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Nb nanocomposite wires
NanoStructured Materials, Vol. 11, No. 8, pp. 1031–1039, 1999 Elsevier Science Ltd Copyright © 2000 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter
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FIM AND 3D ATOM PROBE ANALYSIS OF Cu/Nb NANOCOMPOSITE WIRES X. Sauvage1, L. Thilly2,3, F. Lecouturier3, A. Guillet1 D. Blavette1 1
Groupe de Me´tallurgie Physique, UMR CNRS 6634, Faculte´ des Sciences, Universite´ de Rouen, 76821 Mont-Saint-Aignan Cedex France 2 Laboratoire de Physique de la Matie`re Condense´e, INSA-UPS-CNRS, Complexe Scientifique de Rangueil, 31077 Toulouse Cedex 4, France 3 Service National des Champs Magne´tiques Pulse´s, UMS CNRS, 118 Rte de Narbonne, 31068 Toulouse Cedex France. (Received May 5, 1999) (Accepted August 30, 1999) Abstract—Two kinds of Cu/Nb nanocomposite wires were investigated using field ion microscopy (FIM) and 3D atom probe. These two techniques revealed for the first time the nanoscale microstructure of nanocomposite wire cross sections. FIM investigations confirmed the Cu and Nb texture and the disorientation between (111) Cu and (110) Nb planes. Low angle Nb/Nb grain boudaries were also observed. Thanks to 3D atom probe, parts of niobium fibres and copper channels a few nanometer width were mapped out in 3D. Smooth Cu/Nb interfaces were attributed to stress-induced diffusion. Shear bands, observed perpendicular to the wire axis, were attributed to tracks of moving dislocations in a copper channel. ©2000 Acta Metallurgica Inc.
Introduction Cu/Nb nanocomposite wires seem to be promising for the achievement of non-destructive magnets for high pulsed magnetic fields (1). The tensile strength of such conductors has already been tested close to 2GPa at 77K (1,2,3,4,5,6,8,9). Such a resistance is necessary to hold out against Lorentz’s forces. The electrical resistivity remains below one ohm.cm at 77K (7,8). Such wires are produced by large strain drawing of either in-situ melted Cu-Nb mixtures or continuous Nb fibres in a copper matrix. Transmission Electron Microscopy (TEM) observations (4,5,8,9) revealed the microstructure of these nanocomposite wires. Nb dendrites or continuous filaments are elongated along the wire axis and their average width in the cross section is commonly less than 100nm. Moreover, niobium grains exhibit a ⬍110⬎ texture parallel to the wire axis, while copper grains present a ⬍111⬎ texture (5,8). The aim of this work is to demonstrate the capability of field ion microscopy (FIM) (10) and of tomographic atom probe (TAP) (11) in the investigation of such nanostructured materials in the cross section of wires. Experimental Material Two different Cu/Nb nanocomposite wires were investigated. The first type of wire, W1, was provided by the Bochvar All Russia Scientific Research Institute of Inorganic Materials. These wires were produced by drawing an in-situ melted mixture of Cu-Nb0.18-Hf0.002 (%vol.). The wire diameter was 0.3 1031
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mm and the average width of niobium fibre cross sections in the range of 10 to 60nm (9). The other type, W2 wire, was provided by the Service National des Champs Magne´tiques Pulse´s (SNCMP) which developed an original process to elaborate Cu/Nb conductors. This process is based on the repeated drawing of aligned and continuous niobium fibres in a copper matrix and restacking. The as-produced Nanocomposite wires contain Nb nanowhiskers (8). The niobium content of W2 wires is 17%vol. The wire diameter is 0.5 mm and the average diameter of niobium fibres is 67 nm. More details about these two processes are given elsewhere (1,8,9). Specimen Preparation FIM and TAP investigations require the preparation of sharply pointed needles with an apex radius (R) in the range of 10 to 50nm. Specimen was prepared by sharpening wires using a standard electropolishing method. The tip axis was therefore parallel to that of wires. The composition of the electrolyte was 5%vol. HNO3 ⫹ 5%vol. H2SO4 in methanol. Electropolishing was performed at ⫺40°C with a continuous voltage in the range of 3 to 10 volts. After this preparation, FIM specimen was immediately introduced into the UHV (10⫺11 Pa) storage chamber of the TAP in order to avoid oxidation. Analysis Conditions During FIM and TAP investigations, the specimen temperature was kept in the range of 25 to 80 K and the high voltage (V) applied to the tip was between 2 and 15 kV. Neon was used as imaging gas at a pressure of 5 10⫺7 Pa for FIM observations. The half angle of view of the field ion microscope was 32° (maximum projection angle on the visualisation panel). TAP analysis were performed under UHV conditions (10⫺13 Pa) with a 1.7 kHz pulse repetition rate and a pulse to standing voltage ratio of 20%. Experimental Results Field Ion Microscopy Figure 1 and 2 show typical field ion micrographs of a W1 wire. Bright and dark areas are exhibited. This contrast may be interpreted as a phase contrast. The evaporation field (E) of Nb is indeed known to be much higher than that of Cu (12). This leads to a higher protuberance of niobium regions as compared to copper zones which have a larger curvature radius (R ⬃ V/E). FIM investigations of tips cooled between 25 to 80K revealed that the higher the temperature, the more contrasted the micrographs. Moreover, if the temperature was increased to 40K, the difference between the evaporation field of Cu and Nb dramatically increased and copper atoms were not imaged anymore. The spot density in Figure 1 seems to be higher in copper areas than in niobium ones. The atomic density of copper, known to be 30% higher than that of niobium, explains partly this difference. However, local magnification effects are also likely to contribute to this density difference. The larger local tip radius of copper zones leads to a smaller magnification which, in turn, gives rise to a smaller average distance between spots in FIM micrographs. In Figures 1 and 2, the average diameter of niobium fibres and the width of copper channels are both about 20nm. However, local magnification effect may enlarge the image of niobium fibres, while scaling down copper zones. The FIM micrograph of the Figure 1 exhibits a pole contrast in both niobium and copper regions. The pole contrast in the niobium rich region is quite unusual. Several low index poles (large concentric circles) appear very close to each other. This may indicate that several niobium grains with different
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Figure 1. FIM micrograph of a W1 wire. T ⫽ 25K, V ⫽ 11.9kV. The diameter of the observed surface is close to 100 nm.
