The genesis of the cellular precipitation reaction
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FOURNELLE
AND
J. B. CLARK
The m o r p h o l o g y of c e l l u l a r p r e c i p i t a t i o n in a Cu-9.5 at. pct In a l l o y h a s been i n v e s t i g a t e d by light and e l e c t r o n m i c r o s c o p y . Both c e l l u l a r and g e n e r a l p r e c i p i t a t i o n w e r e o b s e r v e d to o c c u r s i m u l t a n e o u s l y in quenched and a g e d a l l o y s while only c e l l u l a r p r e c i p i t a t i o n w a s o b s e r v e d to o c c u r in i s o t h e r m a l l y a g e d a l l o y s . B e c a u s e of the p r e s e n c e of w i d e , s o l u t e r i c h , p r e c i p i t a t e f r e e z o n e s in the v i c i n i t y of g r a i n b o u n d a r i e s in the quenched and a g e d a l l o y s , the e a r l y d e v e l o p m e n t of c e l l u l a r p r e c i p i t a t i o n w a s found to be i d e n t i c a l for both t y p e s of h e a t t r e a t m e n t . F r o m light and e l e c t r o n m i c r o s c o p y o b s e r v a t i o n s of the e a r l y s t a g e s of c e l l u l a r p r e c i p i t a t i o n a m e c h a n i s m for the f o r m a t i o n of c e l l s w a s d e v e l o p e d . At the s t a r t of aging, the unoccupied g r a i n b o u n d a r y b e g i n s to m i g r a t e u n d e r the influence of g r a i n b o u n d a r y m i g r a t i o n f o r c e s a s if it w e r e in a single p h a s e a l l o y . As the b o u n d a r y m i g r a t e s , s o l u t e s e g r e g a t e s along it to f o r m a l l o t r i o m o r p h s which pin the b o u n d a r y . The b o u n d a r y continues to m i g r a t e and bows between the s i m u l t a n e o u s l y f o r m i n g a l l o t r i o m o r p h s . With f u r t h e r aging, the a l l o t r i o m o r p h s lengthen following the bowing b o u n d a r y and the a l l o t r i o m o r p h s b e c o m e the i n i t i a l p r e c i p i t a t e l a m e l l a e of the developing c e l l a s a s t e a d y - s t a t e l a m e l l a r s t r u c t u r e d e v e l o p s . A s s u m i n g that the c r i t i c a l s t e p in the d e v e l o p m e n t of a cell i s the a b i l i t y of the b o u n d a r y to bow between the i n i t i a l a l l o t r i o m o r p h s , a c r i t e r i o n for the o c c u r r e n c e of c e l l u l a r p r e c i p i t a t i o n w a s d e v e l o p e d .
ACCORDINGto Smith, ~ s o l u t e
segregates from supers a t u r a t e d s o l i d solution by c e l l u l a r p r e c i p i t a t i o n when the b o u n d a r y b e t w e e n two a d j a c e n t s u p e r s a t u r a t e d g r a i n s b e c o m e u n s t a b l e and m i g r a t e s f r o m one g r a i n into the o t h e r . As the b o u n d a r y m i g r a t e s an a g g r e g a t e of p r e c i p i t a t e p h a s e and d e p l e t e d s o l i d solution is f o r m e d behind the a d v a n c i n g i n t e r f a c e . T h i s a g g r e g a t e i s c h a r a c t e r i z e d by a r e g u l a r a r r a n g e m e n t of the p r e c i p i t a t e and d e p l e t e d m a t r i x p h a s e and by the d e p l e t e d m a t r i x p h a s e having the s a m e c r y s t a l l o
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