Nucleation in undercooled Ag-Ge alloys
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G. A. COLLIGAN
E x p e r i m e n t s with bulk s a m p l e s of Ag-Ge a l l o y s have shown that the s i l v e r solid solution will u n d e r c o o l ~ l l ~ below T e in the p r e s e n c e of p r i m a r y g e r m a n i u m . T h e s e data a g r e e with the r e s u l t s of Sundquist and Mondolfo. x On the o t h e r hand, a modified single c r y s t a l s u b s t r a t e method s i m i l a r to that used by W a g n e r ~ h a s shown that l a r g e d r o p l e t s (0.25 to 2.5g) of h y p e r e u t e c t i c Ag-Ge alloy m e l t e d on a single c r y s t a l of g e r m a n i u m u n d e r c o o l e d to a m a x i m u m of 56~ below T e . T h i s r e s u l t i s c l o s e r to W a g n e r ' s data and in a p p a r e n t conflict with the bulk sample r e s u l t s and those of Sundquist and Mondolfo. z An e x p l a n a t i o n is p r o p o s e d for this d i s crepancy.
T H E r e s u l t s of an e x t e n s i v e i n v e s t i g a t i o n of n u c l e a t i o n p h e n o m e n a in the l i q u i d - t o - s o l i d t r a n s f o r m a t i o n in e u t e c t i c s y s t e m s have been r e p o r t e d by Sundquist and Mondolfo. 1 The only published r e s u l t which conflicts with those of Sundquist and Mondolfo 1 is the l a r g e r und e r c o o l i n g r e p o r t e d by W a g n e r 2 for the s i l v e r solid solution in the Ag-Ge s y s t e m . The a u t h o r s s'4 have r e c e n t l y applied a method for d e t e r m i n i n g n u c l e a t i o n c a t a l y s i s which u t i l i z e s u n d e r cooling data for hulk s a m p l e s as a function of the s a m ple c o m p o s i t i o n . The p u r p o s e of this p a p e r i s to r e p o r t the r e s u l t s obtained when this technique is applied to the Ag-Ge s y s t e m . Also included are the r e s u l t s of e x p e r i m e n t s c a r r i e d out by applying W a g n e r ' s technique to s m a l l s a m p l e s cut f r o m bulk u n d e r c o o l i n g s a m p l e s . The r e s u l t s of both s e t s of e x p e r i m e n t s provide a b a s i s for e x p l a i n i n g the a p p a r e n t d i s c r e p a n c y between W a g n e r ' s r e s u l t and that of Sundquist and Mondolfo. z E X P E R I M E N T A L PROCEDURE AND RESULTS Bulk S a m p l e s Bulk s a m p l e s (40 g) of Ag-Ge alloy were p r e p a r e d by m e l t i n g s i l v e r and g e r m a n i u m in a l u m i n a c r u c i b l e s u n der a pyrex g l a s s c o v e r as d e s c r i b e d p r e v i o u s l y , s'8 Melting was c a r r i e d out in a tube f u r n a c e in an argon a t m o s p h e r e . All s a m p l e s at each c o m p o s i t i o n were i n i t i a l l y held for 24 h r at 50~ to 100~ s u p e r h e a t to e n sure h o m o g e n i z a t i o n . All u n d e r c o o l i n g v a l u e s were c o n f i r m e d in three s u c c e s s i v e u n d e r c o o l i n g r u n s at i n t e r v a l s of 12 h r . The i n i t i a l c o m p o s i t i o n was 10 wt pct Ge. The p o s i tion of this h y p o e u t e c t i c alloy with r e s p e c t to the e u t e c t i c c o m p o s i t i o n is sho
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