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THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPERTIES OF Al-Li
ALLOYS
D. Webster
To cite this version:
D. Webster. THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPER- TIES OF Al-Li ALLOYS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-685-C3-691.
�10.1051/jphyscol:1987379�. �jpa-00226610�
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C o l l o q u e C 3 , s u p p l 6 m e n t au n 0 9 , Tome 4 8 , s e p t e m b r e 1 9 8 7
THE EFFECT OF LOW MELTING POINT IMPURITIES ON THE PROPERTIES OF A1-Li ALLOYS
D . WEBSTER
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 8 7 7 , P l e a s a n t o n , C A 94566, U . S . A .
A b s t r a c t
The e f f e c t of low m e l t i n g p o i n t i m p u r i t i e s , s u c h a s s o d i u m a n d p o t a s s i u m , o n t h e 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 A1-Li a l l o y s h a s b e e n i n v e s t i g a t e d . T h e s e i m p u r i t i e s h a v e b e e n shown t o o c c u r a s d i s c r e t e l e n t i c u l a r p a r t i c l e s a l o n g g r a i n b o u n d a r i e s . I n u n d e r a g e d a n d peak-aged c o n d i t i o n s , t h e i m p u r i t y p a r t i c l e s a r e l i q u i d a n d t h e t o u g h n e s s a n d d u c t i l i t y o f a l l o y s i n t h i s c o n d i t i o n were f o u n d t o be i m p r o v e d by l o w e r i n g t h e t e s t t e m p e r a t u r e b e l o w t h e f r e e z i n g p o i n t o f t h e i m p u r i t y . I n t h e o v e r a g e d c o n d i t i o n , t h e s o d i u m a n d p o t a s s i u m o c c u r a s a s o l i d p h a s e a n d m a t e r i a l s i n t h i s c o n d i t i o n were f o u n d t o f o l l o w t h e p a t t e r n o f m o s t aluminum a l l o y s a n d show a d e c r e a s e i n C h a r p y i m p a c t t o u g h n e s s a s t h e t e s t i n g t e m p e r a t u r e w a s l o w e r e d . The a d d i t i o n o f sodium a n d p o t a s s i u m t o A1-Li a l l o y s r e d u c e d t o u g h n e s s a n d d u c t i l i t y a t room t e m p e r a t u r e i n t h e r a n g e o f c o m p o s i t i o n s i n v e s t i g a t e d ( u p t o 476 ppm Na a n d u p t o 2 3 ppm K ) . Low m e l t i n g p o i n t i m p u r i t i e s were f o u n d t o h a v e l i t t l e e f f e c t o n t h e t o u g h n e s s o r d u c t i l i t y of h i g h p u r i t y aluminum.
I n t r o d u c t i o n
A t t h e p r e s e n t s t a g e o f t h e i r d e v e l o p m e n t , A1-Li a l l o y s g e n e r a l l y compare u n f a v o r a b l y w i t h c o n v e n t i o n a l aluminum a l l o y s i n terms of t o u g h n e s s a n d d u c t i l i t y a t e q u i v a l e n t s t r e n g t h l e v e l s . F o r e x a m p l e , A 1 7 0 7 5 a n d A 1 2024 e x t r u s i o n s made from c o m m e r c i a l i n g o t s h a v e s i g n i f i c a n t l y g r e a t e r (2-3 times) i m p a c t t o u g h n e s s t h a n i d e n t i c a l A 1 8090 a n d A 1 2090 e x t r u s i o n s . T h e r e i s e v i d e n c e i n t h e l i t e r a t u r e t h a t t h i s d i f f e r e n c e i n t o u g h n e s s may b e r e l a t e d t o low m e l t i n g p o i n t i m p u r i t i e s s u c h a s o d i u m a n d p o t a s s i u m . P a y n e a n d Eynon, i n 1957 B r i t i s h P a t e n t 787665, s u g g e s t e d t h a t e m b r i t t l e m e n t o f A1-Li a l l o y s o c c u r r e d i f t h e s o d i u m c o n t e n t w a s o v e r 5 0 ppm.
