MATEC Web of Conferences 14, 06001 (2014)DOI: 10.1051/matecconf/20141406001c© Owned by the authors, published by EDP Sciences, 2014

Thermomechanical fatigue in single crystal superalloys

Johan J. Moverare1,a and Roger C. Reed2

1 Engineering Materials, Dept. of Management and Engineering, Linkoping University, 581 83 Linkoping, Sweden2Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK

Abstract. Thermomechanical fatigue (TMF) is a mechanism of deformation which is growing in importancedue to the efficiency of modern cooling systems and the manner in which turbines and associated turbomachineryare now being operated. Unfortunately, at the present time, relatively little research has been carried outparticularly on TMF of single crystal (SX) superalloys, probably because the testing is significantly morechallenging than the more standard creep and low cycle fatigue (LCF) cases; the scarcity and relative expenseof the material are additional factors. In this paper, the authors summarise their experiences on the TMFtesting of SX superalloys, built up over several years. Emphasis is placed upon describing: (i) the nature ofthe testing method, the challenges involved in ensuring that an given testing methodology is representativeof engine conditions (ii) the behaviour of a typical Re-containing second generation alloy such as CMSX-4,and its differing performance in out-of-phase/in-phase loading and crystallographic orientation and (iii) thedifferences in behaviour displayed by the Re-containing alloys and new Re-free variants such as STAL15. Itis demonstrated that the Re-containing superalloys are prone to different degradation mechanisms involving forexample microtwinning, TCP precipitation and recrystallisation. The performance of STAL15 is not too inferiorto alloys such as CMSX-4, suggesting that creep resistance itself does not correlate strongly with resistance toTMF. The implications for alloy design efforts are discussed.

1. IntroductionNickel-based superalloys are designed to withstandextreme conditions of temperature and loading duringoperation. Their performance in critical hot gas pathcomponents such as vanes, blades and combustors largelylimits the durability of modern gas turbines. Suchcomponents experience complex thermal and mechanicalloading, especially during engine start-up and shut down,when the thermal gradients in conjunction with mechanicalconstraints and the time variation of both the temperatureand the stresses eventually may lead to fatigue damage,a phenomenon often referred to as thermomechanicalfatigue (TMF). Broadly speaking, TMF failure is promotedwhen plastic strains cannot be accommodated at lowtemperatures and creep deformation in combinationwith oxidation occurs at high temperature. However,microstructural stability plays an important role and itis now established that there are significant interactionsbetween the degradation mechanisms occurring at high andlow temperatures. In addition the effect of coatings must beconsidered.

Although material for critical hot components hastraditionally been evaluated with respect to creep, thedeformation characteristics under TMF conditions are nowjust as important and need to be understood and quantifiedif component lifetime estimates are to be accurate. Indeed,one of the major causes of failure of hot gas path

a Corresponding author: johan.moverare@liu.se

components in current gas turbines is thermally inducedcracking aggravated by oxidation. The main load cycleof such components will always be of the type start-up,operation and shut down, but the length and detailed formof such a load cycle will depend on the turbine application:i.e. whether it is a jet engine used for aircraft propulsion,or alternatively a stationary gas turbine situated in a power-plant. The detrimental effect of a load cycle can thereforevary significantly between different types of operatingconditions.

Thermomechanical fatigue in superalloys is growingin importance since gas turbines are being operatedunder ever more arduous conditions, to reduce fuelusage and CO2 emissions and to maximise performance.The introduction of thermal barrier coatings (TBCs) andsophisticated and more efficient cooling systems togetherwith higher operating temperatures have lead to a tendencyfor thinner wall thicknesses which means that the thermalgradients are steeper. In addition, the changes in demandsand competition within the power generation marketforces the power plants in many countries to operateunder cyclic conditions. Today, it is common to havegas turbines in intermittent operation in order to balanceirregular fluctuations of the peak load requirements of agrid. This means that the turbines either start and stopon a daily basis or have significant variations in poweroutput over a very short period of time. This exacerbatesthe importance of the thermomechanical fatigue (TMF)effect.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution,and reproduction in any medium, provided the original work is properly cited.

Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20141406001

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2. TMF testing methodology

Constraint of free thermal expansion and contraction isan intrinsic ingredient of the thermally induced fatigueprocess. It is therefore this which is of interest to reproduceon a laboratory scale. This can be achieved by meansof thermal fatigue (TF) tests on specimens with internalconstraints or by thermomechanical fatigue (TMF) testson specimens with external constraints. TMF testing onspecimens with external constraints was introduced inthe early 70’s; the pioneering work on superalloys wasconducted at Pratt & Whitney Aircraft on both smoothspecimens and specimens suitable for studying TMF crackpropagation [1,2]. However, testing of materials in TMF isstill the subject of considerable debate, and standards for itare relatively new.

Most TMF tests are conducted in strain control usingservo-electric or servo-hydraulic testing machines in whichthe stress and strains are measured using a load cell andhigh temperature extensometry, as in a normal LCF testat elevated temperature. Several methods such as directand indirect induction heating, resistive heating as wellas radiation heating have been successfully used for non-isothermal TMF testing, with direct induction heatingbeing the most frequently employed [3].

In practice, several aspects require careful considera-tion if TMF testing is to be meaningful and accurate. In themechanical strain-controlled TMF tests, the measurementand control of temperature is considered to be a mostcritical issue; this is because an error in the temperaturegives rise to an error in the mechanical strain, whichwill in turn affect the stress response and the cyclic lifemeasured. Thus, the temperature measurement was foundto be the major origin of scatter in a recent round robinexercise [4]. Furthermore, as the specimen is continuouslyheated and cooled during TMF testing, thermal gradientsare inevitable, and if not kept low enough these gradientswill lead to premature failure of the test piece. Goodcontrol of the gradients both during cooling and heatingis thus vital if testing conditions are to be appropriate. InTMF testing it is also, as for LCF-testing, important toreduce the misalignment in the load train to a minimum.

The fact that single crystal superalloys are highlyanisotropic and also sometimes show a very localizeddeformation behaviour add an extra complexity to thetesting of these materials. If deformation primarily occuralong deformation bands that extend across the completecross section of the specimen, as seen in Fig. 1, theinterpretation of the results can be difficult if there also isa gradient in stress and temperature along the deformationband.

Apart from the difficulty to do the actual testing itis also of high importance to carefully select the testconditions to ensure that it is representative of real engineoperation. This often requires very good knowledge ofthe component of interest with respect to stress, strainand temperature fields over the component and how thesefields vary with time. In addition it is necessary to havegood knowledge about the material behaviour in order tointerpret the results; sometimes it is necessary to take thedegradation effects that occur during long terms serviceinto account [5,6].

Figure 1. Deformation bands in STAL15 due to TMF.

Tem

per

atu

reMechanical Strain

Tmax

T

ε ε

min

max min

Out-of-Phase (OP)

In-Phase (IP)

ClockwizeDiamond (CD)

Counter ClockwizeDiamond (CCD)

Figure 2. Illustration of different TMF-cycles.

TMF testing provides a powerful tool to assess thesynergistic effects of combined thermal and mechanicalloads and involves the cycling of the temperature T andmechanical strain εmech with different phase shifts. Thetesting can thus be very component-specific and mimicmore or less exactly the situation in a critical location of acomponent. However, in order to generate generic materialdata, it is common to stick to some simplified generalcycles such as the in-phase (IP), the out-of-phase (OP)cycle or diamond cycles, see Fig. 2.

Focusing on the IP-cycle, we have a situation inwhich the tensile stress/strain loading coincides with thehigh-temperature part of the cycle. The deformation isthus characterized by high temperature creep in tensionand low temperature plasticity in compression. For purethermal loadings, this situation is found at “cold-spotareas” where the temperature is typically lower comparedto the surrounding, e.g. near cooling holes or at structuresinside cooling channels. However, the stresses/strains fromthermal gradients can also be superimposed by externalloads such as internal pressure and centrifugal forceswhich will further increase the tensile stress at elevatedtemperature. The combination of high tensile stresses at anelevated temperature and significant hold times can havea very detrimental effect on fatigue performance even forrather moderate temperatures.

