NASA Contractor Report 174628 

The Role of Cobalt on the Creep of Waspaloy 

Robert N. Jarrett, Lori Chin, and John K. Tien 

Columbia University 

New York, New York 

February 1984 

Prepared for 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Lewis Research Center 

Under Grant NAG3-57 
Trade names or manufacturers’ names are used in this report for identification 

only. This usage docs not constitute an official endorsement, either expressed or 

implied, by the National Aeronautics and Space Administration. 
1 

As part of the NASA Conservation of Strategic Aerospace 

Materials (COSAIG) program lD2 we have studied the role of cobalt 

variations on the static properties of Waspaloy. Previously, we 

reported the effects of cobalt on Udimet TOO3? 

study on Waspaloy is interesting since this alloy has the same 

alloying elements as Udimet 700, except that Waspaloy has less 

aluminum and titanium (and hence, less 7' precipitates) and less 

molybdenum for solid solution strengthening, For these reasons 

Waspaloy belongs to a lower strength class of alloys. 

A comparitive 

In the study on Udimet 700, systematic creep and tensile tests 

were correlated with microstructure and microchemistry results to 

determine the effects of replacing cobalt with nickel in the alloy 

which normally contains 18% cobalt. These alloys were given a 

sub-solvus heat treatment used to retain some primary y* to pin 

grain boundaries. This fine grain material is designed for lower 

temperature, but higher strength applications--such as the gas 

turbine disk. 

The yield and tensile strengths of low cobalt Udimet 700 were 

found to be slightly lower than the standard alloy. This small 

decrease in tensile strength was attributed to th3 slight decrease 

in strengthening y' fraction even though the total 7' fraction was 

found to be independent of alloy cobalt content 3,4s6 4 
2 

In contrast to the small effect of cobalt on tensile 

properties, creep and stress rupture strength of the low cobalt 

alloys fell drastically. Besides increasing the effective ye 

fraction, we showed that cobalt partitions to the y matrix, SO it 

would be expected to decrease the stacking fault energy (SFE) of 

this matrix. This decrease in the SFE could increase the 

separation of partial dislocations and strengthen the alloy by 

reducing cross-slip. 

To determine whether SPE or the effective y' fraction is 

responsible for the loss in creep resistance, the Udimet 700 alloys 

were given a second type of heat treatment which included a 

solution above the y' solvus. The total 7' fraction and the matrix 

cobalt content were found to be nearly identical to the alloys 

after the sub-solvus heat treatment--only the y' morphology was 

changed. After this heat treatment all y' was uniformly 

distributed, and hence, contributed to the alloy strength. Not 

unexpectedly, the tensile strength and the yield strength after 

this second heat treatment were independent of cobalt. But more 

importantly, with the microstructural variable removed, the creep 

and stress rupture properties of Udimet 700 also were not 

affected by cobalt. We conclude that cobalt may lower the stacking 

fault eaerey in Udimet 700, but that the SFE effect on properties 

is found to be minor once this variable is separated from cobalt's 

effect on the strengthening precipitate content. 

Concerning higher strength cast superalloys, Nathal and 

Maier' showed that removing cobalt from conventionally cast 
3 

WarK247 lowered the yp fraction and the creep resistance of this 

alloy. By removing cobalt from single crystal alloys erived from 

GarK247, Natbal and Ebert' showed the opposite effect of cob 

on ye fraction and stress rupture. Although there may be some 

matrix SFE component to the creep behavior in these studies, 

microstructural constraints have preverrt~d isolating that 

variable. It does, however, appear that the SFE effects in these 

alloys are also fairly minoro 

At the other end of the strength spectrum, two studies have 

shown that cobalt greatly improves the stress rupture resistance 

of both Niaonic 80A9 and Vaspaloylo. 

increases the volume fraction of 7' by decreasing the matrix 

9 solubility for aluminum and titanium. Additionally, Heslop 

showed qualitatively that cobalt lowers the stacking fault energy 

of the matrix in Nimonic 80A. The cobalt-free matrix after a 1% 

deformation shows tangled dislocations as evidence of cross-slip 

in contrast to straight partial dislocations separated by wide 

In both alloys cobalt 

10 stacking faults in the cobalt containing matrix, MaUEer 

alluded to SFE effects by calculating that a 2 volume percent 

change in =yw fraction would not totally account for the change in 

stress rupture behavior. NeitBer author attempted to separat 

microstructural effects from the matrix chemistry (SFE) effects of 

cobalt. In this paper we discuss our efforts to distinguish the 

effects of cobalt on SFE from the 7' solubility effects. 
4 

EXPERIMENTAL PROCEDURE 

Cobalt was systematically replaced with nickel in four alloys 

based on Waspaloy while the alloy contents of all other components were kept constant, see Table I. Heats of 68 kg. (150 Pbs.) 

of each alloy were vacuum induction melted (VIM), vacaua arc 

remelted (VAR) into a 6 inch (152 mm) ingot, and hot rolled to 3/4 

inch (19 mm) diameter bar by Special Metals Corporation,of New 

Hartford, New York. The y' solvus determined by differential 

thermal analysis-(DTA) did not vary with cobalt content, so the 

alloys were given identical heat treatments, Table 11, resulting in 

equivalent microstructures with an average grain size of 10-13 

microns. 