V a y n b l a t , e t a l , ( R e f . 1 ) i n 1 9 7 6 f o u n d a 1 6 % d r o p i n t o u g h n e s s o f A1-2 Li-0.13 Z r a l l o y s when t h e s o d i u m c o n t e n t was i n c r e a s e d from 3 1 t o 7 6 ppm. More r e c e n t l y , V a s u d e v a n , e t a l , i n 1982 ( R e f . 2 ) o b s e r v e d l i n e a r d e c r e a s e s i n t o u g h n e s s i n A1-Li a l l o y s c o n t a i n i n g u p t o 480 ppm s o d i u m , a n d W e b s t e r ( R e f . 3 ) o b s e r v e d t o u g h n e s s d e c r e a s e s i n A1-Li a l l o y s c o n t a i n i n g u p t o 434 ppm s o d i u m a n d 2 3 ppm p o t a s s i u m . The a d d i t i o n o f s o d i u m a n d p o t a s s i u m p r o m o t e s t h e g r a i n b o u n d a r y
f r a c t u r e mode i n A1-Li a l l o y s ( R e f . 2 ) a n d t h e s e e l e m e n t s have b e e n f o u n d by s u r f a c e a n a l y s i s t o s e g r e g a t e t o t h e g r a i n b o u n d a r i e s
( R e f s . 2 , 3 , 4 , , 5 ) . t h e aim o f t h e p r e s e n t work i s t o i d e n t i f y t h e n a t u r e of t h e g r a i n b o u n d a r y s e g r e g a t i o n a n d t h e c h a n g e s l t u n d e r g o e s d u r i n g a g i n g .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987379
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E x p e r i m e n t a l T e c h n i q u e
Most of t h e a l l o y s e v a l u a t e d i n t h i s work w e r e from a l l o y s c a s t i n t o 1 2 7 mm d i a m e t e r i n g o t s and e x t r u d e d t o f l a t b a r 5 8 x 1 4 mm.
F o u r a l l o y s w i t h a d d e d Na a n d K were c a s t a s b o t h 127 mm d i a m e t e r i n g o t s , e x t r u d e d t o f l a t b a r a n d a s 1 0 0 mm x 25 mm i n g o t s w h i c h were h o t r o l l e d t o 6.4 mm p l a t e . The a l l o y s were m e l t e d u n d e r a n a r g o n a t m o s p h e r e .
R e s u l t s
When l i t h i u m m e t a l i s a d d e d t o a m o l t e n aluminum a l l o y , t h e r e is a n i n c r e a s e i n sodium, p o t a s s i u m a n d h y d r o g e n l e v e l s i n t h e a l l o y . T h e s e e l e m e n t s a r e p r e s e n t i n t h e l i t h i u m m e t a l ( 5 3 ppm Na, 22ppm K a n d 280 ppm H). A d d i t i o n s o f sodium, p o t a s s i u m a n d l i t h i u m g r e a t l y i n c r e a s e t h e h y d r o g e n c o n t e n t o f aluminum a l l o y s . T h i s i s n o t s u r p r i s i n g s i n c e h y d r o g e n h a s a h i g h s o l u b i l i t y i n a l l t h r e e a l k a l i metals When t h e d a t a a r e measured a s a t o m i c p e r c e n t a g e s t h e h y d r o g e n c o n t e n t i s e q u a l t o t h e c u b e r o o t o f t h e sum o f t h e a l k a l i m e t a l s . I n a l l t h e a l l o y s examined t h e r e i s a n e x c e s s of a l k a l i m e t a l s above t h a t r e q u i r e d t o form h y d r i d e s , e v e n b e f o r e t h e a l l o y s a r e d e g a s s e d .
