The effect of rapid thermal fluctuations on the creep rate in Inconel 718
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void s p e c i m e n c o n t a m i n a t i o n , the e x p e r i m e n t s w e r e c a r r i e d out in a v a c u u m c h a m b e r (-~10 -6 t o r r ) . The p l a s t i c s t r a i n s w e r e l e s s than 2 pct so one can c o n s i d e r each test as a constant s t r e s s e x p e r i m e n t , although it is in fact of the d e a d - l o a d type. The exp e r i m e n t s w e r e c a r r i e d out at two s t r e s s l e v e l s of 25.9 kg m m -2 and 36.1 kg m m -z. T y p i c a l c r e e p c u r v e s for Inconel 718 obtained in the e x p e r i m e n t s at t h e s e s t r e s s e s and at t e m p e r a t u r e s of about 600~ a r e shown in Fig. 2(a). A f t e r about 3 h r the slope of the c r e e p curve in this t e m p e r a t u r e r a n g e r e m a i n e d cons t a n t ( i s = constant) and all s u b s e q u e n t t e s t s r e p o r t e d h e r e w e r e i n i t i a t e d as i s o t h e r m a l c r e e p t e s t s for 4 h r b e f o r e cycling began or before i s (defined h e r e as the effective s t e a d y - s t a t e c r e e p rate) was m e a s u r e d from the slope of the r e c o r d e d l i n e a r c r e e p curve. Fig. 3 shows the dependence of this effective s t e a d y - s t a t e c r e e p rate ( i s ) on t e m p e r a t u r e at the two different s t r e s s l e v e l s . The open c i r c l e s r e p r e s e n t c r e e p r a t e s obtained f r o m individual i s o t h e r m a l c r e e p e x p e r i m e n t s , Fig. 2(a). It was found that the s a m e t e m p e r a t u r e dependence of the m e a s u r e d c r e e p r a t e was obtained f r o m t e s t s conducted on a single s p e c i m e n by i n c r e a s i n g the t e m p e r a t u r e a f t e r the s p e c i m e n had r e a c h e d the effective s t e a d y - s t a t e c r e e p r a n g e , Fig. 2(b). This data is shown in Fig. 3 by the full c i r c l e s . A t h i r d type of e x p e r i m e n t was a l s o c a r r i e d out: the s p e c i m e n was subjected to t h e r m a l cycling between constant t e m p e r a t u r e t e s t s and the i s o t h e r m a l steady | TEMPERATURE T HEASURED HIGH VALUES AT T= 650 =C LOW VALUES AT T= 450=C
TI
I
/'~.T'=10"15 '=C
METALLURGICALTRANSACTIONS
I~T = 3ยง
T
T
~.T= 150- 200 ~
L I I I I
N. NIR (KUSHNIR), D. EYLON, and A. ROSEN are with the Department of Materials Engineering,Technion - Israel Institute of Technology, Technion City, Haifa, Israel. Manuscript submitted May 21, 1970.
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-~ effe~ve | gauge length l_
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Fig. 1--The temperature gradient along the specimen. VOLUME 2, AUGUST 1971-2237
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60
=
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T-610eC
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T=610 :C
T=550~C
T 560
C
THERMAL CYCLING RANGE
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t ( hours
T:580 ~ __ ~
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(a) T, < Tz < T=
0" = CONST 550
CONSTANT TEMPERATURE T1
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I 600
I 610
I 620
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