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FATIGUE CRACK INITIATION AND
PROPAGATION PROPERTIES OF AL-Li-Cu ALLOYS IN AIR AND IN AQUEOUS CORROSIVE
SOLUTIONS
T. Magnin, P. Rieux, C. Lespinasse, C. Bathias
To cite this version:
T. Magnin, P. Rieux, C. Lespinasse, C. Bathias. FATIGUE CRACK INITIATION AND PROPA- GATION PROPERTIES OF AL-Li-Cu ALLOYS IN AIR AND IN AQUEOUS CORROSIVE SOLU- TIONS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-817-C3-822. �10.1051/jphyscol:1987396�.
�jpa-00226628�
JOURNAL D E PHYSIQUE
Colloque C3, supplkment au n09, Tome 48, septembre 1987
FATIGUE CRACK INITIATION AND PROPAGATION PROPERTIES OF Al-Li-Cu ALLOYS I N AIR AND I N AQUEOUS CORROSIVE SOLUTIONS
T. MAGNIN, P. RIEUX, C.
L E S P I N A ~ S E *and C. BATHIAS*
E c o f e d e s M i n e s , 158 C o u r s F a u r i e l , F - 4 2 0 2 3 S a i n t - E t i e n n e C e d e x ,
Trance
U n i v e r s i t e d e Technologic d e C o m p i e g n e , D e p a r t e m e n t G e n i e M e c a n i q u e ,
B . P233, F - 6 0 2 0 6 C o m p i e g n e C e d e x , F r a n c e
SYNOPSIS
F a t l g u e t e s t s a r e performed on a n Al-Li-Cu CP271 a l l o y i n a i r and i n d i f f e r e n t c o r - r o s i v e s o l u t i o n s on smooth and n o t c h e d specimens t o d e t e r m i n e t h e c r a c k i n i t i a t i o n and c r a c k p r o p a g a t i o n p r o p e r t i e s o f t h e CP271 a l l o y i n t h e peak aged c o n d i t i o n s . C r a c k p r o p a g a t i o n p r o p e r t i e s a r e shown t o b e v e r y i n t e r e s t i n g i n r e g a r d s w i t h t h e c o n v e n t i o - n a l Al-Zn-Mg a l l o y s , Crack i n i t i a t i o n p r o p e r t i e s a r e shown t o b e v e r y s e n s i t i v e t o t h e t e x t u r e o f t h e specimens b o t h i n a i r and i n a 3.5% NaCl s o l u t i o n . The a n o d i c d i s s o l u - t i o n a c c e l e r a t e s c r a c k i n i t i a t i o n i n t h e c o r r o s i v e s o l u t i o n b u t t h e c o r r e s p o n d i n g r e - d u c t i o n o f t h e f a t i g u e l i f e i s l e s s i m p o r t a n t t h a n f o r c o n v e n t i o n a l Al-Zn-Mg a l l o y s . INTRODUCTION
-
k c o n s i d e r a b l e amount of r e s e a r c h h a s b e e n r e c e n t l y d e v o t e d t o t h e aluminium-lithiun ( c o p p e r ) a l l a y s p r o p e r t i e s [ 1 , 2 , 3 1 . I t i s w e l l known t h a t t h e a d d i t i o n o f l i t h i u m t o aluminium r e s u l t s i n a s u b s t a n t i a l r e d u c t i o n o f t h e d e n s i t y and a n i n c r e a s e i n e l a s - t i c modulus when compared w i t h c o n v e n t i o n a l aluminium a l l o y s . Modern a i r w o r t h i n e s s r e q u i r e m e n t s f o r a r e o s p a c e v e h i c l e s s p e c i f y t h a t environment e f f e c t s b e c o n s i d e r e d . It i s r e c o g n i z e d t h a t c o r r o s i o n and f a t i g u e f r e q u e n t l y d e t e r m i n e t h e u s e f u l l i f e o f a i r c r a f t s t r u c t u r e s . However, l i t t l e a t t e n t i o n h a s b e e naid