The alloys with four different cobalt levels were creep and 

stress rupture tested at 448, 517 and 551 HPa (65, 75 and 80 ksi) 

and 76OOC (1400°F) to determine the phenomenological backstress 

component to creep at a constant temperature. Tests were also run 

at 704, 732 and 760°C (1300,1350 and 14006F) and 551 MPa (80 ksi) 

to determine the creep activation energy at a constant applied 

stress. Additionally room temperature tensile tests were used to 

characterize the effects of cobalt on the low temperature strength 

of Waspaloy. 

Gamma prime phase extractions' determined that the fraction 

of this strengthening phase decreased from 22% to 20% as cobalt was 

removed. To compensate for this decrease in ye fraction, two 7 kg 
5 

beats .of cobalt-free material witb additional aluminum and titanium, Table 118 ere VIM cast and rolled to 2.5x0.5 inch (63x13 mm) 

plate by Special Metals Corporation. Simee the ~'solv~s in 

superalloys increases sbarply with aluminum and titanium content, 

the solution temperatures in the heat treatment of these alloys 

were increased accordingly, Table 11. 
6 

EXPERTMENTAL RESnLTS AND DISCUSSION 

Microstructure 

The heat treated microstructures of Waspaloy with varying 

cobalt levels were examined optically and with SEM. The small 

effect of cobalt on the y' fraction in this material is barely 

noticeable in SEM micrographs, Fig. 1. The carbide morphology 

does appear to be slightly affected by cobalt as more, and coarser, 

carbides appear at the grain boundaries in the low cobalt alloys 

than the standard Vaspaloy. The effect of cobalt on the carbides 

3-10 in nickel-base superalloys was demonstrated in earlier papers 

usually with the arguement that cobalt increases the carbon solubility in the matrix. Bowever, recent experimental evidence shows 

that cobalt can change the transformation temperature of the 

carbides5 but that the effect on the carbon solubility (which 

is extremely low at the operating temperatures) appears to be 

negligible . 11 

Tensile Behavior 

The minor microstructural effects of cobalt on y' fraction and 

carbides result in little or no change in the room temperature 

strength or ductility of Vaspaloy, Fig. 2 and Table III. The 

tensile strength and the yield strength are nearly constant at 

rong&ly 1400 MPa (200 ksi) and 1000 MPa (145 ksi), respectively. 

The tensile ductilities of the alloys are essentially unaffected by 

cobalt, all are in the range of 25 to 30% elongation and 40 to 45% 

reduction of area. These results are not surprising since in the 

cases of Udinet 700~'~~ MaiM2477"8, Nimonic $06' and an 
7 

earlier study on Waspaloy" cobalt was found to have little, if 

any, effect on tbe tensile behavior of nickel-base superalloys. 

Creep and Stress R~~ture Be_h_myj-ox 

In contrast to the small effect of cobalt on microstructure 

and properties in tensile tests, the alloys with less cobalt 

had lower stress rupture and creep resistance than the standard 

alloy at all applied stresses and temperatures tested. See Fig. 3 

for the rupture lives and Fig. 4 for the minimum creep rates as a 

function of cobalt content. For all alloys the rupture ductilities were scattered in a band from 15 to 30% elongation with no 

systematic effect of cobalt, Table IV. However, the reduction 

of area for the cobalt-free alloy (20-358) was roughly half the 

reduction (40-65%) for the three cobalt containing alloys. 

Cobalt resides mainly in the matrix of Waspaloy (the matrix 

cobalt content is approximately 15% compared to 5% in the y') as 

has been shown previously in this alloy and a range of other 

super a 11 oys 3'7D12. Cobalt in the matrix affects the creep 

behavior of the alloy in two separate ways. First, as mentioned 

above, cobalt decreases the matrix solubility limit for the ye 

formers, aluminum and titanium. In Waspaloy, 13% cobalt produces 

an increase from 20 to 22% yt6. Using the lever rule applied to 

a pseudo-binary phase diagram 4D9 for this system the y' fraction 

increase translates to a decrease in (Al+Ti) solubility of 0.35 

atomic percent. 

1 

The second effect of cobalt in the matrix is its solid sola- 
tion strengthening. Unlike other alloying elements in superalloys, 

the cobalt mismatch witb nickel is less than I%, Therefore, as a 

misfitting element its role is negligible. Its effect on the 

coherency strains of 7/7' mismatch have been shown to be small e 

However, Eeeston, et d3, Heslop , and Law, et a1.14 have 

6 

9 

shown that cobalt substantially lowers stacking fault energy in 

binary nickel-cobalt alloys, in Ni-Cr-Co ternaries, and in the 

matrix of a complex nickel-base superalloy, MERL 76, respectively. 

From these results we conclude that the 13% cobalt in Waspaloy 

lowers the stacking fault energy in the matrix. By usin4 the various linear approximation^^^-^^ we will show that this cobalt 

contczt lowers the SFE of the matrix by about 43%. 

To determine which cobalt effect has the dominant role in 

the creep behavior of Waspaloy, we separated the particle contribntion from the matrix (or SFE) contribution using existing semiempirical equations 15*16. 

data using the standard creep equation (1) was done to determine 

the stress exponent, n, and the apparent activation energy, 

for each of the cobalt contents, see Table V. 

