alloys. it is now being considered to introduce them also in land … · 2014. 5. 16. · peralloy...

Deformation behaviour of polycrystalline

(IN78LC) and single crystal (SC16) nickel

base superalloys under creep-fatigue loading

W. Chen, H. Chen, G. Schumacher, R.P. Wahi

Hahn-Meitner-Institut Berlin GmbH,

D-l4109 Berlin, Germany

Email: [email protected]

Email: [email protected]

Abstract

Deformation behaviour of polycrystalline (IN738LC) and single crystal (SCI6)superalloys has been investigated under creep-fatigue loading at 1223 K. Boththe alloys show qualitatively a similar deformation behaviour: In the compres-sive part of the cycles a stable cyclic stress response was observed under all testconditions. The stress amplitude in compression increases with decreasing holdstress. The average creep rate during the hold periods as a function of number ofload cycles shows a three stage behaviour: A rapid increase in the creep rate inthe first stage. This increase in the creep rate is dependent on the levels of holdstresses. The second stage is characterised by a much slower increase in thecreep rate with increasing number of cycles. At higher hold stress levels thecreep rate remains approximately constant. In the last stage, the creep rate in-creases again rapidly. Based on the results of microscopic examinations thisbehaviour is discussed in the light of dislocation substructure, internal stresses,stability of precipitate morphology and formation of macro cracks.

1 Introduction

For more than a decade now single crystal nickel base superalloys arebeing used as blade materials in advanced aerospace engines due to theirbetter high temperature properties in comparison with the polycrystalline

Transactions on Engineering Sciences vol 19, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533

Damage and Fracture Mechanics

alloys. It is now being considered to introduce them also in land-basedgas turbines for large electricity generating plants. The single crystal su-peralloy SCI6 is a recent development to replace the polycrystalline su-peralloy IN738LC, which is a widely used blade material in land-basedgas turbines \ Initial tests on the single crystal superalloy SCI6 revealedthat under tensile and creep loadings this alloy provided improved hightemperature mechanical properties^, as compared to IN738LC. Underlow cycle fatigue (LCF) conditions, SCI6 showed a cyclic life at varioustemperatures, which is about 10 times higher than that of IN738LC at thesame axial strain amplitudes' . In the present study the creep-fatigue

behaviour of both the alloys at 1223 K is investigated. The dependence of

the cyclic life of these alloys under creep-fatigue conditions was found tobe similar to that under pure LCF loading^, i.e. much larger lives for thesingle crystal alloy than those for the polycrystalline one. Therefore theelongation of single crystal blades due to creep deformation under cen-trifugal stresses could become potential life limiting factor for bladesinstead of crack initiation and fracture. In this paper the deformation be-haviour of both the alloys under creep-fatigue conditions are compared

and discussed.

2 Experimental

Both the alloys IN738LC and SCI6 have similar chemical compositions,given in Table 1. The polycrystalline specimens were subjected to hotisostatic pressing to reduce the micro porosity in the alloy. The singlecrystal specimens were roughly [001] oriented with a maximum deviationof about 6° in the specimen axis from the [001] crystal orientation. Boththe alloys have similar initial microstructure after the heat treatment (de-veloped individually for each of thenr ) consisting of unimodally dis-tributed ordered (LI2) y' precipitates embedded in fee y matrix. The cu-boidal y' precipitates have an average edge length of about 450 nm and avolume fraction of about 40 %.

Table 1. Nominal chemical composition in wt. %

AlloyIN738LCSC16

Al3.403.50

C0.11-

Co8.60

-

Cr15.916.0

Mo1.753.0

Nb0.82

-

Ta1.813.50

Ti3.473.50

W2.61-

NiBal.Bal.

The mechanical tests were conducted in a dedicated MTS servohydraulicmechanical testing system under total axial strain control at 1223 K, a


total axial strain amplitude of 0.4 % and a constant cyclic strain rate of

10"* s"'. Dwell periods were introduced only in the tensile part of the cy-

cles. During dwell periods, the stress (hold stress) was kept constant.

Different hold stresses between 150 and 230 MPa were employed in thepresent study. Since the total axial strain amplitude was maintained con-stant, in all the tests the creep strain is a function of hold stress level. Thelower the hold stress, the higher is the creep strain per load cycled So far

only on IN738LC some interruped tests at different deformation levels

could be carried out to study microstructural evolution during creep-fatigue loading.

The microstructures of the deformed specimens were analysed usingtransmission (TEM) and scanning electron microscopy (SEM). Tech-nique of specimen preparation for the TEM analysis can be found in Ref.6 and 7.

