Effect of hydrogen on fatigue crack propagation in vanadium
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T H E effects of h y d r o g e n on monotonic m e c h a n i c a l b e h a v i o r of the group Va r e f r a c t o r y m e t a l s (Ta, V and Nb) have been e x t e n s i v e l y i n v e s t i g a t e d . S t r e n g t h ening by hydrogen in solution and hydride p r e c i p i t a t e s in t h e s e m e t a l s occur within c e r t a i n t e m p e r a t u r e r a n g e s , but r e d u c e d ductility (hydrogen e m b r i t t l e merit) at low t e m p e r a t u r e also has been o b s e r v e d . 1-3 Little attention, however, has been d i r e c t e d to the influence of hydrogen on cyclic d e f o r m a t i o n and fatigue c r a c k p r o p a g a t i o n in the r e f r a c t o r y m e t a l s . Wilcox 4 showed that h y d r o g e n in solution and in the f o r m of h y d r i d e s s i g n i f i c a n t l y r e d u c e d the fatigue life of t a n t a l u m and concluded that hydrogen e m b r i t t l e m e n t was a s s o c i a t e d with i n c r e a s e d ease of both c r a c k i n i t i a t i o n and p r o p a g a t i o n . In c o n t r a s t , Lee and Stoloff5 r e c e n t l y r e p o r t e d s i g n i f i c a n t i m p r o v e m e n t s in high cycle t e n s i o n - c o m p r e s s i o n (T-C) fatigue life when hydrogen was i n t r o d u c e d into v a n a d i u m as h y d r i d e s . F u r t h e r m o r e , fatigue l i v e s of hydrided v a n a d i u m were s e v e r a l t i m e s g r e a t e r than for an alloy with 132 ppm hydrogen in solution when t e s t e d at a s i m i l a r s t r e s s level. The p r e s e n t i n v e s t i g a t i o n extends the work of Lee and Stoloff ~ to take into account the s e p a r a t e effects of hydrogen content on the time for c r a c k i n i t i a t i o n and the r a t e of c r a c k p r o p a g a t i o n in v a n a d i u m . The study u t i l i z e s six different h y d r o g e n contents in p o l y c r y s t a l l i n e vanadium; unalloyed, two alloys with hydrogen in solution and t h r e e hydrided alloys. S t r e s s r a n g e s and m e a n s t r e s s e s have been i n v e s t i g a t e d in s t r e s s c o n t r o l l e d t e s t s at r o o m t e m p e r a t u r e . F a t i g u e c r a c k growth c h a r a c t e r i s t i c s are c o m p a r e d with t e n s i l e p r o p e r t i e s and f r a c t u r e modes were o b s e r v e d by m e a n s of m e t a l l o g r a p h y and s c a n n i n g e l e c t r o n microscopy. E X P E R I M E N T A L PROCEDURE Vanadium was r e c e i v e d f r o m Wah Chang Corp. in the f o r m of annealed sheet with a g r a i n size ~50 ~ m . F a t i g u e s p e c i m e n s of the s i n g l e - e d g e notch type: 0.3
c m thick • 2.54 c m gage length • 1.43 cm wide, (notch depth = 0.127 cm) were m a c h i n e d f r o m the o r i g i n a l m a t e r i a l . Following m e c h a n i c a l p o l i s h i n g to 6 ~ m d i a m o n d compound, e l e c t r o l y t i c p o l i s h i n g was c o n ducted in a HzSO4-H20 solution, applying 30 volts at 0~ All s a m p l e s were then annealed for two h o u r s at 760~ in a v a c u u m of 10 -7 t o r r to obtain the
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