Lattice imaging and optical microanalysis of a Cu-Ni-Cr spinodal alloy
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A s p i n o d a l l y d e c o m p o s e d p r o d u c t m a y be c h a r a c t e r i z e d by s m a l l c o m p o s i t i o n f l u c t u a t i o n s o v e r l a r g e d i s t a n c e s . Studies by X - r a y d i f f r a c t i o n 1 r e v e a l only the w a v e l e n g t h of the e a r l y s t a g e m o d u l a t i o n and the a v e r age modulation amplitude through diffracted intensit i e s , which a r e a v e r a g e d o v e r a l a r g e s p e c i m e n v o l u m e . Conventional b r i g h t - f i e l d o r d a r k - f i e l d i m a g e s o b t a i n e d 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 (TEM) cannot p r o v i d e any c o m p o s i t i o n a l i n f o r m a t i o n about the two p h a s e s . 2'3 It is only p o s s i b l e to d e t e c t the l o c a l ized, p l a n e - b y - p l a n e v a r i a t i o n s in c o m p o s i t i o n d i r e c t l y by c o r r e s p o n d i n g l a t t i c e p a r a m e t e r m o d u l a t i o n s t h r o u g h the l a t t i c e i m a g i n g technique, a'5 G r o n s k y et al 4,s f i r s t u s e d l a t t i c e i m a g i n g f o r s t u d y ing s p i n o d a l a l l o y s . In p r e l i m i n a r y r e s u l t s of s p i n o d a l l y d e c o m p o s e d Au-77 at. p c t Ni, l a t t i c e p a r a m e t e r d i f f e r e n c e s (Aa) w e r e c l e a r l y d e t e c t e d f r o m one r e gion to a n o t h e r by m e a s u r e m e n t s on m i c r o d e n s i t o m e t e r t r a c e s a c r o s s the l a t t i c e f r i n g e s . In addition, the a v e r a g e w a v e l e n g t h d e t e r m i n e d f r o m this t e c h nique was in e x a c t a g r e e m e n t with that d e t e r m i n e d f r o m the p o s i t i o n s of s a t e l l i t e r e f l e c t i o n s in the c o r r e s p o n d i n g d i f f r a c t i o n p a t t e r n s . T h u s , the l a t t i c e i m a g e of t h i s m a t e r i a l r e v e a l s the p r i n c i p a l f e a t u r e s e x p e c t e d of a s p i n o d a l l y d e c o m p o s e d a l l o y , and l a s e r o p t i c a l d i f f r a c t i o n f r o m the l a t t i c e i m a g e n e g a t i v e s a l s o showed c h a r a c t e r i s t i c s a t e l l i t e r e f l e c t i o n s . T h i s l a t t e r method a l l o w s a r e a s a s s m a l l a s 10/~ (1 nm) in d i a m to be c h o s e n to give d i f f r a c t i o n d a t a which a r e f a r s u p e r i o r in t e r m s of r e s o l u t i o n to those f r o m 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 . 5 T h e s e o p t i c a l diff r a c t i o n p a t t e r n s can a l s o be u s e d to obtain f r i n g e s p a c i n g c h a n g e s 6,7 and a r e an a l t e r n a t i v e to the m i c r o d e n s i t o m e t e r m e a s u r e m e n t s for obtaining f r i n g e spacing information. F o r the p r e s e n t r e s e a r c h , C u - N i - C r was s t u d i e d p a r t i c u l a r l y b e c a u s e of its c o m m e r c i a l i m p o r t a n c e a s an a l l o y with good s t r e n g t h and c o r r o s i o n r e s i s t a n c e in m a r i n e e n v i r o n m e n t s .
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