Generation of dislocations in a precipitation-hardened alloy
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d o m i n a n t s o u r c e s of d i s l o c a t i o n s a r e p r e s e n t at the grain boundaries. Strips of 24 Ni-17 C r - 1 . 9 4 T i - 0 . 0 4 C s t e e l were solution h e a t - t r e a t e d at 1150~ and then aged at 650~ for 1400 h r to produce a fine d i s p e r s i o n of o r d e r e d Ni3Ti p r e c i p i t a t e s . The s p e c i m e n s a r e u n d e r a g e d and d u r i n g d e f o r m a t i o n the p r e c i p i t a t e p a r t i c l e s a r e i n t e r s e c t e d by moving d i s l o c a t i o n s f l The s p e c i m e n s were rapidly quenched into an ice w a t e r m i x t u r e to introduce a s m a l l a m o u n t of d e f o r m a t i o n . Some s p e c i m e n s were d e f o r m e d 1 pct u n d e r t e n s i o n by an I n s t r o n m a c h i n e . Thin foils were p r e p a r e d and e x a m i n e d under an e l e c t r o n m i c r o s c o p e o p e r a t i n g at 100 kv. On e l e c t r o n m e t a l l o g r a p h i c e x a m i n a t i o n of thin f o i l s , a fine d i s p e r s i o n of NisTi p r e c i p i t a t e s was obs e r v e d in the m a t r i x . Large p r e c i p i t a t e s of (Fe,Cr)23C6 were o b s e r v e d on the g r a i n b o u n d a r i e s . T h e r e was a p r e c i p i t a t e free region n e a r the g r a i n b o u n d a r i e s . The d e n s i t y of d i s l o c a t i o n s was low and m o s t of the d i s l o c a t i o n s were confined to a r r a y s close to g r a i n b o u n d a r i e s , Fig. 1. Single d i s l o c a t i o n s a r e o b s e r v e d in p r e c i p i t a t e free r e g i o n s and p a i r s of d i s l o c a t i o n s a r e o b s e r v e d in r e g i o n s c o n t a i n i n g p r e c i p i t a t e s . That d i s l o c a t i o n of the p a i r which is away from the g r a i n b o u n d a r y is c h a r a c t e r i z e d by c u r v e d s e g m e n t s , while the other which is towards the g r a i n b o u n d a r y is n e a r l y straight. It was c o n f i r m e d from the b e h a v i o r of d i s location i m a g e s in a p a i r for o p e r a t i n g r e f l e c t i o n s of + g and - g , that the d i s l o c a t i o n s c o n s t i t u t i n g a p a i r have the s a m e sign. Since the m o v e m e n t of a d i s l o c a tion in a m a t e r i a l c o n t a i n i n g o r d e r e d p r e c i p i t a t e s r e s u l t s in the f o r m a t i o n of a high e n e r g y antiphase b o u n d a r y , the d i s l o c a t i o n s tend to move in p a i r s . 7,B The leading d i s l o c a t i o n p r o d u c e s an a n t i p h a s e b o u n d a r y within each p a r t i c l e and the second d i s l o c a t i o n of the p a i r e l i m i n a t e s it. A c c o r d i n g to G l e i t e r and Hornbogen 9 the shape of l e a d i n g and t r a i l i n g d i s l o c a t i o n s and the spacing between them is d e t e r m i n e d by i) F o r c e s of a t t r a c t i o n due to a n t i p h a s e b o u n d a r i e s . ii) Repulsive f o r c e s between d i s l o c a t i o n s . iii) E x t e r n a l l y applied s h e a r s t r e s s (due to m e c h a n i cal t e s t i n g or t h e r
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