METAL MATRIX COMPOSITES BASED ON ALUMINIUM LITHIUM AND SILICON CARBIDE

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Submitted on 1 Jan 1987

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METAL MATRIX COMPOSITES BASED ON ALUMINIUM LITHIUM AND SILICON CARBIDE

J. White, I. Hughes, T. Willis, R. Jordan

To cite this version:

J. White, I. Hughes, T. Willis, R. Jordan. METAL MATRIX COMPOSITES BASED ON ALU- MINIUM LITHIUM AND SILICON CARBIDE. Journal de Physique Colloques, 1987, 48 (C3), pp.C3- 347-C3-353. �10.1051/jphyscol:1987340�. �jpa-00226571�

JOURNAL DE PHYSIQUE

C o l l o q u e C 3 , s u p p l g m e n t au n 0 9 , Tome 4 8 , s e p t e m b r e 1 9 8 7

METAL MATRIX COMPOSITES BASED ON ALUMINIUM LITHIUM AND SILICON CARBIDE

J. WHITE, I.R. HUGHES, T.C. WILLIS and R.M. JORDAN

Alcan International Ltd, Southam Road, Banbury.

GB-Oxon OX16 7SP, Great-Britain

ABSTRACT

The p r e s e n t s t u d y t e s t s t h e f e a s i b i l i t y of producing m e t a l m a t r i x c o m p o s i t e s based on aluminium-lithium a l l o y s . The f i r s t s t e p i n t h i s p r o c e s s h a s been t o produce an MMC based on 8090 w i t h S i c . T h i s h a s been s u c c e s s f u l l y produced by Alcan I n t e r n a t i o n a l u s i n g t h e "Osprey" s p r a y d e p o s i t i o n p r o c e s s . The raw b i l l e t s have been p r o c e s s e d i n i t i a l l y by e x t r u s i o n . The t e n s i l e p r o p e r t i e s of t h i s m a t e r i a l h a v e been d e t e r m i n e d and t h e s t r e n g t h compares f a v o u r a b l y w i t h DC c a s t a l l o y s . Modulus i s improved t o 100 GPa w i t h a c c e p t a b l e d u c t i l i t y . D e n s i t y h a s been measured a s 2.62 Pfg/m3. The s t i f f n e s s t o d e n s i t y r a t i o of t h e composite i s 18% g r e a t e r t h a n 8090 and 50% g r e a t e r t h a n c o n v e n t i o n a l aluminium a l l o y s . The r e s u l t s of t h e s t u d y d e m o n s t r a t e t h e f e a s i b i l i t y and p o t e n t i a l of A1-Li based- p a r t i c u l a t e r e i n f o r c e d composite m a t e r i a l s .

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

There h a s been c o n s i d e r a b l e e f f o r t devoted t o t h e development of aluminium based a l l o y s w i t h h i g h s t i f f n e s s aimed p r e d o m i n a n t l y a t a e r o s p a c e a p p l i c a t i o n s . Two main approaches have been used i n t h i s c o n t e x t ; t h e u s e of l i t h i u m a s a n a l l o y i n g a d d i t i o n and t h e r e i n f o r c e m e n t by c e r a m i c s of h i g h modulus. The former approach h a s c o n c e n t r a t e d on t h e development of c o n v e n t i o n a l i n g o t c a s t i n g t e c h n i q u e s t o produce aluminium a l l o y s c o n t a i n i n g up t o = 3 wt% l i t h i u m . The c u r r e n t s t a t u s of t h e development of t h e s e a l l o y s c a n be found i n t h e p r o c e e d i n g s of t h e f i r s t s e s s i o n o f t h i s c o n f e r e n c e ( l , 2 , 3 ) . These a l l o y s a c h i e v e improvements i n modulus of

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The second approach t o t h e development of m a t e r i a l s w i t h improved

s t r e n g t h / s t i f f n e s s t o weight r a t i o s h a s been t h a t of m e t a l m a t r i x composites. T h i s group of m a t e r i a l s o f f e r s t h e p o t e n t i a l f o r much g r e a t e r i n c r e a s e s i n modulus compared w i t h aluminium l i t h i u m a l l o y s ( > 150 GPa). The development of t h e s e m a t e r i a l s h a s p r i m a r i l y c o n c e n t r a t e d on t h e p r o d u c t i o n of r e i n f o r c e d a l l o y s c o n t a i n i n g e i t h e r w h i s k e r s o r f i b r e s ( c o n t i n u o u s o r d i s c o n t i n u o u s ) . These have r e q u i r e d complex p r o c e s s i n g r o u t e s and t h i s , combined w i t h e x p e n s i v e s t a r t i n g

