On the columnar to equiaxed transition in small ingots
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P. D. COOPER,
M. H. BRADBURY,
AND
F. J. HONEY
The c o l u m n a r to equiaxed t r a n s i t i o n (CET) in s m a l l ingots of a l u m i n u m a l l o y s was found to o c c u r m o r e e a s i l y for alloys with a l a r g e r v a l u e of the c o n s t i t u t i o n a l s u p e r c o o l i n g p a r a m e t e r ( - m C o ( 1 - te)/k). The CET was found to be c o m p l e t e l y s u p p r e s s e d by i n c r e a s e s in the mold t e m p e r a t u r e by p r e h e a t i n g b e f o r e c a s t i n g . T h e s e r e s u l t s a r e d i s c u s s e d in t e r m s of the m o d e l p r o p o s e d by B u r d e n and Hunt that the CET o c c u r s by the effect of the t h e r m a l gradient, a r i s i n g f r o m the slow s o l i d i f i c a t i o n of equiaxed d e n d r i t e s , which inc r e a s e s the u n d e r c o o l i n g of the c o l u m n a r d e n d r i t e s . The a p p l i c a t i o n of the model due to B u r d e n and Hunt is shown to r e q u i r e the u s e of the ' b i g b a n g ' model for equiaxed n u c l e a tion on p o u r i n g . A higher d e n s i t y of the n u c l e i , that grow into equiaxed g r a i n s , f o r m e d by p o u r i n g with lower s u p e r h e a t and into a cold mold, gives a higher t h e r m a l g r a d i e n t i m m e d i a t e l y in f r o n t of the growing c o l u m n a r g r a i n s . O t h e r e v i d e n c e in favor of the model is b r i e f l y d i s c u s s e d .
T H E d e t a i l s of the m a c r o s t r u c t u r e o f cast ingots have b e e n of i n t e r e s t for m a n y y e a r s ~-3 and m a n y exp e r i m e n t a l s t u d i e s and r e v i e w a r t i c l e s have been p u b l i s h e d . 4 D e s p i t e this effort t h e r e is as yet no way that the m a c r o s t r u c t u r e of an ingot can be p r e dicted nor even any c l e a r a g r e e m e n t on how the c o l u m n a r to equiaxed t r a n s f o r m a t i o n (CET) a c t u a l l y occurs.5 Much a t t e n t i o n has been paid to the i m p o r t a n t p r o b l e m of how the equiaxed g r a i n s a r e f i r s t f o r m e d and two m a i n hypotheses have b e e n put f o r w a r d : i) h e t e r o g e n e o u s nucleation; and ii) f r a g m e n t a t i o n of e x i s t i n g d e n d r i t e s . The h e t e r o g e n e o u s n u c l e a t i o n model has two v a r i a n t s , f i r s t l y that n u c l e a t i o n can occur any t i m e d u r i n g s o l i d i f i c a t i o n due to r e s i d u a l c o n s t i t u t i o n a l s u p e r c o o l i n g , 6 or a l t e r n a t i v e l y only d u r i n g the i n i t i a l p o u r i n g into the mold when the liquid m e t a l is d r a s t i c a l l y c h i l l e d 7 - t h e s o - c a l l e d 'big b a n g ' model. The d e n d r i t e f r a g m e n t a t i o n model a l s o has s e v e r a l v a r i a n t s - d e n d r i t e a r m m e l t - o f f due to t h e r m a l effects such as convection, 8 or f r a g m e n t a t i o n due to m e c h a n i c a l damage ~ or to fallout f r o m the upper s u r f a c e of the ingot. ~~ Many e x p e r i m e n t a l s t u
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