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HAL Id: jpa-00222444

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EFFECT OF ISOTHERMAL ANNEALING ON THE STRUCTURE OF AN AMORPHOUS Pd-Si ALLOY

M. Laridjani, J. Sadoc

To cite this version:

M. Laridjani, J. Sadoc. EFFECT OF ISOTHERMAL ANNEALING ON THE STRUCTURE OF AN AMORPHOUS Pd-Si ALLOY. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-83-C9-86.

�10.1051/jphyscol:1982915�. �jpa-00222444�

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Colloque C9S supplément au n°12, Tome 43, décembre 1982 page C9-83

EFFECT OF ISOTHERMAL ANNEALING ON THE STRUCTURE OF AN AMORPHOUS P d - S i ALLOY

M. L a r i d j a n i * a n d J . F . Sadoc**

"Laboratoire de Magnétisme, C.N.R.S., 92190 Meudon Bellevue, France

**Laboratoire de Physique des Solides, V.P.S., Bâtiment 510, 91405 Orsay, France

Résumé. - L ' e f f e t d'un chauffage sur la structure d'alliages Pd-Si amorphe est présenté. Des effets quasi-reversibles sont observés avant qu'apparais- sent les effets de relaxation structurale.

A b s t r a c t . - The e f f e c t o f h e a t i n g on the s t r u c t u r e o f amorphous Pd-Si a l l o y s i s presented. Q u a s i - r e v e r s i b l e e f f e c t s are observed p r i o r t o s t r u c - t u r a l r e l a x a t i o n .

I n t r o d u c t i o n . - In the past ten years the p r o p e r t i e s o f glassy a l l o y s ( l i q u i d quenched) have been e x t e n s i v e l y s t u d i e d . I t become c l e a r t n a t many o f these p r o - p e r t i e s are changed by annealing before c r y s t a l l i z a t i o n . This phenomenon i s known as " S t r u c t u r a l r e l a x a t i o n " . The mechanism o f t h i s s t r u c t u r a l r e l a x a t i o n i s not w e l l understood.

Recent experiments have been done by Egami ( 1 ) which suggest t h a t the s t r u c t u r a l r e l a x a t i o n occurs by change i n t o p o l o g i c a l s h o r t range o r d e r . I t a l s o has been proposed t h a t r e l a x a t i o n a t low temperature i n v o l v e s l i g h t re-arrangements so c a l - led i n c i p i e n t stage o f c r y s t a l l i z a t i o n ( 2 ) . In. t h i s paper X-ray d i f f r a c t i o n and D.T.A. experiments on s p u t t e r e d and l i q u i d quenched amorphous Pd-Si a l l o y s are presented. The composition o f both a l l o y s checked by micro-probe a n a l y s i s i s : Pd 79 Si 21 and PdgQ Si go f o r the l i q u i d quenched a l l o y and the s p u t t e r e d one r e s p e c t i v e l y .

Experimental methods a) X-ray d i f f r a c t i o n

The i n t e r f e r e n c e f u n c t i o n I ( K ) was obtained by energy d i s p e r s i v e X-ray d i f f r a c t i o n . S t r u c t u r a l thermal v a r i a t i o n s o f the i n t e r f e r e n c e f u n c t i o n s were determined from runs a t three s c a t t e r i n g angles (26 = 2 5 ° , 4 3 ° , 7 5 ° ) . The h e a t i n g was done i n s i t u , using a c o n t r o l l e d atmosphere f u r n a c e . The s c a t t e r e d X-rays were detected by an energy s e n s i t i v e d e t e c t o r Si ( L i ) . D e t a i l s o f the experiment were reported e l s e - where ( 3 ) .

b) D i f f e r e n t i a l thermal a n a l y s i s (D.T.A)

The thermal e f f e c t was measured by a thermo analyser m e t t l e r . This equipement d e s i - gned f o r simultaneous analysis o f D.T.A. and thermo g r a v i m e t r y . Typical D.T.A thermograms f o r PdgQ Si20 a l l o y s are reproduced on f i g . l . For a l l a l l o y s temperature transformations were depending upon h e a t i n g r a t e : when heating r a t e was changed by 10° t o l°C/mn the temperature o f c r y s t a l l i z a t i o n f a l l s o f 6°C.

