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Texture and mechanical properties of cold deformed and annealed multilayer Ni base substrate tapes prepared by a powder metallurgy route
Materials Science and Engineering A 488 (2008) 84–91
Texture and mechanical properties of cold deformed and annealed multilayer Ni base substrate tapes prepared by a powder metallurgy route P.P. Bhattacharjee a,∗ , R.K. Ray b , A. Upadhyaya c a
ARC Centre of Excellence-Design in Light Metals, Deakin University, Geelong, VIC 3217, Australia b R&D Division, TATA Steel, Jamshedpur 831001, India c Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 22 September 2007; received in revised form 24 October 2007; accepted 24 October 2007
Abstract An innovative powder metallurgy based technique has been investigated to fabricate multilayer tapes having configurations Ni/Ni–5 at.% W and Ni/Ni–5 at.% W/Ni for use as mechanically strong and textured substrates for coated superconductor applications. Development of cube texture ({0 0 1}1 0 0) following heavy cold rolling (∼95%) and annealing has been studied in the Ni side(s) of these multilayer tapes and this has been compared to a monolithic Ni tape. The deformation textures in the Ni side(s) of the tapes are found to be quite similar to that of monolithic Ni strained to similar level of deformation. However, the cube texture upon annealing has been found to be significantly stronger in the Ni side(s) of the multilayer tapes as compared to the monolithic Ni tape after different annealing treatments. Cross-sectional EDS analyses in form of X-ray area mapping of the multilayer tapes reveal significant diffusion of W from the alloy side to the Ni side(s). All these facts amply demonstrate the beneficial role of the alloying element W on the development of cube texture in Ni. Finally, the mechanical properties of the multilayer tapes have been evaluated to ascertain their suitability in the actual application scenario. © 2007 Elsevier B.V. All rights reserved. Keywords: Nickel alloys; Powder metallurgy; Multilayer substrate; Cube texture; EBSD
1. Introduction The rolling assisted biaxially textured substrates (RABiTSTM ) method developed at ORNL, USA is a very promising route for making highly cube textured ({0 0 1}1 0 0), long, flexible substrate tapes for coated high temperature superconductor (HTS) applications [1,2]. A necessary condition for improving the critical current densities of the superconducting tapes is to remove high angle grain boundaries which typically show the ‘weak link’ behaviour [3]. In the RABiTSTM method this is achieved by the epitaxial growth of the buffer and superconducting layers on highly cube textured metallic substrates. The sharp cube texture of the substrate material ensures the presence of grains of almost a single orientation and thus the grain boundary network consists of predominantly low angle boundaries.
∗
Corresponding author. Tel.: +61 3 5227 2112; fax: +61 3 5227 1103. E-mail address: [email protected] (P.P. Bhattacharjee).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.065
Pure Ni received the initial attention for use as a substrate material due to the possibility of developing a sharp cube texture after heavy cold rolling and annealing [4]. The oxidation resistance of pure Ni and its small lattice mismatch with overlying buffer layers are also favourable factors for the epitaxial growth of the buffer and superconducting layers. Current densities in excess of 106 A/cm2 have been reported in Ni base substrate tapes with certain buffer layer configurations [5]. To increase the engineering current density (i.e. the total current carried by the HTS film divided by the total cross-sectional area of the whole tape) the substrate thickness should be kept to a minimum. Unfortunately, poor mechanical strength of pure Ni in annealed condition makes it difficult to prepare very thin coated Ni tapes involving a reel-to-reel deposition process of the buffer and superconducting layers. Hence, it becomes imperative to increase the mechanical strength of pure Ni through alloying addition via the solid solution strengthening mechanism. However, the addition of alloying elements and impurities in general are considered detrimental to the development of sharp cube texture. Therefore, the choice of alloying elements should
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Fig. 1. Flow diagram of the double layer compact preparation.
