Creep fracture mechanisms in single crystal superalloys

Creep fracture mechanisms in single crystal superalloys

Scripta METALLURGICA et MATERIALIA Vol. 26, pp. 579-584, 1992 Printed in the U.S.A. Pergamon Press plc All rights reserved C R E E P F R A C T U R ...

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Scripta METALLURGICA et MATERIALIA

Vol. 26, pp. 579-584, 1992 Printed in the U.S.A.

Pergamon Press plc All rights reserved

C R E E P F R A C T U R E M E C H A N I S M S IN S I N G L E C R Y S T A L S U P E R A L L O Y S S.H. Ai*, V. Lupine* * a n d M. M a l d i n i " * State Key L a b o r a t o r y of Fatigue and Fracture for Materials Institute of M e t a l Research, A c a d e m i a Sinica, S h e n y a n g 110015



•*

CNR-ITM

Via l n d u n o 10, 20092 CiniseUo B.fMI) Italy

(Received July 25, 1991) (Revised December 4, 1991) Introduction Single crystal rotating blades with excellent high t e m p e r a t u r e creep and low cycle fatigue resistance in the [0O li crystalline directions have been a d o p t e d in aeroengine and efforts have being made to use them in industri. al gas turbines also. Several microstructural parameters, i.e., the volume fraction, size and distribution o f precipitate phase. dendritic parameters, casting porosity, etc., can strongly affect the material properties. In particular, casting pores are of u t m o s t importance for t I C F ; they influence LCF, a n d can also strongly influence creep fracture behaviour, a l t h o u g h such effects have been studied only rarely (1). While nucleation and g r o w t h of grain b o u n d a r y cavities in polycrystalline materials at high temperatures have been widely investigated (2-6), very little a t t e n t i o n has been paid to creep fracture p h e n o m e n o l o g y in single crystal superalloys. T h a t is the purpose of the present paper. Experimental Procedures The chemical compositions of C M S X - 2 , SRR 99 and R R 2000 alloys examined in this work are given in Table 1, while the heat treatments are reported in Table 2. Table 1. Chemical C o m p o s i t i o n in wt.% !

]

AHoy

Cr

Mo

Ti

Ta

W

Co

A1

C

Ni

[

CMSX-2

8

0.6

1

6

8

4..6

5.6

15ppm

Bal.

f

SRR 99

8.5

825

2.2

2.8

9.5

5.0

5.5

150ppm

Bal.

i

RR2008

10

3

15

5.6

150ppm

Bal.

I

i l

4.0 I

Table 2.

! i

-

-

Solution a n d Ageing Heat T r e a t m e n t s

Alloy

Solution

Ageing

CMSX-2

1315U / 3h

1080U / 4 h + 8 7 0 U / 20h

S R R 99

1280-1305U / 4h

8 7 0 U / 16h

R R 2000

1260U / 4h

,

l l 0 0 U / l h + 8 5 0 U / 16h

C o n s t a n t load creep tests were performed on single crystal s p e c i m e n s having ori entati ons within 12 ° o f

579 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc

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<01)l > , with 5.64 m m gauge diameter a n d 28 m m gauge length, in air between 750 a n d 1000"C at applied stresses between 140 a n d 790 M Pa. Creep strain was continuously m o n i t o r e d using capacitive t r a n s d u c e r s con. netted to eztensometers clamped to the shoulders o f the specimens. M o s t tests were carried o u t to failure, while others were i n t e r r u p t e d after different p o r t i o n s of creep life in order to observe the creep d a m a g e accumulation. The fractured specimens were sectioned parallel to the applied stress axis a n d examined by SEM to observe creep crack d a m a g e a n d to c o m p a r e it to the porosity in a s - r e c e i v e d materials. Results a n d Discussion 3.1

