Effects of environment on high-temperature fatigue crack growth in a superalloy
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A N D R. H. K A N E
Fatigue c r a c k growth r a t e m e a s u r e m e n t s were made on I n c o n e l alloy 718 s a m p l e s at 650~ in f o u r t e e n different g a s e o u s e n v i r o n m e n t s . M i n o r a m o u n t s of e i t h e r oxygen or s u l f u r b e a r i n g s p e c i e s in the e n v i r o n m e n t produced large i n c r e a s e s in c r a c k growth r a t e s . A g g r e s s i v e e n v i r o n m e n t s p r o m o t e d i n t e r g r a n u l a r c r a c k growth. Kinetic f a c t o r s r a t h e r than t h e r m o d y n a m i c ones a p p e a r to be the v a r i a b l e s d o m i n a t i n g the effects of a n e n v i r o n m e n t on c r a c k growth. E n v i r o n m e n t s that m a r k e d l y i n c r e a s e d the c r a c k growth r a t e did not produce s i g n i f i c a n t c o r r o s i o n a t t a c k on u n s t r e s s e d s a m p l e s . Thus c o n v e n t i o n a l high t e m p e r a t u r e c o r r o s i o n t e s t s may not be useful for p r e d i c t i n g s e r v i c e p e r f o r m a n c e of s t r e s s e d c o m p o n e n t s .
IT has b e e n r e c o g n i z e d for
some t i m e that the e n v i r o n m e n t can s e r i o u s l y affect the e l e v a t e d . t e m p e r a t u r e c r a c k growth in N i - b a s e alloys. 1-9 A i r , in p a r t i c u l a r , has f r e q u e n t l y b e e n o b s e r v e d to be a g g r e s s i v e c o m p a r e d to v a c u u m or i n e r t gas e n v i r o n m e n t s . In addition, the e n v i r o n m e n t often has b e e n s u s p e c t e d of being the cause of the i n c r e a s e d r a t e of c r a c k growth when the f r e q u e n c y is r e d u c e d in high t e m p e r a t u r e fatigue t e s t s . D e s p i t e the w i d e s p r e a d r e c o g n i t i o n that the e n v i r o n m e n t can play a m a j o r role in high t e m p e r a t u r e c r a c k growth, t h e r e have been few s t u d i e s of the effects of d i f f e r e n t g a s e o u s e n v i r o n m e n t s . Some t e s t i n g has been done in specific g a s e o u s a t m o s p h e r e s , such as jet engine fuel exhausts .lo While these t e s t s a r e i n f o r m a tive, they have g e n e r a l l y been conducted on d i f f e r e n t alloys and with d i f f e r e n t t e s t p r o c e d u r e s so that d i r e c t c o m p a r i s o n of the e n v i r o n m e n t a l effects is difficult. In the p r e s e n t study the influence of 14 d i f f e r e n t g a s e o u s e n v i r o n m e n t s on the high t e m p e r a t u r e fatigue c r a c k growth rate of a n i c k e l - b a s e alloy has b e e n d e t e r mined. A l l t e s t s were r u n on the s a m e alloy using the s a m e loading conditions so that d i r e c t c o m p a r i s o n of the e n v i r o n m e n t effects could be m a d e . The object i v e s of the study were to identify which g a s e o u s s p e c i e s affected c r a c k growth and, m o r e g e n e r a l l y , to e x p l o r e the m e c h a n i s m s and the p o s s i b i l i t y of p r e dicting whether a g a s e o u s e n v i r o n m e n t wou
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