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Development of cube textured Ni–5at.%W alloy substrates for YBCO coated conductor application using a powder metallurgy process
Physica C 463–465 (2007) 604–608 www.elsevier.com/locate/physc
Development of cube textured Ni–5at.%W alloy substrates for YBCO coated conductor application using a powder metallurgy process S.-S. Kim a, J.-S. Tak a, S.-Y. Bae a, J.-K. Chung b, I.-S. Ahn a, C.-J. Kim a, K.-W. Kim a, K.-K. Cho a,* a
School of Nano and Advanced Materials Engineering, K-MEM R&D CLUSTER and i-cube Center, Gyeongsang National University, 900 Gazwa-dong, Jinju, Gyeongnam 660-701, Republic of Korea b Institute of Industrial Technology, Changwon National University, Changwon, Kyungnam 641-120, Republic of Korea Received 30 October 2006; accepted 10 April 2007 Available online 24 May 2007
Abstract In this paper, Ni–5at.%W alloy substrate for YBCO coated conductor was fabricated by a dry powder metallurgy process including powder compaction, cold isostatic pressing (CIP), cold rolling and annealing for recrystallization. Ni and W powders were ball-milled at this process for various times of 10, 30, 50 and 100 h in argon atmosphere. The rod-like Ni–W alloy compacts were sintered at 1150 C for 1 h in 96%Ar–4%H2 atmosphere. The sintered rods were cold rolling into thin tape of 70–90 lm thickness with 5% reduction at each path. The Ni–W alloy tapes were annealed at 800–1200 C in an atmosphere of 96%Ar–4%H2 mixing gas for the development of cube texture. The tape with the best properties of low surface roughness, small grain size and strong cube texture was obtained at the condition annealed at 1200 C using ball-milled powder for 30 min. The W addition to Ni improved the mechanical properties by solid solution hardening and inhibited grain growth for annealing heat treatment. The tapes were characterized by X-ray pole-figure, optical microscopy (OM), scanning electron microscopy (SEM) and scanning probe microscopy (SPM). 2007 Elsevier B.V. All rights reserved. PACS: 84.71.Mn; 81.20.Ev Keywords: Ni–5at.% alloy tape; Cube texture; Powder metallurgy
1. Introduction The rolling assisted biaxially textured substrate (RABiTS) method developed at ORNL, USA demonstrated the tremendous possibility of using highly cube textured nickel as a substrate for coated superconductor applications [1,2]. Pure Ni substrate has workability and forms a strong cube texture by cold rolling and annealing. However, pure Ni has low mechanical quality and thermal grooving after recrystallization heat treatment. So much research has been directed toward Ni-alloy substrate, and
*
Corresponding author. Tel.: +82 055 751 6292; fax: +82 055 759 1745. E-mail address: [email protected] (K.-K. Cho).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.258
it was suggested that W, Cr, Mo, V, Al, and Cu are among the more suitable alloying elements for obtaining those improved properties [3,4]. Tungsten was good candidate for the alloying elements because of the high solubility of these elements to Ni. Generally, the Ni-alloy rods which are used for the fabrication of the substrate are made by the melting techniques and powder metallurgy. When the Ni-alloy rods are made by melting, the grain size control is not easy and the impurity can enter into the alloy from a crucible, which affects negatively the recrystallization behavior of Ni. A powder metallurgy process has advantages of an easy control of the initial grain size and lowering the processing temperature compared to the melting process. Powder metallurgy process was classified into two large groups which
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
are wet and dry process. In a wet process, mixing powder before compacting is prepared by milling of Ni, addition element, solution (alcohol, acetone, etc.) and balls in jar. There is no difference between the two processes, except for whether the solution would be used or not. The wet process has been mainly used in the previous work. But the wet process has disadvantages, such as an influx of impurities from solution and long ball milling time caused by low impact energy of balls, compared to the dry process. In this study, therefore, the rod of Ni–5at.