Creep D e f o r m a t i o n and Initiation o f Cracks

Typical p h o t o g r a p h s of a s - r e c e i v e d a n d crept specimens are s h o w n in Fig.l. One sees t h a t after a creep strain of 15%, the a m o u n t that occurred in C M S X - 2 a t 8 5 0 t a n d 520 M P a after 95% of the creep life of the specimen, the initial r o u n d casting pores (Fig.la), located between dendrite arms, elongate much more than ex. pected from the average creep d e f o r m a t i o n a n d exhibit small cracks perpendicular to the load direction, as up. parent in F i g . l b . T h e n u m b e r of pores in advanced tertiary creep, just before fracture, is equivalent to the n u m b e r in the as-received material, showing that n o new cavities nucleate during creep. In a d d i t i o n to pores, fractured car. bides a n d interfaces between carbides a n d matrix can also act as crack initiation sites in alloys c o n t a i n i n g higher c a r b o n levels, i.e., S R R 99 a n d R R 2000 (Fig.2a). 3.2

Creep C r a c k G r o w t h and F r a c t u r e

Figure 2 shows the cracking m o r p h o l o g y in fractured material after 2317 h o f creep at 750U / 680 M P a in SRR 99 alloy a n d in prerafted C M S X - 2 alloy after 44 h o f creep at 850 ~ / 520 M P a (creep life fraction = 95% ). It can be observed that cracks nucleate at casting pores and they p r o p a g a t e at y / , / i n t e r f a c e s , Fig.2b. As local creep strain and stress increase, the cracks c o n t i n u o u s l y p r o p a g a t e until some o f t h e m coalesce a n d successively link up by ductile overload (Fig.3). Two typical tensile creep fracture surfaces are s h o w n in Fig.4. They consist mainly of three different types of crystallographic facets oriented on (001), {'111 } a n d (110) or (110) planes. It is revealed from Fig.4 t h a t a (0O 1) facet is always associated with the casting pores a n d t h a t creep cracks nucleated at pores a n d p r o p a g a t e d to create the (001) facets. M etallographic m e a s u r e m e n t s o f crystalline o r i e n t a t i o n showed t h a t the four edges of the nominally square (001) q uasicleavage facets were in < 110 > a n d < 110 > directions. This creep crack b e h a v i o u r is consistent with the fractography of low cycle fatigue fracture surfaces o f a high m o l y b d e n u m , N i - b a s e single crystal alloy (7). W h e n the overload condition is obtained, due to loss o f external section produced by b o t h h o m o g e n e o u s elongation a n d necking, a n d due to loss o f internal section d u r i n g creep crack p r o p a g a t i o n , the (001) s q u a r e - l i k e facets link up by shear: i) o n the [111} planes and ii) o n the (110) and (110) planes, in particular at higher tern. verature. The fracture surface m o r p h o l o g y , a n d possibly also fracture m e c h a n i s m s after creep, do n o t change substantially with testing t e m p e r a t u r e a n d stress for the e x a m i n e d three single crystal superalloys, a l t h o u g h at the highest temperature, 1 0 0 0 t , the crystallographic facets on the fracture surfaces a p p e a r hidden by oxide, Fig.5. In Fig. 6 h i s t o g r a m s describing the size distribution of (001) facets o n the fracture surface are reported for the case o f creep at 950U a n d different stresses. The h i s t o g r a m s show t h a t in general larger facets grow when overload is reached at low stress, while smaller facets a p p e a r o n the fracture surface at high stress. In Fig.7 the t o t a l area fraction, F, o f (001) facets projected on the c r o s s - s e c t i o n a l plane is plotted as a function o f applied stress for the three alloys examined. The figures indicate t h a t in general S R R 99 a n d R R 2000,

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containing higher c a r b o n , contain more creep cracking d a m a g e a c c u m u l a t i o n s t h a n C M S X - 2 alloy and t h a t the values of F for the latter alloy arc higher after creep at 950~ t h a n at 850U because stresses arc lower at higher temperature. As a first a p p r o x i m a t i o n F represents the K a c h a n o v type internal loss of section creep damage, and the m e a s u r e m e n t o f F after rupture gives i n f o r m a t i o n a b o u t the critical value reached by this p a r a m e t e r when overload takes place. In a n a l o g y to the creep d a m a g e evaluation by M a t e r a (8) o n a M n - C r polycrystal, the fol. lowing simple relation for the real c r o s s - s e c t i o n area, A, d u r i n g creep is adopted: A mAo(I-Z)(I

