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A TECHNIQUE FOR IMPROVING THE
TOUGHNESS OF Al-Li POWDER METALLURGY ALLOYS
D. Webster
To cite this version:
D. Webster. A TECHNIQUE FOR IMPROVING THE TOUGHNESS OF Al-Li POWDER METALLURGY ALLOYS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-335-C3-340.
�10.1051/jphyscol:1987338�. �jpa-00226569�
JOURNAL DE PHYSIQUE
C o l l o q u e C3, s u p p l e m e n t a u ' n o 9 , T o m e 4 8 , s e p t e m b r e 1 9 8 7
A TECHNIQUE FOR IMPROVING THE TOUGHNESS OF A 1 - L i POWDER METALLURGY ALLOYS
K a i s e r Aluminum a n d C h e m i c a l C o r p o r a t i o n , P.O. Box 877.
P l e a s a n t o n , C A 94566, U . S . A .
A b s t r a c t
A t e c h n i q u e h a s b e e n d e v e l o p e d f o r i n c r e a s i n g t h e t o u g h n e s s o f A1-Li p r o d u c t s made b y powder m e t a l l u r g y . The t e c h n i q u e w h i c h i n v o l v e s t h e a d d i t i o n o f u n a l l o y e d aluminum powder t o A1-Li powder b e f o r e c o m p a c t i o n was e v a l u a t e d w i t h A1-Li-Cu-Mg-Zr a l l o y s ( A 1 8 0 9 0 ) , a n d Al-Li-Zn-Cu-Mg-Zr a n d Al-Li-Mg-Si-Cr a l l o y s . T h e a d d i t i o n of 15%
aluminum t o A18090 a g e d a t 422K f o r 40 h p r o d u c e d a n i n c r e a s e i n i m p a c t t o u g h n e s s o f 215% a t t h e e x p e n s e o f a d r o p i n y i e l d s t r e n g t h o f 11%. The Al-Li-Mg-Si-Cr a l l o y w i t h 1 5 % aluminum a d d e d w a s r e s i s t a n t t o stress c o r r o s i o n i n t h e s h o r t t r a n s v e r s e d i r e c t i o n a t s t r e s s e s a s h i g h a s 345 MPa ( 5 0 k s i ) . The A 1 8090 powder a l l o y s w i t h a d d e d aluminum h a d s u p e r i o r l o n g i t u d i n a l s t r e n g t h - t o u g h n e s s c o m b i n a t i o n s t o i n g o t m a t e r i a l s o f s i m i l a r c o m p o s i t i o n p r o c e s s e d i d e n t i c a l l y .
1.0 I n t r o d u c t i o n
The i n t e n t o f t h e p r e s e n t work was t o p r o d u c e A1-Li p o w d e r a l l o y s w i t h p r o p e r t i e s s u p e r i o r t o t h e l e s s e x p e n s i v e i n g o t m a t e r i a l s . The a p p r o a c h c h o s e n was t o p r o d u c e a ' c o m p o s i t e ' powder c o n s i s t i n g o f a n A1-Li powder mixed w i t h 10-30% o f u n a l l o y e d aluminum. The v e r y h i g h t o u g h n e s s of t h e u n a l l o y e d aluminum powder w a s e x p e c t e d t o i m p r o v e t h e t o u g h n e s s o f a l l o y s by a c t i n g t o b l u n t a g r o w i n g c r a c k . A s m a l l l o s s o f s t r e n g t h a n d a n i n c r e a s e i n d e n s i t y were e x p e c t e d a s a r e s u l t of t h e r e d u c e d a v e r a g e l i t h i u m l e v e l o f t h e powder m i x t u r e . T h i s t e c h n i q u e (U.S. P a t e n t 4,597792 i s s u e d 1 9 8 6 ) is a p p l i c a b l e t o b o t h aluminum a l l o y s a n d m e t a l m a t r i x c o m p o s i t e s . The m e c h a n i c a l p r o p e r t i e s a n d m i c r o s t r u c t u r e o f n i n e A1-Li a l l o y s made by t h i s t e c h n i q u e a r e d e s c r i b e d b e l o w .
2.0 M a t e r i a l
The c o m p o s i t i o n s o f t h e a l l o y p o w d e r s a r e g i v e n i n T a b l e I, t o g e t h e r w i t h t h e c o m p o s i t i o n s o f t h e powder m i x t u r e s t e s t e d .
