Fracture at elevated temperature
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T HE d e m a n d for s t r u c t u r a l m a t e r i a l s for high t e m p e r a t u r e a p p l i c a t i o n s in the last two d e c a d e s has led to the d e v e l o p m e n t of s o p h i s t i c a t e d m a t e r i a l s such as v a r i o u s n i c k e l - b a s e a l l o y s . The c r e e p s t r e n g t h in these m a t e r i a l s is u s u a l l y obtained by the d i s p e r s i o n of fine second phase p a r t i c l e s which inhibit d i s l o c a t i o n mobility, and by the p r e c i p i t a t i o n of p a r t i c l e s (usually c a r b i d e s ) in the g r a i n b o u n d a r y which o b s t r u c t sliding. It is c o m m o n l y observed, however, that an i n c r e a s e in c r e e p s t r e n g t h is a c c o m p a n i e d by a d e c r e a s e in the f r a c t u r e toughness of the m a t e r i a l at e l e v a t e d t e m p e r a t u r e . A s i t u a t i o n has a l m o s t b e e n r e a c h e d where the f r a c t u r e toughness r a t h e r than c r e e p r e s i s t a n c e has b e c o m e the l i m i t i n g factor in the s e r v i c e life of these m a t e r i a l s . C r a c k p r o p a g a t i o n and fatigue s t u d i e s a r e c u r r e n t l y underway 1 which follow a f r a c t u r e - m e c h a n i c s a p p r o a c h to the c h a r a c t e r i z a t i o n of these m a t e r i a l s . A l l of t h e s e s t u d i e s show a g r e e m e n t on one point: the ductility and the f r a c t u r e toughness a r e the lowest when the mode of f a i l u r e is i n t e r g r a n u l a r . T h i s has led to a r e c o n s i d e r a t i o n of e a r l y e x p e r i m e n t s by B e n gough, 2 R h i n e s and Wray, 3 Greenwood, 4 Garofalo, 5 Skelton ~ and o t h e r s , and m o r e r e c e n t l y V e n k i t e s w a r a n and T a p l i n , v on i n t e r g r a n u l a r f r a c t u r e in m a t e r i a l s r a n g i n g f r o m copper, a l u m i n u m , and i r o n to the modern super alloys. I n t e r g r a n u l a r f r a c t u r e in t h e s e s t u d i e s has b e e n i n t e r p r e t e d in t e r m s of the i n i t i a t i o n and linkage of c a v i t i e s in the g r a i n b o u n d a r i e s . The cavity i n i t i a t i o n has p o s i t i v e l y b e e n identified with second phase p a r t i cles, 8-1~ although g r a i n b o u n d a r y ledges have also b e e n W. PAVINICH,formerly a Graduate Student in the Department of Materials Science and Engineeringat Corneli University, is now with the General Electric Company, Wilmington,N.C. and R. RAJ is Assistant Professor, Materials Science and Engineering,Cornell University, Ithaca, N.Y. 14853. Manuscript submitted March 17, 1977. METALLURGICALTRANSACTIONSA
s u g g e s t e d as n u c l e a t i o n s i t e s . 5 Most c o m m o n l y , the growth of c a v i t i e s has b e e n explained by a d i f f u s i o n a l t r a n s p o r t m e c h a n i s m which was f i r s t a n a l y z e d t h e o r e t i c a l l y by Hull and R i m m e r n and m o r e r e c e n t l y in det a i l by one of us.12 T h i s growth m e c h a n i s m p r e d i c t s a l i
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