Refining by fractional solidification
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R E F I N I N G by p a r t i a l s o l i d i f i c a t i o n is b a s e d on the w e l l - k n o w n fact that when a m e l t c o n t a i n i n g one or m o r e s o l u t e s is cooled, c r y s t a l s that solidify have a d i f f e r e n t i m p u r i t y content than the liquid. A s s o l i d i f i cation c o n t i n u e s , liquid m a y b e c o m e e n t r a p p e d by the growing d e n d r i t e c r y s t a l s . S e p a r a t i o n of e n t r a p p e d liquid f r o m the solid will leave behind a r e s i d u a l solid portion, the i m p u r i t y content of which is d i f f e r e n t than that of the o r i g i n a l alloy, depending on the phase d i a g r a m . F o r the u s u a l case of the p a r t i t i o n r a t i o , k, l e s s than unity, the r e s i d u a l solid p o r t i o n is lower in solute. One of the most widely used p r o c e s s e s of r e f i n i n g of alloys by p a r t i a l s o l i d i f i c a t i o n is zone r e f i n i n g . In this method a s m a l l c r o s s - s e c t i o n of a b a r is m e l t e d and slowly moved a l o n g its length. The i n t e r f a c e b e tween the liquid and solid is m a i n t a i n e d p l a n a r by s o l i d i f y i n g at a slow growth r a t e with a s t e e p t e m p e r a t u r e g r a d i e n t . F o r k < 1 solute is r e j e c t e d into the liquid, l e a v i n g behind a r e s i d u a l p u r i f i e d solid. The p r o c e s s can be r e p e a t e d m a n y t i m e s , but is slow, e x p e n s i v e , and p r a c t i c a l only for e x p e n s i v e m a t e r i a l s of r e l a t i v e l y low i n i t i a l s o l u t e content (e.g., s e m i c o n ductor materials). 1 When s o l i d i f i c a t i o n is c a r r i e d out m o r e r a p i d l y than in s p e c i a l p r o c e s s e s such as zone m e l t i n g , o r when the i m p u r i t y content is higher, the liquid is i n t i m a t e l y i n t e r m i x e d with the solid. S i m i l a r p u r i f i c a t i o n t a k e s place, but on the l o c a l s c a l e of the d e n d r i t e s t r u c t u r e , and for c o m m e r c i a l s e p a r a t i o n , m e a n s m u s t be found to p h y s i c a l l y s e p a r a t e the liquid and s o l i d . Figure 1 illustrates schematically some processes that a r e or can be applied to m o l t e n m e t a l s when only a s m a l l f r a c t i o n solid is p r e s e n t . The f i r s t , p r o c e s s I, is s i m p l e , i n e x p e n s i v e and is widely used in i n d u s t r y f o r e x a m p l e , in r e m o v i n g high m e l t i n g point i m p u r i A. L. LUX is Senior Research Scientist, Consolidated Aluminum Corp., Science Center, St. Louis, MO 63141, and M. C. FLEMINGS is Ford Professor of Engineering,Massachusetts Institute of Technology, Cambridge, MA 02139. This paper is based on doctoral thesis work of A. L. Lux. Manuscript submitted May 31, 1978. METALLURGICAL TRANSACTIONS B
t i e s f r o m a l u m i n u m a l l o y s and r e m o v i n g d r o s s f r o m v a c u u m m e l t e d s u p e r a l l o y s . As in p r o c e s s II, the s e d i m e n t a t
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