4.0 R e s u l t s
4 . 1 M i c r o s t r u c t u r e
I n t h e h o t p r e s s e d c o n d i t i o n , t h e a l l o y b i l l e t s w i t h n o a d d e d aluminum have a n e q u i a x e d g r a i n s i z e w i t h a mean l i n e a r i n t e r c e p t o f a b o u t 30 pm. The a d d i t i o n o f u n a l l o y e d aluminum p r o d u c e s l o c a l a r e a s o f t h e p r e s s i n g w h e r e t h e c o m p o s i t i o n a l c h a n g e a l l o w s some g r a i n s t o
(l'~onsultant for Advanced Material Development at 20565 Verde Vista Lane. Saratoga. CA 95070, USA.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987338
J O U R N A L DE PHYSIQUE
Table I. Composition of Atomized Powders and Powder ?fixtures
Alloy Pct. Aluminum Powder Added
0 10 15 20 30 15 0 10 15 15 10
C ~ m p ~ s i t I ~ n ut. Pet.
&g&znpe
0.74 0.005 0.11 0.01
0.68 0.004 0.11 0.01
0.63 0.005 0.10 0.02 0.59 0.005 0.10 0.02 0.52 0.007 0.08 0.03 0.61 0.004 0.10 0.02 0.72 0.003 0.12 0.004 0.65 0.002 0.11 0.004
0.85 2.78 0.15 ND
1.1 ND ND ND
1.2 ND ND ND
Ut. PPU Na K
- -
<0.5 <0.5
<0.5 <0.5 ND ND ND ND ND ND ND ND
<0.5 <0.5
<0.5 <0.5 ND ND ND ND ND ND
grow up t o 1 0 0 urn d i a m e t e r d u r i n g t h e c o n s o l i d a t i o n p r o c e s s . T h i s i s c l e a r l y s e e n i n F i g u r e 1 f o r a l l o y s 1 6 1 4 and 1 6 1 5 . I n F i g u r e l a , a l l o y 1614 w i t h 3 . 3 9 Li h a s a u n i f o r m g r a i n s i z e a n d a u n i f o r m d i s p e r s i o n o f i n t e r m e t a l l i c p a r t i c l e s up t o 20 pm l o n g . I n c o n t r a s t , F i g u r e l b shows i s o l a t e d l a r g e g r a i n s i n a m a t r i x o f f i n e r g r a i n s w h i c h a r e t h e same s i z e a s t h o s e i n a l l o y 1 6 1 4 . I n a d d i t i o n , t h e r e i s a d e n u d e d a r e a 200-300 p m w i d e a r o u n d e a c h c o a r s e g r a i n where t h e l a r g e r i n t e r m e t a l l i c p a r t i c l e s s e e n u n i f o r m l y d i s p e r s e d i n a l l o y 1614 a r e n o t p r e s e n t . T h i s d e n u d e d a r e a would a p p e a r t o be t h e a p p r o x i m a t e e x t e n t o f s i g n i f i c a n t d i f f u s i o n o f a l l o y i n g e l e m e n t s d u r i n g h o t c o m p a c t i o n .
4.2 T e n s i l e a n d T o u g h n e s s P r o p e r t i e s 4.2.1 Al-Li-Cu-Mg-Zr A l l o y s
The a d d i t i o n of 10-30% o f u n a l l o y e d aluminum powder t o a n A l - 2.6%Li-Cu-Mg-Zr a l l o y r e d u c e s l o n g i t u d i n a l t e n s i l e y i e l d s t r e n g t h b u t s i g n i f i c a n t l y i m p r o v e s i m p a c t t o u g h n e s s (L-T n o t c h ) when a g e d a t 394K ( F i g . 2 ) a n d f o r a l l t h e o t h e r a g i n g c o n d i t i o n s i n v e s t i g a t e d . I n t h e t r a n s v e r s e d i r e c t i o n , t h e t e n s i l e e l o n g a t i o n shows n o i m p r o v e m e n t s w i t h 1 0 % a d d e d aluminum b u t t h e r e i s a s i g n i f i c a n t improvement w i t h 15-30%. The t r a n s v e r s e e l o n g a t i o n s a r e h i g h e r t h a n t h o s e i n t h e l o n g i t u d i n a l d i r e c t i o n . The t r a n s v e r s e t e n s i l e y i e l d s t r e n g t h i s l o w e r t h a n i n t h e l o n g i t u d i n a l d i r e c t i o n a n d i s l o w e r e d i n a n a l m o s t l i n e a r manner by aluminum a d d i t i o n s .
