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Nb wires
Acta mater. 49 (2001) 389–394 www.elsevier.com/locate/actamat
SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES X. SAUVAGE1*, L. RENAUD1, B. DECONIHOUT1, D. BLAVETTE1, D. H. PING2 and K. HONO2 1
Groupe de Physique des Mate´riaux-UMR CNRS 6634, Faculte´ des Sciences, Universite´ de Rouen, 76821 Mont-Saint-Aignan cedex, France and 2National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan ( Received 2 August 2000; received in revised form 24 September 2000; accepted 3 October 2000 )
Abstract—The microstructure of cold drawn Cu/Nb nanocomposite wires was investigated using a three dimensional atom probe (3D-AP) and transmission electron microscopy (TEM). Although there is no solubility between Nb and Cu in the equilibrium state, atom probe analysis results revealed that intermixing occurs between Nb and Cu filaments as a result of cold drawing with a large strain. High resolution transmission electron microscopy (HRTEM) results revealed that an amorphous layer is formed along some Cu/Nb interfaces. This solid state amorphization is compared with similar reactions observed in Cu–Nb multilayers. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Cu/Nb nanocomposites; Cold working; Atom probe; Transmission electron microscopy (TEM)
1. INTRODUCTION
2. EXPERIMENTAL PROCEDURES
Cu/Nb nanocomposite wires have been studied by a number of authors [1–8]. These metal-matrix composites (MMC) exhibit very good mechanical properties while keeping a low electrical resistivity. That is why they were developed for electrical conducting wires of non-destructive magnets for high pulsed magnetic fields. Such wires can be processed by repeated drawing of an in-situ melted mixture of copper and niobium, or by the repeated drawing of aligned and continuous niobium fibres in a copper matrix and restacking [3]. During the plastic deformation, niobium dendrites or continuous fibres are elongated along the wire axis. Since the true strain is about 10, their average thickness decreases below 100 nm. This nanostructure leads to a very high tensile strength, up to 2 GPa [1–3,5–7]. In such wires, Raabe et al. [8] pointed out amorphous zones in niobium fibres containing up to 3.5 wt% copper. In this work, the three dimensional atom probe (3D-AP) technique [9] and high resolution transmission electron microscopy (HRTEM) were employed to clarify the link between amorphous zones and their nano-chemistry.
The Cu/Nb metal matrix composite wire produced by drawing a cast ingot of Cu–Nb18–Hf0.2 (%vol.) up to a true strain of 11. Materials were provided by the Bochvar All Russia Scientific Research Institute of Inorganic Materials. More details about the wire production process are given elsewhere [3]. The average width of the niobium fibres in the drawn wire was in a range of 10–60 nm. 3D-AP specimens were prepared along the wire axis with a standard electropolishing procedure: the wire was sharpened in a solution of 5% vol. HNO3⫹5% vol. H2SO4 in methanol, cooled to ⫺40°C, at 5 V d.c. Field Ion Microscopy (FIM) observations were performed at 40 K, using neon as an imaging gas. 3D-AP analyses were carried out on the new generation of tomographic atom probe (TAP) developed in the GPM (University of Rouen). The optical TAP (oTAP) [10] relies on a new positionsensitive detector based on a fast CCD camera. This allows a FIM image of the specimen to be obtained on the detector prior to an analysis. Moreover, the multi-hit capabilities of this detector enhance the reliability of composition data measurements. 3D-AP analyses were performed at 50 K with a 1.7 kHz pulse repetition rate and a pulse to standing voltage ratio of 20%. Transmission electron microscopy (TEM) and HRTEM specimens were prepared along two different directions in the wire: in the cross section
* To whom all correspondence should be addressed. Fax: ⫹33-002-35-14-66-52. E-mail address: [email protected] (X. Sauvage)
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 3 3 8 - 4
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 1. Bright field image of a longitudinal specimen showing the elongation of Cu and Nb grains along the wire axis.
(electron beam parallel to the wire axis) and along the wire axis (electron beam perpendicular to the wire axis). Because of the very small diameter of the wire (0.3 mm), the wire was nickel-coated and sliced with a saw to prepare cross section specimen. Then, conventional preparation techniques were used to make both longitudinal and cross sectional specimens, i.e. mechanical thinning and dimpling to a thickness of 20 µm and, finally, ion milling to reach electron transparency. Specimens were examined with a Philips CM200 microscope operating at 200 kV and with a JEOL JEM-4000EX microscope operating at 400 kV for high resolution observations. 3. RESULTS
3.1. TEM and HRTEM investigations Figure 1 shows a bright field image of a longitudinal specimen. As expected, copper and niobium grains have undergone a strong elongation along the wire axis. However, because of the heterogeneity of the original material, the filament width distribution appears quite large (in the range 10–100 nm). Figure 2 exhibits, with a higher magnification, an area where the thickness of niobium fibres is in the range 5–10 nm. Along copper–niobium interfaces, a periodic contrast appears, the result of residual stresses generated by misfit dislocations. The pres-
Fig. 2. Bright field image of a longitudinal specimen showing Nb fibres surrounded by copper.