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be such that they do not impair the possibility of developing a sharp cube texture in the substrate. Ni base alloys such as Ni–W and Ni–Mo have previously been suggested which can develop sharp cube texture after cold deformation and annealing and these alloys also have sufficient mechanical strength at the deposition temperatures of the buffer and superconducting layers [6,7]. A novel approach to improve the sharpness of cube texture and mechanical strength simultaneously is to design substrates with laminated or multilayer architecture. The advantage of this approach is that the different constituent layers can serve different purposes. Recently, tapes with such multilayer architecture have been reported with sharp cube texture intensity, good oxidation resistance and mechanical strength [8,9]. Sarma et al. [8,9] prepared a variety of multilayer substrates by inserting a highly strengthened alloy rod (core) into a Ni–W tube (outer layer) followed by the hot rolling, cold rolling and finally annealing of the whole assembly. The hot rolling step is introduced to develop the
Fig. 2. (a) A typical cross-sectional area of Ni/Ni–5 at.% W sintered compact and (b) the X-ray mapping of the same area showing the distribution of W.
Fig. 3. SEM micrographs of typical cross-sectional area of Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C and (b) 1000 ◦ C.
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ing and casting route which can inadvertently introduce some impurities during the melting stage itself. The development of textures and mechanical properties has been studied to ascertain the suitability of such multilayered tapes for use as substrates for coated superconductor applications. 2. Experimental
Fig. 4. SEM micrographs of typical cross-sectional area of Ni/Ni–5 at.% W/Ni tape after annealing at (a) 500 ◦ C and (b) 1000 ◦ C.
requisite bond strength between the different layers. However, possibility of deterioration in the intensity of the cube texture upon cold rolling and annealing owing to the introduction of the hot rolling step cannot be ruled out completely. In contrast, the current work explores the possibility of employing an innovative powder metallurgy based technique to prepare multilayer substrates having configurations Ni/Ni–5 at.% W and Ni/Ni–5 at.% W/Ni, respectively. This method has the advantage that it does not employ hot rolling which was introduced in the earlier processes [8,9] to develop strong bonding between the outer layer and the inner core. In addition, powder metallurgy route has the advantage of exercising greater control over the purity of material by starting out with high purity elemental powders as compared to the melt-
Elemental powders of Ni and (∼99.99% purity), W (∼99.6% purity) were used as the starting materials. The important characteristics of the powders have been given elsewhere [10]. The elemental powders were first bended in appropriate proportion to prepare the Ni–5 at.% W alloy composition. The multilayer compacts were then prepared by co-pressing the constituent powder layers arranged in appropriate sequence at 250 MPa in a die having a rectangular cross-section of dimension 8 mm × 25 mm followed by sintering at 1100 ◦ C for one hour in flowing H2 atmosphere in a horizontal tube furnace. The process has been shown schematically in Fig. 1 for the double layer compact preparation. These sintered compacts were subsequently cold rolled to ∼95% reduction in thickness. The cold rolled multilayer tapes were then annealed at temperatures ranging between 500 and 1000 ◦ C for one hour. In addition to single step heat treatment, the effect of a two step annealing (TSA) treatment comprising of holding the sample at the lowest end of the temperature for 15 min (e.g. 500 ◦ C) followed by holding at the highest temperature (e.g. 1000 ◦ C) for 45 min was also investigated. The purpose of the TSA treatment was to nucleate the cube grains at lower temperatures and letting it grow at faster rates at higher temperatures to obtain a very sharp cube component, as suggested by some authors [11]. Microtextures and bulk textures of the samples were measured on the Ni side(s) only as this would only act as the template for the epitaxial growth of the buffer and superconducting layers. The bulk textures of the samples were measured with a Siemens ˚ radiaD5000 texture goniometer using Co K␣ (λ = 1.789 A) tion. The {1 1 1}, {2 0 0}, {2 2 0} and {3 1 1} pole figures were obtained for each sample and ODFs were calculated from them using the Bunge method [12]. Microtextures of the annealed samples were measured with a fully computer controlled automated electron back scatter diffraction (EBSD) system attached to a FEI scanning electron microscope (QUANTA 200) using the TSL’s OIM analysis Version 4 software. Volume fractions of the texture components in the annealed materials were determined within a spread of 15◦ around the respective ideal orientations. The room temperature mechanical properties of the tapes were evaluated with an INSTRON Universal Floor Model tester (Model 1195, 100 KN capacity). 3. Results and discussion 3.1. Microstructural observation on the cross-section of the composite tapes Fig. 2a and b, respectively, shows a representative SEM micrograph (back scatter electron mode) of typical crosssectional areas of the Ni/Ni–5 at.% W sintered compact and the
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Fig. 5. The (1 1 1) pole figures of the Ni side of the multilayer tape Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment.
