NASA USAAVSCOMTechnical Memorandum 83769 Technical Report 84-C-16

! Low Cycle Fatigue Behavior of Conventionally._ -

Cast MAR-M 200 at 1000° C

NASA-TM-83769

Walter W. MilliganGeorgia Institute of TechnologyAtlanta, Georgia

and

Robert C. BillPropulsion LaboratoryAVSCOM Research and Technology LaboratoriesLewis Research CenterCleveland, Ohio _[ _ r_t_,a ",_

1' ' 1''''_ '<_ "I_: )884LANGLEYRESEARCHCENTER

LIBRARY,NASAHA,",_PTON,VII_GII'IIA

September 1984I

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THE LOW CYCLE FATIGUEBEHAVIOROF CONVENTIONALLYCAST MAR-M 200 AT lO00° C

Walter W. MilllganGeorgia Instituteof Technology

Atlanta,Georgia 30332

and

Robert C. BillPropulsionLaboratory

AVSCOM Researchand TechnologyLaboratoriesLewis ResearchCenterCleveland,Ohio 44135

SUMMARY

The low cycle fatiguebehaviorof the nlckel-basedsuperalloyMAR-M 200 inconventionallycast form was studiedat lO00° C. Continuouscycllngtestswithout hold times, were conductedwith inelasticstrain rangesof from 0.04 too0.33 percent. Tests were also conductedwhich includeda hold time at peakstrain in either tensionor compression. For the conditionsstudied,it wasI

determinedthat impositionof hold times did not significantlyaffect the fa-tigue llfe. Also, for continuouscyclingtests, increasingor decreasingthecycle frequencydid not affect llfe. Metallographlcanalysis revealedthat themost significantdamagemechanisminvolvedenvironmentallyassisted inter-granularcrack initiationand propagation,regardlessof the cycle type.Changes in the y' morphology.(raftingand rod formation)were observed,butdid not significantlyaffect the failure.

INTRODUCTION

High-temperature,low-cyclefatigue is a complexprocessthat is studiedfrom severaldifferentpoints of view. Many investigatorsare currentlyat-temptingto correlateactual physicaldamage mechanismswith observedquantita-tive fatiguebehavior. An understandingof these physicalmechanismsshouldbe useful not only in llfe prediction,but also in alloy developmentand inmaterialsapplicationssituations. Gell and Leverantoffer an excellentreviewof fatiguedamagemechanisms in nickel-basedsuperalloys(ref. l).

A recentlydeveloped llfe predictiontechniquethat is useful in gainingan understandingof physicalmechanismsis the StrainrangePartitioning(SRP)approach (ref. 2). In this approachthe effectsof creep and plasticity,andtheir interactionswith each other can be documented. The SRP approach hasgained fairlywide acceptanceas a frameworkfor low cycle fatiguetesting.One possibledrawback to SRP, however,Is that environmentaldegradationcansometimesbe mistaken for creep damage.

In this study,a turbine blade alloy, MAR-M 200, was investigated(primar-ily at I000° C) to try to identifyrelevantphysicaldamagemechanisms. Theobjectivewas to relateobservedmicrostructuralchangesto the processofcrack initiationand fatiguellfe, and to providean experimentaland mecha-nistic basis againstwhich resultsobtained from more complexnonisothermalcycling experimentsmight be compared.

MATERIAL _

MAR-M 200 is a cast nlckel-basedsuperalloyoriginallydevelopedfor tur-bine Flade application. The compositionis outllnedIn table I. The micro-structureconsistsof approximately60 vol_ primary y' in a y matrix,wlth both M23C6 and MC type carbides. The alloy was conventionallycast intosolid "hourglass"specimensas illustratedin figure l, which resultedin aradial grain structurewith grain sizes of approximately1 mm by 2 to 3 mm.The specimenswere tested in the as cast and machinedcondition.

APPARATUSAND PROCEDURE

Testingwas conductedon a closed-loop,servohydraullctest system,in thedlametralstraln-controlledmode (ref. 3). Specimenswere heated using adirect resistanceunit. The "hourglass"specimendesign is shown in figure I.

