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T H E R E is a v e r y l a r g e n u m b e r of r e p o r t s on the phase e q u i l i b r i a in the solid state of the F e - N s y s t e m and b e tween s o l i d F e - N alloys and an a t m o s p h e r e of N2 or HzN/H v The p r e s e n t p a p e r r e p r e s e n t s an a t t e m p t to r a t i o n a l i z e all the i n f o r m a t i o n in t e r m s of f r e e e n e r g y functions for the i n d i v i d u a l p h a s e s . The s u c c e s s of such a p r o j e c t depends upon the v a l i d i t y of the t h e r m o d y n a m i c m o d e l s c h o s e n and, in the p r e s e n t case, t h e r e a r e s e v e r a l p h a s e s which m u s t be t r e a t e d as i n t e r s t i t i a l s o l u t i o n s of n i t r o g e n in i r o n . It is hoped that the p r e s e n t a n a l y s i s of the F e - N s y s t e m can f o r m the b a s i s for s i m i l a r a n a l y s e s of F e - N - M s y s t e m s , where M r e p r e s e n t s v a r i o u s s u b s t i t u t i o n a l alloying e l e m e n t s . As a c o n s e q u e n c e , it was decided to choose a model which has r e c e n t l y been s u g g e s t e d by H i l l e r t and Staffansson 1 for t e r n a r y p h a s e s where one c o m p o n e n t is d i s s o l v e d i n t e r s t i t i a l l y and the o t h e r two a r e r a n d o m l y m i x e d on the l a t t i c e s i t e s . F o r the b i n a r y c a s e the model r e duces to the following e x p r e s s i o n s for the c h e m i c a l potentials --

2

GN= ~ N + R T In [yN/(1-- YN)] -- c "

G-Fe = ~

c + a"

YN" L

1 R T In [ 1 - YN] + -a--" (YN)2" L

[la] [lb]

L is a r e g u l a r - s o l u t i o n type p a r a m e t e r . YN is the m e a s u r e of the n i t r o g e n content and is defined as

YN

a =

XN 1 -

--~ G N =~

+ R T In [ y ~ A 1 - y~)]

[3a]

G~e = ~ + 3 R T In [1 - y~] [3b] In p r a c t i c e it is m o r e c o m m o n to r e f e r the c h e m i c a l p o t e n t i a l of n i t r o g e n to a s t a n d a r d state defined by a n i t r o g e n gas of one a t m o s p h e r e , which m a y be denoted b y ~1 oG N2 and for low p r e s s u r e s one w r i t e s G--N = 89176 2 + R T In Pv~N2

[41

The d i f f e r e n c e b e t w e e n the two s t a n d a r d s t a t e s can be e v a l u a t e d f r o m the e x p e r i m e n t a l i n f o r m a t i o n of the s o l u b i l i t y of n i t r o g e n in f e r r i t e . T h e r e a r e m a n y such i n v e s t i g a t i o n s and the data have r e c e n t l y b e e n c o m p i l e d b y G r i e v e s o n and Turkdoganfl In the high t e m p e r a t u r e r a n g e of 5 - f e r r i t e and in the r e g i o n of 900 to 700~ t h e r e a r e d i r e c t m e a s u r e m e n t s of the e q u i l i b r i u m b e t w e e n f e r r i t e and n i t r o g e n gas at 1 a t m p r e s s u r e . Below 600~ t h e r e a r e only i n d i r e c t m e a s u r e m e n t s , c a r r i e d out with gas m i x t u r e s of h y d r o gen and a m m o n i a . By using the known e q u i l i b r i u m c o n s t a n t for the a m m o n i a r e a c t i o n , one can c a l c u l a t e the