corresponding X-ray area map depicting the distribution of W in the constituent layers. The X-ray area mapping in Fig. 2b clearly indicates that the alloying element W has diffused into the Ni layer from the alloy side during the sintering stage itself. Similar observations were made on the multilayer tape having triple layer configuration. Fig. 3a and b shows the SEM micrographs of typical crosssectional areas of Ni/Ni–5 at.% W tape after annealing at 500 and 1000 ◦ C, respectively. The Ni layer of the Ni/Ni–5 at.% W tape has clearly recrystallized at 500 ◦ C (Fig. 3a). However, the Ni–5 at.% W layer has retained the cold worked structure in this annealed condition due to its higher recrystallization temperature. At 1000 ◦ C, however, both these layers have recrystallized (Fig. 3b). Fig. 4a and b represents SEM micrographs of typical crosssectional areas of the tri-layer tape after annealing at 500 and 1000 ◦ C, respectively. At lower recrystallization temperature the core material has retained its deformation structure. At 1000 ◦ C all the constituent layers have recrystallized. In all cases, the interface(s) between the constituent layers are very sharp and continuous as may be clearly seen from
these microstructures indicating that during the deformation process the individual layers have maintained their identity. Evidently, excellent bond strength between the constituent layers is achieved after sintering and delamination of the layers does not occur.
3.2. Textural and mechanical properties The deformation textures measured on the Ni side(s) of the multilayer tapes in all cases are found to be quite similar to the deformation texture that of a stand-alone monolithic pure Ni tape (prepared using the same Ni powder and strained to similar level of deformation by cold rolling) showing typical pure metal or ‘Cu type’ deformation texture [10]. Due to this reason the deformation textures have not been shown here separately. Fig. 5a–c shows the calculated (1 1 1) pole figures measured from the Ni side (determined by XRD) of the double-layer Ni/Ni–5 at.% W after annealing at 500, 1000 ◦ C and after the TSA treatment. All these figures indicate the development of a cube texture.
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Fig. 6. The (1 1 1) pole figures of the Ni side of the multilayer tape Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment.
The (1 1 1) pole figures from the Ni side(s) of the tri-layer tape have been shown in Fig. 6a–c. The formation of cube texture is also quite evident from the pole figures. In order to have a clearer idea of the development of cube texture and the grain boundary character distributions in all the annealed materials, each of them was subjected to a thorough EBSD study. The volume fractions of the cube component and its twin ({2 2 1}1 2 2) component were determined within a spread of 15◦ around the respective ideal orientations. Fig. 7a–c shows the crystal orientation maps of the Ni side of Ni/Ni–5 at.% W tape showing the volume fractions and spatial distributions of the cube oriented grains and their twins after annealing at 500, 1000 ◦ C and after the TSA treatment. The volume fraction of the cube component after annealing at 500 and 1000 ◦ C are almost similar. The volume fraction of the cube component increases, though not much, after the TSA treatment. The volume fractions of the twin of the cube component are also similar after annealing at 500 and 1000 ◦ C. The twin volume fraction decreases in the TSA treated condition. Similar crystal orientation maps from the Ni side of the trilayer tapes in the three annealed states have been shown in Fig. 8a–c. Here also, the volume fraction of the cube component
does not show any substantial increase after high temperature annealing. However, the most striking point is the enhancement of cube texture on the Ni side(s) of these multilayer tapes as compared to a similarly deformed and recrystallized monolithic pure Ni tape prepared from the same Ni powder. Fig. 9 shows the crystal orientation map for the Ni tape after annealing at 500 ◦ C. A comparison of the crystal orientation maps shown in Figs. 7a, 8a and 9 should clearly reveal the increase in volume fraction of the cube component in the Ni layer(s) of the multilayer tapes. Earlier studies on the development of deformation and recrystallization textures on pure Ni and Ni–5 at.% W alloy by the present authors [10] have shown that the presence of W in solid solution in Ni enhances the formation of cube texture although the deformation texture of the two materials do not reveal any perceptible difference, qualitatively as well as quantitatively. The difference in the recrystallization textures of the two materials is attributed to the presence of a strong RD rotated cube component in pure Ni as compared to the Ni–W alloy. The presence of W is found to enhance the nucleation of more near cube oriented grains in the Ni–W alloy in contrast to the nucleation of predominantly RD-rotated cube grains in pure Ni at a very
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Fig. 7. Crystal orientation maps depicting the spatial distribution and volume fractions of cube grains (blue) and their twins (red) in the Ni side of Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
