INFLUENCE OF RAPID QUENCHING AND SAMPLE TREATMENT ON THE STRUCTURE OF MELT SPUN Al-Si ALLOYS

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INFLUENCE OF RAPID QUENCHING AND

SAMPLE TREATMENT ON THE STRUCTURE OF

MELT SPUN Al-Si ALLOYS

Joël Chevrier, P. Sainfort, P. Germi, D. Pavuna

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, supplément au n°12, Tome 46, décembre 1985

page C8-533

INFLUENCE OF RAPID QUENCHING AND SAMPLE TREATMENT ON THE STRUCTURE OF MELT SPUN A l - S i ALLOYS

J . C h e v r i e r , P. S a i n f o r t , P . Germi and D. Pavuna

LEPES, CNRS, B.P. 166, 38042 Grenoble Cedex, France

+CEGEDUR Pechiney, Centre de Recherehe et Developpement, B.P. 27,

S8340 Voreppe, France

++Laboratoire de Cristallographie, CNRS, B.P. 166, 38042 Grenoble, France

Résumé :

Nous avons étudié les caractéristiques structurales d'alliages AlSi prépa-rés par trempe sur roue. L'effet principal que nous dégageons est une solution soli-de largement sursaturée soli-de Silicium dans la matrice d'Aluminium, ce qui permet d'ex-pliquer les propriétés électroniques et thermiques de ces systèmes et montre leur métastabilité importante.

Abstract :

We have studied the structural characteristics of the range of melt spun Al-Si alloys. We find that the dominant feature is the existence of sursaturated Si

in f.c.c. Al grains which well explains the metastability of these systems as well as related thermal and electronic properties.

I - INTRODUCTION

In the past two decades there have been numerous studies of the structure of AlSi alloys prepared by a variety of rapid quenching techniques /1-7/. It was clearly observed by all researchers that the microstructure of these alloys is very sensitive to the cooling rate used in the preparation process.

The principal objective of our study has been to determine :

i) the structural characteristics of a range of AlSi alloys prepared by maxi-mum cooling rate obtainable by an improved melt spinning process /8-10/,

ii) the influence of thermal treatment on the actual structure.

In this short paper we present the results obtained by X-ray diffraction and Scanning Transmission Electron Microscope (STEM), resistivity at high tempera-tures and Differential Scanning Calorimetry ( D S C ) . Chemical microanalysis was performed in S T E M using X-ray energy dispersive spectrometer ( X E D S ) . II - EXPERIMENTAL RESULTS

The metallic ribbons of Al with up to 30 at % Si were produced in the he-lium atmosphere of the multimode melt spinning apparatus /8-10/. We always used the highest available roller speed of < 50ms-l and obtained ribbons ~ 30pm thick and 2 mm wide.

Detailed studies of electronic properties (that aire presented in Ref. 11) show that the cooling rate of our melt-spinner, which is of the order of 106K/s , produces essentially the same effects in all our samples independently of concen-tration of Si.

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The X-ray d i f f r a c t i o n s t u d i e s show two phases i n a l l

mi

c o n c e n t r a t i o n s t u d i e d :

i ) An f . c . c . phase which d i f f r a c t i o n p a t t e r n i s v e r y s i m i l a r t o t h a t of p u r e Aluminium ; however, t h e l i n e s a r e n o t a s sharp,which j.s an i n d i c a t i o n of s t r a i n

and s m a l l g r a i n s i z e .

Our a n a l y s i s shows t h a t t h e l a t t i c e parameter d e c r e a s e s from 4,049 i n pure A 1 t o 4,035 i n AlSi 20 %.

i i ) A second phase, which corresponds t o diamond s i l i c o n ( s e e f i g . 1 ) . From She p r o f i l l i n e a n a l y s i s , we e s t i m a t e t h e diameter of t h e s i l i c o n g r a i n s t o b e 50-100 A.

Fig. 1

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X-ray r e f l e x i o n (111) of S i l i c o n i n a ) a s quenched and b) annealed samples.

I n o r d e r t o g e t more i n f o r m a t i o n on t h e s t r u c t u r e of our a l l o y s , we a l s o used S T E M . The samples were t h i n n e d e i t h e r by e l e c t r o c h e m i c a l p r o c e s s o r by u l - t r a t h i n microtomy.

General morphology of t h e AlSi a l l o y s s t u d i e d i s shown on micrograph 2 which i s a d e t a i l of t r a n s v e r s e s e c t i o n of A1.20 a t % S i r i b b o n c u t by microtomy.

The micrograph e x h i b i t s a complex s t r u c t u r e . Homogeneous A 1 f . c . c . den- d r i t i c g r a i n s a r e imbedded i n a continuous i n t e r g r a n u l a r phase. The A 1 g r a i n s i z e v a r i e s a l o n g t h e c r o s s s e c t i o n of t h e r i b b o n from 100

A

a t t h e wheelside (where t h e c o l l i n g r a t e i s t h e h i g h e s t ) t o l u m a t t h e f r e e s u r f a c e .

Fig. 2

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a

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Cross s e c t i o n of A1-20 a t % S i a l l o y thinned by microtomy. Dark f i e l d on t h e (111) S i r i n g (x 12000).

b

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T y p i c a l d i f f r a c t i o n p a t t e r n of A1-20at% S i .

The S i c o n t e n t i n s o l i d s o l u t i o n d e c r e a s e s from t h e wheel s u r f a c e t o t h e f r e e ribbon s u r f a c e , hence t h i s a l s o confirms t h e g r a d i e n t i n c o o l i n g r a t e along t h e c r o s s s e c t i o n .