A preliminary analysis of the creep 

QA * 

n (-QA/RT) 

6=Aoc e (1) 

b is the minimum creep rate, Q is the applied stress, 110 is a 

structure dependent constant and T is the absolute temperature. 

Both the stress exponent and the apparent creep activation 

energy increase (from 7.0 to 7.9 and from 64 to 74 kcal/mole, 
9 

respectively) with increasing cobalt content in Waspaloy. 

A further analysis of the creep results using the creep 

equation derived to separate the particle contribution from the 

alloy matrix contribution of complex particle strengthed alloys. 

In this equation A' is a new material constant, k is a factor 

between 0 and 1 relating the matrix solid solation drag stress, 

to the effective stress on a dislocation Q = k (a - a ), 

u is the applied stress, u is the particle back stress, E is the 

temperature dependent elastic modulus, QO is the true activation 

energy for creep which is roughly the activation energy for pelf 

diffusion in nickel. 

s8 s P 

P 

Values of k and a for Vaspaloy with various cobalt levels 

P 

are included in Table V. Note the particle component, u increases from 207 MPa (30.0 ksi) for the cobalt-free alloy to 243 

MPa (35.4 ksi) for the standard alloy. This increase is consistent 

with the small increase in y' fraction as the cobalt level 

P, 

increases. In Waspaloy, in contrast to Udimet 700', Table VI, the 

matrix contribution, k, also increases markedly with cobalt content 

from .651 in the cobalt-free alloy to -800 in standard Waspaloy. 

Xit et presented a variation of the creep equation 

(2) based on the work of Sherby and co-workers conelading 
10 

that stacking fault energy is the major matrix contribution to 

the creep strength of particle strengthened systems. 

is SFE In this equation A is a new material constant and y 

the stacking fault energy of the matrix relative to pure nickel. 

Tbe SFE exponent comes directly from Equation (2) by claiming that 

the matrix drag effect is due entirely to the SFE. This exponent 

is slightly higher than the values of 2-3.5 reported by Law14 or 

Sherby 18-20 for single phase alloys. The other variables have 

the same meaning as in equation (2). Interpreting the present 

creep results with this new equation, the SFE of the Waspaloy 

matrix is found to be reduced by 43% due to cobalt. This value 

is quite reasonable and consistent with the linear approximation 

of SFE in Refs. 14 and 15. 

The creep activation energies determined in this study 

provide further evidence that SFE is playing the dominant role in 

cobalt's effect on the creep resistance of Waspaloy. The 

apparent creep activation energy in Waspaloy increases with 

cobalt content (see Table V). The true creep activation 

energy, Qo, can be calculated from the apparent value, QA, by 

including the temperature effect on the elastic modulus in 

equation (1). In a range of particle strengthened nickel-base 

alloys previous researchers 16D21-23 have shown that the trae 

creep activation energy is alloy independent and nearly equal to 
11 

tbe self diffusion activation energy for nickel. 

24 For Waspaloy at 76OoC, the elastic modulus is 168 GPa 

and (dE/dT) is roughly -S5 HPa/K, giving QO the value of 66 

kcal/mole. We have not experimentally determined the values of 

E(T) and dE/dT for cobalt modified Waspaloy.. However, the 

elastic moduli would not be expected to differ much from the 

standard alloy, as the case with Nimonic 80A and Indeed, 

the difference between the elastic moduli for any two nickel-base 

superalloys would not be sufficient to explain the these 

differences in the creep activation energies with cobalt. This 

indicates that cobalt's effect on the creep activation energy 

cannot be explained by a simple modulus effect in equation (1). 

A more thorough analysis of the activation energies in 

particle strengthened alloys using equations (2) or (3) includes 

23,16 temperature effects on the particle and drag back stresses 

in addition to the modulus effect. The true activation energy, 

Qo takes the form, 

or, alternatively, in'terms of the stacking fault energy, 
12 

The variables are defined as in equation (3) where no was given 

the value of 4. In this form the particle back stress has the same 

temperature dependence as the elastic modulus and, as such, 22 

should not be greatly affected by cobalt. However, the stacking 

fault energy term would be expected to show a cobalt effect. 

Eric~son~~ and Tisone26 have shown that the SFE of Co-Ni 

alloys increases with temperature, so the last term in equation 

(5b) becomes more negative with increasing cobalt. Thus, although 

the true activation energy does not change with Cobalt, the 

apparent activation energy in cobalt containing Waspaloy would be 

expected to be higher than in the cobalt-free alloy. We can calculate the temperature dependence of the SFE assuming this effect 

accounts for the 10 kcal/mole difference in the activation energies 

between the standard and cobalt-free Waspaloy. Using 240 erg/crnt 

as the SFE of nickel”, dy/dT at 760°C is found to be .056 

erg/cm2-oC. This is within a factor of two of the value .03+.02 

erg/cmZ-OC determined by Ericsson using the node area method in 25 

the binary Co-Ni .alloys. 

From the results of the analyses of both the matrix drag back 

stress and the creep activation energy we conclude that the effect 

of cobalt on the creep of Waspaloy is due the reduction of the 

matrix stacking fault energy. Although cobalt does cause some 

microstructural changes in the y@ and carbides of Waspaloy, these 

effects are decidedly small when compared to the SFE effect. 
13 

Nodifjed Waspaloy 

The next phase of this project was to attempt to remedy the 

loss of creep resistance in Waspaloy due to the removal of cobalt. 