Fig. 3. HRTEM image of a longitudinal specimen showing (111)Cu and (110)Nb planes. A partly semi-coherent Cu/Nb interface with misfit dislocations (indicated by arrows) is exhibited.
ence of such semi-coherent interfaces was reported previously [3, 7]. During the plastic deformation, copper grains develop a strong ⬍111> texture parallel to the wire axis and niobium grains a ⬍110> texture along the same direction. The lattice misfit between (111)Cu and (110)Nb planes leads to the formation of periodic misfit dislocations. Such misfit dislocations are displayed in the HRTEM image in Fig. 3. This picture shows a copper–niobium interface, where copper is located on the top of the picture and niobium on the bottom. (111)Cu and (110)Nb planes are clearly resolved. On the left of the picture, the interface is semi-coherent. On the right, however, the periodic contrast seems to disappear and a zone with an amorphous contrast is located at the interface. The HRTEM picture in Fig. 4 exhibits the interfaces on both sides of a niobium fibre. The right side is semi-coherent while the left side appears to be layered by an amorphous phase. The cross-sectional microstructure (Fig. 5) is much more complicated than along the wire axis: niobium fibres appear elongated and curled. Such a microstructure results from the non-axisymmetric elong-
Fig. 4. HRTEM image of a longitudinal specimen showing an Nb fibre surrounded by copper. The Cu/Nb interface on the right is semi-coherent (misfit dislocations are indicated by arrows), while on the left, the interface is layered by an amorphous phase.
SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 5. Bright field image of a cross-sectional specimen showing Nb fibres surrounded by Cu.
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such an amorphous phase at the interfaces of two niobium fibres in a cross-sectional specimen. The thickness of the amorphous layer is about 2 nm. It is worth noticing that such amorphous zones hardly appear in HRTEM on longitudinal specimens, perhaps because of the bending of niobium fibres exhibited in Fig. 5. In the cross-sectional HRTEM micrograph (Fig. 7), grain boundaries very close to each other are indicated by arrows in the niobium filaments. Such features, which have already been pointed out in previous FIM investigations [4], may result from large strains. During the axisymmetric drawing, the Nb bcc phase develops a strong single component ⬍110> fibre texture and a curled structure of interfolding ribbon-like fibres in the cross-section (see Fig. 5). The folding of Nb ribbons imposed a permanent storage of geometrically necessary dislocations [11]. The observed grain boundaries may therefore be the result of the accumulation of these geometrical necessary dislocations. 3.2. FIM and 3D-AP investigations
Fig. 6. SAED pattern of the area shown in Fig. 5.
ation of the Nb bcc phase and is usually reported as ‘Van Gogh Sky structure’ [11]. Close to the thinnest niobium filaments, some white zones are exhibited. These regions do not show diffraction contrast, they are attributed to the amorphous phase. This is further confirmed by the selected area diffraction pattern in Fig. 6 which exhibits a diffuse halo ring. The HRTEM micrograph in Fig. 7 clearly reveals
Fig. 7. HRTEM image of a cross-sectional specimen showing an amorphous phase located at Cu/Nb interfaces. Grain boundaries in Nb fibres are indicated by arrows.
The cross section of the Cu/Nb wire was observed using FIM (Fig. 8). Cu and Nb grains appear respectively as darkly and brightly imaged zones because of the higher evaporation field of Nb [4]. Note that the magnification of the FIM image from the Nb filament is larger than that from the Cu filament, because of the local magnification effect originating from the smaller curvature radius of high-field Nb regions. The cross-section of a 3D reconstructed volume involving a copper–niobium interface is exhibited in Fig. 9. The apparent width of the chemical interface is about 0.7 nm. Taking into account the lateral resolution of the apparatus (about 0.5 nm) [9], it seems clear that the interface is chemically abrupt. This analysis gives the evidence for the capability of the 3D-AP to exhibit sharp Cu/Nb interfaces, in spite of potential experimental artefacts such as local magni-
Fig. 8. FIM micrograph showing an Nb fibre surrounded by Cu; Cu/Nb interfaces are indicated by arrows.
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 11. 3D reconstruction of a small analysed volume (7.2⫻7.2⫻10 nm3). Only copper atoms are plotted. Sampling box: 0.4⫻3⫻5 nm3.
