Fatigue crack propagation in two classes of directionally solidified eutectic alloys
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ALTHOUGH
the g e n e r a l f a t i g u e b e h a v i o r of d i r e c t i o n a l l y s o l i d i f i e d (DS) e u t e c t i c a l l o y s has b e e n r e ported for several systems, linear elastic fracture m e c h a n i c s has been a p p l i e d to only one a l l o y of c o n s t a n t i n t e r l a m e l l a r s p a c i n g , i.e., the b i n a r y s y s t e m + 5.1 In t h i s p r o g r a m , the i s o t h e r m a l fatigue b e h a v i o r of two c l a s s e s of DS e u t e c t i c s was s t u d i e d with i n t e r l a m e l l a r and i n t e r f i b e r s p a c i n g a s t h e p r i n c i p a l v a r i a b l e . E m p h a s i s was p l a c e d on d e f i n i n g f a t i g u e c r a c k p r o p a g a t i o n in t e r m s of f r a c t u r e m e c h a n i c s c o n c e p t s and on c h a r a c t e r i z i n g the p r e f e r r e d fatigue fracture paths. M A T E R I A L AND E X P E R I M E N T A L P R O C E D U R E A l l o y s of m a r k e d l y d i f f e r e n t r e i n f o r c i n g p h a s e m o r phology w e r e c h o s e n f o r t h i s p r o g r a m . T h e s e i n c l u d e d ~/~,' + 5, a n i c k e l - b a s e a l l o y r e i n f o r c e d by p l a t e l e t s ( l a m e l l a e ) of Ni3Cb(5), and a c o b a l t - b a s e a l loy, 73C, r e i n f o r c e d by f i b e r s of Cr7C3. A l l o y 73C w a s d i r e c t i o n a l l y s o l i d i f i e d at growth r a t e s of 1 and 25 cph t h r o u g h t h e r m a l g r a d i e n t s of 80 and 120 K / c m , r e s p e c t i v e l y . The 1 cph m a t e r i a l had a n o m i n a l c o m p o s i t i o n of C o - 4 N i - 3 5 C r - 2 . S A 1 - 2 . 0 C while the 25 cph m a t e r i a l d i f f e r e d s l i g h t l y in c a r b o n content, 2.15 wt p c t . T h e c r y s t a l l o g r a p h i c d i r e c t i o n s p a r a l l e l to t h e g r o w t h d i r e c t i o n s had b e e n p r e v i o u s l y e s t a b l i s h e d a s [0001] and [211] f o r the Cr7C3 and m a t r i x , r e s p e c t i v e l y . 2 It was a l s o found that the f a c e t e d f a c e s of the c a r b i d e s a r e of the {1100} type but no s i m p l e i n t e r r a c i a l r e l a t i o n s h i p e x i s t s b e t w e e n t h e s e f a c e t s and the c o b a l t m a t r i x . 2 The m i c r o s t r u c t u r e s f o r the two g r o w t h conditions a r e shown in F i g . 1. The p r e s e n c e of p r i m a r y c a r b i d e s in the 25 cph m a t e r i a l i n d i c a t e d t h a t the a l l o y was s l i g h t l y h y p e r e u t e c t i c . I n t e r f i b e r s p a c i n g (X) was 5.5 and 1.5 g m f o r the 1 and 25 cph m a t e r i a l s , r e s p e c t i v e l y . Two c o m p o s i t i o n s of ~,/y' + 5 w e r e e v a l u a t e d : one with no c h r o m i u m a d d i t i o n and one containing 6 wt A. YUEN, formerly of Pratt & Whitney Aircraft, is presently attending the Albany Medical College, Albany, NY. G. R. LEVERANT is affiliated with the Materials Engineering and Research Laboratory, Pratt & Whitney Aircraft, East Hartford, CT 06108. Manuscript submitted November 26, 1975. METALLURGICAL TRANSACTI
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