The Effect of Crystallographic Orientation on the

Oxidation Behavior of a Single-Crystal Nickel-base

Superalloy

F. H. Yuan,*z E. H. Han,* C. Y. Jo,y T. F. Li*, and Z. Q. Hu*

Received July 3, 2002; revised March 28, 2003

The oxidation kinetics of DD100 nickel-base single-crystal alloy, with (221)and (100) surface orientations were tested in this study by thermogravimetry.A pronounced anisotropy in both the cyclic and isothermal oxidation resistanceof DD100 alloy was observed. The (221) crystallographic surface of DD100had a slower oxidation rate than the (100) surface when isothermally exposedto stationary air at 9508C, whereas the opposite results were obtained at10508C. When cyclically oxidized at 11008C, the weight loss of samples with(221) surfaces was significantly greater than that of (100) surfaces, so thatthe cyclic-oxidation anisotropy was more remarkable at 11008C than at 9508C.The different spatial alignment of the � 0=� interface is thought to beresponsible for the anisotropic oxidation behavior of the nickel-base singlecrystal alloy.

KEY WORDS: Single-crystal nickel-base superalloy; high-temperature oxidation; oxidationkinetics; oxidation anisotropy.

INTRODUCTION

Turbine inlet temperatures have now approached 16508C at maximum

power for the latest large, commercial-turbofan engines, resulting in high

fuel efficiency and thrust levels approaching or exceeding 445 kN.1 This level

of performance has been brough about by a combination of advances in air-

*State Key Laboratory for Corrosion and Protection, Institute of Metal Research, ChineseAcademy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China.yHigh Temperature Materials Laboratory, Korea Institute of Machinery and Materials, 66Sangnam Dong, Changwon, Kyungnam 641-010, Korea.zTo whom all correspondence should be addressed; E-mail: fhyuan@imr.ac.cn

Oxidation of Metals, Vol. 60, Nos. 3/4, October 2003 (# 2003)

211

0030-770X/03/1000–0211/0 # 2003 Plenum Publishing Corporation

cooling for turbine blades and vanes, computerized design technology forstress and airflow, and the development and application of nickel-base,single-crystal superalloys with a high � 0 volume fraction. Particularly, thesesuperalloys have been the subject of much attention over the past decadesaround the world, because of their more desirable mechanical properties atelevated temperatures compared to conventionally-cast or directionally-solidified nickel-base superalloys. The single-crystal alloys were developedto improve their stress-rupture properties as well as oxidation resistance.2,3

Hence hydrogen annealing has been proposed to desulfurize the single-crystal alloys, thereby improving oxide-scale adhesion on the alloysurfaces.4,5

Mechanical properties of nickel-base, single-crystal superalloys can bewell characterized by crystallographic anisotropy6 since their fatigueproperties7,8 and creep behavior9,10 are evidently crystallographic-orienta-tion dependent. On the other hand, it is found that the oxidation behavior ofsingle-crystal Ni11 and the alloy NiAl12 are all affected by crystallographicorientations of the corresponding oxidized surfaces. Studies on the effects oforientations have been primarily concentrated on pure Fe, Ni, and Cu singlecrystals.13 Recently, it was reported that oxidation anisotropy is alsoapparent in the initial oxidation stage of polycrystalline Ni3Al alloys, in thatcrystal grains in the sample surfaces were notably oxidized to varyingdegrees.14 Therefore, the discrepancy in the oxidation behavior of differentcrystallographic surfaces is quite distinct even in the case of polycrystallinealloys.

Although a lot of relevant work has been done in recent years,15–20

investigations about the oxidation behavior of nickel-base, single-crystalalloys have still not been thoroughly carried out. Moreover, the influence ofcrystallographic orientation on the high-temperature oxidation resistance ofnickel-base, single-crystal superalloys has not yet been published in the openliterature. Therefore, the present work was done on the isothermal andcyclic behavior of DD100 nickel-base, single-crystal alloy with (100) and(221) crystallographic-orientation surfaces, to explore the role played byexternal surface orientation during high-temperature oxidation. It wasfound that the surface orientation had a noteworthy influence on theoxidation kinetics of DD100 alloy.