S m a l l p a r t i c l e s r i c h i n Na a n d K were o b s e r v e d i n t r a h s m i s s i o n e l e c t r o n m i c r o s c o p y i n a l l t h e a l l o y s examined w i t h a d d e d a m o u n t s o f t h e s e e l e m e n t s . The p a r t i c l e s were l e n t i c u l a r i n s h a p e w i t h t h e i r l o n g e s t d i m e n s i o n (0.3-1.7 pm) a l o n g t h e g r a i n b o u n d a r y . I n many c a s e s t h e r e was a s m a l l p a r t i c l e , r i c h i n Pb w i t h i n o f t h e l e n t i c u l a r Na-K p a r t i c l e , s u g g e s t i n g t h a t t h i s was a c t i n g a s a n u c l e a t i n g a g e n t f o r sodium a n d p o t a s s i u m s e g r e g a t i o n . The d i h e d r a l a n g l e o f t h e l e n t i c u l a r Na-K p a r 6 i c l e s a t t h e g r a i n b o u n d a r i e s was f o u n d t o be i n t h e r a n g e 100-120 . One o f t h e s m a l l e s t ( 0 . 3 p m ) s o d i u m - r i c h p a r t i c l e s o b s e r v e d was f o u n d a t a g r a i n b o u n d a r y t r i p l e j u n c t i o n i n a l l o y 2BLKN a n d i s shown i n F i g u r e 1. I n s i d e t h i s p a r t i c l e i s a l e a d - r i c h p a r t i c l e . The EDX a n a l y s i s from t h i s p a r t i c l e i n d i c a t e s i t i s 81% Na, 1 9 % K. A TEM e x a m i n a t i o n o f a l l o y 2BLK, w h i c h c o n t a i n s s l i g h t l y more p o t a s s i u m t h a n 2BLKN, b u t o n l y 6% a s much s o d i u m , shows s i m i l a r g r a i n b o u n d a r y p a r t i c l e s r i c h i n Na a n d K which a l s o a p p e a r t o h a v e n u c l e a t e d o n l e a d p a r t i c l e s . EDX a n a l y s e s t a k e n from t h e g r a i n b o u n d a r i e s a d j a c e n t t o t h e Na-K p a r t i c l e s show o n l y t h e p r e s e n c e o f aluminum, i n d i c a t i n g t h a t t h e sodium a n d p o t a s s i u m a r e n o t p r e s e n t a s a u n i f o r m g r a i n b o u n d a r y f i l m b u t a r e c o n c e n t r a t e d i n i s o l a t e d p a r t i c l e s .
B e c a u s e o f t h e s m a l l volume o f e a c h s o d i u m - r i c h p a r t i c l e , a q u a n t i t a t i v e a n a l y s i s of t h e c o m p o s i t i o n c a n n o t b e made by EDX i n t h e STEM. However, t h e Na:K r a t i o c a n be o b t a i n e d , a n d i f t h e a s s u m p t i o n i s made t h a t a p a r t from o c c a s i o n a l n u c l e a t i n g p a r t i c l e s t h e sodium- r i c h p a r t i c l e c o n t a i n s o n l y sodium p l u s p o t a s s i u m , t h e n t h e c o m p o s i t i o n o f e a c h p a r t i c l e a n d t h e r e f o r e i t s l i q u i d u s a n d s o l i d u s t e m p e r a t u r e c a n b e e s t i m a t e d . T h i s h a s b e e n d o n e f o r a l l o y s
2BLK(llppmNa, 23ppm K ) , 2BLKN(476ppm Na, 15ppm K and 2BLKNC( lloppm
Na) F i g u r e 2. The c o m p o s i t i o n o f e a c h p a r t i c l e i s p l o t t e d o n t h e Na- K p h a s e d i a g r a m a t t h e l o w e s t t e m p e r a t u r e a t w h i c h a l i q u i d p h a s e c a n e x i s t i n a p a r t i c l e o f t h a t c o m p o s i t i o n . I t c a n be s e e n t h a t a l i q u i d p h a s e would b e e x p e c t e d a t room t e m p e r a t u r e i n a l l t h r e e a l l o y s a n d t h a t p o t a s s i u m was i d e n t i f i e d i n t h e g r a i n b o u n d a r y p a r t i c l e s e v e n when t h e amount was t o o low t o b e d e t e c t e d by b u l k a n a l y s i s (2BLKNC).
I n m o s t o f t h e s o d i u m - r i c h p a r t i c l e s o b s e r v e d a f t e r a g i n g a t 394K, s m a l l b u b b l e s were o b s e r v e d moving s l o w l y j u s t i n s i d e t h e p e r i m e t e r o f t h e p a r t i c l e i n d i c a t i n g t h a t t h e p h a s e i s i n d e e d l i q u i d
<Ref. 6>.
Figure 1. TEM of Na-K particle in grain boundary triple junction of alloy 2BLKN.Dark particle contains Pb.