early stage of recrystallization leading to a much sharper cube texture in the former material. Coming back to the present situation, the cross-sectional EDS analyses in form of X-ray area mapping of the double as well as triple layer compacts in the sintered condition clearly reveal the diffusion of W from the alloy side to pure Ni side(s) (see Fig. 2b). The concurrent increase in the cube volume fraction on the Ni side(s) of the tapes as compared to a stand alone monolithic Ni tape thus elucidates the beneficial effect of the alloying element W in increasing the volume fraction of the cube component in pure Ni. Apart from the requirement of sharp cube texture another important consideration is the mechanical strength of the substrate materials. Thus, room temperature tensile properties of the annealed multilayer tapes were determined and compared to a monolithic Ni tape with similar average grain size. Fig. 10 summarizes the engineering stress–strain plots for the double and tri-layer tapes after annealing at 1000 ◦ C and the monolithic Ni tape in the 800 ◦ C annealed condition. The yield strength of
the double (∼120 MPa) and triple (∼175 MPa) layer tapes show an increase by a factor of about 1.5 and 2, respectively, over that of the pure Ni (∼80 MPa) tape. However, the tensile elongation of the monolithic Ni tape is found to be considerably higher than that of the multilayer tapes. The possibility of development of sharp cube texture and improved mechanical properties of these tapes opens up new opportunities for preparing multilayer substrate tapes through this simple powder metallurgy based technique investigated in the current study. One such possibility could be using an outer layer(s) more rich in W content (e.g. a Ni–5 at.% W layer) and a core material with even higher alloy content. With the presence of W actually proven to be beneficial for development of cube texture such configuration can lead to very sharp cube texture in the outer layer and a significant strengthening of the derived tapes. The intensity of the cube texture in such multilayer tapes can be further enhanced by an optimized two stage rolling (TSR) procedure investigated by these authors earlier [10]. Work is currently under progress along this line.
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Fig. 8. Crystal orientation maps depicting the spatial distribution and volume fractions of cube grains (blue) and their twins (red) in the Ni side of Ni/Ni–5 at.% W/Ni tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 10. Engineering stress–strain plots for different substrate materials.
4. Conclusions
Fig. 9. Crystal orientation map of monolithic Ni tape annealed at 500 ◦ C.
(i) A simple powder metallurgy based technique has been developed which can successfully be employed to fabricate multilayer tapes for use as substrate for coated superconductor applications. (ii) The cube texture in the Ni layer(s) of the multilayer tapes is found to be much stronger than monolithic pure Ni tapes although the deformation texture appears to be similar.
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(iii) The cross-sectional EDS analyses in form of X-ray area mapping of the as-sintered compacts reveal considerable diffusion of W from the alloy side to the Ni side(s) which elucidates the beneficial role of W in enhancing the cube volume fraction in Ni, amply corroborating the earlier observation by the present authors. (iv) The yield strength of the derived tapes shows significant improvement as compared to monolithic pure Ni tape although the tensile elongation drops.
[2]
[3] [4] [5]
[6]
Acknowledgement [7]
The authors are grateful to Professor Francis Wagner, Director, LETAM, University of Metz, France, for kindly providing laboratory facilities for the bulk texture measurement of a number of samples. References [1] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Mar-
[8] [9] [10] [11] [12]
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tin, C.E. Klabunde, E. Hartfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795–1797. D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Q. He, B. Saffian, M. Paranthaman, C.E. Klabunde, D.F. Lee, B.C. Sales, F.A. List, Science 274 (1996) 755–757. D. Dimos, P. Chaudhari, J. Mannhart, F.K. Legoues, Phys. Rev. Lett. 61 (1988) 219–222. H.G. M¨uller, Z. Metallkd. 31 (1939) 161–167. A. Goyal, F.A. List, J. Mathis, M. Paranthaman, E.D. Specht, D.P. Norton, C. Park, D.F. Lee, D.M. Kroeger, D.K. Christen, J.D. Budai, P.M. Martin, J. Supercond. 11 (1998) 481–487. J. Eickemeyer, D. Selbmann, R. Opitz, B. de Boer, B. Holzapfel, L. Schultz, U. Miller, Supercond. Sci. Technol. 14 (2001) 152–159. E. Varesi, V. Boffa, G. Celentano, L. Ciontea, F. Fabbri, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, A. Vannozzi, Phys. C 372 (2002) 763–766. V.S. Sarma, J. Eickemeyer, A. Singh, L. Schultz, B. Holzapfel, Acta Mater. 51 (2003) 4919–4927. V.S. Sarma, B. de Boer, J. Eickemeyer, B. Holzapfel, Scr. Mater. 48 (2003) 1167–1171. P.P. Bhattacharjee, R.K. Ray, Mater. Sci. Eng. A 459 (2007) 309–323. V.S. Sarma, J. Eickemeyer, L. Schultz, B. Holzapfel, Scr. Mater. 50 (2004) 953–957. H.J. Bunge, Mathematische Methoden der Texturanalyse, Academic Press, Berlin, 1969.
