The relationship between discontinuous yielding and cyclic behavior in polycrystalline molybdenum
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The fatigue p r o p e r t i e s of p o l y c r y s t a l l i n e m o l y b d e n u m at room t e m p e r a t u r e have b e e n d e t e r m i n e d by axial t e n s i o n - c o m p r e s s i o n t e s t s in both s t r e s s and s t r a i n c o n t r o l m o d e s to give lives of up to 106 c y c l e s . U n d e r the a p p r o p r i a t e conditions, the e n d u r a n c e l i m i t can be c o n s i d e r a b l y reduced, e.g., the s t r e s s to cause f a i l u r e in 106 cycles d e c r e a s e s from ~42,000 psi at 5 cps to -28~000 psi at 0.5 cps. The fatigue r e s i s t a n c e as a s s e s s e d by e i t h e r s t r a i n c o n t r o l or by s t r e s s c o n t r o l is d i s c u s s e d in r e l a t i o n to the t e n s i l e and c r e e p b e h a v i o r , and it is concluded that the r e d u c t i o n in the long life fatigue r e s i s t a n c e with d e c r e a s e in the c y c l i c f r e q u e n c y is due to the time dependent g e n e r a t i o n of a m o b i l e d i s l o c a t i o n population. The p r e v i o u s l y a c c e p t e d value of 0.6 for the r a t i o FL/UTS f o r bcc m e t a l s can be a t t r i b u t e d to this f r e q u e n c y effect. A l o w e r value for this r a t i o , 0.4, can be d e r i v e d f r o m the high f r e q u e n c y data by the use of an e x t r a p o l a t i o n technique. This lower value is c o n f i r m e d by long life, low f r e q u e n c y t e s t s , in a g r e e m e n t with the value for fcc m e t a l s . The i m p o r t a n c e of obtaining fatigue data u n d e r c o m p a r a b l e s t a t e s of mobile d i s l o c a t i o n d e n s i t y i s e m p h a s i z e d , p a r t i c u l a r l y for m a t e r i a l s e x h i b i t i n g p r o n o u n c e d t e n sile yield points, in o r d e r that m e a n i n g f u l c o m p a r i s o n s can be made b e t w e e n different m a t e r i a l s .
T H E b e h a v i o r of fcc m e t a l s u n d e r cyclic loading cond i t i o n s has b e e n the s u b j e c t of n u m e r o u s i n v e s t i g a t i o n s and both single and p o l y c r y s t a l s have b e e n c h a r a c t e r ized in t e r m s of s u b s t r u c t u r a l changes and c r a c k growth p r o c e s s e s . 1-3 C o m p a r a b l e s t u d i e s on bcc m e t a l s have not b e e n made n e a r l y to the s a m e extent. Most of the a v a i l a b l e i n f o r m a t i o n on bcc m e t a l s c o n s i s t s p r i m a r i l y of e i t h e r f r a c t u r e m e c h a n i c s or e n v i r o n m e n t a l effects in i r o n - b a s e alloys. The s m a l l a m o u n t of work which has b e e n r e p o r t e d on bcc m e t a l s o t h e r than the i r o n b a s e alloys also is c o n c e r n e d p r i m a r i l y with these two p a r t i c u l a r a s p e c t s . 4-9 In c o n s e q u e n c e , r e l a t i v e l y little is known of the f u n d a m e n t a l changes which o c c u r d u r ing fatigue in bcc m e t a l s , c o m p a r e d to those for fcc metals. T h e r e a r e three b a s i c d i f f e r e n c e s between the m e c h a n i c a l r e s p o n s e
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