FIG. 2 . Na-K PHASE DIAGRAM SHOWING RELATIVE CONCENTRATIONS OF Na-K ElEASURED BY EDX FROM PHASES OBSFBVED BY 'TI31
WEIGHT PCT. SODlL?l
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I n o r d e r t o show t h e a l k a l i m e t a l d i s t r i b u t i o n i n a l l o y s w i t h o n l y r e s i d u a l a m o u n t s , a more s e n s i t i v e m i c r o - a n a l y t i c a l t e c h n i q u e , t h e Cameca I o n M i c r o p r o b e was u s e d . The d i s t r i b u t i o n o f Na, K , Rb a n d Cs was examined o n b o t h p o l i s h e d s a m p l e s a n d s a m p l e s f r a c t u r e d i n t e r g r a n u l a r l y a l o n g t h e S-T d i r e c t i o n . The K d i s t r i b u t i o n i n a l5Oum d i a m e t e r c i r c l e o n a f r a c t u r e s u r f a c e o f a semi c o m m e r c i a l A 1 8 0 9 1 a l l o y i n t h e peak a g e d c o n d i t i o n is shown i n F i g u r e 3. The Na a n d K d i s t r i b u t i o n s v a r y from p a r t i c l e t o p a r t i c l e b u t t h e l o c a t i o n o f t h e h i g h c o n c e n t r a t i o n s o f e a c h e l e m e n t a r e t h e same a s i n d i c a t e d by t h e TEM work a b o v e . F o r t h e t h r e e l a r g e p a r t i c l e s shown i n F i g u r e 3 t h e a v e r a g e Na a n d K a t o m i c c o n c e n t r a t i o n s were a b o u t e q u a l . The Cs l e v e l s were 24 times l o w e r a n d t h e Rb l e v e l s were 300 times l o w e r t h a n t h e Na a n d K l e v e l s . I n t h e c a s e o f t h e K and p e r h a p s a l s o t h e Cs a n d Rb t h e r e i s e n r i c h m e n t i n t h e p a r t i c l e s compared t o t h e b u l k c o m p o s i t i o n .
FIGURE 3 . PfYFASSIU? DISTRIBUTION ON SEIORT ?EWEVERSE FRACI'LJRE SURFACE O F Al 8091 AUQY EXTRUSION AS SH@,JN BY THE ION .PUCROPROBE.
TIE DARK AREAS ARE REGIONS OF HIGH POTASSIW4 C O N ~ R A T I O N S . N a , R b AND C s WE ALSO CONCEJTRA"D I N THE S R I E NZEAS.
Na AMD K LFVELS BY BULK ANALYSIS ARE 3 PPM AND 4 0 . 5 PPM RESPECTIVELY. THE NUMBEPS INDICATE TEE REXATIVE POTASSIUq C O N ~ T I O N S I N TI= YATIIIX AND I N THE PAF3ICLE CENTERS.
The t e n s i l e a n d C h a r p y i m p a c t p r o p e r t i e s o f u n d e r a g e d 6 mm A1-Li p l a t e w i t h v a r i o u s a d d i t i o n s ' o f sodium a n d p o t a s s i u m a r e a s f o l l o w s . A t 295K t h e r e i s a s l i g h t d e c r e a s e i n y i e l d s t r e n g t h w i t h i m p u r i t y c o n t e n t a n d a s i g n i f i c a n t d e c r e a s e i n d u c t i l i t y a n d t o u g h n e s s . The h i g h p o t a s s i u m a l l o y s have l o w e r t o u g h n e s s t h a n e x p e c t e d a b o v e 77K, b a s e d o n t h e a s s u m p t i o n t h a t Na a n d K a r e o f e q u a l e f f e c t i v e n e s s i n r e d u c i n g t o u g h n e s s . A b e t t e r c o r r e l a t i o n i s o b t a i n e d i f t h e p o t a s s i u m c o n t e n t a l o n e i s assumed t o b e t h a c o n t r o l l i n g f a c t o r . F o r t h e r e s u l t s a t 295K t h i s i s a l o g i c a l a s s u m p t i o n s i n c e i n a l l t h e a l l o y s , Na w h i c h is t h e m a j o r a l k a l i m e t a l i m p u r i t y , i s a s o l i d a t 295K. I n t h e c o m p l e t e a b s e n c e o f o t h e r a l k a l i m e t a l s t h e r e f o r e n o e m b r i t t l e m e n t s h o u l d b e o b s e r v e d a t 295K. I n t h i s c a s e t h e amount o f
l i q u i d p r e s e n t w i l l b e c o n t r o l l e d e n t i r e l y by t h e s e c o n d m a j o r a l k a l i m e t a l i m p u r i t y i . e . p o t a s s i u m w h i c h when i n t h e p r e s e n c e of s o d i u m f o r m s p h a s e s w h i c h a r e l i q u i d below 295K. To b e c o n s i s t e n t t h e c h a r p y d a t a f o r 373K s h o u l d b e p l o t t e d a g a i n s t t h e sum o f Na+K s i n c e t h i s t e m p e r a t u r e i s a b o v e t h e m e l t i n g p o i n t o f Na. When t h i s i s d o n e t h e number o f l i n e s o n t h e p l o t is r e d u c e d t o two s i n c e t h e 373K d a t a becomes a n e x t e n s i o n o f t h e 295K d a t a < F i g . 4). I n o r d e r t o
FIG.4.EFFECT OF LIQUID IMPURITY LEVELS ,K AT 77K AND Na+K AT 3733 ON CHARPY VALUES OF Al-Li ALLOYS WITH ADDED Na+K.