EXPERIMENTAL PROCEDURES

The nickel-base, single-crystal alloy DD100 was used in the presentwork. The nominal composition (in wt.%) of DD100 is 10Cr, 5Co, 3Mo,2Ta, 4.8Al, 4.7 Ti, 0:1 � 0:15Hf; and balance Ni. To reduce the amount of� 0 þ � eutectic and elemental segregation, the as-cast alloy was solutionized,

212 Yuan et al.

followed by an aging treatment as follows: 1240�C=3 h ACþ 1270�C=3 h ACþ1277�C=3 h ACþ1080�C=4 h ACþ870�C=20 h AC; where ACstands for air-cooling. The microstructure of the alloy contains two phasescharacterized by 0:5�m precipitates of the cuboidal � 0ðNi3AlÞ phaseuniformly distributed in the matrix � phase.

Samples 15� 10� 2mm; with the two principal parallel plane surfacesperpendicular to the specified crystallographic orientations, were cut byelectrical-discharge machining from the heat-treated castings, then werepolished with SiC paper to 800 grit. The principal oxidation surfaces ofsingle-crystal samples were identified to be within �5� of the desiredorientations using the Laue X-ray diffraction (XRD) technique. Prior tooxidation testing, all samples were ultrasonically cleaned in acetone andthen rinsed in alcohol. The samples were weighed to a sensitivity of 10�5 g:

The samples were oxidized isothermally in static air at 9508C and10508C, respectively. Cyclic oxidation was performed in a vertical tubefurnace equipped with a time-programmable controller for moving samplesinto and out of the furnace automatically. Every thermal cycle of the cyclicoxidation was 50-min high-temperature exposure in the furnace, followed bya 10-min ambient cooling. Samples were reweighed after varying numbers ofoxidation cycles, and triplicate samples were used under one test condition.

Some of the oxidized samples were examined to determine the oxidemorphologies developed on the surfaces. Some were cross-sectioned andthen examined metallographically by means of scanning-electron micro-scopy (SEM) in combination with energy-dispersive X-ray spectroscopy(EDS). The oxides formed were also analyzed by X-ray diffraction.

RESULTS

Isothermal Oxidation

The isothermal-oxidation kinetics of DD100 nickel-base superalloy at9508C and 10508C are given in Fig. 1. It can be noted from this figure thatthe (221) surface oxidized slower than the (100) surface at 9508C. At 10508C,the opposite result was observed. The oxidation of both the (221) and (100)surfaces was still in the transient stage after oxidation for 100 hr at 9508C,because the mass gain was still increasing quickly. At 10508C both surfaceswith the above-mentioned orientations reached a stable stage when oxidizedfor about 50 hr. The morphologies of the (221) and (100) surfaces oxidizedat 9508C for 100 hr are shown in Fig. 2. A significant amount of TiO2

clusters developed on both the (221) and (100) surfaces. Nonetheless, onecan note that the amount of TiO2 formed on the (221) surface was much lessthan that on the (100) surfaces. EDS analysis indicated that the oxideformed on the (221) surface was rich in aluminum in addition to small

Effect of Crystallographic Orientation 213

amounts of titanium, chromium and nickel. Distinctive, wrinkled-oxide

scales developed on the (221) surfaces because of predominant lateral

growth of the alumina scale, while the oxide morphologies on (100) surfaces

are relatively smooth as a result of a small amount of aluminum oxide in the

scale (Fig. 3(a) and (b)). The oxide products generated on the (100) surfaces

were primarily nickel oxide when exposed to atmospheric air at 9508C for

100 hr.The surfaces of samples with both the (221) and (100) orientations were

all rugged when oxidized at 10508C for 100 hr, resulting from severe local

scale spallation. Composition analysis by EDS of the spalled-scale area

indicated that the substrate surfaces were markedly titanium depleted. The

black area in Fig. 2(c) and (d) was rich in aluminum, while the white oxide

Fig. 1. Isothermal-oxidation kinetics of (221) and (100) surfaces ofDD100 single-crystals (a) at 9508C and (b) at 10508C for 100 hr.

