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TARSHIS, J . L. W A L K E R , A N D J . W . R U T T E R

Two r e l a t e d experimental p r o g r a m s o n the solidification s t r u c t u r e of a l l o y c a s t i n g s are rep o r t e d . In t h e f i r s t , t h e g r a i n s t r u c t u r e o f c a t a l y t i c a l l y c l e a n N i - C u a l l o y s i s e x a m i n e d a s a f u n c t i o n o f t h e d e g r e e o f s u p e r c o o l i n g b e l o w t h e equilibrium l i q u i d u s . F o r supercoolings g r e a t e r than 8 5 ° C , t h e N i - C u a l l o y s e x h i b i t a s t r u c t u r e w h i c h i s i n a c c o r d w i t h p r e v i o u s o b s e r v a t i o n s i n p u r e n i c k e l , i.e., t h e s t r u c t u r e i s f o u n d t o be c o a r s e a n d d e n d r i t i c i n t h e r a n g e 85 ° t o 150°C s u p e r c o o l i n g , but f i n e a n d e q u i a x e d f o r s u p e r c o o l i n g s g r e a t e r t h a n 1 5 0 ° C . H o w e v e r , i n t h e l o w e r r a n g e o f s u p e r c o o l i n g ( 1). D u e t o t h e c h e m i c a l s i m i l a r i t y of c o b a l t t o n i c k e l o n e m a y n o t be a b l e t o d e t e c t t h i s d i s a g r e e m e n t i n N i - C o alloys. M a c r o s c o p i c e x a m i n a t i o n s h a v e also been p e r f o r m e d for each of t h e s e alloy s y s t e m s with 5 a t . pct b i n a r y

additions. T h e g r a i n s i z e s a g a i n follow the g e n e r a l t r e n d , Fig. 6, with a v a l u e o f P f o r e a c h s y s t e m a p p r o x i m a t e l y five t i m e s as g r e a t . I n Fig. 7 , t h e m a c r o s t r u c t u r e s of t h r e e of t h e s e c a s t i n g s a r e p r e s e n t e d a along with the casting of pure nickel. T h e s t r u c t u r e s i n the 5 p c t a l l o y s , F i g . 7 , a r e t o be c o m p a r e d w i t h t h e p h o t o g r a p h s in Fig. 5 w h i c h c o n t a i n t h e s a m e a l l o y i n g c o n s t i t u e n t s but a t t h e l o w e r s o l u t e c o n c e n t r a t i o n (1 p c t ) . T h e l i n e a r - a v e r a g e d g r a i n size is r e d u c e d , c o n s i s t e n t w i t h t h e p r e d i c t i o n s b a s e d on a p a r a m e t e r s u c h a s P w h i c h d e p e n d s linearly o n composition. To test the generality of the a b o v e r e s u l t s , s i m i l a r experiments w e r e p e r f o r m e d with a l u m i n u m a s the s o l v e n t m a t e r i a l . E x p e r i m e n t s to d e t e r m i n e t h e s u p e r cooling potential of the a l u m i n u m and a l u m i n u m a l l o y s were n o t p e r f o r m e d b e c a u s e o f t h e inherent difficulty o f s u p e r c o o l i n g bulk a l u m i n u m . H o w e v e r , C i b u l a6

Table III. Nominal Aluminum Composition, W t Pct

Si Fe Cu Mg

0.001 0.002 0.002 0.001

Fig. 7--Comparison of the a s - c a s t g r a i n s t r u c t u r e s a s a function of the p a r a m e t e r P : (a) P = 0 (pure nickel); (b) P = 7.5 (Ni + 5 a t . pct Cr); (c) P - 31.5 (Ni + 5 a t . pct Si); a n d (d) P >> 250 (Ni + 5 a t . pct Ce). This f i g u r e s h o u l d be c o m p a r e d with Fig, 5 . Magnification