A practical method for identifying inoculants

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A r e l a t i v e l y p u r e m e t a l o r low c o n c e n t r a t i o n a l l o y c a s t i n g which s o l i d i f i e s a s a s i n g l e p h a s e will have an i n i t i a l a s - c a s t g r a i n s i z e often d e t e r m i n e d b y n u c l e a tion at the c h i l l zone. 1 Subsequent c h a n g e s in g r a i n s h a p e (such a s S t r u c t u r e s r e s u l t i n g f r o m the c o l u m n a r to e q u i a x e d t r a n s i t i o n ) o r g r a i n s i z e d e p e n d in m a n y c a s e s upon d e n d r i t e d e t a c h m e n t and c r y s t a l m u l t i p l i cation. This m u l t i p l i c a t i o n m a y o c c u r n a t u r a l l y o r be f o r c e d b y d i s t u r b i n g the liquid. 2 On the o t h e r hand the n u c l e a t i o n c h a r a c t e r i s t i c s of m e l t s t r e a t e d b y i n o c u l a n t s a r e d i f f e r e n t f r o m t h o s e of u n t r e a t e d m e l t s , s i n c e t h e y contain s e c o n d p h a s e s a l m o s t b y definition. It is t h e s e s e c o n d p h a s e s which affect n u c l e a t i o n (or c r y s t a l m u l t i p l i c a t i o n ) and thus g r a i n s i z e . An inoculant is a s o l u t e addition at a low c o n c e n t r a tion which a f f e c t s the p r i m a r y m e t a l (or alloy) by inc r e a s i n g the n u m b e r of a s - s o l i d i f i e d g r a i n s . * The *The word inoculant is also used to describe small additions to alloys containing two phases, where the addition affects characteristics of the minor phase (e.g., Na affects the high silicon phase in AI-Sialloys). Such inoculation can be included with our study since a second phase is present from the beginning and does itself affect the primary metal grain size.

" l o w " c o n c e n t r a t i o n is n e v e r t h e l e s s high enough so that an i n s o l u b l e s e c o n d p h a s e i n v a r i a b l y e x i s t s o r is f o r m e d in the m e l t i m m e d i a t e l y p r i o r to s o l i d i f i c a t i o n . J u s t how the s e c o n d p h a s e c o m e s into b e i n g will not c o n c e r n us h e r e . F o r e x a m p l e , we s h a l l not c a r e w h e t h e r Ti a d d i t i o n s c a u s e n u c l e a t i o n of A1 by f o r m ing A13Ti through a p e r l t e c t i c r e a c t i o n 3-5 o r by f o r m ing TiC through r e a c t i o n with c a r b o n i m p u r i t y in the a l u m i n u m m e l t 6 o r both, 7 a s r e c e n t l y r e v i e w e d . 8 We have p r e v i o u s l y r e p o r t e d a s t u d y of a l u m i n i m i n o c u l a t e d with c o m m e r c i a l a d d i t i o n s containing Ti and B a s the a c t i v e e l e m e n t s ( F o s e c o t y p e s ) . 9 In that p a p e r we s t u d i e d n u c l e a t i o n a s it was a f f e c t e d by c h a n g e s in the fluid flow e n v i r o n m e n t . O b s e r v a t i o n of the c o l u m n a r - t o - e q u t a x e d t r a n s i t i o n was used a s a tool which r e v e a l e d the f r e q u e n c y and e f f e c t s of new g r a i n s n u c l e a t e d b y the inoculant. Without i n o c u l a n t s this t r a n s i