Prediction of rupture lifetime in thin sections of a nickel base superalloy

S~iptaMctall~gicaet Matefiafi~V01. 3I,No. 6,pp. 719-722,1994 Copyright ©1994 Elsevier ScienceLtd Printed inthe USA. All tights reserved 0956-716X/94 $6.00 + 00

Pergamon

PREDICTION

OF R U P T U R E L I F E T I M E IN THIN A N I C K E L BASE S U P E R A L L O Y M.C.

P a n d e y ÷ and D.M.R.

+Defence Metallurgical Kanchanbagh, H y d e r a b a d - 5 0 0

SECTIONS

OF

T a p l i n ++

Research 258, India

Laboratory,

+VSchool of M a n u f a c t u r i n g , M a t e r i a l s and M e c h a n i c a l Engineering, U n i v e r s i t y of Plymouth, Plymouth, PL4 8AA, U K (Received (Revised

March 25, 1994) May 1 1 , 1 9 9 4 )

Introduction Several studies on the influence of section size on the creep behaviour of nickel base superalloys have shown that life to fracture is shortened and creep rate is enhanced by reducing the specimen size [1-3]. These findings have technological implications as fracture lifetime of a component with a varying section size depends on the creep behaviour of the lowest section thickness. It is, therefore, important that creep data for components with different section sizes be generated on the specimen size equal to that of lowest f h i r k n e s s of the c o m p o n e n t s . An i n t e r e s t i n g e x a m p l e of how a thin section can influence the r u p t u r e l i f e t i m e has been found in the case of aeroengine blades w h e r e i n m o d i f i c a t i o n of these blades p r o v i d i n g cooling channels for improving the e n g i n e e f f i c i e n c y led to p r e m a t u r e failure. The m o d i f i c a t i o n in the b l a d e s r e d u c e d the s e c t i o n size at the leading edge as a result of w h i c h the r u p t u r e l i f e t i m e d e c r e a s e d [4]. The p r e d i c t i o n of r u p t u r e l i f e t i m e of a c o m p o n e n t can be b a s e d on the data g e n e r a t e d from round, flat and t u b u l a r specimens. An i n v e s t i g a t i o n on the influence of specimen g e o m e t r y on the c r e e p b e h a v i o u r of Inconel alloy X-750 [5] showed that the t u b u l a r s p e c i m e n e x h i b i t e d b e t t e r creep p e r f o r m a n c e when rupture l i f e t i m e data were c o m p a r e d on the basis of section size. However, the time to rupture data of all three s p e c i m e n g e o m e t r i e s m e r g e d t o g e t h e r when c o m p a r e d on the basis of the v o l u m e to surface area ratio (V/S) indicating that there is a d e f i n i t e r e l a t i o n s h i p b e t w e e n V/S and the rupture lifetime. This a n a l y s i s is now e x t e n d e d in a n o t h e r g a m m a prime s t r e n g t h e n e d nickel base superalloy, w h i c h was i n v e s t i g a t e d by R i c h a r d s [1], and the finding is reported here. Experimental

Procedure

The c h e m i c a l c o m p o s i t i o n of the a l l o y [I] in wtg, is as follows: 0.07C0.66Si-0.07Cu-0.79Fe-19.1Cr-2.35Ti-I.S2AI and r e m a i n d e r nickel. The alloy a v a i l a b l e in the c o n d i t i o n s of both h o t - r o l l e d and e x t r u d e d sheet bars was s o l u t i o n t r e a t e d at I150"C for 5 and 30 m i n u t e s r e s p e c t i v e l y and c o o l e d in air. The a l l o y was further aged at 700"C for 16 hours. The g r a i n size of the alloy, s o l u t i o n t r e a t e d for 5 and 30 minutes, was found to be 55 and 250 ~m, respectively. A f t e r the s o l u t i o n treatment, flat s p e c i m e n s w i t h a w i d t h of 8.4 mm and 25.4 m m gauge l e n g t h were m a c h i n e d from the h o t - r o l l e d sheet and the e x t r u d e d sheet bar. S p e c i m e n t h i c k n e s s v a r i e d from 0.9 to 3.2 mm. C r e e p tests were c a r r i e d out at 750"C and 2 3 1 M P a . A d e t a i l e d e x p e r i m e n t a l p r o c e d u r e can be found in r e f e r e n c e I.