This can be accomplished by raising the matrix drag stress (by 

adding solid solution strengtheners), or by raising the particle 

resisting stress (by increasing the y 'fraction), or a combination of 

both, 

effects were the origin of a loss of creep strength. Modifying the 

In the earlier study on Udimet 7003, microstructural 

heat treatment and thermomechanical processing to compensate for 

the y' solvus effects of cobalt restored the creep resistance 

without any chemistry alterations. 

The effect of cobalt on Waspaloy is not microstructural so 

thermal treatment options do not apply, Therefore, we either 

1) replace cobalt with an equivalent amount of another BCP metal to 

lower the stacking fault energy, or 2) increase the solid solution 

strengthening by adding refractory elements, or 3) increase the y' 

fraction by adding y' formers. 

The approach we decided to use was increasing the y'formers, 

aluminum and titanium. Since these elements are potent strengtheners, minor changes in chemistry can result in substantial gains 

in strength. We ruled out increasing the chromium or molybdenum 

contents to lower the SFE because the alloy matrix in nearly 

saturated with these elements and such an addition could lead to 

phase instabilities with a marginal effect on the solid solution 

strength. Also, we eliminated adding an alternative HCP metal to 

replace the cobalt singe introducing, say. 10% of a new element to 
14 

the alloy is the equivalent of designing an entirely new alloy, 

which is no1 the intent of this study. 

The alloy D1-1948-1 was modified with an addition of 0.42 

atomic % (Al+Ti) to compensate for the 2% y' volume fraction 

decrease due to removing cobalt. This alloy showed essentially no 

improvement in the 551 MPa/7320C (80 ksi/135OOF) stress rupure life 

over the cobalt-free Waspaloy with the standard y' content . However, tbe alloy with 1.23 atomic k (Al+Ti) added did show the 

expected increase in the rupture life so we concentrated on this 

alloy in our study. (As a reference, the aluminum and titanium 

contents of this alloy are 0.10 w/o and 0.15 w/o above the 

specification maximum.) 

17 

The additional estimated to be about 5 percent) raised the 

448 MPA/7600C (65 ksi/l4OOOF rupture life of the cobalt-free alloys 

from 3.48 to 18.85 hours, reducing the minimum creep rate from 

6.3~10-~ to 0.25~10-~ per sec, see Table 111. Note that the 

creep rates of this alloy at 760°C are nearly identical to the 

standard alloy. The trade-off for this additional strength is a 

drop in ductility (from 15-309 for the standard alloy to 11-14s for 

this low cobalt alloy with more y'). The combination of equivalent 

minimum creep rates and reduce ductility results in shorter rupture 

lives for this modified alloy compared to the standard 

cob a 1 t-con t a ining Wa sp aloy . 

These results indicate that although cobalt is certainly 

necessary for the high temperature creep strength of Waspaloy, 
15 

some conservation of tbis strategic element may be achieved by 

increasing the y' content of a low cobalt version of the alloy, 

However, to achieve the creep resistance in 8 cobalt-free alloy a 

decrease in the rupture ductility results. A possible compromise 

alloy with, say, half tbe cobalt content and maximal aluminum and 

titanium contents may be an effective alternative alloy with 

adequate creep and stress rupture resistance. 
16 

Waspaloy and Udimet 700 belong to the the same family of 

wrought nickel-base alloys containing Cr, eo, Mo, A1 and Ti as the 

major alloying elements. Waspaloy is at the low end of the 

strength scale with roughly 20% y' and Udimet 700 at the high end 

with 45%. In this study and the previous work on Udimet 700 

we have systematically replaced the cobalt with nickel to isolate 

cobalt's effect on properties and microstructure. 

304 

In these alloy systems we have identified the primary effects 

of cobalt. Because it partitions to the 7 matrix, cobalt predominently affects this phase. Cobalt lowers the matrix stacking fault 

energy and decreases the aluminum and titanium solubilities. Since 

cobalt and aluminum do not form Co Al, substituting cobalt for 

nickel lowers the y' solvus in superalloys. Cobalt also lowers the 

terminal matrix alaminum solubility so a cross-over of the cobalt 

containing and cobalt-free solvus lines results. Indeed, Waspaloy 

is at this cross-over composition while Udimet 700 is above' and 

3 

Nimonic 90 is below' the alnminnm content of the cr'oss-over. 

As zi consequence of this solvus effect, the carbide solvus and 

the processing temperatures of Bigh yg fraction alloys like Udimet 

700 decrease with decreasing cobalt content. However, after the 

proper processing modifications cobalt effects on microstractare 

oan be essentially eliminated. 
17 

In Raspaloy, as in Udimet 700, cobalt does not affect the 

tensile or yield strengths. This result is quite reasonable since 

the strengthening methods for short times or at low temperatures in 

these alloys are determined by 7' fraction, anti-phase boundary 

energy, coherency strains, and, of conrse, microstructure * 27-29 

Cobalt's effect on microstructure is, again, minimal. Jts 

effects on mismatch or on coherency strains are also very small-- 

essentially below the resolvable limit . Cobalt can increase the 

7' fraction by decreasing the solubility for (Al+Ti). In Waspaloy, 

the y' prime fraction iccreased from 20 to 22% which would not have 

a measurable effect on properties. In Udimet 700, a smaller volume 

increase of less than 1% Fesalted. The effect of cobalt on APB 

would be expected to be very small since the y' cobalt content is 

much smaller (roughly one-third) the matrix content. Should cobalt 

have any effect it would be expected to lower the APB since it 

tends to de-stabilize the 7'. 

