In- and Out-of Phase Thermomechanical

Fatigue of a Ni-Based Single-Crystal Superalloy

Mikael Segersäll, Johan Moverare, Daniel Leidermark and Sten Johansson

Linköping University Post Print

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Original Publication:

Mikael Segersäll, Johan Moverare, Daniel Leidermark and Sten Johansson, In- and Out-of

Phase Thermomechanical Fatigue of a Ni-Based Single-Crystal Superalloy, 2014,

EUROSUPERALLOYS 2014 – 2nd European Symposium on Superalloys and their

Applications, MATEC Web of Conferences, (14), 19003.

http://dx.doi.org/10.1051/matecconf/20141419003

Copyright: Owned by the authors, published by EDP Sciences, 2014

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Postprint available at: Linköping University Electronic Press

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MATEC Web of Conferences 14, 19003 (2014)DOI: 10.1051/matecconf/20141419003c© Owned by the authors, published by EDP Sciences, 2014

In- and out-of-phase thermomechanical fatigue of a Ni-basedsingle-crystal superalloy

Mikael Segersall1,a, Johan J. Moverare1,2, Daniel Leidermark1, and Sten Johansson1

1 Department of Management and Engineering, Linkoping University, 58183 Linkoping, Sweden2 Siemens Industrial Turbomachinery AB, 16283 Finspang, Sweden

Abstract. In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue(TMF) cycling from 100 to 750 ◦C has been investigated for the Ni-based single-crystal superalloy MD2. Inaddition, two different crystal orientations were studied, the 〈001〉 and 〈011〉 orientations respectively. Whencomparing IP and OP TMF lives, a strain range dependency is found for the 〈001〉 direction. For high strainranges, IP cycling leads to a higher number of cycles to failure compared to OP. However at lower strain ranges,OP cycling leads to a higher number of cycles to failure compared to IP. Microstructure investigation shows thatfor the 〈001〉 direction, deformation twinning within the γ /γ ′-microstructure is much more pronounced duringOP conditions compared to IP. However for the 〈011〉 direction, the opposite is observed; twinning is morepronounced during IP TMF. From the microstructure investigation it is also visible that intersections betweentwins seems to trigger formation of TCP phases and recrystallization. These intersections also work as initiationpoints for TMF damage.

1. IntroductionNi-based superalloys casted in single-crystal form areoften used for first row turbine blades in gas turbinesand aero engines. First row turbine blades are subjectedto the highest temperatures (above 1000 ◦C) of all bladesand at the same time, high centrifugal forces. Excellentfatigue and creep properties are of great importancefor turbine blades, and both properties are enhanced byuse of single-crystal instead of poly-crystal material [1].Superalloys consist of the typical γ /γ ′-microstructurewhere the γ -matrix is surrounding precipitates of cuboidalγ ′-phase which has an L12-ordered structure. Performingthermomechanical fatigue (TMF) testing in the laboratoryis one way to investigate component near serviceconditions. Several types of cycles can be used duringTMF testing. For example, an in-phase (IP) TMF cycle,will lead to creep relaxation in tension at maximumtemperature and plastic deformation in compression atminimum temperature. This condition corresponds to coldspots on the turbine blade, which often are found close tocooling holes or inside cooling channels [2]. On the otherhand, an out-of-phase (OP) TMF cycle, creep relaxationwill occur in compression at maximum temperature andplastic deformation in tension at minimum temperature.OP TMF is more connected to hot spots on the turbineblade where the hot spots desire to expand by thermalexpansion, but are prevented by their cooler surroundings.The hot spots are often found on the blade airfoil or

a Corresponding author: mikael.segersall@liu.se

platform. TMF damage on gas turbine blades is oftenmore connected to hot spots, meaning that OP TMF isa more component near testing condition. Hence, studiesconcerning OP TMF are more common in the literaturecompared to IP TMF.

Han et al. [3] studied the difference between IP andOP TMF and found that OP fatigue lives were shorterthan IP fatigue lives. This was explained by that themaximum tensile stress is obtained in the cold end ofthe OP TMF cycle. Other research has found that thedifference between IP and OP TMF life seems to be strainrange dependent [4]. In that study IP TMF life was foundto be shorter than OP at high strain ranges. A betterunderstanding regarding the deformation and damagemechanisms that occur during TMF is of great importanceto study in order to understand what governs the TMF life.Previous studies [5,6] have shown that the deformationwithin the γ /γ ′-microstructure is very localized and thattwinning is a major deformation mechanism during OPTMF. Another study [7] showed that parallel twin platesgrow on {111} planes close to the crack tip and thatthose twins create a path for crack propagation. Kovariket al. [8,9] performed extensive research regarding theformation of microtwins during deformation of superalloysat intermediate temperatures (660–700 ◦C). It seems thatthe twinning partial dislocations propagate through theL12-ordered γ ′-precipitates on {111} planes. The ratelimiting process of microtwinning seems to be re-orderingwithin the γ ′-phase.