-

w)

(!)

where A e is the original c r o s s - s e c t i o n area, Z is the external reduction in area due to creep d e f o r m a t i o n a n d necking a n d w is a K a c h a n o v - t y p c creep damage, defined as loss o f load b e a r i n g capacity due to cracks. If the tensile strength of the material does n o t c h a n g e d u r i n g the creep process, then the true ultimate tensile stress a,.., as m e a s u r e d in a tensile test at the creep temperature, c o r r e s p o n d s to the overload stress in the residual ligaments o f the creep specimen; hence, the following relation holds: P ~1#

=

-

(2)

A

where P is the creep test load a n d A is the real c r o s s - s e c t i o n area at fracture. W h e n plastic instability occurs, the residual ligaments collapse, freezing the a c c u m u l a t e d value o f the creep crack d a m a g e at the value c o r r e s p o n d i n g to the onset o f the fast rupture. F r o m Eqs. 1 a n d 2, s u p p o s i n g t h a t the a d d i t i o n a l reduction in the cross section area, which occurs during fast fracture, is neglegible with respect to the creep reduction o f area (this a s s u m p t i o n is reasonable because the many creep cracks present can distribute the necking a m o n g the m a n y ligaments existing between adjacent cracks), one can o b t a i n the following expression for creep crack d a m a g e at fracture:

w

oB

= I

(3)

o .. (I -- Z ,)

where • e is the initial applied stress o f the creep test and Z, is the value o f Z measured after fracture. A d o p t i n g Eq. 3, an expression similar to the one p r o p o s e d by M a t e r a (8) for cavitation o f grain b o u n d a r i e s in an austenitic steel, o n e can evaluate creep d a m a g e using high t e m p e r a t u r e tensile test d a t a (9) and reduction in area at fracture after creep. Creep test parameters, calculated values o f d a m a g e w~ a n d measured values of p a r a m e t e r F for one o f the single crystal alloys examined are reported in Table 3 a n d Fig.8. As a p p a r e n t from Fig.8, the calculated w~ values are c o n g r u e n t with m e a s u r e d values of F, consistent with the hypothesis that the (U01)cracked facets repre. sent the creep d a m a g e t h a t directly p r o d u c e s the fracture in single crystals. Table 3. T(~ )

Creep Test P a r a m e t e r s of C M S X - 2 A L l o y

#o(MPa) '

tr(h)

sf

z,

273

0.24

0.29

460

385

0.26

0.39

850

380

1638

0.33

950

270

170

0.35

950

230

333 1036

850

520

850

i

F 0.30

,

950

1

190

J I

l

We I

0.33

I

[

0.35

0.31

.0.44

0.31

0.38

0.39

0.34

0.40

0.36

0.48

0.44

0.40

0.35

0.50

0.43

0.48

Conclusions The analysis o f fracture b e h a v i o u r in three single crystal superalloys led to the foUowing conclusions:

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- C r e e p cracks always initiated at casting pores a n d also at carbides in alloys containing a higher c a r b o n level. - C r a c k p r o p a g a t i o n occurred anisotropicaily a l o n g (001) crystallographic planes perpendicular to the ap. plied load and preferred to p r o p a g a t e a l o n g 7 / 7'interfaces. - T h e area fraction of (001) facets at fracture surface seems to be a good p a r a m e t e r to represent creep crack damage. - C r e e p crack d a m a g e m e c h a n i s m s are i n d e p e n d e n d e n t o f test t e m p e r a t u r e a n d stress. Acknowledgments The specimens used in this investigation were kindly provided by Alfa R o m e o Avio, PomigUano d~Aeco in the framework of the Progetto Finalizzato Energetica ]], C N R / E N E A a n d by F I A T Aviazione, Torino. A. Bianchessi skilfully conducted the creep testing. One of the authors, S.H. Ai is thankful for the financial support of the International Centre of Theoretical Physics, Trieste that allowed her to stay at C N R - I T M .

