THE EFFECT OF FTMT ON THE GRAIN BOUNDARY MICROSTRUCTURE OF ALUMINIUM-LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM ALLOYS

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THE EFFECT OF FTMT ON THE GRAIN

BOUNDARY MICROSTRUCTURE OF ALUMINIUM- LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM

ALLOYS

J. White, W. Miller

To cite this version:

J. White, W. Miller. THE EFFECT OF FTMT ON THE GRAIN BOUNDARY MICROSTRUC-

TURE OF ALUMINIUM-LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM ALLOYS. Journal de

Physique Colloques, 1987, 48 (C3), pp.C3-425-C3-431. �10.1051/jphyscol:1987348�. �jpa-00226579�

JOURNAL DE PHYSIQUE

C o l l o q u e C 3 , s u p p l e m e n t

au

n 0 9 , Tome 48,

septembre

1 9 8 7

THE EFFECT OF FTMT ON T H E GRAIN BOUNDARY MICROSTRUCTURE OF ALUMINIUM-LITHIUM-COPPER-MAGNESIUM-ZIRCONIUM ALLOYS

J. WHITE and W.S. MILLER

A l c a n

I n t e r n a t i o n a l

L t d ,

Southam

R o a d , B a n b u r y , GB-Oxon, OX16

7 S P ,

G r e a t - B r i t a i n

ABSTRACT

T h i s s t u d y e x p l o r e s t h e e f f e c t of s o l u t i o n h e a t t r e a t m e n t , d e f o r m a t i o n p r i o r t o a g e i n g and a g e i n g t e m p e r a t u r e on t h e g r a i n boundary m i c r o s t r u c t u r e of A1-Li a l l o y s . Emphasis i s p l a c e d on 8090 b u t comparison i s made w i t h o t h e r a l l o y systems where a p p r o p r i a t e . It i s c o n c l u d e d t h a t a l l t h r e e v a r i a b l e s a f f e c t g r a i n boundary p r e c i p i t a t i o n and t h a t t h e l a t t e r two c a n be used t o r e f i n e t h e g r a i n boundary m i c r o s t r u c t u r e .

INTRODUCTION

The o b j e c t i v e of t h i s s t u d y i s t o examine t h e i n f l u e n c e of f i n a l

thermo-mechanical t r e a t m e n t (FTMT) on t h e g r a i n boundary m i c r o s t r u c t u r e of complex aluminium a l l o y s c o n t a i n i n g l i t h i u m . Emphasis w i l l b e p l a c e d on 8090 b u t

comparisons w i l l be made w i t h o t h e r a l l o y s y s t e m s where a p p r o p r i a t e . The f r a c t u r e b e h a v i o u r of aluminium a l l o y s h a s been shown t o be dominated by g r a i n boundary f a i l u r e ( 1 , Z ) . T h i s h a s been a t t r i b u t e d t o b o t h d e f o r m a t i o n b e h a v i o u r w i t h i n t h e m a t r i x and t o g r a i n boundary s t r u c t u r e . I n t h e former t h e p r e s e n c e of t h e c o h e r e n t , o r d e r e d 6' (A13Li) phase w i t h i n t h e m a t r i x l e a d s t o i n t e n s e p l a n a r s l i p r e s u l t i n g i n s t r e s s c o n c e n t r a t i o n s a t h i g h a n g l e g r a i n b o u n d a r i e s . I n t h e l a t t e r c a s e , l a r g e q u a s i - e q u i l i b r i u m p h a s e s a r e p r e c i p i t a t e d a t h i g h a n g l e g r a i n b o u n d a r i e s w h i l s t a s s o c i a t e d 6' PFZ's a r e formed. A l l of t h e s e f a c t o r s would b e e x p e c t e d t o r e d u c e g r a i n boundary s t r e n g t h t h u s l e a d i n g t o a d e c r e a s e i n f r a c t u r e r e s i s t a n c e i n t h i s r e g i o n .

