Low cycle fatigue behaviour of cast nickel-base superalloy IN738LC at room temperature

Low c y c l e fatigue behaviour of cast n i c k e l - b a s e superalloy IN738LC at room temperature Guo Jiantingand D. Ranucci The cyclic stress~strain response and the low cycle fatigue life of cast nickel-base superalloy IN738LC were studied. Fully reversed strain-controlled tests were performed at room temperature and at two different strain rates. Optical and electron microscopy were used to study the processes of deformation and cracking during cycling. A power.law relationship between life-time and total (plastic as well as elastic) strain range was obtained, which is not influenced by the strain rate and by the frequency. Cracking was generally initiated at the surface microporosities and propagated along interdendritic paths. During cyclic deformation, only cyclic hardening occurred at room temperature. Keywords: fatigue; optical microscopy; electron microscopy; low cycle fatigue; microporosities; cyclic hardening; nickel-base alloys

IN738LC is a high strength cast nickel-base superalloy which is used extensively in the gas turbine industry as a gas turbine blade material, and low cycle fatigue (LCF) is an important consideration in the design of turbine components such as blades and discs. So there is considerable interestin the L C F of superalloy IN738LC, not only from a fundamental point of view, but also from the practical one. The high temperature L C F characteristicsand the effect of environments on them have been well documented. 1-5 However, the room temperature L C F characteristicsof this alloy have not been studied, in spite of these characteristics also being important design parameters for the use of this engineering alloy. The present study reports on the L C F behaviour of IN738LC at room temperature, in order to add to the body of information on the L C F of IN738LC and nickel-base superalloys.

Experimental procedure Strain controlled low cycle fatigue tests were performed on cast hourglass specimens at room temperature. The composition of the alloy is given in Table 1. The heat treatment schedules consisted of 2 h at 1120°C in vacuum or hydrogen followed by cooling to room temperature, then 24h at 845°C in argon or vacuum followed by cooling to room temperature. The heat treatment employed resulted in two various 7' sizes (Fig. 1).

The large T' particles were nearly cuboidal. Their cube edges were about 0.43/zm, while the small T' phase consisted of spherical particles of approximately 0.11 pro. The grain size was found to be approximately 2 grains per mm as determined by linear intercept. The machined specimens were tested using a 25T closed loop servohydraullc MTS machine. A diametral extensometer was placed on the specimen and all tests were carried out in diametral strain control at strain rate 10 - 2 or 10-3s -1, with a triangular wave form and with a mean strain of zero. During testing, the load was continually monitored and hysteresis loops were recorded at appropriate intervals. The triangular wave form was chosen for this study largely because of the large body of information that has been accumulated; it is common to look upon the triangular wave form tests as a standard from which to predict other wave forms and to compare material behaviour. After testing, the specimens were extensively examined by optical, electron and scanning electron microscopy. The fractured specimens were sectioned on a plane containing the stress axis and crack initiation site in order to examine microstructural features as wen as the relationship between the crack path and the microstructure.

Table 1. Composition of the alloy (balance is nickel) Element

Weight (%)

Element

Weight (%)

C Si Mn Cr Mo Co W Nb

0.12 <0.10 <0.10 15.90 1.60 8.30 2.50 0.96

Ta AI Ti Zr B P Cu Ni

1.72 3.40 3.30 0.07 0.22 0.002 0.001 <0.10

Fig. 1 Transmission electron micrograph showing two different '7' sizes

0142-1123/83/020095-03 $3.00 © 1983 Butterworth & Co (Publishers) Ltd Int J Fatigue Vol 5 No 2 April 1983

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Results and discussion Ae Versus Nf T h e d i a m e t r a l s t r a i n range was c o n v e r t e d t o t o t a l ]ongi-

tudinai strain range Aet, longitudinal plastic strain range Aep and longitudinal elastic strain range Ace; their plots against life were constructed (Fig. 2). The data determined from an analysis of the hysteresis loop at approximately half life are found to have a linear relationship with the three kinds of strain range and cycles to fracture, which is independent of the strain rate and of the frequency. This is in agreement with results obtained for the alloy at 900°C in vacuum. 6 It must be pointed out that there is also a linear relationship between the total strain range and cycles to fracture for alloy 901, alloy 718 and Waspaloy.7 The CoffinManson equations representing these data are given by: Act = 0.036Nf 0"17

(I)

Aep =

(2)

0.094Nf -°'58

ACe = 0 . 0 1 3 N f - ° ' ° 4 8

(3)

An examination of the fatigue fracture surface shows that crack initiation was predominantly transgranular and nucleation sites were always at the surface microporosities (Fig. 3). Crack propagation from these nucleation sites is

I 0 -~

~

o.o~~ \

~

•

^_

<3

Fig. 4 SEM image o f the fracture showing cracking along an interdendritic path

essentially transgranular, and there is a definite tendency for cracking to occur along an interdendritic path (Fig. 4). Metallographic observation of longitudinal sections of the test specimens shows similar results. Because there are no effects of temperature, creep, environment and microstructurai instability on fatigue behaviour at room temperature, the deformation behaviour within the plastic zone is cycle dependent ordy.8 So the low cycle fatigue of IN738LC may be expressed in terms of a Coffin-Manson type equation relating total strain range, plastic strain range and elastic strain range to cycles to fracture. Such an equation is not influenced by the strain rate or by the frequency.

