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γ-Al2O3 interfaces
ScriptsMetallurgicaet Mat&ah,
Vol. 33, No. 7. pp.1043-1048,199s Elsevia Science Ltd cqyight 0 1995 Acta Mct.allllrgicaInc. PkkdintkUSA.AlIIi&&-d 09%716x195 $9.50 + .oo
Pergamon 0956-716X(9!5)00341-X
ELECTRON MICROSCOPY STUDIES OF NiAl/y-Al,O, INTERFACES J. C. Yang, K. Nadarzinski, E. Schumann and M. Riihle Max-Planck-Institut fur Metallforschung Institut fbr WerkstotRvissenschatt D-701 74 Stuttgart GERMANY (Received February 27,199s) Introduction
Intermetallics belong to a group of interesting materials which may find high temperature applications, such as jet turbine engines, due to their combination of desirable properties, such as low density, high melting &npen&m and high thermal conductivity. Their durability will depend on good corrosion resistance. NiAl is an intermetallic material which demon&ratesexcellent oxidation resistance due to the slow growth of A&O, on its surface during high temperature oxidation. Presently, NiAl is used as a coating material on superalloys because of its excellent oxidation resistance [ 11. A better under&ml& of the oxidation mechanisms requires knowledge of the structure and composition of the intermetallic,oxide scale and interfacial region during the oxidation process. There have been several mvestigationsof the altia scale growth on intermetallic materials using a wide variety of techniques, such as SEM, TEM, XPS, SIMS, etc. [2-71. However, there have been only a few investigations of the t?mdamental aspects of the initial oxide scale formation [S- 141. Obviously more work is needed to elucidate the structure ofNiAl and its initiallyformed oxide scale. With the improvement of electron microscopy sample preparation techniques, we can now examine the NiAl/oxide scale at the subnanometer level. Emerimental
Procedure
Single crystals of NiAl containing 0.01 wt. % Y was oriented with respect to the (001) pole within lo. The sample was sliced with a spark errosion cutter to dimensions of 2 mm x 20 mm and polished down to 1mm diamond paste. The sample was placed into a tube furnace at 1223°K for 0.1 hr in air. The cross-sectional TEM samples were prepared by a technique developed by Strecker et al [ 151. The oxidized samples were satldwichedtogether, embedded in brass, sliced, ground and polished to approximately 3mm diameter discs and 1OOmmthickness. They were ion-milled in a Bal-Tee precision ion miller at a low angle of 4Ountil perforation. CTEM investigationswere performed on a JEOL JEM 2OOOFX,analytical electron micricoscopy was performed on a dedicated VG HB501 STEM equipped with a Gatan PEELS detector, and HREM investigations were pe&rmed on a JEOL JEMARMl250 (atomic resolution microscope) operated at 1.25MV, which has a demonstrated resolution of 1A [ 161.
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Nii+1203
INTERFACES
Vol. 33, No. 7
Figure1.~~~-fieldTEMhnageaftheNiAVy-~~O,interface~theN~w~oxidized~~~95OaCfor6~U~. Many faceted voids are observedat the interface.