crystallographic orientations join each other (as indicated by arrows). A close examination of the copper rich regions reveals the presence of five low-index poles. Because of the strong ⬍111⬎ texture of copper parallel to the wire axis, the pole close to the centre of the image was assumed to be a (111) pole. A plausible indexation of other poles is proposed in the FIM micrograph.
Figure 2. FIM micrograph of a W1 wire. T ⫽ 50K, V ⫽ 7.7kV. The diameter of the observed surface is close to 70 nm.
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Figure 3. FIM micrograph of a W2 wire. T ⫽ 25K, V ⫽ 5.5kV. The diameter of the observed surface is close to 70 nm.
Figure 3 shows a high magnification field ion micrograph of a W2 wire. The left side of the picture appears brighter: it is thought to be a niobium fibre. The darkly imaged right part is likely to be copper. The Cu/Nb interface is indicated by a dotted line. At the centre of the micrograph, i.e. along the wire axis, half a low index pole of niobium faces half a low index pole of copper. As niobium grains have a ⬍110⬎ texture parallel to the wire axis, while copper grains present along the same direction a ⬍111⬎ texture (5,8), these two poles were identified as a (111) Cu and a (011) Nb poles. This hypothesis is in good agreement with the FIM simulation calculated for the same magnification and proposed Figure 4. The estimated pole centres were plotted on the Figure 3: they do not merge into each other. An FIM micrograph is a quasi-stereographic projection, the distance between these two spots is therefore directly proportional to the misorientation angle between (111) Cu and (011) Nb planes. As the half angle of view of the field ion microscope is 32°, this slight disorientation was measured to be about 5°. Tomographic Atom Probe Investigations Figure 5 shows a typical mass spectrum obtained during the analysis of a W2 wire. Copper and niobium phases were simultaneously analysed. Niobium (Nb93) was detected as doubly and triply charged ions. The copper isotopes (Cu63 and Cu65) were detected as simply and doubly charged ions with the expected ratio of 69% Cu63 and 31% Cu65. Note the slight overlap of Nb3⫹ (m/n ⫽ 31 a.m.u.) and Cu632⫹ (m/n ⫽ 31.5 a.m.u.) peaks. Each ion detected with a mass to charge ratio over 31.4 u.m.a. was attributed to copper. Consequently, in the following 3D reconstructions a low amount of copper atoms may be niobium in reality. Figure 6 shows the cross section of the 3D reconstruction of an analysed volume in a W2 wire. Copper channels and niobium fibres are exhibited. In order to avoid a projection artefact, a thin layer of only 1.5nm depth was represented. Thus, even if a sharp interface is not perfectly plane and parallel to the tip axis, it would appear sharp on the cross section projection. Despite these precautions,
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Figure 4. Simulation of Cu and Nb FIM micrographs with (111) Cu and (011) Nb poles in the center of the picture.
interfaces between the 2 materials appear diffuse and Nb atoms are present in copper, while niobium fibres contain Cu atoms. Figure 7 shows a scanning electron micrograph of a Cu/Nb nanocomposite wire. As a consequence of the plastic deformation, niobium fibre cross sections are not circular anymore, but exhibit thin branches. Niobium regions, as shown in Figure 6, may be interpretated as very thin Nb branches which
Figure 5. Mass spectrum related to the analysis of a W2 wire.