3 Results and Discussion

3.1 Cyclic stress response

Figures la and Ib show the cyclic compressive stress response of SCI6and IN738LC at different hold stress levels, including the results obtained

400

M LO 8to 8 1

A 175 MPaO 200 MPaO 225 MPaV no hold

1223 K

AE 12 0.4 %

10" 10' 10* 10-'Number of Cycles

(a)

IN738LC

CUs

%3#H£

J3C/3

S o

Hold Stress0 ISOMPa^ 170 MPaO 190MPaO 230MPaV no hold

1223K

AE^/2 0.4%

10 100Number of Cycles

(b)Fig. 1 Cyclic compressive stress response of superalloys (a) SCI6 and

(b) IN738LC.



from the LCF tests on the two alloys. As compared to the stress responsebehaviour under LCF condition, which shows a slight cyclic softening

(specially in the single crystal alloy) at the beginning of the test, a rela-tively stable cyclic deformation behaviour was observed in both of the

alloys under creep-fatigue conditions. A negligibly small cyclic harden-ing can be observed at the higher hold stresses in the single crystal alloySCI6. Since the total strain amplitude in the tests carried out in this studywas kept constant, the inelastic strain (sum of the creep strain and cyclic

plastic strain) becomes a function of hold stress level. For the tests atlower hold stress levels the inelastic strain is higher, as compared to thoseat higher hold stress levels at the same total strain amplitude. To over-come the additional inelastic strain, higher compressive stresses areneeded. This leads to the observed behaviour: The lower the hold stress,

the higher is the compressive stress amplitude, see Figure la and Ib.

3.2 Creep deformation

For both the alloys the hold time required to obtain a total strain of 0.4%decreases with increasing number of cycles, i.e. the material softens. Thevariation in the averge creep strain rate with the number of cycles fordifferent hold stresses is shown in Fig. 2a (SCI6) and 2b (IN738LC).Three deformation stages can be observed:

10 10' 10Number of Cycles

(a)

Number of Cycles

(b)Fig. 2 Average creep rate vs. number of cycles at various hold stresses:

(a)SC16and(b)IN738LC.


Stage 1: From the first to the second cycle, the creep rate increases con-

siderably. The extent of the increase is the largest at the lowesthold stress.

Stage 2: The creep rate remains stable at the higher hold stresses or in-creases slowly at the lower hold stresses. In the case of SCI6,

only a negligibly small hold-stress-dependence of creep rate isobserved.

Stage 3: Shortly before fracture the creep rate again increases rapidly.

An examination of the deformation microstructure of the fracturedspecimens revealed the following characteristic features:

i) Under all the testing conditions, a low density of homogeneouslydistributed dislocations was found essentially in the y matrix and atthe y/y' interfaces. Since the tests were always stopped due to frac-ture in the tensile part of the cycle, the above observation impliesthat the creep strain is the result of deformation essentially in the y-matrix.

ii) A directional coarsening and coalescence of the y' precipitates wasobserved in both the alloys. In IN738LC the effect was most pro-

nounced in specimens subjected to the smallest hold stress (Fig. 3a).This directional growth of the y' precipitates represents the initialstages of y'-rafting observed in this alloy under pure and cyclic creep

|500nm

(a) (b)Fig. 3 Micrographs showing y' rafts formation in (a) IN738LC (hold

stress GH = 150 MPa) and (b) SCI6 (GH = 200 MPa).



and cyclic creep ®. In the single crystal alloy the most pronouncedy' rafts formation was found in the intermediate range of holdstresses used (Fig. 3b).

For a better understanding of the softening behaviour observed during thehold period, some interrupted tests on both the alloys after reaching dif-

ferent degrees of deformation were conducted. The results obtained so far

on IN738LC are as follows:

i) At the end of any hold period, the dislocations were found to bedistributed homogeneously, essentially in the y matrix and at the y/y'interfaces, and had a lower density as compared to that after thecompressive cycle. This was especially true for the smallest holdstress (Fig. 4a).

ii) After a hold period and the following compressive deformation (at

1(T* s"') there existed a high density of dislocations produced byglide processes both in y and y' phases. The deformation was essen-tially planar (Fig. 4b).

iii) The directional coarsening of the y' precipitates, in case of the

smallest hold stress, was visible already after the first hold period.

t

(a)

[111]

/

(b)

Fig. 4 Micrographs showing (a) homogeneous dislocation substructureafter hold preiod and (b) planar slip bands after compressive part

of cycle.