Results Structural Observations
1 represents a bright-field TEM micrograph of a cross-section of the N&l/oxide scale interface a&r the NiAl was oxidized in air at 1223“K for 0.1 hr. The oxide scale was observed to be a 40-50 nm thick layer all along the interface. The scale contains many defects, possibly dislocations and/or twin boundaries. Faceted 200-300 nm voids and facets were observed to form along [OO1],ti and [Ol llNiAlinterfaces. Figure 2 is an electron ditbaction pattern taken from the NiAl/oxide scale interface. The dominant spots are due to (100) NiAl. Extra spots can be indexed as (110) y-A&O,. Some of the dithaction spots from the oxide scale have been indexed in figure 2. The presence of long facets and the excellent epitaxy between the NiAl and oxide scale suggested high resolution investigations. Figure 3 is a HREM micrograph of the interface between (0 11&M and the oxide scale. The two crystals have an excellent relative orientation since all of the lattice planes, including the 0.139nm (T4O),,,Q, are resolved. The following orientation relationship could be revealed: (0 1l),, // and [lOO],A // [llOIY_AIzq.The interface is atomically flat. Both the y-A&O, and NiAl appear 01 l),,, defect free. The lattice mismatch between the (lT2),,,,, and (01 l& is calculated to be 17%. However, no misfits or stand-off distance were observed in this interface. Figure 4 is a HREM micrograph of the NiAl/oxi& scale interface where the interfacial plane is parallel to (00 1h. The noise in the image has been reduced by a Fourier tilter by adapting a new method suggested by Mobus ef al. [ 171. There is some slight preferred faceting along (001 ),, and (0 1 l)NiA1, but, on average, this interface is observed to be rough The lattice mismatch was measured by counting the number of (020),, lattice fringes and the @40),_,+9 lattice tiinges for the same distance and found to be 4%, which is close to the calculated lattice mismatch of 3%. In contrast to the (01 l>NiAI interface, the interface here is observed to be coherent, except for one monolayer at the interface. It has been previously observed that the relative geometry between the metal and oxide can significantly alter the interfacial structure [ 181. The interfacial width, measured t+om the last (004) I _AZ%plane to the first (OOl),,, plane, is 0.21 f 0.02nm. The distance between the (OlO),, planes was measured to be 0.280 f 0.02nm instead of 0.288 run. The angle between the (00 1h and the (O04)Y_A)L0J lattice planes at four di&rent regions along the interface was measured to be
Figure
Vol.
33, No. 7
NiAVy-AI,O,INTERFACES
Figunz 2. Eledron diBia&m pattan oftk NWy-A&O, tion spots t%omthe oxide scale have been indexed
interhe.
1045
The relative orientation is the Bain relationship. Some of the dihc-
0 - 4iO.5 “. A region of a width of -10 (1 OOhti planes and next to the interface contains many dislocations, with the Burger’s vector perpendicular to the interface. Microtwins were observed in the y-A&O, which originated at interfacial steps (Figure 5). The twin boundaries are along (11 I),,~. Chemical Information
The normal mode EELS signalrevealed no nickel present in the oxide scale. For greater chemical sensitivity, EELS f%stdifFerencemode was used [ 19,201. NiO was used as the reference material. The oxygen 1st dif-
Fiye3.HREMimageofthe(O11~o~&scaleirderfacewlleretherelativeorientationrelatiooshipis:(OllX,//(ll),.y~and [loo], // [110]r-Y4
1046
Figure 4. HREM image oft& (OOl~oxide /I (001X.4% and [lOO]~// [110],.~4
NM/y-A&O,INTERFACES
Vol. 33, No. 7
scale inter&e where the relative orientation relationship is the Bain relationship: (OOlh
ference peaks of the oxide scale was matched to the standard, NiO. The magnification necessary to match the Ni peaks between the oxide scale and the standard, NiO, gave the Ni to 0 ratio. The 1st di&rence spectrum was taken at five different regions in the oxide scale using a box of size 18x25~1~. The Ni to 0 ratio was found to be in the range 0 to 0.01, with an average of 0.005. This signal may be due to a surface layer of nickel on the oxide scale due to ion milling. Therefore, an upper limit to the amount of nickel in the oxide scale is 1%.