I n o r d e r t o improve f r a c t u r e r e s i s t a n c e t h e r e a r e s e v e r a l c o u r s e s of a c t i o n ; modify e i t h e r t h e d e f o r m a t i o n c h a r a c t e r i s t i c s of t h e a l l o y , t h e g r a i n s t r u c t u r e o r t h e g r a i n boundary m i c r o s t r u c t u r e . These a r e n o t a l t e r n a t i v e c o u r s e s of a c t i o n and may b e combined t o produce t h e optimum c o m b i n a t i o n of p r o p e r t i e s i n a f i n a l

product(3)'.

The main a t t e m p t t o a l t e r t h e d e f o r m a t i o n c h a r a c t e r i s t i c s of Al-Li a l l o y s h a s been t o i n c o r p o r a t e a d d i t i o n a l p r e c i p i t a t i o n . The prime example of t h i s i s S' (A12CuMg) p r e c i p i t a t i o n i n Al-Li-Cu-Mg-Zr a l l o y s ( 4 ) . T h i s phase i s p r e c i p i t a t e d h e t e r o g e n e o u s l y on s u b - g r a i n b o u n d a r i e s and d i s l o c a t i o n s and hence t h e r e i s c o n s i d e r a b l e scope f o r m o d i f i c a t i o n of t h e d i s t r i b u t i o n of t h i s phase u s i n g m e c h a n i c a l working t r e a t m e n t s p r i o r t o a g e i n g .

The e f f e c t of g r a i n s t r u c t u r e on toughness h a s - b e e n d i s c u s s e d p a r t i c u l a r l y i n r e l a t i o n t o s h e e t p r o d u c t s ( 5 ) . It h a s been shown t h a t under p l a n e s t r e s s c o n d i t i o n s t o u g h n e s s i s c o n t r o l l e d by t h e amount of p l a s t i c d e f o r m a t i o n o c c u r r i n g i n t h e p l a s t i c zone. Under t h e s e c o n d i t i o n s of " s t r a i n c o n t r o l " a f i n e r e c r y s t a l l i s e d g r a i n s i z e is r e q u i r e d t o g e n e r a t e maximum s t r a i n s t o f a i l u r e i n t h e p l a s t i c zone.

The s i t u a t i o n i s d i f f e r e n t f o r p l a t e p r o d u c t s . Under t h e e x t r a c o n s t r a i n t of p l a n e s t r a i n , f a i l u r e i s i n i t i a t e d ahead of t h e induced f a t i g u e c r a c k a t t h e n e a r e s t weak f e a t u r e . T h i s w i l l be t h e n e a r e s t g r a i n boundary p a r t i c l e o r p r e c i p i t a t e . ~ h t h e ~ s i d e a l s t r u c t u r e f o r t h i c k p r o d u c t s i s a c o a r s e u n r e c r y s t a l l i s e d s t r u c t u r e . I n t h i c k p r o d u c t forms t h e r e f o r e , g r a i n boundary m i c r o s t r u c t u r e i s of primary i m p o r t a n c e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987348

C3-426 JOURNAL

DE

PHYSIQUE

P r e c i p i t a t e s on g r a i n b o u n d a r i e s a r e of two t y p e s ; c o n s t i t u e n t p h a s e s ( p r i m a r i l y 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 ) and p r e c i p i t a t e s a r i s i n g d u r i n g t h e s o l u t i o n t r e a t m e n t and a g e i n g p r a c t i c e s . The e f f e c t of c o n s t i t u e n t p h a s e s on m e c h a n i c a l p r o p e r t i e s h a s been c h a r a c t e r i s e d by Owen(6). D e c r e a s i n g t h e volume f r a c t i o n of c o n s t i t u e n t p h a s e s i s a c h i e v e d by m i n i m i s i n g t h e i r o n and s i l i c o n c o n t e n t . I n commercial a l l o y s t h e p r a c t i c a l l i m i t s f o r t h e s e e l e m e n t s i s t y p i c a l l y 0.1 wt%. The most p r a c t i c a l method of modifying t h e g r a i n boundary m i c r o s t r u c t u r e , t h e r e f o r e , i s t o c o n t r o l g r a i n boundary p r e c i p i t a t i o n d u r i n g h e a t t r e a t m e n t . The p r e s e n t p a p e r e x p l o r e s t h e e f f e c t of s o l u t i o n h e a t t r e a t m e n t p r a c t i c e , d e f o r m a t i o n p r i o r t o a g e i n g and a g e i n g t e m p e r a t u r e on t h e g r a i n boundary m i c r o s t r u c t u r e . The observed changes i n g r a i n boundary m i c r o s t r u c t u r e a r e r e l a t e d t o m e c h a n i c a l p r o p e r t i e s .

