James L. Smialek, Charles A. Barrett, and Ralph G. Garlick

Glenn Research Center, Cleveland, Ohio

The Effect of Hydrogen Annealing on the

Oxidation Resistance of Four EPM Single

Crystal Superalloys

NASA/TM—2001-211064

August 2001
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National Aeronautics and

Space Administration

Glenn Research Center

James L. Smialek, Charles A. Barrett, and Ralph G. Garlick

Glenn Research Center, Cleveland, Ohio

The Effect of Hydrogen Annealing on the

Oxidation Resistance of Four EPM Single

Crystal Superalloys

NASA/TM—2001-211064
Acknowledgments

Helpful discussions with Dr. Rebecca MacKay, Dr. Robert Dreshfield, and Dr. John Schaeffer

are greatly appreciated.

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NASA/TM—2001-211064 1

THE EFFECT OF HYDROGEN ANNEALING ON THE

OXIDATION RESISTANCE OF FOUR EPM SINGLE

CRYSTAL SUPERALLOYS

James L. Smialek, Charles A. Barrett, and Ralph G. Garlick

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

SUMMARY

Four single crystal EPM developmental airfoil superalloys were hydrogen annealed at 1300 °C for up to

100 hr to remove sulfur and improve oxidation resistance. Although the 1100 and 1150 °C cyclic oxidation

resistance was remarkably improved by annealing for 24 or 100 hr, the behavior was still considerably inferior

to that of commercially available single crystal superalloys, especially those that are either Y-doped or hydrogen annealed. Excessive degradation in the developmental alloys appeared to be correlated with low Cr contents and, to a lesser extent, high Co and Re contents.

INTRODUCTION

The oxidation resistance of single crystal superalloys has improved greatly over the last decade. The addition of

high levels of tantalum appears to be associated with slow growing mixed scales of CrTaO4 and NiTa2O6 with the

more commonly observed Al2O3 and Ni(Al,Cr)2O4 phases formed on oxidation resistant NiCrAl alloys. Furthermore, the addition of Hf (PWA 1484, Rene'N5, and CMSX 4) and Y (PWA 1487 and Rene'N5) produce even

greater oxidation resistance by the well known reactive element effect. The role of bulk sulfur content and sulfur

segregation in Al2O3 scale adhesion has also been the topic of many studies. In the past ten years, a number of

hydrogen annealing studies have shown that desulfurization of superalloys from 10 ppmw S down to 0.1 ppmw S is

readily possible and that the resultant oxidation resistance is comparable to Y-doped alloys, many of which are

covered in a recent review article (ref. 1).

Since the EPM turbine airfoil alloy development program was primarily directed toward improved high

temperature mechanical properties, Y additions were postponed to a later date, and scale adhesion issues for the

uncoated alloys were not specifically addressed. The purpose of this study is to provide baseline cyclic oxidation

resistance of early EPM alloys and to demonstrate what level of improvements are possible through desulfurization

by hydrogen annealing. A concurrent objective is to determine what annealing schedule is necessary for sufficient

desulfurization. The response of desulfurized alloys in cyclic oxidation tests should be indicative of the level of

improvements that can be achieved with Y additions.

MATERIALS AND EXPERIMENTAL PROCEDURES

EPM alloys #29, 35, 39, and 41 were obtained from excess slab material cast at EMTL, GE Aircraft Engines.

The nominal (target) compositions are shown in table I (ref. 2). The slab material was EDM machined into 1-mm

thick coupons with surface dimensions of about 1/4 by 1/2 to 1 in. (0.6 by 1.2 to 2.0 cm) and polished to a 600 grit

finish. Duplicate samples were annealed in a 5 percent H2/Ar atmosphere (<1 ppm H2O) at 1300 °C for 4, 24, and

100 hr.