Figure5,HREMimageoftheo~&scale~amicrotwinisobservedintheo~&scaleandnearaninterfacialstep. boundaries are along (Tl l),,,
Thetwin
Vol. 33, No. 7
NiAl/y-AI,o,
INTERFACES
1047
The NiAl was doped with a small amount of yttrium, 0.01 wt. %. For an understanding if yttrium intluences the growth of the oxide scale, EDS (energy dispersive X-ray spectroscopy) was used to detect the presence of Y. Y was not de&ted at the interface, with a calculated detection limit of 0.05 monolayers [2 11. We conclude that the small amount of Y in the sample plays no role during the short oxidation. Therefore, our results are similar to undoped NiAl. Discussion
Gur observations of the structure of the initial oxide scale is in excellent agreement with other investigators. Doychak et al. studied the oxide formation on (OOl)NiAl+Zr due to oxidation in air at 1073 “K and 0.1 hr by planar TEM techniques [ 111. They observed the formation of cubic alumina which was epitaxially oriented withthe underlying metal, i.e. (OOl),, // (OOl),,,,, and [lOO],, // [l lO],,,, @in orientation). Prussner et al. [6] had also noticed, by scanning electron microscopy (SEM), a transient oxide scale when a (1OO)NiA.l was oxidized at low oxygen activities (4x 10-14Pa)at 1223 “K for 0.1 hr. The (1OO)NiAlsurface was investigated by high resolution electron energy loss spectroscopy @REELS), low energy electron dithaction (LEED) and auger electron spectroscopy @ES) by Gassmann et al. [ 141. In reasonable agreement to our studies, they concluded that in the temperature range, 700- 1200”K, the initial oxide scale is y -Al,O, which has the Bain orientation with respect to the NiAl. Jaeger et al. [ 131 studied the formation of a well-ordered oxide on (1lO)NiAl by electron spectroscopic techniques. They also observed y-A&O, with the same relative orientation relationship. Rilhle et al. examined the oxidation behavior of another N&Al system, N&Al, by high resohttion electron microscopy (HREM) [22]. They also noticed y-A&O, formed epitaxially with respect to the underlying substrate. Contlicting data exist in the litemture concemmg the chemistry of the oxide scale. Doychak et al. detected Ni in the oxide scale with EDS (energy dispersive x-ray spectroscopy) and concluded that the oxide scale consisted of N&&O4and y-&O, in solid solution [ 11. However, Jaeger et al. believed that there is no indication of NiA&O,near the intertace [ 131. Young et al. observed, by XPS (X-ray photoelectron spectroscopy) that above an oxidation temperature of 750°K a transient form of A&O, formed with 0.5 at% Ni in solution [8]. Our results of the oxide scale containing ~1% Ni is in agreement with most other investigator’s results. The microstructure near and at the metal/oxide interface provide insight into the inter-facial strength and possible modes of relieving the inter-facialstress. For example, the formation of voids is clearly detrimental to the adhesion between the metal and oxide. We observed -200mn faceted voids atter only 0.1 hr oxidation in air, which is in agreement with the observations of Pr-ussner et al. of NiAl oxidized at low oxygen activities at 1223 “K for 0. lhr [6]. Doychak et al. had noticed -0.2mm faceted voids where the void planes are also along (110) after the NiAl was oxidized in air at 1373 “K for 1 hr [ 121. These observations suggest that voids form rapidly and grow during the initial oxidation process. A possible method to increase the inter-facial strength is to reduce void formation, such as by reactive element additions [lo]. The coherency and defect structure of the interface will signiticantly intluence the inter-facial strength. The (0Ol)NiAl interface was observed to be mostly rough and coherent. We also noted that the lattice parameter is reduced by 3% near the interface. The lattice parameter of NiAl is dependent on the composition [23]. However, for a 10% increaseintherliCkelcom;enhrltion, the change in lattice parameter is only 0.7%. Bobeth et al. measured the NiAl to be slightlynickel rich, 2-3%, close to the interface [24]. Hence, it is not expected that the lattice parameter changes due to nickel enrichment near the interface. We conclude that the NiAl is under compression near the interf’. The coherency and the compression of the (00 1)NiAl near the interface suggest that the NiAl/Al,O, interface is extremely strong, and may help explain why NiAl is an excellent oxidation-r-e&ant coating material. The presence of microtwins in the oxide scale near interfacial steps suggests that a method for relieving interfacial stress is by twinning in the oxide. A 3nm region in the NiAl close to the interface, where them existed many dislocations, was also observed. Clearly this region will play a role in the inter-facial strength, but its role is not yet understood and needs further investigations.
