The morphology and crystallography of directionally solidified pb-sn eutectic alloys
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e u t e c t i c a l l o y s have b e e n s t u d i e d e x t e n s i v e l y o v e r the p a s t s e v e r a l y e a r s a s a m o d e l s y s t e m to v e r i f y v a r i o u s a s p e c t s of e u t e c t i c g r o w t h t h e o r y . H o w e v e r , t h e r e have b e e n r e l a t i v e l y few s t u d i e s of the c r y s t a l l o g r a p h y of t h i s e u t e c t i c , p r o b a b l y b e c a u s e of the soft n a t u r e of the ~lloy and the d i f f i c u l t y of p r e p a r i n g thin s a m p l e s for the t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e (TEM). Hopkins and K r a f t ~ have p r e s e n t e d the only c r y s t a l l o g r a p h i c study on bulk grown P b - S n e u t e c t i c a l l o y s . T h e y u s e d an X - r a y t e c h n i q u e d e v e l o p e d by K r a f t e and r e p o r t e d the o r i e n t a t i o n r e l a t i o n s h i p s given in T a b l e I for a l l o y s grown by both the v e r t i c a l p u l l i n g technique and a h o r i z o n t a l boat t e c h nique. T h e e x p e r i m e n t a l w o r k of T a k a h u s h i and S c h i n u m a ~ a s w e l l a s that of L a b u l l e and P e t i p a s ~ was done on a l l o y s s o l i d i f i e d in the s h a p e of thin f i l m s . T h e o r i e n t a t i o n r e l a t i o n s h i p s of Tu and T u r n b u l l 5 w e r e m e a s u r e d on l a m e l l a r P b - S n c o m p o s i t e s grown by d i s c o n t i n u o u s p r e c i p i t a t i o n . A s t u d y of the r e l a t i o n s h i p s given in T a b l e I shows that t h e r e i s l i t t l e a g r e e m e n t b e t w e e n the t h r e e eut e c t i c s t u d i e s , x,3,~ T h i s l a c k of a g r e e m e n t h a s b e e n a t t r i b u t e d ~ to the d i f f e r e n t t e c h n i q u e s u s e d in the g r o w t h of the a l l o y s . T h e r e f o r e , we had a s s u m e d , a l o n g with o t h e r s , %~ that bulk P b - S n e u t e c t i c a l l o y s would follow the r e l a t i o n s h i p r e p o r t e d by Hopkins and K r a f t . ~ H o w e v e r , we found that we w e r e unable to r e p r o d u c e the r e s u l t s of Hopkins and K r a f t . T h i s l e a d us to e x a m i n e the c r y s t a l l o g r a p h y of many P b Sn e u t e c t i c a l l o y s which w e r e p r e p a r e d in our l a b o r a t o r y a few y e a r s ago in s t u d i e s of o t h e r a s p e c t s of e u t e c t i c growth. T h e s e e x p e r i m e n t s w e r e done by unid i r e c t i o n a l s o l i d i f i c a t i o n in 5 to 6 m m d i a m q u a r t z t u b e s in an a p p a r a t u s which h a s p r e v i o u s l y b e e n d e s c r i b e d . ~'9 In a l l c a s e s , the l!quid t e m p e r a t u r e g r a d i ent was quite high, 120 to 360 C / c m , the s o l i d / l i q u i d i n t e r f a c e was p l a n a r , and the c o m p o s i t i o n s w e r e e i t h e r e u t e c t i c o r o f f - e u t e c t i c , m a i n l y on the high Sn s i d e . M e t a l l o g r a p h i c e x a m i n a t i o n of t r a n s v e r s e s e c t i o n s of t h e s e s a m p l e s v i
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