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Figure 6. Cross section of a 3D reconstruction of a small analysed volume in a W2 wire at 25K (cross section: 17 ⫻ 17 nm2, depth 1.5 nm). Copper atoms are represented in small grey dots, while large black dots correspond to niobium.
thickness has decreased close to 10nm because of the higher level of plastic deformation. The copper channel between these Nb branches, on the top right corner, is about 4 nm width. Figures 8a and 8b show two views of a 3D reconstruction exhibiting interesting features related to W2 wires. The view of the section (Figure 8b) exhibits a niobium fibre in the top corner. The view along the wire axis (Figure 8a) partially reveals another Nb fibre in the bottom of the picture. As indicated by arrows, the copper channel exhibits tow steps perpendicular to the wire axis.
Figure 7. SEM micrograph of the cross section of a nanocomposite Cu/Nb wire W2 elaborated at the SNCMP(1) - 10 ⫻ 7 m - (Nb amount: 29.5% vol., section reduction: 99.44%, Nb fibres average diameter: 524 nm).
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Figure 8. 3D reconstruction of a small analysed volume in a W2 wire at 40K (8.8 ⫻ 8.8 ⫻ 8 nm3). Copper atoms are represented in small grey dots, while large black dots correspond to niobium. Figure 8a is a view along the wire axis, while Figure 8b is the cross section.
The high dot density in the copper region in the bottom of Figures 8a and 8b is attributed to local magnification effects which are more pronounced at 40K than at 25K (Figure 6). Discussion FIM micrographs have confirmed X-ray studies concerning the ⬍110⬎ texture of niobium grains and the ⬍111⬎ texture of copper grains along the wire axis (5,8). Niobium fibre diameters and copper channel widths measured on FIM micrographs are, as well, in good agreement with TEM investigations (9). Moreover, as already observed in High Resolution Electron Microscopy in W2 wires (7,8), a slight disorientation of about 5° between (111) Cu and (011) Nb was here confirmed. A close examination of the FIM micrograph of the Figure 1 seems to indicate that several niobium grains with different crystallographic orientations join each other. This suggests the existence of numerous low-angle Nb/Nb grain boundaries which may be the consequence of the ⬍110⬎ texture development along the wire axis. Such poly-crystalline niobium fibres have never been revealed by TEM observations as foils were prepared along the wire axis (9). 3D Atom Probe data have exhibited diffuse Cu/Nb interfaces and a possible slight inter-diffusion of Cu and Nb atoms (Figure 6). Due to the mass resolution of the TAP, some atoms labelled as copper may be in fact niobium. There is however no ambiguity concerning the attribution of niobium. The inter-solubility of Cu and Nb is known to be insignificant whatever the temperature, while no compounds or intermediate phases occur in the Cu-Nb system (13). The observed inter-diffusion through Cu/Nb interfaces could be explained as the direct consequence of large strains. Indeed, the plastic deformation strongly increase the area of Cu/Nb interfaces, i.e. the energy of the Cu/Nb system. Hence, the thermodynamical equilibrium may be changed, and the intersolubility of Cu/Nb modified. The interdiffusion may be promoted by moving dislocations and a possible increase of the vacancy density. A similar phenomenon in a quite different system was already pointed out in cold drawn pearlitic steel wires (14,15). It has to be noted that the study of the microstructure of Cu-20w.% Nb in-situ composite revealed the presence in the Nb filaments of amorphous or glassy areas containing enhanced Cu content (up to 3.5mass%) (16). Moreover, in multilayered structures, if the layer thickness is reduced down to the nanometer scale, a significant fraction of atoms resides at interfaces and grain
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boundaries. The excess of free energy associated to interfaces may therefore be relatively large and increase the energy of the system, so that alloying becomes possible. Studies have been conducted to investigate the possibility of amorphization by solid state inter-diffusion in vacuum-deposited Cu-Nb multilayers with nanoscale thickness (17). Different heat treatments were applied to the thin films (Cu0.25Nb0.75) and analysis by selected area electron diffraction revealed the formation of amorphous Cu-Nb alloy films. If the amorphous phase was heated at 400°C, the Cu-Nb phase changed into a crystalline structure. The W2 conductors can locally be considered as nanoscale multilayers with a high amount of Cu-Nb interfaces. Since the structure was heavily deformed and heat-treated during the elaboration process, some amorphous or metastable crystalline phases may therefore have been formed through a solid state reaction. Moreover, HREM studies of the W2 wires pointed out coherent Cu/Nb interfaces, with (111)Cu planes parallel to (110)Nb planes. Fourier filtering of the observations revealed that misfit dislocations are present at the interface every 8 planes. Since the crystalline coherency is respected at the interface, one can assume that the Nb atoms in Cu are substitution atoms and reciprocally. However, instrumental issues must be taken into consideration. The 3D reconstruction algorithm used is based on simple geometrical considerations. Local magnification effects caused by the large difference between the evaporation field of copper and that of niobium were not taken into account in the reconstruction procedure. These effects give rise to additional aberrations in the ion trajectories in the vicinity of interfaces. Divergent trajectories originating from low curvature Nb regions are therefore likely to lead to a spread of niobium in the copper phase. This may hence give rise to apparent diffuse Cu/Nb interfaces. Another noticeable