214 Yuan et al.

was a mixture of chromium, nickel, and titanium oxide products. It could be

identified by XRD analysis that the oxides were primarily NiO,

Al2O3; TiO2, and spinels of NiTiO3; NiCr2O4, andNiAl2O4 when oxidized

at 9508C for 100 hr.In comparison with 9508C, the amount of nickel oxide in the scale at

10508C was reduced. The inner layer of oxide scale consisted essentially of

TiO2 and Al2O3; while the outer layer of the scale was composed principally

of NiTiO3; NiCr2O4, and NiAl2O4 spinels. The amount of Al2O3 formed

on the (100) surface was larger than that on (221) when oxidized at 10508Cfor 100 hr. EDS analysis of oxide scales detached from the surfaces of (221)

and (100) samples indicated that the scales were composed of nickel,

aluminum, chromium, and titanium oxides. The oxide products detached

from (221) samples had more aluminum, titanium, and chromium oxides

than those from (100) samples (Fig. 3(c) and (d)). Therefore, it was much

easier for (221) surfaces to generate aluminum, titanium, and chromium

oxides in the initial oxidation stage as compared to (100) surfaces.

Fig. 2. Surface morphologies of (a), (c) (100) surface and (b), (d) (221) surface of DD100single-crystal after isothermal oxidation at (a), (b) 9508C and (c), (d) 10508C.

Effect of Crystallographic Orientation 215

Cyclic Oxidation

The cyclic-oxidation kinetics of DD100 single-crystal samples with

(221) and (100) surfaces at 9508C for 300 hr, and at 11008C for 250 hr are

presented in Fig. 4. At 9508C, samples with (221) and (100) surfaces did not

exhibit a significant discrepancy in cyclic-oxidation kinetics during 300 hr,

even though the weight change of the samples with (221) surfaces was

roughly similar to that of (100) surfaces. The cyclic-oxidation kinetics could

all be characterized by a two-stage oxidation process, i.e., at the beginning

of the test, although occasional spallation took place, the oxide scale was

still adherent and protective, so that the plots in Fig. 4 indicate weight gains

of the samples were similar after 220 hr oxidation. After 220 hr, the scales

formed spalled severely. As a result, both types of samples, (221) and (100)

surfaces, began to lose weight. When oxidized at 11008C, all the samples

showed a remarkable weight loss. Moreover, the cyclic-oxidation resistance

of (221) surfaces was markedly inferior to that of (100) samples after 250 hr.

Fig. 3. EDS analysis of oxide scales other than nodule products corresponding respectively tothat shown in (a), (b), (c), and (d) of Fig. 2.

216 Yuan et al.

The oxide morphology of (221) surfaces oxidized at 9508C for 300 hr is

presented in Fig. 5. It is notable that there were quite a few TiO2 clusters on

the surface. EDS analysis showed that the continuous scale beneath was rich

in titanium, chromium and nickel. No cracking was found in the scale, and

little of the scale had spalled. The morphology of the oxide scale on the (100)

sample was similar to that on (221) surfaces.After cyclic oxidation at 11008C for 250 hr, significant spallation

occurred on (221) surfaces so that the surface became rougher with

oxidation time. On the other hand, little of the scale on the (100) surface

became detached when subjected to thermal cycling. Examination of the

corresponding cross sections indicated that most of the scales formed on

Fig. 4. Cyclic-oxidation kinetics of (221) and (100) surfaces of DD100single-crystals (a) at 9508C and (b) at 11008C.

