Rheological behavior of Sn-15 pct Pb in the crystallization range
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T H E r h e o l o g i c a l b e h a v i o r of p a r t i a l l y s o l i d i f i e d m e t a l a l l o y s has long been of f o u n d r y i n t e r e s t . It is known t h a t in fine g r a i n e d a l l o y s , s h r i n k a g e in the e a r l y s t a g e s of s o l i d i f i c a t i o n i s c o m p e n s a t e d for by s i m u l t a n e o u s m o v e m e n t of liquid and s o l i d . T h i s is t e r m e d " m a s s f e e d i n g . ''1 A t l a t e r s t a g e s of s o l i d i f i c a t i o n the g r o w i n g d e n d r i t e s f o r m a continuous s k e l e t o n within the l i q u i d - s o l i d z o n e . A s cooling c o n t i n u e s , this s k e l eton is s u b j e c t e d to s t r a i n f r o m t h e r m a l c o n t r a c t i o n . T h e s t r a i n can be highly l o c a l i z e d . Within l o c a l r e gions of high s t r a i n , bonds b e t w e e n d e n d r i t e s o r d e n d r i t e a r m s a r e b r o k e n , the d e n d r i t e s s e p a r a t e , and if s u f f i c i e n t fluid i s not a v a i l a b l e to f i l l the s p a c e , open " h o t t e a r s " r e s u l t . If fluid d o e s fill the s p a c e , s e g r e g a t e d r e g i o n s , s o m e t i m e s t e r m e d " f i l l e d hot t e a r s " a r e p r o d u c e d . 2-4 D e p e n d i n g s o m e w h a t on g r a i n s i z e , the continuous s o l i d s k e l e t o n f o r m s at f r a c t i o n s s o l i d g r e a t e r than a few t e n t h s , and t h e r e a f t e r t e n s i l e s t r e n g t h i n c r e a s e s r a p i d l y . In an a l u m i n u m a l l o y , for e x a m p l e , s t r e n g t h s of the o r d e r of 100 p s i (7 • 10 + d y n e s p e r sq cm) w e r e found when f r a c t i o n s o l i d r e a c h e d 0.5.3 In this p a p e r we show that v i g o r o u s a g i t a t i o n of a m e t a l a l l o y p o s t p o n e s the f o r m a t i o n of a continuous s o l i d n e t w o r k to much h i g h e r f r a c t i o n s s o l i d . With this a g i t a t i o n we show that a l e a d - t i n a l l o y can be m a d e to b e h a v e a s a t h i x o t r o p i c s l u r r y to f r a c t i o n s s o l i d in e x c e s s of 0.5. Within the r a n g e of about 0 to 0.5 f r a c t i o n s o l i d , the r h e o l o g i c a l b e h a v i o r of the a l loy is s i m i l a r to that of a n u m b e r of m o r e c o n v e n tional slurries described below. A p p a r e n t v i s c o s i t y of s l u r r i e s i n c r e a s e s with i n c r e a s i n g s o l i d c o n c e n t r a t i o n . 5'6 C o l l e c t e d d a t a for a v a r i e t y of d i s p e r s e d Newtonian s u s p e n s i o n s a r e shown
in F i g . 1. The a p p a r e n t v i s c o s i t y i n c r e a s e s to v e r y high v a l u e s b e t w e e n 0.4 and 0.6 f r a c t i o n s o l i d . When p l o t t e d a s r e l a t i v e v i s c o s i t y vs t r a c t i o n s o l i d , the d a t a a l l f a l l on the s a m e g e n e r a l c u r v e r e g a r d l e s s of m a t e r i a l . In t h e s e e x p e r i m e n t s the p a r t i c l e s w e r e s p h e r i c a l in s h a p e with a r a n g
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