consequence is the lower dot density of the Nb rich zone in the reconstructed volumes. The depth resolution of the instrument (0.1nm) being much better than the lateral resolution, it would be preferable to prepare the tip in the wire cross sections. Interfaces would be thus perpendicular to the tip axis. 3D Atom Probe data have also exhibited a Cu/Nb interface with steps perpendicular to the wire axis (Figure 8a). As the voltage and the evaporation flow measured did not reveal any discontinuity during the analysis, these steps are not the result of small tip fractures. Instead, because of the strong ⬍111⬎ texture of copper along the wire axis, these steps may be sliding (111) Cu planes activated during the plastic deformation. However, this hypothesis is not confirmed by tracks of (111) Cu planes. These do not appear in the 3D reconstruction probably because of the occurrence of pronounced local magnification effects. Step heights are about 0.4 nm (⫾ 0.1 nm), while the Burger vector length of dislocations along the ⬍110⬎ direction in (111) Cu planes is known to be 0.256nm. Consequently, if these steps are assumed to be shear bands, they may be both the result of 1 or 2 moving dislocations along the ⬍110⬎ direction in (111) Cu planes. It is physically not realistic to consider that the Hall-Petch mechanism is responsible for the deformation of the Cu channel and the creation of such steps, since two dislocations cannot be considered as a pileup. The study of Cu-Nb multilayers hardness (18) led to the calculation of the number of dislocations n in a pileup to explain their enhanced mechanical properties. For a layer thickness d of 50nm, n was found to be 6, while for d ⫽ 10 nm, n was less than 2. The authors concluded therefore that the Hall-Petch hardening is no more possible in such system, while the Orowan mechanism can be considered: the enhancement of the mechanical properties of thin films and multilayers may be the result of the gliding and bowing of a single dislocation in the Cu channel. Such hypothesis can reasonably be applied to the Cu-Nb nanocomposites, which mechanical properties exhibit a strong deviation to the rule of mixture. If the steps observed by TAP are confirmed to be the result of the gliding of single dislocations that cross the interface and glide in the channels, then the interpretation as well as the modelling of the mechanical properties of the W2 nanocomposites will be possible.
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Conclusion FIM and TAP investigations of two different Cu/Nb nanocomposite wires have revealed their nanoscale microstructure in the cross sections of wires. In contrast to TEM where foils are observed along the wire axis, FIM as well as 3D Atom Probe make it possible to examine the cross sections of wires. These two latter techniques enable to reveal nanostructural features not accessible to more conventional instruments. The strong texture of both copper and niobium as well as a slight disorientation about 5° between (111) Cu and (011) Nb were confirmed. FIM investigation revealed as well low angles Nb/Nb grain boundaries. Using 3D Atom Probe, the three dimensional distribution of Cu and Nb has been mapped out. Smooth Cu/Nb interfaces were attributed either to stress-induced atomic diffusion or to local magnification effects in the 3D reconstructions. This possible increase of the Cu-Nb intersolubility was correlated to the increase of the system free energy, resulting from the plastic deformation. For the first time nanometer scale cross sections of Nb fibres were imaged. Steps in copper regions, attributed to shear bands, resulting of one or two moving dislocations along the ⬍110⬎ direction in (111) Cu planes, have also been observed. Through a comparison with studies of Cu/Nb multilayered structures, the enhancement of wire mechanical properties were discussed in terms of the Orowan mechanism. More refined treatments of atom-probe data as well as additional experiments will be soon conducted in order to elucidate the role of interdiffusion and shear bands in such nanocomposite wires. Acknowledgments The authors would like to thank the Bochvar All Russia Scientific Research Institute of Inorganic Materials for providing materials. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
F. Dupouy, Doctoral Thesis, INSA Toulouse, France (1995). J. Bevk, J. P. Harbison, and J. L. Bell, J. Appl. Phys. 49, 6031 (1978). W. A. Spitzig and P. D. Krotz, Scripta Metall. 21, 1143 (1987). W. A. Spitzig, A. R. Pelton, and F. C. Laabs, Acta Metall. 35, 2427 (1987). S. I. Hong, M. A. Hill, Y. Sakai, J. T. Wood, and J. D. Embury, Acta Metall. 43, 3313 (1995). D. Raabe and D. Mattissen, Acta Metall. 46, 5973 (1998). F. Heringhaus, D. Raabe, and G. Gottstein, Acta Metall. 43, 1467 (1995). F. Dupouy, E. Snoeck, M. J. Casanove, C. Roucau, J. P. Peyrade, and S. Askenazy, Scripta Metall. 34, 1067 (1996). E. Snoeck, F. Lecouturier, L. Thilly, M. J. Casanove, H. Rakoto, G. Coffe, S. Aske´nazy, J. P. Peyrade, C. Roucau, V. Pantsyrny, A. Shikov, and A. Nikulin, Scripta Metall. 38, 1643 (1998). E. W. Mu¨ller, Z. Phys. 131, 136 (1951). D. Blavette, A. Bostel, J. M. Sarrau, B. Deconihout, and A. Menand, Nature. 363, 432 (1993). M. K. Miller, A. Cerezo, M. G. Hetherington, and G. D. W. Smith, Atom Probe Field Ion Microscopy, p. 493, Clarendon Press, Oxford (1996). T. B. Massalski, Binary Alloy Phase Diagrams, p. 936, ASM, Metals Park, OH (1986). J. Languillaume, G. Kapelski, and B. Baudelet, Acta Mater. 45, 1201 (1997). X. Sauvage, J. Copreaux, F. Danoix, and D. Blavette, Phil. Mag. A. in press (1999). D. Raabe, F. Heringhaus, U. Hangen, and G. Z. Gottstein, Metallkd. 86, 405 (1995). F. Pan, Z. F. Ling, K. Y. Gao, and B. X. Liu, Mater. Res. Soc. Symp. Proc. 398, 337 (1996). T. E. Mitchell, Y. C. Lu, A. J. Griffin, M. Nastasi, and H. J. Kung, Am. Ceram. Soc. 80, 1673 (1997).