6 

Under elevated temperature creep and stress rupture conditions 

the matrix (and, hence, cobalt) would be expected to play a larger 

role than in short time tensile tests. During creep deformation, 

matrix dislocations by-pass the y' precipitates by thermally 

activated cro~s-slip~~~~~ and climb mechanisms32 as well as 

catting the particle 23*33s34e 

important role in creep by lowering the stacking fault energy of 

the matrix and apparently inhibiting the cross-slip and climb 

processes. Bowever, in Udimet 700, with nearly an identical matrix 

In Waspaloy cobalt plays an 
18 

as Piaspaloy, cobalt has no effect on creep. 

Resolving the difference in behavior of cobalt between low 

volume fraction Waspaloy and high volume fraction Udimet 700 is key 

to understanding not only the role of cobalt in superalloys, bat 

more generally, understanding the role of SFE in particle 

strengthened systems. The major differences between the two alloy 

systems are the y' volume fraction and particle size. In Waspaloy 

the 20% volume fraction of 7' is precipitated mostly as very small 

spheroids 40-60nm in diameter and a small fraction of larger 

particles 150-200nm in diameter. In contrast, the yl in Udimet 700 

precipitates as alligned cubes 300-500nm on a side and a small 

fraction of round very fine particles about 60-100nm in diameter. 

Due to the difference in volume fraction the mechanical 

behavior of Waspaloy would be dominated by the matrix while the 

behavior of Udimet 700 would be dominated by the y'. gear, et al. 

(Refs. 31,33,34) present a model of the y' cutting behavior of 

dislocations in the high y' fraction alloy Mar M200. They explain 

that the cutting mechanism is provided by a pair of g<iOl>(lll) 

dislocations. The first forms an anti-phase boundary (APB) in the 

7# and the second removes that APB leaving behind a perfect crystal, 

The lowest energy configuration of these dislocations is as a coplanar pair. The separation of these co-planar pairs is far greater 

than the stacking fault width between the partials of the individual 

dislocations. Thus cross-slip or climb of the pairs would be 

fairly insensitive to changes in stacking fault energy (which 

determines the separations of the partials but not the distance 

2 
19 

between t1;e dislocation pairs) e 

This pairing and cutting has been observed in Udimet 700 but 

is not seen in Waspaloy 23s35, For this reason the effect of 

cobalt on the stacking fault energy of the matrix does not affect 

the creep behavior of Udimet 700 while it bas a considerable effect 

on the creep of Waspaloy. In Waspaloy, the results of the present 

study indicate that raising the SFE by removing cobalt may significantly increase creep rates by reducing the barriers to crossslip by reducing the partial dislocation separation. 

Since the alloy chemistry determines the matrix stacking fault 

energy, fen alternatives are available to lower this energy without 

making considerable changes in the alloy. Such changes may far 

outweigh the subtle effect on SFE. The alternative method we used 

to correct this decreased creep resistance is to increase the 

number of obstacles (i.e,, the y' fraction), A cobalt-free alloy 

with the creep resistance of standard Waspaloy was produced by 

adding 0.25 weight percent (Al+Ti) above the specification 

maximum. The trade-off of reduced ductility for the increased 

strength resulted. Optimizing Faspaloy with an intermediate cobalt 

level and the aluminum and titanium contents at the specification 

maximum may be a reasonable means of reducing cobalt consumption in 

this alloy. 

The effects of cobalt on creep deformation in wrought 

nickel-base superalloys can be summarized as follows: 

1) Cobalt decreases the matrix solubility for aluminum and 
20 

titanium which slightly increases tfie ye volume fraction in 

low volume fraction alloys. This effect is usually too 

small to affect the properties of the alloys (as seen in 

Waspaloy and Udimet 700). 

2) Cobalt lowers the matrix stacking fault energy of all 

nickel-base alloys. 

3) SFE effects of cobalt have been shown to control the 

creep resistance of Waspaloy and the results fit a 

semi-empirical equation which includes the SFE term 

in the resisting stress model for creep in particle 

strengthened systems. 

4) The same SFE effect in the high y' fraction alloy, 

Udimet 700, has no effect on creep. 

5) We attribute this difference between the two alloys to 

the dislocation pairing in the high volume fraction alloys 

overshadowing the effects of SFE on partial dislocation 

separation. 

6) The loss of creep resistance in low cobalt, high strength 

alloys is not generally an intrinsic effect and can be 

corrected with proper thermomechanical processing or heat 

treatment. In the low y' froct.ion alloys such as Waspetoy, 

the effect of cobalt on creep is intrinsic to the matrix 

chemistry so conpasitionaf changes aze a necessity to 

restore properties. 