The situation during an OP TMF-cycle is completelydifferent. In this case the material undergoes creeprelaxation in compression at high temperatures and plasticdeformation in tension at low temperatures. This is thesituation found at “hot-spot areas”, i.e. small regionswith higher temperature than the near environment whichmay occur on e.g. on a turbine blade airfoil or platformdue to insufficient cooling. Since these areas will be in

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-1000

-800

-600

-400

-200

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600 700 800Stress,M

Pa

Cycles

Maximum Stress

Mean Stress

Minimum Stress

Figure 3. Evolution of maximum, mean and minimum stressduring IP TMF testing (100–750 ◦C) of MD2.

compression during the high-temperature running periodof the engine, high-temperature creep relaxation willcause the material to be in tension at shut-down. Sincerafting and other degradation mechanisms occurring athigh temperature are found to have a higher effect on thelow-temperature properties than on the high-temperatureones, it can be readily concluded that the OP TMF-loadingcondition is quite different from an iso-thermal LCF-loading.

In case of both IP and OP-TMF, the magnitude of thestresses in the cold end of the TMF cycle will increasewith the amount of stress relaxation in the hot-end ofthe TMF cycle. Thus, the possibility for having reversedplasticity at engine shut-down will increase with theamount of stress relaxation during service. Thus the lengthof the time spent at high temperature will (apart fromother degradation mechanisms) be important for the TMFlife of the component. As creep always is present whenengineering components are subjected to TMF conditions,it is strongly recommended also to introduce a dwell periodat the maximum temperature in the TMF-test. The lengthof the dwell period often varies from a couple of secondsto several hours, but for practical reasons the dwell timein laboratory tests is often much shorter than the typicaltime of operation for the component of interest. Thisis especially the case for hot components in industrialturbines where the average time between start and stop canbe up to 500 hours.

From a testing perspective, it is of course problematicto use very extended dwell times. Short dwell times arehowever also problematic when evaluating the results sincethe time dependent creep relaxation in one end of the TMFcycle can shift the mean stress significantly cycle by cycleduring the test, as shown in Fig. 3. As a consequence nostabilized stress values will be achieved. For single crystalmaterials this is typically a problem for lower values ofTmax and short dwell times. One way to partly overcomethis problem is to use a significantly longer dwell time inthe first cycle. The industrial standard for some companiesis to apply a 20 hour dwell time in the first cycle and5 minutes dwell time in the following cycles [5]. Also theR-value has to be selected carefully. By assuming that theminimum temperature is the reference temperature where

Figure 4. Microstructure of MD2 in virgin and degradedcondition tensile tested at 20 ◦C in the 〈001〉 direction [9].

the mechanical strain is defined as “zero” the definition ofthe IP and OP cycles implies Rε = 0 for the IP cycle andRε = −∞ for the OP cycle. Since both these R-values arenon symmetric they will lead to a significant redistributionof stresses during the first cycle, i.e. the amount of plasticdeformation is much larger in the first cycle compared tothe following cycles. This partly contribute to the problemwith the drift of mean stress as illustrated in Fig. 3 butthis redistribution of stresses is the actual situation formost critical areas sensitive to fatigue. For single crystalsuperalloys, the plastic deformation in the first cycle is alsocrucial in order to enhance microstructural changes such asrafting. In fact rafting has been found to occur very rapidlyduring TMF testing [6]. Thus, to fully take the degradationeffect into account it is often necessary to also do testing onservice exposed material or to simulate service conditionsby doing interrupted testing with an intermediate artificiallong term ageing treatment [5].

Thus there are a number of testing parameters that haveto be selected carefully in order to do TMF testing that isrelevant from the component perspective. One parameterthat is not included here but will be covered in the nextsection is the influence of the minimum temperature.

3. Microstructural aspects of TMF3.1. Influence of rafting & minimum temperature

The yield strength anomaly for the pure γ ′-phase(Ni3Al) is rather well known [7] but the implicationsof this phenomena on TMF testing of γ -γ ′ alloysand the influence of minimum temperature is oftenoverseen. Both the γ matrix and the γ ′-precipitate arerelatively weak at room temperature. The remarkablyhigh virgin material yield strength is a consequence of afavourable matrix/precipitate interaction associated withthe carefully optimised, but thermodynamically unstable,virgin microstructure. Therefore it is only natural that adeviation from this carefully optimised microstructure willresult in a reduction of the yield strength [8]. One examplecan be seen in Fig. 4, where the virgin microstructurewith a room-temperature yield strength of 934 MPa iscompared to the microstructure of a degraded material witha room-temperature yield strength of 729 MPa. Even if theorientation and composition is identical there is a reductionof approximately 25% due to the degradation [9].