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987340

C3-348 JOURNAL DE PHYSIQUE

m a t e r i a l s , h a s r e s u l t e d i n s u b s t a n t i a l c o s t p e n a l t i e s a l b e i t with i m p r e s s i v e improvements i n modulus. The u s e of r e i n f o r c e m e n t w i t h a h i g h a s p e c t r a t i o a l s o l e a d s t o c o n s i d e r a b l e a n i s o t r o p y . More r e c e n t l y a t t e n t i o n has been focussed on p a r t i c u l a t e r e i n f o r c e d m e t a l m a t r i x composites which, a l t h o u g h producing

c o m p a r a t i v e l y modest improvements i n modulus ( t h e y u s u a l l y have a much lower Vf of r e i n f o r c e m e n t ) , are i s o t r o p i c . T h i s t y p e of MMC has been produced by a v a r i e t y of r o u t e s t h e most widely r e p o r t e d of which has been t h a t of powder mixing(7). A t Alcan I n t e r n a t i o n a l c o n s i d e r a b l e a t t e n t i o n has been devoted t o t h e p r e p a r a t i o n o f aluminium a l l o y composites u s i n g SIC a s t h e p a r t i c u l a t e r e i n f o r c e m e n t . These composites have been produced u s i n g a s p r a y d e p o s i t i o n t e c h n i q u e i n c o r p o r a t i n g S i c a s an i n t e g r a l p a r t of t h e p r o c e s s . It has been shown t h a t s i g n i f i c a n t improvements i n b o t h modulus and s t r e n g t h can be achieved w i t h good i s o t r o p y u s i n g c o n v e n t i o n a l aluminium a l l o y s a s t h e m a t r i x . Spray d e p o s i t i o n a s a t e c h n i q u e f o r t h e p r o d u c t i o n of composite m a t e r i a l s has a n a t u r a l advantage over t h e more u s u a l powder t e c h n i q u e i n t h a t t h e i n h e r e n t l y dangerous and expensive s t a g e of mixing powders i s n o t n e c e s s a r y hence l e a d i n g t o a r e l a t i v e l y i n e x p e n s i v e p r o d u c t .

The p r e s e n t s t u d y endeavors t o combine t h e above two approaches t o t h e development of aluminium based m a t e r i a l s w i t h an improved s t r e n g t h / s t i f f n e s s t o weight r a t i o by i n c o r p o r a t i n g ceramic p a r t i c u l a t e i n t o an A1-Li a l l o y m a t r i x . The f i r s t s t e p has been t o t a k e a c o n v e n t i o n a l A1-Li a l l o y (8090) and r e i n f o r c e w i t h a low volume f r a c t i o n of SIC p a r t i c u l a t e . The p r e s e n t paper d e s c r i b e s t h e p r e l i m i n a r y d a t a d e r i v e d from t h i s e x e r c i s e .

Experimental P r o c e d u r e

The composite m a t e r i a l was produced u s i n g i n e r t g a s m e t a l s p r a y - d e p o s i t i o n technology. Equipment was purchased from Osprey Metals, Neath and f u r t h e r developed a t t h e Banbury L a b o r a t o r i e s of Alcan I n t e r n a t i o n a l Limited. The r e i n f o r c e m e n t m a t e r i a l ( S i c ) was i n c o r p o r a t e d i n t o t h e m a t r i x a l l o y a s a n i n t e g r a l p a r t of t h e m e t a l s p r a y i n g p r o c e s s i n such a manner a s t o produce a uniform d i s t r i b u t i o n

throughout t h e f i n i s h e d m a t e r i a l . The S i c used was p a r t i c u l a t e of mean diameter = 13 p and s i z e range 5-20 pa. The shape of t h e as-sprayed i n g o t s was chosen t o b e c y l i n d r i c a l so t h a t minimum machining was r e q u i r e d t o produce e x t r u s i o n b i l l e t . The average weight of as-sprayed i n g o t was 8.0 kg.