Effect of Crystallographic Orientation 217

(221) surfaces spalled, whereas scales formed on (100) surfaces were stilladherent. There were evidently a few internal oxides present in the substratebeneath the surfaces of (221) and (100) samples. Furthermore, aconspicuous band of a � 0-depleted area developed underneath the oxidescale in the substrate (Fig. 6). Based on EDS analysis, the internal-oxideproducts were identified as blocky ALN or acicular TiN (Fig. 6(d)). On theother hand, a continuous inner-Al2O3 scale developed under the outer layerwhich contained NiO, (Ni,Co)Cr2O4; TiO2; and Al2O3; NiAl2O4, etc. inthe case of (100) surface, and internal-Al2O3 particles also developed in thesubstrate beneath the outer oxide scale (Fig. 6(c)).

DISCUSSION

Isothermal Oxidation

It has been known for many years that the oxidation behavior of single-crystal metals is crystallographic orientation dependent.21,22 Thus, one canclaim that the oxidation behavior of single-crystal metals and alloys isclosely correlated with crystallographic orientations of the specific samplesurfaces, i.e., the orientation relationships between the alloy substrate andits surface oxide scale should develop towards the minimal mismatchbetween parallel close-packed directions in the metal or alloy and the cationsublattice of the oxide scale.12

The oxidation behavior of (221) and (100) surfaces of DD100 was stillin the transient stage after oxidation for 100 hr at 9508C, as evidenced by arapid increase in the specimen mass. In general, a transient oxidation stageoccurs prior to formation of the stable �-Al2O3 layer since at temperatureslower than 10008C it is very difficult for the healing alumina to developbecause of the slow diffusion rate of aluminum atoms. The (221) surfaces

Fig. 5. Surface morphologies of (221) oriented DD100 single-crystal alloy after cyclic oxidationat 9508C for 300 hr.

218 Yuan et al.

are more oxidation resistant than (100) surfaces of DD100. This

phenomenon can be attributed to a much-more protective Al2O3 developed

on (221) surfaces. On the other hand, more TiO2 oxide is formed on (100)surfaces of the alloy. TiO2 oxide is not an effective barrier to inhibit

oxidation so that the oxidation weight gain of the (100) samples is larger

than that of (221) samples.At 10508C, more oxide scale on (221) surface is cracked and seriously

spalled, so the oxidation rate of (221) surface is higher than that of (100)samples. The oxide scales formed on the surfaces of (221) and (100) samples

were further examined by the inverse-pole figure method. Unfortunately, we

did not find scales having a noticeable preferred orientation. Scales formed

in the initial stage should be oriented with respect to the substrate, and then

a transition to a randomly-oriented scale occurs. It can be concluded thatafter a long oxidation time, 100 hr in our work, the oxide scale is too thick to

retain its growth epitaxy, i.e., the degree of preferred oxide orientation

decreases with oxidation time.The samples of DD100 exhibit a remarkable discrepancy in the

oxidation kinetics and oxide products when oxidized at 950 and 10508C,

Fig. 6. Surface morphologies and BSE images of cross-sections of (a), (c) (100), and (b), (d) (221)samples cyclically oxidized at 11008C for 250 hr.

Effect of Crystallographic Orientation 219

testifying to a significant relationship between the scaling rate andcrystallographic orientation of the oxidized surfaces at elevated tempera-tures. Likewise, the oxidation behavior of the outer surface of single-crystalnickel metal is highly epitaxial so that short-circuit diffusion paths aredecreased, resulting in slower scale growth on the surfaces.11 Therefore,scales having a higher degree of preferential orientation grow more slowly.

Cathcart and co-workers thought that the oxide scales formed on thesurfaces of nickel single crystals possess a large number of short-circuitdiffusion paths, which play a role, as a function of the crystallographicorientations of the single crystal surfaces, so the surfaces with differentcrystallographic orientations showed various oxidation rates.23 These short-circuit diffusion paths are produced because the oxide scale is subjected toan epitaxial-growth force during its development on surfaces of single-crystal metals or alloys during oxidation.24–26 The short-circuit diffusionpaths are actually non-coincident, twin-crystal grain boundaries. Thefunction of these paths during oxidation is similar to that of high-anglegrain boundaries in view of short-circuit diffusion, so they can exert aremarkable effect on the oxidation resistance of a nickel single crystal.27,28