The d i f f e r e n t i a l thermal a n a l y s i s curve ( f i g . l ) of s p u t t e r e d a l l o y shows two zones:

on the f i r s t one, a hump extends from 275°C up to 370°C which corresponds t o an exothermic r e a c t i o n . I n the second zone a sharper peak i n d i c a t e the c r y s t a l l i z a - t i o n ; i t appears at Tx = 395°C. I f the a l l o y i s pre-heated a t 320CC (a temperature s i t u a t e d i n the f i r s t zone) and then quenched q u i c k l y , i t keeps i t s amorphous cha- Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982915

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C 9-8 4 JOURNAL DE PHYSIQUE

r a c t e r and i t s thermogram remains i d e n t i c a l t o t h a t o t non-heated sample. Un the c o n t r a r y t h e c r y s t a l l i z a t i o n appears a f t e r an annealing a t 375°C. Thermogram o f an amorphous a l l o y aged f o r 10 mn a t 350°C shows t h a t the exothermic trump

Fig.1. D.T.A. Thermogram f o r Pd-Si a l l o y s : I. Sputtered sample

11. L i q u i d quenched sample 111. Preheated s p u t t e r e d sample

(at3502Cfor 10mn)

completely disappears, and t h e w i d t h o f the c r y s t a l l i z a t i o n peak f o r s p u t t e r e d a1

-

l o y s remains g r e a t e r than f o r t h e m e l t quenched one. N e l t quenched and s p u t t e r e d a1 loys heated a t T > Tx completely c r y s t a l l i z e ; they are composed o f a m i x t u r e o f two e q u i l i b r i u m phase (PdqSi

,

Pd3Si).

X Ray d i f f r a c t i o n experiments

For t h e s p l a t cooled and f o r the s p u t t e r e d samples i n t e r f e r e n c e f u n c t i o n s were measured a t room temperature, then a t 200°C, and a t 300°C and again a t room temperatu- r e . I n order t o observe q u a s i - r e v e r s i b l e e f f e c t s each sample was heated l e s s than h a l f an hour. But t o observe the e v o l u t i o n o f i n t e r f e r e n c e f u n c t i o n s d u r i n g a l o n g p e r i o d a t 300°C samples were heated a t t h i s temperature d u r i n g 24 hours. Several i n t e r f e r e n c e f u n c t i o n s were successively recorded a t t h i s temperature f o l l o w e d by another one a f t e r a r e t u r n a t room temperature. The same experiment was a l s o p e r - form d u r i n g 10 days a t a lower temperature (250°C). F i n a l l y r a d i a l d i s t r i b u t i o n functions were deduced by F o u r i e r transform.

I n t e r f e r e n c e and r a d i a1 d i s t r i b u t i o n functions-

The h e i g h t o f the f i r s t peak o f I ( K ) s l i g h t l y increases by agehg a t h i g h temperatu- r e (a 3%). Nevertheless when measurements are done a t h i g h e r temperature t h e i n t e n - s i t y o f t h e f i r s t peak decrease consequently t o the increase o f t h e thermal d i s o r d e r . The most i m p o r t a n t e f f e c t seems t o appear i n the second peak. The shoulder i n c r e a - ses on h e a t i n g . T h i s e v o l u t i o n i s more s e n s i t i v e on t h e s p u t t e r e d sample than one t h e s p l a t cooled one. When the sample i s maintained a t h i g h temperature d u r i n g a s h o r t time, t h i s e f f e c t appears t o be r a t h e r r e v e r s i b l e : the curve obtained again a t room temperature i s s i m i l a r t o the f i r s t room temperature one. Nevertheless t h i s i s more e a s i l y observed w i t h a s p u t t e r e d sample. Some d e t a i l s seem a l s o quasi- r e v e r s i b l e on the 3 r d peak o f I ( K ) f u n c t i o n s ( F i g . 2 - 3 ) When the h i g h temperature h e a t i n g i s l o n g (300°C d u r i n g 1 day o r 250°C d u r i n g 10 days), the i n t e r f e r e n c e func- t i o n s obtained a f t e r r e t u r n t o room temperature a r e m o d i f i e d w i t h r e s p e c t t o the i n i t i a l curved obtained a t t h i s temperature. These new curves present a r e i n f o r c e d shoulder on t h e second peak f o r both types o f samples.