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FIM AND 3D ATOM PROBE ANALYSIS OF Cu/Nb NANOCOMPOSITE WIRES X. Sauvage1, L. Thilly2,3, F. Lecouturier3, A. Guillet1 D. Blavette1 1
Groupe de Me´tallurgie Physique, UMR CNRS 6634, Faculte´ des Sciences, Universite´ de Rouen, 76821 Mont-Saint-Aignan Cedex France 2 Laboratoire de Physique de la Matie`re Condense´e, INSA-UPS-CNRS, Complexe Scientifique de Rangueil, 31077 Toulouse Cedex 4, France 3 Service National des Champs Magne´tiques Pulse´s, UMS CNRS, 118 Rte de Narbonne, 31068 Toulouse Cedex France. (Received May 5, 1999) (Accepted August 30, 1999) Abstract—Two kinds of Cu/Nb nanocomposite wires were investigated using field ion microscopy (FIM) and 3D atom probe. These two techniques revealed for the first time the nanoscale microstructure of nanocomposite wire cross sections. FIM investigations confirmed the Cu and Nb texture and the disorientation between (111) Cu and (110) Nb planes. Low angle Nb/Nb grain boudaries were also observed. Thanks to 3D atom probe, parts of niobium fibres and copper channels a few nanometer width were mapped out in 3D. Smooth Cu/Nb interfaces were attributed to stress-induced diffusion. Shear bands, observed perpendicular to the wire axis, were attributed to tracks of moving dislocations in a copper channel. ©2000 Acta Metallurgica Inc.
Introduction Cu/Nb nanocomposite wires seem to be promising for the achievement of non-destructive magnets for high pulsed magnetic fields (1). The tensile strength of such conductors has already been tested close to 2GPa at 77K (1,2,3,4,5,6,8,9). Such a resistance is necessary to hold out against Lorentz’s forces. The electrical resistivity remains below one ohm.cm at 77K (7,8). Such wires are produced by large strain drawing of either in-situ melted Cu-Nb mixtures or continuous Nb fibres in a copper matrix. Transmission Electron Microscopy (TEM) observations (4,5,8,9) revealed the microstructure of these nanocomposite wires. Nb dendrites or continuous filaments are elongated along the wire axis and their average width in the cross section is commonly less than 100nm. Moreover, niobium grains exhibit a ⬍110⬎ texture parallel to the wire axis, while copper grains present a ⬍111⬎ texture (5,8). The aim of this work is to demonstrate the capability of field ion microscopy (FIM) (10) and of tomographic atom probe (TAP) (11) in the investigation of such nanostructured materials in the cross section of wires. Experimental Material Two different Cu/Nb nanocomposite wires were investigated. The first type of wire, W1, was provided by the Bochvar All Russia Scientific Research Institute of Inorganic Materials. These wires were produced by drawing an in-situ melted mixture of Cu-Nb0.18-Hf0.002 (%vol.). The wire diameter was 0.3 1031
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mm and the average width of niobium fibre cross sections in the range of 10 to 60nm (9). The other type, W2 wire, was provided by the Service National des Champs Magne´tiques Pulse´s (SNCMP) which developed an original process to elaborate Cu/Nb conductors. This process is based on the repeated drawing of aligned and continuous niobium fibres in a copper matrix and restacking. The as-produced Nanocomposite wires contain Nb nanowhiskers (8). The niobium content of W2 wires is 17%vol. The wire diameter is 0.5 mm and the average diameter of niobium fibres is 67 nm. More details about these two processes are given elsewhere (1,8,9). Specimen Preparation FIM and TAP investigations require the preparation of sharply pointed needles with an apex radius (R) in the range of 10 to 50nm. Specimen was prepared by sharpening wires using a standard electropolishing method. The tip axis was therefore parallel to that of wires. The composition of the electrolyte was 5%vol. HNO3 ⫹ 5%vol. H2SO4 in methanol. Electropolishing was performed at ⫺40°C with a continuous voltage in the range of 3 to 10 volts. After this preparation, FIM specimen was immediately introduced into the UHV (10⫺11 Pa) storage chamber of the TAP in order to avoid oxidation. Analysis Conditions During FIM and TAP investigations, the specimen temperature was kept in the range of 25 to 80 K and the high voltage (V) applied to the tip was between 2 and 15 kV. Neon was used as imaging gas at a pressure of 5 10⫺7 Pa for FIM observations. The half angle of view of the field ion microscope was 32° (maximum projection angle on the visualisation panel). TAP analysis were performed under UHV conditions (10⫺13 Pa) with a 1.7 kHz pulse repetition rate and a pulse to standing voltage ratio of 20%. Experimental Results Field Ion Microscopy Figure 1 and 2 show typical field ion micrographs of a W1 wire. Bright and dark areas are exhibited. This