Herchl et al. found that samples of (100)-oriented nickel single crystalsoxidized more rapidly than the (110) and (111) nickel crystal faces.29

Cathcart found that when oxidized at 900 and 10008C, crystalline grainswith different orientations in the surfaces of polycrystalline Ni3Al samplesshow notable scale-growth anisotropy during the early stage of oxidation,namely the scales on various grains grow at significantly different rates.30

The oxide products developed on grains with different crystallographicorientations also vary greatly. Therefore, the crystallographic orientationsnot only affect oxidation kinetics, but also influence oxide productsdeveloped on surfaces of NiAl3 samples when oxidized at elevatedtemperatures.

Although the diffusion in crystals is generally crystallographicdependent, diffusion coefficients for different directions of cubic metalcrystals have similar values.31 Hence the oxidation kinetics of DD100 singlecrystal with different surface crystallographic orientations are hardlyaffected by lattice diffusion of elements in the alloy. There may be tworeasons for the difference in oxidation kinetics of (221)- and (100)-orientedDD100 alloy: one is even though an orientation relationship between theoxide scale and the DD100 substrate was obviously not detected, thestructure and defects in the oxide scale formed in the initial stage should stillexert an influence on subsequent scale development in view of the growthrate and oxide products. The other reason is that the nickel-base, single-crystal superalloy is composed primarily of � 0 and � phases. Thestrengthening � 0 precipitates are uniformly distributed in the � matrix.

220 Yuan et al.

The orientation relationship between the � 0 and � phases is ð100Þ� 0==ð100Þ� :Interfacial alignment of the phase boundaries between � 0 and � is differentfor the outer surfaces of DD100 single-crystal samples with (221) and (100)crystallographic orientations. For samples with (100) surfaces, the phaseboundaries are always perpendicular to the primary outer surfaces, whereasin the case of (221)-oriented, single-crystal samples, the phase boundariesmeet the external surfaces at an angle of about 48 or 70 degrees. Theboundaries between the � 0 and � phases act as rapid paths for elementdiffusion during oxidation, so the spatial alignment of these boundaries inrelation to the outer surfaces must have remarkable effects on the oxidationresistance of the single-crystal superalloys. In brief, scales initially growepitaxially with respect to the substrate surface during the initial stage, but atransition of the epitaxial-oxide scale to randomly-oriented scale occurs atsome time afterwards.

Moreover, a significant amount of TiO2 formed on both the (221) and(100) surfaces when isothermally oxidized at 9508C for 100 hr. This can beattributed to a high concentration of titanium in DD100. It was claimed thattitanium does not affect oxidation resistance at concentrations of 1wt.% orlower. However, adverse effects would result at higher titanium concentra-tions,33 because oxides of titanium are formed on the surfaces ofsuperalloys, and this oxide is ineffective in protecting the underlying alloysfrom oxidation at high temperatures.

Cyclic Oxidation

The (221)- and (100)-oriented surfaces of DD100 did not show markeddifference in cyclic-oxidation resistance when tested at 9508C for 300 hr.Nonetheless, oxide scales on the above two surfaces showed a notabledifference in spallation resistance when cyclically oxidized at 11008C for250 hr, namely the cyclic-oxidation resistance of (221) samples was muchinferior to that of (100) samples. Cross-section examination of the samplesafter cyclical oxidation at 11008C indicated that almost all of the oxide scaleformed on (221) surface had spalled, whereas some of the scale on (100)-oriented surfaces was still intact. In addition to small amounts of aluminumand cobalt, this scale is rich in titanium and nickel. Aluminum and cobaltwere depleted in the substrate beneath the scale as a result of repeated scalebreakaway from the substrate surfaces. Consequently, a � 0-phase depletedband developed underneath the surface so that the cyclic-oxidationresistance of the DD100 sample deteriorated with oxidation time, exhibitingincreasing mass loss rate with increasing time. XRD analysis of the detachedoxide scales showed that predominant NiO, (Ni,Co)Cr2O4, and TiO2

developed in addition to small amounts of Al2O3 and NiAl2O4: Thedetection of a significant amount of NiO in the spalled oxide scale indicates

Effect of Crystallographic Orientation 221

that after some cycles of oxidation, protective alumina and chromia scalecannot form any more, hence the rapid growth of NiO oxide plays anincreasing role with time during the cyclic-oxidation process.