%W alloy was prepared by a dry powder metallurgy process including powder packing, cold isostatic pressing and sintering. The rod was made into tape with a thickness of 70–90 lm and biaxial texture by cold rolling and annealing for recrystallization. We evaluated the effects of annealing temperature on the texture, grain size and surface morphology. In addition, we examined the effects of W content in Ni on texture, grain size and surface morphology. And the results in this work were compared to those of tape fabricated by existing wet powder metallurgy and melting technique. 2. Experiments procedure Nickel powder of purity 99.9% of 5 lm size and tungsten powder of purity 99.9%, 1 lm size was used as the starting material. Ni alloy of composition Ni–5at.%W was prepared by a dry powder metallurgy method. Ni and W powders were ball-milled at this process for various times of 10, 30, 50 and 100 h in Argon atmosphere. Ball-milled powder was loaded into a urethane mold (20 mm diameter and 130 mm length), and cold isostatic pressing (CIP) of 200 MPa was applied to form a rod-type compact. The green compact was placed at the center of a tubular furnace, sintered at 1150 C for an hour atmosphere of 96%Ar and 4%H2. The sintered Ni–5at.%W alloy rods were cut by milling machine. After cutting the thickness, the width and length was 7, 10 and 80 mm, respectively. The cutting rectangular bar was cold-rolled at 5% reduction to the final thickness of 70–90 lm using a two-high rolling mill. Finally, the sample was heat treated at temperatures between 800 and 1200 C for recrystallization in 96%Ar–4%H2 atmosphere. Annealing for recrystallization held 30 min in all experiment condition. Microstructure observation was performed by optical microscopy (OM) and scanning electron microscopy (SEM). The texture of the Ni alloy tapes were analyzed by X-ray diffraction (XRD). The textures were determined by measuring polefigures by means of an automated X-ray texture goniometer (BRUKER, D8 discover). Surface roughness and thermal grain grooving were evaluated by scanning probe microscopy (SPM) over a 40 · 40 lm2 area. 3. Results and discussion Ni–5at%W powders were milled for 10, 30, 50 and 100 h using general ball milling process. SEM images of powders
605
Fig. 1. SEM photographs of Ni–5at.%W powder on ball-milling time: (a) 10 h, (b) 30 h, (c) 50 h and (d) 100 h.
ball-milled at various milling times are shown in Fig. 1. In the cases of ball-milled powder for 10 h, W particles with diameter of 1 lm were frequently observed (marked with arrowed line). But the particles were not observed in powder ball-milled for 30 h. This means that homogeneous Ni– W alloy powder can be obtained after ball milling for 30 h. For the powders ball-milled for 30, 50 and 100 h, there are almost no significant change with the increase of milling time. In order to obtain optimum ball milling time, each ball-milled powder was compacted at pressure of 140 kg/ cm2 using general bi-axial compacting process, and then were sintered at 1150 C for 1 h in atmosphere of 96%Ar and 4%H2. The green compact density of powders ballmilled for 10, 30, and 50 h was 6.14, 6.36 and 6.02 g/cm3, respectively. In powder ball-milled for 100 h, but, the compacts were not formed due to the high strain energy stored within each particles. Fig. 2 shows the pore characteristics of the sintered Ni–W alloy. The porosity is the lowest in powder ball-milled for 30 h, as shown in Fig. 2b. Therefore, we hereafter use powder ball-milled for 30 h in all experiments. Fig. 3 shows the log-scale XRD patterns of as coldrolled and texture-annealed Ni–W tape at various temperatures after cold rolling. The XRD pattern of the as cold-rolled samples consists of various peaks such as (1 1 1), (2 0 0) and (2 2 0). To understand the effect of annealing temperature on the development of the (2 0 0) texture (the ‘‘cube texture or annealing texture’’), annealing temperature was changed from 800 to 1200 C. With increasing annealing temperature, the intensity of (1 1 1) and (2 2 0) peaks, which means deformation texture decreased, and that of (2 0 0) peak which means annealing texture increased. As can be seen in Fig. 3, the XRD pattern of the annealed Ni–W tape at 1200 C shows only the (2 0 0) peak, which indicates that (2 0 0) texture was formed by texture annealing. We evaluated the texture of the cold-rolled tape and the annealed substrate of both Ni–5at.%W alloys; selected (1 1 1) pole-figure are presented in Fig. 4. In Fig. 4a, the tex-
606
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
Fig. 2. OM images of sintered Ni–5at.%W on ball-milling time: (a) 10 h, (b) 30 h and (c) 50 h.
