Dendritic solidification of Cu-Ni alloys: Part II. The influence of initial dendrite growth temperature on microsegregat
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FEEST
AND
R. D. DOHERTY
M i c r o s e g r e g a t i o n m e a s u r e m e n t s and c a l c u l a t i o n s w e r e c a r r i e d out for s o l i d i f i c a t i o n of s m a l l m e l t s of Cu-40 wt p c t Ni u n d e r conditions in which both the cooling r a t e and the l o c a l r a t e of heat e x t r a c t i o n d u r i n g d e n d r i t e g r o w t h w e r e v a r i e d . T h e s e d e m o n s t r a t e d that the s u p e r c o o l i n g r e q u i r e d to s u p p o r t d e n d r i t e growth s i g n i f i c a n t l y r e d u c e s the m i c r o s e g r e g a t i o n in the c o m p l e t e l y f r o z e n ingot. The i m p o r t a n c e of t h i s for the d i f f e r e n t m i c r o s e g r e g a t i o n in c o l u m n a r and e q u i a x e d r e g i o n s of l a r g e ingots i s b r i e f l y d i s c u s s e d .
IN P a r t
11 e v i d e n c e w a s p r e s e n t e d which i n d i c a t e d that t h e r e w a s a m e a s u r a b l e growth r a t e d e p e n d e n c e of b i n a r y a l l o y , d e n d r i t e g r o w t h t e m p e r a t u r e . T h i s p a p e r i s c o n c e r n e d with the i m p l i c a t i o n s of t h i s effect on the u n d e r s t a n d i n g of c e r t a i n d e n d r i t e g r o w t h p h e n o m e n a of p r a c t i c a l i m p o r t a n c e , the m o s t p r o m i n e n t of which i s m i c r o s e g r e g a t i o n . The d i s c u s s i o n and e x p e r i m e n t s a r e l i m i t e d to the c a s e of b i n a r y a l l o y s o l i d i f i c a t i o n and the n o m e n c l a t u r e 9u s e d i s c o n s i s t e n t with that u s e d in P a r t I. A s u m m a r y of the s y m b o l s u s e d i s given in A p p e n d i x B. MICROSEGREGA TION THEORIES The m o s t f r u i t f u l t h e o r e t i c a l a p p r o a c h e s to the d e s c r i p t i o n of m i c r o s e g r e g a t i o n have c o n s i d e r e d a c o n t r o l v o l u m e of m a t e r i a l at a fixed p o s i t i o n within the s o l i d i f y i n g m a s s and have c a l c u l a t e d the r e s u l t a n t m i c r o s e g r e g a t i o n for a given l o c a l t h e r m a l h i s t o r y and liquid c o m p o s i t i o n , Co. At p r e s e n t , the m o s t s a t i s f a c t o r y of t h e s e a n a l y s e s a r e b a s e d on the m o d e l d e v e l o p e d by F l e m i n g s and h i s c o w o r k e r s at M. I. T. The c o n t r o l v o l u m e of t h e s e a n a l y s e s h a s a length equal to half the final a r m s p a c i n g , d, g r o w t h is c o n s i d e r e d u n i d i r e c t i o n a l and s o l u t e i s c o n s e r v e d within this v o l u m e . F l e m i n g s 2 h a s l i s t e d the m a i n a s s u m p t i o n s of the m o d e l a s : no s u p e r c o o l i n g b e f o r e n u c l e a tion, n e g l i g i b l e i n t e r f a c e u n d e r c o o l i n g due to a t o m i c k i n e t i c s , c u r v a t u r e o r s o l u t e e n r i c h m e n t , ko, a p p l y i n g at the i n t e r f a c e and s o l u t e g r a d i e n t s not b e i n g p r e s e n t in the l i q u i d . If t h e r e is no s o l i d s
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