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198400062,19

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Effects of Cobalt in Nickel-BaseSuperalloys

John K. Tien and Robert N, Jarett

Columbia University

New York, New York

December1983

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EFFECTS OF COBALT IN NICKEL-BASE SUPERALLOYS*

John K. Tien andRobert N. Jarrett

Columbia University

Center for Strategic Materials

Henry Krumb School of Mines

New York, New York 10027

The role of cobalt in a representative wrought nickel-base

superalloy was recently determined. The results show cobalt affecting the solubility of elements in the y matrix, resulting in

enhanced y' volume fraction, in the stabilization of MC-type carbides, and in the stabilization of sigma phase. In the particular

alloy studied, these microstructural and microchemistry changes

are insufficient in extent to impact on tensile strength, yield

strength, and in the ductilities. Depending on the heat treatment,

creep and stress rupture resistance can be cobalt sensitive. In

the coarse grain, fully solutioned and aged condition, all of the

alloy's 17% cobalt can be replaced by nickel without deleteriously

affecting this resistance. In the fine grain, partially solutioned

and aged condition, this resistance is deleteriously affected only

when one-half or more of the initial cobalt content is removed.

The structure and property results are discussed with respect to

existing theories and with respect to other recent and earlier

findings on the impact of cobalt, if any, on the performance of

nickel-base superalloys.

*Material previously presented as an invited contribution

at the High Temperature Alloys for Gas Turbines

Conference, Liege, Belgium, October 4-6, 1982.
.

I. INTRODUCTION

In 1980, spurred by the 1978-1980 sharp increase in the world

price of cobalt, Tien et el. (I) reviewed the role of cobalt in

superalloys and concluded that:

a. The superalloysystem, includingthe nickel-basesuperalloys, is the single largest consumerof cobalt in the United

States. Cobalt as a major alloying element in superalloys accounted for about 30% of the cobalt consumed in the United States

in 1979. This percentage increased to over 40% in 1980-1981.

b. Systematic information on the role of cobalt in nickelbase superalloysin developingthe approPriate strengthening

microstructuresand mechanicalpropertiesis scanty and not-at

el! comprehensive. _ : _

c. Analyticalcalculationsshow, however, that the roie of

c0b@!t, the nearest chemicaland physicalneighbor to nickel, may

not be pivotal and the present levels of cobalt (i0 to 20 percent)

in nlckel-basesuperalloys,a priori, are not technicallyJustified.

It was prudentlyrecognized,however, that the last statement

is itself based on minimal empiricalinformationand that systematic studies must be undertakento understandthe role of cobalt

in nickel-basesuperalloys. Since 1980, a study in our group has

concentratedon understandingthe role of cobalt in Udimet 700,

which is a representativeintermediatey' volume fractionnickelbase superalloy. Udimet 700, containingabout 18% cobalt,has

many versatileuses. In the cast form and/or coarse-gralned

wrought form, it is used in rotating turbine blade applications.

In the flne-grainedwrought form, it is used as turbine disks.

With the appropriatealloy modifications,it is also a powder

alloy.

Our results (2) will be discussedwith respect to the findings of other related studies. Particularattentionwill be given

to two other very recent studies (3,4). The study by Nathal and

Meier (3) concentratedon Mar-M247,which is representativeof

the high strength cast alloys for high temperaturevane and blade

applicationsand can be used in polycrystalline,directionally

solidifiedand monocrystallineforms. The study of Maurer et el.

(4) was on Waspaloy,which is a high sales volume, moderate

strengthwrought alloy used as turbinedisks. The compositions

and the y' fractionsof the three alloys are given in Table I.

As can be seen, together they span the y' volume fraction spectrum, and hence, the elevated temperaturestrengthspectrum of

today's superalloys.

The common strategyfor studying the cobalt effect in nickel-
"1 I • e

Table I: Nominal Compositions of Several Nickel-Base Superalloys*

% y' Ni Co Cr A1 Ti Mo W Ta Hf B Zr C

w/o 58 13.5 19.5 1.3 3.0 4.3 .006 .06 .08

Waspaloy 4 20

a/o 56 13.0 21.4 2.7 3.6 2.6 .03 .04 .38

w/o 53 18.5 15.0 4.3 3.5 5.2 .030 .08

Udimet 7002 45

a/o 50 17.4 16.0 8.8 4.1 3.0 .15 .37

w/o 60 i0.0 8.2 5.5 1.0 0.6 10. 3. 1.5 .020 .09 .16

Mar-M247 3 55

a/o 61 i0.i 9.2 12.2 1.2 0.4 3. I. 0.5 .Ii .06 .79

*From the International Nickel Company, Inc. Handbook on "High Temperature, High

Strength Nickel-Base Alloys," 1979.
.

base superalloys is to systematically replace cobalt by nickel.

Processing and experimental procedure details for the results

discussed below are published elsewhere (2).

2. PARTITIONING AND PHASE EQUILIBRIA

2.1 Partitioning of Cobalt

In Udimet 700, in situ STEM/EDS microchemistry analysis of

the T' and T phases has shown that cobalt partitions to the matrix (2); see Table II. The concentration of cobalt in the

matrix was found to be about 2.5 times that in the T' precipitate

for the coarse grained, fully solutioned alloys and approximately

3 times that in the T' for the fine grained, partially solutioned

alloys. This partitioning ratio did not vary with cobalt content.

