NASA Technical Memorandum 192404 ..... - . , ..=

, High Temperature Fatigue Behavior

of Tungsten Copper Composites

Michael J. Verrilli, Yong-Suk Kim, and Timothy P. Gabb

Lewis Research Center .......

Cleveland, Ohio

Prepared for the - -

Symposium on Fundamental Relationships Between Microstructure

and Mechanical Properties of Metal-Matrix Composites

-cosponsored by The Metallurgical Society

, and the American Society for Metals

Indianapolis, Indiana, October !-5, 1989

_:_(NA_A-TM-I02404) HT6_ TEMPrRATURF FATIGUE

6EHAW[JR nF TUNGSTLN COPPER CnMmQSITE£

(NAeA) 18 p CSCL llO

N90-21138

G3/z6

HIGH-TEMPERATURE FATIGUE BEHAVIOR OF TUNGSTEN-COPPER COMPOSITES

Michael J. Verrllli, Yong-Suk K1m, Timothy P. Gabb

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

I

SUMMARY

The hlgh-temperature fatigue behavlor of a 9 vol %, tungsten fiberreinforced copper matrix composlte was investigated. Load-controlled Isothermal fatigue experiments at 260 and 560 °C and thermomechanlcal fatigue (TMF)

experlments, both In phase and out of phase between 260 and 560 °C, were performed. The stress-straln response dlsplayed conslderable Inelasticity under

all conditions. Also, strain ratcheting was observed during a11 the fatlgue

experlments. For the Isothermal fatigue and In-phase TMF tests, the ratchetIng was always In a tensile direction, continuing untll failure. The ratchetIng during the out-of-phase TMF test shifted from a tensile direction to a

compressive direction. This behavior was thought to be assoclated with the

observed bulging and the extensive cracking of the out-of-phase speclmen. For

all cases, the fatigue lives were found to be controlled by damage to the copper matrix. Grain boundary cavltatlon was the domlnant damage mechanism of the

matrix. On a stress basis, TMF 1oadlng reduced lives substantially, relative

to Isothermal cycling. In-phase cycling resulted in the shortest lives, and

Isothermal fatigue at 260 °C, the longest.

INTRODUCTION

Tungsten flber-reinforced copper matrix (W-Cu) composites have been

studied for over 30 years as model metal matrix composites (MMC's). Interest

In thls material stems from its relatively easy manufacture, its strong fiberto-matrix bond, and an apparent absence of Interracial products. Early Investigations of mechanical behavlor concentrated on such basic propertles of W-Cu

composites as tensile behavior (refs. 1 to 6) and creep behavior (ref. 6).

Later research involved isothermal fatigue (refs. 7 to 9) and thermal fatigue

(refs. I0 to 15).

A concern about the effect that the thermal coefflcient of expansion (_)

mismatch between the fibers and the matrix may have on hlgh-temperature behavIor led to an interest _n thermal fatigue. Durlng thermal cycling of a fiberreinforced MMC, internal stresses are generated because of thls difference.

When W-Cu composites undergo thermal cycling, these Internal stresses can

result in such damage to the composlte as Inelastlc deformation of the matrix

vla sl_p, grain boundary migration, void formation, grain boundary slidlng

(ref. lO), thermal ratchetlng of the matrix relative to the flbers (ref. 15),

and cavitation at the flber-matrlx interface (ref. 12).

As a structural material for a hlgh-temperature appllcation, a fiberreinforced MMC such as W-Cu would experience both mechanlcal and thermal cyclic

loadings, or thermomechanical fatigue (TMF). Many monollthlc materials have

shorter lives from TMF than from Isothermal fatigue at the temperature extremes

of the TMF cycle (ref. 16). In a recent Investigation (ref. 17), nonisothermal
fatigue 1oadlng degraded the fatigue resistance of a sillcon carbide fiberreinforced titanium matrix composite substantially more than isothermal loading

did. This suggests that characterization of TMFbehavior of a MMCis important

for elevated-temperature applIcat|ons.

A W-Cu composite Is being considered as a combustor liner material for the

space shuttle maln engine. In this app11cation the composite would experience

cyc11c thermal and mechanical loading. The purpose of our work was two-fold.

(1) to determine the effect of simultaneous thermal and mechanical cyc11ng on

the low-cycle fatigue llfe of this composite system, and (2) to understand the

mechanisms of deformation and failure under high temperature Isothermal fatigue

and TMF. Tensile tests, Isothermal fatigue tests, and thermomechanIcal fatigue

tests were performed on tungsten fiber reinforced copper composites. This

paper deals with the behavior of the composite, and it presents data that were

generated to provide guidance for the development of MMC fatigue 11fe prediction schemes.