4.22 Al-Li-Zn-Cu-Mg-Zr a n d Al-Li-Mg-Si-Cr A l l o y s
The l o n g i t u d i n a l y i e l d s t r e n g t h and t o u g h n e s s p r o p e r t i e s a r e shown i n F i g u r e 3 . The s t r e n g t h - t o u g h n e s s c o m b i n a t i o n o f a l l o y 66 is i n f e r i o r t o t h a t o f t h e Al-Li-Cu-Mg-Zr a l l o y s t e s t e d a b o v e b u t d i r e c t c o m p a r i s o n s c a n n o t be made b e c a u s e o f t h e d i f f e r e n c e i n e x t r u s i o n s i z e a n d t h e r e d u c e d amount o f s t r e t c h ( 1 . 5 % ) g i v e n t o a l l o y 66.
F r a c t u r e t o u g h n e s s d e t e r m i n a t i o n s o n 1 . 9 1 cm t h i c k compact t e n s i o n s p e c i m e n s were o b t a i n e d u s i n g L-T a n d T-L n o t c h e s a n d a r e g i v e n i n T a b l e 11. P r o p e r t i e s o f a l l o y 81 a r e g i v e n i n F i g u r e 4.
Figure 1. (a) Microstructure of Alloy 1614 (3.3% Li) in the 'As Hot Pressed Condition'.
(b) Microstructure of Alloy 1615 (3% Li) in the 'As Hot Pressed Condition' Showing Isolated Large Grains Surrounded by Areas of Alloy Denudation.
C3-338 JOURNAL DE PHYSIQUE
T a b l e 11. F r a c t u r e T o u g h n e s s V a l u e s D e t e r m i n e d f r o m Compact T e n s i o n S p e c i m e n s f o r A l l o y 6 6 A g i n g
C o n d i t i o n s
N o t c h Q
O r i e n t a t i o n (MPa-iffi
L-T T-L L-T T-L L-T T-L 4.3 S t r e s s C o r r o s i o n R e s i s t a n c e
4.3.1 Al-Li-Cu-Mg-Zr A l l o y s
The a n d K v a l u e s i n t h e s h o r t t r a n s v e r s e d i r e c t i o n f o r 1 1 8090 z 3 - L i-Cu-Mg-Zr) a l l o y s w i t h u p t o 30% a d d e d a l u m i n u m w e r e m e a s u r e d . The a d d i t i o n o f a l u m i n u m i n c r e a s e s b o t h t h e KIA an?,
K I v a l u e s f o r a l l a g i n g c o n d i t i o n s . ~ ~ ~
The s h o r t t r a n s v e r s e KIA a n d K I v a l u e s f o r t h e h i g h e r l i t h i u m ~ ~ ~ a l l o y s 1 6 1 4 a n d 1 6 1 5 w e r e m e a s u r e d . The 1 0 % a d d e d a l u m i n u m i n 1 6 1 5 h a d o n l y a s m a l l e f f e c t o n t h e stress c o r r o s i o n t h r e s h o l d .
4.3.2 Al-Li-Zn-Cu-Mg-Zr A l l o y
T h e KIA a n d R I v a l u e s i n t h e s h o r t t r a n s v e r s e d i r e c t i o n ~ ~ ~ f o r a l l o y 6 6 a r e g i v e n i n T a b l e 111.
