Effect of thermotransport upon off-eutectic composite growth in Sn-Pb alloys
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s e r i e s of e x p e r i m e n t s i n v e s t i g a t i n g o f f - e u t e c t i c c o m p o s i t e growth on the tin side of the S n - P b e u t e c t i c we have e n c o u n t e r e d an i n t e r e s t i n g p h e n o m e n o n . At the s t a r t of an e x p e r i m e n t with a n alloy of c o m p o s i t i o n xo, one expects the c o m p o s i t i o n of the solid to r i s e f r o m an i n i t i a l value of koxoto a s t e a d y - s t a t e v a l u e of x0. T h i s i n i t i a l t r a n s i e n t is expected to o c c u r in two s t a g e s a s d i s c u s s e d by M o l l a r d and F l e m i n g s . ' We have found, however, that f r e q u e n t l y the c o m p o s i t e c o m p o s i t i o n quickly r i s e s s i g n i f i c a n t l y above xo and then slowly d r o p s to Xo o v e r d i s t a n c e s on the o r d e r of i n c h e s . In t h i s p a p e r we p r e s e n t a study of this p h e n o m e n o n on S n - P b a l l o y s which shows that the effect i s due to t h e r m o t r a n s p o r t (Soret effect), and that the effect will be s i g n i f i c a n t in m a n y o f f - e u t e c t i c c o m p o s i t e growth e x p e r i m e n t s . EXPERIMENTAL S n - P b a l l o y s of n e a r 31.7 wt pct (21 at. pct) Pb w e r e u s e d in all of the e x p e r i m e n t s . The tin and lead had quoted p u r i t i e s of 99.999 pct. The e x p e r i m e n t s w e r e c a r r i e d out in a v e r t i c a l s o l i d i f i c a t i o n a p p a r a t u s which h a s been d e s c r i b e d in detail elsewhere.2 The alloy was contained in a 5 m m ID v y c o r tube which was coated with a thin l a y e r of colloidal g r a p h i t e . The g e n e r a l exp e r i m e n t a l p r o c e d u r e was a s follows. A 20 cm length of the alloy was m e l t e d in the v y c o r tube and then thoroughly m i x e d with a p i s t o n s t i r r i n g device. 3 The w a t e r jacket was quickly r a i s e d quenching 5 cm of the alloy. The t e m p e r a t u r e g r a d i e n t w a s then a d j u s t e d to the d e s i r e d value and the r e m a i n i n g 15 cm of liquid was s t i r r e d again. T e n m i n u t e s a f t e r this final s t i r r i n g the t r a v e l was begun. A f t e r a p p r o x i m a t e l y 7.5 cm of t r a v e l the r e m a i n i n g liquid was r a p i d l y quenched by r a i s i n g the w a t e r j a c k e t . The solidified alloy was pulled f r o m the v y c o r tube a f t e r i m m e r s i n g in liquid n i t r o g e n , and the i n i t i a l and final i n t e r f a c e s w e r e located by e l e c t r o p o l i s h i n g the e n t i r e rod. Sections w e r e m a c h i n e d f r o m the rod at v a r i o u s i n t e r v a l s along its length and t h e s e w e r e c h e m i c a l l y a n a l y z e d . T r a n s v e r s e s e c t i o n a l views of each region of the rod b e t w e e n the a n a l y z e d s e c t i o n s w e r e m o u n t e d and e x a m i n e d m e t a l l o g r a p h i c a l l y for the p r e s e n c e of d e n d r i t e s . Only J. D. VERHOEVEN, J. C. WARNER, and E. D. GIBSON are Senior
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