contrast may be interpreted as a phase contrast. The evaporation field (E) of Nb is indeed known to be much higher than that of Cu (12). This leads to a higher protuberance of niobium regions as compared to copper zones which have a larger curvature radius (R ⬃ V/E). FIM investigations of tips cooled between 25 to 80K revealed that the higher the temperature, the more contrasted the micrographs. Moreover, if the temperature was increased to 40K, the difference between the evaporation field of Cu and Nb dramatically increased and copper atoms were not imaged anymore. The spot density in Figure 1 seems to be higher in copper areas than in niobium ones. The atomic density of copper, known to be 30% higher than that of niobium, explains partly this difference. However, local magnification effects are also likely to contribute to this density difference. The larger local tip radius of copper zones leads to a smaller magnification which, in turn, gives rise to a smaller average distance between spots in FIM micrographs. In Figures 1 and 2, the average diameter of niobium fibres and the width of copper channels are both about 20nm. However, local magnification effect may enlarge the image of niobium fibres, while scaling down copper zones. The FIM micrograph of the Figure 1 exhibits a pole contrast in both niobium and copper regions. The pole contrast in the niobium rich region is quite unusual. Several low index poles (large concentric circles) appear very close to each other. This may indicate that several niobium grains with different
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Figure 1. FIM micrograph of a W1 wire. T ⫽ 25K, V ⫽ 11.9kV. The diameter of the observed surface is close to 100 nm.
crystallographic orientations join each other (as indicated by arrows). A close examination of the copper rich regions reveals the presence of five low-index poles. Because of the strong ⬍111⬎ texture of copper parallel to the wire axis, the pole close to the centre of the image was assumed to be a (111) pole. A plausible indexation of other poles is proposed in the FIM micrograph.
Figure 2. FIM micrograph of a W1 wire. T ⫽ 50K, V ⫽ 7.7kV. The diameter of the observed surface is close to 70 nm.
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Figure 3. FIM micrograph of a W2 wire. T ⫽ 25K, V ⫽ 5.5kV. The diameter of the observed surface is close to 70 nm.
Figure 3 shows a high magnification field ion micrograph of a W2 wire. The left side of the picture appears brighter: it is thought to be a niobium fibre. The darkly imaged right part is likely to be copper. The Cu/Nb interface is indicated by a dotted line. At the centre of the micrograph, i.e. along the wire axis, half a low index pole of niobium faces half a low index pole of copper. As niobium grains have a ⬍110⬎ texture parallel to the wire axis, while copper grains present along the same direction a ⬍111⬎ texture (5,8), these two poles were identified as a (111) Cu and a (011) Nb poles. This hypothesis is in good agreement with the FIM simulation calculated for the same magnification and proposed Figure 4. The estimated pole centres were plotted on the Figure 3: they do not merge into each other. An FIM micrograph is a quasi-stereographic projection, the distance between these two spots is therefore directly proportional to the misorientation angle between (111) Cu and (011) Nb planes. As the half angle of view of the field ion microscope is 32°, this slight disorientation was measured to be about 5°. Tomographic Atom Probe Investigations Figure 5 shows a typical mass spectrum obtained during the analysis of a W2 wire. Copper and niobium phases were simultaneously analysed. Niobium (Nb93) was detected as doubly and triply charged ions. The copper isotopes (Cu63 and Cu65) were detected as simply and doubly charged ions with the expected ratio of 69% Cu63 and 31% Cu65. Note the slight overlap of Nb3⫹ (m/n ⫽ 31 a.m.u.) and Cu632⫹ (m/n ⫽ 31.5 a.m.u.) peaks. Each ion detected with a mass to charge ratio over 31.4 u.m.a. was attributed to copper. Consequently, in the following 3D reconstructions a low amount of copper atoms may be niobium in reality. Figure 6 shows the cross section of the 3D reconstruction of an analysed volume in a W2 wire. Copper channels and niobium fibres are exhibited. In order to avoid a projection artefact, a thin layer of only 1.5nm depth was represented. Thus, even if a sharp interface is not perfectly plane and parallel to the tip axis, it would appear sharp on the cross section projection. Despite these precautions,
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Figure 4. Simulation of Cu and Nb FIM micrographs with (111) Cu and (011) Nb poles in the center of the picture.