Fig. 9. Cross section of the 3D reconstruction of a small analysed volume (6⫻6⫻3 nm3). Only copper atoms are plotted to exhibit the Cu/Nb interface.
fication effects or ion trajectory aberrations which may occur near interfaces at the tip surface. The depth of the analysed volume shown in Fig. 9 is only about 3 nm. The volume of the Cu/Nb interface analysed is therefore too small to get a composition profile with reasonable statistical reliability across this interface. It is worth noticing that a very small amount of copper seems to be detected in the niobium fibre. It may be attributed to the slight overlap of Nb3⫹ and Cu2⫹ peaks in the TAP mass spectra [4]. The FIM micrograph in Fig. 10 exhibits the crosssection of a region of the wire where the microstructure is very similar to that in Fig. 7. The dark lamellae indicated by arrows are thought to be copper and the bright regions which exhibit a concentric ring contrast are attributed to crystalline niobium. In this lamellar region, the width of Cu and Nb channels is only about a few nanometers. This specimen was analysed with
Fig. 10. FIM micrograph showing thin copper channels (dark zones) surrounded by niobium fibres (bright zones). Copper channels are indicated by arrows.
the 3D-AP in a darkly imaged zone, between two crystalline Nb lamellae. The analysis area is delimited by the white square displayed in Fig. 10. Figure 11 shows the 3D reconstruction of the analysed volume. In this picture, only copper atoms are plotted. Between two niobium rich zones, a thin copper lamella is observed along the wire axis. The concentration depth profiles in Fig. 12 was obtained in the perpendicular direction to the lamella (as indicated by the arrow in Fig. 11), where the small inset box indicates the sampling box for calculating a local composition. The copper and niobium concentrations are plotted as a function of the sampling box position in Fig. 12a. The concentration of copper detected in the niobium rich zones is about 12 at.%. On the other hand, the 3D-AP data indicate that the copper lath contains only 30 at.% copper. So, this analysis reveals first that darkly imaged zones in the FIM micrograph are not pure copper zones. On the other hand, in the
Fig. 12. (a) Composition profile across the copper channel from Fig. 11. (b) Number of atoms in the sampling box as a function of the profile depth.
SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
regions where the microstructure is finest, interdiffusion of Cu and Nb has occurred. Because of the large difference between the evaporation fields of copper and niobium, local magnification effects and trajectory overlaps are likely to occur. Indeed, the evaporation field of Cu atoms is much lower than the evaporation field of Nb. On the tip surface, Cu-rich zones therefore exhibit a larger curvature radius than Nb-rich zones. Consequently, atoms coming from Cu-rich zones are more convergent, i.e. the magnification is lower and the local density is higher in the 3D reconstructed volume. The total number of atoms contained in the sampling box is plotted as a function of box position in Fig. 12b. As expected, this profile reveals that the local density of the 3D reconstructed volume is not constant: it is almost four times higher in the copper-rich lath. Another interesting feature appears: the number of niobium atoms in the sampling box is three times larger in the copper-rich channel than in the other part of the reconstructed volume. Consequently, most of the niobium atoms detected in the copper-rich lath should not be the result of overlaps resulting from the large difference between the curvature radius of Curich and Nb-rich zones. Moreover, the percentage of copper (vs. niobium) reaches a steady state in the copper-rich lath, contrary to the number of copper (vs. niobium) atoms in the sampling box. It is interesting to notice that the number of niobium (vs. copper) atoms starts to increase before any composition change (as indicated by dotted lines). These two features seem to indicate that the curvature radius of the analysed tip increases continuously from the niobiumto the copper-rich zones. Local magnification effects seem, therefore, not to affect the estimated compositions in the copper-rich channel. These local magnification effects, however, give rise to errors in the scale of the 3D elemental mapping. For instance, low-field Cu-rich regions in which evaporated ions are more convergent, are narrowed, contrary to high-field Nb-rich regions which are enlarged. The apparent width of the copper-rich channel (about 1 nm) revealed by the composition profile in Fig. 12a is therefore underestimated. A corrected composition profile (Fig. 13) was computed based on a sampling box similar to that shown in Fig. 12. To
Fig. 13. Corrected composition profile across the copper-rich channel in Fig. 9.