Intensity (log-scale)
1200°C
1000°C
800°C (220)
(111)
as rolled (200)
40
50
60
70
80
90
2 Theta (degree) Fig. 3. XRD diffraction patterns of the Ni–5at.%W alloy tapes annealed at various temperatures for 30 min.
deformation texture and better symmetry of poles for the Ni alloy tape is partly related to the reduction of stacking fault energy resulting from the presence of tungsten in nickel. Fig. 4b–d are selected sets of (1 1 1) pole-figures of substrates annealed at various temperature of 800– 1200 C. Ni–5at.%W substrate annealed at 800 C has some bass deformation texture, because annealing temperature does not enough. However, Ni–5at.%W substrates annealed at 1000 C and 1200 C have good cube texture. The full-width half maximum (FWHM) of in-plane (uscan) and out-of-plane (x-scan) texture values are plotted in Fig. 5. The average FWHM values of in-plane texture for various temperatures of 800, 1000 and 1200 C are 10.73, 10.06 and 9.05, respectively. The FWHM values of out-of-plane texture for various temperatures of 800, 1000 and 1200 C are 7.3, 7.0 and 5.85, respectively. The variation in FWHM values for Ni–5at.%W substrate annealed at various temperatures show the obvious dependence on annealing temperature. The FWHM values are similar to that reported in literature elsewhere [5]. Fig. 6 shows optical micrographs of top views of annealed Ni–W substrates at various temperatures. Grain size increased with increasing annealing temperature. The average grain size of Ni–W substrates annealed at 800, 1000 and 1200 C was 9.0, 12.1 and 20.5 lm, respectively. The average grain size is smaller than those of tape fabricated by existing a melting technique [6] and a wet powder metallurgy process [7]. In general, abnormal grain growth
In-plane
14
out-of-plane
FWHM (deg)
12
Fig. 4. (1 1 1) Pole-figures of the Ni–5at.%W alloy tapes annealed at: (a) as rolled and (b) 800 C, (c) 1000 C and (d) 1200 C for 30 min.
10
8
6
4
ture pattern of cold-rolled sample was typical brass deformation texture and the poles were symmetric. The texture components, usually observed in deformed FCC materials, are known to transform well to cube texture after annealing for recrystallization. It is supposed that stronger brass
800
1000
1200
Annealing Temperature(°C) Fig. 5. Change of the in-plane and out-of-plane of the Ni–5at.%W alloy tapes annealed at various temperatures for 30 min.
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
607
Fig. 6. OM images of etched Ni–5at.%W alloy tapes annealed at: (a) 800 C, (b) 1000 C and (c) 1200 C for 30 min.
Fig. 7. SPM images of Ni–5at.%W alloy tapes annealed at: (a) as rolled and (b) 800 C, (c) 1000 C and (d) 1200 C for 30 min.
is often observed when grain growth is inhibited by the presence of inclusions, relative grain size effects, or development of a strong preferred orientation [8]. In our samples, however, abnormal grain growth and severe increase of grain size were not observed. It may be related to the existence of solute atom (W), which restrict grain growth during heat treatment (the ‘‘solute drag effect’’). The surface roughness of the substrate after annealing is rather important because a flat and thin tape has to be used for the further deposition of epitaxial buffer layers and superconducting layers. SPM was used to examine the surface morphology and roughness. Fig. 7 shows SPM images of the Ni–5at.%W alloy substrates annealed at various temperatures. The SPM scan on Ni–5at.%W substrates after cold-rolled and annealed at various temperature of 800, 1000 and 1200 C gave a mean roughness over a 40 · 40 lm area of about 11, 7, 6 and 3 nm, respectively. With increasing annealing temperature, the surface roughness decreased, which can be due to grain growth. The surface roughness of substrates fabricated in this work, is
better than those of substrates obtained by wet powder metallurgy and melting method reported in literature elsewhere [9,10]. In this work, the grooves are however observed in the substrate annealed at 1200 C, as shown in Fig. 7d. A problem arising with high temperature annealing is high angle grain boundary grooving [11,12]. The width and depth of these grain boundary grooves depend on annealing temperature and time. A deep groove in the substrate could give rise to a disruption of the epitaxy of YBCO, resulting in decrease of critical current density in the final YBCO coated conductors. In order to have fabrication of a substrate with no groove, small grain size and the optimum texture, carefull control of the annealing temperature and time is necessity. 4. Conclusion In conclusion, we applied the dry powder metallurgy to the fabrication of Ni–W tape for YBCO coated conductors. The Ni–W alloy tape with highly (2 0 0) texture (the
608
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
‘‘cube texture or annealing texture’’) was fabricated by cold rolling and texture annealing at 1200 C. The mean grain size of annealed tape for at 1200 C for 30 min is about 20 lm. The X-ray pole-figure analysis showed that the full width half maximum of in-plane and out-of-plane texture was 9.05 and 5.85, respectively. The SPM surface roughness of the Ni–5at.%W alloy tape was 3 nm. In this work, we obtained Ni–5at.%W alloy tape with low surface roughness, excellent cube texture and small grain size compared to tape fabricated by existing a melting technique and a wet powder metallurgy process. Acknowledgements
[2] [3] [4]
[5]
[6] [7] [8]
This work was supported under Grant No. RTI04-01-03 form the Regional Technology Innovation Program founded by the Ministry of Commerce, Industry and Energy (MOCIE) and Sang-Suk Kim is grateful to the Brain Korea (BK) 21 Project for supporting a Fellowship.
