The effects of microstructure on elevated temperature crack growth in nickel-base alloys
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y e a r s much of the e m p h a s i s in a i r c r a f t gas t u r b i n e d e v e l o p m e n t has b e e n d i r e c t e d to i n c r e a s i n g t h r u s t - t o - w e i g h t r a t i o s . T h i s t r e n d has r e s u l t e d in a g r e a t d e m a n d for h i g h e r s t r e n g t h s u p e r a l l o y s . A l though the new a l l o y s p o s s e s s h i g h e r t e n s i l e and c r e e p c a p a b i l i t i e s , o t h e r p r o p e r t i e s c o n t r i b u t i n g to co m p o n en t life, such a s f at i g u e c r a c k initiation and p r o p a g a t i o n , have not, in many c a s e s , kept pace with s t a t i c p r o p e r t y i m p r o v e m e n t s . R e c e n t engine d e s i g n s , t h e r e f o r e , have b e e n l i m i t e d by fatigue and f r a c t u r e m e c h a n i c s c o n s i d e r a t i o n s . As a r e s u l t , t h e r e is c o n s i d e r a b l e i m petus to c h a r a c t e r i z e s u p e r a l l o y s in t e r m s of t h e i r r e s i s t a n c e to c r a c k i n g . Th e a p p l i c a t i o n of f r a c t u r e m e c h a n i c s to study c r e e p c r a c k growth in n i c k e l - b a s e a l l o y s has b e e n d i s c u s s e d p r e v i o u s l y . 1-3 T h e o b j e c t i v e of the p r e s e n t w o r k is to e x a m i n e the effect of m i c r o s t r u c t u r e on the f a t i g u e c r a c k initiation and p r o p a g a t i o n in c o n v e n t i o n a l l y c a s t and wrought Waspaloy* and P / M A s t r o l o y . * *Trademark. In the c a s e of W a s p a l o y , heat t r e a t m e n t s have b e e n a p p l i ed so that the y ' s i z e and d i s t r i b u t i o n could be v a r i e d while m a i n t a i n i n g a constant g r a i n s i z e . Th e e f f e c t of g r a i n s i z e and m o r p h o l o g y was e v a l u a t e d f o r P/M Astroloy.
2.54 c m w e r e m a c h i n e d f r o m heat t r e a t e d m a t e r i a l . S a m p l e s f o r fatigue c r a c k initiation t e s t s w e r e made with a 0.32 cm m a c h i n e d notch r a d i u s . C r a c k p r o p a gation t e s t s a m p l e s w e r e m a c h i n e d to a c h e v r o n shaped notch and fatigue p r e c r a c k e d at r o o m t e m p e r a t u r e at low AK v a l u e s so that m o r e than 50,000 c y c l e s w e r e r e q u i r e d to f o r m the p r e c r a c k s . Th e s a m p l e s w e r e T - L o r i e n t e d such that the c r a c k s w e r e p a r a l l e l to the r o l l i n g d i r e c t i o n . T h e s p e c i m e n t h i c k n e s s was 0.64 cm f o r W a s p a l o y and 0.95 cm f o r P / M A s t r o l o y . Test Procedure F a t i g u e c r a c k initiation t e s t s w e r e r u n at 650~ at a m a x i m u m load of 1600 lbs, a f r e q u e n c y of 1.0 Hz and an R r a t i o of 0.05. C r a c k i n i t i a t i o n was d e t e c t e d by changes in the d i s p l a c e m e n t of the c l o s e d loop hyd r a u l i c t e s t m a c h i n e , and by v i s u a l e x a m i n a t i o n s t h r o u g h q u a r t z windows in the f u r n a c e s u r r o u n d i n g the s a m p l e . C r a c k i n i
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