Changes are a l s o observed on t h e r a d i a l d i s t r i b u t i o n f u n c t i o n s (Fig.4,5). F o r t h e f i r s t peak, v a r i a t i o n s a r e o f t h e order o f experimental u n c e r t a i n t i e s . C a r a c t e r i s - t i c behaviours a r e observed on t h e second peak and a l s o f o r g r e a t e s t distances.

The s p l i t t i n g o f t h e

znd

peak

is

m o d i f i e d b y temperature: t h e d i s t a n c e corresponding t o r = 6

8

s l i g h t l y increases and a small o s c i l l a t i o n between the two maximums i n d i c a t e s a t h i r d distance which i s a l s o more pronounced a t h i g h e r temperature. A change i n shape o f the 4 t h peak i s a l s o observed: a t low temperature t h i s peak i s symmetrical and some d e t a i l s are increased by heating. These behaviour

is

a l s o qua- s i r e v e r s i b l e f o r s h o r t annealing time, b u t f o r l o n g annealing time t h e r e i s a

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r e v e r s i b l e behaviours a r e e a s i e r t o observe on l a r g e r distances than on t h e s m a l l e r ones.

Discussion

These r e s u l t s can be i n t e r p r e t e d u s i n g a t e t r a h e d r a l packing model f o r a l l t h e amorphous s t a t e s o f Pd-Si a l l o y s . The change o f the s t r u c t u r e i s exnlained: by change o f the i n t e r a t o m i c distance f l u c t u a t i o n s , n o n - a f f e c t i n g t h e l o c a l t e t r a h e - d r a l topology, and by change o f the d e f e c t density, a f f e c t i n g t h e l o c a l topology (co-ordinance). As described i n a r e c e n t paper using a curved space d e s c r i p t i o n ( 4 ) these two c o n t r i b u t i o n s t o the l o c a l order are i n competition: an increase o f the d e f e c t d e n s i t y l e a d i n g t o a more important d i s o r d e r f o r atoms i n the neighbo- r i n g o f t h e d e f e c t , tends t o decrease the d i s o r d e r f o r o t h e r atoms (increase o f the t e t r a h e d r a l p e r f e c t i o n )

.

The thermal v i b r a t i o n s s o f t e n distance f l uctua- t i o n s and t h e r e f o r e l o c a l symmetry increases on heating. T h i s i s supported by the e v o l u t i o n o f the I(K) second peak shoulder and by the re-inforcement o f some w e l l d e f i n e d distances on t h e P ( r ) functions*. A f t e r r e t u r n t o room temperature thermal v i b r a t i o n decreases: the e f f e c t i s r e v e r s i b l e w i t h o u t any change o f the l o c a l topology. But i f t h e temperature i s k e p t h i g h enough f o r a l o n g t i m e (several hours), we observe an annealing e f f e c t w i t h f e a t u r e s corresponding t o an i n c r e a s i n g o f the l o c a l symmetry. Rearrangement o f the d e f e c t s can e x p l a i n these observations. I n these c o n d i t i o n s the chanqe i n the l o c a l symmetry i s n o t o n l y due t o the thermal averaging e f f e c t b u t corresponds t o a r e a l s t r u c t u r a l m o d i f i c a t i o n as i t remains when t h e temperature i s lowered.