MATERIAL AND EXPERIMENTAL PROCEDURE

Materlal and Specimens

We studied a tungsten flber-reinforced copper matrix composite, which was

manufactured by using an arc-spray process. The fibers employed were

200-Nm-dlam General Electric 218 tungsten wire. The four-ply composite plates

contalned 9 vol % of unidirectional tungsten flbers in a matrix of oxygen-free,

hlgh-conductlvlty copper.

Specimens were electro-dlscharge machined from composite plates into the

geometry shown in figure I. All fibers were aligned parallel to the load axis.

The machined surfaces of the specimen gage length were hand polished prior to

testlng.

Mechanical Test Procedures

All the mechanical tests were performed by using a 90-kN servohydrauIIc

test system fitted with an environmental chamber that allowed hlgh-temperature

mechanical testing in a vacuum (<SxIO -6 torr), In flowing Ti-gettered argon, or

in laboratory air. Strain was measured by a 12.7-mm gage-length axial extensometer. Waveforms were generated and data were acquired with a minicomputer.

Load, strain, and temperature were recorded during the tests by the test control software, Specimens were heated by Induction, and temperature was measured wlth either Chromel-Alumel therFm_couples or an Infrared pyrometer.

Straln-contro!led tensile tests were performed In vacuum at 260 and 560 °C

with a strain rate of 2.0xlO -_ In./In./sec.

Load-controlled Isothermal fatigue tests were also performed in vacuum at

260 and 560 °C. Load was chosen as the control mode for all the fatigue experiments because the composite specimens were thln, and therefore, stralncontrolled tests would result In significant compresslve loads, which would

cause specimen buckling. A triangular waveform was employed, with R-ratlos of

about 0.05 (R : minimum load/maximum load). A cycle frequency of 3 cpm was
chosen to approximate the strain rate employed in the tensile tests. The failure criterion for all fatigue tests was separation of the specimen into two

pieces.

Load-controlled TMF tests were performed between 260 and 560 °C. A triangular waveform with a cycle frequency of 0.25 cpm was employed for both the

load and temperature. The experiments were performed In flowing argon in order

to increase the cooling rates of the thermal cycle. A load R-ratio of 0.065

was used. For In-phase TMF tests, the maxlmum load was attained at 560 °C, and

the minimum load occurred at 260 °C. For the out-of-phase tests, the maximum

load was reached at 260 °C, and the minimum load, at 560 °C. Prior to beginnlng each TMF experiment, the temperature of the specimen was cycled while

maintaining zero load In order to measure the thermal expansion strain as a

function of temperature. The thermal expansion strain of the composite was

assumed to be constant throughout the test. After completion of the test, the

thermal strain data were used to determine the applied mechanical strains from

the recorded total (thermal plus mechanical) strains as follows: Temperature

as a function of thermal expansion straln was fitted to a plecewlse linear

function; for each stress-total strain (Ctota l) data point, the corresponding

temperature was Input into the plecewlse linear function to calculate the thermal expansion strain, etherma I and then mechanical strain (Cmech) was determined by using the relationship

Cmech = Ctota I - Ctherma I

(1)

Metallography and Fractography

Fracture surfaces and polished sections of specimens from all experiments,

both tested to failure and interrupted, were examlned with optical and scanning

electron microscopy. Results of the isothermal fatlgue experlments, which

reported elsewhere (ref. 18), wil] be summarized herein.

RESULTS

Tenslle Behavior

Stress-straln behavior. - The tensile curves at 260 and 560 °C are plotted In figure 2, and tenslle properties are summarized in table I. Note that

the composite strain hardens until failure during tensile deformation at

260 °C, but at 560 °C the stress decreases after reaching its ultimate

strength. The strain at which the composite failed was slgnlflcantly hlgher

at 260 °C than at 560 °C.

