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I N the L12 s t r u c t u r e , s t a c k i n g faults a r e of t h r e e b a s i c t y p e s - - s u p e r l a t t i c e i n t r i n s i c (or e x t r i n s i c ) faults, a n t i p h a s e boundary f a u l t s , and complex faults. S u p e r lattice i n t r i n s i c (S-ISF) and e x t r i n s i c (S-ESF) s t a c k i n g faults a r e r e l a t e d to L12 through s h e a r d i s p l a c e m e n t s of the type {111}~-(112), or c l i m b d i s p l a c e m e n t s of the type {111}~-(111), and have the following c h a r a c t e r istics : 4 l a y e r s of SnNia (DO~9) s t r u c t u r e S-ESF 7 l a y e r s of TiNis fl3024) s t r u c t u r e S - I S F + S - E S F -- 10 l a y e r s of VCos s t r u c t u r e S-ISF

e x t r i n s i c s t a c k i n g fault. T h u s , a d i s l o c a t i o n with net B u r g e r s v e c t o r 2BA = a[1]0], u s i n g the c u s t o m a r y o r i e n t a t i o n for the r e f e r e n c e t e t r a h e d r o n ABCD, may be r e p r e s e n t e d as 2BA = 2B6 + 26A for an i n t r i n s i c d i s s o c i a t i o n , and 2BA = 2B6 + 26A for an e x t r i n s i c dissociation. 1) O r d i n a r y G l i s s i l e P a i r s - - a ( 1105 B u r g e r s Vector The f u n d a m e n t a l d i s s o c i a t i o n s a r e :7-9

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In the s t a c k i n g s e q u e n c e s a p p r o p r i a t e for such ABa p h a s e s , see T a b l e I, each A atom is s u r r o u n d e d by 12 n e a r e s t neighbor B a t o m s , i . e . in all c a s e s the c o o r d i n a t i o n n u m b e r (CN = 12) is ideal. ~The e n e r g i e s of S - I S F and S - E S F , t h e r e f o r e , should be r e l a t i v e l y low, s i n c e t h e r e a r e no n e a r e s t n e i g h b o r bond v i o l a t i o n s a s s o c i a t e d with such s t a c k i n g f a u l t s . Antiphase bounda r y (APB) faults a r e r e l a t e d to LI~ through d i s p l a c e m e n t s of the type a / 2 ( l 1 0 ) . The e n e r g y of { l l l } a / 2 (1105 s h e a r - t y p e APB should be c o n s i d e r a b l y l a r g e r than that of S - I S F and S - E S F f a u l t s , since a c e r t a i n f r a c t i o n of the a t o m s in the APB have i n c o r r e c t n e a r est n e i g h b o r s . The e n e r g y of an APB depends s t r o n g l y on c r y s t a l l o g r a p h i c o r i e n t a t i o n and is a m i n i m u m for a { 1 0 0 } a / 2 ( 1 1 0 5 s h e a r - t y p e b o u n d a r y J '3 Complex (CF) f a u l t s a r e r e l a t e d to L12 through {111}~ 4112) s h e a r - t y p e d i s p l a c e m e n t s , and can be r e g a r d e d a s the s u p e r p o s i t i o n of antiphase b o u n d a r y and s u p e r l a t tice i n t r i n s i c fault, i . e . , CF = APB + S - I S F . T h i s r e l a t i o n s h i p i n d i c a t e s that the e n e r g y of CF should be somewhat l a r g e r than that of APB. I. DISSOCIATED DISLOCATIONS In what follows B u r g e r s v e c t o r s of s u p e r l a t t i c e d i s l o c a t i o n s a r e defined after the m a n n e r proposed by F r a n k 4 and T h o m p s o n . 5 F u r t h e r m o r e , following W e e r t m a n , 8 to d