Hot stage TEM investigations of dislocation climb in NiAl
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t a k e n d u r i n g the w a r m up p e r i o d w e r e f r e q u e n t l y b l u r r e d due to s m a l l d r i f t s of the s p e c i m e n . M i c r o g r a p h s w e r e t a k e n at v a r i o u s t i m e s d u r i n g the a n n e a l ing p e r i o d , which v a r i e d b e t w e e n 60 and 200 rain.
RESULTS AND DISCUSSION Loop Growth D u r i n g Heating Stage E x p e r i m e n t s The p r i s m a t i c d i s l o c a t i o n l o o p s l i e on { 100} with b = ( 1 0 0 ) . Thus when v i e w e d in a (100) foil the (100) l o o p s a p p e a r s q u a r e while only the edge of l o o p s on (010) and (001) can be s e e n . When v i e w e d f r o m the [110] d i r e c t i o n the (100) and (010) l o o p s a p p e a r a s r e c t a n g l e s while o n l y the edge of (001) loops can be s e e n . Slow c o o l i n g of the s a m p l e p r i o r to hot s t a g e e x a m i n a t i o n s a l l o w s the d e v e l o p m e n t of the p r i s m a t i c d i s l o c a t i o n loop a r r a y s . G e n e r a l l y , s l o w e r c o o l i n g p r o m o t e s m o r e fully d e v e l o p e d a r r a y s than f a s t e r cooling, while q u e n c h e s can be used to s u p p r e s s loop f o r m a t i o n . 1 F i g . l ( a ) shows the d i s l o c a t i o n l o o p s f o r m e d d u r i n g a 1000~ slow cool in a 47 at. pct Ni s a m p l e in the (100) o r i e n t a t i o n . The i d e n t i f i c a t i o n s c h e m e for all o b s e r v a t i o n s and growth c u r v e s is shown at s o u r c e 3. Each s o u r c e is n u m b e r e d and the e m a n a t i n g l o o p s a r e l e t t e r e d in o r d e r of t h e i r g e n e r a t i o n , i.e, 3A i s f o r m e d f i r s t and is l a r g e r than 3B and 3C, and so forth. D u r i n g the h e a t i n g s t a g e t r e a t m e n t at 515~ the l o o p s u n d e r w e n t a g r a d u a l growth t h r o u g h o u t the 120 m i n e x p e r i m e n t and r e m a i n e d e s s e n t i a l l y s q u a r e with s h a r p c o r n e r s . Fig. l(b) shows the loop s t r u c t u r e a f t e r 32 min. The growth of the loops on p l a n e s n o r m a l to the foil s u r f a c e can b e s e e n a s a l e n g t h e n i n g of the s t r a i g h t s e g m e n t s n e a r m o s t of the b l a c k dot s o u r c e s . Fig. 1(c) and (d), an e n l a r g e d view of the d i s l o c a t i o n loop a r r a y s at s o u r c e 5, d e m o n s t r a t e s one of the c o m m o n p h e n o m e n a r e l a t e d to g r o w t h of the l o o p s . The o u t e r l o o p s f r o m the s o u r c e s c o a l e s c e d d u r i n g the p r e l i m i n a r y slow cool. The s e c o n d l a r g e s t l o o p s f r o m each s o u r c e a p p r o a c h e d and p a s s e d one a n o t h e r in Fig. l ( c ) i n d i c a t i n g they a r e not c o p l a n a r but r a t h e r l i e on p a r a l l e l (100) p l a n e s . Continued growth of the l o o p s o c c u r s until c o p l a n a r l o o p s c o a l e s c e d , Fig. l(d). The g r a d u a l growth of t h e s e l o o p s is shown in Fig. 2. The growth of a l l l o o p s was s i m i l
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