On the short time creep rupture
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PUGH
Short t i m e t e n s i l e c r e e p r u p t u r e t e s t s w e r e made on 7 m i l lamp f i l a m e n t w i r e and on 7 rail undoped t u n g s t e n w i r e in the t e m p e r a t u r e r a n g e 2500 to 3100 K. S t e a d y - s t a t e c r e e p r a t e was d e s c r i b e d by an e x p r e s s i o n having the f o r m : = AD(a/E)
m
where A is a c o n s t a n t , D is the s e l f - d i f f u s i o n coefficient, a is the c r e e p s t r e s s , E is the e l a s t i c m o d u l u s , and rn is 5 for undoped t u n g s t e n and 25 for lamp grade w i r e . A c t i v a t i o n e n e r g y for t i m e to f a i l u r e of l a m p wire was found to v a r y f r o m 150 to 185 kcal in the r a n g e 2500 to 3100 K. F a i l u r e mode in this r a n g e was slip at high s t r e s s e s and i n t e r g r a n u l a r b r i t t l e f r a c t u r e at low s t r e s s e s . The v e r y high c r e e p - s t r e n g t h o b s e r v e d for lamp wire was a t t r i b u t e d to a c o m b i n a t i o n of r e s i s t a n c e to diffusional c r e e p at low s t r e s s e s and r e s i s t a n c e to d i s l o c a t i o n c r e e p at high s t r e s s e s . The d i s p e r s o i d s t r u c t u r e of lamp w i r e was suggested as the o r i g i n of s t r e n g t h in both c a s e s .
ALTHOUGHl a m p f i l a m e n t f a i l u r e s
are frequently c r e e p - r u p t u r e f a i l u r e s , little is known about the n a t u r e of this m e c h a n i s m in lamp w i r e . The p e r f o r m a n c e of f i l a m e n t s i n d i c a t e s r e m a r k a b l e high t e m p e r a t u r e c r e e p s t r e n g t h in the w i r e . T h i s s t r e n g t h is a t t r i b u t e d to a unique m i c r o s t r u c t u r e whose h a l l m a r k is a v e r y long, o v e r l a p p i n g g r a i n m o r p h o l o g y . The a r t and s c i e n c e i n volved in the doping and p r o c e s s i n g which is r e s p o n s i b l e have r e c e i v e d much a t t e n t i o n . 1-8 Somewhat l e s s attention has b e e n given to e x p l a i n i n g the r e l a t i o n s h i p between s t r u c t u r e and p r o p e r t i e s and l e a s t a t t e n t i o n has b e e n given to c h a r a c t e r i z a t i o n in t e r m s of specific m e c h a n i cal p r o p e r t i e s . T h i s p a p e r is a i m e d at i m p r o v i n g this b a l a n c e of u n d e r s t a n d i n g by e x a m i n i n g c r e e p and r u p t u r e characteristics.
alloying e l e m e n t s (dopants) were p r e s e n t at the c o n c e n t r a t i o n s : K-80 ppm, AI-10 ppm, Si-3 ppm. Fig. 2, a p h o t o m i c r o g r a p h of the wire after a n n e a l i n g for 2 rain at 3000 K, i l l u s t r a t e s the elongated r e c r y s t a l l i z e d g r a i n s , which have an a s p e c t r a t i o of about 10, and the i r r e g u l a r c h a r a c t e r of the g r a i n b o u n d a r i e s . Fig. 3, a t r a n s m i s sion e l e c t r o n p h o t o m i c r o g r a p h for the wire in the s a m e QUARTZ
SIGHT
STRAINING FRAME'--~---*-
E X P E R I M E N T A L PROCEDURE The t e s t i n g
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