The pearlite reaction
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F o r i s o t h e r m a l p e a r l i t e t h e s e p e r t u r b a t i o n r e s u l t s c o r r e s p o n d c l o s e l y to the s t a t e of m a x i m u m e n t r o p y p r o d u c t i o n r a t e while for f o r c e d v e l o c i t y p e a r l i t e the c o r r e s p o n d e n c e is a l s o s a t i s f a c t o r y . A d e t a i l e d a n a l y s i s of the d a t a l e a d s us to r e a f f i r m the a u t h o r ' s c o n c l u s i o n s that the e u t e c t o i d r e a c t i o n s in Cu-12 p e t A1 and s o m e r e l a t e d t e r n a r y a l l o y s r e p o r t e d by A s u n d i and W e s t a r e c o n t r o l l e d by volume diffusion and that the e u t e e t o i d r e a c t i o n in A1-78 Zn r e p o r t e d by C h e e t h a m and R i d l e y is c o n t r o l l e d by b o u n d a r y diffusion. We c o n elude f u r t h e r a f t e r c a r e f u l a n a l y s i s that the p e a r l i t e r e a c t i o n in F e - 0 . 8 C is c o n t r o l l e d for the h i g h e r t e m p e r a t u r e s by v o l u m e diffusion of c a r b o n in a u s t e n i t e . We a r e a l s o led to s t a t e t h a t the p e a r l i t e t r a n s f o r m a t i o n s in F e - C - M n and F e - C - N i o c c u r for the m o s t p a r t in a n o p a r t i t i o n r e g i m e and a r e t h e r e f o r e c o n t r o l l e d by volume diffusion of c a r b o n in a u s t e n i t e , while the t r a n s f o r m a t i o n s in F e - C - C r and F e - C - M o , being f o r c e d by t h e r m o d y n a m i c s to s u s t a i n p a r t i t i o n of c h r o m i u m and m o l y b d e n u m , a r e c o n t r o l l e d by p h a s e b o u n d a r y diffusion of the l a t t e r e l e m e n t s .
FEw m e t a l l u r g i s t s
of today r e c a l l , or w e r e indeed e v e r told, why the e u t e c t o i d s t r u c t u r e u n d e r d i s c u s s i o n r e c e i v e d the p a r t i c u l a r n a m e " p e a r l i t e " . Yet the n a m e i s s t r i k i n g l y a p p r o p r i a t e , for the m o t h e r - o f - p e a r l a p p e a r a n c e of a c a r e f u l l y p o l i s h e d and e t c h e d s a m p l e s t e m s d i r e c t l y f r o m the d i f f r a c t i o n g r a t i n g - l i k e r e l i e f of the p a r a l l e l m i c r o n - s p a c e d l a m e l l a e of FesC which p r o j e c t s l i g h t l y f r o m the e t c h e d s u r f a c e . The r e m a r k a b l e u n i f o r m i t y of the l a m e l l a e in s e l e c t e d c o l o n i e s has n e v e r f a i l e d to i n t r i g u e the o b s e r v e r and to incite h i m to a s e a r c h for an e l e g a n t and c o m p l e t e t h e o r e t i c a l d e s c r i p t i o n . F i g . 1 shows a p e a r l i t e in s t e e l p r o d u c e d in a f o r c e d v e l o c i t y e x p e r i m e n t by Boiling and R i c h m a n . ~ It is i d e n t i c a l to the u s u a l p e a r l i t e s f o r m e d isothermally. In this c o n t r i b u t i o n we r e v i e w and c r i t i c a l l y a s s e s s the e x p e r i m e n t a l and t h e o r e t i c a l p r o g r e s s wh
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