The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron
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U s i n g a s i m i l a r h o t - s t a g e t e c h n i q u e , Sundquist and Mondolfo 1~ studied the n u c l e a t i o n b e h a v i o r of sixty b i n a r y e u t e c t i c s . T h e i r m a i n o b s e r v a t i o n was that in a e u t e c t i c s y s t e m one phase n u c l e a t e s the other but not v i c e v e r s a . T h e y also studied v a r i o u s o r i e n t a t i o n r e l a t i o n s h i p s in the h e t e r o g e n e o u s n u c l e a t i o n of lead d r o p l e t s by s m a l l single c r y s t a l s of Ni, Cu, Ag, and Ge 6 a n d found that for any given n u c l e a t i n g agent the s u p e r c o o l i n g was f a i r l y c o n s t a n t and not dependent on the o r i e n t a t i o n r e l a t i o n s h i p s of d i s r e g i s t r y . They p o s t u l a t e d that h e t e r o g e n e o u s n u c l e a t i o n s t a r t s with the f o r m a t i o n of an a b s o r b e d l a y e r of the n u c l e a t e d phase on the s u b s t r a t e and that c a v i t i e s in the subs t r a t e act as s i t e s for h e t e r o g e n e o u s n u c l e a t i o n . R e y n o l d s and Tottle n i n v e s t i g a t e d h e t e r o g e n e o u s n u c l e a t i o n in Z n , A1, Mg, Sn, P b , Cu, and Sb c a s t i n g s by applying v a r i o u s m e t a l powders to the s u r f a c e of sand and m e t a l l i c m o l d s . The e f f e c t i v e n e s s of the p o w d e r s as n u c l e a t i n g a g e n t s was d e t e r m i n e d by c o m p a r i n g the g r a i n size at the c a s t i n g s u r f a c e exposed to the powders with the g r a i n size at an unexposed s u r face. They found that the m e t a l powders that had a c r y s t a l s t r u c t u r e s i m i l a r to that of the m e l t m a t e r i a l were effective in p r o m o t i n g n u c l e a t i o n when the lattice s p a c i n g difference between the powder and the m e l t m a t e r i a l was l e s s than 10 pct. If the lattice m i s fit was g r e a t e r than 10 pct, positive r e s u l t s were not obtained. G l i c k s m a n and Childs12 studied the effect of catalytic s u b s t r a t e s on the s u p e r c o o l i n g of liquid tin. Supercooling m e a s u r e m e n t s were made in a v a c u u m - i n d u c tion f u r n a c e s i m i l a r to the one u s e d in the p r e s e n t i n v e s t i g a t i o n . They obtained r e p r o d u c i b l e s u p e r c o o l i n g m e a s u r e m e n t s with and without the p r e s e n c e of the s u b s t r a t e in the m e l t . Among the s u b s t r a t e s t e s t e d were c a r b i d e s , sulfides, oxides, and individual e l e m e n t s . W o r k i n g with a single c r y s t a l of y t t r i u m because its lattice s p a c i n g is s i m i l a r to that of tin, they obtained two c h a r a c t e r i s t i c s u p e r c o o l i n g v a l u e s , depending on whether the p r i s m a t i c or the b a s a l plane was exposed to the melt. Wallace and c o w o r k e r s ~s- 17 have studied the effect of v a r i o u s i n o c u l a n t s on the n u c l e a t i o n b e h a v i o r of i r o n - b a s e a l l o y s .
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