[9] [10] [11]
References [12] [1] A. Goyal, D.P. Norton, D.K. Christen, E.D. Specht, M. Paranthaman, D.M. Kroeger, J.D. Budai, Q. He, F.A. List, R. Feenstra, H.R.
Kerchner, D.F. Lee, E. Hatfield, P.M. Martin, J. Park, Appl. Phys. Lett. 69 (1996) 1795. D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Science 274 (1996) 755. R.I. Tomov, A. Kursumovic, M. Matoros, D.J. Kang, B.A. Glowacki, J.E. Evatts, Supercond. Sci. Technol. 14 (2001) 152. B. de Boer, N. Reger, L. Fernandez, G.-R. Fernandez, J. Eickemeyer, B. Holzapfel, L. Schultz, W. Prusseit, P. Berberich, Physica C 351 (2001) 38. E. Varesi, V. Boffa, G. Celentano, L. Ciontea, F. Fabbri, V. Galluzzi, U. Gambardella 3, A. Mancini, T. Petrisor, A. Rufoloni, A. Vannozzi, Physica C 372–376 (2002) 763. J. Eickemeyer, D. Selbmann, R. Opitz, H. Wendrock, E. Maher, U. Miller, W. Prusseit, Physica C 372–376 (2002) 814. B.K. Ji, D.-W. Lee, M.-W. Kim, B.-H. Jun, P.Y. Park, K.-D. Jung, C.-J. Kim, Physica C 412–414 (2004) 853. Robert E. Reed-Hill, Reza Abbaschian, Physical Metallurgy Principles, PWS Publishing Co, Boston, MA, 1994, p. 268. K.T. Kim, J.H. Lim, J.H. Kim, J.-h. Joo, W.-S. Nah, B.K. Ji, B.-H. Jun, C.-J. Kim, G.-W. Hong, Physica C 412–414 (2004) 859. Y. Zhao, H.-L. Suo, M. Liu, D. -M Liu, Y.-X. Zhang, M.-L. Zhou, Physica C 440 (2006) 10. D.P. Norton, C. Park, C. Prouteau, D.K. Christen, M.F. Chisholm, J.D. Budai, S.J. Pennycook, A. Goyal, E.Y. Sun, D.F. Lee, D.M. Kroeger, E. Specht, M. Paranthaman, Mater. Sci. Eng. B 56 (1998) 86. B. De Boer, J. Eickemeyer, N. Reger, L. Fernandez, G.-R. Fernandez, J. Richter, B. Holzapfel, L. Schultz, W. Prusseit, P. Berbeerich, Acta Mater. 49 (2001) 1421.