All of the duplicate samples were tested in cyclic oxidation using 1 hr at temperature and 20 min cooling to

about 60 °C. Duplicate samples of each alloy were oxidized in the as-received condition, (with a 600 grit finish), and

after annealing for 4, 24, and 100 hr. The annealed samples were tested in the as-annealed condition, except for alloy

29; here one of the duplicate annealed samples was repolished to a 600 grit finish before oxidation, removing an
NASA/TM—2001-211064 2

average of 7.2 mg/cm2 or an 8 mm (1/3 mil) layer, assuming a density of about 9 gm/cm3. Samples of alloys #29

and #35 were cyclic furnace oxidation tested at 1150 °C for 80 to 100 hr, when severe weight loss necessitated

termination of the test. Alloys #39 and #41 were oxidized at either 1150 °C for 100 hr or at 1100 °C for 200 hr.

Weight changes were recorded throughout the test. The oxide scale phases formed on the oxidized coupons were

analyzed by X-ray diffraction.

RESULTS

Hydrogen Annealing

The annealing process produced average weight changes of only +0.001, and – 0.010 mg/cm2, respectively, for

the three annealing times. All samples were extremely reflective and metallic in appearance after annealing,

indicating that no oxide scale formed during annealing. Therefore, no barrier layer existed and desulfurization should

have proceeded as expected.

Cyclic Oxidation Weight Change

The specific weight change data for 1150 °C cyclic oxidation of alloy #29 is shown in figure 1. Rather severe

weight losses were exhibited for all the samples. No differences were noted between samples tested with the as-annealed surface condition and those re-polished to a 600 grit surface finish. Hydrogen annealing at 1300 °C for 24 to

100 hr did reduce the weight loss somewhat from 180 to 190 mg/cm2 down to 110 to 135 mg/cm2. However, essentially all the annealing times were ineffective in producing a truly protective scale at 1150 °C, despite some

improvement. In comparison, second generation SX superalloys, Rene’N5 and PWA 1484, with either Y–doping or

hydrogen annealing, gain <1 mg/cm2 after 1000 1-hr cycles to 1150 °C (refs. 3 to 5).

The weight change data for 1150 °C cyclic oxidation of alloy #35 is shown in figure 2. Again large weight

losses (100 mg/cm2) were exhibited by the as-received samples test for only 100 hr at 1150 °C, but these were

somewhat less severe than those obtained for alloy #29. Furthermore, a more significant effect of 24 and 100 hr

hydrogen annealing occurred for alloy #35, decreasing the weight loss to only 15 to 18 mg/cm2 after 100 hr of

oxidation. Identical behavior was displayed by the duplicate samples, both tested in the as-annealed surface

condition.

The 1150 °C data for alloys #39 and #41 are shown in figures 3 and 4, respectively. Both alloys show similar

performance, falling between those of the previously discussed alloys. The duplicate samples of these alloys were not

tested as pairs. Rather, because of the poor overall performance and absence of any surface condition effects, these

duplicate samples were tested at a less severe temperature to produce more meaningful results. Accordingly, the

results of 1100 °C cyclic oxidation testing is shown in figures 5 and 6. Although both as-received alloys still

exhibited substantial weight loss (>150 mg/cm2), alloy #39, when hydrogen annealed for 24 or 100 hr, achieved a

weight loss of only 6 mg/cm2 after 200 hr at 1100 °C.

The comparative behavior of the four alloys can be readily assessed in figures 7 and 8, where the final weight

change from the 1150 and 1100 °C tests (for as-annealed surfaces) are plotted as a function of hydrogen annealing

time. A number of points are clear from this figure: (1) hydrogen annealing at 1300 °C produced substantial

improvements for all the alloys; (2) alloys #35 and #39 are superior to alloys #41 and #29 after hydrogen annealing;

and (3) hydrogen annealing for 24 hr produced most of the benefit, with little further improvement demonstrated for

100 hr annealing.

X-ray Diffraction

No initial protective scales were preferentially formed from the hydrogen annealing process. The scale phases

present at the end of the oxidation test were characterized by X-ray diffraction on a select number of specimens. The

results are summarized in table II. The major scale phases were NiO, NiTa2O6, and Ni(Al,Cr)2O4 spinel. Very little

Al2O3 was identified, primarily because of the masking effect of the other heavily absorbing oxide phases and the

severity of attack sustained at the end of testing. Thus, the protective nature of the scale was limited. Surprisingly,
NASA/TM—2001-211064 3

no trend in scale type was observed as a function of performance (annealing time or alloy composition). It should be

noted that the lattice parameter of NiAl2O4 is 8.05Å, and that for NiCr2O4 is 8.30Å. Thus the observed spinel

represents a mixed composition somewhat closer to nickel aluminate than to nickel chromate.