t o t h e i n t e r a c t i o n s between f a t i g u e and c o r r o s i o n f o r Al-Li a l l o y s r 3 , 4 1 .Thus, t h e aim o f t h e p r e s e n t p a p e r i s t o a n a l y s e and compare t h e f a t i g u e p r o p e r t i e s o f a p Al-Li-Cu CP271 a l l o y i n d i f f e r e n t e n v i r o n m e n t s ( d r y a r g o n , humid a i r , d i s t i l l e d w a t e r , NaCl s o l u t i o n s ) , t a k i n g i n t o a c c o u n t b o t h t h e c r a c k i n i t i a t i o n and che c r a c k p r o p a g a t i o p r e s i s t a n c e s . Moreover a comparison w l t h t h e b e h a v i o u r o f c o n v e n t i o n a l Al-Zn-Mg a l l o y s u s e d f o r a i r c r a f t s t r u c t u r e s w i l l b e e s t a b l i s h e d i n e a c h c a s e . EXPERIMENTAL TECHNIQUE
The i n v e s t i g a t e d CP271 a l l o y ( s u p p l i e d by P e c h i n e y i n f o r m o f 13 mm t h i c k e x t r u d e d p l a t e s ) c o n t a i n s ( i n w e i g h t p e r c e n t ) 2 . 5 L i , 1 . 3 Cu, 1 . 0 Mg, 0 . 0 9 Z r , 0 . 0 2 Fe and ( 5 0 . 2 5 ) Zn, The p l a t e s have b e e n s o l u t i o n t r e a t e d a t 5 3 5 ' ~ , t h e n quenched i n c o l d w a t e r , s t r e t c h e d by 2 % and a r t i f i c i a l l y aged f o r 12h a t 190°c (T651 h e a t t r e a t m e n t ) . The a l l o y i s h i g h l y t e x t u r e d and e x h i b i t s a n u n r e c r y s t a l l i z e d s t r u c t u r e w i t h v e r y l o n g g r a i n s a l o n g t h e L o r i e n t a t i o n ( F i g . I ) .
Smooth s p e c i m e n s (5 mm d i a m e t e r , 15 mm gauge l e n g t h ) and n o t c h e d specimens (CTW75 BIZ) were machined from t h e r o l l i n g p l a t e s ( w i t h t h e t e n s i l e a x i s ( i ) p a r a l l e l t o t h e L o r i e n t a t i o n and ( i i ) p a r a l l e l t o t h e TL o r i e n t a t i o n ) t o d e t e r m i n e t h e c r a c k i n i t i a - t i o n r e s i s t a n c e and t h e c r a c k p r o p a g a t i o n r e s i s t a n c e r e s p e c t i v e l y . T e s t s were conduc- t e d i n d r y a r g o n , i n a 60 % h u m i d i t y a i r , i n d i s t i l l e d w a t e r and i n a 30 g / l NaCl s o l u t i o n a t pH 6 and a t f r e e c o r r o s i o n p o t e n t i a l Eo. Smooth s p e c i m e n s were-hycled i n t e n s i o n - c o m p r e s s i o n a t imposed p l a s t i c s t r a i n a m p l ~ t u d e (10-' < AE-12 < 10 ) and
- 7 -1
s t r a i n r a t e (10 s < E < I O - ~ S - ' ) u n d e r ~ o t e n t i o s t a t i c c o n t r o l (:or more d e t a i l s , s e e r 4 1 ) . F a t i g u e c r a c k p r o p a g a t i o n e x p e r i m e n t ; w e r e c o n d u c t e d f o r d i f f e r e n t l o a d r a t i o s R = 0 . 0 1 , R = 0 . 1 and R = 0 . 5 a t f r e q u e n c i e s f = 10 and 20 H z . The c r a c k l e n g t h was m o n i t o r e d u s i n g a n o p t i c a l method ( f o r more d e t a i l s , s e e r 5 1 ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987396
J O U R N A L
DE
PHYSIQUEEXPERIMENTAL RESULTS M i c r o s t r u c t u r e s
P r e c i p i t a t i o n i n t h e Al-Li-Cu s y s t e m h a s b e e n e x t e n s i v e l y s t u d i e d [ 1 , 2 , 3 1 . I n t h e p r e s e n t p a p e r t h e a l l o y s a r e c o n s i d e r e d f o r t h e T651 h e a t t r e a t m e n t which c o r r e s p o n d s t o t h e peak aged c o n d i t i o n s . The c o r r e s p o n d i n g m i c r o s t r u c t u r e s a r e shown o n F i g . 2 and F i g . 3 (From K h i r e d d i n e , Rahouadj and C l a v e l , p r i v a t e communication). It can b e o b s e r v e d t h a t :