First, to establishthe conventionalCoffin-Mansonrelationship,isother-mal, continuouscycling(PP type) tests were conductedat lO00° C in the fullyreversed,strain controlledmode. The tests were conductedat a frequencyof0.5 Hz, with plastic strain rangesfrom 0.04 to 0.33 percent. Next, a seriesof tests was conductedat a constant Inelastlc'stralnrange of 0.2 percentwithvaryingcycle parameters: continuouscyclingtests were conductedat frequen-cies of 0.033 and l.O0 Hz; CP tests,whlch includeda 2.5 mln hold time at themaximumtensilestrain,and a PC test,which includeda 2.5 mln hold time atthe maximum compressivestrain,were also conducted. Finally,severalPP typetests were conductedat lower temperatures(870° and 760° C). A metallographlcand fractographlcstudy was undertakento identifydamage mechanisms. Also, astudy of oxidationand diffusionwas conductedto help clarifythe role of theenvironment. This consistedof exposingsamplesunder no load in air atlO00° C for periodsrangingfrom 15 mln to 96 hr. The sampleswere sectioned,mounted in mineral filled epoxy (for good edge retention),polished,anddocumentedmetallographlcally.

RESULTS

Life Relationships

As seen from Figure2, the straln-controlledfatiguedata are fairly con-sistent,and the alloy followsthe Coffln-MansonLaw In the range of inelasticstralnrange(a_ in) studied. Also, for the conditionsstudied(ACp = 0.2percent,T = lO00° C), the fatiguellfe is not affectedby changingthe fre-quency or imposinga hold time. Resultsfrom SRP tests of hollow tubularMARM-200 specimensreportedby Manson,Halfordand Oldrleve (ref. 4) at 927° Care also shown for comparison. An approximatelyfourfold increasein llfe isseen for the lO00° C data comparedto the 927° C resultsfrom reference4.This is believed to be due to two effects. First, considerthe mechanicalproperties. For a given inelasticstrain range, peak stressesat lO00° C areabout 25 percent lower than those at 927° C, therebyreducingthe dynamiccrackgrowth drivingforce. Also, ductilityof MAR-M 200 at lO00° C is lO to 20percenthigher than at 927° C, therebyincreasingthe low cycle fatiguellfeone would predicton the basis of cyclic inelasticstrain. Second,all other

things being equal, the lives of hollow tube specimens are observed to be some-what lower than those of soltd specimens (as used in the present Investiga-tion), consistent with the general observation of crack initiation on the innersurface of the hollow tube specimens.

Crack Initiation and Propagation

At I000° C, crack initiationalways occurred in grain boundarieson thespecimensurface,as shown in figures3 to 6. It was also observedthat crackinitiationwas always accompaniedby preferentialoxidationand alloy depletionat the grain boundary.

In all tests that includeda hold time, crack propagationat lO00° C waslO0 percentIntergranular. In continuouscycling (PP type) tests, crack prop-agationat fOOD° C was observed to be mixed Intergranularand transgranularwith the Intergranularmode being the most dominantas illustratedin figures4and 5. The highestdegree of transgranularcrackingwas observed in Specimen_G2, a PP type test (Frequency= l Hz), in which about 25 percentof the thecrackingpresentwas transgranular. At 870° C and 760° C, however, trans-granular propagationbecamemuch more dominant.

OxidationBehavior

The samplesthat were oxidizedunder no load were comparedto the fatiguedspecimensmetallographlcally.The degree of oxidationand depletionwasInhomogeneous,due possiblyto differencesin the adherenceof the oxide scalein grains of differentorientation,or to dendriticsegregation. It was deter-mined, however,that the rates of oxidation,alloy depletion,and Intergranularattack are essentiallya functionof time only, and not of the loadingh_story.There was one notable exception,as illustratedin figure 6. Althoughthedegree of diffusionand oxidationwas about the same for the fatiguedsampleand the unstressedsample,the oxide scale tended to "grow" into the base metalgrain boundaryduring fatigue. In the unstressedsamples,oxide "spikes"werenucleatedin the alloy depletionzones,but the scale itselfdid not appear tobe continuous. (The "spikes"were also nucleatedin the depleted zones of thefatiguedsamples.)