Development of cube textured Ni–5at.%W alloy substrates for YBCO coated conductor application using a powder metallurgy process S.-S. Kim a, J.-S. Tak a, S.-Y. Bae a, J.-K. Chung b, I.-S. Ahn a, C.-J. Kim a, K.-W. Kim a, K.-K. Cho a,* a
School of Nano and Advanced Materials Engineering, K-MEM R&D CLUSTER and i-cube Center, Gyeongsang National University, 900 Gazwa-dong, Jinju, Gyeongnam 660-701, Republic of Korea b Institute of Industrial Technology, Changwon National University, Changwon, Kyungnam 641-120, Republic of Korea Received 30 October 2006; accepted 10 April 2007 Available online 24 May 2007
Abstract In this paper, Ni–5at.%W alloy substrate for YBCO coated conductor was fabricated by a dry powder metallurgy process including powder compaction, cold isostatic pressing (CIP), cold rolling and annealing for recrystallization. Ni and W powders were ball-milled at this process for various times of 10, 30, 50 and 100 h in argon atmosphere. The rod-like Ni–W alloy compacts were sintered at 1150 C for 1 h in 96%Ar–4%H2 atmosphere. The sintered rods were cold rolling into thin tape of 70–90 lm thickness with 5% reduction at each path. The Ni–W alloy tapes were annealed at 800–1200 C in an atmosphere of 96%Ar–4%H2 mixing gas for the development of cube texture. The tape with the best properties of low surface roughness, small grain size and strong cube texture was obtained at the condition annealed at 1200 C using ball-milled powder for 30 min. The W addition to Ni improved the mechanical properties by solid solution hardening and inhibited grain growth for annealing heat treatment. The tapes were characterized by X-ray pole-figure, optical microscopy (OM), scanning electron microscopy (SEM) and scanning probe microscopy (SPM). 2007 Elsevier B.V. All rights reserved. PACS: 84.71.Mn; 81.20.Ev Keywords: Ni–5at.% alloy tape; Cube texture; Powder metallurgy
1. Introduction The rolling assisted biaxially textured substrate (RABiTS) method developed at ORNL, USA demonstrated the tremendous possibility of using highly cube textured nickel as a substrate for coated superconductor applications [1,2]. Pure Ni substrate has workability and forms a strong cube texture by cold rolling and annealing. However, pure Ni has low mechanical quality and thermal grooving after recrystallization heat treatment. So much research has been directed toward Ni-alloy substrate, and
*
Corresponding author. Tel.: +82 055 751 6292; fax: +82 055 759 1745. E-mail address: [email protected] (K.-K. Cho).
0921-4534/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.258
it was suggested that W, Cr, Mo, V, Al, and Cu are among the more suitable alloying elements for obtaining those improved properties [3,4]. Tungsten was good candidate for the alloying elements because of the high solubility of these elements to Ni. Generally, the Ni-alloy rods which are used for the fabrication of the substrate are made by the melting techniques and powder metallurgy. When the Ni-alloy rods are made by melting, the grain size control is not easy and the impurity can enter into the alloy from a crucible, which affects negatively the recrystallization behavior of Ni. A powder metallurgy process has advantages of an easy control of the initial grain size and lowering the processing temperature compared to the melting process. Powder metallurgy process was classified into two large groups which
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
are wet and dry process. In a wet process, mixing powder before compacting is prepared by milling of Ni, addition element, solution (alcohol, acetone, etc.) and balls in jar. There is no difference between the two processes, except for whether the solution would be used or not. The wet process has been mainly used in the previous work. But the wet process has disadvantages, such as an influx of impurities from solution and long ball milling time caused by low impact energy of balls, compared to the dry process. In this study, therefore, the rod of Ni–5at.%W alloy was prepared by a dry powder metallurgy process including powder packing, cold isostatic pressing and sintering. The rod was made into tape with a thickness of 70–90 lm and biaxial texture by cold rolling and annealing for recrystallization. We evaluated the effects of annealing temperature on the texture, grain size and surface morphology. In addition, we examined the effects of W content in Ni on texture, grain size and surface morphology. And the results in this work were compared to those of tape fabricated by existing wet powder metallurgy and melting technique. 2. Experiments procedure Nickel powder of purity 99.9% of 5 lm size and tungsten powder of purity 99.9%, 1 lm size was used as the starting material. Ni alloy of composition Ni–5at.%W was prepared by a dry powder metallurgy method. Ni and W powders were ball-milled at this process for various times of 10, 30, 50 and 100 h in Argon atmosphere. Ball-milled powder was loaded into a urethane mold (20 mm diameter and 130 mm length), and cold isostatic pressing (CIP) of 200 MPa was applied to form a rod-type compact. The green compact was placed at the center of a tubular furnace, sintered at 1150 C for an hour atmosphere of 96%Ar and 4%H2. The sintered Ni–5at.%W alloy rods were cut by milling machine. After cutting the thickness, the width and length was 7, 10 and 80 mm, respectively. The cutting rectangular bar was cold-rolled at 5% reduction to the final thickness of 70–90 lm using a two-high rolling mill. Finally, the sample was heat treated at temperatures between 800 and 1200 C for recrystallization in 96%Ar–4%H2 atmosphere. Annealing for recrystallization held 30 min in all experiment condition. Microstructure observation was performed by optical microscopy (OM) and scanning electron microscopy (SEM). The texture of the Ni alloy tapes were analyzed by X-ray diffraction (XRD). The textures were determined by measuring polefigures by means of an automated X-ray texture goniometer (BRUKER, D8 discover). Surface roughness and thermal grain grooving were evaluated by scanning probe microscopy (SPM) over a 40 · 40 lm2 area. 3. Results and discussion Ni–5at%W powders were milled for 10, 30, 50 and 100 h using general ball milling process. SEM images of powders
605
Fig. 1. SEM photographs of Ni–5at.%W powder on ball-milling time: (a) 10 h, (b) 30 h, (c) 50 h and (d) 100 h.