No p a r t i c u l a r d i f f i c u l t y was experienced i n machining e x t r u s i o n b i l l e t s which were prepared u s i n g c o n v e n t i o n a l machining t e c h n i q u e s . I n o r d e r t o reduce t o o l wear c a r b i d e t i p p e d t o o l s were used f o r rough machining. F i n i s h machining, such a s t h a t needed f o r t e n s i l e t e s t p i e c e s , was c a r r i e d o u t u s i n g diamond t i p p e d t o o l s . The f i n i s h e d b i l l e t s were 80 mm i n diameter and 228 mm i n l e n g t h . Homogenisation was c a r r i e d o u t u s i n g s t a n d a r d p r a c t i c e developed f o r DC c a s t 8090 i n c o r p o r a t i n g a s l o w h e a t up t o 5 4 0 ' ~ and a hold f o r 24 hours.

E x t r u s i o n was c a r r i e d o u t a t NPL, Teddington U.K. The f i n a l c r o s s - s e c t i o n was round b a r of 18 mm d i a m e t e r , g i v i n g an e x t r u s i o n r a t i o of

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R e s u l t s and D i s c u s s i o n

1. M i c r o s t r u c t u r e and Composition

The composition of t h e m a t r i x i n t h e as-sprayed form w a s determined by chemical a n a l y s i s f o l l o w i n g d i s s o l u t i o n and i s shown i n t h e t a b l e below.

The composition i s a t t h e lower end of t h e 8090 s p e c i f i e d c o m p o s i t i o n a l range(3).

The d e n s i t y of t h e m a t e r i a l i n t h e as-extruded form was determined a s 2.62 big/m3.

This i s t o be compared with

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p r e s e n t a u t h o r s ) . Matrix Alloy Composition (wt%)

L

L i 2 - 2 3

Cu 1.08

Mg 0.50

Zr 0.12

F e 0.08

S i 0.04

A 1 Balance

The S i c was found t o b e u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t t h e as-sprayed i n g o t a l t h o u g h o c c a s i o n a l c l u s t e r s of SIC were found ( o f t e n a s s o c i a t e d w i t h p o r o s i t y ) . F i g u r e 1 shows t h e as-sprayed m i c r o s t r u c t u r e imaged i n back s c a t t e r e d mode i n t h e 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 (BEI). The phases i n t h e as-produced m a t e r i a l have n o t been p o s i t i v e l y i d e n t i f i e d but q u a l i t a t i v e l y a p p e a r s i m i l a r t o t h o s e found i n a s - c a s t i n g o t 8090. The as-produced phases a r e e v e n l y d i s t r i b u t e d t h r o u g h o u t t h e m a t r i x and a r e n o t a s s o c i a t e d w i t h t h e i n t e r f a c e between t h e m a t r i x and t h e S i c . The " h a l o " e f f e c t s u r r o u n d i n g t h e S i c p a r t i c l e s i s caused bjr t h e d i f f e r e n c e i n h e i g h t between t h e p a r t i c l e and s u r r o u n d i n g m a t r i x . The phase d i s t r i b u t i o n is c o n s i d e r a b l y r e f i n e d when compared w i t h c o n v e n t i o n a l l y c a s t 8090. The r e f i n e m e n t of m i c r o s t r u c t u r e i s a l s o observed i n t h e f i n e as-produced g r a i n s i z e which i s

a p p r o x i m a t e l y 50 p. F i g u r e 2 shows t h e homogenised m i c r o s t r u c t u r e imaged u s i n g BEI. The homogenisation p r o c e d u r e used, which i s t h e s t a n d a r d f o r i n g o t m e t a l l u r g y 8090, h a s c l e a r l y been s u c c e s s f u l , r e s u l t i n g i n d i s s o l u t i o n of v i r t u a l l y a l l t h e as-produced phases w i t h t h e e x c e p t i o n of i r o n c o n t a i n i n g i n t e r m e t a l l i c s . The o v e r a l l volume f r a c t i o n of S i c was e s t i m a t e d a t 10% on t h e b a s i s of a r e a f r a c t i o n measurements on o p t i c a l s e c t i o n s and d i s s o l u t i o n of t h e m a t r i x followed by weight f r a c t i o n measurements. The p o r o s i t y i n t h e as-sprayed i n g o t was e s t i m a t e d t o be 10% i n e a r l y i n g o t . T h i s i s h i g h e r t h a n t h a t n o r m a l l y found i n non-lithium c o n t a i n i n g c o m p o s i t e s produced by t h i s r o u t e ( < 5 % ) . 8090 i n g o t c o n t a i n i n g S i c sprayed s u b s e q u e n t t o t h i s i n i t i a l work have shown much l e s s p o r o s i t y .

F i ure 1: Back s c a t t e r e d e l e c t r o n image BE1 of a s produced composite.

f r Figure 2: BE1 of homogenised composite.