Alloy Composition Effects

The major compositional variations in this set of four alloys occur for Cr, Co, Ru, and Re (table I). These are

also the elements that have substantial differences in level from those in the two commercial alloys listed. The

alloys are also ranked according to the final weight changes. This ranking remained relatively consistent for all the

hydrogen pre-anneals, as well as for the as received material, figure 7. (The weight change data in table I is that for

100 hr oxidation at 1150 °C, for EPM samples having 100 hr hydrogen annealing at 1300 °C, and for the

commercial alloys annealed at 1250 °C for 50 to 100 hr.). There is only one compositional change in going from

the best to second best alloy (#35 to #39, i.e., an increase in Co from 5 to 20 percent. In surveying an extensive

database for Ni(Co) superalloys, there is rarely any correlation with performance and Co content, (ref. 6).

A more significant performance loss is incurred in going from alloys #39 to #41 by reducing Ru from 4.5 to

1.5 percent. And finally, the worst performance of alloy #29 is obtained by reducing Cr from 2.3 to 1.5 percent,

decreasing Ru from 1.5 to 1.0 percent, and increasing Re from 5.7 to 6.25 percent. There is some reasonable

precedent to suggest that low Cr can be deleterious, in that some Cr is needed to assist in a transient oxidation

period during which a protective Al2O3 scale is being built up. Cr has been weakly correlated with improved cyclic

oxidation resistance in superalloys overall, but generally at levels higher than 5 percent (ref. 6). Previous EPM

experience (Dr. Jon C. Schaeffer) would indicate that Cr and Ru may act in concert to produce beneficial effects.

However these performance changes must be evaluated in the context of the excellent performance of current

oxidation resistant single crystal alloys, containing 5 to 7 percent Cr, 8 to 10 percent Cr, 3 percent Re, 5.6 to

6.2 percent Al, and no Ru. For example, weight loss of only 5 to 7 mg/cm2 were achieved for as-received Rene'N5

and PWA 1484 (without Y) after 100 hr of cyclic oxidation at 1150 °C or with weight gains of only 0.6 to

2 mg/cm2 after 1000 hr for hydrogen annealed Rene'N5 and PWA 1484 (refs. 3 to 5). Similar results were obtained

for 1100 to 1200 °C tests of hydrogen annealed PWA 1480 and CMSX-4 (refs. 7 to 9).

These alloys possess substantial compositional differences with the four EPM alloys, as presented in table I.

The most critical difference is believed to be the Cr content, which is only 1.5 to 2.3 percent for the EPM alloys

compared to 5 to 6 percent in current single crystal superalloys. Aluminum content is always very important, and

the EPM alloys do contain 5.5 percent Al. The Co levels at least overlap current alloys, but 20 percent Co appears

to be relatively high. The Re levels did not vary appreciably in the four alloy set, but are considerably higher than

present alloys, which contain 3 percent. Finally, higher Ru levels were identified in the best EPM alloys, but no Ru

is present in the superior conventional single crystal alloys. Thus Ru may help in the EPM alloys, but is not

needed for the good oxidation resistance of current superalloys.

In summary, low Cr levels are believed to have had the most detrimental effect on oxidation resistance. Also,

high Re may have degraded performance.

