n c e s .

(1) J.L. BESSON, P. BOCH, M. VARDELLE, L ' I n d u s t r i e Cdram. 4 , 727 (1979)

(2) P. BOCH, D. FARGEOT, C. GAULT, F. PLATON. Rev. I n t . Haute Temp. 18, 1 (1981) (3) P . BOCH, C . GAULT, F. PLATON

-

3 r d European Conf. on I ~ t e r n a l F r i c t i o n and U l t r a s o n i c A t t e n u a t i o n i n s o l i d s . Manchester (1979) P r o c . p 389, Pergamon (1980)

( 4 ) D. FARGEOT, C. GAULT, F. PLATON, M6m. S c i . Rev. Met. 2, 145 (1980) (5) G.F. HURLEY, F.D. GAC, Am. Ceram. Soc. Bul. 5 8 , 5 , 509 (1979)

( 6 ) R. DUCLOS

-

J o u r . de Phys. L e t . 40, L 109 (1979) (7) G.K.BANSAL, A.H. HEUER..Phil. Mag. 29, 4 709 (1974)