( i ) The
6 '
(A13Li) p r e c i p i t a t e s ( 1 5 0 t o 200 A d i a m e t e r ) a r e p r o f u s e i n t h e m a t r i x and n u c l e a t e p r e f e r i e n t a l l y on A13Zr p r e c i p i t a t e s .( i i ) The S ' (AlzCuMg) p r e c i p i t a t e g t a k e p l a c e p r e f e r e n t i a l l y a t d i s l o c a t i o n s i n t h e m a t r i x , in,form o f p l a t e s of 150 A l e n g t h w i t h a n a v e r a g e s p a c e between them o f a b o u t 700 A.
( i i i ) The
6
(Al L i ) p r e c i p i t a t e s h a v e b e e n e n c o u n t e r e d a t g r a i n b o u n d a r i e s b u t t h e T (AlnCuLi) p r e c i p i t a t e s a r e v e r y r a r e , i n t h e m a t r i x ( o n l y a t g r a i n b o u n d a r i e s ) .1
Crack i n i t i a t i o n r e s i s t a n c e i n a i r and i n aqueous s o l u t i o n s
An examole o f t h e e v o l u t i o n o f t h e c v c l i c ~ e a k s t r e s s A012 w i t h the nunher of c v c l e s N i s shown on F i g . 4 f o r t h e CP271 a l l o y i n a i r and i n t h e 30 g / l NaCl s o l u t i o n a t f r e e c o r r o s i o n p o t e n t i a l Eo. A s e n s i t i v e e f f e c t o f t h e o r i e n t a t i o n o f t h e specimen c a n be o b s e r v e d . The f a t i g u e l i v e s and t h e s a t u r a t i o n s t r e s s e s a r e lower i n t h e TL o r i e n t a t i o n zompared w i t h t h e L o r i e n t a t i o n . N e v e r t h e l e s s t h e peak s t r e s s o f t h e CP271 a l l o y i s a l w a y s h i g h e r t h a n t h e peak s t r e s s o f t h e c o n v e n t i o n a l 7020 T6DR a l l o y
C41
even i f t h e f a t i g u e l i v e s o f t h e 7020 T6DR a l l o y a r e more l o n g e r . The e f f e c t o f t h e c o r r o s i v e s o l u t i o n i s t h e n much more i m p o r t a n t f o r t h e 7020 a l l o y ( r e d u c t i o n by a f a c t o r 5 of t h e f a t i g u e l i f e i n t h e NaCl s o l u t i o n compared w i t h a i r ) ; i t d e c r e a s e s f o r t h e CP271 L a l l o y ( r e d u c t i o n by a f a c t o r 2 . 5 ) and i s n o n - e x i s t e n t f o r t h e CP271 TL a l l o y . F i n a l l y , i t h a s b e e n n o t i c e d t h a t , c o n t r a r i l y t o t h e 7020 a l l o y , t h e CP271 L and TL a l l o y s do n o t e x h i b i t any r e d u c t i o g 3 0 f t h e f a t i g u e l i v e s i n t h e NaCl c o r r o s i v e s o l u t i o n a t h i g h e r s t r a i n s (A€ / 2>
10 ) : t h i s can b e a l s o r e l a t e d t o t h e v e r y s h o r t f a t i g u e l i f e s o f t h e ~ ~ 2 7 1 ~ a l l o ~ a t h i g h s t r a i n a m p l i t u d e s .The c y c l i c s t r e s s - s t r a i n c u r v e s o f t h e CP271 a l l o y s i n a i r and i n t h e NaCl c o r r o - s i v e s o l u t i o n ( F i g . 5 ) c o n f i r m t h a t t h e s t r e s s e s i n t h e CP271Lalloy a r e h i g h e r t h a n t h o s e i n t h e CP271 TL a l l o y and h i g h e r t h a n t h o s e o f t h e 7020 L a l l o y .
