A fractographic study of Stage I fatigue cracking in a nickel-base superalloy single crystal
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s u r f a c e s for p o s s i b l e d i f f e r e n c e s in a p p e a r a n c e and to u s e these o b s e r v a t i o n s to obtain a b e t t e r u n d e r s t a n d i n g of the Stage I f r a c t u r e p r o c e s s .
EXPERIMENTAL
PROCEDURE
A nickel-base superalloy, low-carbon Mar-M200, having the composition 4.7 AI, 1.7 Ti, 8.5 Cr, 0.7 Cb(Nb), 10.7 Co, 11.8 W, 0.04 Zr, 0.005 B, 0.008 C, balance Ni, was tested in single crystal form. The crystal was solutionized at 2250~ (1230~ for I00 hr, followed by aging at 1600~ (870~ for 32 hr. This heat t r e a t m e n t r e s u l t e d in a 0.2 pct offset y i e l d s t r e s s of 150,000 p s i at r o o m t e m p e r a t u r e . The m i c r o s t r u c t u r e of the m a t e r i a l has b e e n d e s c r i b e d in p r e v i o u s s t u d i e s . 2'3 The t e n s i l e axis of the s p e c i m e n was app r o x i m a t e l y 30 deg f r o m a d i r e c t i o n . Most of the f r a c t o g r a p h i c w o r k was c a r r i e d out on a s i n g l e 1/8 in. (3.2 m m ) diam c y l i n d r i c a l s p e c i m e n that was c y c l i c a l ly s t r e s s e d b e t w e e n 5 and 45 ksi at 1020 cps in a r e s onance fatigue m a c h i n e and had a life of 3.5 • 106 cyc l e s . The f r a c t u r e s u r f a c e s of other s p e c i m e n s cycled at 10 cps and at h i g h e r s t r e s s r a n g e s showed s i m i l a r results. R e p l i c a s of the f r a c t u r e s u r f a c e s w e r e p r e p a r e d u t i l i z i n g a t w o - s t e p p l a s t i c - c a r b o n t e c h n i q u e . All r e p l i c a s w e r e c h r o m i u m shadowed at a c o n s t a n t angle, with the i n i t i a t i o n site on each f r a c t u r e s u r f a c e p o s i tioned in the s a m e way r e l a t i v e to the shadowing source. Experimental Results Fig. 1 s c h e m a t i c a l l y shows the two p i e c e s of the s p e c i m e n following f r a c t u r e . The l e t t e r s A and B i n dicate opposing f r a c t u r e h a l v e s . Fig. 2 is an o p t i c a l m i c r o g r a p h showing both f r a c t u r e s u r f a c e s . The e n t i r e a r e a in the f i g u r e i s the fatigue p o r t i o n of the f r a c t u r e . The c r a c k o r i g i n a t e s at a s u r f a c e - c o n n e c t e d c a s t i n g p o r e ( a r r o w at top of m i c r o g r a p h ) and p r o p a g a t i o n o c c u r s on {111} p l a n e s , the slip p l a n e s in this m a t e r i a l . The f r a c t u r e s u r f a c e s contain f r a c t u r e s t e p s and r i v e r l i n e s , and show no VOLUME 1, NOVEMBER 1970-3107
e v i d e n c e of rubbing. T h e s e r e s u l t s a r e in a g r e e m e n t with p r e v i o u s o b s e r v a t i o n s . 2 A t t h i s low m a g n i f i c a t i o n , the f e a t u r e s on the f r a c t u r e h a l v e s a p p e a r the s a m e
[010],TENSILAXI ES
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TENSIOVERLOAD LE FRACTURE SURFACE
INITIATIOSINTE
Fig. 1 - - O r i e n t a t i o n of fatigue f r a c t u r e s u r f a c e r e l a t i v e to the tensile axis.
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