In the Mar-M247 study by Nathal and Maier (3), the T' phase was

electrolytically extracted from the matrix and its composition

determined by emission spectroscopy. The partitioning ratio calculated from the volume fractions showed the matrix cobalt content

was about 2 times the T' content, again regardless of the alloy

cobalt content. The partitioning ratio for Waspaloy (4), as determined from SEM/EDS analysis of extracted T', was found to be

roughly 4. These partiaioning ratios are consistent with the

values cited much earlier by Kriege and Baris (5); see Table II.

TaBle II. Matrix Partitioning of CoBalt

Partitioning Ratios from

Alloy Kriege and Baris (5)

Udimet 700 (disk) 3.0 (2)

Udimet 700 (blade) 2.5 (2) 2.8

Mar-M247 2.0 (3) 1.7"

Waspaloy 4. (4) 5.5

* This value is for the alloy Mar-M200, which is the

sister alloy of Mar-M247.

Most commercial alloys contain between I0 and 20 percent

cobalt (I). Hence, the T cobalt content would usually range

between 15 and 25 percent and the T' would contain up to about

10% cobalt (5). The cobalt in the ordered T' phase apparently

substitutes for nickel (2,5,6), and when present in higher concentrations, it can also displace chromium (2,7). The solubility

of cobalt in the T' increases with temperature (8), and thus, the
6

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TEMPERATURE (°C)

Fig. I. Partitioning ratios of cobalt (w/o cobalt in y: w/o cobalt in y') versus temperature.
.

partitioning ratio between T and T' decreases with temperature.

This trend is fairly alloy insensitive, and the T:T' cobalt partitioning ratio falls on a single curve as a function of the final

aging temperature, Fig. i.

2.2 The Effect of Cobalt on the Microchemistry of Other Alloying

Elements

Maurer et al. (4) found that removing cobalt from Waspaloy

decreased the T' fraction from 18% in the standard alloy to 16

volume percent in the cobalt-free version. A smaller decrease

of less than 1 volume percent was observed in Udimet 700 (2).

In a 1964 paper (9), Heslop observed roughly a 5% increase in

the T' volume fraction of Nimonic 80A with the addition of 17%

cobalt. Nathal and Maier (3) observed an 18% decrease in the

complex alloy Mar-M247, as cobalt was completely removed.

We believe that this effect is related to cobalt's reducing

the matrix solubility for other alloying elements. In this case

cobalt has forced aluminum and titanium out of the T solution,

resulting in more T'. The ternary Ni-Co-AI diagram, Fig. 2

(I0,II), supports this conclusion. For example, adding 30% cobalt

to nickel reduces the T matrix solubility for aluminum from 6% to

about 3% at 700°C. Furthermore, Heslop (9) used pseudo-blnary

phase diagrams of AI+TI in a Ni-2OCr matrix and a Ni-20Cr-20Co

matrix to explain the volume fraction effect in Nimonlc 90. In

the superposition of the two diagrams, Fig. 3, the AI+TI solubility at 800°C can be seen to decrease from 1.8 w/o to 1.2 w/o with

the addition of 20% cobalt to the matrix.

Indeed, this diagram illustrates that cobalt's effect in

increasing volume fraction will be less, due to the lever rule,

as the alloy's y' volume fraction is more. This is consistent

with the observed cobalt effect in Nimonic 80A/90, Waspaloy, and

Udlmet 700.

One additional solubility argument may help to explain the

reduced effect of cobalt on volume fraction in the molybdenum

containing alloys, Udimet 700 and Waspaloy. From Ni-AI-X ternary

phase diagrams--where X is either chromium (12), titanium (12),

molybdenum (13), or cobalt (10,11)--the third element is seen to

always reduce the solubility of T for aluminum. We maintain that

the combined effects of many alloying elements would result in a

greater reduction of the solubility for aluminum than cobalt alone.

It is perhaps for this reason that the aluminum solubility at 800°C

in the NI-Co-AI ternary decreases 2.0% for a 20% cobalt addition

(ii), whereas the (AI+Ti) solubility decreases by only 0.8% for

the same 20% cobalt addition to the Ni-Cr-Co-(AI+Ti) pseudo-blnary

diagram (9).
J

°0

I00 _, \ 0

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o 5 io i5 5 to t5 5 lo i5

ALUMINUM Ln w/o ALUMINUM in wlo ALUMINUM in w/o

(bl (el (dl

Fig. 2. The Ni-Co-AI ternary phase diagram. Fig. 2a

shows a composite of isothermal sections at

700°C and 800°C from Davies et al. II Fig. 2b

is the Ni-AI binary, Fig. 2c is the 80Ni-20Co

compositional section, and Fig. 2d is the CoA1 binary. I0
1200 1200

I ! I { I mllllillll{l_II

II00 _ II00.