7) We have demonstrate that the creep resistance of a low 

cobalt version of Waspaloy can be restored to the level of 

the standard alloy with minor additions of (Al+Ti) to 

increase the Y~ fraction. 
21 

A CK N OWL ED G EGI EN T S 

This study was sponsored by NASA-Lewis Research Center (Grant 

NAG 3-57). We thank the project monitors, Joseph R. Stephens and 

Coulson Scheaermann; Gern E. laurer and Stephen H. Reichman of 

Special IUetals Corporation; John F. Radavich and Mayer A. Engel of 

Purdue University; and Juan M. Sanchez, Vincent Nardone, Jeffrey 

Barefoot and John Collier for their contributions to this study 

and interesting discussions. Waspaloy, Udimet, MarM and Nimonic 

are trademarks or designations of United Technologies Corporation, 

Special Metals Corporation, Martin-Marietta Corporation and Henry 

Wiggin and Co., Ltd., respectively. 
22 

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C.C. Law, L.S. 1,in and X.J. Blackburn, 'Reduction of Gtrategic Elements in Turbine Disk Alloys'a U.S.A.F Off. of Sci. 

Research Report #AFOSR-81-0554, 1981. 

X.S. Xie, G.L. Cben, P.J. WcRugh and J.K. Tien, 'Including 

Stacking Fault Energy into the Resisting Stress lode1 for 

Creep of Particle Strengthened Alloys', Scripta Met., vol. 16 

(19821, pp. 483-488. 

0. Ajaja, T.E. Bowson, S. Purushothaman and J.K. Tien, 'The 

Role of the Alloy Matrix in tbe Creep Behavior of ParticleStrengthened Alloys', Mat. Sci. and Eng., vol. 44 (19801, 

pp. 165-172. 

G.E. Maurer, Special Metals Corp., New Hartford, NY, private 

c onimunica t ion . C.R. Barrett and O.D. Sberby, 'Influence of Stacking-Fault 

Energy on Bigh-Temperature Creep of Pure Metals', Trans. 

AILE, vol. 233 (19651, pp. 1116-1119. 

R.M. Bonesteel and O.D. Sherby, 'Influence of Diffusivity, 

Elastic Modulus and Stacking-Fault Energy on the Bigh 

Temperature Creep Behavior of Alpha Brasses', Acta Yet., vol. 

14 ($9661, pp. 385-391. 

C.R. Barrett and O.D. Sherby, 'Re-interpretation of the Influence of Stacking-Fault Energy on High Temperature Creep of 

Cu-A1 Solid Solutions', Scripta Yet., vol. 3 (19691, pages 

297-300. 

C.R. Barrett, A.J. Ardell and O.D. Sherby, 'Influence of 

Modulus on the Temperature Dependence of the Activation 

Energy for Creep at High Temperatures', Trans. AIMED vol. 230 

(1964), pp. 200-204. 

R.W. Land and W.D. Nix, 'On High Creep Activation Energies 

f o r D i s p e r s i o n Strengthened Metals', Met. Trams. A, vol. 6A 

(1975) pp. 1329-1333. 

S. Purushothaman and J.K. Tien, 'Role of Back Stress in the 

Creep Behavior of Particle Strengthened A11oys', Acta Wet., 

vol. 26 (19781, pp. 519-528. 

Binh Temperature, Fi~h Strength Nickel-Base Alloys, Third 

Edition, International Nickel Company, Inc., 1977: 

T. Bricsson, 'The Temperature and Concentration Dependence of 

Stacking Fault Energy in the Co-Ni System', Acta Yet,, vol. 

14 (19661, pp. 853-865, 

T,C. Tisone, @The Concentration and Temperature Dependence 
24 

of the Stacking Fault Energy in Face-Centered Cubic Co-Fe 

Alloys', Acta let., vole 21 (1973), pp. 229-236. 

27, L.M. Brown and RaKe Bam, 'Dislocation-Particle Interactions', 

in S t r e n g t h e n i n g C e t h o d s in Crystals., eds. A. Kelly and R. 

E. Nicbolson, J. Wiley and Sons, New York, 1971, pp. 12-135. 

28. E.R. Jensen and J.K. Tien, 'Creep and High Temperature Deformation oE S i m p l e M e t a l s and Superalloys', in l e t a l l a r u i c a l 

Treatis-e2, TMS-AIME, Warrendale, PA (1981), pp. 529-550. 

29, D.A. Grose and G.S. Ansell, 'The Influence of Coherency 

Strain on the Elevoted Temperature Tensile Behavior of Ni15Cr-Al-Ti-Mo Alloys', Met. Trans. A, vol. 12A (19811, pp. 

1631-1645. 

30. MoS. Dnesbery and P.B. Hirsch, 'On the Mechanisn, of 

Cross-Slip of Dislocations at Particles', in Fundanrentat. 

Aspects of Dislocation the or^, Eds. J.A. Simmons, et al., 

NBS Special Publication #317, 1970, pp. 1115-1135. 

31. S.M. Copley and BOBo gear, 'The Dependence of Width of a 

Dissociated Dislocation on Dislocation Velocity', Acta Met., 

vol. 16 (19681, pp. 227-231. 

32, R. Lagneborg and B. Bergman, 'The Stress Rupture/ Creep 

Behavior of Precipitation-Hardened Alloys', Met. Sci. 

(19761, pp. 20-28. 