Fractoqraphy and metalloqraphy. - Examination of the fracture surfaces

revealed that at both temperatures the tungsten fibers failed in a ductlle

manner, having a reduction In area of about 65 percent. Some longitudinal

spllttlng of the flbers, whlch was much more prevalent at 260 °C than at

560 :C, was observed. Copper scales still adhered to the exterior of necked

fibers, thus indi:ating good Interfacial bonding. At 260 °C, the copper matrix

failure was predominantly transgranular via mlcrovold coalescence, whereas at

560 °C, matrix failure was predominantly Intergranular.
Fatigue Behavior

Stress-straln behavior. - The typlca] stress-straln response of the composite during the isothermal and thermomechanlcal fatigue cycling is shown In

figures 3 to 6. Hysteresis loops that were measured throughout the fatigue

_ests are shown. For all hysteresis loops, both the loading and unloading

responses have portions of nonlinearity, that is, inelastic flow of the composire. The amount of this tensile Inelasticity In the hysteresis loops decreased

as the test continued. The accumulatlon of tensile inelastic strain (ratchetIng) continued until failure; consequently, the hysteresls loops never completely stabilized. At 260 °C the hysteresis loops eventually consisted almost entirely of elastic strain, with a small component of inelastic strain

(O.03-percent inelastic strain at half life). Inelastic strain is represented

by the maximum width of the hysteresis loop. At 560 °C the composite experienced significantly more inelastlclty, as evidenced by the larger width of the

loops (O.08-percent Inelastic straln at half llfe). Also, no slgniflcant

change of the elastic modulus was detected during the Isothermal fatigue tests,

except Just prior to failure.

The stress-mechanlcal strain response of the composite during In-phase

thermomechanIcal cycling (fig. 5) was similar to that at 260 °C (fig. 3). Just

as in the 560-°C Isothermal tests (fig. 4), significant cyclic inelastlcity

occurred during out-of-phase cycling (flg. 6); however, the shape of the hysteresls loop changed dramatically near composite failure. The Inelastlc-straln

range decreased by 65 percent, and the composlte stiffness decreased 32 percent (from 21.4 to 14.6 GPa), from half life (lO00 cycles) to near failure

(2000 cycles). Also, the loop that was measured at cycle 2000 was curved at

the two extremes but llnear In the middle.

The nature of the ratcheting behavior is more clearly shown in a plot of

the maximum cyclic strain as a function of cycle number (fig. 7). The curves

for specimens tested at 260 and 560 °C and for the In-phase specimen resemble

typlcal secondary and tertiary creep response of a monolithic material. The

ratchetlng behavior for the out-of-phase specimen was markedly different. The

maximum strain Increased during the first 3 percent of fatigue life (0.03 Nf)

but then steadily decreased until about 0.4 Nf (Nf = cycles to failure). Subsequently the maximum strain contlnually increased until failure.

Fatigue life. - Note that while the stress range is held constant, the

strain range and maximum strain vary in complex ways among the different

fatlgue cycles. In a first attempt to understand what controls fatigue behavior, these variables will still be employed in comparing the fatigue lives.

Fatlgue llfe was longest for specimens Isothermally cycled at 260 °C

(flg. 8). Although only two In-phase and one out-of-phase TMF tests were run

to failure, the data show that the lives of thermomechanically fatigued composIres are conslstently shorter than the lives of those Isothermally fatlgued at

either temperature extreme of the TMF cycle. As indlcated by the very shallow

slopes of the lifmllnes, the fatigue lives were very sensitive to small

changes In stress range in thls life regime.

Llfe was decisively shorter for In-phase rather than out-of-phase TMF

tested specimens. To Illustrate the effect of phaslng on composite llfe, an

4
In-phase TMF test was performed wlth a stress range of 108 MPa, and an outof-phase TMF test was done wlth a stress range of 109 MPa. The In-phase

speclmen failed after 581 cycles whereas the out-of-phase test was stopped at

1788 cycles before fallure.

Fatigue life was also examined on a strain basis by plottlng the mechanical strain range measured at half llfe with respect to cycles-to-failure

(flg. 9). On thls basis, the two Isothermal data sets are close together. The

out-of-phase TMF tests would appear to yield the longest lives and the in-phase

TMF tests, the shortest. As will be discussed later, the strain measurements

in out-of-phase TMF tests were deemed to be erroneous because of cracking and

barreling of the gage section during cycling.

The strain at failure as a function of fatigue life was also plotted

(flg. lO). For each fatlgue condition, failure stralns were hlghest for the

shortest fatlgue llves. Life was longest on this basls for the specimens

tested at 260 °C. The hlgh-temperature isothermal cycling yielded the shortest 11ves on thls basis, and the TMF life appeared slightly longer than the

560 °C Isothermal life.