E

o With Co o

"-'iO00_ _ i000_ /i

With Co

_900_ 900_ o =

f',i

_Without Co

W _Without Co 800 800

W

s

700 I I I ! I 700 .... _ , , , _ , , , ,

o _ 2 s 4 5 B 0 5 1o 15

A1 + Ti CONTENT w/o A1 + Ti CONTENT. a/o

(a) (b)

Fig. 3. Pseudo-binary phase diagrams. Fig. 3a: Heslop's 9 pseudo-binary of the Ni-20Cr and

Ni-20Cr-20Co matrices. Fig. 3b: Composite from y' solvus results.2,4, 9 In both

diagrams note that cobalt decreases the solubility of (AI+Ti) and in the higher y'

volume fraction alloys cobalt decreases the y' solvus temperature.
J

- The combined effect of additional components on the solubility of aluminum and the T' fraction may also explain stress rupture

results of Peter et al. (14). In this study on the effect of

. molybdenum on Nimonic 80A and Nimonic 90, they found that adding

4.5% molybdenum to both systems reduces the effect of cobalt on

_he 750°C 100-hour or !000-hour rupture stress by approximately

one-third ..........

2.3 Gamma Prime Solvus Effect

The Y' solvus in Udimet 700 increases by 35°C (2) when cobalt is removed, Fig. 4a. Similarly, Natbal and Haler (3) found

the solvus in Mar-M247 increases by 30°C, Fig. 4b. In Waspaloy,

with a smaller y' volume fraction, Maurer et al. (4) observed

that the solvus temperature is unaffected by cobalt. The T'

solvus in Nimonic 90, with yet a smaller y' fraction, is found

to decrease by 60°C when cobalt is completely removed (9).

This seemingly complicated solvus effect can also be explained

by the ternary Ni-Co-AI phase diagram, Fig. 2 (i0). Since cobalt

and aluminum do not form a Co3Ai intermetallic, cobalt destabilizes

the Ni3AI Y' when it substitutes for nickel, lowering the solution

temperature. Recalling that cobalt lowers the matrix solubility

for aluminum, a cross-over of the cobalt containing solvus and the

cobalt-free solvus must exist. Heslop's pseudo-binaries, Fig. 3,

illustrate this cross-over (9). Mar-M247 and Udimet 700 are in

the region with a large T' fraction where cobalt decreases the

solvus temperature; Waspaloy is in the intermediate, cross-over

region; and Nimonic 90 is in the low volume fraction region where

cobalt raises the solvus temperature.

This y' solvus effect of cobalt results in microstructural

changes in Udimet 700 during aging. For example, after the blading heat treatment, the fraction of fine, cubic y' is larger and

the fraction of residual ultrafine y' formed is much smaller in

the cobalt-free alloy than in a cobalt-containing alloy, Fig. 5.

After solutioning, both alloys were aged at i079°C for 4 hours,

which produced cubic 7' roughly .3 _m to a side. At this aging

temperature, which is above the cross-over in the pseudo-binary

phase diagram, more y' precipitates in the cobalt-free alloy. In

the remaining aging steps, less ultrafine y' forms in this alloy

since the corresponding pseudo-binary boundary has a steeper temperature dependence than the solvus boundary of the alloy containing cobalt.

In Udimet 700, although the aluminum and titanium solubilities in the matrix at low temperatures are reduced by cobalt, at

temperatures above the cross-over (ca. 1000°C) the solubilities

are increased by cobalt. For this reason the values of the aluminum and titanium contents in the matrix determined by STEM/EDS
I0.

in Reference (2) show a trend of increasing solubility with cobalt

content. STEM/EDS analysis of thin foils includes ultrafine y'

particles in the determination of the matrix composition.

Cobalt also affects the solidus temperature in these alloys.

For example, as cobalt is completely removed from Udimet 700 (2),

the solidus is observed to increase by approximately lO°C. Accordingly, the temperature range of solutioning decreases as

cobalt is removed from Udimet 700. A similar trend was found

for the higher y' fraction Mar-M247 (3). This effect can potentially result in heat treatment problems when cobalt is removed

from alloys which already have narrow solutioning ranges.

2.4 Phase Stability

Udimet 700 containing 17% cobalt exposed to 815°C for I000

hours resulted in the formation of -1.5% feathery sigma phase (2).

Alloys with 12.8% cobalt and 8.6% cobalt contained only about 1%

sigma and a trace of sigma, respectively. No sigma phase formed

in alloys with yet lower cobalt contents. This is not surprising

since cobalt was previously determined to be a major constituent

in the sigma phase in Udimet 700 (2,15) and in other superalloys

(7,16).

Again the ternary and quaternary phase diagrams can describe

the behavior seen in superalloys. Cobalt lowers the solubility

of chromium and molybdenum in a nickel solid solution (17). The

sigma phase that forms is based on a Co-Cr intermetallic.

PHACOMP (16,18), the semi-empirical method of forecasting

sigma instabilities based on electron vacancy numbers, also predicts that cobalt will promote sigma instabilities. The electron

vacancy number for cobalt is 1.71 compared to 0.66 for nickel.

Thus, when cobalt replaces nickel in an alloy, the electron vacancy number is increased and the alloy is expected to be less

stable with respect to sigma phase formation.

In a 1968 paper on Mar-M421, Lund et al. (12) observed sigma

phase in cobalt-free 713C and in another experimental cobalt-free

alloy. They also observed no sigma formation in three experimental alloys that contained cobalt to varying levels. On that basis

they concluded that cobalt stabilized the alloys with respect to

sigma instability. However, we maintain that such a conclusion

cannot be made, since unfortunately the cobalt-free alloys also

had more of the alloying elements tungsten, molybdenum, and columbium as well as aluminum and titanium. These elements promote

sigma formation (19).

