Dislocation damage in tantalum after cyclic loading
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The d i s l o c a t i o n d i s t r i b u t i o n in t a n t a l u m foil a f t e r fatigue t e s t i n g in the r e g i o n of the y i e l d point has b e e n d e t e r m i n e d q u a n t i t a t i v e l y u s i n g thin f i l m e l e c t r o n m i c r o s c o p y . Ch an g es in the m e a s u r e d m i c r o s t r u c t u r a l p a r a m e t e r s as a function of s t r e s s l e v e l , t e s t t e m p e r a t u r e , and n u m b e r of loading c y c l e s have b e e n r e l a t e d to a m i c r o s c o p i c m o d e l of the fatigue p r o c e s s . The m o d e l l e a d s to a c o n s i s t e n t c r i t e r i o n for fatigue f a i l u r e b a s e d on the g r a d u a l growth of i m p e n e t r a b l e subbounda r y groups of i m m o b i l e d i s l o c a t i o n s . T H E dislocation substructure resulting from cyclic l o ad i n g h as b e e n c o m p r e h e n s i v e l y i n v e s t i g a t e d in a n u m b e r of f c c m e t a l s and a l l o y s ~- n although the i n f o r m a t i o n a v a i l a b l e f o r b c c m e t a l s is much l e s s c o m p l e t e . ~2-~4 In m o s t c a s e s the f o r m a t i o n of d i s l o c a t i o n loops o r c e l l s t r u c t u r e h a s b e e n d e m o n s t r a t e d and the q u a l i t a t i v e d e p e n d e n c e of the d a m a g e on the load a m p l i t u d e , n u m b e r of c y c l e s , and t e s t t e m p e r a t u r e h as b e e n studied. In addition to the d i s i o c a t i o n d e n s i t y , q u a n t i t a t i v e m e a s u r e m e n t s have b e e n made of the d e n s i t y of d i s l o c a t i o n loops b9 and the c e l l d i a m e t e r . 6 The p r e s e n t w o r k is an a t t e m p t to t r e a t the d i s l o c a tion population as an e n s e m b l e and not just a s indiv i d u a l d i s l o c a t i o n s i n t e r a c t i n g , s'5'9 A new s t a t i s t i c a l m e t h o d 1~ has been u s e d to d e t e r m i n e the d i s l o c a t i o n d i s t r i b u t i o n in t a n t a l u m a f t e r u n i d i r e c t i o n a l c y c l i c loading. The o b s e r v e d d i s t r i b u t i o n and its d e p e n d e n c e on the loading conditions is a n a l y z e d in t e r m s of m i c r o s t r u c t u r a l p a r a m e t e r s d e r i v e d f r o m the d i s t r i b u t i o n and r e l a t e d to the o v e r a l l b e h a v i o r of the d i s l o c a t i o n population. EXPERIMENTAL
PROCEDURE Material
Annealed polycrystalline tantalum with a grain size of 8 pm was used for this investigation. The composition is given in Table I. The specimen cross-section was 5 x 0.I mm with a preferred orientation of (111)(112> ((112> along the a x i s of the s t r ip ) . Testing U n i d i r e c t i o n a l c y c l i c loading w a s p e r f o r m e d in a s p e c i a l r i g 18 at a f r e q u e n c y of 80 c y c l e s rain -1. V a r i ous s t r a i n a m p l i t u d e s a r e a v a i l a b l e at t e m p e r a t u r e s up to 900~ under a v a c u u m of b e t t e r than 10 -5 t o r r . T e s t i n g was l i m i t e d to m a x i m u m s t r e s s l e v e l s amax of
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