On the formation of the central equiaxed zone in ingots
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O N E g e n e r a l l y d i s t i n g u i s h e s between t h r e e z o n e s in a c a s t ingot: the chill zone of equiaxed g r a i n s , the zone of c o l u m n a r g r a i n s , and the c e n t r a l zone of equiaxed g r a i n s . The o r i g i n of t h e s e z o n e s has b e e n d i s c u s s e d r e p e a t e d l y in the l i t e r a t u r e . Even in 1906, Stead 1 s u g g e s t e d that s e d i m e n t a t i o n of floating c r y s t a l s p l a y s an i m p o r t a n t role in the f o r m a t i o n of the c e n t r a l equiaxed zone. In 1916, Howe 2 s u g g e s t e d that floating c r y s t a l s could indeed f o r m in the i n t e r i o r of the m e l t b e c a u s e the u n d e r c o o l i n g t h e r e could be l a r g e r than at the f r o n t of the growing c o l u m n a r c r y s t a l s due to the e n r i c h m e n t of a l l o y i n g e l e m e n t s at the front, provided that the diff e r e n c e in t e m p e r a t u r e i s s m a l l . In the following y e a r s it s e e m s to have b e e n g e n e r a l l y b e l i e v e d that the colu m n a r zone would grow a s long as t h e r e i s a s u f f i c i e n t l y l a r g e t e m p e r a t u r e g r a d i e n t . The e o l u m n a r - t o equiaxed t r a n s i t i o n would then take place when the t e m p e r a t u r e of the m e l t h a s d e c r e a s e d to a r a t h e r c o n s t a n t l e v e l , a p p r o x i m a t e l y equal to the l i q u i d u s of the alloy. T h e fact that the t e m p e r a t u r e of the m e l t could fall r a p i d l y to a r a t h e r c o n s t a n t l e v e l was shown e x p e r i m e n t a l l y by H e n s e l in 19293 who explained t h i s a s due to c o n v e c t i o n . F u r t h e r m o r e , H e n s e l found that the col u m n a r - t o - e q u i a x e d t r a n s i t i o n did not take p l a c e i m m e d i a t e l y when the t e m p e r a t u r e level had b e e n r e a c h e d . I n s t e a d it o c c u r r e d at s o m e l a t e r m o m e n t , which could be i n f l u e n c e d by e x p e r i m e n t a l p a r a m e t e r s . In r e c e n t y e a r s t h e r e has b e e n c o n s i d e r a b l e i n t e r e s t in the c o l u m n a r - t o - e q u i a x e d t r a n s i t i o n , and the effect of m a n y v a r i a b l e s on the length of the c o l u m n a r g r a i n s , and the size of the c e n t r a l equiaxed zone have b e e n i n v e s t i g a t e d ; for e x a m p l e a l l o y c o m p o s i t i o n and p u r i t y , c a s t i n g t e m p e r a t u r e , c o n v e c t i o n , s t i r r i n g , and the a p p l i c a t i o n of a hot top and v a r i o u s m e c h a n i c a l b a r r i e r s i n the m e l t . The m a i n a t t e n t i o n h a s b e e n devoted to the m e c h a n i s m of n u c l e a t i o n of the c e n t r a l equiaxed g r a i n s . The m e c h a n i s m by which the continued growth of the c o l u m n a r g r a i n s is i m p e d e d by the equiaxed g r a i n s h a s b e e n d i s c u s s e d in much l e s s d e t a i l . It s e e m s to be g e n e r a l l y a g r e e d that the t
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