Texture and mechanical properties of cold deformed and annealed multilayer Ni base substrate tapes prepared by a powder metallurgy route P.P. Bhattacharjee a,∗ , R.K. Ray b , A. Upadhyaya c a
ARC Centre of Excellence-Design in Light Metals, Deakin University, Geelong, VIC 3217, Australia b R&D Division, TATA Steel, Jamshedpur 831001, India c Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 22 September 2007; received in revised form 24 October 2007; accepted 24 October 2007
Abstract An innovative powder metallurgy based technique has been investigated to fabricate multilayer tapes having configurations Ni/Ni–5 at.% W and Ni/Ni–5 at.% W/Ni for use as mechanically strong and textured substrates for coated superconductor applications. Development of cube texture ({0 0 1}1 0 0) following heavy cold rolling (∼95%) and annealing has been studied in the Ni side(s) of these multilayer tapes and this has been compared to a monolithic Ni tape. The deformation textures in the Ni side(s) of the tapes are found to be quite similar to that of monolithic Ni strained to similar level of deformation. However, the cube texture upon annealing has been found to be significantly stronger in the Ni side(s) of the multilayer tapes as compared to the monolithic Ni tape after different annealing treatments. Cross-sectional EDS analyses in form of X-ray area mapping of the multilayer tapes reveal significant diffusion of W from the alloy side to the Ni side(s). All these facts amply demonstrate the beneficial role of the alloying element W on the development of cube texture in Ni. Finally, the mechanical properties of the multilayer tapes have been evaluated to ascertain their suitability in the actual application scenario. © 2007 Elsevier B.V. All rights reserved. Keywords: Nickel alloys; Powder metallurgy; Multilayer substrate; Cube texture; EBSD
1. Introduction The rolling assisted biaxially textured substrates (RABiTSTM ) method developed at ORNL, USA is a very promising route for making highly cube textured ({0 0 1}1 0 0), long, flexible substrate tapes for coated high temperature superconductor (HTS) applications [1,2]. A necessary condition for improving the critical current densities of the superconducting tapes is to remove high angle grain boundaries which typically show the ‘weak link’ behaviour [3]. In the RABiTSTM method this is achieved by the epitaxial growth of the buffer and superconducting layers on highly cube textured metallic substrates. The sharp cube texture of the substrate material ensures the presence of grains of almost a single orientation and thus the grain boundary network consists of predominantly low angle boundaries.
∗
Corresponding author. Tel.: +61 3 5227 2112; fax: +61 3 5227 1103. E-mail address: [email protected] (P.P. Bhattacharjee).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.065
Pure Ni received the initial attention for use as a substrate material due to the possibility of developing a sharp cube texture after heavy cold rolling and annealing [4]. The oxidation resistance of pure Ni and its small lattice mismatch with overlying buffer layers are also favourable factors for the epitaxial growth of the buffer and superconducting layers. Current densities in excess of 106 A/cm2 have been reported in Ni base substrate tapes with certain buffer layer configurations [5]. To increase the engineering current density (i.e. the total current carried by the HTS film divided by the total cross-sectional area of the whole tape) the substrate thickness should be kept to a minimum. Unfortunately, poor mechanical strength of pure Ni in annealed condition makes it difficult to prepare very thin coated Ni tapes involving a reel-to-reel deposition process of the buffer and superconducting layers. Hence, it becomes imperative to increase the mechanical strength of pure Ni through alloying addition via the solid solution strengthening mechanism. However, the addition of alloying elements and impurities in general are considered detrimental to the development of sharp cube texture. Therefore, the choice of alloying elements should
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Fig. 1. Flow diagram of the double layer compact preparation.
