Structure of undercooled Ni-Sn eutectic
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two types of eutectic. Within the i r r e g u l a r eutectic r e g i o n s the m i n o r phase a p p e a r s to be p r e s e n t a s d i s c r e t e p a r t i c l e s . However, s u c c e s s i v e polishing and exa m i n a t i o n of p a r a l l e l s e c t i o n s a p p r o x i m a t e l y 10 a p a r t showed that both phases were i n t e r c o n n e c t e d along a p o l y h e d r a l network. Fig. 2 i s a p h o t o m i c r o g r a p h of an u n d e r c o o l e d s a m ple at low m a g n i f i c a t i o n showing that the i r r e g u l a r eutectic r e g i o n s have a " d e n d r i t e - l i k e " morphology. The m a r k e d effect of c h i l l i n g a f t e r n u c l e a t i o n in
Fig. 1--Photomicrographs of Ni-32 pct Sn eutectic undercooled alloy specimens. Magnification 340 times. No chilling was employed. Degrees of undercooling are" (a) 0~ (b) 75~ (c) 240~ VOLUME 1,MAY 1970-1449
(a)
Fig. 2--Photomicrograph of a Ni-32 pct Sn eutectic specimen undercooled 75~ Magnification 18 times. Light regions are irregular eutectic. Note dendrite-like morphology of these regions.
r e f i n i n g the s t r u c t u r e a n d r e d u c i n g the a m o u n t of l a m e l l a r eutectic is shown in Fig. 3. The s a m p l e taken f r o m a location c l o s e s t to the chill (and h e n c e , cooled m o s t rapidly) shows no l a m e l l a r e u t e c t i c , while the s a m p l e f u r t h e s t f r o m the chill shows s i g n i f i c a n t a m o u n t s of e u t e c t i c and a c o a r s e r s t r u c t u r e . The foregoing o b s e r v a t i o n s a r e m o s t r e a d i l y e x p l a i n e d by a s s u m i n g that i n i t i a l growth is of a s u p e r s a t u r a t e d s i n g l e - p h a s e o~ s o l i d solution (of e u t e c t i c c o m p o s i t i o n ) . This phase grows d e n d r i t i c a l l y d u r i n g r e c a l e s c e n c e . Its a p p r o x i m a t e morphology n e a r the end of r e c a l e s c e n c e i s the d e n d r i t e - l i k e s t r u c t u r e in F i g . 2. At some point d u r i n g or n e a r the end of r e c a l e s c e n c e , this s i n g l e - p h a s e d e n d r i t e then p a r t i a l l y d e c o m p o s e s b e c a u s e it is highly s u p e r s a t u r a t e d with r e s p e c t to tin. The d e c o m p o s i t i o n i s such that the s e c o n d - p h a s e f o r m i n g ( t i n - r i c h ~? phase) i s d i s t r i b u t e d in a c l o s e l y spaced n e t w o r k through the d e n d r i t e to m i n i m i z e r e q u i r e d diffusion d i s t a n c e d u r i n g the t r a n s f o r m a t i o n . The d e n d r i t e m u s t e i t h e r decompose dir e c t l y to the ~ + 77 s t r u c t u r e , or d e c o m p o s e s f i r s t to + a n e t w o r k of liquid, with s u b s e q u e n t n u c l e a t i o n and growth of 77 throughout this liquid n e t w o r k . The r e l a t i v e c o a r s e n e s s of the i r r e g u l a r s t r u c t u r e c o m p a r e d with the l a m e l l a r s t r u c t u r e , Fig. l(b), m a k e s the l a t t e r path s e e m m o r e likely to us. At the end of r e
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