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Simplified preparation of thin films and foils for TEM
Metallography
451
SHORT COMMUNICATIONS
Simplified Preparation of Thin Films and Foils for TEM G. HECKMUS Union Carbide European Research Associates s.a., Brussels, Belgium
The understanding of the oxidation mechanism of superalloys implies a complete characterization of (1) the external oxide layer; (2) the physical state of the metal-oxide interface; and (3) the precipitates formed by internal oxidation, as shown schematically in Fig. 1. The use of transmission electron microscopy could be of the greatest help if the difficulties associated with specimen preparation could be overcome. The effort to develop such experimental procedures is summarized in this report, and micrographs are shown to illustrate the type of information which has been obtained on the CoCrNi system. No electrolytic technique is available which permits the direct thinning of the oxide itself, so we chose to restrict our experiments strictly to those cases where the external oxide is thin enough to be observed directly after stripping (see also ref. 1). The external oxide layer is stripped by dissolving the metallic matrix at the alloy-oxide interface by electrolytic etching. The electrolyte consisted of a 3 : 1 mixture by volume of perchloric acid (d = 1.60) and acetic acid; it was used at room temperature, and the applied potential was 10 volts, z For each composition of the matrix the best conditions always coincided with those for good electrolytic polishing of the matrix. This point is of importance, as an incorrect etching treatment will generally induce many artifacts in the electron microscope image.
J
External oxide Precipitates formed by internal oxidation Intercrystalline precipitates formed by internal oxidation
Fro. 1. Schematic representation of external and internal oxides. Metallography, 3 (1970) 451-456
Copyright © 1970 by American Elsevier Publishing Company, Inc.
452
G. Heckmus
FIG. 2. Transmission micrograph of an oxide film stripped from the surface of a cobalt-base superalloy oxidized for 30 seconds in air at I I00°C. Both spinel and Cr203 phases are present. The growth of the oxide has been influenced by the alloy substrate and the oxide crystallites are preferentially oriented. Magnification 5000×. Very thin oxide films are conveniently reinforced by an evaporated layer of carbon. Figure 2 is an example of stripped oxide layers as observed in the microscope. Although it is generally recognized that the physical state of the metal-oxide interface is of importance in oxidation resistance, very little is known of its actual physical state. The method described in this paragraph permits a direct observation by transmission electron microscopy through a thin section of the alloy together with the external oxide. T h e method consists in protecting the "external" oxide with a lacquer and then creating a wedged section using the preferential dissolution which occurs on the edges of a droplet of lacquer deposited on the "internal" side of the specimen. The practical steps are as follows: 1. It is recommended to start with an oxidized sample of which the thickness does not exceed 0.I ram. In preparing the ribbons, cold rolling with frequent intermediate annealing is always preferred to hot rolling to avoid large inclusions. 2. The oxidation conditions must be carefully controlled in order to obtain thin (trasparent to electron) external oxide layers. 3. The "external" side as well as the edges of the specimen are completely
453
TEM of Foils and Films Lacomit // Detail A
View in section Lacquer ~ External oxide Wedged internal side Lacquer drop
External oxide side Matrix Internal side Lacomit
View in p l a n e ~
~
side a
Matrix side b
I
Samples protected by Lacomit lacquer drop
Side b Side b Side b Side b II III IV V FIG. 3. (I) The two sides, external (a) and internal (b), before electrolytic thinning. (II) First step of thinning occurring on the top of the window of side b. (III) Little drops of Lacomit lacquer are deposited on the thinning edge of side b. (IV) Second step of thinning. Progression of the thinning edge and obtaining the first series of samples protected by the Lacomit lacquer drops. (V) Final stage of thinning when the thinning front arrives at the lower edge of the window painted on side b. covered by painting with Lacomit lacquer (a proprietary product of W. Canning, Ltd., England) so that a large "window" is left unprotected on the internal side (Fig. 3, I). 4. The lacquer is allowed to dry at 65°C for 15 minutes in an oven. 5. The specimen is dipped into the electrolytic bath, and the dissolution is allowed to proceed until perforation along the top edge of the window occurs. At that point (Fig. 3, II) the specimen has a wedge shape and part of the edge is transparent to electrons. 6. The specimen is washed carefully in methanol and dried. 7. Little drops of Lacomit lacquer are painted along the wedged edge of the specimen and allowed to dry for 15 minutes under the same conditions as in step 4 (Fig. 3, III).
