Formation of Amorphous Metallic Alloys by Solid-State Reactions
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Abstract F o r t h e last 2 5 y e a r s , a m o r p h o u s metallic alloys h a v e b e e n p r e p a r e d by t h e rapid q u e n c h i n g of m e l t s . Recently, n e w m e t h o d s of s y n t h e s i s based o n i s o t h e r m a l solids t a t e r e a c t i o n s h a v e b e e n d e v e l o p e d . It h a s further been s h o w n that the reaction p r o d u c t s can be p r e d i c t e d from free e n e r g y d i a g r a m s t h a t t r e a t t h e a m o r p h o u s alloy as an u n d e r c o o l e d liquid. T h e s e discoveries h a v e o p e n e d n e w w i n d o w s to t h e s y n t h e s i s of novel m e t a s t a b l e m a t e r i a l s , b o t h a m o r p h o u s and c r y s t a l l i n e . T h i s p a p e r r e v i e w s t h e basic c o n c e p t s b e h i n d a m o r p h i z a t i o n by s o l i d - s t a t e r e a c t i o n s and d i s c u s s e s o u r c u r r e n t u n d e r s t a n d i n g of t h e n u c l e a t i o n and g r o w t h of t h e a m o r p h o u s alloy. Introduction At t e m p e r a t u r e s well b e l o w t h e liquidus, binary metallic s y s t e m s w i t h n e g a t i v e h e a t s of m i x i n g a r e m o s t stable in t h e form of c r y s t a l l i n e i n t e r m e t a l l i c s at a d i s c r e t e n u m b e r of s t o i c h i o m e t r i c c o m p o s i t i o n s . In g e n e r a l , s u c h s y s t e m s can also exist as m e t a s t a b l e a m o r p h o u s alloys t h a t h a v e free e n e r g i e s h i g h e r t h a n t h o s e of a c r y s t a l line i n t e r m e t a l l i c of t h e s a m e c o m p o s i t i o n o r a h e t e r o g e n e o u s m i x t u r e of i n t e r m e t a l lics. T h e m e t a s t a b l e s t a t e , w i t h f r e e e n e r g y G2, can be r e a c h e d s t a r t i n g from a s t a t e of h i g h e r free e n e r g y , G3, and c h o o s i n g a t h e r m o d y n a m i c reaction p a t h t h a t r e m o v e s t h e excess free e n e r g y , G3-G2, w h i l e avoiding a reaction i n t o t h e crystalline s t a t e of free e n e r g y G 1 , w h e r e G i < G 2 < G 3 . In t h e m o r e t r a d i t i o n a l m e t h o d s of s y n t h e s i s of a m o r p h o u s metallic alloys, t h e initial h i g h - e n e r g y s t a t e h a s b e e n e i t h e r t h e v a p o r p h a s e o r t h e liquid p h a s e of t h e alloy. In b o t h cases t h e initial s t a t e h a s t h e s a m e chemical c o m p o s i t i o n as t h e a m o r p h o u s alloy to be p r o d u c e d . F u r t h e r m o r e , t h e a t o m i c s t r u c t u r e in t h e liquid p h a s e is r o u g h l y t h e desired o n e . T h e a m o r p h o u s alloy is a c h i e v e d by t h e fast r e m o v a l of t h e excess t h e r m a l e n e r g y at cooling r a t e s b e t w e e n 1 0 4 and 1 0 1 2 K s" 1 . T h i s reaction p a t h d o e s n o t allow sufficient t i m e for t h e chemical p a r t i t i o n i n g t h a t is n e c e s s a r y to n u c l e a t e and g r o w c r y s t a l l i n e alloy p h a s e s . Schwarz and Johnson have recently f o u n d 2 t h a t a m o r p h o u s me
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