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correct the local magnification effects, the depth of the profile was weighted by the local density. The following procedure details this correction. The sampling box was translated, step by step, of an elemental distance ⌬X. At each step, the Cu and Nb concentrations were computed. Then, the corrected depth of the profile at the ith step Xi (X0 ⫽ 0, beginning of the profile) was weighted by the atomic density in the sampling box at the ith step Di, using the following equation: Xi ⫽ Xi⫺1 ⫹ ⌬X⫻Di/D where D is the average density of the whole reconstructed volume. Using this correction, the copper-rich lath is estimated to be about 2 nm wide (Fig. 13). 4. DISCUSSION
HRTEM and FIM pictures in Figs 7 and 10 exhibit two regions of the wire where the microstructure is very similar. This lamellar nanostructure was analysed using a 3D-AP (Fig. 11) and an unexpected intermixing of Cu and Nb was observed. At the equilibrium, there is no mutual miscibility of Cu and Nb whatever the temperature [12]. It is therefore reasonable to assume that amorphous zones observed in the HRTEM micrographs (Fig. 7) are the result of intermixing of Cu and Nb induced by severe plastic deformation. In such Cu/Nb nanocomposite wires, amorphous areas containing up to 3.5 wt% Cu have already been reported in Nb fibres by other authors [8]. The results presented in this paper clearly demonstrate, however, that this solid state amorphization mostly occurs at Cu/Nb interfaces and in the regions of the wire where the microstructure is finest. 3D-AP data revealed a Cu/Nb sharp interface (Fig. 9) in a region where the cross-sectional grain size was coarser (Fig. 8). Such a sharp interface may be attributed to a semi-coherent Cu/Nb interface exhibited in HRTEM micrographs Figs 3 and 4. Atzmon et al. [13] reported such a solid state amorphization resulting from severe plastic deformation in cold-rolled Cu–Er and Ni–Er. They assumed that strains play a role in enhancing the diffusion rates, thus allowing reaction at near room temperature. But, they have also investigated a cold-rolled Cu–Nb nanocomposite which did not transform into the amorphous phase during plastic deformation. The microstructure, shown in Figs 7 and 10, looks like a Cu/Nb multilayer. Pan et al. [14] studied vacuum-deposited Cu/Nb multilayers with nanoscale thickness and demonstrated that nanoscale layers may transform into the amorphous state through solid state diffusion during heat treatment at 250°C. The extremely high interfacial energy of the system, combined with a negative Cu/Nb enthalpy of mixing, is
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
the driving force for this solid state amorphization. And, because Cu and Nb atom mobilities increase at elevated temperatures, kinetics of reaction is promoted by the heat treatment. A similar phenomenon may have occurred locally in the Cu/Nb nanocomposite wire. The temperature remains below 50°C during the plastic deformation, but the drawing process includes intermediate annealing treatments at 300°C. It is therefore reasonable to assume that some regions of the wire may have a nanoscaled structure before the last annealing treatment, so that solid state amorphization occurs locally during the last intermediate heat treatment. The amorphous phase may then remain in the nanocomposite wire up to the last drawing stage. 5. CONCLUSIONS
HRTEM clearly revealed two kinds of Cu/Nb interfaces. Most of them appeared semi-coherent, while in regions of the wire where the microstructure is finest, an amorphous layer appeared between Cu and Nb. This solid state amorphization was correlated to a strong interdiffusion of Cu and Nb revealed by 3DAP data. This phenomenon was compared with amorphization in multilayer systems and in cold rolled Cu– Er composites. Finally, it turns out that the amorphization in the finest regions of the Cu/Nb nanocomposite wires may have occurred during the last intermediate annealing treatment included in the drawing process. Acknowledgements—The authors would like to thank the Bochvar All Russia Scientific Research Institute of Inorganic
Materials for providing materials. The authors are indebted to S. Aske´nazy, L. Thilly and F. Lecouturier from the Service National des Champ Magne´tiques Pulse´s (Toulouse, France) for the fruitful discussions we have had on the present results.
REFERENCES 1. Bevk, J., Harbison, J. P. and Bell, J. L., J. Appl. Phys., 1978, 49, 6031. 2. Raabe, D. and Mattissen, D., Acta metall., 1998, 46, 5973. 3. Snoeck, E., Lecouturier, F., Thilly, L., Casanove, M. J., Rakoto, H., Coffe, G., Aske´nazy, S., Peyrade, J. P., Roucau, C., Pantsyrny, V., Shikov, A. and Nikulin, A., Scr. metall., 1998, 38, 1643. 4. Sauvage, X., Thilly, L., Lecouturier, F., Guillet, A. and Blavette, D., Nanostruct. Mater., 2000, 8, 1031. 5. Spitzig, W. A., Pelton, A. R. and Laabs, F. C., Acta metall., 1987, 35, 2427. 6. Hong, S. I., Hill, M. A., Sakai, Y., Wood, J. T. and Embury, J. D., Acta metall., 1995, 43, 3313. 7. Dupouy, F., Snoeck, E., Casanove, M. J., Roucau, C., Peyrade, J. P. and Askenazy, S., Scr. metall., 1996, 34, 1067. 8. Raabe, D., Heringhaus, F., Hangen, U. and Gottstein, G. Z., Metallkd., 1995, 86, 405. 9. Blavette, D., Deconihout, B., Bostel, A., Sarrau, J. M., Bouet, M. and Menand, A., Rev. Sci. Instrum., 1993, 64, 2911. 10. Deconihout, B., Renaud, L., Da Costa, G., Bouet, M., Bostel, A. and Blavette, D., Ultramicroscopy, 1998, 73, 253. 11. Gil Sevillano, J. and Flaquer Fuster, J., in Ceramic and Metal Matrix Composites Proceedings, Key Engineering Materials, 1997, pp. 123, 127. 12. Massalski, T.B. Binary alloy phase diagrams, edited by ASM, 1986, p. 936. 13. Atzmon, M., Unruh, K. M. and Johnson, W. L., J. Appl. Phys., 1985, 58, 3865. 14. Pan, F., Ling, Z. F., Gao, K. Y. and Liu, B. X., Mater. Res. Soc. Symp. Proc., 1996, 398, 337.
SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES X. SAUVAGE1*, L. RENAUD1, B. DECONIHOUT1, D. BLAVETTE1, D. H. PING2 and K. HONO2 1
Groupe de Physique des Mate´riaux-UMR CNRS 6634, Faculte´ des Sciences, Universite´ de Rouen, 76821 Mont-Saint-Aignan cedex, France and 2National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan ( Received 2 August 2000; received in revised form 24 September 2000; accepted 3 October 2000 )
Abstract—The microstructure of cold drawn Cu/Nb nanocomposite wires was investigated using a three dimensional atom probe (3D-AP) and transmission electron microscopy (TEM). Although there is no solubility between Nb and Cu in the equilibrium state, atom probe analysis results revealed that intermixing occurs between Nb and Cu filaments as a result of cold drawing with a large strain. High resolution transmission electron microscopy (HRTEM) results revealed that an amorphous layer is formed along some Cu/Nb interfaces. This solid state amorphization is compared with similar reactions observed in Cu–Nb multilayers. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Cu/Nb nanocomposites; Cold working; Atom probe; Transmission electron microscopy (TEM)
1. INTRODUCTION
2. EXPERIMENTAL PROCEDURES
Cu/Nb nanocomposite wires have been studied by a number of authors [1–8]. These metal-matrix composites (MMC) exhibit very good mechanical properties while keeping a low electrical resistivity. That is why they were developed for electrical conducting wires of non-destructive magnets for high pulsed magnetic fields. Such wires can be processed by repeated drawing of an in-situ melted mixture of copper and niobium, or by the repeated drawing of aligned and continuous niobium fibres in a copper matrix and restacking [3]. During the plastic deformation, niobium dendrites or continuous fibres are elongated along the wire axis. Since the true strain is about 10, their average thickness decreases below 100 nm. This nanostructure leads to a very high tensile strength, up to 2 GPa [1–3,5–7]. In such wires, Raabe et al. [8] pointed out amorphous zones in niobium fibres containing up to 3.5 wt% copper. In this work, the three dimensional atom probe (3D-AP) technique [9] and high resolution transmission electron microscopy (HRTEM) were employed to clarify the link between amorphous zones and their nano-chemistry.
The Cu/Nb metal matrix composite wire produced by drawing a cast ingot of Cu–Nb18–Hf0.2 (%vol.) up to a true strain of 11. Materials were provided by the Bochvar All Russia Scientific Research Institute of Inorganic Materials. More details about the wire production process are given elsewhere [3]. The average width of the niobium fibres in the drawn wire was in a range of 10–60 nm. 3D-AP specimens were prepared along the wire axis with a standard electropolishing procedure: the wire was sharpened in a solution of 5% vol. HNO3⫹5% vol. H2SO4 in methanol, cooled to ⫺40°C, at 5 V d.c. Field Ion Microscopy (FIM) observations were performed at 40 K, using neon as an imaging gas. 3D-AP analyses were carried out on the new generation of tomographic atom probe (TAP) developed in the GPM (University of Rouen). The optical TAP (oTAP) [10] relies on a new positionsensitive detector based on a fast CCD camera. This allows a FIM image of the specimen to be obtained on the detector prior to an analysis. Moreover, the multi-hit capabilities of this detector enhance the reliability of composition data measurements. 3D-AP analyses were performed at 50 K with a 1.7 kHz pulse repetition rate and a pulse to standing voltage ratio of 20%. Transmission electron microscopy (TEM) and HRTEM specimens were prepared along two different directions in the wire: in the cross section
* To whom all correspondence should be addressed. Fax: ⫹33-002-35-14-66-52. E-mail address: [email protected] (X. Sauvage)
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 0 ) 0 0 3 3 8 - 4
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 1. Bright field image of a longitudinal specimen showing the elongation of Cu and Nb grains along the wire axis.