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be such that they do not impair the possibility of developing a sharp cube texture in the substrate. Ni base alloys such as Ni–W and Ni–Mo have previously been suggested which can develop sharp cube texture after cold deformation and annealing and these alloys also have sufficient mechanical strength at the deposition temperatures of the buffer and superconducting layers [6,7]. A novel approach to improve the sharpness of cube texture and mechanical strength simultaneously is to design substrates with laminated or multilayer architecture. The advantage of this approach is that the different constituent layers can serve different purposes. Recently, tapes with such multilayer architecture have been reported with sharp cube texture intensity, good oxidation resistance and mechanical strength [8,9]. Sarma et al. [8,9] prepared a variety of multilayer substrates by inserting a highly strengthened alloy rod (core) into a Ni–W tube (outer layer) followed by the hot rolling, cold rolling and finally annealing of the whole assembly. The hot rolling step is introduced to develop the
Fig. 2. (a) A typical cross-sectional area of Ni/Ni–5 at.% W sintered compact and (b) the X-ray mapping of the same area showing the distribution of W.
Fig. 3. SEM micrographs of typical cross-sectional area of Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C and (b) 1000 ◦ C.
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ing and casting route which can inadvertently introduce some impurities during the melting stage itself. The development of textures and mechanical properties has been studied to ascertain the suitability of such multilayered tapes for use as substrates for coated superconductor applications. 2. Experimental
Fig. 4. SEM micrographs of typical cross-sectional area of Ni/Ni–5 at.% W/Ni tape after annealing at (a) 500 ◦ C and (b) 1000 ◦ C.
requisite bond strength between the different layers. However, possibility of deterioration in the intensity of the cube texture upon cold rolling and annealing owing to the introduction of the hot rolling step cannot be ruled out completely. In contrast, the current work explores the possibility of employing an innovative powder metallurgy based technique to prepare multilayer substrates having configurations Ni/Ni–5 at.% W and Ni/Ni–5 at.% W/Ni, respectively. This method has the advantage that it does not employ hot rolling which was introduced in the earlier processes [8,9] to develop strong bonding between the outer layer and the inner core. In addition, powder metallurgy route has the advantage of exercising greater control over the purity of material by starting out with high purity elemental powders as compared to the melt-
Elemental powders of Ni and (∼99.99% purity), W (∼99.6% purity) were used as the starting materials. The important characteristics of the powders have been given elsewhere [10]. The elemental powders were first bended in appropriate proportion to prepare the Ni–5 at.% W alloy composition. The multilayer compacts were then prepared by co-pressing the constituent powder layers arranged in appropriate sequence at 250 MPa in a die having a rectangular cross-section of dimension 8 mm × 25 mm followed by sintering at 1100 ◦ C for one hour in flowing H2 atmosphere in a horizontal tube furnace. The process has been shown schematically in Fig. 1 for the double layer compact preparation. These sintered compacts were subsequently cold rolled to ∼95% reduction in thickness. The cold rolled multilayer tapes were then annealed at temperatures ranging between 500 and 1000 ◦ C for one hour. In addition to single step heat treatment, the effect of a two step annealing (TSA) treatment comprising of holding the sample at the lowest end of the temperature for 15 min (e.g. 500 ◦ C) followed by holding at the highest temperature (e.g. 1000 ◦ C) for 45 min was also investigated. The purpose of the TSA treatment was to nucleate the cube grains at lower temperatures and letting it grow at faster rates at higher temperatures to obtain a very sharp cube component, as suggested by some authors [11]. Microtextures and bulk textures of the samples were measured on the Ni side(s) only as this would only act as the template for the epitaxial growth of the buffer and superconducting layers. The bulk textures of the samples were measured with a Siemens ˚ radiaD5000 texture goniometer using Co K␣ (λ = 1.789 A) tion. The {1 1 1}, {2 0 0}, {2 2 0} and {3 1 1} pole figures were obtained for each sample and ODFs were calculated from them using the Bunge method [12]. Microtextures of the annealed samples were measured with a fully computer controlled automated electron back scatter diffraction (EBSD) system attached to a FEI scanning electron microscope (QUANTA 200) using the TSL’s OIM analysis Version 4 software. Volume fractions of the texture components in the annealed materials were determined within a spread of 15◦ around the respective ideal orientations. The room temperature mechanical properties of the tapes were evaluated with an INSTRON Universal Floor Model tester (Model 1195, 100 KN capacity). 3. Results and discussion 3.1. Microstructural observation on the cross-section of the composite tapes Fig. 2a and b, respectively, shows a representative SEM micrograph (back scatter electron mode) of typical crosssectional areas of the Ni/Ni–5 at.% W sintered compact and the
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Fig. 5. The (1 1 1) pole figures of the Ni side of the multilayer tape Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment.
