Nucleation in undercooled Bi-Te alloys
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AND
G. A. COLLIGAN
T h e r m a l a n a l y s i s of bulk s a m p l e s of B i - T e alloys a s a function of c o m p o s i t i o n a l change from h y p o - e u t e c t i c to h y p e r - e u t e c t i c c o m p o s i t i o n has proved that the i n t e r m e d i a t e phase BiTTe3 with a r h o m b o h e d r a l c r y s t a l s t r u c t u r e is a v e r y efficient n u c l e a n t for b i s m u t h . T h i s technique is a m o d i f i c a t i o n of an e a r l i e r one a p p l i e d by the a u t h o r s a s a c r i t i c a l e x p e r i m e n t to t e s t J a c k s o n ' s objection to Sundquist and Mondolfo's droplet technique for d e t e r m i n a t i o n of n u c l e a tion c a t a l y s i s . The modified method d e s c r i b e d in this paper is unique and e l i m i n a t e s a n o t h e r p o s s i b l e s o u r c e of e r r o r in the d e t e r m i n a t i o n of n u c l e a t i o n capacity in eutectic s y s t e m s .
IT has
long been r e c o g n i z e d that the study of c a t a lyzed n u c l e a t i o n of the l i q u i d - s o l i d t r a n s f o r m a t i o n in m e t a l s and a l l o y s i s plagued by e x p e r i m e n t a l difficult i e s . Sundquist and Mondolfo 1 a t t e m p t e d to o v e r c o m e u n c e r t a i n t i e s r e g a r d i n g the n a t u r e of the n u c l e a t i n g h e t e r o g e n e i t y by f o r m i n g the n u c l e a t i n g s p e c i e s within the s a m p l e . To do this and to produce l a r g e d e g r e e s of u n d e r c o o l i n g , they were r e q u i r e d to use v e r y s m a l l s a m p l e s , typically d r o p l e t s 100 to 300 m~t d i a m . T h e i r method has b e e n c r i t i c i z e d . 2 However, the a u t h o r s 3 have r e c e n t l y applied an e x p e r i m e n t a l method in which the n u c l e a t i n g agent is f o r m e d within the s a m p l e but which is not s u b j e c t to the objection 2 p e r t a i n i n g to Sundquist and Mondolfo's method. The p u r p o s e of this c o m m u n i c a t i o n is to show how the technique applied p r e v i o u s l y by the a u t h o r s 3 has b e e n modified to e l i m inate a n o t h e r potential s o u r c e of e r r o r . T h i s technique i s b r i e f l y d e s c r i b e d with r e f e r e n c e to Fig. 1. The technique is b a s i c a l l y that d e s c r i b e d p r e v i o u s l y 3 but with a r e f i n e m e n t to e l i m i n a t e a poss i b l e e r r o r r e s u l t i n g f r o m u n i n t e n t i o n a l i m p u r i t y addition. Bulk s a m p l e s of pure A a r e s u b s t a n t i a l l y u n d e r cooled. Component B is added to the s a m p l e s to change the c o m p o s i t i o n f r o m pure A to that r e p r e s e n t e d by X. If, a f t e r this addition, the alloy of c o m p o s i t i o n X exh i b i t s l a r g e and c o n s i s t e n t u n d e r c o o l i n g below the A l i q u i d u s , then it can be i n f e r r e d that the B addition has not i n t r o d u c e d i m p u r i t i e s which a r e efficient n u c l e a n t s for A. More component B i s added to change the c
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