T a b l e 111. S h o r t T r a n s v e r s e KIA a n d KIs,, V a l u e s f o r A l l o y 6 6 T e s t e d b y A l t e r n a t e I m m e r s i o n i n 3 1 / 2 % NaCl S o l u t i o n
A g i n g K K T o t a l C r a c k
T r e a t m e n t (MPaTm) - ( MPa fa G r o w t h (mm)
394K 1 6 h 1 1 . 7 , 1 1 . 6 1 0 . 3 , 10.8 2 . 8 , 2.3
422K 1 6 h 1 2 . 1 , 1 2 . 2 1 1 . 8 , 1 2 . 1 0.8, 0.3
42213 lOOh 8 . 1 , 8 . 5 8 , 8.5 0.3, 0
4.3.3 Al-Li-Mg-Si-Cr A l l o y s
T a b l e I V . S h o r t T r a n s v e r s e KIA a n d KIsc, V a l u e s f o r A l l o y s 8 3 a n d 8 1 T e s t e d b y A l t e r n a t e I m m e r s i o n i n 3 1 / 2 % NaCl S o l u t i o n
A g i n g K K ' T o t a l C r a c k
A l l o y C o n d i t i o n s -- - ( ~ ~ a c m ) (MPa J-iai' G r o w t h (mm) 8 3 ( + l o % A l ) 394K 1 6 h 9.4, 8.3 8 . 5 , 8 . 3 0 . 3 , 0
422K 1 6 h 9.8, 9 9.6, 9 0.3, 0
422K lOOh 8 . 7 , 8.5 8 . 4 , 8 . 1 0 . 5 , 0.8
8 1 ( + 1 5 % A l ) 394K 1 6 h 17.8* 1 7 . 8 , 0
422K 1 6 h 1 4 . 2 , 1 3 . 1 1 3 . 6 , 1 2 . 9 0 . 8 , 0.5
422K lOOh 1 0 . 7 , 9.6 10.1, 9 1 , 1 . 3
* I n v a l i d b y 2 . 5 ( K / 0 . 2 % YS) c r i t e r i o n .
S h o r t t r a n s v e r s e t u n i n g f o r k s p e c i m e n s of a l l o y s 83 a n d 8 1 g i v e n t h e same a g i n g t e a t m e n t s l i s t e d i n T a b l e I V showed n o f a i l u r e s i n a l t e r n a t e i m m e r s i o n t e s t s a t stresses u p t o 345 MPa f o r 90 d a y s . 5.0 D i s c u s s i o n
Two i m p o r t a n t o b s e r v a t i o n s c a n b e made from t h e a b o v e r e s u l t s . F i r s t l y , t h e c o m p o s i t i o n o f A1-Li powder a l l o y s c a n b e c o n v e n i e n t l y m a n i p u l a t e d by t h e a d d i t i o n o f u n a l l o y e d aluminum powder w i t h o u t d e t r i m e n t t o t h e m e c h a n i c a l p r o p e r t i e s . S e c o n d l y , a n d more i m p o r t a n t , t h e m e c h a n i c a l p r o p e r t i e s s u c h a s t o u g h n e s s a n d stress c o r r o s i o n r e s i s t a n c e , a r e i n c r e a s e d by t h e aluminum a d d i t i o n s . T h e s e i n c r e a s e s a r e , however, a t t a i n e d a t t h e e x p e n s e of s m a l l d e c r e a s e s i n s t r e n g t h a n d i n c r e a s e s i n d e n s i t y . One q u e s t i o n d h i c h a r i s e s f r o m t h i s work i s how much o f t h e i m p r o v e m e n t i n m e c h a n i c a l p r o p e r t i e s is d u e t o t h e i n t e n t i o n a l l y s e g r e g a t e d m i c r o s t r u c t u r e w i t h h i g h l i t h i u m a n d low l i t h i u m a r e a s , a n d how much is d u e s i m p l y t o t h e r e d u c e d a v e r a g e l i t h i u m c o n t e n t . A s e p a r a t i o n o f t h e two e f f e c t s c a n b e o b t a i n e d b y p l o t t i n g t h e t o u g h n e s s v a l u e f o r a g i v e n a g i n g t r e a t m e n t v e r s u s t h e a v e r a g e l i t h i u m c o n t e n t o f t h e a l l o y s . I f d i l u t i o n o f t h e l i t h i u m i s t h e o n l y mechanism o p e r a t i n g , t h e n a l l t h e d a t a s h o u l d l i e o n o n e l i n e . I f t h e ' t w o p h a s e ' s t r u c t u r e i m p r o v e s t o u g h n e s s by p r o d u c i n g , o n a m i c r o s t r u c t u r a l s c a l e , a r e a s of low l i t h i u m c o n t e n t w h i c h a r e a b l e t o r e t a r d c r a c k p r o p a g a t i o n , t h e n t h e r e w i l l b e a d i s c o n t i n u i t y i n t h e d a t a w i t h a l l o y s 1 6 1 1 a n d 1 6 1 4 , w h i c h have n o a d d e d aluminum, l y i n g b e l o w t h e rest o f t h e d a t a . A d i s c o n t i n u i t y w a s o b s e r v e d f o r a l l t h r e e a g i n g t e m p e r a t u r e s . The d e g r e e o f i m p r o v e m e n t d u e t o t h e d u p l e x s t r u c t u r e v a r i e d from a b o u t 70% f o r a 394K a g e t o 358 f o r a 444K a g e .
