The electron metallography of ordering reactions in Fe-Al alloys
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SWANN,
W. R. DUFF,
AND
R.M.
FISHER
The m i c r o s t r u c t u r a l changes d u r i n g the a ~ FeA1, FeA1 ~ Fe3A1, and a - - FesA1 t r a n s i t i o n s w e r e studied by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . The o r d e r i n g of f e r r o m a g n e t i c a was obs e r v e d to o c c u r in a c l a s s i c a l m a n n e r by the n u c l e a t i o n and growth of p a r t i c l e s of the FeA1 o r FeaA1 type p h a s e s . However, the o r d e r i n g of p a r a m a g n e t i c t~ to FeA1 and p a r a m a g n e t i c FeA1 to Fe~A1 o c c u r r e d by a m e c h a n i s m which showed m a n y of the c h a r a c t e r i s t i c s expected of a second or h i g h e r d e g r e e t r a n s i t i o n . T h e s e included c r i t i c a l point f l u c t u a t i o n s in the deg r e e of l o n g - r a n g e o r d e r which a l s o a p p e a r e d to be g r e a t e r in the v i c i n i t y of a n t i p h a s e domain b o u n d a r i e s . The FeA1 phase was a l s o o b s e r v e d to p a r t i a l l y d i s o r d e r in the i n i t i a l s t a g e s of the FeA1 ~ a + FeA1 t r a n s i t i o n and this effect a p p e a r s to a c c o u n t for the a n o m a l o u s m a g n e t i c b e h a v i o r of s o m e FeA1 a l l o y s .
T W O o r d e r e d s t r u c t u r e s with s t o i c h i o m e t r i c c o m p o s i t i o n s FeA1 and Fe3A1 can f o r m f r o m the d i s o r d e r e d bcc a phase in Fe-A1 alloys.1 The s t r u c t u r e s can exist over a wide r a n g e of c o m p o s i t i o n s and in a c e r t a i n c o m p o s i t i o n r a n g e a d i r e c t t r a n s i t i o n f r o m Fe3A1 to F e A l - t y p e o r d e r is p o s s i b l e . The FeA1 p h a s e is f o r m e d by o r d e r i n g a m o n g n e a r e s t n e i g h b o r a t o m s of the a phase and a f u r t h e r o r d e r i n g a m o n g s e c o n d n e a r est n e i g h b o r a t o m s p r o d u c e s the FesA1 s t r u c t u r e with a unit cell having twice the a l a t t i c e p a r a m e t e r . The two s t r u c t u r e s a r e i l l u s t r a t e d in Fig. 1 and it is s e e n that FeA1 and Fe3A1 have s i m p l e cubic and f a c e - c e n t e r e d - c u b i c B r a v a i s l a t t i c e s r e s p e c t i v e l y . T h u s , diff r a c t i o n p a t t e r n s of [e3A1 contain only r e f l e c t i o n s with u n m i x e d i n d i c e s w h e r e a s all hkl r e f l e c t i o n s a r e p o s s i b l e in diffraction p a t t e r n s of FeA1. Fig. 2 ~hows that t h e r e is c o n s i d e r a b l e d i s a g r e e m e n t in the l i t e r a t u r e 2-7 c o n c e r n i n g the e q u i l i b r i u m r e l a t i o n s h i p s between the o r d e r e d p h a s e s . Fig. 2(c) was d e t e r m i n e d t h e o r e t i c a l l y by R u d m a n 4 who did not c o n s i d e r the i n t e r a c t i o n between m a g n e t i z a t i o n and o r d e r . However, Sato, 8 Houska, 9 and L a v i s and Bell 1~ have shown that such i n t e r a c t i o n s can have a n i n f l u ence on phase s t a b i l i t y and this is in a g r e e m e n t with the p r
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