Grain boundary segregation of carbon in iron
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D. N. BESHER~
The d i s t r i b u t i o n of C 14 in a s e r i e s of ~ - p h a s e F e - C a l l o y s was studied a u t o r a d i o g r a p h i c a l l y . E x c e s s c a r b o n was p r e s e n t n e a r g r a i n b o u n d a r i e s in s p e c i m e n s e q u i l i b r a t e d at 500 ~ and 600~ the a m o u n t of c a r b o n n e a r the g r a i n b o u n d a r i e s was found to d e c r e a s e a s the e q u i l i b r a t i o n t e m p e r a t u r e i n c r e a s e d . E l e c t r o n m i c r o s c o p y of the s p e c i m e n s s u b s e q u e n t to the a u t o r a d i o g r a p h y showed that no g r a i n b o u n d a r y p r e c i p i t a t e s were p r e s e n t . The r e s u l t s a r e c o m p a r e d with the t h e o r i e s of n o n e q u i l i b r i u m v a c a n c y - a i d e d s e g r e g a t i o n and G i b b s i a n a d s o r p t i o n , and also with M c L e a n ' s m o d e l of g r a i n b o u n d a r y s e g r e g a t i o n . It is shown that the technique m e a s u r e s d i r e c t l y the quantity of t h e r m o d y n a m i c i n t e r e s t , the total e x c e s s solute per unit a r e a of g r a i n b o u n d a r y .
T H E s e g r e g a t i o n of c a r b o n to g r a i n b o u n d a r i e s in i r o n has been s u s p e c t e d for some t i m e ; but the e x p e r i m e n t s which g e n e r a t e this s u s p i c i o n a r e m a i n l y i n d i r e c t and do not lead to a c l e a r o r c o n s i s t e n t p i c t u r e of the s e g r e gation. C u r r e n t i n t e r e s t in the e f f e c t s of g r a i n b o u n d a r y s e g r e g a t i o n e m p h a s i z e s the need for a d i r e c t e x p e r i m e n t in this s y s t e m ; thus, this p a p e r r e p o r t s an a t t e m p t to p e r f o r m a c o n t r o l l e d , d i r e c t i n v e s t i g a t i o n of g r a i n boundary s e g r e g a t i o n in high purity F e - C a l l o y s . P r e v i o u s e x p e r i m e n t s include work on g r a i n bounda r y e m b r i t t l e m e n t , 1-3 g r a i n b o u n d a r y h a r d n e s s , 4-6 v a r i a t i o n in Petch p a r a m e t e r s , 7-9 and the r e l a t i o n s h i p between m e c h a n i c a l p r o p e r t i e s and t h e r m a l history.l~ A l m o s t without exception these e x p e r i m e n t s s u g g e s t that g r a i n b o u n d a r y s e g r e g a t i o n of c a r b o n o c c u r s . In addition to the above work on m e c h a n i c a l p r o p e r t i e s , e x p e r i m e n t s on the c h e m i c a l o r e l e c t r o c h e m i c a l b e h a v i o r of g r a i n b o u n d a r i e s in i r o n have led to s i m i l a r s u g g e s t i o n s . P h i l l i p s 12 r e v i e w e d the p r e v i o u s work in this a r e a and pointed out the lack of a g r e e m e n t a m o n g the v a r i o u s i n v e s t i g a t o r s . I n t e r n a l f r i c t i o n m e a s u r e m e n t s have also b e e n i n t e r p r e t e d as i n d i c a t i n g g r a i n b o u n d a r y s e g r e g a t i o n of c a r b o n ; 13-1S this i n t e r p r e t a t i o n has r e c e n t l y been i
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