(electron beam parallel to the wire axis) and along the wire axis (electron beam perpendicular to the wire axis). Because of the very small diameter of the wire (0.3 mm), the wire was nickel-coated and sliced with a saw to prepare cross section specimen. Then, conventional preparation techniques were used to make both longitudinal and cross sectional specimens, i.e. mechanical thinning and dimpling to a thickness of 20 µm and, finally, ion milling to reach electron transparency. Specimens were examined with a Philips CM200 microscope operating at 200 kV and with a JEOL JEM-4000EX microscope operating at 400 kV for high resolution observations. 3. RESULTS
3.1. TEM and HRTEM investigations Figure 1 shows a bright field image of a longitudinal specimen. As expected, copper and niobium grains have undergone a strong elongation along the wire axis. However, because of the heterogeneity of the original material, the filament width distribution appears quite large (in the range 10–100 nm). Figure 2 exhibits, with a higher magnification, an area where the thickness of niobium fibres is in the range 5–10 nm. Along copper–niobium interfaces, a periodic contrast appears, the result of residual stresses generated by misfit dislocations. The pres-
Fig. 2. Bright field image of a longitudinal specimen showing Nb fibres surrounded by copper.
Fig. 3. HRTEM image of a longitudinal specimen showing (111)Cu and (110)Nb planes. A partly semi-coherent Cu/Nb interface with misfit dislocations (indicated by arrows) is exhibited.
ence of such semi-coherent interfaces was reported previously [3, 7]. During the plastic deformation, copper grains develop a strong ⬍111> texture parallel to the wire axis and niobium grains a ⬍110> texture along the same direction. The lattice misfit between (111)Cu and (110)Nb planes leads to the formation of periodic misfit dislocations. Such misfit dislocations are displayed in the HRTEM image in Fig. 3. This picture shows a copper–niobium interface, where copper is located on the top of the picture and niobium on the bottom. (111)Cu and (110)Nb planes are clearly resolved. On the left of the picture, the interface is semi-coherent. On the right, however, the periodic contrast seems to disappear and a zone with an amorphous contrast is located at the interface. The HRTEM picture in Fig. 4 exhibits the interfaces on both sides of a niobium fibre. The right side is semi-coherent while the left side appears to be layered by an amorphous phase. The cross-sectional microstructure (Fig. 5) is much more complicated than along the wire axis: niobium fibres appear elongated and curled. Such a microstructure results from the non-axisymmetric elong-
Fig. 4. HRTEM image of a longitudinal specimen showing an Nb fibre surrounded by copper. The Cu/Nb interface on the right is semi-coherent (misfit dislocations are indicated by arrows), while on the left, the interface is layered by an amorphous phase.
SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 5. Bright field image of a cross-sectional specimen showing Nb fibres surrounded by Cu.
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such an amorphous phase at the interfaces of two niobium fibres in a cross-sectional specimen. The thickness of the amorphous layer is about 2 nm. It is worth noticing that such amorphous zones hardly appear in HRTEM on longitudinal specimens, perhaps because of the bending of niobium fibres exhibited in Fig. 5. In the cross-sectional HRTEM micrograph (Fig. 7), grain boundaries very close to each other are indicated by arrows in the niobium filaments. Such features, which have already been pointed out in previous FIM investigations [4], may result from large strains. During the axisymmetric drawing, the Nb bcc phase develops a strong single component ⬍110> fibre texture and a curled structure of interfolding ribbon-like fibres in the cross-section (see Fig. 5). The folding of Nb ribbons imposed a permanent storage of geometrically necessary dislocations [11]. The observed grain boundaries may therefore be the result of the accumulation of these geometrical necessary dislocations. 3.2. FIM and 3D-AP investigations
Fig. 6. SAED pattern of the area shown in Fig. 5.
ation of the Nb bcc phase and is usually reported as ‘Van Gogh Sky structure’ [11]. Close to the thinnest niobium filaments, some white zones are exhibited. These regions do not show diffraction contrast, they are attributed to the amorphous phase. This is further confirmed by the selected area diffraction pattern in Fig. 6 which exhibits a diffuse halo ring. The HRTEM micrograph in Fig. 7 clearly reveals
Fig. 7. HRTEM image of a cross-sectional specimen showing an amorphous phase located at Cu/Nb interfaces. Grain boundaries in Nb fibres are indicated by arrows.