Sulfur Content

The sulfur contents were not measured, but the trend with hydrogen annealing could be estimated from the thin

slab diffusion equation, as demonstrated previously (ref. 3). Assuming sulfur diffusion control of desulfurization, the

thin slab diffusion solution provides the following approximation (for values of

Dt/L2 >0.05):

Cs/Co = 8/p2 exp(–p2Dst/L2) (1 )

where Cs and Co are the final and original average sulfur contents (<<1 percent), Ds is the diffusion coefficient of

sulfur in nickel, t is time, and L is thickness. The results of the two best oxidation runs (#35 at 1150 °C and #39 at

1100 °C) are shown in figure 9 as a function of the estimated sulfur content, as determined for the various annealing

times. Here the oxidation response can be seen to steadily improve as the estimated sulfur content falls below
NASA/TM—2001-211064 4

1 ppmw, assuming a starting level of 6.3 ppmw. (This sulfur level was taken from a previous study of Rene’142

(ref. 3), more recent studies show sulfur levels of 2.6 ppmw for Rene’N5 [5].)

(It should be noted that the 100 hr anneal would not actually produce the calculated level of 10–12 ppm sulfur.

Hydrogen annealing has been known to produce a minimum as low as 0.01 ppmw sulfur, but usually about

0.1 ppmw, both of which are nonetheless very low. In any event, the conclusion reached from this type of plot is

that the major effect of desulfurizing a 1-mm thick coupon, as determined in cyclic oxidation tests, occurs at

durations between 4 and 24 hr of hydrogen annealing.)

The plot is consistent with published summaries of hydrogen annealed superalloys which showed diminishing

returns below about 0.1 to 0.3 ppmw. This is also about the level that is equivalent to one monolayer of sulfur, if

all the bulk impurity were to segregate to the interface (refs. 3, 4, and 10). Finally, it is very similar in form to plots

of weight change versus calculated and actual sulfur content for PWA 1480 (refs. 4 and 11). But in these studies the

1100 °C oxidation resistance was much better overall, such that the final weights attained after 1000 1-hr cycles were

+0.5 to +1 mg/cm2.

CONCLUSIONS

This screening study has shown that some representative EPM single crystal alloys presently possess

significant debits compared to the oxidation resistance of commercially available superalloys. It is believed that the

single most important cause of this debit is the low Cr content, with lesser contributions possible from the high Co

and Re contents. Nonetheless, hydrogen annealing (desulfurizing) did have a strong positive effect, suggesting that

low sulfur alloys and Y additions would provide some useful benefits to the oxidation behavior of this class of

alloys. However these modifications would not produce oxidation resistance comparable to that of current second

generation single crystal superalloys.

REFERENCES

1. J.L. Smialek, “Maintaining Adhesion of Protective Al2O3 Scales,” JOM, January 2000, pp. 22–26.

2. Enabling Propulsion Materials Program: Annual Technical Progress Report, Vol. 5, Task J—Long Life

Turbine Airfoil Material System, Jan. 1, 1994 to May 1, 1995, NASA Contract NAS3–26385, June 25,

1995.

3. J.L. Smialek, D.T. Jayne, J.C. Schaeffer, and W.H. Murphy, “Effects of Hydrogen Annealing, Sulfur Segregation and Diffusion on the Cyclic Oxidation Resistance of Superalloys: a Review,” Thin Solid Films,

253, 1994, pp. 285–292.

4. J.L. Smialek, “Toward Optimum Scale and TBC Adhesion on Single Crystal Superalloys,” High Temperature Corrosion and Materials Chemistry, E.J. Opila, P.Y. Hou, D. Shores, M. McNallan, and R. Oltra, eds.

The Electrochemical Society Proceedings, Vol. 98–9, Pennington, NJ, 1998, pp. 211–220.

5. J.L. Smialek and B.A. Pint, “Optimizing Scale Adhesion for Single Crystal Superalloys,” NASA/

TM—2000–210362, August, 2000.

6. J.L. Smialek, C.A. Barrett, and J.C. Schaeffer, “Design for Oxidation,” in Design for Properties, ASM

Handbook, 20 , ASM, Materials Park, OH, 1997, pp. 589–602.

7. J.L. Smialek and B.K. Tubbs, “Effect of Sulfur Removal on Scale Adhesion to PWA 1480,” Metall. and

Mat. Trans., 26A, 427 (1995).

