Substructure characteristics of titanium alloy weldments
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III AND
D. W. BECKER
T h e m o r p h o l o g i c a l c h a r a c t e r i s t i c s of p r i o r b e t a g r a i n and s u b g r a i n b o u n d a r i e s in T i 6 A1-6 V-2 Sn and T i - 6 A1-2 Sn-4 Z r - 6 Mo f u s i o n w e l d m e n t s have b e e n s t u d i e d using a unique d e c o r a t i o n t e c h n i q u e . S u b g r a i n b o u n d a r y c h a r a c t e r i s t i c s w e r e o f t en found to d i f f e r f r o m t h o s e of the m o r e c o m m o n l y o b s e r v e d m i c r o s e g r e g a t i o n - r e v e a l e d s o l i d i f i c a t i o n s u b s t r u c t u r e . D i f f e r e n c e s r e s u l t e d both f r o m a l t e r a t i o n s in the t r u e c e l l b o u n d a r y m o r phology and f r o m the c r e a t i o n of s e c o n d a r y s u b g r a i n b o u n d a r i e s d u r i n g w e l d m e n t c o o l ing. D e c o r a t e d s u b g r a i n s t r u c t u r e s c r e a t e d d u r i n g w e l d m e n t cooling w e r e often o b s e r v e d to be s t a b l e in w e l d m e n t s which had u n d e r g o n e s u p e r - t r a n s u s p o s t - w e l d heat t r e a t m e n t , and c o n s e q u e n t l y which had e x p e r i e n c e d s e v e r a l a l l o t r o p i c t r a n s f o r m a t i o n s . A l t h o u g h a c o m p l e t e u n d e r s t a n d i n g of the d e c o r a t i o n t e c h n i q u e r e m a i n s e l u s i v e , c o n c l u s i o n s obtained c o n c e r n i n g the v a l i d i t y of the s u b g r a i n d e c o r a t i o n t e c h n i q u e and w e l d ment s u b s t r u c t u r e c h a r a c t e r i s t i c s w e r e found to be c o m p l e m e n t a r y .
S U B S T R U C T U R E d e l i n e a t i o n in fusion w e l d m e n t s using c o n v e n t i o n a l m e t a l l o g r a p h i c t e c h n i q u e s is n o r m a l l y a t t r i b u t e d to a l l o y i n g e l e m e n t c o m p o s i t i o n a l variations residual from microsegregation experie n c e d d u r i n g c e l l u l a r o r d e n d r i t i c s o l i d i f i c a t i o n . 1-~ T he r e l a t i o n s h i p which e x i s t s b e t w e e n s u c h " s e g r e g a t i o n - i n d u c e d " s u b s t r u c t u r e s and the " t r u e " s u b g r a i n b o u n d a r i e s p r e s e n t d u r i n g w e l d m e n t c o o li n g and u l t i m a t e l y at r o o m t e m p e r a t u r e is not w e l l u n d e r s t o o d . St udi es by Savage and c o - w o r k e r s 6'7 have i n d i c a t e d that the m o r p h o l o g i c a l c h a r a c t e r i s t i c s of s e g r e g a t i o n induced b o u n d a r i e s in a l l o y s which do not e x p e r i e n c e a l l o t r o p i c t r a n s f o r m a t i o n d u r i n g c o o l in g should c l o s e l y r e s e m b l e t h o s e of t r u e s u b g r a i n b o u n d a r i e s . T h e y have s u g g e s t e d that c e l l b o u n d a r i e s c r e a t e d at the c o m p l e t i o n of s o l i d i f i c a t i o n exhibit a m u t u a l l y - s t a b i l i z i n g r e l a t i o n s h i p with solute that s e g r e g a t e d to t h e s e r e g i o n s , t h e r e b y p r o m o
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