The occurring deformation and damage mechanismsin superalloys during TMF conditions are not yet fullyunderstood and have to be further investigated. In this

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study, both IP and OP TMF tests have been performed intwo crystal orientations, the 〈001〉 and 〈011〉 orientationsrespectively. The aim is not only to investigate thedifference in TMF behaviour but also to study ifthere are any differences in deformation and damagemechanisms between the two loading conditions andcrystal orientations. To study both the 〈001〉 and 〈011〉orientations is of importance since crack propagation onthe blade platform sometimes is dependent on propertiesin different crystal orientations, where the 〈011〉 directionis one of the most important.

2. ExperimentalThe Ni-based single-crystal superalloy MD2 with chem-ical composition Ni, 5.1 Co, 6.0 Ta, 8.0 Cr, 8.1 W, 5.0Al, 1.3 Ti, 2.1 Mo, 0.1 Hf and 0.1 Si in wt.% was usedfor this study. The material was solution heat treated at1290 ◦C for 8 h before a two-stage ageing process with 3 hat 1100 ◦C and 24 h at 850 ◦C. Specimens oriented alongtheir 〈001〉 and 〈011〉 crystallographic orientations witha diameter of approximately 6 mm were machined fromcast bars. The deviation from the ideal crystal orientationwas less than 10◦ for all specimens. A servo-hydraulicTMF machine from Instron with induction heating andforced cooling was used for the TMF tests. Prior TMFtesting, the machine was carefully aligned to preventbuckling of the specimens. A strain-controlled TMF cyclefrom 100–750 ◦C was applied for all specimens with atemperature rate of 5 ◦C/s. Figure 1 displays a simplifiedplot of the TMF cycles used in this study. For each TMFcycle, a hold time of 5 min was applied at Tmax. TheIP tests were performed with Rε = 0 while for the OPtests Rε = −∞. For the 〈001〉 direction three differentmechanical strain ranges (�εmech) were tested: 1.1, 1.25and 1.4% respectively. For each �εmech, one specimenwas subjected to IP TMF cycling and another specimento OP TMF cycling. Hence, altogether six 〈001〉 orientedspecimens were tested. However for the 〈011〉 direction,only one �εmech was tested: 0.8%. Both IP and OP TMFwere studied also for the 〈011〉 direction, hence for thisdirection only two specimens were tested. All specimenswere cycled to failure where the number of cycles to failure(Nf) was defined at 60% load drop.

After the tests, all specimens were examined bystereomicroscopy in order to investigate the fracturesurfaces. Subsequently, the specimens were cut parallelto the loading direction for microstructural investigationsby scanning electron microscopy (SEM). The SEM wereprepared by grinding and mechanical polishing and nosamples were etched. A Hitachi SU70 FEGSEM, with anannular electron backscatter detector making it possible toproduce images by electron channelling contrast imaging(ECCI) with acceleration voltages from 10 to 20 kV, wasused.

3. Results and discussion3.1. The TMF behaviour

Figure 2 displays results from the TMF tests where themechanical strain range, �εmech, is plotted versus cycles

Strain

OP TMF

IP TMF

750 100

Figure 1. A simplified plot showing the strain-temperature cycleduring the TMF tests performed in this study.

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Figure 2. Results from the TMF tests; mechanical strain range,�εmech, versus cycles to failure, Nf.

to failure, Nf. Due to its lower Young’s modulus, the 〈001〉direction shows better fatigue properties during strain-controlled fatigue compared to the 〈011〉 direction. Inthis case, better fatigue properties means a higher numberof cycles to failure when equivalent strain ranges arecompared. For the 〈001〉 direction, it can be seen thatfor a high strain range (�εmech = 1.4%), IP TMF cyclingleads to a higher number of cycles to failure comparedto OP cycling. For the medium strain range (�εmech =1.25%), IP still shows a longer fatigue life compared toOP. However, for the lower strain range (�εmech = 1.1%)IP leads to a lower number of cycles to failure. Hence, theTMF life seems to be strain range dependent in this study.Regarding the 〈011〉 direction where only one strain rangewas tested (�εmech = 0.8%), IP cycling leads to a highernumber of cycles to failure compared to OP cycling.