interfaces between the 2 materials appear diffuse and Nb atoms are present in copper, while niobium fibres contain Cu atoms. Figure 7 shows a scanning electron micrograph of a Cu/Nb nanocomposite wire. As a consequence of the plastic deformation, niobium fibre cross sections are not circular anymore, but exhibit thin branches. Niobium regions, as shown in Figure 6, may be interpretated as very thin Nb branches which
Figure 5. Mass spectrum related to the analysis of a W2 wire.
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Figure 6. Cross section of a 3D reconstruction of a small analysed volume in a W2 wire at 25K (cross section: 17 ⫻ 17 nm2, depth 1.5 nm). Copper atoms are represented in small grey dots, while large black dots correspond to niobium.
thickness has decreased close to 10nm because of the higher level of plastic deformation. The copper channel between these Nb branches, on the top right corner, is about 4 nm width. Figures 8a and 8b show two views of a 3D reconstruction exhibiting interesting features related to W2 wires. The view of the section (Figure 8b) exhibits a niobium fibre in the top corner. The view along the wire axis (Figure 8a) partially reveals another Nb fibre in the bottom of the picture. As indicated by arrows, the copper channel exhibits tow steps perpendicular to the wire axis.
Figure 7. SEM micrograph of the cross section of a nanocomposite Cu/Nb wire W2 elaborated at the SNCMP(1) - 10 ⫻ 7 m - (Nb amount: 29.5% vol., section reduction: 99.44%, Nb fibres average diameter: 524 nm).
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Figure 8. 3D reconstruction of a small analysed volume in a W2 wire at 40K (8.8 ⫻ 8.8 ⫻ 8 nm3). Copper atoms are represented in small grey dots, while large black dots correspond to niobium. Figure 8a is a view along the wire axis, while Figure 8b is the cross section.
The high dot density in the copper region in the bottom of Figures 8a and 8b is attributed to local magnification effects which are more pronounced at 40K than at 25K (Figure 6). Discussion FIM micrographs have confirmed X-ray studies concerning the ⬍110⬎ texture of niobium grains and the ⬍111⬎ texture of copper grains along the wire axis (5,8). Niobium fibre diameters and copper channel widths measured on FIM micrographs are, as well, in good agreement with TEM investigations (9). Moreover, as already observed in High Resolution Electron Microscopy in W2 wires (7,8), a slight disorientation of about 5° between (111) Cu and (011) Nb was here confirmed. A close examination of the FIM micrograph of the Figure 1 seems to indicate that several niobium grains with different crystallographic orientations join each other. This suggests the existence of numerous low-angle Nb/Nb grain boundaries which may be the consequence of the ⬍110⬎ texture development along the wire axis. Such poly-crystalline niobium fibres have never been revealed by TEM observations as foils were prepared along the wire axis (9). 3D Atom Probe data have exhibited diffuse Cu/Nb interfaces and a possible slight inter-diffusion of Cu and Nb atoms (Figure 6). Due to the mass resolution of the TAP, some atoms labelled as copper may be in fact niobium. There is however no ambiguity concerning the attribution of niobium. The inter-solubility of Cu and Nb is known to be insignificant whatever the temperature, while no compounds or intermediate phases occur in the Cu-Nb system (13). The observed inter-diffusion through Cu/Nb interfaces could be explained as the direct consequence of large strains. Indeed, the plastic deformation strongly increase the area of Cu/Nb interfaces, i.e. the energy of the Cu/Nb system. Hence, the thermodynamical equilibrium may be changed, and the intersolubility of Cu/Nb modified. The interdiffusion may be promoted by moving dislocations and a possible increase of the vacancy density. A similar phenomenon in a quite different system was already pointed out in cold drawn pearlitic steel wires (14,15). It has to be noted that the study of the microstructure of Cu-20w.% Nb in-situ composite revealed the presence in the Nb filaments of amorphous or glassy areas containing enhanced Cu content (up to 3.5mass%) (16). Moreover, in multilayered structures, if the layer thickness is reduced down to the nanometer scale, a significant fraction of atoms resides at interfaces and grain
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boundaries. The excess of free energy associated to interfaces may therefore be relatively large and increase the energy of the system, so that alloying becomes possible. Studies have been conducted to investigate the possibility of amorphization by solid state inter-diffusion in vacuum-deposited Cu-Nb multilayers with nanoscale thickness (17). Different heat treatments were applied to the thin films (Cu0.25Nb0.75) and analysis by selected area electron diffraction revealed the formation of amorphous Cu-Nb alloy films. If the amorphous phase was heated at 400°C, the Cu-Nb phase changed into a crystalline structure. The W2 conductors can locally be considered as nanoscale multilayers with a high amount of Cu-Nb interfaces. Since the structure was heavily deformed and heat-treated during the elaboration process, some amorphous or metastable crystalline phases may therefore have been formed through a solid state reaction. Moreover, HREM studies of the