EXPERIMENTAL

A l l of t h e 8090 a l l o y used i n t h e p r e s e n t s t u d y h a s been l a b o r a t o r y produced w i t h c o m p o s i t i o n s w i t h i n t h e p u b l i s h e d s p e c i f i c a t i o n r a n g e s ( 7 ) . The p r o d u c t forms and p e r t i n e n t f a b r i c a t i o n d e t a i l s a r e d e s c r i b e d w i t h t h e a s s o c i a t e d e x p e r i m e n t a l r e s u l t s when t h i s i s a p p r o p r i a t e . T E M f o i l s were p r e p a r e d u s i n g a commercial twin j e t p o l i s h i n g u n i t ( S t r u e r s Tenupol). The e l e c t r o l y t e was 25% n i t r i c a c i d i n methanol and t h e specimens were t h o r o u g h l y washed i n methanol f o l l o w i n g p e r f o r a t i o n . The specimens w e r e examined i n a

JEOL

2000FX t r a n s m i s s i o n microscope f u l l y e q u i p p e d w i t h m i c r o a n a l y t i c a l f a c i l i t i e s . P r e c i p i t a t e f r e e zone w i d t h s were measured w i t h t h e boundary v e r t i c a l w i t h r e s p e c t t o t h e e l e c t r o n beam.

RESULTS

1. E f f e c t of C o o l i n g R a t e from S o l u t i o n Heat T r e a t m e n t

The e f f e c t of c o o l i n g r a t e f o l l o w i n g s o l u t i o n h e a t t r e a t m e n t (SHT) on t h e t o u g h n e s s of 13 m i t h i c k e x t r u d e d 8090 h a s been i n v e s t i g a t e d i n u n p u b l i s h e d work by t h e a u t h o r s . F i g u r e 1, t a k e n from t h i s s t u d y , shows t h e e f f e c t of c o o l i n g r a t e on Charpy i m p a c t e n e r g y f o r c o n s t a n t a g e i n g c o n d i t i o n s . The two extremes of c o o l i n g r a t e a r e t y p i c a l of s t i l l a i r c o o l i n g and w a t e r quenching. D e c r e a s i n g t h e c o o l i n g r a t e , a s i n an i n e f f i c i e n t quench, produces a d e c r e a s e i n impact r e s i s t a n c e . T y p i c a l g r a i n boundary m i c r o s t r u c t u r e s i n s l o w c o o l i n g r a t e and h i g h c o o l i n g r a t e samples a r e shown i n F i g u r e 2. The g r a i n boundary p r e c i p i t a t i o n i s c l e a r l y a f f e c t e d by t h e change i n c o o l i n g r a t e . Slow c o o l i n g h a s r e s u l t e d i n b o t h an i n c r e a s e i n a p p a r e n t volume f r a c t i o n and a change i n morphology of t h e p r e c i p i t a t e s a t t h e g r a i n boundary. E x a m i n a t i o n of s i m i l a r specimens i n t h e quenched b u t unaged c o n d i t i o n c l e a r l y shows t h a t t h i s a d d i t i o n a l p r e c i p i t a t i o n a r i s e s d u r i n g t h e quenching ( o r r a t h e r a i r c o o l i n g ) p h a s e of t h e h e a t t r e a t m e n t c y c l e . The p r e s e n c e of t h e s e p r e c i p i t a t e s i n b o t h t h e quenched and t h e aged c o n d i t i o n s i n d i c a t e s t h a t t h e y do n o t d i s s o l v e d u r i n g a g e i n g .

2 I m p a c t e n e r g y

Figure 1 The effect of cooling from SHT on charpy-impact energy.