The effect of cobalt on the coarsening of y' in Udimet 700
Ii.

1400 • 1400 ! i i ! _ ! ! ! i ! e ! ! ! ! ! ! ! i ! ! ! I ! ! I | i

- SOLIDUS

Udimet 700 2 i

i

0

o t300 t30C_

rr

SOLIDUS

• • Y' SOLVUS

n't20G_ • t200_

lLI I n •

Z •

W y' SOLVUS • •

_- Mar-M 247 4

ttOC .... I .... I .... I , , 1100 .... I .... I ,

0 5 t0 t5 0 5 t0

WEIGHT % COBALT WEIGHT % COBALT

(a) (b)

Fig. 4. y' solvus and solidus temperatures versus cobalt

content for (a) Udimet 7002 and (b) Mar-M247. 3

(a) (b)

Fig. 5. SEM micrograph of blade heat treated Udimet 700,

showing more cubic y' and less ultrafine y' in

the cobalt-free alloy (a) than the 17 w/o cobaltcontaining alloy (b).
12.

(2,20) was shown to be negligible for the disk (fine-grained) material at 815°C and also for fully solutioned material with homogeneously nucleated _' particles coarsened at 982°C. The y' in

the cobalt-free blade heat treated material aged at 982°C developed

a rod-like appearance as the particles coalesced. This behavior

was not observed in the cobalt-contalnlng alloys. The effect of

such shape changes in properties has not been examined. The Insensitivity of y' coarsening kinetics to cobalt content is believed

to be due to the small effect which cobalt has on y' misfit in the

Udimet 700-type alloy (2).

2.5 Carbides

Although cobalt is not a constituent in either the MC-type

or in the M23C6-type carbide in Udimet 700 (2,15), it plays a

major role in the relative stabilities of these carbides. Titanium-rich MC forms as the primary carbide in Udimet 700, and during

alloy aging additional chromlum-rlch M23C 6 carbides precipitate at

the grain boundaries. In cobalt-free Udimet 700, predominantly

primary M23C 6 forms in the lower solution temperature (dlsk-type)

heat treatment. No additional grain boundary carbides form even

after a i000 hour exposure at 815°C (2). These primary M23C 6 carbides can be dissolved in the full-solutionlng blade-type heat

treatment, with grain boundary carbides precipitated during aging.

The resulting cobalt-free Udlmet 700 microstructure is very similar to the standard alloy except that there are fewer MC-type

carbides and more of the stable M23C6-type carbides.

Maurer (4) also found that removing cobalt destabilizes MC

in Waspaloy so that more grain boundary M23C 6 forms during aging.

Likewise, Nathal and Maier (3) observed more grain boundary carbides in lower cobalt and cobalt-free Mar-}_47.

In M23C 6 carbon combines with roughly four times as many metal

atoms as in MC carbides. Accordingly, a fourfold increase in the

volume fraction of carbides can result if M23C 6 becomes the predominant primary and secondary carbide. In cobalt-free Udimet 700

containing only 0.07 w/o carbon such excesses in carbides are seen

as stringers. Although these carbide stringers did not hinder the

hot workability of lower cobalt and cobalt-free Udimet 700 (2), an

excess of primary carbides can result in decreased workability in

superalloys with generically higher carbon content. Interestingly,

Heslop (6) showed that removing cobalt from Nimonic llb-type alloys

(which normally contain .15 w/o carbon) reduced their hot ductility

in the workabillty-simulating hot twist tests.

2.6 Solid Solution Effects of Cobalt

As discussed above, cobalt in solid solution reduces the solubility of the other alloying additions. In the y matrix, cobalt
13.

is expected to contribute little to direct strengthening since

its atomic misfit is only 1% in nickel. Not surprisingly, in

Udimet 700 (2), Waspaloy (4), and Mar-M247 (3), the effect of

" cobalt on yield strength is minimal (see below).

In creep deformation where the effect of stacking fault

energy on thermally activated recovery by climb or cross-slip

is important (21), cobalt in the matrix may play a significant

role (22). In Ni-Co binary alloys, Beeston et al. (23) showed

that the adding of 30% cobalt resulted in the lowering of the

stacking fault energy (TSFE) by about 50%. Whether cobalt would

decrease the SFE of a complex superalloy matrix to the same extent is uncertain at this time (22-24).

3. MECHANICAL BEHAVIOR

3.1 Tensile Behavior

The average room temperature tensile and yield strengths of

Udimet 700 and Waspaloy (based on duplicate tests) are presented

in Fig. 6 as a function of cobalt content (2,4). Examples of

elevated temperature tensile properties of Waspaloy and Mar-M247

are compared in Fig. 7 (3,4) for discussion purposes. In all

cases the strength dependence on cobalt is extremely slight. No

more than a 10% decrease in the strengths is observed when cobalt

was removed completely from Waspaloy and from the disk heat treated

Udimet 700 (2,4). Blade heat treated Udimet 700 basically showed

an invariance of yield strength with cobalt content (2). In the

Mar-M247 case, the strengths show a slight but discernible peak

at one-half the standard cobalt level.

