Interactions between creep, fatigue, and strain-aging in two refractory metal alloys
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A r e c e n t study has shown that s i g n i f i c a n t a l t e r a t i o n s could be made to the c r e e p b e h a v i o r of a m o l y b d e n u m alloy (TZC) by the a p p l i c a t i o n of l o w - a m p l i t u d e , highf r e q u e n c y fatigue v i b r a t i o n d u r i n g c r e e p . ~ T h e s e t e s t s indicated that m o d e r a t e v a r i a t i o n s of the applied load (• pct) at a f r e q u e n c y of about 19 kHz would c a u s e the c r e e p r a t e to i n c r e a s e by as much as two o r d e r s of m a g n i t u d e over that obtained u n d e r static s t r e s s at the s a m e peak v a l u e s . T h e s e r e s u l t s a r e s i g n i f i c a n t s i n c e many t u r b i n e a p p l i c a t i o n s involve s t r e s s c o n d i tions where a high f r e q u e n c y d y n a m i c f l u t t e r load is s u p e r i m p o s e d on a steady c e n t r i f u g a l force. On this b a s i s , conventional c r e e p data obtained at the peak s t r e s s condition would r e s u l t in design p r e d i c t i o n s that would be a l m o s t two o r d e r s of m a g n i t u d e l e s s than that e x p e r i e n c e d when a d y n a m i c load of t 4 5 pct was applied. In m o r e r e c e n t t e s t s , a s i m i l a r effect has been found in a t a n t a l u m alloy (T-111). T h e s e l a t e r t e s t s also showed that the effect extended to load v a r i a t i o n s as s m a l l as ~: 10 pct in both m a t e r i a l s , and to f r e q u e n c i e s as low as 10 kHz in TZC. D e t a i l s of t h e s e new findings a r e r e p o r t e d in this p a p e r , tog e t h e r with a p o s s i b l e i n t e r p r e t a t i o n of the r e s p o n s i ble m e c h a n i s m .
sheet m a t e r i a l the hafnia p r e c i p i t a t e s were found to be d i s t r i b u t e d p r e f e r e n t i a l l y at the g r a i n b o u n d a r i e s . The TZC m a t e r i a l t e s t e d in this p r o g r a m was forged plate which was r e c r y s t a l l i z e d 1 hr at 3092~ (1700~ C o m p o s i t i o n s of the TZC t e s t m a t e r i a l s a r e shown in T a b l e II, and an optical p h o t o m i c r o g r a p h of the r e c r y s t a l l i z e d TZC alloy is shown in Fig. 3. The d i s c r e t e c a r b i d e p r e c i p i t a t e s t y p i c a l of this m a t e r i a l a r e r e a d i l y a p p a r e n t even at the • magnification. The 2000~ (1093~ t e n s i l e p r o p e r t i e s of both the TZC and the T - 1 1 1 alloys a r e l i s t e d in Table III. To e l i m i n a t e the p o s s i b i l i t y of e n v i r o n m e n t a l con-
Table I. Chemical Analysis of T-111 Heat 650038 Studied in the Program
WtPet
Ppm
W
Hf
Ta
C
O
N
H
8.6
2.0
Balance
25
100 19
20 41
2.8 Pretest 9.0 Post Test
55
F
E X P E R I M E N T A L DETAILS The T-111 alloy was t e s t e d in the f o r m of 0.600 in. plate c o l d - r o l l e d and r e c r y s t a l l i z e d 1 hr at 3000~ (1649~ C h e m i c a l a n a l y s i s of this plate is given in T a b l e I, while light and e l e c t r o n m i c r o g r a p h s a r e shown in Figs. 1 and 2
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