454
G. Heckmus
8. Electrolytic operation is repeated, and dissolution is allowed to proceed. The metallic region dissolves preferentially along the edges of the lacquer until the thin samples protected by each lacquer drop are electrically insulated and dissolution stops. Thin specimens (ready for observation) remain attached as shown in Fig. 3, III. 9. During step 8 a new wedged section forms along the dissolution front X X ' of Fig. 3, IV. Another series of lacquer drops is painted, and step 8 is repeated. 10. Figure 3, V, represents the system at the final step of preparation. The lacquer is dissolved in acetone, and the thin samples are washed several times in methanol. The method does not apply only to the oxide-metal interface. Actually the level of observation can easily be adjusted by dissolving the external side to a given depth before the protective lacquer is applied on the external side. In the case of thicker external oxide scales it is recommended (in order to obtain a smooth electrolytic dissolution) to eliminate the external scale by a light mechanical polishing before electrolytic dissolution. Figure 4 is an example of the metal-oxide interface in a cobalt-base superalloy oxidized for 30 seconds in air at 1100°C. A network of misfit dislocations is observed, some of which show extended nodes.
Fro. 4. Transmission electron microscopy of the metal-oxide interface in cobaltbase superalloy oxidized for 5 minutes at a pressure of 10-1~ torr. A network of misfit dislocations is observed with extended nodes. Magnification 23,500×.
TEM of Foils and Films
455
FIG. 5. Cobalt-base superalloy oxidized for 3 hours at I100°C under an atmosphere of air-argon mixture. Transmission electron micrograph of a thin section immediately underneath the metal-oxide interface. (The external oxide has been stripped off by electrolytic etching; see text.) Filamentary precipitates formed by internal oxidation are seen. It is not possible to say with any certainty whether the curved precipitates are still embedded in the metallic matrix. The dark spots seen on the micrograph are relies of external oxide which became globular under the influence of the electron beam. Magnification 21,000 x. Figure 5 is a micrograph taken at a level immediately underneath the m e t a l oxide interface. Numerous filamentary precipitates formed by internal oxidation are observed. This method is also of interest for other electrolytic thinning techniques because it protects the electron transmission specimens from mechanical deformation due to cutting. For other alloys or metals, the bath and the conditions of thinning must be determined in each case in order to apply the method described here. This work has been developed to study the oxidation mechanism in superalloys. T h e observations and their interpretation will be published in a future paper.
I should like to thank particularly Dr. H. Hatwell who has directed me toward electron microscopy and who has constantly advised me.
456
G. Heckmus
1. T. E. Bauer and R. H. Beauchamp, "Mechanical Thinning of Ceramic Materials for Transmission Electron and Optical Microscopy." AEC Research and Development Report (BNWL-652), 1968, Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Washington 99352. 2. P. Jacquet, Met. Rev., 1 (1956) 216.
Accepted May 19, 1970
451
SHORT COMMUNICATIONS
Simplified Preparation of Thin Films and Foils for TEM G. HECKMUS Union Carbide European Research Associates s.a., Brussels, Belgium
The understanding of the oxidation mechanism of superalloys implies a complete characterization of (1) the external oxide layer; (2) the physical state of the metal-oxide interface; and (3) the precipitates formed by internal oxidation, as shown schematically in Fig. 1. The use of transmission electron microscopy could be of the greatest help if the difficulties associated with specimen preparation could be overcome. The effort to develop such experimental procedures is summarized in this report, and micrographs are shown to illustrate the type of information which has been obtained on the CoCrNi system. No electrolytic technique is available which permits the direct thinning of the oxide itself, so we chose to restrict our experiments strictly to those cases where the external oxide is thin enough to be observed directly after stripping (see also ref. 1). The external oxide layer is stripped by dissolving the metallic matrix at the alloy-oxide interface by electrolytic etching. The electrolyte consisted of a 3 : 1 mixture by volume of perchloric acid (d = 1.60) and acetic acid; it was used at room temperature, and the applied potential was 10 volts, z For each composition of the matrix the best conditions always coincided with those for good electrolytic polishing of the matrix. This point is of importance, as an incorrect etching treatment will generally induce many artifacts in the electron microscope image.
J
External oxide Precipitates formed by internal oxidation Intercrystalline precipitates formed by internal oxidation
Fro. 1. Schematic representation of external and internal oxides. Metallography, 3 (1970) 451-456
Copyright © 1970 by American Elsevier Publishing Company, Inc.