Ham and Brown's equation (25), which recognizes both the

bowing between and the cutting of coherent y' precipitates, apparently can satisfactorily explain the yield strength results

for all three alloys where,

ATy = (FAPB/2b) [ (4FAPBrof/_) 2_f] (i)

ATy is the difference in yield strength between y+y' and the y

phase, FAp B is the APB energy, b is Burger's vector, ro is the

average y, size, f is the volume (or weight) fraction of y', and

is the strain energy of the mobile dislocations bowing between

and/or cutting through the coherent T' precipitates. In the case

of disk heat treated Udimet 700, a quantitative analysis was performed (2). The conclusion was that the slight drop in strength

as cobalt is removed can be attributed to the observed decrease

in the volume fraction of the fine, strengthening y' precipitates.

Conversely, the invariance in strength with cobalt after the fully

solutioning, blade-type heat treatment is consistent with the ob-
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50C

i,i

Disk Udimet 700 2 " Blade Udimet 700 _ Haspaloy 4

t , I , I , , , , ! , , ,, I , , n , nm I, , , , I, , , , I ' ' C , , , , I, , , , !,, , m I, ,

0 5 iO 15 0 5 iO i5 0 5 iO i5

HEIGHT _ COBALT HEIGHT _ COBALT HEIGHT _ COBALT

('a} {b) ('o)

Fig. 6. Room temperature tensile properties of Udimet 7002 [both (a) blade and (b) dlsk heat

treated] and Waspaloy 4 as a function of cobalt content.
iS00 150( IS0(

. ; J | i . i . i . | , | i | . o i i . i . i | i i a i | l | i | i i !

538 C 760 C 982 C

O'*!JTS ,,

-- OUTS

"_tOOC_ iOOC , tOO(

0"0.2 (_UTS

oi

m 500 50O 50C

Ill

--. rr" 00. 2

I--

Haspaloy4 Mar-M 2473 Mar-M 247 3

O' ,, , q I,,, , I .... 0 , , , _ I = , , , I , 0 , , , , I q , , , I ,

0 5 LO L5 0 5 iO 0 5 tO

WEIGHT % COBALT WEIGHT % COBALT WEIGHT % COBALT

(a) (b) ('el

Fig. 7. Elevated temperature properties of (a) Waspaloy 4 at 538°C and Mar-M2473 at (b) 760°C

and (c) 982°C.
16.

served invariance of the strengthening _' fraction with cobalt

after this treatment. Volume fraction dependence can likewise

explain the observed yield strength trend in Waspaloy. Equation

(i) also can qualitatively explain the slight strength peak observed at the intermediate cobalt level in the high y' volume

fraction Mar-M247 (3). Nathal and Maier (3) observed the opposing

trend of the y' APB strengthener tungsten increasing in the ?'

and the volume fraction of ?' decreasing as cobalt was removed

from the alloy. Equation (i) would predict a strength maximum at

intermediate cobalt levels for such element partitioning and phase

equilibria behavior. Interestingly, Nathal and Maier observed no

peak at 982°C (3). This is consistent with the decrease in the

effectiveness of APB's at temperatures above 800°C.

Earlier studies on cobalt effects also show the relative insensitivity of strength on cobalt content. For example, the yield

strength:of a Soviet high strength nickel-base superalloy was observed to be insensitive to cobalt content (26). Generally, it is

tempting to conclude that increasing cobalt content has a small

effect in increasing y' fraction and that through a parabolic

relationship (see Eq. (i)) this small effect is translated to an

even smaller impact on yield strength. Furthermore, for a chemically well-endowed alloy, such as the tungsten containing Mar-M247,

where y' precipitate cutting and FAp B can be the predominant

strengthening mode, the effect of increasing cobalt content on

decreasing the solubility of, say, tungsten in the _' precipitates

can in effect counterbalance any positive _' fraction effect.

The tensile ductility of all three alloys is found also to

be either Invariant or only marginally dependent on cobalt content. In Udimet 700, there is a slight increase in ductility as

cobalt is increased above 8.6 w/o (2). The increase in ductility,

although resolvable, is small--from about 9% to 13% in the coarse

grain material and from about 22% to 28% in the fine grain material. Maurer et al. (4) observed basically no effect of cobalt on

room temperature and elevated temperature tensile ductilities.

Nathal and Maler found no trends between tensile ductility and

cobalt over the temperature range of 649°C to 982°C.

The slightly lower ductilities in Udimet 700 may be related

to the previously discussed increase in carbide inclusion fraction

as the cobalt content in the alloy is decreased by more than onehalf the initial content (2). More systematic analysis on tensile

fracture and crack initiation behavior is needed before any firm

conclusions can be made.