At higher temperatures, this class of alloys does notneed to rely as much on a favourable matrix-particle

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Figure 5. Influence of dwell and Tmin on the TMF crack initiationlife of CMSX-4, from [8].

interaction, because the γ ′ imparts strength regardless ofthe shape and size of the particles. This is associated withthe fact that the γ ′ shows an increasing yield strength fromroom temperature to about 700 ◦C [7]. Because raftingoccurs during TMF tests with high Tmax and dwell times,it follows that the material will yield at a lower stress valuethan in the virgin condition but this effect will also increasewith decreasing value of Tmin. When the material yieldsat a lower stress level, it will result in a higher inelasticstrain range and a correspondingly reduced crack initiationlife. This has been demonstrated for CMSX-4, see Fig. 5,where the TMF life was reduced by a factor of 3 whenthe Tmin was reduced from 400 ◦C to 100 ◦C [8]. Similarbehaviour has also been reported for directional solidifiedmaterial [10].

3.2. Slip and twinning

During OP-TMF testing of second generation singlecrystals such as CMSX-4, highly localized twinning arefound to be the main deformation mechanism [6,11–13].In the study by Segersall [14] it was found that for highstrain ranges the twins often extend across the completecross section of the specimen, which leads to a highlycrystallographic fracture surface as seen in Figs. 6a and 6c.For lower strain ranges the fracture surface is more non-crystallographic and typically perpendicular to the fracturesurface, as seen in Figs. 6b and 6d.

However, even for the non-crystallographic fracturesurfaces in Figs. 6b and 6d, twinning was found to bethe main deformation mechanism. This is illustrated inFig. 7 were it can be seen that the crack is surrounded bycrystallographic deformation and twins. During OP TMFtesting of specimens oriented along the 〈001〉 direction,twinning will occur during compression at elevatedtemperature as a result of activity of the 〈112〉{111}type slip system which requires the passage of a/3〈112〉dislocations through both the matrix and superlattice onadjacent {111} planes [15].

In the same study by Segersall [14] OP TMF testingwas also performed on 〈011〉 oriented samples and forthis direction twinning was less frequently observed;especially at high strain ranges, deformation occurred invery localized deformation bands but without formation of

<

<

High

<001>

<011>

(a)

(c)

h strain range Low stra

(b)

(d)

ain range

Figure 6. Influence of strain range on the crack appearance forCMSX-4 during OP-TMF 100–850 ◦C [14].

Figure 7. Crystallographic deformation and twins surrounding asecondary crack in CMSX-4 after OP-TMF 100–850 ◦C testing(low strain range for the 〈001〉-direction) [14].

twins. The deformation occur by slip and cutting of theγ ′ particles along the {111} planes but seems to also beaccompanied by a local rafting mechanism as illustratedin Fig. 8. The orientation of the single crystal relative tothe applied load is thus important for the formation ofdeformation twins during TMF.

In addition to the influence of crystallographicorientation, single crystal superalloys are known to displaya tension/compression asymmetry in both plasticity andcreep [9,16,17]. Consequently, different deformationbehaviour can be expected for IP and OP cycling duringTMF. This has been illustrated in a recent study bySegersall [18] where a clear difference between IP andOP TMF was found: for the 〈001〉 direction twinning wasmuch more pronounced during OP TMF while for the〈011〉 direction IP TMF leads to twinning.

While the observations described above are typicalfor second generation single crystal superalloys with ahigh volume fraction of γ ′ such as CMSX-4, a somewhatdifferent behaviour is observed in Re-free single crystalswith a lower fraction of γ ′, such as SCA425 and STAL15(Ni-5Co-1Mo-3.7W-15Cr-4.55Al-8Ta). Even if twinningsometimes is observed, the main deformation mechanismis slip along the {111} planes even during OP TMF testingalong the 〈001〉 direction [19,20]. Slip deformation isconcentrated within a small number of bands along thespecimen gauge length. The thickness of these bands canbe up to 2 µm and they often develop across the entire

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5 µm

Figure 8. Deformation band formed in CMSX-4 during OP-TMF(100–850 ◦C) testing along the 〈011〉 direction [14]. Stress axis isthe vertical direction.

Figure 9. Deformation band formed in SCA425Hf during OP-TMF (100–950 ◦C) testing along the 〈001〉 direction [19].

cross-section of the specimen. The structure inside thebands can be seen in Fig. 9.

