The dissolution kinetics of Fe 3 C in ferrite
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TRADITIONALLY,
the m o s t widely used a s s u m p t i o n i n q u a l i t a t i v e or q u a n t i t a t i v e t r e a t m e n t s of phase bounda r y motion is that of local i n t e r f a c i a l e q u i l i b r i u m . This a s s u m p t i o n is e q u i v a l e n t to the s t a t e m e n t that long range diffusion of one or m o r e e l e m e n t s is rate c o n t r o l l i n g . O c c a s i o n a l l y , i n v e s t i g a t o r s have p e r f o r m e d e x p e r i m e n t s which y i e l d e d r e s u l t s i n c o n s i s t e n t with a model b a s e d upon local e q u i l i b r i u m . T h e s e data led to the postulate that an i n t e r r a c i a l r e a c t i o n m a y be rate c o n t r o l l i n g r a t h e r than long r a n g e diffusion. In p r o c e s s e s of this type, the a c t i v i t y , a, of one or m o r e e l e m e n t s is d i s c o n t i n u o u s at the phase bounda r y (local n o n e q u i l i b r i u m ) and the flux, J , of a given e l e m e n t a c r o s s the b o u n d a r y is s o m e function of its a c t i v i t y d i s c o n t i n u i t y , Aa. Heckel ~ i n v e s t i g a t e d the growth of p r o e u t e c t o i d c e m e n t i t e a l l o t r i o m o r p h s . One of the F e - C a l l o y s used contained ~0.2 wt pct Si. C e m e n t i t e growth r a t e s m e a s u r e d on this m a t e r i a l were an o r d e r of m a g n i tude below those p r e d i c t e d by a model which a s s u m e d local e q u i l i b r i u m at the F e s C - a u s t e n i t e i n t e r f a c e and long range c a r b o n diffusion as the rate c o n t r o l l i n g step. Heckel a t t r i b u t e d this d i s c r e p a n c y to the p r e s of s i l i c o n . This e l e m e n t is i n s o l u b l e in FeaC and will be pushed ahead of the growing c e m e n t i t e . Since the diffusivity of s i l i c o n is much s m a l l e r than c a r b o n in a u s t e n i t e , a b u i l d - u p of the e l e m e n t o c c u r s in f r o n t of the growing FesC which would r e s u l t in growth r a t e s lower than those p r e d i c t e d by c a r b o n diffusion (see DarkenS). The s a m e e x p e r i m e n t s were s u b s e q u e n t l y p e r f o r m e d on a m a t e r i a l c o n t a i n i n g 0.01 wt pct Si. FRANK V. NOLFI, JR., formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, Pa., is Assistant Metallurgist,Argonne National Laboratory, Argonne, Ill. PAULG. SHEWMONis DivisionDirector, Metallurgy Division,Argonne National Laboratory. JAMES S. FOSTER is Assistant Professor, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity. This paper is based on a thesis submitted by FRANK V. NOLFI, JR., to Carnegie-MellonUniversity, in partial fulfillment of the requirements of the Ph.D. degree. Manuscript submitted July 17, 1969. METALLURGICALTRANSACTIONS
The o b s e r v e d r a t e s were a f a c t o r of two f a s t e r than the e a r l i e r v a l u e s but were s t i l l a f a c t o r of five s l o w e r than the p r e d i c t e d v a l u e s
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