8090: aged 48h/150°C.

0 1

^{1 I I 1}

0.1

1

.o 10 100 1000

Q u e n c h r a t e ( ' ~ / s e c )

F i ure 2 The e f f e c t of cooling r a t e on g r a i n boundary m i c r o s t r u c t u r e +aged 48hr a t 150°Cj. ( a ) Water quenched, (b) Air cooled.

Two t y p e s o f g r a i n boundary p r e c i p i t a t e a r e o b s e r v e d . The more -on of t h e s e . ( F i g u r e 3 ) e x h i b i t s f i v e f o l d symmetry i n s e v e r a l zone a x i s e l e c t r o n d i f f r a c t i o n p a t t e r n s and i s p r o b a b l y t h e i c o s o h e d r a l phase t h a t h a s r e c e n t l y been t h e s u b j e c t of much a t t e n t i o n ( s e e f o r example B a l l e t a l ( 8 ) ) . The o t h e r p h a s e , which h a s been

s e e n o n l y r a r e l y , e x h i b i t s zone a x i s d i f f r a c t i o n p a t t e r n s c o n s i s t e n t w i t h a c u b i c s t r u c t u r e . T h i s may be t h e C-phase(9). Both p h a s e s have s i m i l a r morphology and e x t e n d a l o n g t h e g r a i n boundary o f t e n n u c l e a t i n g a t t r i p l e p o i n t s . It would t h u s b e e x p e c t e d t h a t t h e y would have a p a r t i c u l a r l y d e l e t e r i o u s e f f e c t on b o t h impact e n e r g y and f r a c t u r e t o u g h n e s s ( l 0 ) . EDX a n a l y s i s i n t h e TEM shows t h e p r e s e n c e of aluminium and copper: a n a s s o c i a t e d 6' PFZ s u g g e s t s t h e p r e s e n c e of l i t h i u m i n t h e p r e c i p i t a t e s .

Figure 3 High angle g r a i n boundary p r e c i p i t a t e nucleated during a i r cooling from SHT.

(8090: aged 48 hr a t 150°C).

The g r a i n boundary m i c r o s t r u c t u r e i n s o l u t i o n t r e a t e d m a t e r i a l h a s been examined i n a v a r i e t y of p r o d u c t forms i n o r d e r t o a s s e s s t h e e f f e c t of s e c t i o n t h i c k n e s s on t h e quench s e n s i t i v i t y of 8090. I n r e l a t i v e l y t h i n s e c t i o n m a t e r i a l , t h a t i s s h e e t o r t h i n e x t r u s i o n ( t y p i c a l l y l e s s t h a n 6 ma), t h e c o n v e n t i o n a l c o l d w a t e r quenching t r e a t m e n t f o l l o w i n g s o l u t i o n h e a t t r e a t m e n t e n t i r e l y s u p p r e s s e s g r a i n boundary p r e c i p i t a t i o n . I n t h i c k e r s e c t i o n s up t o 50 mm ( t y p i c a l of p l a t e , f o r g i n g s and e x t r u s i o n s ) c o l d w a t e r quenching s u p p r e s s e s g r a i n boundary

p r e c i p i t a t i o n t o t h e e x t e n t t h a t i s o l a t e d p r e c i p i t a t e s a r e o b s e r v e d o n l y

i n f r e q u e n t l y i n ' t h e c e n t r e of t h e t h i c k e s t s e c t i o n s . They t h u s a r e n o t e x p e c t e d t o have a s i g n i f i c a n t e f f e c t on f r a c t u r e .

C3-428 J O U R N A L DE PHYSIQUE

The amount of g r a i n boundary p r e c i p i t a t i o n induced d u r i n g t h e quench becomes s i g n i f i c a n t a t s e c t i o n s i z e s of 2 100 m ( F i g u r e 4). At t h e s e s e c t i o n t h i c k n e s s e s t h e e f f e c t c a n n o t be s u p p r e s s e d by t h e u s e of a more e f f i c i e n t quenching medium (e.g. i c e d b r i n e ) a s t h e r a t e of h e a t e x t r a c t i o n i s governed by t h e h e a t f l o w t h r o u g h t h e m a t e r i a l r a t h e r t h a n t r a n s f e r t o t h e quenching medium. The e x t e n t of p r e c i p i t a t i o n i s , however, much l e s s t h a n i n t h e a i r c o o l e d samples from t h e i n i t i a l s t u d y and i n t h e f u l l y aged c o n d i t i o n t h e g r a i n boundary p r e c i p i t a t i o n is dominated by t h e a g e i n g t r e a t m e n t r a t h e r t h a n t h e quench.