While during creep, dislocation activity is mainlyrestricted to the γ matrix phase, TMF failure is promotedby localized shear banding with the γ ′ phase penetratedby dislocations. Consistently it was found in [20] thatthe size of the γ ′-precipitates influences creep and TMFdifferently. The TMF resistance increased with increasingprimary γ ′ size while in creep the finer primary γ ′microstructure gave better performance. In addition it hasalso been found that Si which can be believed to strengthenthe γ ′-phase significantly improves the resistance to TMFfailure [11].

3.3. TCP formation and recrystallization

The plastic and elastic energy stored inside the de-formation bands described in the previous section canbe expected to be rather high. This will explain theirsusceptibility to TCP formation and recrystallization. Inreference [6] it was found that within the deformationbands in CMSX-4 after OP TMF testing with a maximumtemperature of 1000 ◦C, small precipitates of topologicallyclose-packed (TCP) phases could be observed, see Fig. 10.WDS analysis revealed that the precipitates are richin Ta, W and Re, consistent with CMSX-4’s knownpropensity for precipitation of the µ-phase. Since the TCPphases were observed only inside or at the twin/matrixinterface, the precipitation must have occurred after thetwinning. Thus the precipitation of the TCP phases mustbe promoted by the plastic and elastic energy associated bythe deformation band. In the case of twinning there might

Figure 10. Deformation band with TCP phases formed inCMSX-4 during OP-TMF (100–1000 ◦C) testing along the 〈001〉direction [6].

also be the possibility that thermally-activated diffusionalprocesses that must occur during the formation of the twincan lead to segregation of heavy elements [21,22] whichalso promotes the formation of TCP-phases.

When long term ageing at 1000 ◦C for 4000 hours wasperformed on CMSX-4 a lot of needle-shaped precipitateswere found, which act as obstacles for the slip ortwin bands. As a consequence, the slip and twin bandscannot extend across the complete cross-section of thespecimen and thus prevent the deformation from beingas highly localized as for the virgin material. In orderto accommodate the plastic deformation in the agedmaterial, more slip or twin bands are activated comparedto virgin material, giving a more dispersed deformationbehaviour [6].

In addition to TCP formation it has frequently beenobserved that recrystallization eventually will occur withinthe bands of localized deformation. The absence ofgrain boundary strengthening elements in single crystalmaterials leads to easy crack propagation along therecrystallized bands. The recrystallization is illustrated inFig. 11 for an OP TMF test on CMSX-4 with a maximumtemperature of 1000 ◦C [6]. Similar observations have alsobeen found in TMF tests with a maximum temperature of850 ◦C [14] and in MD2 even in test with 750 ◦C as themaximum temperature [18].

The starting point for the recrystallization processhas been found to be the intersection of twins growingin different directions. Once recrystallization has started,it can develop further along the twin directions [6].Interestingly, no recrystallization is observed for CMSX-4in the long term aged condition [6] due to the smaller widthand length of the twins and the more dispersed deformationbehaviour.

4. Conclusions• During TMF testing if the highest temperature

is above 900 ◦C, rafting occurs rather rapidly.Consequently, the low temperature yield strength isreduced and there is a strong influence on the TMFlife when the minimum temperature is altered from400 ◦C to 100 ◦C.

• Twinning is an important deformation mechanismduring TMF in single crystal superalloys. Twinningis typically more pronounced during OP TMF

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Figure 11. Left: micrograph showing TCP formation andrecrystallization near a crack formed during OP TMF (100–950 ◦C) testing of CMSX-4. Right: region within a bandof localized deformation characterized by EBSD, confirmingrecrystallization [6].

testing along the 〈001〉 direction and during IP TMFtesting along the 〈011〉 direction.

• In alloys with a lower fraction of γ ′, twinning is lessfrequently observed but still the deformation is verylocalized to a few deformation bands.

• Deformation bands formed by TMF are sus-ceptible to TCP formation and recrystallization.Recrystallization is particularly detrimental to TMFresistance and crack formation occurs readily alongthe recrystallized deformation bands.

• When recrystallization occurs in this way, thenucleation sites are often the intersection of twopropagating twins of different orientations. DuringOP-TMF testing recrystallization has been observedin tests with a maximum temperatures as low as750 ◦C.

The work has been financially supported by Siemens IndustrialTurbomachinery AB in Finspang, Sweden and the SwedishEnergy Agency, via the Research Consortium of MaterialsTechnology for Thermal Energy Processes, Grant No. KME-502.

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