corresponding X-ray area map depicting the distribution of W in the constituent layers. The X-ray area mapping in Fig. 2b clearly indicates that the alloying element W has diffused into the Ni layer from the alloy side during the sintering stage itself. Similar observations were made on the multilayer tape having triple layer configuration. Fig. 3a and b shows the SEM micrographs of typical crosssectional areas of Ni/Ni–5 at.% W tape after annealing at 500 and 1000 ◦ C, respectively. The Ni layer of the Ni/Ni–5 at.% W tape has clearly recrystallized at 500 ◦ C (Fig. 3a). However, the Ni–5 at.% W layer has retained the cold worked structure in this annealed condition due to its higher recrystallization temperature. At 1000 ◦ C, however, both these layers have recrystallized (Fig. 3b). Fig. 4a and b represents SEM micrographs of typical crosssectional areas of the tri-layer tape after annealing at 500 and 1000 ◦ C, respectively. At lower recrystallization temperature the core material has retained its deformation structure. At 1000 ◦ C all the constituent layers have recrystallized. In all cases, the interface(s) between the constituent layers are very sharp and continuous as may be clearly seen from
these microstructures indicating that during the deformation process the individual layers have maintained their identity. Evidently, excellent bond strength between the constituent layers is achieved after sintering and delamination of the layers does not occur.
3.2. Textural and mechanical properties The deformation textures measured on the Ni side(s) of the multilayer tapes in all cases are found to be quite similar to the deformation texture that of a stand-alone monolithic pure Ni tape (prepared using the same Ni powder and strained to similar level of deformation by cold rolling) showing typical pure metal or ‘Cu type’ deformation texture [10]. Due to this reason the deformation textures have not been shown here separately. Fig. 5a–c shows the calculated (1 1 1) pole figures measured from the Ni side (determined by XRD) of the double-layer Ni/Ni–5 at.% W after annealing at 500, 1000 ◦ C and after the TSA treatment. All these figures indicate the development of a cube texture.
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Fig. 6. The (1 1 1) pole figures of the Ni side of the multilayer tape Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment.
The (1 1 1) pole figures from the Ni side(s) of the tri-layer tape have been shown in Fig. 6a–c. The formation of cube texture is also quite evident from the pole figures. In order to have a clearer idea of the development of cube texture and the grain boundary character distributions in all the annealed materials, each of them was subjected to a thorough EBSD study. The volume fractions of the cube component and its twin ({2 2 1}1 2 2) component were determined within a spread of 15◦ around the respective ideal orientations. Fig. 7a–c shows the crystal orientation maps of the Ni side of Ni/Ni–5 at.% W tape showing the volume fractions and spatial distributions of the cube oriented grains and their twins after annealing at 500, 1000 ◦ C and after the TSA treatment. The volume fraction of the cube component after annealing at 500 and 1000 ◦ C are almost similar. The volume fraction of the cube component increases, though not much, after the TSA treatment. The volume fractions of the twin of the cube component are also similar after annealing at 500 and 1000 ◦ C. The twin volume fraction decreases in the TSA treated condition. Similar crystal orientation maps from the Ni side of the trilayer tapes in the three annealed states have been shown in Fig. 8a–c. Here also, the volume fraction of the cube component
does not show any substantial increase after high temperature annealing. However, the most striking point is the enhancement of cube texture on the Ni side(s) of these multilayer tapes as compared to a similarly deformed and recrystallized monolithic pure Ni tape prepared from the same Ni powder. Fig. 9 shows the crystal orientation map for the Ni tape after annealing at 500 ◦ C. A comparison of the crystal orientation maps shown in Figs. 7a, 8a and 9 should clearly reveal the increase in volume fraction of the cube component in the Ni layer(s) of the multilayer tapes. Earlier studies on the development of deformation and recrystallization textures on pure Ni and Ni–5 at.% W alloy by the present authors [10] have shown that the presence of W in solid solution in Ni enhances the formation of cube texture although the deformation texture of the two materials do not reveal any perceptible difference, qualitatively as well as quantitatively. The difference in the recrystallization textures of the two materials is attributed to the presence of a strong RD rotated cube component in pure Ni as compared to the Ni–W alloy. The presence of W is found to enhance the nucleation of more near cube oriented grains in the Ni–W alloy in contrast to the nucleation of predominantly RD-rotated cube grains in pure Ni at a very