F i g u r e 4 High a n g l e g r a i n boundary p r e c i p i t a t i o n a t t h e f t p o s i t i o n i n SHT and c o l d w a t e r quenched 100 rnrn p l a t e .

2. E f f e c t of Mechanical Work P r i o r t o Ageing

The e f f e c t of a 2% s t r e t c h p r i o r t o a g e i n g on t h e l e v e l of g r a i n boundary p r e c i p i t a t i o n i n 8090 i s i l l u s t r a t e d i n F i g u r e 5 which shows 8090 p l a t e i n t h e underaged c o n d i t i o n . Q u a l i t a t i v e l y t h e r e a p p e a r s t o be a d e c r e a s e i n t h e volume f r a c t i o n of p r e c i p i t a t e s a t h i g h a n g l e g r a i n b o u n d a r i e s i n t h e s t r e t c h e d compared w i t h t h e u n s t r e t c h e d m a t e r i a l . I n an a t t e m p t t o q u a n t i f y t h i s d i f f e r e n c e t h e number of p r e c i p i t a t e s p e r u n i t l e n g t h of g r a i n boundary was measured i n f o i l s t a k e n from s t r e t c h e d and u n s t r e t c h e d p l a t e aged f o r d i f f e r e n t t i m e s a t 190°C. The r e s u l t s of t h i s s t u d y a r e shown i n F i g u r e 6 . The d i f f e r e n c e s i n t h e l e v e l of g r a i n boundary p r e c i p i t a t i o n a r e n o t a s marked a s i n i t i a l q u a l i t a t i v e o b s e r v a t i o n would s u g g e s t and c l e a r l y a more s y s t e m a t i c s u r v e y i s r e q u i r e d t o improve t h e s t a t i s t i c s ( t h i s would

i n v o l v e a c c u r a t e measurements of f o i l t h i c k ' n e s s which have n o t been made i n t h i s i n s t a n c e ) . There i s , however, a s t a t i s t i c a l l y s i g n i f i c a n t d e c r e a s e i n g r a i n boundary p r e c i p i t a t i o n w i t h s t r e t c h i n g i n t h e underaged temper. The d i f f e r e n c e i s n o t so marked i n t h e peak and overaged tempers. The l e v e l of g r a i n boundary p r e c i p i t a t i o n a l s o s a t u r a t e s i n t h e peak aged c o n d i t i o n .

F i g u r e 5 H i g h a n g l e g r a i n boundary p r e c i p i t a t i o n on 8090 p l a t e aged

5 h r a t 190°C. ( a ) u n s t r e t c h e d , ( b ) s t r e t c h e d 2%.

3.0

1

R e l a t i v e

U n s t r e t c h e d 0 S t r e t c h e d 2 %

F i g u r e 6 Comparison o f t h e number o f h i g h a n g l e g r a i n b o u n d a r y p r e c i p i t a t e s p e r u n i t l e n g t h o f boundary i n s t r e t c h e d a n d u n s t r e t c h e d 8090 a g e d a t 190°C.