References l} ?,~ 3) ¢) 5) 0"~ 7) 8) 9)

Fritzcmcicr, L.G., in #Superalloys 1988 #, S. Rcichman et al. (eds.),The Metallurgical Society,pp. 265-274. Cocks. A.C.F. and Ashby, M.F.. Frog. Mat. Sci., 27, (1982) pp. 189-244. Anderson, P.M. and Rice. J.R., Acta Metall., 33. (1985) pp. 409-422. Wilkinson, D.S.. Acta Metall., 35, 0987)pp. 1251-1259. Tvcrgaard, V , Acta Mctall., 35, (1987) pp.923-933. Chokshi, A . H , Mater. Sci. Technol., 3, (1987) pp. 656-664-. A n i o n . D L.. Acta Metall., 32, (1984) pp. 1669-1679. M a t e t a . R., Scripta Metall., 23, (1989) pp. 6 5 - 7 0 . C a r o n . P. and K h a n . T., in T. K h a n a n d A. Lasalmonie (eds.), A d v a n c e d Materials a n d Processing Tech. niquc~ fo~ Structureal Applications, Proc. ASM ]~urope Conf., Paris, (1987) pp. 59-70.

...............~..........~,~i~'i~!!~!~,~i~!i!~ii~i!~ii~iiiiJii~!i~i~m ~,,~i,~iill~i~i~i~~!i~i,~~~i~,i!~i~~!~, i~ ~,i~,~iii~ii!!~ii~!~,,i~i~,,~i~i!~ Fig,l

i •

~' ~i~~:~¸

~i~ ~a

Typical casting porosity in a s - r e c e i v e d C M S X - 2 (a) a n d crack initiation a t porosity after prerafting at 1 0 5 0 t / 120 M P a / 6 0 h and creeping at 8 5 0 t / $20 M P a / 4 4 h and then stopped at 15% total creep strain before necking started Co).

4

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4

FII.2

Crack Initiation and propa|stion at pores and ~trbldes In SRR 99 rraetured after 2317 h at 750~ / 680 ]U[Pa (a) and at a pore in preraf~d C M S X - 2 after 44 h and IS%strain at 8501g / 520 MPa (b).

Fii~.3

Fil.4

Creep crack link up in Fractured C M S X - 2 miter 340 h at I0001~ / 130 MPa.

Fracture surface morpholoiw o / ' C M S X - 2 after 273 h at 8501~ / 520 MPa (a) and SRR 99 aPasr 2317 h at 750~ / 680 MPa (b).

Fill.5

Fracture 8urfaee and longitudinal sect/on morphology of C M S X - 2 after 273 h at 850~ / 520 MPa (a) and after l ~ 6 h at 1000~ / 190 MPa (b).

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,~

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CMSX-2 9SO'C CMSX-2 850"C SRR99 8~0"C RR2000 850"C

o o

g u

" 0.2

!

0

I

,

200 Initial

Fig,7

Applied

I

,

400 Stress (MPa)

600

Dependence of total area fraction of (00 I) facets on stress and temperature for different alloys.

tL

o,6 CMSX-2 950"C

u

[] 27oMP~ u

0.15

[]

~.

190 M P a

o

0.4

o

3

0.10[~--~--~/~

c o

K:

.o_

~o.o~

~

~

•"

0

60 120 180 240 Range of Area (Urn2)

i ~"

300

~ [_~ ~/

/

~ ,

0

Fig.6

~_ 0.2

~

o

/ 0

'

0

'

1

,

,

0.2 0.4 Creep D a m a g e , we

eso'c

,

0.6

Distributions of area fraction of (Q01) facets in FIB.8 Correlation between measures total area fraction

C M S X - 2 for two different stress values at 950113.

of (001) facets and calculated creep crack damage.