3.2 Creep and Stress Rupture

The minimum creep rates and stress rupture lives of Udlmet 700

and Mar-M247 are summarized as a function of cobalt content in
-5 -5 -8 I i I I I . i I I I i I I i I I I • i I i i i i I i i i i i I i i i I I I I I i i I I I I I

T Udimet 700 (Disk) .' Udimet 700 {Bladel Mar-M2473

o 7600C i 760 °C 690 MPa 760°C

"-" 1"II_..._ I'I 689 MPo"_ 0 .--.I

1U

I-- . 0

t-I 5aa -e,L "

w

m 0

n- [] I 586 MPo

0 _ O-- 0 __0

(.'3 X 483 MPa I 0

0

.._1 0 _.0 _.---,.._

0

-- , , , , I , , , , I , , . , I , , -71 , , , , I , , , , I , , , , I , , -8 . , ' ' I , , , , t ,

0 5 iO 15 0 5 iO .!5 0 5 tO

HEIGHT % COBALT HEIGHT % COBALT HEIGHT % COBALT

{al (bl (cl

Fig. 8. Minimum or steady state creep rates of Udimet 7002 [both (a) disk and (b) blade] and

and Mar-M2473 at 760°C.
°

80 IIiiiiiiiiiiiiill 81 t5C IIiiiiiiiii

Udimet 700 (Disk) Mar-M2473

oc 760oc • 760 oc/

0 0

"-'40 O 4{ 586 MPa 10G • / o

/ 483 MPa O

H _ 690 MPa

W

_20 /

O

2¢ 50 /

/

586 MPa

_ _0 689 MPa /0_0 . 0 -

0 / ) 1 0 0 _ 0 l

0 .... I .... ! .... I , , OI , , , , I , , , , I ,

0 5 iO i5 0 5 iO i5 0 5 I0

WEZGHSTCOSALT WEZGHT_ COBALT WEZGHTS COBALT

(a) (b) (c)

Fig. 9. Stress Rupture Life of Udimet 700z [both (a) disk and (b) blade] and Mar-M2473

at 760°C.
19.

Figs. 8 and 9. The creep and stress rupture resistance of Waspaloy,

like those of Mar-M247 (3) and the disk heat treated Udimet 700 (2),

decreases when cobalt is removed (4). We have concluded that cobalt

effects on creep resistance can be accounted for by the modified

creep equation developed for superalloys (2,22)

Sm = A'(YSFE )4{[° - °p (f)]/E}4exp[-(Q°/RT)] (2)

where Sm is the minimum creep rate, YSFE is the already defined

stacking fault energy, _ is the applied stress, Cp(f) is the particle contribution to creep resistance and is a function of y'

volume fraction, E is Young's Modulus, Qo is the true activation

energy for creep or self-diffusion, A' is an alloy constant, and

RT has the usual meaning, As can be seen, creep resistance, and

corresponding stress rupture resistance (since rupture lives are

generally inversely pr0porti0nalto minimum creep rates in suReralloys), increases when either 7SFE decreases or the volume fraction of _' increases.

By analyzing the creep results of Udimet 700 after the disktype heat treatment and the blade heat treatment, respectively,

it was concluded that cobalt's effect on y' volume fraction, and

not on YSFE, affected creep and stress rupture resistance as cobait was removed (2), the argument being that the y matrix composition, and hence YSFE, was not affected by the two solutioning

temperatures that distinguish blade heat treatment from disk heat

treatment, but that they did affect cobalt's effect on the fraction

of the strengthening y' precipitates. In particular, cobalt was

determined not to affect the strengthening y' fraction after the

blade-type heat treatment, consistent with the observed insensitivity of creep rates to cobalt content, but after the disk-type

heat treatment, the strengthening y' was determined to have decreased from about 36% to 29%, which was considered sufficient to

lower the creep resistance (or increase the creep rate) by the

observed one order of magnitude (2,27,28). For the case of the

lower y' fraction Waspaloy, Maurer et al. argued that volume

fraction alone could not fully account for the decrease in creep

resistance as cobalt was fully removed from the alloy, but that

YSFE had to play a role (4). Nathal and Maier argued that in the

very high y' fraction Mar-M147 case, the observed threefold increase in creep rates as the cobalt was removed was due to the

observed decrease in the y' fraction (3). That the observed large

decrease in _' fraction, from 59% to 41%, causing only a threefold

maximum increase in creep rates is perturbing given the Udimet 700

° results, where a smaller strengthening y' fraction decrease, from

36% to 29%, resulted in a tenfold increase in creep rates. The

rationalization of this apparent inconsistency may rest either in

an error in y' fraction determination or in that for the high y'

fraction, cast alloys creep is not as sensitive to y' fraction

since y' cutting would be the predominant strengthening mode and
20.

-8 -8 ! l I ! l l ! l l i l l I ! ! ! | I l ! i !

Fig. i0. Creep Rates of Mar-M2473 at (a) 871°C and

(b) 982°C.
21.

. that such cutting would occur at both the high end and the low

end of the observed %' fractions in Mar-M247. Whereas, in Udimet

700, the bowing mechanism would be sensitive to the _' fraction

ranges observed in the cobalt-containing and cobalt-free alloys.

The results of Nathal and Maier also show that at the highest

temperature (about 982°C), the cobalt effect decreases to less

than twofold (3), see Fig. i0. This is consistent with earlier

Japanese observations (29), where cobalt was seen not to affect

creep and stress rupture in a high strength nickel-base superalloy,

also at about 982°C. The observations that cobalt can enhance

creep and stress rupture resistance at lower temperatures also are

consistent with results from earlier studies (30,31).