1048
NiAlly-A&O, INTERFACES
Vol. 33, No. 7
Conclusion
For one of the tirst times, an external oxide scale which formed due to high temperature oxidation has been studied by cross-sectional TEM techniques, including HREM. The interfaces between single crystal (00 1hti and the oxide scale formed at 1223 “K in 0.1 hr were investigated. The following were observed: 1. Approximately 2OOnmwide voids with facets along (OOl),, and (01 l)EliA1 were observed. A few of the facets contained an oxide scale. 2. Cubic alumina formed and was epitaxial with the NiAl with the Bain relationship: (OOl),, // (OO1),_Az% The interface was mostly rough with some preferential faceting along and [lOOI,, // [l lOI,,,. The interface is coherent. [OOl]-and [Ol l]-. 3. The (1 IO)& oxide scale interface was planar at the atomic scale. The relative orientation is: (01 l)m /I(1 l),,,,,and NL,, iI1 10lr_,,,zq.No misifit dislocations were observed at or near the interface. 4. Many defects were observed in the Nii near the NiAl/oxide scale interface. Microtwins in the y -Al,O, were observed at interfacial steps. Acknowledrment
This work was supported by Deutsche Forschuugsgemeinschaft (DFG). JY gratefully acknowledges the National Science Foundatiou (NSF) for support The authors thank Ute Salzberger for the excellent TEM samples and Frank Ernst for useful discussions on the high resolution microscopy of y -Al,O,. The experimental assistance of Hui Gu and Harald Mullejans with the analytical electron microscopy is appreciated. Ram Darolia and Jon C. Schaefer (Geueral Electrics) kiudly provided the single crystal NiAl. Reference 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
T. N. Rhys-Jones, Corrosion Science 29 623 (1989). M. W. Brumm and H. J. Grabke, Corros. Sci. 33 1677 (1992). H. M. Hindam and W. W. Smelker, J. Ele&o&em. SIX. 127 1630 (1980). J. L. Smialek and R G&ala, Met. Trans. A 14 A 2143 (1983). E. Schumann and M. RtIhle, Acta metall. mater. 42 1481(1994). K Pri&er, J. Bndey, U. Salzberger, H. Zweygatt, E. Schumann and M. Riihle, Proc. 2nd Int. Conf. on Micmscopy of Oxidation 435 (1993). E. Schumann, oxidation of Metals 43 157 (1995). E. W. A Young, J. C. Riviere and L. S. Welch, Applied Surface Science 28 71(1987). E. W. A. Young, J. C. Riviere and L S. Welch, Applied Surface Science 31370 (1988). H. J. Grabke, D. Wiemer and H. Viefhauq Applied Surf. Sci. 47 243 (1991). J. Doychak, J. L. Smialek and T. E. Mitchell, Met. Trans. A 20 A 499 (1989). J. Doychak and M. Rtihle, Gxid Met. 3143 l(1989). R M. Jaeger, H. Kuhlenbeck, H. Freud, M. Wurtig, W. Hoffmann R Franchy and H. Ibach, Surface Science 259 235 (1991). P. Gaamann, R Franchy and H. Iback, Surface Science 319 95 (1994). k Strecker, U. Salzberger and J. Mayer, Practical Metallography 30 482 (1993). F. Phillipp, R H&&en, M. Gsaki, G. M&us and M. ROhle, Ultramicroscopy 56 l(1994). G. Mabus, G. Necker and M. Rtlhle, Ultramicroscopy 49 46 (1991). J. C. Yang, Y. Lu and S. L. Sass, Mater& Science and Engineering Al62 97 (1993). R D. Leapmao and C. R Swyt, Ultramicroscopy 26 393 (1988). H. Shuman and k P. Somlyo, Ultramicroscopy 2123 (1987). E. Schumann, J. C. Yang M. Graham and M. Rllhle, Materials and Corrosion 46 2 18 (1995). M. Riihle, U. Salzberger and E. Schumann, Proc. 2nd. M. Conf. on Microcopy of Gxidation 3 (1993). RD.Noebe,RRBowman aud M. V. Nathal, Inter&ional Materials Review 38 193 (1993). M. Bobeth, E. Biihoe M. Rock&h, E. Sdumann and M. RUhle, submilted to Corrcsion Science.