3. The e f f e c t of Ageing T e m p e r a t u r e

The e f f e c t of a g e i n g t e m p e r a t u r e on g r a i n boundary m i c r o s t r u c t u r e has been a s s e s s e d u s i n g 25 mm p l a t e s o l u t i o n h e a t t r e a t e d , c o l d w a t e r quenched and g i v e n a 2%

s t r e t c h p r i o r t o a g e i n g . T h r e e a g e i n g t e m p e r a t u r e s were u s e d ; 150°C, 170°C and 190°C. Q u a l i t a t i v e o b s e r v a t i o n s , u s i n g b r i g h t f i e l d TEM, c o n f i r m t h e e x p e c t e d i n c r e a s e i n g r a i n boundary p r e c i p i t a t i o n a s t e m p e r a t u r e o r time i s i n c r e a s e d . I n o r d e r t o q u a n t i f y t h e s e o b s e r v a t i o n s , t h e w i d t h of t h e 6' PFZ h a s been used a s a measure of t h e v a r i a t i o n i n t h e g r a i n boundary m i c r o s t r u c t u r e . T h i s i s because o f t h e d i f f i c u l t y i n o b t a i n i n g s t a t i s t i c a l l y v a l i d measurements of t h e number of g r a i n boundary p r e c i p i t a t e s ( a s was d e m o n s t r a t e d i n t h e p r e v i o u s s e c t i o n ) .

I n agreement w i t h p r e v i o u s w o r k e r s ( l l , L Z , 1 3 , 1 4 ) t h e h a l f PFZ w i d t h has been p l o t t e d a g a i n s t t h e s q u a r e r o o t of time and t h e r e s u l t s a r e shown i n F i g u r e 7. The r e s u l t s a p p e a r t o f i t t h e p a r a b o l i c r e l a t i o n s h i p s e e n i n o t h e r Al-Li a l l o y s w i t h i n t h e r a n g e of e x p e r i m e n t a t i o n . Much l o n g e r a g e i n g t i m e s , however, would be r e q u i r e d t o c o n f i r m t h a t t h e PFZ v a r i e s w i t h t h e s q u a r e r o o t r a t h e r t h a n t h e cube r o o t of time. Such a g e i n g t i m e s a r e u n l i k e l y t o be used i n a commercial h e a t t r e a t m e n t . F o r t h e r a n g e of t e m p e r a t u r e and t i m e i n v e s t i g a t e d , t e m p e r a t u r e h a s a f a r more pronounced e f f e c t t h a n t i m e on g r a i n boundary m i c r o s t r u c t u r e . W i t h i n t h e l i m i t s o f e x p e r i m e n t a l e r r o r t h e d a t a can be e x t r a p o l a t e d t o c o n c l u d e t h a t no PFZ i s p r e s e n t f o l l o w i n g s o l u t i o n h e a t t r e a t m e n t . T h i s , combined w i t h t h e c o n t i n u o u s growth of t h e zones d u r i n g a g e i n g , i n d i c a t e s t h a t t h e PFZ's a r e formed by a s o l u t e r a t h e r t h a n a vacancy d e p l e t i o n mechanism. I n o r d e r t o c o n f i r m t h i s r a t e c o n s t a n t s were

d e t e r m i n e d f o r each t e m p e r a t u r e and used t o c a l c u l a t e t h e a c t i v a t i o n e n e r g y f o r PFZ growth i n 8090. T h i s was found t o be 1.11 eV (107 k J / m o l ) . T h i s a g r e e s w i t h t h e r a n g e of p u b l i s h e d d a t a f o r t h e a c t i v a t i o n e n e r g y f o r PFZ growth i n A1-Li b i n a r y a l l o y s ( 1 2 , 1 4 ) and A1-Cu-Li a l l o y s ( l 3 ) . T h i s s u g g e s t s t h a t t h e r a t e d e t e r m i n i n g s t e p i n dl t h r e e t y p e s of a l l o y i s t h e same; t h e d i f f u s i o n of l i t h i u m i n aluminium ( a c t i v a t i o n e n e r g y

=

1.43 eV)(15).

J O U R N A L

DE

PHYSIQUE

F i g u r e 7 H a l f PFZ w i d t h p l o t t e d a g a i n s t ti. (8090 p l a t e ) .