Fractography and metallography. - We have reported elsewhere on the

characterization of the isothermal fatigue failure modes through microscopy

(ref. 18). We observed that the deformation of the copper matrix controls the

falIure of the composite. Cracks began In the copper matrix and propagated via

cavity nucleation and coalescence processes. The failure of the matrix that

was cycled at 260 °C was characterized as a combination of Intergranular and

transgranular failure, with intergranular failure predominating near the speclmen surface (flg. 11(a)). At 560 °C the fracture was Intergranular throughout

(flg. 11(b)). The breakage of the tungsten fibers came after failure of the

surrounding matrix (fig. 12). The fibers failed locally at matrix cracks by a

ductile fallure wlth necklng to a large reduction in area (65 percent). A considerable amount of secondary cracking of the composite speclmen was observed

for 260-°C cycling, whereas one domlnant crack appeared to nucleate and grow

during fatigue at 560 °C.

Examlnatlon of the falled TMF specimens Is still In progress; however,

some characteristics of the fracture surfaces (flg. 13) and pollshed sections

(flgs. 12 to 14) have been observed. In specimens subjected to In-phase cycling (flg. 13(a)) the failure of the matrix was similar to that of the 560 °C

Isothermal experiments (fig. 11(b)). Intergranular cavitation of the copper

matrix dominated, wlth clearly formed cavities evident on the grain surfaces.

A11 of the tungsten flbers necked to about a ?6-percent reduction of area at

failure, nearly the same as observed in the Isothermal fatlgue and tensile

failures. Unlike the Isothermal specimens, the fracture surfaces and polished

sections revealed localized matrix cracking near the flber-matrix interfaces.

These cracks were confirmed to be parallel to the flbers and could be found at

a distance from the failed fiber ends in the longitudinal sections, as shown in

figure 14. Work Is In progress to determine the sequence of events leading to

failure.

Three specimens were subjected to out-of-phase cycling, but only one was

tested to failure. DImenslonal instability of the speclmens in the form of

significant bulging of the gage length, and many secondary cracks both parallel

and normal to the flbers were observed (flg. 15). The thickness of the gage
section of the specimen that was tested to failure Increased by 40 percent.

Intergranular cavitation of the copper matrix predomlnated, although the cavities on the grain surfaces (fig. 13(b)) were not as well formed as those of

In-phase specimens (fig. 13(a)). The fiber failure modewas the sameas observed in isothermal experiments. Most necked fibers were encased by a thicker

layer of copper matrix, unlike those of isothermal specimens. These copper

scales separated from the rest of the matrix because of intergranular cavitatlon. The fibers were often surrounded by deep circumferential matrix cracks

very similar to those of the In-phase specimen. Examinatlon of longitudinal

sections again indicated localized matrix cracking near the fiber-matrix interfaces. More work Is in progress to Investigate this process.

DISCUSSION

Fatigue Stress-Strain Behavior

The Isothermal hysteresis loops can be explained slmply in terms of the

four stages of deformation in a W-Cu composite as described by McDanels

(ref. 6) and Ham and Place (ref. 7). Initial loading of the composite gives

an elastlc response of both the fiber and the matrlx (fig. 3) and, therefore,

linear response of the composite (stage I). On further straining, the lower

strength copper yields; but the tungsten continues to strain elastically, and

the response of the composite Is again llnear but with a smaller slope

(stage II). In stage III behavior both the fiber and the matrix are straining

plastically, and the composite response is nonlinear. On cyclic reversal from

the maximum stress, the flbers and the matrix Inltially contract elastlcally

(stage I). The stress in the matrix surpasses Its compressive yield strength

before the composite reaches the minimum load. At this point the behavior

again becomes stage II. At the end of the first cycle, the matrix is In compression, the fibers are in tension, and the composite is permanently elongated.

Fatlgue hardening of the matrix during subsequent cycles is responsible for the

narrowing of the loops. The loops became narrower at 260 °C than at 560 °C because of the greater straln hardening of copper at 260 °C. Copper exhibits

hlgher strain hardening rates at lower temperatures (ref. 19).

During fatigue of a W-Cu composlte at room temperature, ratchetlng was not

observed (ref. 8). The similarity between plots of the maximum cyclic strain

as a function of cycle number and the creep curves of a monolithic material

suggests that the ratcheting observed during these hlgh-temperature fatigue

tests is the result of a creep-fatigue interaction. To examine this posslblIIty, two creep tests were performed on the composite at 260 °C. For one test

the creep stress was 241MPa, and the time to specimen failure was 18.2 hr.

The second speclmen crept at a stress of 125 MPa for ]500 hr without faillng.

A fatigue test that was performed at 260 °C with a maximum cyclic stress of

241MPa failed after 70 hr. Metallographic analyses Indlcated a signlficant

difference In fal]ure mode between these creep and fatigue specimens (ref. 18);

this implies that a creep-fatigue interactlon was present In the cyclic tests.