f u l l y e v a l u a t e t h e e f f e c t o f Na a n d K o n t h e t o u g h n e s s o f A1-Li a l l o y s , i t i s d e s i r a b l e t o b e a b l e t o e v a l u a t e a n a l l o y w h e r e t h e s e e l e m e n t s a r e e x t r e m e l y low. T h i s i s n o t e a s y t o d o b y i n g o t m e t a l l u r g y , however, t h i s c o n d i t i o n c a n b e a p p r o a c h e d i n a l l o y s made by powder m e t a l l u r g y . Two a l l o y s 1 6 1 1 a n d 1614 w e r e a t o m i z e d a t 1200K i n t o -100 mesh [ < I 4 9 p m ] p a r t i c l e s . The h i g h t e m p e r a t u r e a n d t h e s m a l l p a r t i c l e s i z e would b e e x p e c t e d t o e n a b l e a l m o s t a l l t h e s o d i u m a n d p o t a s s i u m t o e v a p o r a t e . S p e c i m e n s w e r e a g e d a t t e m p e r a t u r e s u p t o 533K a n d t e s t e d a t room t e m p e r a t u r e a n d 77K. The t e m p e r a t u r e d e p e n d e n c e o f t o u g h n e s s f o r t h e s e a l l o y s w a s q u i t e d i f f e r e n t from i n g o t m e t a l l u r g y a l l o y s c o n t a i n i n g h i g h e r Na a n d K.
The powder a l l o y s showed a d e c r e a s e i n t o u g h n e s s a t 77K u n d e r a l l a g i n g c o n d i t i o n s < F i g . 5 > .
D i s c u s s i o n
The a b o v e r e s u l t s show 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 aluminum r e s u l t s i n a n i n c r e a s e i n t h e u n d e r s i r a b l e i m p u r i t i e s Na, K a n d H.
A l l t h r e e o f t h e s e e l e m e n t s a r e c o n c e n t r a t e d i n t h e g r a i n b o u n d a r i e s
< R e f . 3 > , a n d t h e K a n d t o a l e s s e r e x t e n t t h e Na, r e d u c e t h e t o u g h n e s s o f A1-Li a l l o y s a t t e m p e r a t u r e s a b o v e a p p r o x i m a t e l y 200K.
The t e m p e r a t u r e d e p e n d e n c e o f i m p a c t t o u q h n e s s a n d d u c t i l i t y , o f
JOURNAL DE PHYSIQUE
FIG.5 TOUGHNESS AS A FUNCTION OF AGING TEMPERATURE FOR POWDER METALLURGY EXTRUSIONS 1611 AND 1612 TESTED AT 295K and 77K.
a l l o y s w i t h a d d e d Na a n d K , a n d a l l o y s w i t h o n l y r e s i d u a l a m o u n t s o f t h e s e e l e m e n t s , d i f f e r o n l y i n t h e m a g n i t u d e o f t h e e f f e c t <Ref. 3 ) . I t h a s b e e n shown t h a t t h e s e e l e m e n t s o c c u r a s i s o l a t e d g r a i n b o u n d a r y p a r t i c l e s r a t h e r t h a n c o n t i n u o u s g r a i n b o u n d a r y f i l m s . T h e s e l i q u i d p h a s e s c a n a c t a s g r a i n b o u n d a r y c r a c k s , w h i c h c a u s e e x t e n s i v e l o c a l d e f o r m a t i o n i n t h e g r a i n b o u n d a r y r e g i o n . I m p u r i t y p a r t i c l e s c o n t a i n i n g o n l y Na a n d K a r e l i q u i d down t o 260K a c c o r d i n g t o t h e p h a s e d i a g r a m . The a l l o y s i n v e s t i g a t e d i n t h i s work show a n i n c r e a s e i n t o u g h n e s s down t o a t l e a s t 200K. T h i s s u g g e s t t h a t o t h e r i m p u r i t y e l e m e n t s , s u c h a s t h e Cs a n d Rb o b s e r v e d a b o v e i n t h e i o n m i c r o p r o b e , a r e a s s o c i a t e d w i t h Na a n d K. I n t h e t e r n a r y s y s t e m , Na-K-Cs, a l i q u i d p h a s e c a n e x i s t down t o 195K a n d t h e s o l i d u s is l o w e r e d f u r t h e r i n t h e p r e s e n c e of m e r c u r y . I n m o s t A1-Li a l l o y s , a c o n t i n u o u s i n c r e a s e o f t o u g h n e s s would b e e x p e c t e d b e t w e e n t h e f r e e z i n g p o i n t o f Na a n d t h e Na-K-Cs e u t e c t i c . T h i s would p r o d u c e a marked s i m i l a r i t y t o t h e a c t u a l t o u g h n e s s - t e m p e r a t u r e r e l a t i o n s h i p s o b s e r v e d < R e f s . 