719

720

PREDICTION OF RUPTURE LIFETIME

Results

Vol. 31, No. 6

and D i s c u s s i o n

Figure 1 shows time-to-rupture data plotted as a function of specimen thickness f o r 55 a n d 250 pm g r a i n sizes. It is seen that time-to-rupture decreased as the thickness of the specimen became smaller. Creep data shown in Figure 1 were replotted between the volume to surface area ratio (V/S) and time-to-rupture, te, (Figure 2). One c a n s e e t h a t t h e r e e x i s t s a straight line relationship between time-to-rupture and volume to surface area ratio (tf a V/S) for both grain sizes. V to S was further normalized with grain size, d, and plots w e r e made b e t w e e n t i m e - t o - r u p t u r e and (V/S)/d as shown in Figure 3. It is interesting to note that the time-to-rupture data for both grain sizes merged together and are represented by a single curve (compare with Figure 2). This further strengthens our earlier analysis [5] that V/S is the most suitable parameter that controls the rupture lifetime. This means that in the case of a flat specimen, thickness and width both dictate the fracture lifetime. This is because fracture of the specimen will occur only when grain boundary cracks spread" across both the w i d t h and the thickness. V / S appears to be the a p p r o p r i a t e parameter as an i n c r e a s e in the c r o s s - s e c t i o n a l area (A) e n h a n c e s the rupture lifetime, whereas an o p p o s i t e effect is o b s e r v e d with the perimeter(P). This is due to the fact that the f r a c t i o n of c r o s s - s e c t i o n a l area damaged by the g r o w t h and i n t e r l i n k a g e of c a v i t i e s / c r a c k s for a g i v e n amount of damage d e c r e a s e s w i t h A and, on the o t h e r hand, the damage c a u s e d by oxygen interaction along the g r a i n b o u n d a r y i n c r e a s e s w i t h P. Experimental evidence has been found in a n o t h e r nickel base s u p e r a l l o y w h e r e i n d i f f u s i o n of o x y g e n o c c u r r e d in the a l l o y d u r i n g its t e n s i l e t e s t i n g in air at e l e v a t e d t e m p e r a t u r e s and led to poor t e n s i l e p r o p e r t i e s [6]. D e g r a d a t i o n in the p r o p e r t i e s occurred b e c a u s e of d i f f u s i o n of o x y g e n that r e a c t e d with carbide particles present in the grain boundary and causing decohesion of the particles. This r e s u l t e d in g r a i n b o u n d a r y c r a c k i n g at the surface. The p r e s e n c e of these cracks not only a c t e d as a n o t c h but also reduced the load bearing c r o s s - s e c t i o n a l area. This m e a n s that a s the p e r i m e t e r or surface area increases, grain b o u n d a r y area e x p o s e d to an o x y g e n e n v i r o n m e n t increases. The net effect results in the d e g r a d a t i o n of c r e e p p r o p e r t i e s and the r e d u c t i o n in rupture lifetime. Conclusions A n a l y s i s of the rupture l i f e t i m e d a t a of a g a m m a p r i m e s t r e n g h e n e d nickel b a s e s u p e r a l l o y shows that t i m e - t o - r u p t u r e is c o n t r o l l e d by the volume to surface area ratio. F u r t h e r m o r e , the n o r m a l i z a t i o n of the volume to surface area ratio w i t h the grain size shows that the t i m e - t o - r u p t u r e data of two g r a i n sizes, 55 and 250 pm, can be r e p r e s e n t e d by a single curve. Acknowledgements The a u t h o r s would like to t h a n k P r o f e s s o r A.K. Mukherjee, U n i v e r s i t y of California, Davis, USA for u s e f u l d i s c u s s i o n s . One of us (MCP) would like to t h a n k Mr. S . L . N . A c h a r y u l u , Director, DMRL f o r h i s e n c o u r a g e m e n t . References 1. 2.

5.

E.G. Richards, J. Inst. Metals, 95, 365 (1968). G.D. Oxx, Int. Conf. on Creep Fatigue on Elevated Temperature A p p l i c a t i o n s , Inst. of Mech. Eng., p a p e r 212 (1974). T.B. Gibbons, M e t a l s T e c h n o l o g y , 8, 472 (1981). M.C. Pandey, A.M. S r i r a m a m u r t h y and M.L. Bhatia, u n p u b l i s h e d research, DMRL, H y d e r a b a d (1989). M.C. P a n d e y , D . M . R . T a p l i n and P. R a m a Rao, Mat. Sc. Eng., Al18, 33

6.

M.C.

3. 4.

(1989). Pandey,

1763 (1984).

D.M.R.

Taplin

and

A.K.

Mukherjee,

Metall.Trans.

A.,

15A,

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PREDICTION OF RUPTURELIFETIME

300

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Vol. 31, No. 6

PREDICTION OF RUPTURELIFETIME

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