452
G. Heckmus
FIG. 2. Transmission micrograph of an oxide film stripped from the surface of a cobalt-base superalloy oxidized for 30 seconds in air at I I00°C. Both spinel and Cr203 phases are present. The growth of the oxide has been influenced by the alloy substrate and the oxide crystallites are preferentially oriented. Magnification 5000×. Very thin oxide films are conveniently reinforced by an evaporated layer of carbon. Figure 2 is an example of stripped oxide layers as observed in the microscope. Although it is generally recognized that the physical state of the metal-oxide interface is of importance in oxidation resistance, very little is known of its actual physical state. The method described in this paragraph permits a direct observation by transmission electron microscopy through a thin section of the alloy together with the external oxide. T h e method consists in protecting the "external" oxide with a lacquer and then creating a wedged section using the preferential dissolution which occurs on the edges of a droplet of lacquer deposited on the "internal" side of the specimen. The practical steps are as follows: 1. It is recommended to start with an oxidized sample of which the thickness does not exceed 0.I ram. In preparing the ribbons, cold rolling with frequent intermediate annealing is always preferred to hot rolling to avoid large inclusions. 2. The oxidation conditions must be carefully controlled in order to obtain thin (trasparent to electron) external oxide layers. 3. The "external" side as well as the edges of the specimen are completely
453
TEM of Foils and Films Lacomit // Detail A
View in section Lacquer ~ External oxide Wedged internal side Lacquer drop
External oxide side Matrix Internal side Lacomit
View in p l a n e ~
~
side a
Matrix side b
I
Samples protected by Lacomit lacquer drop
Side b Side b Side b Side b II III IV V FIG. 3. (I) The two sides, external (a) and internal (b), before electrolytic thinning. (II) First step of thinning occurring on the top of the window of side b. (III) Little drops of Lacomit lacquer are deposited on the thinning edge of side b. (IV) Second step of thinning. Progression of the thinning edge and obtaining the first series of samples protected by the Lacomit lacquer drops. (V) Final stage of thinning when the thinning front arrives at the lower edge of the window painted on side b. covered by painting with Lacomit lacquer (a proprietary product of W. Canning, Ltd., England) so that a large "window" is left unprotected on the internal side (Fig. 3, I). 4. The lacquer is allowed to dry at 65°C for 15 minutes in an oven. 5. The specimen is dipped into the electrolytic bath, and the dissolution is allowed to proceed until perforation along the top edge of the window occurs. At that point (Fig. 3, II) the specimen has a wedge shape and part of the edge is transparent to electrons. 6. The specimen is washed carefully in methanol and dried. 7. Little drops of Lacomit lacquer are painted along the wedged edge of the specimen and allowed to dry for 15 minutes under the same conditions as in step 4 (Fig. 3, III).
454
G. Heckmus
8. Electrolytic operation is repeated, and dissolution is allowed to proceed. The metallic region dissolves preferentially along the edges of the lacquer until the thin samples protected by each lacquer drop are electrically insulated and dissolution stops. Thin specimens (ready for observation) remain attached as shown in Fig. 3, III. 9. During step 8 a new wedged section forms along the dissolution front X X ' of Fig. 3, IV. Another series of lacquer drops is painted, and step 8 is repeated. 10. Figure 3, V, represents the system at the final step of preparation. The lacquer is dissolved in acetone, and the thin samples are washed several times in methanol. The method does not apply only to the oxide-metal interface. Actually the level of observation can easily be adjusted by dissolving the external side to a given depth before the protective lacquer is applied on the external side. In the case of thicker external oxide scales it is recommended (in order to obtain a smooth electrolytic dissolution) to eliminate the external scale by a light mechanical polishing before electrolytic dissolution. Figure 4 is an example of the metal-oxide interface in a cobalt-base superalloy oxidized for 30 seconds in air at 1100°C. A network of misfit dislocations is observed, some of which show extended nodes.
Fro. 4. Transmission electron microscopy of the metal-oxide interface in cobaltbase superalloy oxidized for 5 minutes at a pressure of 10-1~ torr. A network of misfit dislocations is observed with extended nodes. Magnification 23,500×.
TEM of Foils and Films
455
FIG. 5. Cobalt-base superalloy oxidized for 3 hours at I100°C under an atmosphere of air-argon mixture. Transmission electron micrograph of a thin section immediately underneath the metal-oxide interface. (The external oxide has been stripped off by electrolytic etching; see text.) Filamentary precipitates formed by internal oxidation are seen. It is not possible to say with any certainty whether the curved precipitates are still embedded in the metallic matrix. The dark spots seen on the micrograph are relies of external oxide which became globular under the influence of the electron beam. Magnification 21,000 x. Figure 5 is a micrograph taken at a level immediately underneath the m e t a l oxide interface. Numerous filamentary precipitates formed by internal oxidation are observed. This method is also of interest for other electrolytic thinning techniques because it protects the electron transmission specimens from mechanical deformation due to cutting. For other alloys or metals, the bath and the conditions of thinning must be determined in each case in order to apply the method described here. This work has been developed to study the oxidation mechanism in superalloys. T h e observations and their interpretation will be published in a future paper.
I should like to thank particularly Dr. H. Hatwell who has directed me toward electron microscopy and who has constantly advised me.
456
G. Heckmus
1. T. E. Bauer and R. H. Beauchamp, "Mechanical Thinning of Ceramic Materials for Transmission Electron and Optical Microscopy." AEC Research and Development Report (BNWL-652), 1968, Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Washington 99352. 2. P. Jacquet, Met. Rev., 1 (1956) 216.
Accepted May 19, 1970