MicrostructuralInstabilities

In addition to surfacereactions(oxidationand alloy depletion),the bulkmicrostructureof the alloy changedduring the fatigueprocessat lO00° C. Asshown In figure 7, a tensile strain hold time (CP type test) induced "rafting,"

which is an elongatingand coalescingof the y' particlesin the <lO0> crys-tallographicdirectionmost nearly perpendicularto the tensileaxis. A com-pressive strain hold time (PC type test) induceddirectionalcoarseningin the<lO0> crystallographicdirectionmost nearly parallelto the tenslleaxis.lhese phenomenaare very Inhomogeneous,dependingstronglyon grain orientationrelatlveto the tensileaxis. The y' morphologyof many grains changedverylittle,even during the longestCP type test.

In the continuous cycling (PP type) tests, and In the unstressed samples,very slight y' coarsening in random <100> directions was observed.

DISCUSSION

The SRP behaviorof polycrystalllneMAR-M 200 at lO00° C is typicalof highy' Nlckel-basedsuperalloysused In turbineairfoilapplications. Thesematerialswere designedfor creep resistance,and by additionsof boron, zirco-nium, carbon,and carbideformers,grain boundaryslidinghas been virtuallyeliminated. For this reason,the moderate to hlgh inelasticstrain rangesimposedresultedin CP and PC lives that were about the same as the PP lives.This type of behaviorhas been documentedfor MAR-M 200 at 927° C, (ref. 4)for Rene' BO (whichis mlcrostructurallyand chemicallyvery similarto MAR-M200) at lO00° C, (ref. 5) and for other similaralloys.

Althoughgrain boundaryslldlnghas been eliminated,other creep processeshave not been. In the CP and PC type tests, the partitioned(ref. 2) inelasticstralnrangesconsistof 40 to 50 percentCP or PC type inelasticstrain. Also,the constant inelasticstrain range continuouscycle tests (PP type) illustratethat creep can occur even during this type of cycle. As the continuouscyclefrequencywas increasedfrom 0.033 to l.O0 Hz, the stress range (for the sameinelasticstrain range) increasedfrom 550 to 680 MPa. Thls indicatesthat adynamic stress relaxationcreep mechanismis active. The probablecreepmechanismsinvolvedare dislocationclimb and bulk diffusion.

Althoughcreep is taking place, it does not significantlyaffect the fa-tlgue llfe under the conditionsstudied. The reasonfor this is that thecrackingoccurs in the grain boundaries,while the creep deformationprocessesappear to be occurringwithin the grains.

One side effectof the creep-fatlguedeformationprocess is the directionalcoarseningor rafting,of the y' particles. This behaviorhas been reportedfor singlecrystal superalloys,(ref. 6) and recent Investlgatlonshave sug-gested that "rafting"may be beneficialfor creep resistance. In the conven-tionallycast form, however, the creep-fatlguelives appear to be independentof this behavior. Once again, the Intergranularnature of the cracking seemsto be the reason.

The primarydamage mechanismfor this materialat lO00° C consistsofenvironmentallyassisted Intergranularcrack initiationand propagation,asillustratedIn figures5 and 6. Gell and Leverant(ref. l), McMahonand Coffin(ref. 7), Coffin (ref. B), and Antolovlch,Liu and Baur (ref. 9) have docu-mented thls type of behavior for similaralloys at high temperature. Severalspecificmechanismshave been proposed. Gell and Leverantdescribean oxlda--tlon affected zone ahead of the Intergranularcrack. The oxidationaffectedzone is structurallyand chemicallychanged, specificallyundergoingdepletionof oxide formingelements. Under some circumstancesIntergranularcrack growthmay be acceleratedby the formationof this oxidationaffected zone ahead ofthe crack. An Intergranularcrack initiationmechanismwas observedby McMahonand Coffin in cast Udlmet 500 wherein surfaceoxide ridges that formed athighly stressedgrain boundary sites led to crack initiation. McMahonandCoffin also observedextensiveoxide penetration,independentof cracking,along surfaceconnectedgrain boundariesas well as alloy depletionadjacent to

