The influence of microstructure on fracture of drawn tungsten wire
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F. BACON,
AND
D. LEE
The m i c r o s t r u c t u r e s and longitudinal f r a c t u r e r e s i s t a n c e s of 0.635 m m d i a m l a m p - d o p e d and undoped tungsten w i r e w e r e e x a m i n e d in the a s - d r a w n condition and a f t e r a n e a l i n g at t e m p e r a t u r e s b e t w e e n 600 and 1500~ A v a r i e t y of e x p e r i m e n t a l t e c h n i q u e s w e r e e m ployed, including A u g e r E l e c t r o n S p e c t r o s c o p y , Scanning E l e c t r o n M i c r o s c o p y , 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 and a newly d e v e l o p e d m e c h a n i c a l t e s t i n g technique. The longit u d i n a l f r a c t u r e mode was i n t e r g r a n u l a r for a l l w i r e s and a s e c o n d p h a s e was o b s e r v e d on the g r a i n b o u n d a r i e s of a l l doped w i r e s . High c o n c e n t r a t i o n s of the dopant e l e m e n t p o t a s s i u m w e r e p r e s e n t on the f r a c t u r e s u r f a c e s of doped w i r e s and e x p e r i m e n t a l e v i d e n c e was o b t a i n e d which s u g g e s t s they m a y be due to p o s t f r a c t u r e s u r f a c e diffusion. Doped w i r e s d e m o n s t r a t e d i n c r e a s i n g a m o u n t s of s t r u c t u r e c o a r s e n i n g up to 1500~ w h e r e a s l a r g e e q u i a x e d g r a i n s w e r e f o r m e d in undoped w i r e s a n n e a l e d at 1300 and 1500~ T h e longitudin a l f r a c t u r e r e s i s t a n c e of undoped w i r e was u n a l t e r e d by a n n e a l i n g at 1050~ and below, but d e c r e a s e d d r a m a t i c a l l y a f t e r a n n e a l i n g at 1300 and 1500~ In c o n t r a s t the f r a c t u r e r e s i s t a n c e of doped w i r e d e c r e a s e d a f t e r a n n e a l i n g at 1050 and 1300~ but i n c r e a s e d a f t e r a n n e a l i n g at 1500~ F r a c t u r e r e s i s t a n c e is d i s c u s s e d in t e r m s of m i c r o s t r u c t u r e and f r a c t u r e s u r f a c e c h e m i s t r y .
HEAVILYd r a w n
tungsten wire displays considerable d u c t i l i t y u n d e r a v a r i e t y of d e f o r m a t i o n p r o c e s s i n g m e t h o d s s u c h a s d r a w i n g and coiling. H o w e v e r , this w i r e is s u s c e p t i b l e to b r i t t l e s e p a r a t i o n a l o n g longitudinal grain boundaries. Longitudinal intergranular c r a c k s m a y d e v e l o p when the s t r e s s component n o r m a l to the w i r e a x i s is p r e d o m i n a n t and e x c e e d s c e r t a i n t h r e s h o l d l e v e l s . F o r e x a m p l e , f a i l u r e of w i r e s u n d e r s i m p l e s h e a r i n g conditions is i n v a r i a b l y a c c o m p a n i e d by l o n g i t u d i n a l i n t e r g r a n u l a r c r a c k i n g and w i r e s with s u r f a c e flaws s e p a r a t e l o n g i t u d i n a l l y along g r a i n b o u n d a r i e s when they a r e s i m u l t a n e o u s l y bent and s t r e t c h e d . 1 V a r g a has e s t i m a t e d that g r a i n b o u n d a r y s t r e n g t h is only about 14 pct of w i r
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