A model for ferrite nucleation applied to boron hardenability
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1973 S h a r m a and P u r d y ~ p r o p o s e d that the f e r r i t e n u c l e u s in a u s t e n i t e be m o d e l e d a s having a d i s k s h a p e . T h e j u s t i f i c a t i o n for t h i s m o d e l was the c o h e r e n t int e r f a c e that can f o r m b e t w e e n the o~ and 7' p h a s e s when they s a t i s f y t h e K u r d j u m o v - S a c h s r e l a t i o n s h i p . It w a s r e a s o n e d that the r e s u l t i n g low i n t e r r a c i a l t e n s i o n , actor, would give r i s e to the l a r g e , f l a t f a c e s of the d i s k . T h e edge of the d i s k w a s p r e s u m a b l y s e m i c o h e r e n t , like the t i p of a W i d m a n s t ~ t t e n p l a t e with t e n s i o n ec~"Tsemi_coh. It was p r o p o s e d that the d i s k would nuc l e a t e h e t e r o g e n e o u s l y on i n c o h e r e n t g r a i n b o u n d a r i e s , with the ~ - y i n t e r f a c e on the g r a i n b o u n d a r y b e i n g inc o h e r e n t a s w e l l and having t e n s i o n cr~-y. One o u t c o m e of this m o d e l was the p r e d i c t i o n that s m a l l c h a n g e s in i n t e r f a c i a l t e n s i o n would have l i t t l e influence on the f e r r i t e n u c l e a t i o n r a t e . F o r t h i s r e a s o n S h a r m a and P u r d y concluded that the effect of b o r o n on h a r d e n a b i l i t y could not b e c a u s e d by e q u i l i b r i u m a b s o r p t i o n of b o r o n into the a u s t e n i t e g r a i n b o u n d a r i e s . I n s t e a d t h e y a r g u e d , following a s u g g e s tion b y Zener~ 2 that the f o r m a t i o n of g r a i n b o u n d a r y b o r o c a r b i d e s was r e s p o n s i b l e in b o r o n c o n t a i n i n g steels for retarding ferrite nucleation. Recently M a i t r e p i e r r e , T h i v e l l i e r and T r i c o t s t r i e d to p r o v e t h i s c l a i m with a d i v e r s i f i e d study of b o r o n and nonboron s t e e l s . H o w e v e r , a s m e n t i o n e d by t h e s e i n v e s t i g a t o r s , t h e i r r e s u l t s could not c o n c l u s i v e l y p r o v e what m e c h a n i s m was r e s p o n s i b l e for the b o r o n h a r d e n a b i l i t y . At the t i m e of the S h a r m a and P u r d y s t u d y , i n a d e quate m e t h o d s w e r e a v a i l a b l e f o r t r e a t i n g h e t e r o g e n e ous n u c l e a t i o n of a n i s o t r o p i c n u c l e i , S i n c e t h e i r p r o p o s a l , Cahn and Hoffman 4 have p r e s e n t e d a c o n s t r u c tion for d e a l i n g with h e t e r o g e n e o u s n u c l e a t i o n . With t h i s c o n s t r u c t i o n and with a s o m e w h a t s i m p l i f i e d a s s u m p t i o n , we will show, in the following work, that the S h a r m a and P u r d y m o d e l should be m o d i f i e d . T h e m o d i f i e d m o d e l h a s a d i f f e r e n t s h a p e d n u c l e u s and l e a d s to d i f f e r e n t c o n c l u s i o n s r e g a r d i n g b o r o n h a r d e n ability, The C a h n - H o f f m a n c o n s t r u c t i o n is a r i g o r o u s way o
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