The Effect of Temperature Cycling on Rupture Strength of a Tantalum Carbide Strengthened Cobalt Base Eutectic
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D. A. WOODFORDis Staff Metallurgist,Corporate Research and Development, General Electric Company, Schenectady, NY 12301. Manuscript submitted June 13, 1977. 2016-VOLUME 8A, DECEMBER 1977
ance, t e m p e r a t u r e c y c l i n g may lead to an i n c r e a s e o r a d e c r e a s e depending on the n a t u r e of the cycle.4,6 C l e a r l y , it is i m p o r t a n t at l e a s t to be a w a r e of the magnitude of t h e s e effects; at this stage an a p p r o p r i ate s t r e n g t h m a r g i n could be allowed although a c o m plete d e s i g n a n a l y s i s i n c o r p o r a t i n g the q u a n t i t a t i v e r e s p o n s e of an alloy in a gas t u r b i n e cycle is not yet l e a s ible. The most d r a m a t i c effects on r u p t u r e life so far r e ported have b e e n in cobalt base e u t e c t i c s . 3'4 Dunlevey and W a l l a c e 3 cycled s p e c i m e n blanks in a b u r n e r r i g b e t w e e n 427 and 1093~ and found an a p p r e c i a b l e loss in r u p t u r e s t r e n g t h in a C o - 1 5 C r - 2 5 N i - 1 3 T a C alloy a f t e r 5000 c y c l e s with a 15 s hold t i m e . T h i s was att r i b u t e d to e x t e n s i v e f i b e r d e t e r i o r a t i o n . However, t h e r e were c o n s i d e r a b l e f i b e r s e r r a t i o n s developed a f t e r 2000 c y c l e s with l e s s r u p t u r e life d e g r a d a t i o n than a f t e r 200 c y c l e s . Benz e t al 4 a l s o r e p o r t e d f i b e r s e r r a t i o n s in a Co- 1 5 , 7 C r - 9 . 5 N i - 3 W - 12.77TAC alloy (Co50B3W) after slow cycling (20 min) b e t w e e n 140 and 1090~ However, although the extent of these s e r r a t i o n s i n c r e a s e d p r o g r e s s i v e l y with i n c r e a s i n g n u m b e r of c y c l e s , the r u p t u r e s t r e n g t h showed a m a x i m u m a f t e r about 500 c y c l e s . A m a x i m u m i n c r e a s e of a factor of 17 on life was r e p o r t e d . In that work, the e v e n t u a l d e g r a d a t i o n for a l a r g e n u m b e r of cycles was a t t r i b u t e d p r i n c i p a l l y to r e c o v e r y p r o c e s s e s l e a d i n g to s u b g r a i n f o r m a t i o n in the m a t r i x with a c o n t r i b u t i n g factor b e i n g the f i b e r s e r r a t i o n s . R u p t u r e life d e g r a d a t i o n may o c c u r in n i c k e l b a s e e u t e c t i c s and c o n v e n t i o n a l s u p e r a l l o y s with no a s s o c i ated c a r b i d e s e r r a f i o n s . ~ It is worthwhile, t h e r e f o r e , to r e e x a m i n e the effect of t e m p e r a t u r e c y c l i n g in Co50B3W. The b u r n e r r i g cycle m o s t p r o b a b l y leads to m a c r o s c o p i c as well as m i c r o s c o p i c s t r a i n c y c l i n g and is p r o b a b l y m o r e r e p r e s e n t a t i v e of events o c c u r r i n g d u r i n g heat up and cool down in a gas t u r b i n e . However, the a p p l i c a t i o n also d e m a n d s a hold t i m e at m a x i m u m t e m p e r a t u r e p r o m o t i n g r e l a x a t i o n of r e s i d u a l s t r e s s e s and c o n s e q u
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