The cross section of the Cu/Nb wire was observed using FIM (Fig. 8). Cu and Nb grains appear respectively as darkly and brightly imaged zones because of the higher evaporation field of Nb [4]. Note that the magnification of the FIM image from the Nb filament is larger than that from the Cu filament, because of the local magnification effect originating from the smaller curvature radius of high-field Nb regions. The cross-section of a 3D reconstructed volume involving a copper–niobium interface is exhibited in Fig. 9. The apparent width of the chemical interface is about 0.7 nm. Taking into account the lateral resolution of the apparatus (about 0.5 nm) [9], it seems clear that the interface is chemically abrupt. This analysis gives the evidence for the capability of the 3D-AP to exhibit sharp Cu/Nb interfaces, in spite of potential experimental artefacts such as local magni-
Fig. 8. FIM micrograph showing an Nb fibre surrounded by Cu; Cu/Nb interfaces are indicated by arrows.
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
Fig. 11. 3D reconstruction of a small analysed volume (7.2⫻7.2⫻10 nm3). Only copper atoms are plotted. Sampling box: 0.4⫻3⫻5 nm3.
Fig. 9. Cross section of the 3D reconstruction of a small analysed volume (6⫻6⫻3 nm3). Only copper atoms are plotted to exhibit the Cu/Nb interface.
fication effects or ion trajectory aberrations which may occur near interfaces at the tip surface. The depth of the analysed volume shown in Fig. 9 is only about 3 nm. The volume of the Cu/Nb interface analysed is therefore too small to get a composition profile with reasonable statistical reliability across this interface. It is worth noticing that a very small amount of copper seems to be detected in the niobium fibre. It may be attributed to the slight overlap of Nb3⫹ and Cu2⫹ peaks in the TAP mass spectra [4]. The FIM micrograph in Fig. 10 exhibits the crosssection of a region of the wire where the microstructure is very similar to that in Fig. 7. The dark lamellae indicated by arrows are thought to be copper and the bright regions which exhibit a concentric ring contrast are attributed to crystalline niobium. In this lamellar region, the width of Cu and Nb channels is only about a few nanometers. This specimen was analysed with
Fig. 10. FIM micrograph showing thin copper channels (dark zones) surrounded by niobium fibres (bright zones). Copper channels are indicated by arrows.
the 3D-AP in a darkly imaged zone, between two crystalline Nb lamellae. The analysis area is delimited by the white square displayed in Fig. 10. Figure 11 shows the 3D reconstruction of the analysed volume. In this picture, only copper atoms are plotted. Between two niobium rich zones, a thin copper lamella is observed along the wire axis. The concentration depth profiles in Fig. 12 was obtained in the perpendicular direction to the lamella (as indicated by the arrow in Fig. 11), where the small inset box indicates the sampling box for calculating a local composition. The copper and niobium concentrations are plotted as a function of the sampling box position in Fig. 12a. The concentration of copper detected in the niobium rich zones is about 12 at.%. On the other hand, the 3D-AP data indicate that the copper lath contains only 30 at.% copper. So, this analysis reveals first that darkly imaged zones in the FIM micrograph are not pure copper zones. On the other hand, in the
Fig. 12. (a) Composition profile across the copper channel from Fig. 11. (b) Number of atoms in the sampling box as a function of the profile depth.
SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
regions where the microstructure is finest, interdiffusion of Cu and Nb has occurred. Because of the large difference between the evaporation fields of copper and niobium, local magnification effects and trajectory overlaps are likely to occur. Indeed, the evaporation field of Cu atoms is much lower than the evaporation field of Nb. On the tip surface, Cu-rich zones therefore exhibit a larger curvature radius than Nb-rich zones. Consequently, atoms coming from Cu-rich zones are more convergent, i.e. the magnification is lower and the local density is higher in the 3D reconstructed volume. The total number of atoms contained in the sampling box is plotted as a function of box position in Fig. 12b. As expected, this profile reveals that the local density of the 3D reconstructed volume is not constant: it is almost four times higher in the copper-rich lath. Another interesting feature appears: the number of niobium atoms in the sampling box is three times larger in the copper-rich channel than in the other part of the reconstructed volume. Consequently, most of the niobium atoms detected in the copper-rich lath should not be the result of overlaps resulting from the large difference between the curvature radius of Curich and Nb-rich zones. Moreover, the percentage of copper (vs. niobium) reaches a steady state in the copper-rich lath, contrary to the number of copper (vs. niobium) atoms in the sampling box. It is interesting to notice that the number of niobium (vs. copper) atoms starts to increase before any composition change (as indicated by dotted lines). These two features seem to indicate that the curvature radius of the analysed tip increases continuously from the niobiumto the copper-rich zones. Local magnification effects seem, therefore, not to affect the estimated compositions in the copper-rich channel. These local magnification effects, however, give rise to errors in the scale of the 3D elemental mapping. For instance, low-field Cu-rich regions in which evaporated ions are more convergent, are narrowed, contrary to high-field Nb-rich regions which are enlarged. The apparent width of the copper-rich channel (about 1 nm) revealed by the composition profile in Fig. 12a is therefore underestimated. A corrected composition profile (Fig. 13) was computed based on a sampling box similar to that shown in Fig. 12. To
Fig. 13. Corrected composition profile across the copper-rich channel in Fig. 9.