A s e n s i t i v e e f f e c t o f t h e imposed s t r a i n r a t e
I
on t h e r e d u c t i o n o f t h e f a t i g u e l i f e s i s o b s e r v e d a t A€ / 2 = 3 . 3 . 1 0 ~ ' for-lhelCP271 L a l l o y ( F i g . 6 ) . T h i s r e d u c t i o n i s n o t i c e a b l ~ ~ a t h i g h s e r a i n r a t e( t
>/ 10 s ) b u t i s non e x i s t e n t a t lower s t r a i n r a t e(i:
= 10 t o I O - ~ S - ' ) . I t i s shown e l s e w h e r e C61 t h a t a new r e d u c t i o n o f t h e f a t i g u e l i f e s i s o b s e r v e d a t v e r y low s t r a i n r a t e(i:
< 1 0 - ~ s - ' ) f o r which s t r e s s c o r r o s i o n c r a c k i n g c a n o c c u r . Such a b e h a v i o r i s d i f f e r e n t o f t h e c o r r o s i o n - f a t i g u e b e h a v i o u r o f t h e 7020 T6DR a l l o y a s shown on FigA6611F i n a l l y , i t h a s b e e n o b s e r v e d t h a t f o r
I
> 10 s,
f a t i g u e c r a c k i n i t i a t i o n i n a i r i s t r a n s g r a n u l a r on t h e CP271 L and TL a l l o y s and i s mixed ( t r a n s + i n t e r g r a n u - l a r ) i n t h e NaCl s o l u t i o n (which e m p h a s i z e s t h e r o l e o f d i s s o l u t i o n a t g r a i n boundary) Crack p r o p a g a t i o n r e s i s t a n c e i n a i r and i n c o r r o s i v e s o l u t i o n sA t y p i c a l f e a t u r e o f t h e f a t i g u e c r a c k p r o p a g a t i o n b e h a v i o u r i n a i r o f the-CP271 . . a l l o y can b e o b s e r v e d on F i g . 7 f o r c r a c k gro;th r a t e s da/dN i n t h e r a n g e t o
mm/c. A k i n d o f p l a t e a u i s o b s e r v e d w h a t e v e r t h e a p p l i e d l o a d r a t i o . . R, even i f t h i s p l a t e a u i s s h o r t e r when R i n c r e a s e s . The o b s e r v a t i o n o f t h e f r a c t u r e s u r f a c e i n t h e medium c r a c k growth r a t e s r a n g e e m p h a s i z e s t h e o c c u r r e n c e o f d a r k e n e d a r e a s c o r r e s p o n d i n g t o oxyde d e p o s i t s . Such r e s u l t s a r e q u i t e s i m i l a r t o t h o s e r e c e n t l y o b t a i n e d by P e t e r s e t a 1 l 7 1 who r e p o r t t h a t f o r h i g h l o a d r a t i o s ( t y p i c a l l y R = 0 . 7 ) t h e p l a t e a u i s much l e s s pronounced and t h e c o r r e s p o n d i n g d a r k e n e d a r e a s much s m a l l e r . An oxyde i n d u c e d c r a c k c l o s u r e and a r o u g h n e s s i n d u c e d c r a c k c l o s u r e e f f e c t s a r e t h e n e x p e c t e d C81. It c a n b e a l s o n o t i c e d t h a t , f o r t h e e x p l o r e d c r a c k growth r a t e r a n g e , t h e c r a c k p r o p a g a t i o n r e s i s t a n c e o f t h e CP271 a l l o y i s v e r y s i m i - l a r t o t h a t o f t h e 7075 T651 a l l o y s C91. F i n a l l y no e f f e c t o f t e x u r e h a s been o b s e r -
-i
ved on t h e c r a c k growth r a t e s o f t h e CP271 a l l o y . For da/dN > 10 mm/C.
