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M E T A L L I C p a r t i c l e s have b e e n o b s e r v e d in m e t e o r i re s , 1 and l u n ar r o c k s , and s o i l s . 2 T h e s e m e t a l l i c p a r t i c l e s a r e p r e d o m i n a n t l y i r o n - n i c k e l with s m a l l a m o u n t s of o t h e r e l e m e n t s including Co, P, S, and C. In i r o n m e t e o r i t e s s e v e r a l types of c a r b i d e s have been o b s e r v e d , including cohenite, (FeNi)3C and haxonite, (FeNi)z3C6J R e c e n t l y , cohenite has a l s o been i d e n t i f i e d in lunar r o c k s and s o i l s J A t h e o r e t i c a l ph as e d i a g r a m fo r the F e - N i - C s y s t e m has b e e n d ev e l o p e d by B r e t t 4 in o r d e r to d e s c r i b e the s t a b i l i t y r a n g e of c o h e n i t e . Although s o m e e x p e r i m e n t a l p h ase d i a g r a m data is a l s o a v a i l a b l e , ~-v the e n t i r e i r o n - r i c h c o r n e r of the d i a g r a m has not yet b e e n d e t e r m i n e d in the solid p h as e t e m p e r a t u r e r e g i o n . T h e p u r p o s e of this w o r k is to d e t e r m i n e e x p e r i m e n t a l l y the so l i d ph as e r e l a t i o n s in the F e - N i - C s y s t e m . T h i s p h ase d i a g r a m is n e c e s s a r y in o r d e r to u n d e r s t a n d the d ev e l o p m e n t of m e t a l - c a r b i d e a s s e m b l a g e s o b s e r v e d in e x t r a t e r r e s t r i a l s a m p l e s . Two key p r o b l e m s have p r e v e n t e d the d e t e r m i n a tion of the F e - N i - C d i a g r a m : a) t h e r m o d y n a m i c a l l y unstable Fe3C is r e n d e r e d e v e n l e s s s t a b l e by the g r a p h i t i z i n g e f f e c t of Ni, which g i v e s r i s e to e x p e r i m e n t a l and i n t e r p r e t i v e d i f f i c u l t i e s , and b) the t r a n s f o r m a t i o n y ~ ~ + y is so s l u g g i s h at low t e m p e r a t u r e s that e s t a b l i s h m e n t of e q u i l i b r i u m in a r e a s o n able t i m e is e i t h e r difficult o r i m p o s s i b l e . 8 The f i r s t p r o b l e m can be c o n t r o l l e d by s e l e c t i n g a heat t r e a t ment which f a v o r s the n u c l e a t i o n and i n i t i a l growth of c a r b i d e o v e r g r a p h i t e . T h e s e c o n d p r o b l e m inv o l v e s d e t e r m i n i n g t i e - l i n e s when bulk e q u i l i b r i u m has not yet b e e n a t t a i n e d . T i e - l i n e d e t e r m i n a t i o n un d er such conditions is, h o w e v e r , p o s s i b l e if the a s sumption of l o c a l e q u i l i b r i u m is e m p l o y e d and the two ph as e i n t e r f a c e c o m p o s i t i o n s a r e m e a s u r e d with the electron microprobe. 9

A. D. ROMIG, Jr. is a Research Assistant and J. I. GOLDSTEIN is Professor of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted December 16, 1977. METALLURGICALTRANSACTIONS A

EXPERIMENTAL PROCEDURE Alloy Preparation Th e t e r n a r y a l l o y s used in this study w e r e p r e p a r e d by induction m e l t i n g of we