the grain boundaries. In studies performed on Rene' 80, Coffin again observedcrack initiation at surface grain boundary sites where oxlde ridging occurred.However, crack propagation in Rene' 80 was primarily transgranular, independentof cyclic loading frequency. Antolovtch et. al. reported crack initiation InRene' 80 at deeply penetrating oxide spikes that formed In surface connectedgrain boundaries. An oxidation controlled crack initiation ]lfe predictionmethod was proposed to account for these observations.

In addition to the mechanistic observations cited above, phenomenologicalsupport for the effect of the environment has been found in the literature.Halford and Nachttgall (ref. 5) investigated the low cycle fatigue behavior ofRene' 80 at 1000° C in air, and found essentially the same behavior as docu-mented here for MAR-N200. The four SRP lifelines coincided. Kortovlch andSheinker (ref. 10) studied Rene' 80 from the same heat at 1000° C in a highvacuum. Fatigue lives were improved drastically (approximately doubled) forthe PP type cycle in vacuumcompared to air, resulting in a separation of theSRP lifelines. They reported that crack propagation at 1000° C In vacuum forthe PP cycle was essentially transgranular, while CP and PC type cycles stillresulted in tntergranular cracking. The major effect of the environment seemsto be a promotion of Intergranular fracture, even in PP type tests, resultingtn lower fatigue lives.

Thus, the results obtained here for polycrystalllne MAR-M200 are generallyconsistent with those for other conventionally cast nickel based superalloys.An interesting observation pertaining to RAR-R200 however, is the Insensltlv-tty of cyclic crack growth rate to the modeof crack propagation (lntergranularor mixed transgranular/lntergranular), resulting in cyclic lives that wereindependent of the cycle type. The key role played by oxidation In the crackinitiation and early propagation process provides a basis for better under-standing the strong sensitivity of cyclic life to the phase relationship be-tween the thermal cycle and the mechanical cycle seen In the thermomechanicalfatigue experiments reported In reference 11. Because of crack opening undertensile loading at high temperatures, there was muchmore opportunity for oxi-dation mechanisms to promote crack growth under in-phase cycling (tensile loadat maximumtemperature) than under out-of-phase cycling (compressive load atmaximumtemperature).

CONCLUSIONS

I. For the conditionsinvestigated(T = lO00° C, inelasticstrain range<0.05 percentv < 1Hz), the fatiguellfe (Nf) of conventionallycast MAR-M200 is not significantlyaffectedby frequencyor hold times.

2. The most significantdamagemechanismin this region,regardlessofcycle type, consistsof oxidationacceleratedIntergranularcrack initiationand propagation.

3. Damagingmlcrostructuralchangesobservedincludedpreferentialoxida-tion and alloy depletionimmediatelyadjacentto the grain boundaries. ChangesIn the bulk m_crostructureobservedincludedrafting,rod formation,and _'coarsening,none of which seemedto have a significanteffect on the fatiguellfe (Nf).

REFERENCES J

I. Gell, M.; and Leverant,G. R.: Mechanismsof High TemperatureFatigue.

Fatlgueat ElevatedTemperature. ASTM STP 520, 1972, pp 37-67.

2. Hirschberg,Marvin H.; and Halford,Gary R.: Use of StralnrangeParti-tioningto PredictHigh-TemperatureLow-CycleFatigueLife. NASATN D-B072,1976.

3. Hirschberg,M. H.: A Low Cycle FatigueTesting Facility. Manual on LowCycle FatigueTesting. ASTM STP 465, 196g, pp 67-B6.