33. G.R. Leverant and B.R.Kear, 'The Mechanism of Creep i n G a m m a 

Frime Frecipitation-Hardened Nickel-Base Superalloys', Yet. 

Transo, vol. 1 (1970), pp. 491-498. 

34. B.H. Kear, G.R. Leverant and J.M. Oblak, 'An Analysis of 

Creep Induced Intrinsic/Extrinsic Fault Fairs in a Precipitation Hardened Nickel-Ease Superalloy', Tran. ASM, vol. 62 

(19691, pp. 639-650. 

35. B.V.Miner, J. Gayda and R.D. Maier, 'Fatigue and Creep-Fatigue 

Deformation of Several Nickel-Base Superalloys at 650°C', Ket, 

Trans. A, vole 13A, October 1982, pp. 1755-1765. 
25 

Table Ie Alloy Compositions (in weight percent) 

Heat No. Ni co Cr 2: 0 A1 Ti C I3 Zr 

DJ-1947 bal. <0.1 19.6 4.07 1.30 3.03 .039 .005 0.07 

D5-1948 bal. 4.6 19.5 4.08 1.29 3.02 -042 -005 0.07 

D5-1949 bal. 9.0 19.4 4.12 1.32 2.99 .041 -004 0.07 

D5-1950 bal. 13.4 19.5 4.19 1.33 3.05 ,040 .005 0.07 

Dl-1948-1 bal. 0.0 19.3 4.00 1.50 3.15 .035 .005 0.07 

D1-1948-2 bal. 0.0 19.3 4.00 1.70 3.40 -035 .005 0,07 

For all alloys Fe<.13, Mn, Si, Cu <.lo, S<.OO4 and P<.Ol. 

Table 11. Heat Treatment Schedules. 

Heats D5-1947, 1948, 1949 and 1950 

10IO°C (1850OF) / 4 Hours / Oil Quench 

843OC (1550OF) / 4 Boars / Air Cool 

76OOC (1400OF) /16 Hours / Air Cool 

Heat D1-1948-1 

1029OC (1885OF) / 4 Hours /'Oil Quench 

843OC (1550OF) / 4 Hours / Air Cool 

760OC (1400°F) /16 Hours / Air Cool 

Heat P1-1948-2 

1052OC (1925OF) / 4 Hours / Oil Quench 

843OC (1550OF) / 4 Hours / Air Cool 

76OOC (1400°F) /16 Hours / Air Cool 
26 

Table 111. Room Temperature Tensile Properties of U'aspaloy 

0.2% Yield U1 t ima te Elongation Redact ion 

Numb e r Strength Strength (in %) of Area 

( in bfPa) (in ItSPa) (in W) 

Heat 

D5-1947 

(0 z CO) 

D5-1948 

(4.6% CO) 

D5-1949 

(9.0% CO) 

D5-1950 

(13.4% Co) 

999 

1010 

1012 

1026 

1048 

1026 

1021 

D1-1948 879 

(A1 + Ti) 923 

1375 

1380 

1390 

1410 

1420 

1425 

1420 

1325 

1368 

30.4 

28.9 

28.8 

29.7 

29.5 

29.2 

29.6 

26.2 

26.3 

43.9 

42.9 

39.8 

46.3 

42.8 

43.1 

41.6 

29.2 

31.5 
27 

Table IV. Creep and Stress Rupture Properties of Waspaloy 

Alloy Temperature Applied Rupture liniaam Elongation Reduction 

Eiea t (in OC) Stress Life Creep Rate (in %) of Area 

Number (in IPa) (hours) (per sec) (in %) 

D5-1947 760 

(0% CO) 

732 

704 

D5-1948 760 

(4,6% Co) 

732 

704 

D5-1949 760 

(9.0% CO) 

732 

704 

D 5-1 9 50 760 

(13.4% Co) 

732 

704 

448 

517 

551 

551 

551 

551 

448 

517 

551 

551 

551 

448 

517 

551 

551 

551 

448 

517 

551 

551 

551 

3.48 

1.55 

1.49 

4.03 

4.61 

19.56 

8.81 

1.97 

2.29 

6. 

65.30 

14.15 

5.77 

2.04 

12 . 80 

109.5 

30.01 

11.53 

8.99 

32.0 

225.4 

6 .3x10-6 

1.7x10-3 

4 . 4x1 0-6 

6 . 3x103 

6 .OxlO-7 

2 .3x103 

7 .4xl 0-6 

2 .2XlO'6 

1.5x10-' 

1.0~10-5 

1.0~10-5 

9 .1~10-7 

3 .Ox10-6 

7 . 5x10-6 

l.lxlo-6 

6 .3xlO-S 

3. 5x10-7 

7 . 9x1 0-7 

1. 8~10'~ 

1.7~10' 

1.8~10'8 

22.5 

22.9 

16.5 

18.0 

23.4 

16 .O 

21.7 

11.2 

24.3 

15.7 

37.6 

28.4 

30.9 

29.9 

16.4 

25.3 

15.4 

24.6 

17.2 

29.1 

32.3 

37.2 

26.2 

27.6 

20.7 

21. .3 

56.1 

34.9 

62.8 

54.4 

46.7 

38.7 

48.8 

50.3 

44.7 

39.6 

65.8 

54.7 

59.7 

39.1 

54.3 

D1-1948 760 448 18.85 2.5~10'7 14.0 18.1 

517 4.35 8.3~10'7 11,o 14.6 
Table V. Creep Parameters for Cobalt Modified Waspalop Beat 

# * 

U Q Relative APP P S Cobalt 0. 