To further understand the TMF behaviour of thematerial it is important to study the hysteresis loops.Figures 3 and 4 displays the first two cycles from IP andOP TMF cycling at �εmech = 1.4%, respectively. Here,the typical hysteresis loops for IP and OP TMF cycling isvisible. During IP TMF cycling, creep relaxation occursin tension at the maximum temperature, while plasticdeformation occurs in compression at the cold end of theTMF cycle, see Fig. 3. However, for OP TMF cycling,stress relaxation occurs in compression at the hot end of

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ss[M

Pa]

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Figure 3. The stress-strain curve for the first 2 cycles of the 〈001〉oriented specimen subjected to IP TMF with �εmech = 1.4%.

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Figure 4. The stress-strain curve for the first 2 cycles of the 〈001〉oriented specimen subjected to OP TMF with �εmech = 1.4%.

the TMF cycle, while plastic deformation in tension isobtained at the cold end of the cycle, see Fig. 4.

A significant difference between the 〈001〉 and 〈011〉directions is observed when loading into the first TMFcycle. For the 〈011〉 direction, a serrated yielding isobserved in both IP and OP. Figure 5 shows the two firstcycles for the 〈011〉 oriented specimen subjected to OPTMF. Here it is visible that a serrated yielding behaviour ispresent during the first cycle while the second cycle is morestable. However, for the 〈001〉 direction the yielding ismuch more stable already in the first cycle for both IP andOP cycled material, Figs. 3–4. Hence, the serrated yieldingbehaviour when loading into the TMF cycle seems tobe more extensive for the 〈011〉 direction compared tothe 〈001〉 direction. In literature, a serrated yieldingbehaviour may sometimes be attributed to deformationtwinning within the microstructure. However in this study,microstructural studies show that twinning does occur inboth the 〈001〉 and 〈011〉 directions (please see Sect. 3.3for a detailed discussion regarding the microstructure).Therefore, it is not believed that the serrated yieldingoccurs due to twinning in this case. Instead, previousstudies [10,11] have reported of a serrated yieldingbehaviour for the 〈011〉 direction, and it was discussed that

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Figure 5. Serrated yielding during the first cycle of the 〈011〉oriented specimen subjected to OP TMF with �εmech = 0.8%.

Figure 6. Surface of the 〈011〉 oriented specimen subjected toIP TMF. Both the main crack and crystallographic deformationbands are visible on the specimen surface.

dynamic strain ageing has occurred and also that only oneslip system is active during plastic deformation in the 〈011〉direction. If a serrated yielding behaviour during the firstcycles will influence the TMF life has not been consideredin this paper, but this may be of interest for future research.

3.2. Fracture appearance

The fracture appearance was studied by stereomicroscopy.For all specimens, the fractures seems to be partlycrystallographic and partly random. The investigationshows that specimens subjected to OP TMF conditionshave a higher tendency to show partly crystallographicfracture, while the specimens subjected to IP conditionshave a tendency to show more random fractures. Moreover,crystallographic deformation bands are sometimes visibleon the specimen surfaces. Here the 〈011〉 specimensgenerally show more distinct crystallographic deformationbands compared to the 〈001〉 oriented specimens. Hence,the 〈011〉 direction shows both a serrated yieldingbehaviour and crystallographic deformation bands onthe surface. On some specimens, deformation bands indifferent directions are observed; see Fig. 6 for suchan example. Here the main crack has propagated alonga deformation band going in a different orientation

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Main crack Additional cracks

100 µm Load

direction

Figure 7. An ECCI image showing severe plastic deformationaround both the main crack as well as some additional cracks forthe 〈001〉 IP specimen subjected to �εmech = 1.1%.

compared to other deformation bands. However, alldeformation bands are consistent with slip along one ofthe {111} crystallographic planes.

3.3. Scanning electron microscopy

When studying the specimens by SEM, no rafting(directional coarsening of the γ ′-precipitates) of themicrostructure is observed. It seems that 750 ◦C is a toolow temperature to initiate the rafting phenomena duringTMF cycling in this case. From the SEM investigationof the 〈001〉 oriented specimens, a difference between IPand OP TMF cycling is the appearance of cracks withinthe microstructure. For the 〈001〉 IP specimens, severalsmaller cracks are observed in addition to the main crack.Figure 7 displays such an appearance where the main cracknot only is surrounded by severe plastic deformation butalso several additional cracks. The smaller cracks seem tohave initiated around pores and other defects. On the otherhand, in 〈001〉 oriented specimens subjected to OP TMFcycling less smaller cracks are observe in addition to themain crack.