3,5>. The t o u g h n e s s o f A1-Li a l l o y s i n c r e a s e s m a r k e d l y a s t h e t e s t t e m p e r a t u r e i s d e c r e a s e d , w h i l e
t h e t o u g h n e s s of c o n v e n t i o n a l h i g h s t r e n g t h aluminum a l l o y s d e c r e a s e s a t low t e s t t e m p e r a t u r e s . The r e s u l t o f t h i s i s t h a t t h e 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 of A1-Li a l l o y s a t c r y o g e n i c t e m p e r a t u r e s i s e q u a l t o , o r i n many c a s e s s u p e r i o r t o , t h a t o f c o n v e n t i o n a l aluminum a l l o y s . T h i s a p p e a r s t o c r e a t e a p r o b l e m f o r a l t e r n a t e t h e o r i e s f o r
t h e e m b r i t t l e m e n t o f A1-Li a l l o y s s u c h a s s t r a i n l o c a l i z a t i o n d u e t o t h e o r d e r e d A 1 L i p h a s e . S t r a i n l o c a l i z a t i o n t h e o r i e s d o n o t p r e d i c t a n i n c r e a s e i n t o u g h n e s s a t c r y o g e n i c t e m p e r a t u r e s n o r a n a b s e n c e o f e m b r i t t l e m e n t a t t h e s e t e m p e r a t u r e s .
C o n c l u s i o n s
1. The a d d i t i o n of u p t o 4 7 6 ppm Na a n d 2 3 ppm K t o A1-Li a l l o y s r e d u c e s t h e i r i m p a c t t o u g h n e s s a n d d u c t i l i t y b u t h a s l i t t l e i n f l u e n c e o n s t r e n g t h .
2 . Na a n d K r e d u c e t o u g h n e s s a n d d u c t i l i t y by f o r m i n g d i s c r e t e l e n t i c u l a r g r a i n b o u n d a r y p a r t i c l e s w h i c h a r e l i q u i d a t room
t e m p e r a t u r e . T h e s e l i q u i d p h a s e s p r o m o t e i n t e r g r a n u l a r f a i l u r e . The s o l i d i f i c a t i o n o f t h e l i q u i d p h a s e s a t t e m p e r a t u r e s down t o a b o u t 200K r e s u l t s i n a n i n c r e a s e i n t o u g h n e s s a n d d u c t i l i t y .
Acknowledgements. The a u t h o r would l i k e t o t h a n k t h e f o l l o w i n g p e r s o n n e l o f K a i s e r Aluminum & C h e m i c a l C o r p o r a t i o n l s C e n t e r f o r T e c h n o l o g y i n P l e a s a n t o n , C a l i f o r n i a ; D.A. Nguyen f o r o u t s t a n d i n g t r a n s m i s s i o n m i c r o s c o p y , R. H i n t z e f o r t h e d e v e l o p m e n t o f i m p r o v e d a n a l y t i c a l t e c h n i q u e s f o r a l k a l i m e t a l s a n d a l s o D. Wentz, R.C.
D i c k i n s o n a n d K.S. Dayton. I n a d d i t i o n t h e e x p e r t i s e o f C h a r l e s E v a n s a n d Ron F l e m i n g o f C h a r l e s E v a n s a n d A s s o c i a t e s i n Redwood C i t y , C a l i f o r n i a was i n v a l u a b l e when u s i n g t h e i o n m i c r o p r o b e .
The a u t h o r i s a c o n s u l t a n t f o r Advanced M a t e r i a l D e v e l o p m e n t a t 20565 Verde V i s t a Lane, S a r a t o g a , C a l i f o r n i a 95070, USA. ( 4 0 8 ) 867-6017
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