More creep ratchetlng was observed at 260 °C than at 560 °C because of the

hlgher ductlllty of the composite at the lower temperature, as evidenced by the

tensile response at these two temperatures.

The stress-mechanlcal strain behavior during In-phase TMF tests was slmlfar to that observed durlng the isothermal tests. For a given cyclic load

6
range, the matrix deformed more easily near the maximumapplied composite load

at the higher Instantaneous temperatures encountered during the In-phase test

than It dld during a 260 °C isothermal test. Becauseof the lower temperature,

more hardening occurred near the minimumapplied composite load at the lower

Instantaneous temperatures encountered during the In-phase TMFcycle than occurred during a 560 °C cycle. Hence, for a given load range, the In-phase

fatigue stress-mechanlcal strain response should be a hybrid of the Isothermal

response at the two temperature extremes.

The out-of-phase TMF response was vastly different from the other three

cases. The deformatlon during the first IO to 20 cycles was similar to that

observed during the 560 °C Isothermal tests. During the out-of-phase cycling,

more compresslve flow of the matrix occurred near the minimum applied composite

loads where the Instantaneous temperature approached 560 °C. Contlnued compresslve loading of the matrix at the highest temperature of the cycle caused

bulging, or "barrelling" of the specimen. Thls barrelling apparently caused

the temporary decrease In the maximum tensile strain due to constancy of volume.

Barrelling, which has been observed during hlgh-temperature Isothermal fatigue

of copper alloys, (ref. 20) was found to be most prevalent during strain cycling at low straln rates; it was particularly severe when compressive hold

periods were Involved. Barrelling of the type observed here also occurred during out-of-phase TMF of tantalum base alloys (ref. 21).

Continued out-of-phase cycling caused numerous large edge cracks to be

formed (flg. 14). These edge cracks were normal to the applled stress dlrectlon. The later Increase In maximum tensile straln appeared to be associated

with the Initiation and growth of these edge cracks. Soon the crack opening

displacement became large relatlve to the straln of the composlte, and the

measured strains were erroneously large. The tlme sequence and the mechanlsm

of localized matrix cracklng near the fibers In the out-of-phase and In-phase

specimens Is currently being Investlgated. Localized matrix cracks In Iongltudlnal sectlons away from the fracture surface may Indicate that these cracks

occurred early In the TMF tests. Near the end of the fatlgue life, the damaged

matrix carried little of the applied loads, and the shape of the hysteresis

loop reflects the response of the tungsten fibers.

Fatigue Life

Isothermal life at 260 °C versus 560 °C. - The observed reduction In

fatlgue life with Increased temperature of the Isothermal experiments can be

expIalned by examining the effect of increased temperature on the copper matrix

behavior. Through fractographlc and metallographic examination, the fatigue

damage was found to Initiate In the matrix and propagate vla cavlty nucleation

and growth during the Isothermal tests. Breakage of the flbers, which occurred

after failure of the surrounding matrix, appeared the same at both temperatures. At 260 °C the matrix failed by a comblnatlon of intergranular and

transgranular cracking (ref. 18). The matrix of the speclmens tested at 560 °C

failed almost entirely Intergranularly, thereby suggestlng that void formation

was the dominant damage mechanism (ref. 18). This Implies that there was less

cavitation at 260 °C than at 560 °C. Vold nucleation (ref. 22) and growth

rates (ref. 23) are known to be strongly temperature dependent, both rates

Increaslng wlth Increaslng temperature. Therefore, the reductlon In fatigue
life that occurs between 260 and 560 °C Is due to the increase in cavitation

rate that Is associated wlth an Increase In temperature.

In__nz__phase versus out-of-phase TMF. - The difference between in-phase and

out-of-phase TMF 11fe is also related to the Increase In cavitation rate in

the matrix wlth Increasing temperature. During the in-phase test, the maximum

tenslle stresses were applied at the highest temperatures. For a given temperature and stress range, the cavitation rate was higher when the maximum stress

was applled at the highest temperature of the cycle than it was when the maximum stress was applied at the lowest temperature. Since composite failure during the TMF experiments Is belleved to be controlled by failure of the copper

matrix, composite 11fe should be shorter for the In-phase cycling. The existence of more cavity dimples In specimens cycled in phase than those cycled outof-phase confirmed that the cavitation rate was higher for the in-phase tests.