YSFE Number Content n (kcal/mole) (HPa) 1; (MPa) 

D5-1947 0.0 7 .o 64.1 207 .651 202 1.00 

D5-1948 4.6 7.6 68.7 235 .693 195 0.86 

D5-1949 9.0 8.3 73.1 249 .739 198 0.75 

D5-1950 13.4 7.9 74.1 243 .SO0 219 0.57 

D 1 - 19 4 8-2 0.0 8.4 -- 251 .810 215 0.54 

* 

# The SFE value calculated from equation 3. 

The value for os is based oa an applied stress of 517 MPa. 

Table VI. Creep Parameters for Cobalt Yodified Udimet 700 

Relative # 

Q Q APP P S Heat Cobalt Q 

%FE Numb e r Content n (kcal/mole) (WPa) k (MPa) 

12.3 e- D5-1884 0.0 

12.5 -- D5-1886 8.6 

D5-1933 17 .O 12.1 e227 .744 195 1 .OO 

430 ,754 195 0.96 

423 .739 198 1.02 

* The value for os is based on an applied stress of 689 MPa. 

# The SFE value calculated from equation 3. 
29 

Figure 1. SEM micrographs of Vaspaloy with 13% cobalt (a and c) and 

0% cobalt (b and d). 
30 

I 

t 

e 

B 

C 

L 0 

500 - in’ 

0 I I 

Legend 

U.T.S. 

0 02% YS. 

i 

Pigare 2. Tensile strength of Waspaloy as a function of cobalt. 
31 

30. 

n v1 

L 

S 

0 

U 

I 

W 

5 

l?? 

Y 

Q 

3 Ix 

+ 

0 

250 

200 

150 

m5 10 

Weight % Cobalt 

0 5 10 

Weight % Cobalt 

76OoC 

448 MPa 

0 517 MPa 

D 

- 

551 MPa 

551 MPa 

I732OC 

0 704% 

Figure 3. Stress rupture lives of Waspaloy as a function of cobalt. 
32 

0.1 : I I 

Weight % Cobalt 

0 5 10 

0.01 f 

0 i ;0 

Weight % Cobalt 

760°C 

muam 

0 5l7 MPa 

0 551 

- 

YPa 

551 MPa 

R 760°C 

0 732OC 

D 701% 

Figure 4. Minimum creep rates as a function of cobalt content. 

Note that the cobalt free alloy with additional (Al+Ti) 

shows the same creep rate as the alloy with the standard 

cobalt, aluminum and titanium contents. 
1. Report No. 

NASA CR-174628 

The Role of Cobalt on the Creep of Waspaloy 

2. Government Accession No. 3. Recipient's Catalog ho. 

I 

7. Author@) 8. Performing Organization Report No. 

4. Title and Subtitle 

Robert N. Jarrett, Lori Chin, and John K. Tien 

5. Report Date 

10. Work Unit No. 

B. Performing Organlration Name and Address 

Col umbi a University 

Center for Strategic Materials 

Henry Krumb School of Mines 

New York, New York 10027 

2. Sponsoring Agency Name and Address 

11. Contract or Grant No. 

NAG3-57 

13. Type of Report and Period Covered 

Contractor Report 

National Aeronautics and Space Administration 

Washington, D.C. 20546 

1. Key Wordr (suggested by Author@)) 

Creep 

Cobalt 

Substitution 

Rupture life 

505-33- 1A 

18. Diatrlbution Statement 

Unclassified - unlimited 

STAR Category 26 

I 

5. Supplementary Notes 

Final report. Project Manager, Joseph R. Stephens, Materials Division, NASA Lewis 

Research Center, Cleveland, Ohio 44135. 

a. Security CIWI~. (of this mrt) 

Uncl a s s i f i e d 

5. Abstract 

20. Securlty Clunlf. (of this page) 21. No. of pages 22. Price" 

Unclassified 34 A03 

Cobalt was systematically replaced with nickel in Waspaloy (which normally contains 13% Co) to determine the effects of cobalt on the creep behavior of this 

alloy. Effects of cobalt were found to be minimal on tensile strengths and microstructure. 

in the range from 704" to 760" C (1300" to 1400" C) were found to decrease as 

cobalt was removed from the standard alloy at all stresses and temperatures. 

Roughly a ten-fold drop in rupture life and a corresponding increase in minimum 

creep rate were found under all test conditions. Both the apparent creep activation energy and the matrix contribution to creep resistance were found to increase with cobalt. 

stacking fault energy of the alloy matrix. 

removal of cobalt is shown to be restored by slightly increasing the yl 

fraction. Results are compared to a previous study on Udimet 700 - a higher 

strength, higher yl 

which cobalt did not affect creep resistance. 

ence in behavior based on interparticle spacing and cross-slip is presented. 

The creep resistance and the stress rupture resistance determined 

These creep effects are attributed to cobalt lowering the 

The creep resistance loss due to the 

volume 

volume fraction alloy with similar phase chemistry - in 

An explanation for this differ- 

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