Ordering reactions in unfamiliar systems
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the o r d e r i n g of a t o m s in a s o l i d c a n have a g r e a t influence upon p r o p e r t i e s , i n v e s t i g a t i o n s of o r d e r d i s o r d e r t r a n s i t i o n s a r e f r e q u e n t l y r e q u i r e d to e s t a b l i s h the s u i t a b i l i t y of m a t e r i a l s in s t r u c t u r a l , e l e c t r i c a l o r m a g n e t i c a p p l i c a t i o n s . Such s t u d i e s a r e a l s o v a l u a b l e in the d e v e l o p m e n t of p h a s e - t r a n s f o r m a t i o n t h e o r y b e c a u s e m o s t o r d e r - d i s o r d e r r e a c t i o n s a p p e a r to be d e s c r i b e d b y s i m p l e r p h y s i c a l m o d e l s than can be a p p l i e d to o t h e r k i n d s of t r a n s i t i o n s . F o r t h i s r e a s o n , c o m p a r i son of t h e o r y and e x p e r i m e n t can be r e m a r k a b l y d e tailed. It is n a t u r a l that m a t e r i a l s about which the m o s t is known, the c o m m o n o r " c l a s s i c a l " a l l o y s , a r e c h o s e n f o r the e x p e r i m e n t a l c o n f i r m a t i o n o r the d e v e l o p m e n t of t h e o r y . Thus, the C u - A u s y s t e m h a s b e e n the o v e r w h e l m i n g s e l e c t i o n for o b s e r v a t i o n s of o r d e r i n g k i n e t i c s . In fact, it t u r n s out that the two r e a s o n s for s t u d y ing o r d e r - d i s o r d e r t r a n s i t i o n s , n a m e l y the m e a s u r e m e n t of p r o p e r t i e s o r the e x t e n s i o n of t h e o r y , a l m o s t i n v a r i a b l y s e p a r a t e the u n c o m m o n f r o m the c o m m o n ; the u n c o m m o n i s r a r e l y e m p l o y e d to d i s c o v e r p r i n c i p l e s . In o r d e r to t e s t the p e r t i n e n c e of " c l a s s i c a l " p h a s e t r a n s f o r m a t i o n t h e o r y to o r d e r i n g r e a c t i o n s in l e s s - c o m m o n a l l o y s f o r which i n f o r m a t i o n is s p a r s e , we r e s o r t in p a r t to m e t a l l u r g i c a l l y unfamiliar s y s t e m s such a s n o n m e t a l l i c s o l i d s o l u t i o n s and c o m p o u n d s , in the conviction that s o m e light will b e s h e d on m e t a l l u r g i c a l c o n c e r n s a s well. T h e r e w i l l b e no a t t e m p t h e r e at c o m p l e t e n e s s . What we s h a l l s t r i v e for is a s u f f i c i e n t s a m p l e of the r e c e n t l i t e r a t u r e to i n d i c a t e the s t a t e of knowledge in l e s s c o m m o n and u n f a m i l i a r s y s t e m s , e s p e c i a l l y with r e s p e c t to t r a n s f o r m a t i o n m e c h a n i s m s and i d e n t i f i c a t i o n of the c r i t i c a l i s s u e s . We b e g i n with a b r i e f s u m m a r y of the c o n v e n t i o n a l o r d e r - d i s o r d e r w i s d o m which is R. H. RICHMAN and R. G. DAVIES are on the Scientific Research Staff, Ford Motor Company, Dearborn, Mich. 48121. This paper is based on a presentation made at a symposium on "Phase Transformations in Less Common Metals: A Dialogue," held at the Fall Meetin
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