8. C. Sarioglu, C. Stinner, J.R. Blachere, N. Birks, F.S. Pettit, G.H. Meier, J.L. Smialek, “The Control of Sulfur Content in Nickel-Base, Single Crystal Superalloys and its Effect on Cyclic Oxidation Resistance,” in

Superalloys 96, A.D. Cetel, M.V. Nathal, T.M. Pollock, and D.A. Woodford, eds., TMS-AIME, Warrandale,

PA, 1996, pp. 71–80.

9. M.A. Smith, W.E. Frazier, and B.A. Pregger, “Effect of Sulfur on the Cyclic Oxidation Behavior of a Single Crystalline Nickel-Based Superalloy,” J. Mat. Sci., 203, 328 (1995).

10. J.L. Smialek, “Effect of Sulfur Removal on Al2O3 Scale Adhesion,” Metall. Trans., 22A, 739 (1991).

11. J.L. Smialek, “Oxidation Resistance and Critical Sulfur Content of Single Crystal Superalloys,” Transactions of the ASME, Vol. 120, 1998, pp. 370–374.
NASA/TM—2001-211064 5

TABLE I.—NOMINAL CHEMISTRIES (wt %) OF FOUR SAMPLE EPM DEVELOPMENTAL

SINGLE CRYSTAL AIRFOIL SUPERALLOYS AND TWO COMMERCIAL ALLOYS.

ORDERED ACCORDING TO WEIGHT CHANGE AFTER 100 1-hr CYCLES AT

1150 °C FOR EPM SAMPLES THAT WERE HYDROGEN ANNEALED AT

1300 °C FOR 100 hr, AND FOR COMMERCIAL ALLOYS HYDROGEN

ANNEALED AT 1250 °C FOR 50 TO 100 hr

Commercial

alloy

Al Ta Cr Co Ru Re W Mo Ti DW/A1150 °C, 100 hr,

mg/cm2

Rene'N5 6.2 7 7 8 0 3 5 2 0 0.6

PWA 1484 5.6 9 5 10 0 3 6 2 0 0.6

EPM alloy

 #35 5.5 8.3 2.3 5.0 4.5 5.7 5.6 2.0 0.4 –14.9

 #39 5.5 8.3 2.3 20.0 4.5 5.7 5.6 2.0 0.4 –22.7

 #41 5.5 8.3 2.3 20.0 1.5 5.7 5.6 2.0 0.4 –64.5

 #29 5.6 8.0 1.5 20.0 1.0 6.2 6.0 2.0 0.0 a (–160) aExtrapolated.

TABLE II.—SUMMARY OF SCALE PHASES FORMED AFTER CYCLIC OXIDATION

Alloy #35

600 grit

Alloy #39

as-annealed

Alloy #41

as-annealed

1150 °C, 100 hr 1150 °C, 100 hr 1150 °C, 100 hr

1300 °C anneal time

0 hr N(p), N–T, S(8.10) N–T, N, S(8.10) N, N–T, S(8.10)

4 hr N(p), N–T, S(8.10) N–T, N, S(8.10) N–T, N, S(8.10)

24 hr N(p), N–T, S(8.10) N, N–T, S(8.10) N–T, N, S(8.10)

100 hr N(p), N–T, S(8.10), A N, N–T, S(8.10) N–T, N, S(8.10)

1100 °C, 200 hr 1100 °C, 200 hr

1300 °C anneal time

0 hr N(p), S(8.20), N–T N, N–T, S(8.10)

4 hr N(p), S(8.15), N–T N, N–T, S(8.10)

24 hr N(p), S(8.20), N–T N, N–T, S(8.10)

100 hr N(p), S(8.20), N–T N, N–T, S(8.10)

N = NiO A = Al2O3

N–T = NiTa 206 (p) = preferred orientation

S = Ni(Al,Cr)2O4 (8.10) = ao
NASA/TM—2001-211064 6

–200

20 40 60 80 100

Oxidation time, 1 hr cycles

Alloy 29,

1150 °C

Figure 1.—Effect of hydrogen annealing times on the oxidation resistance of alloy #29.

 (1150 °C, 1 hr cycles). As-annealed surface condition (dashed lines) shows little

 difference with 600 grit polished surfaces (solid lines).

0

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal

Figure 2.—Effect of hydrogen annealing times on the oxidation resistance of alloy #35.