DISCUSSION

H a l f P F Z w l d l h

( n m l

T l m s ( s e c b 1

The o b s e r v a t i o n s r e l a t i n g t o t h e p r e c i p i t a t i o n of g r a i n boundary phases d u r i n g quenching f o l l o w i n g s o l u t i o n h e a t t r e a t m e n t d e m o n s t r a t e t h a t 8090 i s i n s e n s i t i v e t o t h i s phenomenon i n p r o d u c t s w i t h s e c t i o n t h i c k n e s s e s of l e s s t h a n 50 mm. The r e c e n t r e p o r t of B r e t z ( l 6 ) of quench s e n s i t i v i t y i n 2090 i n s e c t i o n t h i c k n e s s e s of g r e a t e r t h a n 25 nrm i s i n broad agreement w i t h t h i s c o n c l u s i o n . The d i f f e r e n c e i n c r i t i c a l s e c t i o n t h i c k n e s s may be i n d i c a t i v e of an e f f e c t of a l l o y c o n t e n t ( p r o b a b l y copper a s t h e main p r e c i p i t a t i n g phase i s copper c o n t a i n i n g ) . I n s e c t i o n s i z e s of g r e a t e r t h a n 50 mm a quench s e n s i t i v i t y i n 8090 would be a n t i c i p a t e d on t h e b a s i s of

m i c r o s t r u c t u r a l o b s e r v a t i o n s and i s observed i n p r a c t i c e . It has been d e m o n s t r a t e d , however, t h a t s a t i s f a c t o r y t o u g h n e s s can be achieved i n 100 mm t h i c k 8090 p l a t e and f o r g i n g by s u i t a b l e m o d i f i c a t i o n of p r o c e s s i n g and h e a t t r e a t m e n t v a r i a b l e s ( l 7 ) .

The d e c r e a s e i n g r a i n boundary p r e c i p i t a t i o n a r i s i n g from s t r e t c h i n g p r i o r t o a g e i n g c a n be r e l a t e d t o improvements i n t h e proof s t r e n g t h 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 observed i n a number of p u b l i c a t i o n s ( 3 , 1 8 , 1 9 ) . A f u r t h e r c o r r e l a t i o n w i t h m e c h a n i c a l p r o p e r t i e s h a s been noted by Gray(2O) who h a s observed a n

improvement i n s t r e s s c o r r o s i o n c r a c k i n g performance i n s t r e t c h e d 8090. I n a

p r e l i m i n a r y e x a m i n a t i o n of 2090 t y p e a l l o y s by t h e a u t h o r s a more marked d e c r e a s e i n g r a i n boundary p r e c i p i t a t i o n w i t h s t r e t c h i n g was found. T h i s p r o b a b l y a r i s e s because of t h e d i f f e r e n t h e t e r o g e n e o u s l y n u c l e a t e d m a t r i x p r e c i p i t a t e s . T1 (AlpCuLi) p r e c i p i t a t i o n i n 2090 w i l l a f f e c t t h e d i f f u s i o n of copper and l i t h i u m t o t h e g r a i n boundary whereas S1(A12CuMg) p r e c i p i t a t i o n i n 8090 w i l l a f f e c t t h e d i f f u s i o n of copper and magnesium. As i t h a s been shown t h a t t h e main g r a i n boundary p r e c i p i t a t e s i n b o t h a l l o y systems c o n t a i n p r e d o m i n a n t l y copper a n d / o r l i t h i u m ( e . g . G(A1Li) and T2(A16Cu(Li,Mg3))(8,9,21) p r e c i p i t a t i o n of TI would be expected t o have a g r e a t e r e f f e c t t h a n S ' . T h i s w i l l make 2090 more s e n s i t i v e t o s t r e t c h i n g t h a n 8090.