A d e t a i l e d d e s c r i p t i o n o f the d e f e c t rearrangement cannot be supported by present experimental r e s u l t s . 6 u t we suppose t h a t it corresponds t o a tendency t o reach an optimum value i n the competition between t h e t o p o l o g i c a l d i s o r d e r and t h e i n t e r a - tomic distance f l u c t u a t i o n s depending on d i s t o r t i o n s o f t e t r a h e d r a l arrangements.

This can be achieved by a change i n t h e d e f e c t d e n s i t y (probably a decrease o f t h e number o f d e f e c t s ) and a l s o by a change i n t h e nature o f these d e f e c t s ( d i s c l i n a - t i o n l i n e s , i n t e r n a l c u t surfaces.. . ) . But experimental r e s u l t s show an increase i n the order o f t h e t e t r a h e d r a l arrangement: c h a r a c t e r i s t i c l o n g distances of these arrangements are w e l l r e s o l v e d on t h e P ( r ) f u n c t i o n s . This i n d i c a t e s t h a t the number o f d e f e c t s a f f e c t i n g these arrangements i s g e t t i n g close t o the optimum va- l u e n e c e s s a r i l y present inanamorphous a l l o y .

Another p o i n t t o be discussed i s the d i f f e r e n c e between l i q u i d quenched and s p u t t e - r e d a l l o y s . The main f e a t u r e s are roughly s i m i l a r b u t some differences can be no- ti ced.

The I(K) second peak shoulder i s more pronounced f o r t h e heated l i q u i d quenched a l - l o y than f o r the s p u t t e r e d a1 l o y . T h i s i n d i c a t e s an increase o f t h e l o c a l order when a l l o y are prepared from t h e melt. This d i f f e r e n c e i s reduced by annealinq.

The D.T.A. experiment a l s o supports t h i s hypothesis.

Reversible e f f e c t s are observed f o r l o n g e r time o f h e a t i n g i n s p u t t e r e d a l l o y s than i n l i q u i d quenched a l l o y s .

These observations suggest t h a t l i q u i d quenched amorphous a l l o y s a r e i n a s t a t e of lower f r e e energy than t h e s p u t t e r e d a l l o y s . 6 u t energy b a r r i e r s between these s t a t e s and s t a t e s o f lowerest energy corresponding t o a r e l a x e d s t a t e are h i g h e r i n t h e s p u t t e r e d a l l o y s case than i n the l i q u i d quenched a l l o y s case. I t i s a l s o shownthat i n any case the e v o l u t i o n ofI(K)and P ( r ) f u n c t i o n s b y h e a t i n g can n o t be ana- l y z e d i n term o f an ~ n c i p i e n t stage o f c r y s t a l l i z a t i o n .

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C9-86 JOURNAL DE PHYSIQUE

I n t e r f e r e n c e f u n c t i o n s f o r s p u t t e r e d Pd-Si a l l o y s . I ( K ) = K.(I(K)-1) - a ) S h o r t thermal cycle(20°,2000, 3OO0,2Cf) -b)Long ageing.

Fig.2. +

I n t e r f e r e n c e f u n c t i o n s f o r t h e l i q u i d quenched a l l o y s (same c o n d i t i o n s a s Fig.2).

Reduced r a d i a l d i s t r i b u t i o n f u n c t i o n s f o r ¹⁶ Pd-Si a l l o y s ^1~

(from Fig.2 I(K) f u n c t i o n s ) .

Id(R)=R.(P(R)-1).

,

Fig.4. + o

L

Reduced radi a1 d i s t r i b u t i o n f u n c t i o n s f o r

l i q u i d quenched ^"

a l l o y s ( f r o m Fig.3

I(K) f u n c t i o n s ) . 22

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BIBLIOGRAPHY

1 ) T. Egami, Journ. o f Mat. S c i . 13 (1978) 2587.

2 ) V . Waseda,

H.

Okazaki

,

T . Masumoto, Journ. of Mat. Sci

.

12 (1977) 1927.

3 ) J . F . Sadoc and M. L a r i d j a n i

,

Journal de Physique L e t t r e s 42 (1981) 1.485.

4) J.F. Sadoc, Journal of Non-Crys.So1. 44 (1981) 1.

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