Isothermal fatigue versus thermomechanlcal fatigue. - The differences

between lifetlmes of composites subjected to TMF versus isothermal fatigue are

difficult to resolve. To explore the effect of the _-mismatch strains imposed

durlng the TMF cycles, let us examine the mechanical strain range w_th respect

to fatigue lives. Figure 9 shows the mechanical strain ranges measured at half

life as a function of fatigue life. Note that the two isothermal data sets

are close together. Thls suggests that axial matrix strain limits Isothermal

fatigue llfe, since the matrix failed before the flbers. For thermomechanlcally fatigued composites, those tested out of phase seem to have the longest

lives, and those tested In phase, the shortest. However, the out-of-phase data

are deceiving. As discussed earller, the strains measured are a combination of

specimen strain and crack opening dlsplacement, so the actual straln values are

lower.

Since axial matrix strain limits the life of an isothermally fatigue composite, it is useful to examine the axial matrix strains experienced durlng

TMF. In the absence of cracking, the composite axlal strain that was measured

during the isothermal tests was the same as the average axial strain In the

matrix. Thermal cycling between 260 and 560 °C at zero load produced an axial

matrix strain ao_aT (T = temperature) of about 0.34 percent, with the maximum

tensile strain occurring at 260 °C and the minimum at 560 °C. Applying this

thermal expansion mismatch correction to obtain the total axial matrix strain

In the worst case, the In-phase TMF cycle, requires subtracting thls component

from the applied axial composite strain. However, this axial matrix strain

correction increases the divergence between TMF and Isothermal fatigue lives.

Therefore, although axial matrix stralns are important in isothermal fatlgue

llfe, this variable alone cannot reconcile TMF-Isothermal fatigue llfe

differences.

Thls simple analysis also suggests that the TMF life Is not controlled

solely by axial matrix strain. Fractography and metallography of the specimens

subjected to TMF suggest that the matrix also cracks locally near the fibers in

a direction parallel to the fibers, rather than cracking normal to the applied

load only, as was observed for the isothermally cycled specimens. The cracks

paralle] to the fibers may be caused by multlaxlal stresses near the f_bers

rather than the simple farfield axial stress due to the applied load, and they

may be Important In determinlng TMF life. A detailed micromechanlcal analysis

of the local stresses In the composite during the TMF cycles could help answer

these questions.
It |s important to note that the difference In composite llfe due to isothermal fatigue as opposed to TMF may be affected by the difference In the

cycle frequencies used for each type of test. However, in 11ght of the disslmllar fallure modes observed, the relatlve positlons of the Isothermal fatlgue

and TMF data on a life curve would probably not change.

CONCLUDING REMARKS

The work reported herein can be summarized as follows:

I. Copper re|nforced with 9 vol % tungsten fibers was tested isothermally

at 260 °C and at 560 °C in tension-tenslon load-controlled fatigue experiments.

Out-of-phase and In-phase thermomechanlcal fatlgue tests were performed between

these two temperatures.

2. Lives of isothermally fatlgued composltes were much longer than the

lives of thermomechanlcally fatigued composites. In-phase TMF tests resulted

in the shortest lives, on a stress basis, and 260-°C isothermal fatigue tests

produced the longest lives.

3. For all cases the stress-straln response of the composite showed conslderable inelasticity. Both the fibers and the matrix falled in a ductlle

manner. The matrix failed through grain boundary cavitation, and the fibers

failed via tensile overload.

4. Ratchetlng was observed during all of the fatigue experlments. For

the isothermal fatigue and in-phase TMF tests, the ratchetlng was always In a

tensile directlon, contlnulng to failure. The ratchetlng observed that was

durlng the out-of-phase TMF tests reversed from a tensile dlrectlon to a compresslve direction. This anomaly was thought to be assoclated with the

bulging and extenslve cracklng of the speclmen.

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Fiber/Copper Composites on Thermal Cycling. Metall. Trans., vol. lOA,

no. 11, 1979, pp. 1796-1798.

15. Yoda, S., et al." Thermal Cycllng-lnduced Deformatlon of Fibrous ComposIres with Particular Reference to the Tungsten-Copper System. Metall.

Trans., vo1. 9A, Sept. 1978, pp. 1229-1236.

16. Halford, G.R.: Chapter 6: Low-Cycle Thermal Fatigue. Thermal Stresses II,

R.B. Hefnarskl, ed., E1sevler Science Publlsher, New York, 1987.

17. Gabb, T.P.; Gayda, J.; and Mackay, R.A." Isothermal and Nonlsothermal

Fatlgue Behavior of a Metal Matrix Composite. To be published in the J.

Compos. Mater., 1990.

18. Kim, Y.-S.; Verri111, M.J.; and Gabb, T.P.: Characterization of Failure

Processes in Tungsten Copper Composites Under Fatigue Loading Conditions.