Vol. 33, No. 7. pp.1043-1048,199s Elsevia Science Ltd cqyight 0 1995 Acta Mct.allllrgicaInc. PkkdintkUSA.AlIIi&&-d 09%716x195 $9.50 + .oo
Pergamon 0956-716X(9!5)00341-X
ELECTRON MICROSCOPY STUDIES OF NiAl/y-Al,O, INTERFACES J. C. Yang, K. Nadarzinski, E. Schumann and M. Riihle Max-Planck-Institut fur Metallforschung Institut fbr WerkstotRvissenschatt D-701 74 Stuttgart GERMANY (Received February 27,199s) Introduction
Intermetallics belong to a group of interesting materials which may find high temperature applications, such as jet turbine engines, due to their combination of desirable properties, such as low density, high melting &npen&m and high thermal conductivity. Their durability will depend on good corrosion resistance. NiAl is an intermetallic material which demon&ratesexcellent oxidation resistance due to the slow growth of A&O, on its surface during high temperature oxidation. Presently, NiAl is used as a coating material on superalloys because of its excellent oxidation resistance [ 11. A better under&ml& of the oxidation mechanisms requires knowledge of the structure and composition of the intermetallic,oxide scale and interfacial region during the oxidation process. There have been several mvestigationsof the altia scale growth on intermetallic materials using a wide variety of techniques, such as SEM, TEM, XPS, SIMS, etc. [2-71. However, there have been only a few investigations of the t?mdamental aspects of the initial oxide scale formation [S- 141. Obviously more work is needed to elucidate the structure ofNiAl and its initiallyformed oxide scale. With the improvement of electron microscopy sample preparation techniques, we can now examine the NiAl/oxide scale at the subnanometer level. Emerimental
Procedure
Single crystals of NiAl containing 0.01 wt. % Y was oriented with respect to the (001) pole within lo. The sample was sliced with a spark errosion cutter to dimensions of 2 mm x 20 mm and polished down to 1mm diamond paste. The sample was placed into a tube furnace at 1223°K for 0.1 hr in air. The cross-sectional TEM samples were prepared by a technique developed by Strecker et al [ 151. The oxidized samples were satldwichedtogether, embedded in brass, sliced, ground and polished to approximately 3mm diameter discs and 1OOmmthickness. They were ion-milled in a Bal-Tee precision ion miller at a low angle of 4Ountil perforation. CTEM investigationswere performed on a JEOL JEM 2OOOFX,analytical electron micricoscopy was performed on a dedicated VG HB501 STEM equipped with a Gatan PEELS detector, and HREM investigations were pe&rmed on a JEOL JEMARMl250 (atomic resolution microscope) operated at 1.25MV, which has a demonstrated resolution of 1A [ 161.
1043
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Nii+1203
INTERFACES
Vol. 33, No. 7
Figure1.~~~-fieldTEMhnageaftheNiAVy-~~O,interface~theN~w~oxidized~~~95OaCfor6~U~. Many faceted voids are observedat the interface.
Results Structural Observations
1 represents a bright-field TEM micrograph of a cross-section of the N&l/oxide scale interface a&r the NiAl was oxidized in air at 1223“K for 0.1 hr. The oxide scale was observed to be a 40-50 nm thick layer all along the interface. The scale contains many defects, possibly dislocations and/or twin boundaries. Faceted 200-300 nm voids and facets were observed to form along [OO1],ti and [Ol llNiAlinterfaces. Figure 2 is an electron ditbaction pattern taken from the NiAl/oxide scale interface. The dominant spots are due to (100) NiAl. Extra spots can be indexed as (110) y-A&O,. Some of the dithaction spots from the oxide scale have been indexed in figure 2. The presence of long facets and the excellent epitaxy between the NiAl and oxide scale suggested high resolution investigations. Figure 3 is a HREM micrograph of the interface between (0 11&M and the oxide scale. The two crystals have an excellent relative orientation since all of the lattice planes, including the 0.139nm (T4O),,,Q, are resolved. The following orientation relationship could be revealed: (0 1l),, // and [lOO],A // [llOIY_AIzq.The interface is atomically flat. Both the y-A&O, and NiAl appear 01 l),,, defect free. The lattice mismatch between the (lT2),,,,, and (01 l& is calculated to be 17%. However, no misfits or stand-off distance were observed in this interface. Figure 4 is a HREM micrograph of the NiAl/oxi& scale interface where the interfacial plane is parallel to (00 1h. The noise in the image has been reduced by a Fourier tilter by adapting a new method suggested by Mobus ef al. [ 171. There is some slight preferred faceting along (001 ),, and (0 1 l)NiA1, but, on average, this interface is observed to be rough The lattice mismatch was measured by counting the number of (020),, lattice fringes and the @40),_,+9 lattice tiinges for the same distance and found to be 4%, which is close to the calculated lattice mismatch of 3%. In contrast to the (01 l>NiAI interface, the interface here is observed to be coherent, except for one monolayer at the interface. It has been previously observed that the relative geometry between the metal and oxide can significantly alter the interfacial structure [ 181. The interfacial width, measured t+om the last (004) I _AZ%plane to the first (OOl),,, plane, is 0.21 f 0.02nm. The distance between the (OlO),, planes was measured to be 0.280 f 0.02nm instead of 0.288 run. The angle between the (00 1h and the (O04)Y_A)L0J lattice planes at four di&rent regions along the interface was measured to be
Figure
Vol.
