The structure of directionally solidified two-phase Sn-Cd peritectic alloys
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T H E " n a t u r a l " f r e e z i n g of b i n a r y alloys having a p e r i t e c t i c r e a c t i o n has b e e n d i s c u s s e d b y m a n y a u t h o r s , n o t a b l y R h i n e s . 1 He has pointed out that c o m p l e t e e q u i l i b r i u m is p r a c t i c a l l y i m p o s s i b l e to a t t a i n ; i.e., the r e a c tion L + a ~ /3 r a r e l y goes to c o m p l e t i o n b e c a u s e of the s l o w n e s s of the solid state diffusion that m u s t n e c e s s a r i l y a c c o m p a n y this r e a c t i o n . He also points out that f r e q u e n t l y the s i m p l e r r e a c t i o n , L ~ /3, a c c o m p l i s h e s the c o m p l e t e s o l i d i f i c a t i o n of a p e r i t e c t i c alloy at t e m p e r a t u r e s below the p e r i t e c t i c t e m p e r a t u r e . The d i r e c t i o n a l s o l i d i f i c a t i o n of p e r i t e c t i c a l l o y s has b e e n d i s c u s s e d by C h a l m e r s , 2 and e x p e r i m e n t s have b e e n p e r f o r m e d by U h l m a n n and Chadwick a which i n d i cate that the d i r e c t i o n a l s o l i d i f i c a t i o n of t w o - p h a s e p e r i t e c t i c alloys* always r e s u l t s in a m i c r o s t r u c t u r e *In this paper,a two-phaseperitecticalloyis definedas one whosebulkcompositionwouldbe in the a +/~two-phasefieldat a temperaturejust belowthe peritectic(L + a ~ 13)reactionisothermin the equilibriumphasediagram(see Fig. 1). c o n s i s t i n g of d e n d r i t e s of a s u r r o u n d e d b y a m a t r i x of /3. C h a l m e r s 4 in an e a r l y work, and m o r e r e c e n t l y L i v ingston, 5 have s u g g e s t e d that if the d e n d r i t i c growth of the ~ phase can be s u p p r e s s e d , a t w o - p h a s e p e r i t e c t i c alloy s o l i d i f y i n g with a flat l i q u i d - s o l i d i n t e r f a c e might e x p e r i e n c e the coupled growth of two solid p h a s e s . This would p r o d u c e a fine e u t e c t i c - l i k e t w o - p h a s e s t r u c t u r e . F l e m i n g s 6 has also s u g g e s t e d the p o s s i b i l i t y of the s o l idification of a t w o - p h a s e p e r i t e c t i c a l l o y with a flat l i q u i d - s o l i d i n t e r f a c e . T h e s e e x p e r i m e n t s were p e r f o r m e d to t e s t these s u g g e s t i o n s . This p a p e r will e s t i m a t e the m i n i m u m value of G/v ( t e m p e r a t u r e g r a d i e n t in the liquid divided b y the growth rate) r e q u i r e d for the s o l i d i f i c a t i o n of a t w o - p h a s e p e r itectic alloy with a p l a n a r l i q u i d - s o l i d i n t e r f a c e u s i n g a simple constitutional supercooling stability criterion. The s t r u c t u r e of Sn-Cd t w o - p h a s e p e r i t e c t i c alloys d i -
r e c t i o n a l l y solidified at v a l u e s of G/v above and below this m i n i m u m v a l u e is r e p o r t e d . Even when s o l i d i f i e d with a p l a n a r i n t e r f a c e , no fine e u t e c t i c - l i k e t w o - p h a s e s t r u c t u r e is o b s e r v e d in t h e s e a l l o
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