Fatigue behavior of niobium-hydrogen alloys
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AND
N. S. STOLOFF
T h e e f f e c t s of h y d r o g e n on r o o m t e m p e r a t u r e fatigue b e h a v i o r of n i o b i u m w e r e i n v e s t i gated u n d e r both high f r e q u e n c y s t r e s s c o n t r o l and low f r e q u e n c y s t r a i n c o n t r o l conditions, in a i r . H y d r o g e n m a r k e d l y i m p r o v e d the fatigue life in high f r e q u e n c y t e s t s , while low f r e q u e n c y t e s t s r e s u l t e d in d e c r e a s e d fatigue life with i n c r e a s i n g h y d r o g e n content. N o t c h e s in h y d r o g e n - c h a r g e d a l l o y s r e d u c e d high c y c l e life s i g n i f i c a n t l y but had l i t t l e eff e c t on low c y c l e t e s t s . F r a c t u r e s u r f a c e s of a n n e a l e d niobium m a i n l y e x h i b i t e d s t r i a t i o n s , with n u m e r o u s c r a c k s o r i g i n a t i n g at t r o u g h s of s t r i a t e d b a n d s in both s t r e s s and s t r a i n c o n t r o l t e s t s . T h e f r a c t u r e m o d e f o r a l l o y s with h y d r o g e n in s o l u t i o n was mixed, with s t r i a t i o n s i n t e r s p e r s e d with c l e a v a g e f a c e t s at high f r e q u e n c i e s but g e n e r a l l y c l e a v a g e s t e p s at low f r e q u e n c i e s . F o r the h y d r i d e d a l l o y s , d i s t i n c t i v e s t e p s of m i x e d d u c t i l e - b r i t t l e a p p e a r a n c e w e r e r e v e a l e d u n d e r high f r e q u e n c y conditions, but l a r g e c l e a v a g e f a c e t s only w e r e o b s e r v e d f o r low f r e q u e n c y t e s t s . T h e r e s u l t s a r e d i s c u s s e d in t e r m s of the e f f e c t s of h y d r o g e n on the c y c l i c s t r a i n h a r d e n i n g r a t e , a s w e l l a s on fatigue s t r e n g t h and d u c t i l i t y of n i o b i u m .
I. INTRODUCTION N U M E R O U S i n v e s t i g a t i o n s have b e e n d i r e c t e d to the e f f e c t s 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). H y d r o g e n in solution o r as h y d r i d e p r e c i p i t a t e s in t h e s e m e t a l s i n c r e a s e s t h e i r s t r e n g t h 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 a l s o r e d u c e s t h e i r d u c t i l i t y (hydrogen e m b r i t t l e m e n t ) at low t e m p e r a t u r e . 1-3 The influence of h y d r o g e n on fatigue b e h a v i o r , h o w e v e r , has not b e e n e x t e n s i v e l y i n v e s t i g a t e d in e i t h e r b c c o r hcp m e t a l s . An e a r l y i n v e s t i g a t i o n of t i t a n i u m and T i - 8 wt pct Mn a l l o y s by B e r g e r e t al 4 r e v e a l e d that h y d r o g e n i m p r o v e d high c y c l e fatigue life, in a c c o r d with an inc r e a s e in t e n s i l e s t r e n g t h . L a t e r , B e e v e r s 5 c o n f i r m e d that h y d r i d e s i n c r e a s e life of a t i t a n i u m , a t t r i b u t i n g t h i s to the r e s t r i c t i o n of twin f o r m a t i o n by the h y d r
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