Deformation twinning within in the γ /γ ′-micro-structure is found in all specimens: in both the 〈001〉and 〈011〉 directions as well as from both IP and OPconditions. However for the 〈001〉 direction, the twinningphenomena seems to be much more pronounced in thespecimens subjected to OP TMF cycling, see Fig. 8.In the 〈001〉 IP specimens, the microstructure instead ischaracterised by a large amount of additional cracks andless twins. According to Zhang et al. [12,13] and Satoet al. [13] twins are created in compression for 〈001〉oriented specimens when the number of TMF cycles isrelatively low. In Ref. [12] it was also stated that twinsarise in the γ -phase and afterwards propagates throughthe γ /γ ′-microstructure by partial dislocation glide alongan {111} plane. In addition, Kovarik et al. [8] discussthat deformation twinning occurs at temperatures around700 ◦C. This may explain why the twinning is muchmore pronounced during OP TMF, where the compressivestresses are obtained at high temperature, compared to IP

3 µmLoad

direction

Figure 8. An ECCI image showing extensive twinning in the〈001〉 OP specimen subjected to �εmech = 1.4%.

TMF, where the compressive stresses are obtained at lowtemperature.

For the 〈011〉 direction the opposite behaviour isobserved; here the twinning is more pronounced in thespecimen subjected to IP TMF compared to OP. In fact,the microstructures of the 〈011〉 IP specimens are verysimilar to the 〈001〉 OP specimens. A previous study [6]has shown that there might be a change in deformationmechanism for the 〈011〉 direction when going from low tohigh strain ranges during OP TMF. In that study, specimenssubjected to low strain ranges showed twinning, whilefor specimens subjected to a higher strain ranges, thedeformation mechanism was rather localised bands with asheared of the γ /γ ′-microstructure. Since the mechanicalstrain range in this study is rather high (0.8%), the resultthat no twinning has occurred in the 〈011〉 OP specimen,seems to agree well with the previous study. From this, onemay conclude that a low mechanical strain range is neededto obtain twinning in the 〈011〉 direction during OP TMF.

All twins have not always propagated in the samedirection. Instead, it is quite common that twins havepropagated in different directions; see Fig. 9 for suchappearance. At the intersection of twins propagating indifferent directions, precipitation of topologically close-packed (TCP) phases often occurs. The TCP phases appearas bright spots in the electron backscattered images and areprobably µ-phase (Fig. 9). From this figure, it also seemsthat the precipitation of TCP phases is preferred to theγ -channels rather than in the γ ′-cuboids. The reason forthis behaviour is not yet understood and has to be furtherinvestigated.

The size of the twins varies and sometimes thinnertwins are created within thicker twins, see Fig. 10 for suchan example. In that image one can see that the largesttwin contains at least five smaller twins, which are all ofnano size. Once again it is visible that precipitation ofTCP phases has occurred at the twin intersections and inFig. 10 it is also visible that recrystallization has occurredat these twin intersections. Figure 11 displays anotherexample where recrystallization and TCP formation haveoccurred at twin intersections. In this image at leastfive recrystallized grains have nucleated at the twin

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1 µm Load

direction

Figure 9. An ECCI image showing how twins have propagatedin different directions in the 〈001〉 OP specimen subjected to�εmech = 1.1%.

80 nm Load

direction

Figure 10. An ECCI image showing how TCP formation andrecrystallization have occurred at a twin intersection in the 〈001〉OP specimen subjected to �εmech = 1.1%. In addition, twinsseem to have grown inside another twin.

intersections. To trigger recrystallization, a great amountof plastic strain has to be induced before the material issubjected to high temperatures. However, recrystallizationat a relatively low temperature as 750 ◦C is still not wellreported in the literature. From this study one may assumethat the intersection of twins induces enough plastic strainand that 750 ◦C is a sufficient temperature to initiaterecrystallization during TMF cycling.

Further investigation shows that the intersection oftwins, where TCP formation and recrystallization haveoccurred, seems to be an initiation point for damage withinthe material. Figure 12 displays two backscattered electronimages where (b) is a magnification of (a). In Fig. 12a, acrack at the specimen surface is observed and the crackhas grown in a crystallographic way along twin plates.From this image one can also see that the deformationis crystallographic to a great extent. Further, in Fig. 12bsevere plastic deformation parallel to where the crack hasinitiated is clearly visible. The image displays extensivetwinning within the γ /γ ′-microstructure where the twin

Load direction 100 nm

Figure 11. An ECCI image showing how TCP formation andrecrystallization have occurred at twin intersections in the 〈001〉OP specimen subjected to �εmech = 1.1%.

