Mechanisms of hydrogen induced delayed cracking in hydride forming materials
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K. NUTTALL,
M. P. PULS,
AND
L. A. SIMPSON
M e c h a n i s m s which have been f o r m u l a t e d to d e s c r i b e delayed hydrogen cracking in hyd r i d e - f o r m i n g m e t a l s a r e reviewed and d i s c u s s e d . P a r t i c u l a r e m p h a s i s is placed on the c o m m e r c i a l alloy Z r - 2 . 5 pct Nb (Cb) which is extensively used in n u c l e a r r e a c t o r core components. A quantitative model for hydrogen cracking in this m a t e r i a l is p r e s e n t e d and c o m p a r e d with a v a i l a b l e e x p e r i m e n t a l data. The kinetics of c r a c k propagation a r e controlled by the growth of h y d r i d e s at the s t r e s s e d c r a c k tip by the diffusive i n g r e s s of hydrogen into this region, The driving f o r c e for the diffusion flux is provided by the local s t r e s s g r a d i e n t which i n t e r a c t s with both hydrogen a t o m s in solution and hydrogen atoms being d i s s o l v e d and r e p r e c i p i t a t e d at the c r a c k tip. The model is developed using concepts of e l a s t o p l a s t i c f r a c t u r e m e c h a n i c s , Stage I c r a c k growth is controlled by hydrides growing in the e l a s t i c s t r e s s gradient, while Stage tI is controlled by hydride growth in the p l a s t i c zone at the c r a c k tip. Recent e x p e r i m e n t a l o b s e r v a t i o n s a r e p r e s e n t e d which indicate that the p r o c e s s o c c u r s in an i n t e r m i t t e n t fashion; hydride c l u s t e r s accumulate at the c r a c k tip followed by unstable c r a c k advance and subsequent c r a c k a r r e s t in r e peated c y c l e s ,
THE
development of the a e r o s p a c e and n u c l e a r ind u s t r i e s has i n c r e a s e d the demand for m a t e r i a l s off e r i n g a combination of high strength and e n v i r o n mental s t a b i l i t y . A phenomenon which has e m e r g e d as a s p e c i a l yet common p r o b l e m in this field is that of strength degradation due to the p r e s e n c e of hydrogen. Hydrogen is a b s o r b e d by most m e t a l s e i t h e r during f a b r i c a t i o n or during s e r v i c e and often c a u s e s component f a i l u r e a f t e r prolonged p e r i o d s at s t r e s s e s which a r e well within n o r m a l load b e a r i n g c a p a b i l i t i e s . The importance of this c l a s s of p r o b l e m is a c c e l e r a t ing within energy r e l a t e d technology. ~ Hydrogen induced delayed cracking in endothermic hydrogen o c c l u d e r s , such as s t e e l s , has been studied extensively. The f i r s t detailed qualitative d i s c u s s i o n of this p r o c e s s was offered by Troiano and his cow o r k e r s 2 who postulated that the p r o c e s s was cont r o l l e d by the long-range diffusion of hydrogen into a c r a c k tip r e g i o n tinder the action of the l o c a l hydros t a t i c s t r e s s gradient. In r e s p o n s e to this suggestion, t h e r e have been s e v e r a l t h e o r e t i c a l investigations 3-8 of the diffusion of point defects to c r a c k tips,
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