To be published in ISTFA 1989 - International Symposium for Testing and

Failure Analysis: Advanced Materlals; Proceedings of the Symposium,

Los Angeles, CA., Nov. 6-I0, 1989, ASM International, Metals Park, OH.

lO
19. Freed, A.D.; and Verr|lll, M.3.: A Viscoplastic Theory Applled to Copper.

NASA TM-I00831, 1988.

20. Conway, J.B.; Stentz, R.H.; and Berllng, J.T.: Hlgh Temperature, Low-Cycle

FatIgue of Copper-Base Alloys In Argon; Part II - Zlrconlum-Copper at 482,

538 and 593 °C. (MTI-RO03-2-2, Mar-Test, Inc., NASA Contract: NAS3-16753)

NASA CR-121260, 1973.

21. Sheffler, K.D.; and Doble, G.S.: Influence of Creep Damage on the Low

Cycle Thermal-Mechanlcal Fatigue Behavlor of Two Tantalum Base Alloys.

NASA CR-12100], 1972. (TRW-ER-7592, TRW EquIpment Lab., NASA Contract:

NAS3-13228).

22. Saegusa, T.; and Weertman, O.R., Influence of Graln-Boundary Morphology on

Vold Formation: Scrip. Metall., vo]. 12, no. 2, 1978, pp. 187-191.

23. Weertman, J.R.: Fatigue Induced Cavitation In Slngle-Phase Material. Can.

Metal]. Q., vol. 18, no. I, 1979, p. 73-81.

TABLE I. - TENSILE PROPERTIES OF 9 vol % W-Cu COMPOSITE

Temperature,

oc

260

560

Tensile

strength,

MPa

290

168

O.05-percent

offset yield

strength,

MPa

143

84

E1astlc

modulus,

GPa

120

8O

Strain at

fallure,

percent

9.68

4.84

,,r- RADIUS, 63.5

I I1-1

139.7 AS- RECEIVED

(2.54)

FRONT SIDE

Figure 1. - Composite specimen; all dimensions are In millimeters.

11
250

150

1--

lOO

50 i_ I 1

2 4 6 8

STRAIN, PERCENT

Figure 2. - Tensile curves for W-Cu composite at 260

and 560 °C.

I

t0

3O0

250

2oo

150

100

50

I/

f

0 I

I

CYCLE NUMBER

5 5O 5OO 1500

I

i

j jl ,

2 3

STRAIN, PERCENT

3O0O

1 I

5 6

Figure 3. - Fatigue stress-strain response of W-Cu

composite at 260 _C (tu_ - 246 MPa, Nf - 3117).

I- CYCLE NUMBER CYCLE NUMBER

1 2 510100500700 _ 1251020 50 70

150

_ _oo _

50

0 1J.5 0 1 2

STRAIN, PERCENT

Figure 4. - Fatigue stress-strain response of W-Cu composite

at 560 _ (z_o= 1S2 MPa, Nf - 939).

I I I

3 4 5

STRAIN, PERCENT

Figure 5. - Stress-mechanical strain response of W-Cu

composite during In-phase TMF test (&o - 116 MPa,

Nf = 100).

12
¢0

o.+

:E

o9

uJ

200--

150

100--

50 --

CYCLE NUMBER

o I

-.5 1.5

-- 700

r

/

/

0

10 2000

//

I I

,5 1

STRAIN, PERCENT

Figure 6. - Stress-mechanical strain response of W-Cu

during out-of-phase TMF test (&o - 133 MPa, Nf = 2135).

TEST MAXIMUM

STRESS,

MPa,

• OUT-OF-PHASE 143

6 r'-- • IN-PHASE 116

| O 560°C 157 _]

/ o+0°o /

0

10 100 1000 10 000

CYCLE NUMBER

Figure 7. - Maximum cydlc strain as a function of

cycle number.

u3

i--

o9

103

102_

102

TEST

- • OUT-OF- PHASE, 5O0 v----=260 °C

• IN-PHASE, 260 ,+-"=_- 560 "C

- O 5O0°C

-- 17 260oc

- SOLID SYMBOLS DENOTE TMF

OPEN SYMBOLS DENOTE

-- ISOTHERMAL FATIGUE

• I v---_ I I

103 104 105

CYCLES TO FAILURE

Figure 8. - Applied stress range as a function of fatigue life.