F i g . 8 e m p h a s i z e s t h e r o l e o f t h e h u m i d i t y , showing t h e b e h a v i o u r i n a i r w i t h 60 % h u m i d i t y i n comparison t o t h e b e h a v i o u r i n d r y a r g o n . The e f f e c t o f h u m i d i t y o n t h e t h r e s h o l d v a l u e i s s e n s i t i v e . Moreover t h e p l a t e a u seems t o d i s a p p e a r i n d r y a r g o n . I n humid atmosphere t h e r e i s a k i n d o f c h a l l e n g e between t h e n e g a t i v e e n v i r o n - ment damage and t h e b e n e f i c i a l c r a c k c l o s u r e e f f e c t s .
F i n a l l y , F i g . 9 shows t h e e f f e c t o f t h e d i s t i l l e d w a t e r on t h e f a t i g u e c r a c k r e s i s - t a n c e of t h e CP271 a l l o y . No e f f e c t i s o b s e r v e d a t low AK, b u t t h e p l a t e a u i s s h o r t e r i n t h e d i s t i l l e d w a t e r
.
DISCUSSION AND CONCLUSIONS
The r e s u l t s r e p o r t e d i n t h e p r e s e n t p a p e r emphasize t h e v e r y good f a t i g u e and c o r r o s i o n f a t i g u e c r a c k p r o p a g a t i o n r e s i s t a n c e of t h e Al-Li-Cu CP271 T651 a l l o y . Moreover a comparison w i t h t h e f a t i g u e p r o p e r t i e s o f t h e c o n v e n t i o n a l Al-Zn-Mg a l l o y s
i n t h e peak aged c o n d i t i o n s c a n b e e s t a b l i s h e d a s i t f o l l o w s :
1
-
The Al-Li-Cu CP271alloy e x h i b i t s h i g h e r c y c l i c s t r e s s e s t h a n t h e 7020 a l l o y a t p r e s c r i b e d p l a s t i c s t r a i n a m p l i t u d e and a c y c l i c s t r e s s - s t r a i n c u r v e c l o s e t o t h a t of t h e 7075 a l l o y r101. N e v e r t h e l e s s t h e f a t i g u e l i f e t i m e s of smooth CP271 specimens a r e v e r y reduced i n comparison t o t h o s e o f t h e 7000 s e r i e s a l l o y s r 4 1 .2 - The e f f e c t o f t e x t u r e i s more s e n s i t i v e on Al-Li-Cu smooth specimens t h a n f o r t h e 7020 a l l o y . The L o r i e n t a t i o n e x h i b i t s h i g h e r c y c l i c s t r e s s and l o n g e r l i f e t i m e s t h a n t h e TL o r i e n t a t i o n . However c r a c k p r o p a g a t i o n p r o p e r t i e s a r e s i m i l a r f o r t h e L and TL o r i e n t a t i o n s .
3 - The c r a c k p r o p a g a t i o n p r o p e r t i e s i n a i r of t h e CP271 a l l o y a r e s i m i l a r t o t h a t of t h e 7000 s e r i e s a l l o y s i n t h e c r a c k growth r a t e r a n g e i n v e s t i g a t e d i n t h i s s t u d y . Moreover a p l a t e a u can b e observed on t h e c u r v e s d a / d ~ = f(AK).