O p t i c a l m i c r o s t r u c t u r e s of t h e e x t r u d e d b a r a r e shown i n F i g u r e 3 . The S i c i s u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t t h e b a r . The e x t r u s i o n p r o c e s s h a s , however, r e s u l t e d i n t h e a l i g n m e n t of t h e p a r t i c u l a t e i n t h e d i r e c t i o n of e x t r u s i o n . The p o r o s i t y observed i n t h e o r i g i n a l i n g o t has c l o s e d up d u r i n g e x t r u s i o n . F i g u r e 4 i s a BE1 of t h e a s - e x t r u d e d m a t e r i a l . A d d i t i o n a l p r e c i p i t a t i o n h a s o c c u r r e d d u r i n g e x t r u s i o n . These r e a d i l y d i s s o l v e on s o l u t i o n h e a t t r e a t m e n t .

Figure 3: Optical m i c r o s t r u c t u r e s of t h e extruded bar ( a ) t r a n s v e r s e , ( b ) l o n g i t u d i n a l .

JOURNAL DE PHYSIQUE

F i g u r e 4: BE1 o f a s - e x t r u d e d composite.

2. Mechanical P r o p e r t i e s

The extruded b a r was s o l u t i o n h e a t t r e a t e d i n an a i r oven f o r 15 min a t 535OC and c o l d water quenched. T h i s t r e a t m e n t was based on t y p i c a l p r a c t i c e s developed f o r i n g o t c a s t 8090 i n t h e extruded form. The bar was s t r e t c h e d 2% p r i o r t o ageing.

Ageing c u r v e s were determined a t t e m p e r a t u r e s of 150°C and 170°C u s i n g h a r d n e s s measurements on p o l i s h e d t r a n s v e r s e s e c t i o n s of t h e e x t r u d e d bar ( F i g u r e 5). The

c u r v e s show an i n i t i a l r a p i d i n c r e a s e i n h a r d n e s s and extended p l a t e a u r e g i o n s t y p i c a l of u n r e i n f o r c e d 8090. The ageing r e s p o n s e of t h e r e i n f o r c e d m a t e r i a l i s n o t s i g n i f i c a n t l y f a s t e r t h a n i n g o t c a s t m a t e r i a l . S i m i l a r maximum h a r d n e s s e s a r e achieved a t both temperatures. Maximum h a r d n e s s a p p e a r s t o be reached a f t e r * 24 hours a t 170°C and * 40 hours a t 150°C. L i t t l e s i g n of overageing was observed w i t h i n t h e range of e x p e r i m e n t a t i o n .

TIME (hours)

Figure 5: Ageing c u r v e s [Hardness ( V H N ) a g a i n s t t i m e ) o b t a i n e d a t 150°C and 170°C.

For t h e i n i t i a l s e t of a g e i n g experiments round b a r t e n s i l e specimens w i t h a 50 mm gauge l e n g t h were t e s t e d i n t r i p l i c a t e i n t h e f o l l o w i n g c o n d i t i o n s : - s o l u t i o n

t r e a t e d and s t r e t c h e d 2%, underaged (8 h r a t 170°C), peak aged (22 h r a t 170°C) and overaged (40 h r a t 170°C). The r e s u l t s a r e r e p o r t e d i n bargraph form i n F i g u r e 6.

As w i t h c o n v e n t i o n a l 8090(8) t h e d u c t i l i t y of t h e composite d e c r e a s e s w i t h i n c r e a s i n g s t r e n g t h a s ageing p r o g r e s s e s . I n DC c a s t and extruded 8090 t h e f o l l o w i n g p r o p e r t i e s can be taken a s t y p i c a l ( 9 , l O ) ; proof s t r e n g t h 480 m a , t e n s i l e s t r e n g t h

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79.5 GPa. The proof s t r e n g t h and t e n s i l e s t r e n g t h of t h e composite a r e comparable with t h e s e v a l u e s . T h i s i s encouraging a s t h e composite m a t r i x used i n t h e p r e s e n t s e r i e s of experiments h a s a composition a t t h e lower end of t h e 8090 s p e c i f i c a t i o n l i m i t s f o r a l l t h e major a l l o y i n g a d d i t i o n s . It would t h u s be expected t h a t w i t h o u t t h e r e i n f o r c e m e n t t h e s t r e n g t h would have been below t y p i c a l . Perhaps a b e t t e r comparison would be t h e p r o v i s i o n a l minimum p r o p e r t y l e v e l s quoted by P e e l e t a l ( 1 0 ) ; PS = 440 m a , TS = 495 m a , Ef = 3% and E = 79.5 GPa. The composite