E l e c t r o n diffraoction p a t t e r n s i n between t h e g r a i n s show d i f f u s e dashed d i f f r a c t i o n r i n g s a t 3.2 A and 1 . 9 which i n d i c a t e c o v a l e n t S i l i c o n . The chemical m i c r o a n a l y s i s of t h i s complex zone r e v e a l s a high S i l i c o n c o n t e n t .

Turning now t o t h e A1-6 a t % S i a l l o y we n o t e t h a t o v e r a l s t r u c t u r e i s v e r y s i m i l a r t o A 1 - 2 0 a t % S i . The major d i f f e r e n c e s come from t h e i n t e r d e n d r i t i c phase volume f r a c t i o n , which i s s m a l l e r i n r e s p e c t w i t h t h e t o t a l S i l i c o n c o n t e n t , and from t h e m i c r o s t r u c t u r e morphology of t h e wheel s i d e . A s a m a t t e r of f a c t , one n o t i c e s a continuous s t r u c t u r e of f . c . c . Aluminium g r a i n i n which an homogeneous d i s t r i b u t i o n of p r e c i p i t a t e s i s embedded. These p a r t i c l e s have a s p h e r i c a l shape w i t h a mean diameter of 100 (micrograph 3 ) .

However, any degree of s u r s a t u r a t i o n of S i i n f . c . c . m a t r i x i s obviously a d i r e c t consequence of r a p i d quenching and t o our knowledge cannot be achieved by c l a s s i c a l techniques.

I t i s of i n t e r e s t t o a s k what happens t o t h e s t r u c t u r e i f t h e ribbon i s t h e r m a l l y t r e a t e d ?

C8-536

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Fig. 3

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A l - 6 a t % S i a l l o y . D e t a i l s of c r o s s s e c t i o n n e a r t h e wheel s i d e , p r e p a r e d by u l t r a t h i n microtomy. Dark f i e l d on (111) S i r i n g . (x 47000).

change i n t h e s t a t e of t h e sample. DS C s c a n shows a n i m p o r t a n t e x o t h e r m a l e f f e c t from which we e s t i m a t e t h e t o t a l r e l e a s e d e n e r g y , A H = 4 K J / m o l e , a s w e l l a s t h e a c t i v a t i o n energy Eoe 1 0 0 K J / m o l e . Moreover, we n o t e t h a t t h e s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e Tc a l s o d r o p s d r a m a t i c a l l y , from 4K b e f o r e a n n e a l i n g down t o

-

1,2 K i n t h e r m a l l y t r e a t e d samples ( s e e f i g . 4 , s e e a l s o r e f . 1 1 ) .

Hence a l t h o u g h s e v e r a l s t r u c t u r a l f e a t u r e s change upon t h e r m a l t r e a t m e n t , t h e most pronounced e f f e c t i s t h e d i f f u s i o n of S i l i c o n o u t o f t h e f.c.c. A1 m a t r i x . T h i s , i n o u r o p i n i o n , i s t h e key e f f e c t t h a t e x p l a i n s t h e changes i n e l e c t r o n i c p r o p e r t i e s of t h e s e a l l o y s ( r e s i s t i v i t y and Tc, s e e Ref. 11). T h e r e f o r e

,

we conclude t h a t m e l t s p i n n i n g of t h e r a n g e of G S i a l l o y s r e s u l t s i n m e t a s t a b l e m i c r o c r y s t a l l i n e s t r u c t u r e w i t h h i g h p e r c e n t a g e s of s u r s a t u - r a t e d S i i n A 1 g r a i n s up t o 8 %. ACKNOWLEDGEMENTS

We acknowledge many s t i m u l a t i n g d i s c u s s i o n s w i t h F r a n ~ o i s e CYROT, S e r g e PAIDASSI, Jean-Paul SIMON, C l a i r e BERGER, and AFME f o r f i n a n c i a l s u p p o r t .

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M. I t a k a g i , B.C. G r i e s s e n and N . J . G r a n t , T r a n s . ASM 61 (1968) 330. S.K. Bose and R. Kumar, J , Mat. S c i . 8 (1973) 1795.

H. Matyja, K.C. R u s s e l l , B.C. G r i e s s e n and N . J . G r a n t , Met. Trans. 6A (1975) 2249.

P. Van Mourik, E.J. M i t t e m e i j e r and T.H. De K e i j s e r , J. Mat. S c i . 18 (1983) 2706. A. B e n d i j k , R. Delhez e t a l . , J. Mat. S c i .

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(1980) 2803. R. Delhez, T.H. K e i j s e r e t a l . , J. Mat. S c i . (1984) 2887. S. P a ' i d a s s i and J. C h e v r i e r , p r o c e e d i n g s o f RQV, ed. S. S t e e b , Wiirzburg 1984, N o r t h Holland 1985, i n p r i n t . D. Pavuna, J . Mat. S c i .

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(1981) 2419. D. P a w n a , P r o c e e d i n g s o f RQ I V , ed. K. S u z u k i , Japan I n s t i t u t e f o r M e t a l s , S e n d a i 1982, p. 81.

D. Pavuna, G. Fourcaudot and J.C. G r i e c o i n "Amorphous M e t a l s and

Non-Equilibrium P r o c e s s i n g " , P r o c e e d i n g s o f MRS-Europe, ed. M. Von Allmen, J. d e P h y s i q u e 1984, p. 45.