4
-
The c r a c k i n i t i a t i o n r e s i s t a n c e o f t h e CP271 a l l o y i n t h e 3 . 5 % NaCl s o l u t i o n a t f r e e c o r r o s i o n p o t e n t i a l i s l e s s reduced t h a n t h a t o f t h e 7020 a l l o y i n t h e same e l e c t r o c h e m i c a l c o n d i t i o n s . At h i g h s t r a i n r a t e (C 3 10-'s-I), t r u e c o r r o s i o n - f a t i g u e i s o b s e r v e d on t h e L o r i e n t a t i o n o f t h e CP271 a l l o y o n l y a t low s t r a i n a m p l i t u d e (Ae 1 2 < a l t h o u g h c o r r o s i o n - f a t i g u e o c c u r s w h a t e v e r AE / 2 on t h e 7020 a l l o y .~ e v g r t h e l e s s i t must b e n o t i c e d t h a t t h e t i m e s i n t h e c o r r o s P v e s o l u t i o n a r e lower f o r t h e CP2715allox bycause o f i t s s h o r t e r f a t i g u e l i f e t i m e s . At i n t e r m e d i a t e s t r a i n r a t e
( t
2 10 -10 s ) no c o r r o s i o n f a t i g u e i s o b s e r v e d f o r t h e CP271 a l l o y . F i n a l l y , a t low s t r a i n r a t e( E <
1 0 - ~ s - l ) c o r r o s i o n f a t i g u e c a n o c c u r b y s t r e s s c o r r o s i o n b o t h f o r t h e 7020 and CP271 a l l o y s .5
-
Crack i n i t i a t i o n d u r i n g t r u e c o r r o s i o n f a t i g u e o c c u r s b o t h a t g r a i n b o u n d a r i e s and a t t h e s l i p band emergence. It h a s b e e n shown e l s e w h e r e r 6 1 t h a t o n l y a n o d i c d i s - s o l u t i o n i s r e s p o n s i b l e o f t h e a c c e l e r a t i o n of t h e c r a c k i n g p r o c e s s i n t h e c o n d i t i o n s o f o u r t e s t s . T h i s e x p l a i n s t h e s t r a i n r a t e e f f e c t : a t h i g h s t r a i n r a t e , t h e d e p a s s i - v a t i o n and t h e d i s s o l u t i o n a r e more pronounced t h a t a t lower s t r a i n r a t e and t h e c o r r e s p o n d i n g f a t i g u e l i f e i s t h e n reduced a t h i g h s t r a i n r a t e .6
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Crack p r o p a g a t i o n p r o p e r t i e s on n o t c h e d specimens a r e no more s e n s i t i v e t o t h e environment t h a n f o r o t h e r 7000 s e r i e s a l l o y s C81, p e r h a p s b e c a u s e of a l e s s pronoun- ced s e n s i t i v i t y t o hydrogen e m b r i t t l e m e n t . The i n f l u e n c e of h u m i d i t y i s p a r t i c u l a r l y s e n s i t i v e f o r t h e low v a l u e s of AK n e a r t h e t h r e s h o l d . Moreover t h e o b s e r v e d p l a t e a u on t h e c u r v e s da/dN = f(AK) i s reduced i n d i s t i l l e d w a t e r .F i n a l l y , i t must be p o i n t e d o u t t h a t a l l t h e c o r r o s i o n f a t i g u e r e s u l t s on t h e CP271 a l l o y were o b t a i n e d a t f r e e c o r r o s i o n p o t e n t i a l and i n such c o n d i t i o n s t h a t o n l y a n o d i c d i s s o l u t i o n can o c c u r . These r e s u l t s c a n b e l a r g e l y m o d i f i e d when t h e hydrogen r e d u c t i o n r e a c t i o n i s f a v o u r e d , s p e c i a l l y a t a p p l i e d c a t h o d i c p o t e n t i a l s a s shown e l s e w h e r e 1 6 1 .
REFERENCES
[ I ] T.H. S a n d e r s Jr and E.A. S t a r k e ( e d s ) , "Aluminium-Lithium a l l o y s " , TMS-AIME (Warrendale) 1981.
C21 T.H. S a n d e r s Jr and E.A. S t a r k e ( e d s ) , "Aluminium-Lithium a l l o y s II", TMS-AIME, 1984.
