Fluid flow through a solid-liquid dendritic interface
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STEWART AND F . WEINBERG
F l u i d flow t h r o u g h a dendritic s o l i d - l i q u i d i n t e r f a c i a l z o n e , during s o l i d i f i c a t i o n , has b e e n obs e r v e d in a s e r i e s of S n - P b a l l o y s . It was found that liquid p e n e t r a t e s only a s h o r t d i s t a n c e into the zone, r e l a t i v e to the total t h i c k n e s s of the zone. The a m o u n t of solid p r e s e n t at the point of m a x i m u m p e n e t r a t i o n v a r i e s f r o m 12 to 22 pct, and depends on the alloy c o n c e n t r a t i o n . Flow due to v o l u m e s h r i n k a g e , t h e r m a l c o n v e c t i o n , o r solute c o n v e c t i o n well inside the z o n e , was not detected. F l u i d flow through a w i r e m e s h model of a thin s e c t i o n of the s o l i d - l i q u i d d e n d r i t i c zone was e x a m i n e d . The r e s u l t s a r e not in a g r e e m e n t with that p r e d i c t e d for flow through a porous b a r r i e r , which had b e e n found a p p l i c a b l e to i n t e r d e n d r i t i c fluid flow. T H E i m p o r t a n c e of i n t e r d e n d r i t i c fluid flow d u r i n g s o l i d i f i c a t i o n has r e c e n t l y been pointed out by a n u m b e r of investigators.~-6 The n a t u r e and extent of this type of flow has b e e n i n f e r r e d from o b s e r v a t i o n s m a d e in t r a n s p a r e n t a m m o n i u m chloride ,cells 2 and f r o m p a r t i c u l a r solute s e g r e g a t i o n p a t t e r n s b e l i e v e d to r e s u l t f r o m i n t e r d e n d r i t i c f l o w , 4-6 No d i r e c t o b s e r v a t i o n s of i n t e r d e n d r i t i c flow in a m e t a l s y s t e m , during s o l i d i f i c a t i o n , have been r e p o r t e d . I n t e r d e n d r i t i c flow can r e s u l t f r o m a n u m b e r of f o r c e s d u r i n g s o l i d i f i c a t i o n including a) volume c o n t r a c t i o n on f r e e z i n g , r e s u l t i n g in liquid being d r a w n into the s o l i d - l i q u i d zone, b) t h e r m a l c o n t r a c t i o n of solid and liquid, c) t h e r m a l c o n v e c t i o n , d) solute c o n v e c t i o n , and e) fluid motion in the r e s i d u a l liquid pool due to p o u r i n g t u r b u l e n c e . The flow is complex, s i n c e it o c c u r s through a n e t w o r k of channels whose s i z e and configuration change with t i m e during s o l i d i f i c a t i o n and whose m o r p h o l o g y is m a r k e d l y dependent on the solidification p a r a m e t e r s . This i n v e s t i g a t i o n was u n d e r t a k e n to e s t a b l i s h dir e c t l y th~ extent of i n t e r d e n d r i t i c fluid flow d u r i n g s o l i d i f i c a t i o n , and to r e l a t e the o b s e r v e d flow to that obtained through a s i m p l i f i e d m o d e l of a s o l i d - l i q u i d dendritic configuration. PROCEDURE AND OBSERVATIONS 1) Fluid Flow T h r o u g h a D e n d r i t i c Solid-Liquid Interface The g e n e r a l e x p e r i m e n t a l p r o c e d u r e used in this i n v e s t i g a t i o n is d e s
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