Arrow Indicates Interrul_led lesL

+_

uJ(3.

z

o --

uJ

:E

.1

102

TEST

• OLK-OF-PHASE, 560 @ 260 "C

• IN-PHASE, 260 _ 560 °C

O 560°C

[] 260°C

SOLID SYMBOLS DENOTE TMF

OPEN SYMBOLS DENOTE

ISOTHERMAL FATIGUE

o%0°@ 8

• I I I

103 104 10 5

CYCLES TO FAILURE

Figure 9. - MechanlcaJ strain range measured at half life as a

function of fatigue life. Arrow Indicates Interrupted lest.

13
(_RI(31NAE PAGE

8LACK AHD Y_LHJ'I'__P_HO_T_OGRAP_H

10 2 --

101 __

!

100 -

__.

q[

u.

TEST

• OUT-OF-PHASE, 560 _ 260 °C

• IN-PHASE,2S0,_S60 _C

[] 260°C

SOLID SYMBOLS OENOTE TMF

OPEN SYMBOLS DENOTE

ISOTHERMAL FATIGUE

D D

• []

OO •

O

O y-_,,- O

10 -1

10 2

I I I

10 3 104 10 5

CYCLES TO FAILURE

Figure 10. - Failure strain wlth respect to fatlgue llfe.

Arrow Indicates Interrupted test.

(a) Isothermal fatigue at 260 "C.

('o) Isothermal fatigue at 560 "C.

Figure 11. - Matrix failure surface due to isothermal fatigue.

14
ORIGINAL' PAGE

BLACK AND W_HJTE PHOT_OGRAP.H

(a) In-phase TMF,

Figure 12. - Polished longitudinal section of a specimen fatigued isothermally

at 260 °C (_ = 246 MPa, Nf = 3117). Arrow Indicates direction of

matrix cracking.

Co)Out-of-phase TMF.

Figure 13. - Matrix failure surface due to TMF.

15
ORIGINAL PAGE

BLACK A__D _LHLT_F-,p.HOT_QGRAP_

Figure 14. - Polished longitudinal section of a specimen fatigued under

in-phase thermomechanical fatigue (,t_ = 1i 6 MPa, Nf = 100). Arrow

Indicates direction of matrix cracking.

Figure 15. - Optical micrograph of an out-of-phase TMF spet;imen (_,G=

10g MPa); test interrupted at 1788 cycles.

16
National Aeronautics and

Space Administration

1. Report No. 2. Government Accession No.

NASA TM- 102404

4. Title and Subtitle

High Temperature Fatigue Behavior of Tungsten Copper Composites

Report Documentation Page ..

7. Author(s)

Michael J. Verrilli, Yong-Suk Kim, Timothy P. Gabb

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

3. Recipient's Catalog No.

5. Report Date

6. Performing Organization Code

8. Performing Organization Report No.

E-5156

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

!10, Work Unit No.

510-01-0A

11. Contract or Grant No.

13. Type of Report and PeriodCovered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared for the Symposium on Fundamental Relationships Between Microstructure and Mechanical Properties

of Metal-Matrix Composites cosponsored by The Metallurgical Society and the American Society for Metals,

Indianapolis, Indiana, October 1-5, 1989. Michael J. Verrilli, and Timothy P. Gabb, NASA Lewis Research

Center. Yong-Suk Kim, National Research Council--NASA Research Associate at Lewis Research Center.

16, Abstract

_=.-

• z/ j ,

The hlgh4emperature fatigue behavior of a 9 vol %, tungsten fiberremforced_copper matrix composite was

investigated. Load-controlled isothermal fatigue experiments at 260 and 560 *-C and thermomechanical fatigue

(TMF) experiments, both in phase and out of phase between 260 and 560 *'C, were performed. The stress-strain

response displayed considerable inelasticity under all conditions. Also, strain ratcheting was observed during all

the fatigue experiments. For the isothermal fatigue and in-phase TMF tests, the ratcheting was always in a tensile

direction, continuing until failure. The ratcheting during the out-of-phase TMF test shifted from a tensile

direction to a compressive direction. This behavior was thought to be associated with the observed bulging and

the extensive cracking of the out-of-phase specimen. For all cases, the fatigue lives were found to be controlled

by damage to the copper matrix. Grain boundary cavitation was the dominant damage mechanism of the matrix.

On a stress basis, TMF loading reduced lives substantially, relative to isothermal cycling. In-phase cycling

resulted in the shortest lives, and isothermal fatigue at 260 _, the longest.

17. Key Words (Suggested by Author(s))

Metal-matrix composites

Thermomechanical fatigue

Low cycle fatigue

18. Distribution Statement

Unclassified - Unlimited

Subject Category 24

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages

Unclassified Unclassified 18

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