Cyclic stress/strain and cyclic hardening 10"~

The cyclic stress/strain curve is plotted in Fig. 5. This curve shows a linear increase of stress range with plastic strain range. The equation representing these data is given by

0,045 Open symbols E = 10-36-' Closed symbols i = 10-Zs-I i

l01

i

i

i I 10 2

,

i

i

i I 10 3

=

i

,

N f (cycles)

Fig. 2 Strain ranges (Ac t, ~e e and Aep) versus fatigue life for IN738LC in air at room temperature and at different strain rates

A o = 3048.51 A e p °'°66

L

(4)

io4

Typical cyclic hardening curves are plotted in Fig. 6. These curves show that the alloy quickly hardens for the firstfew cycles of life,then reaches the steady-state stress

Fig. 3 SEM image of the fracture showing nucleation sites at the surface microporosity (Nf = 2610 cycles, Aep = 0.001, ~ = 10-2sec -~ )

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Int J Fatigue April 1983

have occurred due to the bowing of dislocations around the particles, and a network of dislocation segments extends between each particle (Fig. 7). This would give greater hardening attributable to fatigue. Asia general observation, saturation occurs once the overall network has filled the grain volumes.

~<

1

Conclusions

U) I

1

I

2 3 4 Plastic strain range (%)

I

I

5

6

1)

Fig. 5 Cyclic stress/strain bahaviour of I N738LC at room temperature •

i

2)

205O I ZI( t = 00015

:E b <~

,~

%

2OO0 1950

3) f

0.0125

-"'~

2 ,~ = io"%-'

1700

The low cycle fatigue of IN738LC may be expressed in terms of a Coffin type equation relating total strain range, plastic strain range and elastic strain range with cycles to fracture, which is independent of the strain rate and of the frequency. Fatigue cracks are nucleated principally at the surface microporosities of test specimens, and propagate transgranularly along an interdendritic path. It is obvious that the low cycle fatigue life will increase if the surface microporosities are removed. During testing at room temperature, specimens of IN738LC cyclically harden to a saturation stress value. Cyclic saturation is due to dislocation substructure which fills the grain volumes.

,i = 1 0 " 3 s - I

1650

I

i

I

I

I

t

I

I

i

io

I

I

=

I

i

i

~ Number

of cycles

N

References 1.

Day, M. F. and Thomas, G. B. ' L o w cycle fatigue of two Ni-Cr base gas turbine blading alloys' Int Confon High Temperature Alloys for Gas Turbines (Centres de Recherches Metallurgiques, Belgium, 1978) 10p 641--661

2.

Jianting, G., Ranucci, D. and Pi¢¢o, E. 'The effect of hot corrosion on the low cycle fatigue of IN738LC at elevated temperature' Proc Conf X Convegno AlAS (Universit~ della Calabria, Arcavacata di Rende, Cosenza, 1982) pp 259--269

3:

Marchionni, M., Ranucci, D. and Picco, E. 'High-temperature low-cycle fatigue bahaviour of IN738LC alloy in air and in vacuum' Proc Conf on High Temperature Alloys for Gas Turbines (Centres de Recherches Metallurgiques, Belgium, 1982) pp 791--804 Jianting, G., Ranucci, D. and Picco E. 'Low cycle fatigue behaviour of cast nickel-base superalloy IN738LC in air and hot corrosion environments' Motor Sci Eng 58 (1983) pp 1 2 7 - 1 3 3

Fig. 6 Cyclic hardening behaviour of IN738LC at room temperature

4.

5.

Mmsarelli, L., Remucci, D. and Picco, E. 'Environmental effects on high temperature low-cycle fatigue of cast nickelbase alloys' COST 50, Final Report on Materials for Gas Turbines, 1980

Fig. 7 Transmission electron micrograph showing dislocation substructure

6.

Jianting, G., Ranucci, D., Pi¢¢o, E. and Strocchi, P. M. 'Effect of environment on low cycle fatigue behaviour of cast nickel-base superalloy IN738LC' (to be published)

range and subsequently further hardens cyclically to saturation in the majority of the experiments. A rapid decrease in stress is observed when fatigue crack initiation and propagation occur. Studies of the cyclic stress/strain response in alloys Containing coherent precipitates have emphasized the cyclic softening which occurs when the alloys are in the under'aged condition, while the large cyclic hardening has received much less attention. 6 Stoltz and Pineau point out that the cyclic saturation of Waspaloy is due to a dense dislocation substructure which t'dls the inter-slipband volume.8 Our results show that the dislocation substructure completely f'flls each grain. A large number of dislocation interactions

7.

Merrick, H. F. 'The low cycle fatigue of three wrought nickelbase alloys' Met Trans 5 (1974) pp 8 9 1 - 8 9 7

8.

Stoltz, R, E. and Pineau, A. G. 'Dislocation-precipitate interaction and cyclic stress-strain behaviour of a 3" strengthene~ superalloy' Mater Sci Eng 34 (1978) pp 275--284

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Authors D. Ranucci is at the Institute of Technology for NonTraditional Metals (ITM) - CNR, Via Induno 19, 1-20092 Cinisello Balsamo/Milan, Italy. Guo Jianting is with the Institute of Metal Research, Academia Sinica, Wenhua Road, Shenyang, China. Inquiries should be addressed to Professor Guo Jianting.

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