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Fig. 7. Crystal orientation maps depicting the spatial distribution and volume fractions of cube grains (blue) and their twins (red) in the Ni side of Ni/Ni–5 at.% W tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
early stage of recrystallization leading to a much sharper cube texture in the former material. Coming back to the present situation, the cross-sectional EDS analyses in form of X-ray area mapping of the double as well as triple layer compacts in the sintered condition clearly reveal the diffusion of W from the alloy side to pure Ni side(s) (see Fig. 2b). The concurrent increase in the cube volume fraction on the Ni side(s) of the tapes as compared to a stand alone monolithic Ni tape thus elucidates the beneficial effect of the alloying element W in increasing the volume fraction of the cube component in pure Ni. Apart from the requirement of sharp cube texture another important consideration is the mechanical strength of the substrate materials. Thus, room temperature tensile properties of the annealed multilayer tapes were determined and compared to a monolithic Ni tape with similar average grain size. Fig. 10 summarizes the engineering stress–strain plots for the double and tri-layer tapes after annealing at 1000 ◦ C and the monolithic Ni tape in the 800 ◦ C annealed condition. The yield strength of
the double (∼120 MPa) and triple (∼175 MPa) layer tapes show an increase by a factor of about 1.5 and 2, respectively, over that of the pure Ni (∼80 MPa) tape. However, the tensile elongation of the monolithic Ni tape is found to be considerably higher than that of the multilayer tapes. The possibility of development of sharp cube texture and improved mechanical properties of these tapes opens up new opportunities for preparing multilayer substrate tapes through this simple powder metallurgy based technique investigated in the current study. One such possibility could be using an outer layer(s) more rich in W content (e.g. a Ni–5 at.% W layer) and a core material with even higher alloy content. With the presence of W actually proven to be beneficial for development of cube texture such configuration can lead to very sharp cube texture in the outer layer and a significant strengthening of the derived tapes. The intensity of the cube texture in such multilayer tapes can be further enhanced by an optimized two stage rolling (TSR) procedure investigated by these authors earlier [10]. Work is currently under progress along this line.
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Fig. 8. Crystal orientation maps depicting the spatial distribution and volume fractions of cube grains (blue) and their twins (red) in the Ni side of Ni/Ni–5 at.% W/Ni tape after annealing at (a) 500 ◦ C, (b) 1000 ◦ C and after the (c) TSA treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 10. Engineering stress–strain plots for different substrate materials.
4. Conclusions
Fig. 9. Crystal orientation map of monolithic Ni tape annealed at 500 ◦ C.
(i) A simple powder metallurgy based technique has been developed which can successfully be employed to fabricate multilayer tapes for use as substrate for coated superconductor applications. (ii) The cube texture in the Ni layer(s) of the multilayer tapes is found to be much stronger than monolithic pure Ni tapes although the deformation texture appears to be similar.
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(iii) The cross-sectional EDS analyses in form of X-ray area mapping of the as-sintered compacts reveal considerable diffusion of W from the alloy side to the Ni side(s) which elucidates the beneficial role of W in enhancing the cube volume fraction in Ni, amply corroborating the earlier observation by the present authors. (iv) The yield strength of the derived tapes shows significant improvement as compared to monolithic pure Ni tape although the tensile elongation drops.
[2]
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[6]
Acknowledgement [7]
The authors are grateful to Professor Francis Wagner, Director, LETAM, University of Metz, France, for kindly providing laboratory facilities for the bulk texture measurement of a number of samples. References [1] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Mar-
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