The high temperature creep behavior of doped tungsten wire
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The c r e e p b e h a v i o r of 0.018 c m d i a m doped t u n g s t e n w i r e has b e e n s t u d i e d over a r a n g e of s t r e s s f r o m 30 to 90 M P a and t e m p e r a t u r e f r o m 2400 to 2800 K. G r a i n a s p e c t r a t i o (gar) had a s t r o n g influence on c r e e p and r u p t u r e of the r e c r y s t a l l i z e d w i r e s , and s e p a r a t e d the c r e e p b e h a v i o r into two r e g i m e s with a t r a n s i t i o n a l gar of about 11 b e t w e e n the two. The low gar r e g i m e showed lower s t r e n g t h and c h a r a c t e r i s t i c s t y p i c a l of g r a i n b o u n d a r y s l i d i n g . In the high gar r e g i m e , p r o p e r t i e s were independent of gar, and e v i dence is p r e s e n t e d to show that c r e e p is g o v e r n e d by d i s l o c a t i o n - b u b b l e d i s p e r s i o n strengthening.
DOPEDt u n g s t e n wire
is the p r e f e r r e d m a t e r i a l for a wide r a n g e of i n c a n d e s c e n t lamp f i l a m e n t s , b e c a u s e of its e x c e l l e n t r e s i s t a n c e to c r e e p d e f o r m a t i o n at the highly e l e v a t e d t e m p e r a t u r e s at which m o s t l a m p s o p e r a t e (2000 to 3200 K). D e s p i t e the e x c e p t i o n a l c h a r a c t e r i s t i c s of this m a t e r i a l and its long h i s t o r y of usage (since 1921) only r e c e n t l y has much a t t e n t i o n b e e n devoted to c h a r a c t e r i z i n g and u n d e r s t a n d i n g the n a t u r e of its elevated t e m p e r a t u r e d e f o r mation. It has long b e e n u n d e r s t o o d that the c r e e p r e s i s t a n c e of doped t u n g s t e n is r e l a t e d to the highly elongated and o v e r l a p p i n g g r a i n s t r u c t u r e developed upon r e c r y s t a l l i z a t i o n * of the wire f i l a m e n t s . R e c e n t l y , much
*Theformationof the overlappinggrainstructurewillbe termedthroughout this paperas "recrystallization,"althoughthe exactnatureof the processis one of secondaryor exaggeratedgraingrowthrather than primaryrecrystallization.1,2 work TM has b e e n conducted in d e m o n s t r a t i n g the i m p o r t a n t r o l e that p o t a s s i u m c o n t a i n i n g b u b b l e s , f o r m e d f r o m p o t a s s i u m i n t r o d u c e d in the doping p r o c e s s , play in the d e v e l o p m e n t of the g r a i n s t r u c t u r e . P r e vious s t u d i e s of c r e e p in r e c r y s t a l l i z e d doped t u n g s t e n by Moon and S t i c k l e r s on coils and Pugh 6 on s t r a i g h t wire d e m o n s t r a t e d a m a r k e d d e c r e a s e in c r e e p r a t e c o m p a r e d to undoped m a t e r i a l . A c t i v a t i o n e n e r g i e s of 540 kJ (130 kcal) and s t r e s s exponents of 8.4 to 25 were found. The s t r e n g t h e n i n g was i n t e r p r e t e d as b e ing p a r t l y a r e s u l t of the l a r g e r g r a i n s i z e of the doped wire and p a r t l y due to d i s p e r s i o n s t r e n g t h e n i n g b y the p o t a s s i u m dope b u b b l e s . Although bo
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