Effects of helium on the mechanical properties and microstructure of niobium
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p r o p o s e d D - T f u s i o n power r e a c t o r s will g e n e r a t e high fluxes of 14 MeV n e u t r o n s . B o m b a r d m e n t of m e t a l l i c , i n s u l a t o r , or s u p e r c o n d u c t o r c o m p o n e n t s in the r e a c t o r by t h e s e e n e r g e t i c n e u t r o n s , in most c a s e s , will cause m a t e r i a l d e g r a d a t i o n and will i n c r e a s i n g l y l i m i t fusion r e a c t o r p e r f o r m a n c e with i n c r e a s i n g n e u t r o n fluence. I F i s s i o n n e u t r o n i r r a d i a t i o n is known to change the m e c h a n i c a l p r o p e r t i e s of m a t e r i a l s , u s u a l l y r e d u c i n g component l i f e t i m e and a d v e r s e l y affecting r e a c t o r e c o n o m i c s . F o r this r e a s o n , u n d e r s t a n d i n g and p r e d i c t i n g the effect of fusion n e u t r o n i r r a d i a t i o n on the p r o p e r t i e s of p o t e n t i a l r e a c t o r m a t e r i a l s i s of g r e a t i m p o r t a n c e . High e n e r g y (E ~ 14 MeV) n e u t r o n r a d i a t i o n f r o m fusion r e a c t o r s will cause enhanced damage to r e a c t o r m a t e r i a l s . The d i s p l a c e m e n t damage p e r i n c i d e n t 14 MeV n e u t r o n is g r e a t e r , and c r o s s - s e c t i o n s for (n,a) r e a c t i o n s a l s o a r e higher for f u s i o n n e u t r o n i r r a d i a t i o n than for f i s s i o n n e u t r o n i r r a d i a t i o n . M e c h a n i c a l p r o p e r t i e s of r e a c t o r m a t e r i a l s a r e changed by r e l a t i v e l y s m a l l c o n c e n t r a t i o n s of h e l i u m . 2-5 B e c a u s e of high (n,a) c r o s s - s e c t i o n s , l a r g e c o n c e n t r a t i o n s of h e l i u m will be produced in fusion r e a c t o r m a t e r i a l s i r r a d i ated by 14 MeV n e u t r o n s in a r e l a t i v e l y s h o r t t i m e span. A c o n c e n t r a t i o n of about 25 appm h e l i u m will be produced i n t e r n a l l y in n i o b i u m by a fusion n e u t r o n flux of ~1018 n e u t r o n s m-2s -1 with one y e a r ' s exposure. H e l i u m a t o m s produced in a host m a t r i x having low h e l i u m s o l u b i l i t y a r e t h e r m o d y n a m i c a l l y u n s t a b l e and have a high d r i v i n g force to c o a l e s c e in the m i c r o s t r u c t u r e at v a r i o u s defects. T h u s , h e l i u m a t o m s tend to m i g r a t e to d i s l o c a t i o n s , p r e c i p i t a t e s , g r a i n b o u n d a r i e s , i m p u r i t y p a r t i c l e s , and m i c r o c r a c k s where they c o a l e s c e to f o r m b u b b l e s . T h e s e h e l i u m induced defects g e n e r a l l y cause e m b r i t t l e m e n t . One method for introducing h e l i u m into the m i c r o W. L. BARMORE, E. L. RAYMOND,and C. J. ECHER are Semor Scientist, Technical Specialist,and Technical Associate, respectively, Metals and Ceramics Division,and R. R. VANDERVOORT is Leader, Materials Test and Evaluation, Section of Materials Engineering Division, all wxth Lawrence Livermore
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