Figure 12. A backscattered electron image showing the 〈011〉 IPspecimen subjected to �εmech = 0.8%, (a) crack initiation alongtwin plates, (b) magnification of (a) where damage is visible attwin intersections.

plates have grown in different orientations. It seems thatthe intersection of propagating twins have triggered TCPformation, recrystallization and finally a damage within thematerial (the black voids). Hence, the voids are locatedalong twin plates and it is probable that those voidstogether with the twin plates create a preferential path forcrack propagation. Since superalloys in single-crystal formdo not have any grain boundary strengthening alloying

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elements such as carbon or boron, recrystallized areas canbe considered as weak areas and therefore be assumedas crack initiation points during TMF cycling. Hence isit of great importance to understand the recrystallizationprocess in order to improve the TMF properties ofa single-crystal superalloy. A previous study [5] hasstated that twinning is a major deformation mechanismduring TMF and also that the intersection of twins mighttrigger recrystallization. The same study also found thatcrack propagation is a consequence from recrystallizationand not the other way around, this statement was alsoconfirmed by Segersall et al. [6]. The present studyonce again show indications that the recrystallizationphenomena occurs prior crack propagation and not theother way around.

Since twinning seems to be the major deformationmechanism during TMF cycling, it is of great importanceto study this phenomena in order to understand the TMFbehaviour. According to Kovarik et al. [8,9] microtwinningoccurs at intermediate temperatures, approximately 660–700 ◦C, and re-ordering within the L12-ordered γ ′-phaseis believed to be the rate controlling process. The presentstudy has strengthened the fact that twinning is majordeformation mechanism in single-crystal superalloysduring TMF. However, further studies are needed in orderto fully understand the formation of twins and theirinfluence on TMF properties.

4. Conclusions

From TMF testing at 100–750 ◦C of the Ni-based single-crystal superalloy MD2 the following conclusions can bedrawn:

• IP and OP TMF cycling results in different fatiguelives where IP leads to longer life at high strainrange while OP cycling leads to longer fatigue lifeat a lower strain range.

• No significant differences in deformation anddamage mechanisms were found between thestrain ranges that led to different fatigue lives.

• A clear difference between IP an OP was found:for the 〈001〉 direction twinning seems to be muchmore pronounced during OP TMF while for the〈011〉 direction IP TMF cycling leads to twinning.

• The intersection of twins seems to trigger precip-itation of TCP phases as well as recrystallization.Those areas seem to be weak points where crackinitiation and propagation is preferred.

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.In addition, the support from the Swedish Government StrategicResearch Area in Materials Science on Functional Materialsat Linkoping University (Faculty Grant SFO-Mat-LiU # 2009-00971) is also acknowledged.

References

[1] R. C. Reed, The Superalloys – Fundamentalsand Applications (Cambridge University Press,Cambridge, 2006)

[2] J.J. Moverare, M. Segersall, A. Sato, S. Johansson, R.Reed, Superalloys 2012, ed. E. S. Huron et al., 369(2012)

[3] G.M. Han, J.J. Yu, X.F. Sun, Z.Q. Hu, Mater. Sci.Eng. A, 528, 6217 (2011)

[4] F. Liu, Z.G. Wang, S.H. Ai, Y.C. Wang, X.F. Sun, T.Jin, H.R. Guan, Scr. Mater., 48, 1265 (2003)

[5] J.J. Moverare, S. Johansson, R.C. Reed, Acta Mater.,57, 2266 (2009)

[6] M. Segersall, J.J. Moverare, K. Simonsson, S.Johansson, Superalloys 2012, ed. E. S. Huron et al.,215, (2012)

[7] H.U. Hong, J.G. Yoon, B.G. Choi, I.S. Kim,Y.S. Yoo, C.Y. Jo, Int. J. Fatigue, DOI:10.1016/j.ijfatigue.2013.01.015 (2013)

[8] L. Kovarik, J. Unocic, J. Li, M.J. Mills, JOM, 61, 42(2009)

[9] L. Kovarik, J. Unocic, J. Li, P. Sarosi, C. Shen, Y.Wang, M.J. Mills, Prog. Mater. Sci., 54, 839 (2009)

[10] M. Segersall, J.J. Moverare, Materials, 6, 437 (2013)[11] M. Segersall, J.J. Moverare, D. Leidermark,

K. Simonsson, Metall. Mater. Trans. A,DOI:10.1007/s11661-014-2198-0 (2014)

[12] J.X. Zhang, H. Harada, Y. Ro, Y. Koizumi, T.Kobayashi, Acta Mater., 56, 2975 (2008)

[13] A. Sato, J.J. Moverare, M. Hasselqvist, R.C. Reed,Metall. Mater. Trans. A, 43A, 2302 (2012)

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