Influence of impurities on stacking fault energy determination
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A s t a n d a r d technique used to d e t e r m i n e s t a c k i n g fault e n e r g i e s is the s o - c a l l e d node method in which a m e a s u r e m e n t of the c u r v a t u r e of p a r t i a l s s u r r o u n d i n g an extended node may be r e l a t e d to the fault e n e r g y . A b a s i c a s s u m p t i o n that is made, e i t h e r i m p l i c i t l y or explicitly, when e m ploying this technique is that the d e g r e e of nodal e x t e n s i o n is not i n f l u e n c e d by p r e c i p i t a t i o n or s e g r e g a t i o n effects. C l e a r l y , if the shape o r s i z e of a node is i n f l u e n c e d in this m a n n e r , the m e a s u r e d values of SFE m a y not be m e a n i n g f u l with r e g a r d to i n t e r p r e t i n g p r o p e r t i e s such a s y i e l d s~rength, c r e e p b e h a v i o r , s u s c e p t i b i l i t y to s t r e s s - c o r r o s i o n , a n d so forth. An e x a m p l e of the d r a m a t i c role that i m p u r i t i e s can play in d i s t o r t i n g the e q u i l i b r i u m d i m e n s i o n s of extended nodes through p r e c i p i t a t i o n effects is p r e s e n t e d in this p a p e r . T H E o b s e r v a t i o n by W i l l i a m s I that the addition of s m a l l a m o u n t s of b o r o n can i m p r o v e s u b s t a n t i a l l y the high t e m p e r a t u r e s t r e n g t h of TiC by p r e c i p i t a t i o n h a r d ening p r o m p t e d Venables 2 to i n v e s t i g a t e the n a t u r e of the p r e c i p i t a t e s 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 r e s u l t s of t h e s e s t u d i e s have b e e n useful in i n t e r p r e t i n g the high t e m p e r a t u r e m e c h a n i c a l p r o p e r t i e s of b o r o n - d o p e d TiC but the work has also provided a c l e a r e x a m p l e of the way that i m p u r i t i e s can d i s t o r t the e q u i l i b r i u m s i z e and shape of d i s l o c a t i o n nodes and t h e r e b y influence the a p p a r e n t s t a c k i n g fault e n e r g y as d e t e r m i n e d b y m e a s u r e m e n t s of the node c u r v a t u r e . Thus, although nodes in p u r e TiC a r e tightly c o n s t r i c t e d , b e i n g r e p r e s e n t a t i v e of a m a t e r i a l having a high s t a c k ing fault e n e r g y , 3 the i n t r o d u c t i o n of t r a c e a m o u n t s of b o r o n by diffusion can cause the nodes to b e c o m e widely extended. In this p a p e r an a n a l y s i s of the nodes in (TiC + B) is p r e s e n t e d and the i m p l i c a t i o n s of the o b s e r v a t i o n s with r e g a r d to s t a c k i n g fault e n e r g y m e a s u r e ments is discussed. E X P E R I M E N T A L PROCEDURE TiC s i n g l e c r y s t a l s a m p l e s with a c a r b o n to t i t a n i u m r a t i o > 0.9 were c o m p r e s s e d in v a c u u m and then heated to 2000~ for 1 h r in the p r e s e n c e of e l e m e n t a l b o r o n . E l e c t r o n m i c r o s c o p e foils w e r e p r e p a r e d by j e t t h i n ning at 100 v with an e l e c t r o l y t e c
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