ball-milled at various milling times are shown in Fig. 1. In the cases of ball-milled powder for 10 h, W particles with diameter of 1 lm were frequently observed (marked with arrowed line). But the particles were not observed in powder ball-milled for 30 h. This means that homogeneous Ni– W alloy powder can be obtained after ball milling for 30 h. For the powders ball-milled for 30, 50 and 100 h, there are almost no significant change with the increase of milling time. In order to obtain optimum ball milling time, each ball-milled powder was compacted at pressure of 140 kg/ cm2 using general bi-axial compacting process, and then were sintered at 1150 C for 1 h in atmosphere of 96%Ar and 4%H2. The green compact density of powders ballmilled for 10, 30, and 50 h was 6.14, 6.36 and 6.02 g/cm3, respectively. In powder ball-milled for 100 h, but, the compacts were not formed due to the high strain energy stored within each particles. Fig. 2 shows the pore characteristics of the sintered Ni–W alloy. The porosity is the lowest in powder ball-milled for 30 h, as shown in Fig. 2b. Therefore, we hereafter use powder ball-milled for 30 h in all experiments. Fig. 3 shows the log-scale XRD patterns of as coldrolled and texture-annealed Ni–W tape at various temperatures after cold rolling. The XRD pattern of the as cold-rolled samples consists of various peaks such as (1 1 1), (2 0 0) and (2 2 0). To understand the effect of annealing temperature on the development of the (2 0 0) texture (the ‘‘cube texture or annealing texture’’), annealing temperature was changed from 800 to 1200 C. With increasing annealing temperature, the intensity of (1 1 1) and (2 2 0) peaks, which means deformation texture decreased, and that of (2 0 0) peak which means annealing texture increased. As can be seen in Fig. 3, the XRD pattern of the annealed Ni–W tape at 1200 C shows only the (2 0 0) peak, which indicates that (2 0 0) texture was formed by texture annealing. We evaluated the texture of the cold-rolled tape and the annealed substrate of both Ni–5at.%W alloys; selected (1 1 1) pole-figure are presented in Fig. 4. In Fig. 4a, the tex-
606
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
Fig. 2. OM images of sintered Ni–5at.%W on ball-milling time: (a) 10 h, (b) 30 h and (c) 50 h.
Intensity (log-scale)
1200°C
1000°C
800°C (220)
(111)
as rolled (200)
40
50
60
70
80
90
2 Theta (degree) Fig. 3. XRD diffraction patterns of the Ni–5at.%W alloy tapes annealed at various temperatures for 30 min.
deformation texture and better symmetry of poles for the Ni alloy tape is partly related to the reduction of stacking fault energy resulting from the presence of tungsten in nickel. Fig. 4b–d are selected sets of (1 1 1) pole-figures of substrates annealed at various temperature of 800– 1200 C. Ni–5at.%W substrate annealed at 800 C has some bass deformation texture, because annealing temperature does not enough. However, Ni–5at.%W substrates annealed at 1000 C and 1200 C have good cube texture. The full-width half maximum (FWHM) of in-plane (uscan) and out-of-plane (x-scan) texture values are plotted in Fig. 5. The average FWHM values of in-plane texture for various temperatures of 800, 1000 and 1200 C are 10.73, 10.06 and 9.05, respectively. The FWHM values of out-of-plane texture for various temperatures of 800, 1000 and 1200 C are 7.3, 7.0 and 5.85, respectively. The variation in FWHM values for Ni–5at.%W substrate annealed at various temperatures show the obvious dependence on annealing temperature. The FWHM values are similar to that reported in literature elsewhere [5]. Fig. 6 shows optical micrographs of top views of annealed Ni–W substrates at various temperatures. Grain size increased with increasing annealing temperature. The average grain size of Ni–W substrates annealed at 800, 1000 and 1200 C was 9.0, 12.1 and 20.5 lm, respectively. The average grain size is smaller than those of tape fabricated by existing a melting technique [6] and a wet powder metallurgy process [7]. In general, abnormal grain growth
In-plane
14
out-of-plane
FWHM (deg)
12
Fig. 4. (1 1 1) Pole-figures of the Ni–5at.%W alloy tapes annealed at: (a) as rolled and (b) 800 C, (c) 1000 C and (d) 1200 C for 30 min.
