An electron microscopy study of modes of intermetallic precipitation in Ti-Cu alloys
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POLONIS
The p r e c i p i t a t i o n p r o c e s s e s that a c c o m p a n y t h e aging of s u p e r s a t u r a t e d a - t i t a n i u m so l i d s o l u t i o n s c o n t a i n i n g up to 6 wt pet Cu h a v e b e e n studied by thin foil e l e c t r o n m i c r o s c o p y and X - r a y d i f f r a c t i o n t e c h n i q u e s . Two m e c h a n i s m s of d e c o m p o s i t i o n h a v e b e e n i d e n t i f i e d : i) t h e h e t e r o g e n e o u s n u c l e a t i o n and g r o w t h of Ti2Cu at i n t e r f a c e s and i n t e r n a l s u b s t r u c t u r e such as d i s l o c a t i o n s , and ii) the u n i f o r m n u c l e a t i o n of thin, c o h e r e n t , d i s k - s h a p e d p r e c i p i t a t e s which lie on { 1 0 i l }. T h e c o h e r e n t p r e c i p i t a t e s can f o r m with d e n s i t i e s up to 1017 p e r cu c m ; this and many m o r p h o l o g i c a l f e a t u r e s of the p r e c i p i t a t i o n p r o c e s s in T i - C u a r e a n a l o g o u s to the w e l l - k n o w n b e h a v i o r of A1-Cu a l l o y s . Th e c o h e r e n t p r e c i p i t a t e s f i r s t lose c o h e r e n c y along the edge of the disk and then along the flat f a c e s . Th e m e c h a n i s m s by wh i ch t h e s e two p r o c e s s e s o c c u r a r e c o n s i d e r e d in d e t a i l .
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p r e c i p i t a t i o n of i n t e r m e t a l l i c c o m p o u n d s f r o m s u p e r s a t u r a t e d solid s o l u t io n s of the h e x a g o n a l a-phase in t i t a n i u m a l l o y s has c o n s i d e r a b l e p o t e n t i a l f o r d e v e l o p i n g m a t e r i a l s with high y i e l d s t r e n g t h s . Th e a v a i l able l i t e r a t u r e i n d i c a t e s that such p r e c i p i t a t i o n r e a c t i ons h a v e not been studied in d e t a i l . T h e s t r e n g t h e n i n g of T i - C u a l l o y s by i n t e r m e t a l l i c compound p r e c i p i t a t i o n has been e x a m i n e d by D. N. W i l l i a m s , e t a l . 1 T h e s e w o r k e r s showed that a n n e a l i n g at 760~ r e s u l t e d in s o m e s t r e n g t h e n i n g but the s t r e n g t h l e v e l s a t t a i n e d w e r e not high enough to be commercially promising. More recently Gallaugher, T a g g a r t , and P o l o n i s 2 have r e p o r t e d s i g n i f i c a n t inc r e a s e s in m i c r o h a r d n e s s d u r i n g aging at t e m p e r a t u r e s up to 550~ and that r a p id o v e r a g i n g o c c u r s e v e n at 550~ A c o m p a r i s o n of t h e s e r e s u l t s with the e a r l i e r r e s u l t s of W i l l i a m s , e t a l . shows that s t r e n g t h s r e p o r t e d fo r s a m p l e s aged at 760~ r e p r e s e n t g r o s s overaged properties. T h e p r e s e n t w o r k is c o n c e r n e d with i n t e r m e t a l l i c compound p r e c i p i t a t i o n in a - p h a s e t i t a n i u m a l l o y s that a r e s u p e r s a t u r a t e d with r e s p e c t to c o p p e r . T h e s e s u p e r s a t u r a t i o n s h a v e been obtained by q u e n c h i n g f r o m s
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