Solidification of undercooled Sn-Bi and Pb-Sb alloys
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IN a p r e v i o u s p a p e r , x r e p o r t i n g the
r e s u l t s of n u cleation e x p e r i m e n t s in u n d e r c o o l e d b i s m u t h eutectic a l l o y s , it was r e p o r t e d that hypoeutectic alloys of S n - B i and PbzBi-Bi exhibited a l o w e r e d eutectic f r e e z ing t e m p e r a t u r e while the eutectic m e l t i n g t e m p e r a t u r e was unchanged. Both a r e n o n f a c e t e d - f a c e t e d e u tectic s y s t e m s . A1-Si, a n o t h e r n o n f a c e t e d - f a c e t e d eutectic, d i s p l a y s a l o w e r e d e u t e c t i c f r e e z i n g t e m p e r a t u r e when modified with s o d i u m . T h i s suggested that an i n v e s t i g a t i o n of the p h e n o m e n o n of a d e p r e s s e d e u tectic f r e e z i n g t e m p e r a t u r e in u n d e r c o o l e d S n - B i and P b - S b might provide i n f o r m a t i o n on the role of sodium in the modification of A1-Si eutectic a l l o y s . The P b - S b s y s t e m was chosen in addition to the b i s m u t h s y s t e m to extend the study to a e u t e c t i c s y s t e m where the faceted phase was not b i s m u t h . A n t i m o n y can be r e a d i l y u n d e r c o o l e d a s bulk s a m p l e s . 2 E X P E R I M E N T A L PROCEDURE The eutectic c o m p o s i t i o n s a s shown in H a n s o n 3 were accepted for the s y s t e m s , S n - B i and P b - S b . S a m p l e s weighing 40 g were p r e p a r e d , r a n g i n g in c o m p o s i t i o n from hypoeutectic to e u t e c t i c a l l o y s . The s a m p l e c o m p o s i t i o n s r e l a t i v e to the e u t e c t i c c o m p o s i t i o n a r e shown in Table I. S a m p l e s were p r e p a r e d by m e l t i n g the r e q u i r e d p r o p o r t i o n of c o m p o n e n t s (weighed with a p r e c i s i o n of 0.002 g) in c l e a n p y r e x test t u b e s u n d e r a c o v e r of s i l i c o n e fluid. The p u r i t y of the m e t a l s was 99.999 pct. After m e l t i n g the c o m p o n e n t s , the test tube was shaken v i g o r o u s l y for s e v e r a l m i n u t e s and held at 50~ to 100~ s u p e r h e a t for 24 h r b e f o r e the f i r s t cooling curve was r e c o r d e d . After a cooling c u r v e was r e c o r d e d , the s a m p l e was r e h e a t e d to 50 ~ to 100~ s u p e r h e a t , s h a k e n , and held at this t e m p e r a t u r e for 24 h r before the next cooling c u r v e was taken. T h i s p r o c e d u r e was adopted to i n s u r e that the alloy was c o m p l e t e l y m o l t e n and h o m o g e n e o u s before each cooling c u r v e was taken. D u r i n g this t i m e a s m a l l a m o u n t of oxide f o r m e d on the top s u r f a c e of the m e l t . However, weighing of s a m p l e s a f t e r e x p e r i m e n t a t i o n (weighed with a p r e c i s i o n of 0.002 g) showed that the l o s s of weight due to oxidation was of the o r d e r of the G. L. F. POWELL is a Research Associate and G. A. COLLIGAN, is Professor of Engineering,Thayer School of Engineering, Dartmouth College, Hanover, N. H. Manuscript submi