10
8
6
4
ture pattern of cold-rolled sample was typical brass deformation texture and the poles were symmetric. The texture components, usually observed in deformed FCC materials, are known to transform well to cube texture after annealing for recrystallization. It is supposed that stronger brass
800
1000
1200
Annealing Temperature(°C) Fig. 5. Change of the in-plane and out-of-plane of the Ni–5at.%W alloy tapes annealed at various temperatures for 30 min.
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
607
Fig. 6. OM images of etched Ni–5at.%W alloy tapes annealed at: (a) 800 C, (b) 1000 C and (c) 1200 C for 30 min.
Fig. 7. SPM images of Ni–5at.%W alloy tapes annealed at: (a) as rolled and (b) 800 C, (c) 1000 C and (d) 1200 C for 30 min.
is often observed when grain growth is inhibited by the presence of inclusions, relative grain size effects, or development of a strong preferred orientation [8]. In our samples, however, abnormal grain growth and severe increase of grain size were not observed. It may be related to the existence of solute atom (W), which restrict grain growth during heat treatment (the ‘‘solute drag effect’’). The surface roughness of the substrate after annealing is rather important because a flat and thin tape has to be used for the further deposition of epitaxial buffer layers and superconducting layers. SPM was used to examine the surface morphology and roughness. Fig. 7 shows SPM images of the Ni–5at.%W alloy substrates annealed at various temperatures. The SPM scan on Ni–5at.%W substrates after cold-rolled and annealed at various temperature of 800, 1000 and 1200 C gave a mean roughness over a 40 · 40 lm area of about 11, 7, 6 and 3 nm, respectively. With increasing annealing temperature, the surface roughness decreased, which can be due to grain growth. The surface roughness of substrates fabricated in this work, is
better than those of substrates obtained by wet powder metallurgy and melting method reported in literature elsewhere [9,10]. In this work, the grooves are however observed in the substrate annealed at 1200 C, as shown in Fig. 7d. A problem arising with high temperature annealing is high angle grain boundary grooving [11,12]. The width and depth of these grain boundary grooves depend on annealing temperature and time. A deep groove in the substrate could give rise to a disruption of the epitaxy of YBCO, resulting in decrease of critical current density in the final YBCO coated conductors. In order to have fabrication of a substrate with no groove, small grain size and the optimum texture, carefull control of the annealing temperature and time is necessity. 4. Conclusion In conclusion, we applied the dry powder metallurgy to the fabrication of Ni–W tape for YBCO coated conductors. The Ni–W alloy tape with highly (2 0 0) texture (the
608
S.-S. Kim et al. / Physica C 463–465 (2007) 604–608
‘‘cube texture or annealing texture’’) was fabricated by cold rolling and texture annealing at 1200 C. The mean grain size of annealed tape for at 1200 C for 30 min is about 20 lm. The X-ray pole-figure analysis showed that the full width half maximum of in-plane and out-of-plane texture was 9.05 and 5.85, respectively. The SPM surface roughness of the Ni–5at.%W alloy tape was 3 nm. In this work, we obtained Ni–5at.%W alloy tape with low surface roughness, excellent cube texture and small grain size compared to tape fabricated by existing a melting technique and a wet powder metallurgy process. Acknowledgements
[2] [3] [4]
[5]
[6] [7] [8]
This work was supported under Grant No. RTI04-01-03 form the Regional Technology Innovation Program founded by the Ministry of Commerce, Industry and Energy (MOCIE) and Sang-Suk Kim is grateful to the Brain Korea (BK) 21 Project for supporting a Fellowship.
[9] [10] [11]
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