The dendrite-to-composite transition in off-eutectic Sn-Pb alloys
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IN a r e c e n t
study x we have shown that t h e r e e x i s t s a s i g n i f i c a n t h y s t e r e s i s in the d e n d r i t e - c o m p o s i t e t r a n s i tion in o f f - e u t e c t i c , S n - r i c h S n - P b a l l o y s . The a p p e a r ance of d e n d r i t e s always o c c u r r e d at h i g h e r r a t e s than the r a t e at which the d e n d r i t e s could be made to d i s a p p e a r . The lower t r a n s i t i o n , d e n d r l t i c - t o - c o m p o s i t e , was found to be c o n s i d e r a b l y m o r e r e p r o d u c i b l e and, also, to p o s s e s s some i n t e r e s t i n g m o r p h o l o g i c a l c h a r a c t e r i s t i c s . This t r a n s i t i o n has b e e n f u r t h e r s t u d i e d by the s a m e technique of c o n t i n u o u s l y d e c r e a s i n g the s o l i d i f i cation r a t e until the d e n d r i t e s d i s a p p e a r e d . It was found that the d e n d r i t e s did not s u d d e n l y d i s a p p e a r without w a r n i n g at s o m e c r i t i c a l r a t e . The d i s a p p e a r a n c e is c h a r a c t e r i z e d by a g r a d u a l d e c r e a s e in the n u m b e r of d e n d r i t e s and a l s o by a g r a d u a l d e c r e a s e in the lengths of the d e n d r i t e s . T h e s e c h a r a c t e r i s t i c s have now b e e n studied in s o m e detail and the r e s u l t s of this study a r e
presented here. EXPERIMENTAL
i c a l l y and the r a t e at the d i s a p p e a r a n c e p o s i t i o n was d e t e r m i n e d f r o m the data taken d u r i n g the r u n on r a t e vs position. 1 Each r o d was s e c t i o n e d at the r e g i o n where the d e n d r i t e s d i s a p p e a r e d and two o r t h r e e of these s e c t i o n s w e r e c h e m i c a l l y a n a l y z e d to d e t e r m i n e the c o m p o s i t i o n . The data for the c o m p o s i t i o n s and d i s a p p e a r a n c e r a t e s a r e s u m m a r i z e d in the f i r s t two c o l u m n s of T a b l e s I and II. In m a n y r u n s the t e m p e r a t u r e g r a d i e n t was m e a s u r e d two or t h r e e t i m e s p r i o r to the r u n at r a t e s n e a r the expected d i s a p p e a r a n c e r a t e . The a l l o y was s t i r r e d p r i o r to each g r a d i e n t m e a s u r e m e n t and p r i o r to the r u n itself. In other r u n s the g r a d i e n t was t a k e n to be the s a m e as in r u n s u n d e r i d e n t i c a l t h e r m a l c o n d i t i o n s . The g r a d i e n t r e s u l t s a r e l i s t e d in c o l u m n 3 of T a b l e s I and II with the a s s u m e d g r a d i e n t s b e i n g e n c l o s e d in p a r e n t h e s e s . F o r the high g r a d i e n t e x p e r i m e n t s of this study the a v e r a g e g r a d i ent for all r u n s was 3 1 0 • 7~ p e r cm and for the low g r a d i e n t e x p e r i m e n t s , 110 • 4~ p e r c m where the d e viations are average deviations.
C r i t i c a l Rate of D i s a p p e a r a n c e All of the S n - P b a l l o y s w e r e p r e p a r e d f r o m Sn and Pb having quoted p u r i t
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