33, No. 7
NiAVy-AI,O,INTERFACES
Figunz 2. Eledron diBia&m pattan oftk NWy-A&O, tion spots t%omthe oxide scale have been indexed
interhe.
1045
The relative orientation is the Bain relationship. Some of the dihc-
0 - 4iO.5 “. A region of a width of -10 (1 OOhti planes and next to the interface contains many dislocations, with the Burger’s vector perpendicular to the interface. Microtwins were observed in the y-A&O, which originated at interfacial steps (Figure 5). The twin boundaries are along (11 I),,~. Chemical Information
The normal mode EELS signalrevealed no nickel present in the oxide scale. For greater chemical sensitivity, EELS f%stdifFerencemode was used [ 19,201. NiO was used as the reference material. The oxygen 1st dif-
Fiye3.HREMimageofthe(O11~o~&scaleirderfacewlleretherelativeorientationrelatiooshipis:(OllX,//(ll),.y~and [loo], // [110]r-Y4
1046
Figure 4. HREM image oft& (OOl~oxide /I (001X.4% and [lOO]~// [110],.~4
NM/y-A&O,INTERFACES
Vol. 33, No. 7
scale inter&e where the relative orientation relationship is the Bain relationship: (OOlh
ference peaks of the oxide scale was matched to the standard, NiO. The magnification necessary to match the Ni peaks between the oxide scale and the standard, NiO, gave the Ni to 0 ratio. The 1st di&rence spectrum was taken at five different regions in the oxide scale using a box of size 18x25~1~. The Ni to 0 ratio was found to be in the range 0 to 0.01, with an average of 0.005. This signal may be due to a surface layer of nickel on the oxide scale due to ion milling. Therefore, an upper limit to the amount of nickel in the oxide scale is 1%.
Figure5,HREMimageoftheo~&scale~amicrotwinisobservedintheo~&scaleandnearaninterfacialstep. boundaries are along (Tl l),,,
Thetwin
Vol. 33, No. 7
NiAl/y-AI,o,
INTERFACES
1047
The NiAl was doped with a small amount of yttrium, 0.01 wt. %. For an understanding if yttrium intluences the growth of the oxide scale, EDS (energy dispersive X-ray spectroscopy) was used to detect the presence of Y. Y was not de&ted at the interface, with a calculated detection limit of 0.05 monolayers [2 11. We conclude that the small amount of Y in the sample plays no role during the short oxidation. Therefore, our results are similar to undoped NiAl. Discussion
Gur observations of the structure of the initial oxide scale is in excellent agreement with other investigators. Doychak et al. studied the oxide formation on (OOl)NiAl+Zr due to oxidation in air at 1073 “K and 0.1 hr by planar TEM techniques [ 111. They observed the formation of cubic alumina which was epitaxially oriented withthe underlying metal, i.e. (OOl),, // (OOl),,,,, and [lOO],, // [l lO],,,, @in orientation). Prussner et al. [6] had also noticed, by scanning electron microscopy (SEM), a transient oxide scale when a (1OO)NiA.l was oxidized at low oxygen activities (4x 10-14Pa)at 1223 “K for 0.1 hr. The (1OO)NiAlsurface was investigated by high resolution electron energy loss spectroscopy @REELS), low energy electron dithaction (LEED) and auger electron spectroscopy @ES) by Gassmann et al. [ 141. In reasonable agreement to our studies, they concluded that in the temperature range, 700- 1200”K, the initial oxide scale is y -Al,O, which has the Bain orientation with respect to the NiAl. Jaeger et al. [ 131 studied the formation of a well-ordered oxide on (1lO)NiAl by electron spectroscopic techniques. They also observed y-A&O, with the same relative orientation relationship. Rilhle et al. examined the oxidation behavior of another N&Al system, N&Al, by high resohttion electron microscopy (HREM) [22]. They also noticed y-A&O, formed epitaxially with respect to the underlying substrate. Contlicting data exist in the litemture concemmg the chemistry of the oxide scale. Doychak et al. detected Ni in the oxide scale with EDS (energy dispersive x-ray spectroscopy) and concluded that the oxide scale consisted of N&&O4and y-&O, in solid solution [ 11. However, Jaeger et al. believed that there is no indication of NiA&O,near the intertace [ 131. Young et al. observed, by XPS (X-ray photoelectron