W2 wires pointed out coherent Cu/Nb interfaces, with (111)Cu planes parallel to (110)Nb planes. Fourier filtering of the observations revealed that misfit dislocations are present at the interface every 8 planes. Since the crystalline coherency is respected at the interface, one can assume that the Nb atoms in Cu are substitution atoms and reciprocally. However, instrumental issues must be taken into consideration. The 3D reconstruction algorithm used is based on simple geometrical considerations. Local magnification effects caused by the large difference between the evaporation field of copper and that of niobium were not taken into account in the reconstruction procedure. These effects give rise to additional aberrations in the ion trajectories in the vicinity of interfaces. Divergent trajectories originating from low curvature Nb regions are therefore likely to lead to a spread of niobium in the copper phase. This may hence give rise to apparent diffuse Cu/Nb interfaces. Another noticeable consequence is the lower dot density of the Nb rich zone in the reconstructed volumes. The depth resolution of the instrument (0.1nm) being much better than the lateral resolution, it would be preferable to prepare the tip in the wire cross sections. Interfaces would be thus perpendicular to the tip axis. 3D Atom Probe data have also exhibited a Cu/Nb interface with steps perpendicular to the wire axis (Figure 8a). As the voltage and the evaporation flow measured did not reveal any discontinuity during the analysis, these steps are not the result of small tip fractures. Instead, because of the strong ⬍111⬎ texture of copper along the wire axis, these steps may be sliding (111) Cu planes activated during the plastic deformation. However, this hypothesis is not confirmed by tracks of (111) Cu planes. These do not appear in the 3D reconstruction probably because of the occurrence of pronounced local magnification effects. Step heights are about 0.4 nm (⫾ 0.1 nm), while the Burger vector length of dislocations along the ⬍110⬎ direction in (111) Cu planes is known to be 0.256nm. Consequently, if these steps are assumed to be shear bands, they may be both the result of 1 or 2 moving dislocations along the ⬍110⬎ direction in (111) Cu planes. It is physically not realistic to consider that the Hall-Petch mechanism is responsible for the deformation of the Cu channel and the creation of such steps, since two dislocations cannot be considered as a pileup. The study of Cu-Nb multilayers hardness (18) led to the calculation of the number of dislocations n in a pileup to explain their enhanced mechanical properties. For a layer thickness d of 50nm, n was found to be 6, while for d ⫽ 10 nm, n was less than 2. The authors concluded therefore that the Hall-Petch hardening is no more possible in such system, while the Orowan mechanism can be considered: the enhancement of the mechanical properties of thin films and multilayers may be the result of the gliding and bowing of a single dislocation in the Cu channel. Such hypothesis can reasonably be applied to the Cu-Nb nanocomposites, which mechanical properties exhibit a strong deviation to the rule of mixture. If the steps observed by TAP are confirmed to be the result of the gliding of single dislocations that cross the interface and glide in the channels, then the interpretation as well as the modelling of the mechanical properties of the W2 nanocomposites will be possible.
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Conclusion FIM and TAP investigations of two different Cu/Nb nanocomposite wires have revealed their nanoscale microstructure in the cross sections of wires. In contrast to TEM where foils are observed along the wire axis, FIM as well as 3D Atom Probe make it possible to examine the cross sections of wires. These two latter techniques enable to reveal nanostructural features not accessible to more conventional instruments. The strong texture of both copper and niobium as well as a slight disorientation about 5° between (111) Cu and (011) Nb were confirmed. FIM investigation revealed as well low angles Nb/Nb grain boundaries. Using 3D Atom Probe, the three dimensional distribution of Cu and Nb has been mapped out. Smooth Cu/Nb interfaces were attributed either to stress-induced atomic diffusion or to local magnification effects in the 3D reconstructions. This possible increase of the Cu-Nb intersolubility was correlated to the increase of the system free energy, resulting from the plastic deformation. For the first time nanometer scale cross sections of Nb fibres were imaged. Steps in copper regions, attributed to shear bands, resulting of one or two moving dislocations along the ⬍110⬎ direction in (111) Cu planes, have also been observed. Through a comparison with studies of Cu/Nb multilayered structures, the enhancement of wire mechanical properties were discussed in terms of the Orowan mechanism. More refined treatments of atom-probe data as well as additional experiments will be soon conducted in order to elucidate the role of interdiffusion and shear bands in such nanocomposite wires. Acknowledgments The authors would like to thank the Bochvar All Russia Scientific Research Institute of Inorganic Materials for providing materials. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
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