Mechanisms of damage and fracture in high-temperature, low-cycle fatigue of a cast nickel-based superalloy
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s p e c t i v e l y , and g r a i n b o u n d a r i e s with MC and Mz3C6 c a r b i d e s . The h e a t t r e a t m e n t and m i c r o s t r u c t u r e a r e s t a n d a r d for gas t u r b i n e bucket a p p l i c a t i o n s . H o u r - g l a s s s h a p e d fatigue t e s t s p e c i m e n s w e r e g r o u n d into the c o n f i g u r a t i o n shown in Fig. 2 and the gage s e c t i o n s w e r e g r o u n d to a 5 to 6 ~t in. f i n i s h . F a t i g u e t e s t i n g was done by fully r e v e r s e d t e n s i o n c o m p r e s s i o n l o a d i n g at a c o n s t a n t load o r s t r a i n r a t e u s i n g c l o s e d loop c o n t r o l at s e v e r a l t e m p e r a t u r e s and f r e q u e n c i e s ; t e m p e r a t u r e was m a i n t a i n e d by induction h e a t i n g within + I~ and s t r a i n was m e a s u r e d at the m i n i m u m c r o s s s e c t i o n by d i a m e t r i c a l l y opposed q u a r t z rods a t t a c h e d to s t r a i n - g a g e t r a n s d u c e r s . F u r t h e r d e t a i l s of the m e c h a n i c a l t e s t p r o g r a m will be d e s c r i b e d e l s e w h e r e , x The t e s t c o n d i t i o n s used for m o s t of the p r e s e n t e x p e r i m e n t s w e r e : t e m p e r a t u r e : 815~ f r e q u e n c y : ~ cph, and n o m i n a l s t r e s s a m p l i tude: 70,000 p s i . Heating of u n s t r e s s e d s p e c i m e n s to s t u d y oxidation b e h a v i o r was done in a muffle f u r n a c e in an a i r a t m o s p h e r e . S p e c i m e n s w e r e p l a c e d into the f u r n a c e when the l a t t e r had r e a c h e d the d e s i r e d t e m p e r a t u r e and w e r e cooled b y s t a n d i n g in s t i l l a i r . M i c r o s c o p i c e x a m i n a t i o n s were c a r r i e d out with a s t e r e o m i c r o s c o p e at m a g n i f i c a t i o n s up to 30X on the s u r f a c e s of fatigued s p e c i m e n s and s p e c i m e n s t h e r m a l l y cycled in a i r without s t r e s s . S u r f a c e s of fatigued s p e c i m e n s were a l s o e x a m i n e d at m a g n i f i c a t i o n s up to 2000X by m e a n s of a s c a n n i n g e l e c t r o n m i c r o s c o p e . L o n g i t u d i n a l s e c t i o n s of f a t i g u e d s p e c i m e n s were e x a m i n e d m e t a l l o g r a p h i c a l l y , as were t a p e r e d s e c t i o n s through the oxidized s u r f a c e s of s p e c i m e n s h e a t e d u n s t r e s s e d in a i r .
Table I. Composition of Udimet 500
Solution t r e a t m e n t P r e l i m i n a r y Aging F i n a l Aging
2125~ 1975~ 1400~
4 hr 4 hr 16 h r
The r e s u l t i n g m i c r o s t r u c t u r e , as shown in Fig. 1, c o n s i s t s of l a r g e g r a i n s c o n t a i n i n g c o a r s e and fine 7' f o r m e d at the high and low aging t e m p e r a t u r e s , r e C. J. McMAHON is Associate Professor, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia, Pa. L. F. COFFIN, JR., is Mechanical Engineer, General Electric Research and Development Center, Schenectady, N.Y. Man
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