Determination of local composition by lattice imaging
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u n d e r s t a n d i n g alloy b e h a v i o r at the a t o m i c level the following b a s i c i n f o r m a t i o n is needed: i) the local c h e m i c a l c o m p o s i t i o n , if) v a r i a t i o n s in c r y s t a l l i n e s t r u c t u r e , iii) the role and c o n f i g u r a t i o n of lattice defects. The d i r e c t lattice f r i n g e i m a g i n g technique of 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 (TEM) is capable of p r o viding such i n f o r m a t i o n at the a t o m i c plane l e v e l in c r y s t a l l i n e solids, as has b e e n d e m o n s t r a t e d , e s p e c i a l l y for the l a t t e r two, in s t u d i e s of a v a r i e t y of phase t r a n s f o r m a t i o n s . 1"~ The p u r p o s e of this a r t i c l e is to c o n s i d e r in d e t a i l the f i r s t of the above p a r a m e t e r s . The a d v a n t a g e s and l i m i t a t i o n s of the l a t t i c e i m a g i n g a p p r o a c h for c o m p o s i t i o n d e t e r m i n a t i o n a r e d e m o n s t r a t e d and the p o t e n t i a l of the technique in c o m p a r i son with a l t e r n a t i v e methods is d i s c u s s e d . It should be e m p h a s i z e d that the i n t e r e s t h e r e is in the d e t e r m i n a t i o n of localized r a t h e r than bulk composition. The p o s s i b l e a p p l i c a t i o n s a r e f a r - r e a c h i n g (e.g. e s t i m a t i n g localized c o m p o s i t i o n a l g r a d i e n t s n e a r g r a i n b o u n d a r i e s , i n t e r p h a s e i n t e r f a c e s , and so forth). M o d e r n a n a l y t i c a l t e c h n i q u e s such a s e n e r g y d i s p e r s i v e X - r a y a n a l y s i s and Auger e l e c t r o n s p e c t r o s copy suffer f r o m r e l a t i v e l y poor s p a t i a l r e s o l u t i o n (;~several h u n d r e d s o f / ~ n g s t r o m s ) . S i m i l a r l y d i f f r a c tion data for s u f f i c i e n t l y a c c u r a t e lattice p a r a m e t e r m e a s u r e m e n t s m u s t a l s o be collected f r o m f a i r l y l a r g e v o l u m e s (e.g. ~1018 unit c e l l s in X - r a y and n e u t r o n diff r a c t i o n , ~1012 unit c e l l s for s e l e c t e d a r e a e l e c t r o n d i f f r a c t i o n and ~103 unit c e l l s in s c a n n i n g t r a n s m i s sion e l e c t r o n m i c r o d i f f r a c t i o n methods). T h i s p r o b l e m is one of the m o s t i n t r a c t a b l e in m a t e r i a l s c h a r a c t e r i z a t i o n and one for which l a t t i c e i m a g i n g a p p e a r s to be an exciting new d e v e l o p m e n t .
s t r a i n effects (e.g. at d i s l o c a t i o n lines) a r e not of p r i n c i p a l c o n c e r n . A m a i n p r o b l e m now is that of obt a i n i n g highly localized i n f o r m a t i o n about c h e m i c a l content. The p r e s e n t b a s i s for e s t a b l i s h i n g c o m p o s i tion u s i n g high r e s o l u t i o n t e c h n i q u e s lies in the i n t e r r e l a t i o n of i n t e r p l a n a r s p a c i n g (i.e., lattice p a r a m e t e r ) with c h e
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