Damage and Fracture Mechanics 499

On the basis of the microstructural observations we explain the observedsoftening behaviour during the hold periods as follows:

At the end of the compressive part of any cycle, a high dislocation den-

sity in the specimens corresponding to a strain rate of 1(T* s"* and amaximum stress of about 300 MPa was observed. These dislocationsproduce a stress field which opposes compressive deformation. On rever-sal of the loading direction from the compressive to the tensile part of the

cycle, this stress field favours the tensile creep deformation by raising theeffective creep stress. This effect and the high dislocation density inher-ited from the compressive part appear to be responsible for the large in-crease in the creep rate from the first to the second hold period. Duringthe hold periods, however, the density and distribution of dislocationsinherited from the compressive part change to adjust themselves to thelower hold stresses. This involves annihilation of dislocations throughglide and climb processes. The inherited internal stress also falls. Thus atthe end of a hold period the dislocation density was found to be reduced

and its distribution homogenised (Fig. 4a). Consequently the strain ratefalls continuously over the hold period. The extent of the readjustment ofthe dislocation substructure during the hold period will increase with thedecreasing hold stress and increasing length of the hold period. The pres-

ent experiments are so designed that decreasing hold stress increases thelength of hold period so that it is the hold stress alone which determinesthe extent of softening (Fig. 2). This softening is expected to stabiliseafter a few cycles. Superimposed on these effects is the change in themorphology of the y' precipitates which is also known to cause soften-ing . This morphological change occurs during the hold periods andcontinues until fracture. This effect is a possible cause of the observedslow increase in the strain rate in stage 2 at the lower hold stresses. Theshort compressive regions of the cycle are expected to have little influ-ence on this process. A series of interrupted tests on SCI6 are in progressin order to understand the finer details of the observed creep deformationbehaviour under creep-fatigue condition.

The final increase in the creep rate (stage 3) is due to macrocrack forma-tion and propagation.

4 Conclusions

• The polycrystalline (IN738LC) and the single crystal (S16) superal-loys show qualitatively a similar deformation behaviour under creep-fatigue condition at 1223 K.


500 Damage and Fracture Mechanics

• In the compressive part of the cycles a stable cyclic stress responsewas observed under all test conditions.

• The average creep rate under creep-fatigue condition is much higherthan that under pure creep loading. A three stage behaviour of creepdeformation was observed in both the alloys.

• This creep rate during the hold period is controlled by the i) disloca-tion substructure inherited from the preceding compressive part of thecycle, ii) instability in the y' morphology and iii) formation of macrocracks.

References

[1] Khan, T. & Caron, P. Development of a new single crystal superalloyfor industrial gas turbine blades, High Temperature Materials forPower Engineering 1990, Part II, eds. E. Bachelet, et al., KluwerAcademic Publishers, The Netherlands, pp. 1261-1270, 1990.

[2] Gottschmidt D. Single-crystal blades, Materials for AdvancedPower Engineering 1994, Part I, eds. D. Coutsouradis, et al., KluwerAcademic Publishers, The Netherlands, pp. 661-674, 1994.

[3] Auerswald, J., Mukherji, D., Chen, W. & Wahi, R.P. Deformationbehaviour of the single crystal superalloy SCI6 under low cycle fa-tigue loading, Z. Metallkde., 88, pp. 652-658; 1997.

[4] Wahi, R.P., Auerswald, J., Mukherji, D., Dudka, A., Fecht, H.-J. &Chen, W. Damage mechanisms of single and polycrystalline nickelbase superalloy SCI6 and IN738LC under high temperature LCFloading, J. Fatigue, in press.

[5] Dudka, A., Chen, W., Fecht, H.-J. & Wahi, R.P. High temperatureLCF behaviour of single crystal and polycrystalline nickel base su-peralloys SC16 and IN738LC, Fatigue 96, eds. G. Lutjering and H.Nowack, Pergamon, U.K., pp. 813-818, 1996.

[6] Chen, W. & Wahi, R.P. Mechanical behaviour of polycrystalline andsingle crystal superalloys IN738LC and SCI6 under creep-fatigueconditions, 6*** Liege Conference on Materials for Advanced PowerEngineering 1998, to be published.


Damage and Fracture Mechanics 501

[7] Li, J., Wahi, R.P., Chen, H., Chen, W. & Wever, H. Deformation

substructure in the nickel base alloy IN738LC under superimposed

creep-fatigue loading, Z Metallkde., 84, pp. 268-272, 1993.

[8] Wang, Y., Chen, W., Mukherji, D., Kuttner, T. & Wahi, R.P. Thecyclic creep behaviour of nickel base superalloy EN738LC, Z. Met-o%Wf., 86, pp. 365-370, 1995.

[9] Mukherji, D., Gabrisch, H., Chen, W., Fecht, H.-J. & Wahi, R.P. Me-chanical behaviour and microstructural evolution in the single crys-tal superalloy SC16, Acta mater., 45, pp. 3143-3154, 1997.


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