Active eutectoid decomposition in a near-eutectoid zirconium-copper alloy
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1. INTRODUCTION W HE e u t e c t o i d r e a c t i o n is e x h i b i t e d by many b i n a r y z i r c o n i u m and t i t a n i u m b a s e a l l o y s y s t e m s . In many of t h e s e , the e u t e c t o i d c o m p o s i t i o n s c o r r e s p o n d to low l e v e l s of s o l u t e c o n c e n t r a t i o n so that it is p o s s i b l e to obtain s t r u c t u r e s with l a r g e v o l u m e f r a c t i o n s of the e u t e c t o i d p r o d u c t even in dilute a l l o y s . Some of the b i n a r y s y s t e m s in which the e u t e c t o i d r e a c t i o n o c c u r s n e a r the z i r c o n i u m o r t i t a n i u m r i c h ends 1 a r e l i s t e d in T a b l e I. J a f f e e 2 has c l a s s i f i e d e u t e c t o i d r e a c t i o n s in t i t a n i u m a l l o y s into two c a t e g o r i e s : a c t i v e and s l u g gish. T h e s y s t e m s t i t a n i u m - nickel, t i t a n i u m - c o p p e r , and t i t a n i u m - s i l i c o n b e l o n g to the f o r m e r c l a s s . Comp a r e d to t h e s e , the e u t e c t o i d d e c o m p o s i t i o n of the high t e m p e r a t u r e b e t a - p h a s e in many o t h e r t i t a n i u m b a s e e u t e c t o i d s y s t e m s is r e l a t i v e l y s l u g g i s h . It a p p e a r s that the e u t e c t o i d r e a c t i o n is a c t i v e in t h o s e t i t a n i u m b a s e a l l o y s y s t e m s w h e r e the e u t e c t o i d t e m p e r a t u r e is high, the e u t e c t o i d c o m p o s i t i o n is s o l u t e l e a n and the i n t e r m e t a l l i c p h a s e r e s u l t i n g f r o m the r e a c t i o n is r i c h in the b a s e m e t a l . Not much i n f o r m a t i o n is a v a i l a b l e on a c t i v e e u t e c t o i d d e c o m p o s i t i o n in z i r c o n i u m b a s e a l l o y s y s t e m s . H o w e v e r , in view of the e x p e r i e n c e gained with t i t a n i u m a l l o y s , one would e x p e c t that the e u t e c t o i d r e a c t i o n would b e e x t r e m e l y r a p i d in s y s t e m s like z i r c o n i u m - c o p p e r , z i r c o n i u m - n i c k e l , z i r c o n i u m s i l v e r , and z i r c o n i n m - s i l i c o n . It has b e e n r e p o r t e d 3 that in the t i t a n i u m - c o p p e r s y s t e m , the e u t e c t o i d r e a c t i o n o c c u r s at the p r i o r b e t a g r a i n b o u n d a r i e s in n e a r e u t e c t o i d and h y p e r e u t e c t o i d a l l o y s even d u r i n g r a p i d b e t a quenching; the r e m a i n i n g p o r t i o n s of the g r a i n s u n d e r g o the m a r t e n s i t i c t r a n s f o r m a t i o n . 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 of b e t a quenched t i t a n i u m - c o p p e r a l l o y s of n e a r e u t e c t o i d c o m p o s i t i o n s has b e e n c a r r i e d out b y W i l l i a m s , P o l o n i s and T a g g a r t . 4 T h e y have shown that the d e c o m p o s i t i o n p r o d u c t is a l a m e l l a r a g g r e g a t e of the a l p h a - and the T i z C u - p h a s e s . T h e y have a l s o e s t a b l i
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