m a t e r i a l f u l f i l l s t h e s e r e q u i r e m e n t s . D u c t i l i t y is somewhat lower than t y p i c a l 8090 but t h i s i s n o t unexpected i n composite m a t e r i a l s . At t h i s s t a g e i n t h e development of t h e s e m a t e r i a l s i t i s considered t o be a c c e p t a b l e as no a t t e m p t has been made t o o p t i m i s e p r o c e s s i n g . The major d i f f e r e n c e i n p r o p e r t i e s from c o n v e n t i o n a l 8090 i s t h e i n c r e a s e i n modulus. T h i s i s a g r e a t e r t h a n 20% i n c r e a s e over u n r e i n f o r c e d 8090 and corresponds t o an 18% i n c r e a s e i n the s t i f f n e s s t o d e n s i t y r a t i o . his i s double t h e i n c r e a s e i n s p e c i f i c modulus of c o n v e n t i o n a l p a r t i c u l a t e r e i n f o r c e d aluminium based MMC's over c o n v e n t i o n a l AI-Li a l l o y s .

0 8 2 2 40

AGEING T I M E (hr.) AT 170°C

0.2% PS. (MPal T.S. (MPa)

68X8 E f lo/,l

mmmD E (GPa)

Figure 6: Longitudinal tensile data from composite aged at 170°C.

C3-352 J O U R N A L D E PHYSIQUE

S e v e r a l s t u d i e s on c o n v e n t i o n a l i n g o t m e t a l l u r g y 8090 have i n d i c a t e d t h a t l o w e r a g e i n g t e m p e r a t u r e s can have a b e n e f i c i a l e f f e c t on t h e s t r e n g t h - d u c t i l i t y - f r a c t u r e t o u g h n e s s r e l a t i o n s h i p ( l 1 ) . I n o r d e r t o a s s e s s t h e c a p a c i t y f o r t h e p r e s e n t m a t e r i a l i n t h i s r e s p e c t a f u r t h e r two s e t s o f t e s t - p i e c e were t e s t e d a f t e r ageing a t 150°C f o r 15 h r ( u n d e r a g e d ) and 40 h r ( n e a r peak a g e d ) . The r e s u l t s a r e

p r e s e n t e d i n F i g u r e 7. The l5O0C aged m a t e r i a l e x h i b i t s comparable s t r e n g t h t o t h a t aged a t 170°C w i t h p e r h a p s m a r g i n a l l y h i g h e r d u c t i l i t y . The l a t t e r improvement i n d u c t i l i t y i s l e s s m a r g i n a l when t h e i n c r e a s e d modulus i s t a k e n i n t o a c c o u n t . From t h e l i m i t e d d a t a a v a i l a b l e t h e r e t h u s a p p e a r s t o be scope f o r t h e u s e of lower t e m p e r a t u r e a g e i n g t r e a t m e n t s i n t h e composite. T h i s is i m p o r t a n t a s f l e x i b i l i t y i n t h e a g e i n g b e h a v i o u r may be t h e key t o o b t a i n i n g t h e b e s t c o m b i n a t i o n of primary and s e c o n d a r y p r o p e r t i e s i n t h i s m a t e r i a l .

0 15 40

AGEING TIME ( h r ) AT 1 5 0 ' ~

Figure 7: Longitudinal tensile data from composite aged at 150°C.

3 . C o n c l u s i o n s

The p r e s e n t s t u d y was d e s i g n e d p r i m a r i l y t o t e s t t h e f e a s i b i l i t y of u s i n g a s p r a y d e p o s i t i o n t e c h n i q u e t o m a n u f a c t u r e A1-Li a l l o y ; S i c c o m p o s i t e s . No a t t e m p t . t h e r e f o r e , h a s been made t o o p t i m i s e e i t h e r t h e p r o d u c t i o n r o u t e o r t h e h e a t

t r e a t m e n t .

D e s p i t e t h e p r e l i m i n a r y n a t u r e of t h e m a t e r i a l under i n v e s t i g a t i o n t h e g e n e r a l p r o p e r t i e s o f t h e c o m p o s i t e compare f a v o u r a b l y w i t h t h o s e o f c o n v e n t i o n a l l y c a s t and e x t r u d e d ( u n r e i n f o r c e d ) 8090. The m a j o r d i f f e r e n c e i s t h e s i g n i f i c a n t i n c r e a s e i n e l a s t i c modulus. T h i s c o r r e s p o n d s t o a g r e a t e r t h a n 30% i n c r e a s e i n modulus o v e r c o n v e n t i o n a l aluminium a l l o y s and - 50% i n c r e a s e i n t h e s t i f f n e s s t o d e n s i t y r a t i o . The m a t e r i a l i s t h u s i n a f a i r l y u n i q u e p o s i t i o n a s f a r a s p r o p e r t i e s a r e concerned.