" Stress rupture oductilities are observed tobe very Iinsensitive

to ¢pbaitlco_ent for all thr'ee of the recently studied alloys

(2_4)_ In Udimet 700, this is the case in spite ofthe h!ghe _

vqlume of carbides at the grain boundaries of the lower cobalt

and cobalt-free alloys and the feather-like sigma phases in the

higher cobalt alloys (2). The extra grain boundary carbides may

account for the slightly higher stress rupture resistance of the

cobalt-free Udimet 700 after the blade heat treatment.

In general, it appears that cobalt's effect on creep and

stress rupture resistance can be much more pronounced than its

effect on tensile properties. However, the Udimet 700 experience

also demonstrates that appropriate heat treatments can replace the

positive role of cobalt in promoting creep and stress rupture

resistance.

4. CONCLUDINGREMARKS

This paper discussesthe role of cobalt in Udimet 700, which

is a representativenickel-basesuperalloynominallycontaining

17% or more cobalt. The study spanned the spectrumof microstructural,microchemical,and mechanicalbehavioraspects,which

together form a basis for superalloyperformancein jet engines.

The singlemost significantresult is that eobalt's role in the

alloy is extremelysubtle, and hence, difficultto discern. It

impacts on the partitioningof the elementsbetween the y matrix,

y' precipitate,and carbide phases, but only slightlyso. The

resultantenhancementof the strengtheningy' volume fractionis

small so that it does not significantlyaffect tensileproperties,

. even when all the cobalt in the alloy is replacedby nickel,which

is the alloy-basemetal and the nearest chemicaland physicalkin

to cobalt. Dependingon the details of the heat treatment,cobalt

is seen to promote creep and stress rupture resistance. However,

even there the appropriateheat treatmentcan result in no cobalt

effect. Indeed, an interestingconclusionis that the optimum
22.

Udimet 700 wrought, blade-type alloy is one which contains no

cobalt instead of the standard Udimet 700 which contains at least

17% cobalt.

Less subtle is the discovery that in the alloy studied, and

in two other recently studied nickel-base superalloys, cobalt

promotes fewer carbides by stabilizing the MC-type carbides instead of the M23C6-type carbides. This implies that lowering

cobalt content in nickel-base superalloys may result in the need

to lower carbon content. There are reported indications that,

in alloys with generically higher carbon content, workability

may be affected when cobalt is lowered with no attendent modification of carbon levels. Cobalt is also seen to widen the solid

solution temperature range. Accordingly, it may come to pass

that lowering cobalt content may reduce the heat treatability of

certain nickel-base superalloys.

In terms of long-term alloy stability, we present results

and arguments that show cobalt can have a destabilizing effect

instead, as previously believed, of the stabilizing effect on

sigma phase formation. Also, for the alloy studied, it appears

that cobalt plays a small role in y' coarsening.

Certainly, the effects of cobalt on other properties such

as high cycle fatigue, low cycle fatigue, thermal fatigue, and

oxidation and hot corrosion, require determination and understanding. Such studies are under way. Furthermore, the subtle

effects of cobalt on fine and ultrafine y' precipitation and

their effects on elevated temperature mechanical properties require further study.

5. ACKNOWLEDGMENTS

We thank NASA-Lewis Research Center for sponsoring the

Udimet 700 study as part of the Conservation of Strategic Aerospace Materials (COSAM) Program. We are grateful to the program

monitors, Joseph R. Stephens and Coulson Scheuermann of NASALewis for interesting discussions. We are also grateful to

Timothy Howson and Jeffrey Barefoot for their assistance. We reconize that Udimet, Mar-M, Waspaloy, Nimonic, and Inconel are

trademarks or trade names of Special Metals Corp., Martin Marietta

Corp., United Technologies, Inc., and INCO companies, respectively.

6. REFERENCES

I. J.K. Tien, T.E. Howson, G.L. Chen and X.S. Xie: 1980, Journal

of Metals, Vol. 32, No. i0, pp. 12-20.
23.

2. R.N. Jarrett and J.K. Tien: 1982, Met. Trans. A, Vol. 13A,

No. 6, pp. 1021-1032.

3. M.V. Nathal and R.D. Meier: 1981, NASA Report No. NASA CR-

" 165384, pp. 104.

4. G.E. Maurer, L.A. Jackman and J.A. Domingue: 1980,

"Superalloys 1980," ed. J.K. Tien et el., pp. 43-52.

5. O.H. Kriege and J.M. Baris: 1969, Trans. ASM, Vol. 62,

pp. 195-200.

6. R.W. Guard and J.H. Westbrook: 1959, Trans. Met. Soc. AIME,

Vol. 215, pp. 807-814.

7. C.H. Lund, M.J. Woulds and J. Hockin: 1968, "Structural

Stability in Superalloys," ed. by Joint Committees of ASTM,

ASME, ASM and AIME, pp. 25-46.

8. E.H. Van der Molen, J.M. Oblak and O.H. Kriege: 1971, Met.

Trans., Vol. 2, No. 6, pp. 1627-1633.

9. J. Heslop: Sept. 1964, Cobalt, Vol. 24, pp. 128-137.

i0. J. Schramm: Dec. 1941, Z. Metallkunde, Vol. 33, pp. 403-409.

ii. C.K.L. Davies, P. Nash, R.N. Stevens: 1980, J. Mat. Sci.,

Vol. 15, pp. 1521-1532.