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correct the local magnification effects, the depth of the profile was weighted by the local density. The following procedure details this correction. The sampling box was translated, step by step, of an elemental distance ⌬X. At each step, the Cu and Nb concentrations were computed. Then, the corrected depth of the profile at the ith step Xi (X0 ⫽ 0, beginning of the profile) was weighted by the atomic density in the sampling box at the ith step Di, using the following equation: Xi ⫽ Xi⫺1 ⫹ ⌬X⫻Di/D where D is the average density of the whole reconstructed volume. Using this correction, the copper-rich lath is estimated to be about 2 nm wide (Fig. 13). 4. DISCUSSION
HRTEM and FIM pictures in Figs 7 and 10 exhibit two regions of the wire where the microstructure is very similar. This lamellar nanostructure was analysed using a 3D-AP (Fig. 11) and an unexpected intermixing of Cu and Nb was observed. At the equilibrium, there is no mutual miscibility of Cu and Nb whatever the temperature [12]. It is therefore reasonable to assume that amorphous zones observed in the HRTEM micrographs (Fig. 7) are the result of intermixing of Cu and Nb induced by severe plastic deformation. In such Cu/Nb nanocomposite wires, amorphous areas containing up to 3.5 wt% Cu have already been reported in Nb fibres by other authors [8]. The results presented in this paper clearly demonstrate, however, that this solid state amorphization mostly occurs at Cu/Nb interfaces and in the regions of the wire where the microstructure is finest. 3D-AP data revealed a Cu/Nb sharp interface (Fig. 9) in a region where the cross-sectional grain size was coarser (Fig. 8). Such a sharp interface may be attributed to a semi-coherent Cu/Nb interface exhibited in HRTEM micrographs Figs 3 and 4. Atzmon et al. [13] reported such a solid state amorphization resulting from severe plastic deformation in cold-rolled Cu–Er and Ni–Er. They assumed that strains play a role in enhancing the diffusion rates, thus allowing reaction at near room temperature. But, they have also investigated a cold-rolled Cu–Nb nanocomposite which did not transform into the amorphous phase during plastic deformation. The microstructure, shown in Figs 7 and 10, looks like a Cu/Nb multilayer. Pan et al. [14] studied vacuum-deposited Cu/Nb multilayers with nanoscale thickness and demonstrated that nanoscale layers may transform into the amorphous state through solid state diffusion during heat treatment at 250°C. The extremely high interfacial energy of the system, combined with a negative Cu/Nb enthalpy of mixing, is
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SAUVAGE et al.: SOLID STATE AMORPHIZATION IN COLD DRAWN Cu/Nb WIRES
the driving force for this solid state amorphization. And, because Cu and Nb atom mobilities increase at elevated temperatures, kinetics of reaction is promoted by the heat treatment. A similar phenomenon may have occurred locally in the Cu/Nb nanocomposite wire. The temperature remains below 50°C during the plastic deformation, but the drawing process includes intermediate annealing treatments at 300°C. It is therefore reasonable to assume that some regions of the wire may have a nanoscaled structure before the last annealing treatment, so that solid state amorphization occurs locally during the last intermediate heat treatment. The amorphous phase may then remain in the nanocomposite wire up to the last drawing stage. 5. CONCLUSIONS
HRTEM clearly revealed two kinds of Cu/Nb interfaces. Most of them appeared semi-coherent, while in regions of the wire where the microstructure is finest, an amorphous layer appeared between Cu and Nb. This solid state amorphization was correlated to a strong interdiffusion of Cu and Nb revealed by 3DAP data. This phenomenon was compared with amorphization in multilayer systems and in cold rolled Cu– Er composites. Finally, it turns out that the amorphization in the finest regions of the Cu/Nb nanocomposite wires may have occurred during the last intermediate annealing treatment included in the drawing process. Acknowledgements—The authors would like to thank the Bochvar All Russia Scientific Research Institute of Inorganic
Materials for providing materials. The authors are indebted to S. Aske´nazy, L. Thilly and F. Lecouturier from the Service National des Champ Magne´tiques Pulse´s (Toulouse, France) for the fruitful discussions we have had on the present results.
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