 (1150 °C, 1 hr cycles). As-annealed surface (dashed lines); similar behavior of duplicate

 samples.

–200

20 40 60 80 100

Oxidation time, 1 hr cycles

Alloy 35,

1150 °C

0

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal
NASA/TM—2001-211064 7

–200

20 40 60 80 100

Oxidation time, 1 hr cycles

Alloy 39,

1150 °C

Figure 3.—Effect of hydrogen annealing times on the oxidation resistance of alloy #39.

 (1150 °C, 1 hr cycles; dashed lines: as-annealed surfaces).

Figure 4.—Effect of hydrogen annealing times on the oxidation resistance of alloy #41.

 (1150 °C, 1 hr cycles; dashed lines: as-annealed surfaces).

0

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal

–200

20 40 60 80 100

Oxidation time, 1 hr cycles

Alloy 41,

1150 °C

0

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal
NASA/TM—2001-211064 8

–200

Oxidation time, 1 hr cycles

Alloy 39,

1150 °C

Figure 5.—Effect of hydrogen annealing times on the oxidation resistance of alloy #39.

 (1100 °C, 1 hr cycles; dashed lines: as-annealed surfaces).

Figure 6.—Effect of hydrogen annealing times on the oxidation resistance of alloy #41.

 (1100 °C, 1 hr cycles; dashed lines: as-annealed surfaces).

0 20 40 60 80 100 120 140 160 180 200

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal

–200

Oxidation time, 1 hr cycles

Alloy 41,

1100 °C

–150

–100

–50

0

Weight change, mg/cm2

100 hr

24 hr

4 hr

As-received

1300 °C,

H2 anneal

0 20 40 60 80 100 120 140 160 180 200
NASA/TM—2001-211064 9

–200

Annealing time, hours

1300 °C H2 anneal

1150 °C oxidation

As-annealed surfaces

1300 °C H2 anneal

1100 °C oxidation

As-annealed surfaces

Figure 7.—Effect of alloy type and hydrogen annealing times on the final weight change

 of EPM alloys. (1150 °C, 1 hr cycle oxidation; 1300 °C annealing, as-annealed surfaces).

Figure 8.—Effect of alloy type and hydrogen annealing times on the final weight change

 of EPM alloys. (1100 °C, 1 hr cycle oxidation; 1300 °C annealing, as-annealed surfaces).

0 20 40 60 80 100

–150

–100

–50

0

Final weight change, mg/cm2

–200

Annealing time, hours

–150

–100

–50

0

Final weight change, mg/cm2

40 60 80 100

41

39

0 20

#35

#39

#41

#29
NASA/TM—2001-211064 10

–200

Predicted sulfur content, ppmw

#35, 1150 °C, 100 hr

#39, 1100 °C, 200 hr

As-annealed surface

( ) = anneal time, hours

Figure 9.—Correlation of final weight change with the predicted sulfur content of

 hydrogen annealed EPM alloys. (1300 °C anneal, as-annealed surfaces, 6.3 ppmw

 initial sulfur assumed).

101 10–1 10–3 10–2 10 Minimum 0

–150

–100

–50

0

Final weight change, mg/cm2

(0)

(4)

(24) (100)
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16

The Effect of Hydrogen Annealing on the Oxidation

Resistance of Four EPM Single Crystal Superalloys

James L. Smialek, Charles A. Barrett, and Ralph G. Garlick

Metallic materials

Unclassified -Unlimited

Subject Category: 26 Distribution: Nonstandard

Responsible person, James L. Smialek, organization code 5160, 216–433–5500.

Four single crystal EPM developmental airfoil superalloys were hydrogen annealed at 1300 °C for up to 100 hr to

remove sulfur and improve oxidation resistance. Although the 1100 and 1150 °C cyclic oxidation resistance was

remarkably improved by annealing for 24 or 100 hr, the behavior was still considerably inferior to that of commercially

available single crystal superalloys, especially those that are either Y-doped or hydrogen annealed. Excessive degradation

in the developmental alloys appeared to be correlated with low Cr contents and, to a lesser extent, high Co and Re

contents.