4. Manson, S. S.; Halford, G. R.; and Oldrleve,R. E.: Relationof CyclicLoadingPatternto MicrostructuralFracturein Creep Fatigue.NASA TM-83473,19B3.

5. Halford,G. R.; and Nachtlgall,A. J.: StralnrangePartitioningBehaviorof the Nickel-BaseSuperalloys,Rene' BO and In lO0. CharacterizationofLow Cycle High TemperatureFatigueby the StralnrangePartitioningMethod.AGARD-CP-243,1978, pp. 2-I - 2-14.

6. llen, O. K.; and Copley,S. M.: The Effectof UnlaxialStress on thePeriodicMorphologyof CoherentGamma Prime Precipitatesin Nickel-BaseSuperalloyCrystals. Metall.Trans.,vol. 2, no. "l,3an. 1971,pp 215-219.

7. McMahon,C. O.; and Coffin, L. F.: Mechanismsof Damage and Fracture inHigh-Temperature,Low-CycleFatigueof a Cast Nickel-BasedSuperalloy.Metall.lrans.,vol. l, no. 12, Dec. 1970, pp 3443-3450.

B; Coffin, L. F. 3r.: The Effectof Frequencyon the Cyclic Strain andFatigueBehaviorof Cast Rene' at 1600° F. Metall.Trans.,vol. 5, 1974,no. 5, May pp I053-I060.

g. Antolovlch,StephenO.; Liu, S.; and Baur, R.: Low Cycle FatigueBehaviorof Rene' BO at ElevatedTemperature. Metall.Trans. A, vol. 12, no. 3,Mar. 1981, pp 473-481.

lO. Kortovlch,C. S.; and Sheinker,A. A.: A StralnrangePartitioningAnalysisof Low Cycle Fatigueof Coated and UncoatedRene' BO. Character-izationof Low Cycle High TemperatureFatigueby the StralnrangePartitioningMethod. AGARD-CP-243,197B, pp. l-l - 1-23.

If. Bill, Robert C., et al.: PreliminaryStudy of ThermomechanlcalFatigueof PolycrystalllneMAR-M 200. NASA TP-2280,1984.

TABLE I. - MAR~M 200 ALLOY COMPOSITION(wt % )

NI Al Ti Co W Cr Cb C Fe Zr B S Si Cu Mn

Ba1. 5.2 2.1 I0.3 12.6 9.2 l.O 0.12 0.6 0.043 0.015 0.003 0.073 <O.l <0.02

TABLE II. - FATIGUETEST DATA FOR POLYCRYSTALLINEMAR-M 200 AT lO00° C

Specimen Cycle typea Frequency, Total Total Inelastic Number of cyclesHz stress range, strain range, strain range, to failure,

" MPa xl02 cm/cm xl02 cm/cm Nf

HH33 PP 0.50 370 0.27 0.04 II BOOHH54 PP .50 425 .30 .07 4 060HH69 PP .50 3B5 .28 .07 15 400HH49 PP .50 500 .38 .09 l 960G25 PP .50 625 .48 .15 950G24 PP .033 550 .44 .20 320G30 PP .50 660 .50 .20 520HH41 PP .50 600 .43 .20 760

HH44 PP .50 630 .46 .21 335G2 PP l.OO 6BO .57 .20 630HH4B PP .50 625 .46 .28 340HHI8 PP .50 730 .53 .33 195Gl4 CP b.50 690 .47 .22 450GB CP b.50 630 .43 .20 760G27 PC b.50 610 .46 .20 380

app denotes a continuous test with no hold time. CP denotes a test that includes a hold timeat the maximum tensile strain. PC denotes a test that contains a hold time at the maximum

. compressive strain.bThese tests included a 2.5 mln hold time, with a 2 sec (0.5 Hz) reversal.

518-18UNF-3threadsI

f23

t5.6 Typical

" --T

Figure1. - Specimengeometry. (Note: all dimensions are inmillimeters(exceptthreads).)

