Dislocation damping in high purity molybdenum
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T H E m e c h a n i c a l p r o p e r t i e s of the bcc m e t a l s a r e known for t h e i r s e n s i t i v i t y to s m a l l a m o u n t s of i n t e r s t i t i a l i m p u r i t i e s . L a r g e i n c r e a s e s in the yield and flow s t r e s s e s and d e c r e a s e s in ductility have been a t t r i b u t e d to p r e s e n c e of i n t e r s t i t i a l i m p u r i t y a t o m s . M e t a l l u r g i c a l p h e n o m e n a , e . g . , yield point, s t r a i n aging, and the d u c t i l e - t o - b r i t t l e t r a n s i t i o n in the f r a c t u r e b e h a v i o r with d e c r e a s i n g t e m p e r a t u r e , have been a s s o ciated with the i n t e r s t i t i a l i m p u r i t i e s and t h e i r d i s t r i b u t i o n s in the bcc m e t a l s . Since the m o b i l i t y of d i s l o c a t i o n s in a m a t e r i a l p l a y s a vital r o l e in d e t e r m i n i n g its m e c h a n i c a l p r o p e r t i e s , the i n t e r a c t i o n and p i n n i n g of d i s l o c a t i o n s by i n t e r s t i t i a l i m p u r i t y a t o m s are important processes. A r e c e n t review paper by C h r i s t a i n 1 d i s c u s s e s much of the work c a r r i e d out s i n c e the e a r l y i n v e s t i g a t i o n of Stein e t al. 2 which d e m o n s t r a t e d the potency of the i n t e r s t i t i a l i n t e r a c t i o n s on the m e c h a n i c a l p r o p e r t i e s and dislocation m o b i l i t y . C a r p e n t e r and Baker s'4 have shown that the use of i n t e r n a l f r i c t i o n to m e a s u r e the d i s l o c a t i o n damping is both a s e n s i t i v e and a c o n v e n ient tool to study d i s l o c a t i o n i n t e r s t i t i a l i n t e r a c t i o n s . Unlike m e c h a n i c a l t e s t i n g , damping m e a s u r e m e n t s can continuously m o n i t o r changes caused by t h e s e i n t e r a c t i o n s , and at s m a l l s t r a i n a m p l i t u d e s the motion of the d i s l o c a t i o n is g e n e r a l l y r e v e r s i b l e . In many c a s e s the diffusional energy of the i n t e r s t i t i a l r e s p o n s i b l e for p i n n i n g of the d i s l o c a t i o n s can be d e t e r m i n e d f r o m o b s e r v i n g a d e c r e a s e in the damping with t i m e . The r e s u l t s of a study of the d i s l o c a t i o n i n t e r s t i t i a l i n t e r a c t i o n s in m o l y b d e n u m u s i n g d i s l o c a t i o n damping t e c h n i q u e s a r e p r e s e n t e d in this p a p e r . A d i s l o c a t i o n p i n n i n g p r o c e s s was o b s e r v e d between 100 ~ and 300~ The a c t i v a t i o n e n e r g y of the p r o c e s s has b e e n d e t e r m i n e d . The a c t i v a t i o n e n e r g y c o m p a r e s f a v o r a b l y with
the diffusional e n e r g y of n i t r o g e n in m o l y b d e n u m . A m a x i m u m was o b s e r v e d in the a m p l i t u d e - d e p e n d e n t damping which s a t i s f i e s p r e d i c t i o n s of the Granato and Lficke theory 5'6 of d i s l o c a t i o n damping. The r e s u l t s a r e in q u
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