Variables affecting the nature of the chill zone
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INC~Tm a c r o s t r u c t u r e
g e n e r a l l y c o n s i s t s of t h r e e d i s t i n g u i s h a b l e zones: the " c h i l l z o n e " , the ' ' c o l u m n a r z o n e " a n d a c e n t r a l " e q u i a x e d z o n e " , each of whose o r i g i n has b e e n the s u b j e c t of c o n s i d e r a b l e i n t e r e s t b e c a u s e of the w e l l - k n o w n r e l a t i o n s h i p that exi s t s b e t w e e n s t r u c t u r e and p r o p e r t i e s . The o r i g i n and d e v e l o p m e n t of c o l u m n a r s t r u c t u r e a s t r e a t e d by W a l ton and C h a l m e r s ~ h a s b e e n a c c e p t e d a s the m e c h a n i s m to explain the o r i e n t a t i o n t e x t u r e of the s t r u c t u r e . The equiaxed s t r u c t u r e , which h a s b e e n the s u b j e c t of n u m e r o u s s t u d i e s in the l a s t decade, 2-7 can a l s o b e explained to the extent that the p r i n c i p a l m e c h a n i s m of the s t r u c t u r e f o r m a t i o n is a function of the s o l i d i f i c a tion c o n d i t i o n s i m p o s e d on the s y s t e m . 8 Only r e c e n t l y has t h e r e b e e n a c o r r e l a t i o n b e t w e e n the o r i g i n and d e v e l o p m e n t of the chill zone and the s o l i d i f i c a t i o n c o n d i t i o n s . Bower and F l e m i n g s 9 a n d Biloni and M o r a n d o ~~ s i m u l a t e d the heat e x t r a c t i o n c o n d i t i o n s e x i s t i n g at the r e g i o n w h e r e the chill s t r u c t u r e f o r m s , obtaining q u a l i t a t i v e c o r r e l a t i o n s b e t w e e n d i f f e r e n t v a r i a b l e s that e n t e r in the s o l i d i f i c a t i o n p r o c e s s . Bower and F l e m i n g s 9 p e r f o r m e d t h e i r e x p e r i m e n t s u n d e r c o n d i t i o n s of heat e x t r a c t i o n fixed by a thin fitm of l a m p b l a c k c o v e r i n g a c o p p e r m o l d s u r f a c e . In t h e s e c i r c u m s t a n c e s , the chill m e t a l s u r f a c e is c o m posed of l a r g e g r a i n s with a w e l l - d e v e l o p e d d e n d r i t i c s t r u c t u r e or s m a l l g r a i n s a s p r o d u c t of a m u l t i p l i c a tion m e c h a n i s m , depending on the a m o u n t of c o n v e c t i v e fluid flow p r e s e n t in the s y s t e m . Biloni and Morando, ~~ in addition to e x p e r i m e n t s of the s a m e type, c o n s i d e r e d the effect of the l a m p b l a c k f i l m upon the s t r u c t u r e . Even in the p r e s e n c e of m a x i m u m c o n v e c t i v e fluid flow, a copper or g r a p h i t e s u r f a c e f r e e of l a m p b l a c k gives a s u r f a c e s u b s t r u c t u r e w h e r e p r e d e n d r i t i c n u clei 4 a r e the o r i g i n of the g r a i n s . C o n s i d e r i n g that this was c l e a r i n d i c a t i o n of a h e t e r o g e n e o u s n u c l e a t i o n p r o c e s s o c c u r r i n g within the m e l t 4 r a t h e r than s i m p l y a g r a i n m u l t i p l i c a t i o n m e c h a n i s m , Biloni and M o r a n d o stated that the a m o u n t of the heat
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