10-2

"- Temperature,oC

_- E 1000.__ 10-3N_ o

-g927(ref.4)o,

10-4 ! , I Illlll I I 1,1,11110 102 103 104 10"5

Numberofcyclestofailure, Nf

Figure2. - InelasticstrainrangeversusNfforpolysrystallineMAR-M200at10000C.

(a)Fracturesurfaceof specimenG30(no holdti me).

(b) Polishedandetchedsectionof specimenG30.

Figure3. - Photographsi llustratingthe i ntergranularnatureof cracking.

Figure4. - Photomicrographshowingmixed-modecrackpropagation,specimen030. Theuppercrackinitiated in the grain boundaryandpropagatectransgranularly. Thelowercrackis surface-connected,belowtheplaneof view.

l ] '>

50pm _

Figure5. - Photomicrographshowingthe crackfront propagatingthrough agrainboundary.SpecimenG14(tensile strain hold).

(a)SpecimenGZI(compressivestrain hold, tf - 17hr). Notethe (b) Sampleoxidizedunder no loadfor 16hours.crack initiated atthe lowergrain boundary.

Figure6. - Preferentialoxidationanddiffusionatgrain boundaries.

(a) Untested(startinq) microstructure.

(b) SpecimenG14(tensilestrain hold).

|

Figure7. - Scanningelectronmicrographsillustrating the changesin Y'morphologyduetocreep-fatigue.

(c) Specimen627(compressivestrain hold).

Figure7. - Concluded.

1."Report No. NASATM-83769 =. Government Accesllon No. 3. Recipient'| Catllog No.

USAAVSCOH-TR-84-C-164. Title and Subtitle 5. Report Date

Low Cycle Fatigue=Behaviorof ConventionallyCast September1984

MAR-M200 at 1000° C 6.PerformingOrganizationCode

505-33-227. Author(s) 8. PerformingOrganizationReporlNo.

Walter W. Milliganand Robert C. Bill E-226010. Work Unit No.

g. Performing Organization Name and Address

NASALewis Research Center and Propulsion Laboratory 11.Contrau¢torGrentNo.

U.S. Army Research and Technology Laboratories (AVSCOM)Cleveland, Ohio 44135

13. Type of Report lind Period Covered

12. Sponsoring Agency Name and Address

NationalAeronauticsand Space Administration TechnicalMemorandumWashington,D.C. 20546 and U.S. Army Aviation 14.SponsorlngAgencyCodeSystems Command,St. Louis, Mo. 63120

15. Supplementary Notes

Walter W. Milligan, Georgia Institute of Technology, Atlanta, Georgia 30332;Robert C. Bill, Propulsion Laboratory, AVSCOMResearch and Technology Laboratories,Lewis Research Center, Cleveland, Ohio 44135.

16. Abstract

The low cycle fatigue behaviorof the nickel-basedsuperalloyMAR-M 200 in con-ventionallycast form was studiedat 1000° C. Continuouscyclingtests,withouthold times,were conductedwith inelasticstrainranges of from 0.04 to 0.33percent. Tests were also conductedwhich includeda hold time at peak strain ineither tensionor compression. For the conditionsstudied, it was determinedthat impositionof hold times did not significantlyaffectthe fatigue life.Also, for continuouscyclingtests, increasingor decreasingthe cycle frequencydid not affect life. Metallographicanalysisrevealedthat the most significantdama!]e mechanism involved environmentally assisted intergranular crack initiationand propagation, regardless of the cycle type. Changes in the y' morphology(rafting and rod formation) were observed, but did not significantly affect thefailure.

17. Key WorOs (Suggested by Author(s)) 18. Distribution Statement

Fatigue Unclassified- unlimitedHigh temperature STAR Category26OxidationSuperalloy

19. Security Classlt. (of this report) 20. Security Cllsslf. (of this page) 21. No. of pagee 22. Price"

Unclassified Unclassifled

*For sale by tile National Technical Informalion Service. Springfield, Virginia 22161

N.t,ooa,Aero°au,,c,.n,llllllSpace Administration BOOK r-_.j_llJ_. l

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