A l o g i c a l e x t e n s i o n t o t h e measurement of an a c t i v a t i o n e n e r g y f o r PFZ growth i n 8090 i s t h e development of a n e f f e c t i v e model f o r d i f f u s i o n t o t h e g r a i n b o u n d a r i e s i n t h e a l l o y i n v o l v i n g t h e c a l c u l a t i o n of t h e u n d e r l y i n g d i f f u s i o n c o e f f i c i e n t s . T h i s r e q u i r e s an a c c u r a t e measurement of t h e volume f r a c t i o n of m a t r i x p r e c i p i t a t e s p r e s e n t and i s c u r r e n t l y i n p r o g r e s s . The use of PFZ w i d t h a s a measure o f g r a i n boundary p r e c i p i t a t i o n i s perhaps q u e s t i o n a b l e . The number of g r a i n boundary p r e c i p i t a t e s p e r u n i t l e n g t h of g r a i n boundary, however, i s d i r e c t l y r e l a t e d t o PFZ width and can be shown t o be r e l a t e d t o time through a p a r a b o l i c r e l a t i o n s h i p . To a f i r s t a p p r o x i m a t i o n , t h e r e f o r e , t h e assumption i s v a l i d . The r e s u l t s d e m o n s t r a t e t h a t w i t h i n t h e range of commercially v i a b l e a g e i n g p r a c t i c e s t e m p e r a t u r e has a more s i g n i f i c a n t e f f e c t on t h e g r a i n boundary m i c r o s t r u c t u r e t h a n time. The i m p l i c a t i o n of t h i s i s t h a t i n o r d e r t o o b t a i n t h e optimum combination o f mechanical p r o p e r t i e s , 8090 s h o u l d be aged a t a s low a t e m p e r a t u r e a s p o s s i b l e c o n s i s t e n t w i t h a c h i e v i n g t h e d e s i r e d s t r e n g t h . It i s t h i s p h i l o s o p h y which h a s been used t o d e v e l o p t h e r e c e n t l y r e p o r t e d a g e i n g p r a c t i c e s ( 3 ) i n c o r p o r a t i n g h i g h

l e v e l s of s t r e t c h ( t o a c h i e v e s t r e n g t h ) and low a g e i n g t e m p e r a t u r e s ( t o a c h i e v e h i g h t o u g h n e s s ) .

The e f f e c t of a h i g h l e v e l of s t r e t c h on t h e g r a i n boundary m i c r o s t r u c t u r e of 8090 h a s been i n v e s t i g a t e d i n a p r e l i m i n a r y s t u d y . Q u a l i t a t i v e l y i n c r e a s i n g t h e s t r e t c h from 2% t o 6% h a s no o b s e r v a b l e e f f e c t on t h e l e v e l of p r e c i p i t a t i o n a t h i g h a n g l e g r a i n b o u n d a r i e s . PFZ w i d t h s a r e a l s o u n a f f e c t e d . These o b s e r v a t i o n s d i f f e r from t h o s e of Ashton e t a l ( 1 9 ) who show a d e c r e a s e i n g r a i n boundary p r e c i p i t a t i o n w i t h i n c r e a s i n g s t r e t c h i n 8091 and 2090. T h i s d i f f e r e n c e may be a t t r i b u t a b l e t o t h e h i g h e r copper c o n t e n t s of 2090 and 8091.

CONCLUSIONS

The c o o l i n g r a t e from s o l u t i o n h e a t t r e a t m e n t , s t r e t c h i n g p r i o r t o a g e i n g and a g e i n g t e m p e r a t u r e and t i m e s i g n i f i c a n t l y a f f e c t t h e g r a i n boundary m i c r o s t r u c t u r e of 8090.

Apart from c o o l i n g r a t e t h e s e v a r i a b l e s c a n be used t o r e f i n e t h e g r a i n boandary m i c r o s t r u c t u r e of 8090.

The growth of PFZ's i n 8090 i s c o n t r o l l e d by a s o l u t e d e p l e t i o n mechanism t h e a c t i v a t i o n e n e r g y of which i s

=

1.11 eV and t h e r a t e d e t e r m i n i n g s t e p i s t h e d i f f u s i o n of l i t h i u m i n aluminium.

The r e s u l t s d e s c r i b e d h e r e i n a r e p a r t of a n ongoing s t u d y aimed a t q u a n t i f y i n g t h e e f f e c t of h e a t t r e a t m e n t v a r i a b l e s on g r a i n boundary m i c r o s t r u c t u r e . The u l t i m a t e o b j e c t i v e of t h i s work i s t o r e f i n e t h e model used t o c a l c u l a t e d f r a c t u r e t o u g h n e s s r e f e r r e d t o i n an e a r l i e r p u b l i c a t i o n ( 2 2 ) .

ACKNOWLEDGEMENT

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.

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