spectroscopy) that above an oxidation temperature of 750°K a transient form of A&O, formed with 0.5 at% Ni in solution [8]. Our results of the oxide scale containing ~1% Ni is in agreement with most other investigator’s results. The microstructure near and at the metal/oxide interface provide insight into the inter-facial strength and possible modes of relieving the inter-facialstress. For example, the formation of voids is clearly detrimental to the adhesion between the metal and oxide. We observed -200mn faceted voids atter only 0.1 hr oxidation in air, which is in agreement with the observations of Pr-ussner et al. of NiAl oxidized at low oxygen activities at 1223 “K for 0. lhr [6]. Doychak et al. had noticed -0.2mm faceted voids where the void planes are also along (110) after the NiAl was oxidized in air at 1373 “K for 1 hr [ 121. These observations suggest that voids form rapidly and grow during the initial oxidation process. A possible method to increase the inter-facial strength is to reduce void formation, such as by reactive element additions [lo]. The coherency and defect structure of the interface will signiticantly intluence the inter-facial strength. The (0Ol)NiAl interface was observed to be mostly rough and coherent. We also noted that the lattice parameter is reduced by 3% near the interface. The lattice parameter of NiAl is dependent on the composition [23]. However, for a 10% increaseintherliCkelcom;enhrltion, the change in lattice parameter is only 0.7%. Bobeth et al. measured the NiAl to be slightlynickel rich, 2-3%, close to the interface [24]. Hence, it is not expected that the lattice parameter changes due to nickel enrichment near the interface. We conclude that the NiAl is under compression near the interf’. The coherency and the compression of the (00 1)NiAl near the interface suggest that the NiAl/Al,O, interface is extremely strong, and may help explain why NiAl is an excellent oxidation-r-e&ant coating material. The presence of microtwins in the oxide scale near interfacial steps suggests that a method for relieving interfacial stress is by twinning in the oxide. A 3nm region in the NiAl close to the interface, where them existed many dislocations, was also observed. Clearly this region will play a role in the inter-facial strength, but its role is not yet understood and needs further investigations.
1048
NiAlly-A&O, INTERFACES
Vol. 33, No. 7
Conclusion
For one of the tirst times, an external oxide scale which formed due to high temperature oxidation has been studied by cross-sectional TEM techniques, including HREM. The interfaces between single crystal (00 1hti and the oxide scale formed at 1223 “K in 0.1 hr were investigated. The following were observed: 1. Approximately 2OOnmwide voids with facets along (OOl),, and (01 l)EliA1 were observed. A few of the facets contained an oxide scale. 2. Cubic alumina formed and was epitaxial with the NiAl with the Bain relationship: (OOl),, // (OO1),_Az% The interface was mostly rough with some preferential faceting along and [lOOI,, // [l lOI,,,. The interface is coherent. [OOl]-and [Ol l]-. 3. The (1 IO)& oxide scale interface was planar at the atomic scale. The relative orientation is: (01 l)m /I(1 l),,,,,and NL,, iI1 10lr_,,,zq.No misifit dislocations were observed at or near the interface. 4. Many defects were observed in the Nii near the NiAl/oxide scale interface. Microtwins in the y -Al,O, were observed at interfacial steps. Acknowledrment
This work was supported by Deutsche Forschuugsgemeinschaft (DFG). JY gratefully acknowledges the National Science Foundatiou (NSF) for support The authors thank Ute Salzberger for the excellent TEM samples and Frank Ernst for useful discussions on the high resolution microscopy of y -Al,O,. The experimental assistance of Hui Gu and Harald Mullejans with the analytical electron microscopy is appreciated. Ram Darolia and Jon C. Schaefer (Geueral Electrics) kiudly provided the single crystal NiAl. Reference 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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