12. A. Taylor: Oct. 1956, Trans. AIME, Vol. 206, pp. 1356.

13. V. Ya. Markiv, V.V. Burna_hova, L.I. Pryakhina and K.P.

Myasnikova: 1969_ Iz. Ak. Nauk SSSR, Met. No. 5, pp. 180-185.

14. W. Peter and H. MUller: 1965, Archly fur des Eisenhuttenwesen,

Vol. 36, No. ii, pp. 835-841.

15. W.J. Boesch and H.B. Canada: Apr. 1968, Journal of Metals,

Vol. 20, No. 4, pp. 46-50.

16. L.R. Woodyatt, C.T. Sims and H.J. Beattie, Jr.: Apr. 1966,

Trans. Met. Soc. AIME, Vol. 236, pp. 519.

17. S.P. Rideout and P.A. Beck: 1953 NACA Report NO. 1122.

18. W.J. Boesch and J.S. Slaney: July 1964 Metal Progress,

Vol. 86, No. I, pp. 109-111.

19. J.R. Mihalisin, C.G. Bieber and R.T. Grant: Dec. 1968,

Trans. AIME, Vol. 242, pp. 2399-2414.

20. F. Cosandey, T.J.T. Tang, C. Cheng, R.N. Jarrett and J.K.

Tien, "The Effects of Cobalt on y' Coarsening Kinetics and

Microchemistry in Udimet 700,' to be published.

21. C.R. Barrett and O.D. Sherby: 1965, Trans. Met. Soc. AIME,

Vol. 233, pp. 1116-1119.

22. X.S. Xie, G.L. Chen, P.J. McHugh and J.K. Tien: May 1982,

Scripta Met., Vol. 16, pp. 483-488.

o 23. B.E.P. Beeston, I.L. Dillamore and K.E. Smallman: 1968,

J. Met. Sci., Vol. 2, pp. 12-14.

24. B.E.P. Beeston and L.K. France: 1968, J. Inst. Metals,

Vol. 96, pp. 105-107.

" 25. L.M. Brown and R.K. Ham: 1971, "Strengthening Mechanisms

in Crystals," ed. A. Kelly and R.B. Nicholson, pp. 12-135.

26. N.F. Lashko, O.A. Khromova, A.A. Burmistrova and N.I.

Zhuravleva: 1970, "Structure and Properties of HeatResistant Metals and Alloys," ed. M.V. Pridantsev, pp. 200-

204.
24.

27. V. Luplnc and T.B. Gibbons: 1978, "High Temperature Alloys

for Gas Turbines," ed. D. Coutsouradis et al., pp. 335-358.

28. J.J. Jackson, M.J. Donachle, R.J. Henricks and M. Gell:

1977, Met. Trans. A, Vol. 8A, pp. 1615-1620.

29. R. Watanabe and T. Kuno: 1976, Trans. Iron and Steel Inst.

Jap., Vol. 16, pp. 437-446.

30. J. Heslop: 1962/1963, J. Inst. of Metals, Vol 91, pp. 28- 33.

31. A.M. Borzdyka: Nov-Dec. 1977, Metal Science and Heat Treatment, Vol. 19, pp. 937-939.

1. Report No. 2. Government Accession No. 3. Reclplent's Catalog No.

NASACR-168308

4. Title and Subtitle 5. Report Date

Effects of Cobalt in Nickel-Base Superalloys December 1983

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No,

John K. Tien and Robert N. Jarrett

None

10. Work Unit No.

9. Performing Organization Name and Address

Col umbia Uni versi ty 11. Contract or Grant No.

Center for Strategic Materials

Henry Krumb School of Mines NAG3-57

New York, NewYork 10027 13Typeof Report and Period Covered

12. Sponsoring Agency Name and Address

Contractor Report

National Aeronautics and Space Admini stra ti on 14.SponsorinAgency g Code

Washington, D.C. 20546

505-33-IA

15. Supplementary Notes

Final report. Project Manager, Joseph R. Stephens, Materials Division, NASALewis

Research Center, Cleveland, Ohio 44135. Materials previously presented as an

invited contribution at the High Temperature Alloys for Gas Turbines Conference,

Liege, Belgium, October 4-6, 1982.

16. Abstract

The role of cobalt in a representative wrought nickel-base superalloy was recently

determined. The results show cobalt affecting the solubility of elements in the

¥ matrix, resulting in enhanced y' volume fraction, in the stabilization of

MC-type carbides, and in the stabilization of sigma phase. In the particular

alloy studied, these microstructural and microchemistry changes are insufficient

in extent to impact on tensile strength, yield strength, and in the ductilities.

Depending on the heat treatment, creep and stress rupture resistance can be cobalt

sensitive. In the coarse grain, fully solutioned and aged condition, all of the

alloy's 17% cobalt can be replaced by nickel without deleteriously affecting this

resistance. In the fine grain, partially solutioned and aged condition, this

resistance is deleteriously affected only when one-half or more of the initial

cobalt content is removed. The structure and property results are discussed with

respect to existing theories and with respect to other recent and earlier findings

on the impact of cobalt, if any, on the performance of nickel-base superalloys.

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

Superalloys Unclassified - unlimited

Cobalt STARCategory 26

Nickel

Substitution

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