A c o m p a r i s o n o f t h e stress c o r r o s i o n b e h a v i o r o f t h e t h r e e a l l o y s y s t e m s i n v e s t i g a t e d s h o w s t h a t f o r a l l o y s c o n t a i n i n g 1 5 % a d d e d aluminum < F i g . 5 > , ( 8 1 a n d 6 6 ) KIscc i s s i m i l a r i n e a c h c a s e a t a g i v e n y i e l d s t r e n g t h , w h i l e t h e c r a c k i n i t i a t i o n b e h a v i o r o f a l l o y s 8 1 a n d 66 i s s i g n i f i c a n t l y d i f f e r e n t . The c o p p e r - f r e e a l l o y ( 8 1 ) i s v e r y r e s i s t a n t t o c r a c k i n i t i a t i o n e v e n when s t r e s s e d a t i t s y i e l d s t r e n g t h , w h i l e t h e c o p p e r - c o n t a i n i n g a l l o y ( 6 6 ) c r a c k s a t v e r y low s t r e s s e s e x c e p t i n t h e peak a g e d c o n d i t i o n .
B o t h A 1 8090 t y p e a l l o y s a r e most r e s i s t a n t t o stress c o r r o s i o n c r a c k i n i t i a t i o n i n t h e u n d e r a g e d c o n d i t i o n w i t h t h e optimum a g i n g t e m p e r a t u r e b e i n g 394K f o r a l l o y 1 6 1 1 ( n o a d d e d a l u m i n u m ) a n d 422K f o r a l l o y 1 6 1 2 ( 1 0 % aluminum a d d e d ) . A l l t h e c o p p e r c o n t a i n i n g a l l o y s a r e i n f e r i o r t o t h e c o p p e r f r e e a l l o y s .
T h i s t y p e o f b e h a v i o r h a s b e e n r e p o r t e d b y H o l r o y d , e t a l l <Ref.
2 > who a t t r i b u t e d t h e p o o r p e r f o r m a n c e o f c o p p e r - c o n t a i n i n g A1-Li
a l l o y s t o t h e p r e s e n c e o f T i ( A l ~ c u L i ) , (A12Cu) o r S(A12Cu Mg) w h i c h p r o m o t e d l o c a l g r a i n b o u n d a r y a t t a c k .
7.0 R e f e r e n c e s
1. D. Webs ter, P r o c e e d i n g s o f t h e F i r s t I n t e r n a t i o n a l Aluminum-Li t h i u m C o n f e r e n c e , p u b l i s h e d by AIME, W a r r e n d a l e , P e n n s y l v a n i a , 1 9 8 1 , p p 228- 2 3 9.
2. N . J . H . H o l r o y d , A. G r a y , G. M. Scamans a n d R. Herman, P r o c e e d i n g s o f t h e T h i r d I n t e r n a t i o n a l Aluminum-Li t h i u m c o n f e r e n c e , p u b l i s h e d by t h e I n s t i t u t e o f M e t a l s , London, 1986.
C3-340 J O U R N A L DE PHYSIQUE
Percenr Aluminum Added Figure 3 . Longitudinal Yield Strength-
Figure 2. The Effect of Added Aluminum Powder (LT on the Longitudinal Mechanical Properties of 66' Al-Li-Cu-Mg-Zr Alloys Aged at 394K for 16 hr.
(ft-lb)
0 2 4 6 8 . 10
Longitudinal 0.2% Yield Screnscb (MPa)
K I D = = S W O O T X S P E C I M E N P A I I . U R E S T R E S S
1611 0
1612 v
Figure 5. Short Transverse K I and Crack ~ ~ ~ Initiation Data as-a Function of Longitudinal
Charpy value ( 3 ) Yield Strength for Alloys 66, 81 and 93.
Figure 4. Longitudinal and Transverse Yield Strength-Toughness (LT and TL notches) Relationships for Alloy 81.
- 4 0 0
300 93
f f f A 81 A
2 0 -
LO -
/
0 300
0
400 500
40
30
- A - - A - A- s 66 n
Al-Li-Us-Si-Cr
- Alloy 8 1 txrruslon c c n r r r
Prol'crrlcs
'
- \
