Atomic ordering and structural transformation in the V-Co-Ni ternary alloys
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r e c e n t y e a r s an i n t e r e s t i n g f a m i l y of c l o s e - p a c k e d o r d e r e d s t r u c t u r e s ~ h a s b e e n d i s c o v e r e d in m a n y b i n a r y and p s e u d o b i n a r y a l l o y s y s t e m s . 2-8 All t h e s e s t r u c t u r e s a r e b u i l t up f r o m the c l o s e - p a c k e d o r d e r e d ABa l a y e r s but d i f f e r in s t a c k i n g s e q u e n c e . A g e n e r a l c o r r e l a t i o n b e t w e e n the s t a c k i n g c h a r a c t e r and the e l e c t r o n c o n c e n t r a t l o n (defined a s t h e a v e r a g e p e r a t o m of the n u m b e r of e l e c t r o n s o u t s i d e the i n e r t g a s s h e l l s ) in a l l o y s h a s b e e n n o t e d : 1-4 with an i n c r e a s e in e l e c t r o n c o n c e n t r a t i o n the s t a c k i n g c h a r a c t e r c h a n g e s f r o m p u r e l y c u b i c , through d i f f e r e n t t y p e s of o r d e r e d m i x t u r e s of h e x a g o n a l to cubic l a y e r s , to p u r e l y h e x a g o n a l . The i n t e r m e d i a t e s t r u c t u r e s b e t w e e n the cubic CusAu and h e x a g o n a l NisSn s t r u c t u r e s contain m o r e than 15 l a y e r s in an o r d e r e d unit c e l l . 5,e The c r y s t a l l o g r a p h y and s t a b i l i t y of the v a r i o u s i n t e r m e d i a t e s t r u c t u r e s have b e e n s t u d i e d e x t e n s i v e l y , but v e r y l i t t l e w o r k ' h a s b e e n done on the k i n e t i c a s p e c t s ; c o n s e q u e n t l y s p e c i f i c knowledge i s l a c k i n g about a t o m i c o r d e r i n g and s t r u c t u r a l t r a n s f o r m a t i o n in the c o m p l i c a t e d t r a n s i t i o n structures. F o r m o s t of the o r d e r e d c o m p o u n d s , t h e i r c r i t i c a l o r d e r i n g t e m p e r a t u r e (Tc) r e a c h e s the m e l t i n g point, and t h e r e i s no w a y to o b t a i n the d i s o r d e r e d s t a t e t h r o u g h a c o n v e n t i o n a l h e a t t r e a t m e n t . A l s o , the r a p i d r a t e of o r d e r i n g ' often l i m i t s a c o m p l e t e e x a m i n a t i o n of the t r a n s i t i o n p r o c e s s e s . In t h i s s t u d y , the V - C o - N i t e r n a r y a l l o y s w e r e c h o s e n f o r the following r e a s o n s : 1) The t e r n a r y 8'9 and the p s e u d o b i n a r y VCos-VNi s s y s t e m s ~'a have b e e n s t u d i e d e x t e n s i v e l y ; t h e i r c o n s t i t u t i o n a l d i a g r a m s have b e e n r e p o r t e d to contain v a r i o u s c l o s e - p a c k e d o r d e r e d s t r u c t u r e s , a,6 2) The p h a s e t r a n s f o r m a t i o n i n f o r m a t i o n s in the b i n a r y o r d e r e d c o m p o u n d s VCos and VNi s a r e a v a i l a b l e , " 1o, u and can be u s e d to c o m p a r e with t h o s e o b t a i n e d in the t e r n a r y a l l o y s . 3) The p r e l i m i n a r y d a t a i n d i c a t e d t h a t the k i n e t i c p r o c e s s in the t e r n a r y a l l o y s is slow and that the h i g h - t e m p e r a t u r e d i s o r
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