At 11008C, the (221) surface cyclically oxidized more rapidly than the(100) surface. Calculation of thermal-expansion coefficients of singlecrystals indicated no anisotropy exists for cubic crystal of metals andalloys.32 Since DD100, containing � 0 and � phases, both of which have acubic crystal structure, the surface-orientation-dependent, cyclic-oxidationbehavior of DD100 single crystals should not be closely correlated with itsthermal expansion. On the contrary, the anisotropy of DD100 alloy in cyclicoxidation resistance is derived from different epitaxial-growth processesfrom the very beginning of oxidation. In the case of (221) surfaces, the oxidescale generated has more aluminum, chromium and titanium oxidescompared to scales formed on (100) surfaces. These oxides have smallerthermal-expansion coefficients than NiO, so they are more prone tospallation than NiO when subjected to thermal cycles during cyclicoxidation, hence the (221) surface of DD100 is conspicuously inferior to(100) samples concerning cyclic-oxidation resistance at 11008C.

In the present work, there are noteworthy internal ALN and TiNnitrides that formed in the substrate underneath the (221) surface of DD100during cyclic oxidation. It was claimed by Krupp and co-worker thatnonprotective-oxide formation, during cyclic oxidation in the present work,or cracking and spalling at edges or curved surfaces of samples, enablesnitrogen to penetrate deep into the superalloy, leading to the precipitation ofstable Ti and Al nitrides.34 Besides, in local areas abounding with eutectic,the high Ti concentration in the eutectic phases can also cause intensifiedinternal-nitridation attack.15,16 Similarly, during creep of nickel-base, single-crystal superalloys at high temperature, cracks created in the oxide scale orin the alloy surface during tensile creep, facilitate nucleation and growth ofALN and TiN nitrides in the subsurface area, due to short-circuit diffusionby nitrogen atoms along the cracks deep into the substrate.35

In summary, the crystallographic orientations of outer surfaces ofnickel-base, single-crystal superalloy have a strong influence on theisothermal and cyclic-oxidation resistance. To better understand theoxidation behavior of nickel-base, single-crystal alloys, more work will bedone on the evolution with oxidation time of oxide scales generated onsurfaces with different crystallographic orientations.

CONCLUSION

The (221) surface of DD100 nickel-base, single-crystal alloy has aslower oxidation rate than that of (100) surface when isothermally oxidized

222 Yuan et al.

at 9508C for 100 hr. The opposite result was observed at 10508C. At 9508C,more alumina and less NiO formed on the (221) surface compared to the(100) surface. Alumina-oxide scale formed on the (221) surface is muchmore prone to spallation due to growth stress during isothermal oxidation at10508C. When cyclically oxidized at 11008C, the weight loss of (221)-orientedsamples is markedly greater than that of (100) samples, and this anisotropyin cyclic-oxidation resistance is more obvious at 11008C than at 9508C. Thisphenomenon resulted largely from more alumina-scale formation on the(221) surface. The oxidation anisotropy demonstrated for DD100 isevidence of the different epitaxial defects formed during the initial oxidationstage and variation in oxide products due to different spatial alignment ofthe �0=� interface in relation to crystallographic orientations of the samplesurfaces.

ACKNOWLEDGMENTS

Financial support by the National Natural Science Foundation ofChina and KISTEP Scientific Researcher Exchange program by the KoreanGovernment are gratefully acknowledged. Thanks are also due to ProfessorY. H. Zhang, Shanghai Jiaotong University, for providing the nickel-base,single-crystal alloy samples.

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