C31 C. Baker, P . J . Gregson, S . J . H a r r i s and C.P. P e e l ( e d s ) 'lAluminium-Lithium a l l o y s III", I n s t i t u t e o f M e t a l s , London 1986.
C41 T. Magnin, C . Dubessy and P. R i e u x , Aluminium a l l o y s , Conf. P r o c . C h a r l o t t e s v i l - l e , USA, E.A. S t a r k e and T.H. Sander Ed., EMAS, 1986, p. 1177.
C51 A. C l e r i v e t , T h e s i s , CompiSgne, 1978.
C61 T. Magnin and M. R e b i e r e , T h i s Conference A1-Li IV.
C71 M. P e t e r s , V. Bachmann, K. Welpmann, H. E e l l i n g r o d t , J. E s c h w e i l e r , R. Braun, DFVLR, R e p o r t of The I n s t i t u t e fGr W e r k s t o f f , 1986.
C81 3 . S u r e s h , Met. T r a n s . , 16A, 1985, p. 249.
r 9 1 J. P e t i t , i n F a t i g u e Crack growth t h r e s h o l d c o n c e p t , Davidson and S u r e s h E d . , AIME, 1984, p. 3 .
C3-820 JOURNAL
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PHYSIQUETI01 C. L a i r d , C y c l i c s t r e s s - s t r a i n and p l a s t i c d e f o r m a t i o n a s p e c t s o f f a t i g u e c r a c k growth, ASTM STP 637, 1977, p . 3 .
FIG. 1 . Three d i m e n s i o n a l c o m p o s i t e o p t i c a l p h o t o m i c r o g r a p h i l l u s t r a t i n g t h e m i c r o s t r u c t u r e of t h e CP271 a l l o y (T651).
FIG. 2 . 6 ' p r e c i p i t a t e s i n t h e Al-Li-Cu CP271 a l l o y ( T 6 5 1 ) . Dark f i e l d , s p o t on 100.
FIG. 3 . S' p r e c i p i t a t e s i n t h e Al-Li-Cu 271 a l l o y (T651)
1
FIG.
4.
Evolution of t h e peak s t r e s s A0,2 w i t h O i t 4 163 t h e number of c y c l e s N on t h e CP271 Land TL a l l o y and on t h e 7020L a l l o y a t
A& / 2 = 3.3. ( f o r more d e t a i l s on FIG. 5. C y c l i c s t r e s s - s t r a i n cur-
the P behaviour of the 7020 a l l o y , see ves of t h e Al-Li-Cu CP271
C41. a l l o y i n a i r and i n the
3.5 % NaCl s o l u t i o n .
FIG. 6. I n f l u e n c e of t h e s t r a i n r a t e on t h e c o r r o s i o n f a t i g u e (CF) behaviour of t h e CP271 L a l l o y a t A& / 2 = 3.3.10-'. Ni i s t h e number of c y c l e s t o c r a c k i n i t i a t i o n dorrespon8ing t o a 1 % r a p l d decrease of t h e s a t u r a t i o n s t r e s s 141.
JOURNAL
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PHYSIQUEFIG. 7. I n f l u e n c e o f t h e l o a d r a t i o R on t h e f a t i g u e c r a c k p r o p a g a t i o n o f t h e CP27 1 a l l o y i n laboratory a i r .
FIG. 8. F a t i g u e c r a c k growth r a t e r e s u l t s o f t h e CP271 a l l o y t e s t e d i n a i r and d r y argon a t R = 0.1.
l.OOE-03
I.ooE-04 D 1.m-05 I 0 I.MC-06
I . W - 0 7 I
1.m-m
2 3 4 10
( r r l c l &K W a l m l
air 0 disUllM rrhr
FIG. 9. F a t i g u e c r a c k growth r a t e r e s u l t s of t h e CP271 a l l o y t e s t e d i n a i r and i n d i s t i l l e d w a t e r a t R = 0.01.
