Aligned Ni-In eutectics
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Ni2In, the p r o t o t y p e of the h e x a g o n a l B82 s t r u c t u r e ("filled up" NiAs). However, directionally-solidified s a m p l e s of this c o m p o s i t i o n contained many N i - r i c h d e n d r i t e s . The e u t e c t i c has b e e n r e l o c a t e d at 26.5 (:~ 0.3) at. p c t In, at which c o m p o s i t i o n d e n d r i t e - f r e e s a m p l e s could be p r o d u c e d . A c c o r d i n g to the p h a s e d i a g r a m in F i g . 1, the two p h a s e s of the e u t e c t i c r e a c t below 850~ to f o r m Ni3In (hexagonal DO19 s t r u c t u r e ) . The s t r u c t u r e of d i r e c t i o n a l l y - s o l i d i f i e d s a m p l e s is c o m p l e x , F i g . 2, a s a r e s u l t of p a r t i a l c o m p l e t i o n of this p e r i t e c t o i d r e a c tion d u r i n g cooling. A s a m p l e s o l i d i f i e d at 2 i n . / h r had p h a s e d i m e n s i o n s about o n e - h a l f a s l a r g e a s those in the s a m p l e s o l i d i f i e d at ~ i n . / h . (This is c o n s i s t e n t with the g e n e r a l l y - o b s e r v e d r e l a t i o n ~ that e u t e c t i c s p a c i n g s a r e p r o p o r t i o n a l to the i n v e r s e s q u a r e - r o o t of growth r a t e . ) H o w e v e r , the v o l u m e f r a c t i o n of the v a r i o u s p h a s e s was a p p r o x i m a t e l y the s a m e , i n d i c a t i n g that the d e g r e e of c o m p l e t i o n of the p e r i t e c t o i d r e a c tion was r o u g h l y independent of growth r a t e . T h i s l a t t e r r e s u l t , i n i t i a l l y s u r p r i s i n g , w i l l be e x p l a i n e d below in our d i s c u s s i o n s e c t i o n . To study the p e r i t e c t o i d r e a c t i o n under m o r e c o n t r o l l e d c o n d i t i o n s , it was f i r s t s u p p r e s s e d by heating a d i r e c t i o n a l l y - s o l i d i f i e d s a m p l e at 900~ for 30 min and quenching into w a t e r . T h i s p r o d u c e d a s i m p l e s t r u c t u r e of r i b b o n s of n i c k e l - r i c h s o l i d s o l u t i o n in a
Ni-e Eutectic A c c o r d i n g to H a n s e n , a a e u t e c t i c e x i s t s at 25 at. p c t In b e t w e e n the fcc n i c k e l - r i c h s o l i d s o l u t i o n and c J. D. LIVINGSTON is Physicist, General Electric Research and Development Center, Schenectady, N. Y. 12305. Manuscript submitted April 24, 1972. METALLURGICAL TRANSACTIONS
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400 40 50 60 INDIUM, ATOM PCT Fig. 1--Central portion of the Ni-In phase diagram as r e cently redetermined. 2 The four arrows mark the two eutectic and two eutectoid compositions. On the basis of our results, we have modified the diagram slightly by shifting the first eutectic slightly to the right of the Ni3In composition. 30
VOLUME 3, DECEMBER 1972-3173
Fig. 2--Ni-26.5 at. pct In eutectic solidified at ~- in./h, t r a n s v e r s e section. This complex s
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