R e i n f o r c e m e n t of A1-Li a l l o y s h a s p r e v i o u s l y been a t t e m p t e d u s i n g S i c w h i s k e r ( l 2 ) and A1203(13). These a t t e m p t s were s u c c e s s f u l i n a c h i e v i n g v e r y h i g h moduli ( h i g h volume f r a c t i o n s of r e i n f o r c e m e n t were used) b u t e x h i b i t e d v e r y poor d u c t i l i t y and f r a c t u r e t o u g h n e s s .

The p o t e n t i a l u s a g e of any m a t e r i a l i s d e p e n d e n t on m a t e r i a l s p r o p e r t i e s and c o s t . On t h e s u b j e c t o f t h e f o r m e r , t h e p o t e n t i a l of t h e p r e s e n t s y s t e m h a s been d e m o n s t r a t e d and f u r t h e r g a i n s i n s t i f f n e s s would be e x p e c t e d a t h i g h e r volumwe f r a c t i o n s of S i c ( a l b e i t p r o b a b l y a s a compromise a g a i n s l o s s i n d u c t i l i t y ) . It i s a l s o a n t i c i p a t e d t h a t t h e s e p r o p e r t i e s w i l l be i s o t r o p i c due t o t h e even

d i s t r i b u t i o n and low a s p e c t r a t i o of t h e p a r t i c u l a t e ( c e r t a i n l y more s o t h a n f i b r e r e i n f o r c e d c o m p o s i t e s ) . Work on o t h e r p a r t i c u l a t e based c o m p o s i t e s produced by t h i s r o u t e h a s d e m o n s t r a t e d t h a t t h e r e i n f o r c e m e n t a p p e a r s t o have a g r e a t e r e f f e c t on d u c t i l i t y t h a n t o u g h n e s s , h e n c e any d e c r e a s e i n t o u g h n e s s i n t h e r e i n f o r c e d Al-Li a l l o y would n o t be e x p e c t e d t o be g r e a t e r t h a n t h e p r o p o r t i o n a t e d e c r e a s e i n d u c t i l i t y . I n t h i s r e s p e c t t h e t o u g h n e s s of t h e Al-Li based MMC may prove t o be a c c e p t a b l e , p a r t i c u l a r l y i f underaged tempers a r e used and t h e volume f r a c t i o n of S i c i s m a i n t a i n e d i n t h e p r e s e n t r e g i m e which a p p e a r s t o g i v e a good compromise between s t r e n g t h , modulus and d u c t i l i t y . An improved e l e v a t e d t e m p e r a t u r e

performance f o r a SIC w h i s k e r c o n t a i n i n g Al-Li a l l o y has been r e p o r t e d ( l 2 ) and t h e f a t i g u e r e s i s t a n c e o f c o n v e n t i o n a l aluminium-lithium a l l o y s i s w e l l known. The s e c o n d a r y p r o p e r t i e s of t h i s p a r t i c u l a r c o m p o s i t e a r e t h e r e f o r e e x p e c t e d t o be of g r e a t i n t e r e s t .

On t h e s u b j e c t of c o s t ; t h e p r i c e of a l u m i n i u m - l i t h i u m based c o m p o s i t e s is d i f f i c u l t t o p r e d i c t . Alcan, however, b e c a u s e of i t s p o s i t i o n a s a commercial p r o d u c e r of a l u m i n i u m - l i t h i u m i n g o t a l l o y s would be a b l e t o t a k e a d v a n t a g e of t h e b e n e f i t s of b e i n g an i n t e g r a t e d p r o d u c e r . The o v e r a l l c o s t t h e r e f o r e would n o t be e x p e c t e d t o b e t h a t much g r e a t e r t h a n t h e c o s t of a c o n v e n t i o n a l aluminium based m e t a l m a t r i x c o m p o s i t e s .

Acknowledgements

The a u t h o r s a r e g r a t e f u l t o Alcan I n t e r n a t i o n a l Limited f o r p e r m i s s i o n t o p u b l i s h t h i s work and t o t h e i r c o l l e a g u e s who have a s s i s t e d i n t h e e x p e r i m e n t a l work.

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