Recent Advances in Understanding Helium Embrittlement in Metals
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Figure 1: A t o m p o s i t i o n s s h o w i n g that five h e l i u m a t o m s (small circles) m a y p u s h a nickel atom (purple) off its normal lattice site. T h e large blue circles represent the s u r r o u n d i n g n i c k e l a t o m s . In t h e t o p d r a w i n g (a) the nickel a t o m s are in their normal lattice s i t e s . Upon atomic relaxat i o n t h e nickel a t o m s m o v e away from the h e l i u m cluster (bottom drawing) w i t h the purple n i c k e l a t o m m o v i n g to an i n t e r stitial p o s i t i o n . T h e h e l i u m cluster thus formed is v e r y s t r o n g l y bound and acts as a n u c l e u s for further g r o w t h .
a c t i o n s . Additional helium a t o m s c o n t i n u e this p r o c e s s , i n c r e a s i n g t h e binding e n e r g y t o o v e r 2 eV. D e t a i l s of t h e s e and o t h e r a t o m i s t i c h e l i u m calculations a p p e a r in t h e r e v i e w by Wilson et al. 1 0
Figure 2: Pre-cracked stainless steel s a m ples exposed to (a) h y d r o g e n or (b) tritium s h o w that tritium substantially d i m i n i s h e s t h e resistance of metals to crack g r o w t h due to the h e l i u m that forms w i t h i n the metal by tritium decay. Pre-existing cracks in stainless steel s a m p l e s e x t e n d e d during tritium exposure, w h e r e a s they did not in identical samples exposed to h y d r o g e n and stressed to the same level.
Recent Experiments T w o recent experiments have produced m o r e evidence t h a t helium g e n e r a t e d from t r i t i u m decay can r e s u l t in m e t a l e m b r i t t l e continued
m e n t n t o r s l i g h t l y a b o v e room t e m p e r a t u r e . In t h e first e x p e r i m e n t a pre-cracked s t a i n l e s s steel s a m p l e is e x p o s e d t o h i g h p r e s s u r e t r i t i u m at r o o m t e m p e r a t u r e long e n o u g h to g e n e r a t e 400 a t o m i c ppm of h e l i u m in t h e m a t e r i a l n e a r t h e crack tip. Figure 2b s h o w s t h e crack t h a t formed during the tritium e x p o s u r e while the s a m p l e w a s at c o n s t a n t s t r e s s . For c o m parison, Figure 2a s h o w s an identical sample e x p o s e d to h y d r o g e n and s t r e s s e d to t h e s a m e level. N o crack p r o p a g a t i o n is e v i d e n t . We may c o n c l u d e from this e x p e r i m e n t t h a t t h e p r e s e n c e of h e l i u m allows crack p r o p a g a t i o n at a s t r e s s level l o w e r t h a n t h a t w i t h o u t it. T h e second e x p e r i m e n t investigates t h e loss of d u c t i l i t y in c o p p e r t e n s i l e s p e c i m e n s . C o p p e r s a m p l e s exposed to 1 00 M P a of h y d r o g e n at ~ 2 0 0 ° C for a n u m b e r of m o n t h s s h o w n o loss in ductility and exhi bi t a ductile f r a c t u r e surface similar to u n e x p o s e d c o p p e r . Identical s a m p l e s s u b jected to c o m p a r a b l e e x p o s u r e s of t r i t i u m s h o w s e v e r e e m b r i t t l e m e n t , possibly d u e t o t h e 3 H e g e n e r a t e
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