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Ni–10%Cr–1.5%Al composite substrate for coated conductor application
Scripta Materialia 48 (2003) 1167–1171 www.actamat-journals.com
On the development of high strength and bi-axially textured Ni–3%W/Ni–10%Cr–1.5%Al composite substrate for coated conductor application V. Subramanya Sarma *, B. de Boer 1, J. Eickemeyer, B. Holzapfel Institute for Solid State and Materials Research (IFW), Helmholtz Street 20, 01069 Dresden, Germany Received 24 July 2002; received in revised form 4 November 2002; accepted 27 November 2002
Abstract Ni and Ni base alloys are good candidate substrate materials in the development of coated conductors. The present paper reports the development of a high strength (Ni–3%W/Ni–10%Cr–1.5%Al) composite tape of 80 lm thickness with strong cube texture. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Coated conductors; Recrystallisation; Cube texture; Ni-alloys; Composite
1. Introduction A very promising approach for the profitable production of long lengths of high temperature superconducting (HTS) YBa2 Cu3 O7d (YBCO) tapes capable of carrying high currents in magnetic fields is the rolling assisted biaxially textured substrates (RABiTSe) method developed at the Oak Ridge National Laboratory [1–3]. In this method, the desired strong biaxial texture in the superconducting YBCO film is achieved by epitaxial growth of a buffer and YBCO film on a highly textured substrate. It has been shown that Ni is ideally sui* Corresponding author. Tel.: +49-351-4659203; fax: +49351-4659320. E-mail address: [email protected] (V. Subramanya Sarma). 1 Present address: ThyssenKrupp VDM GmbH, Kleffstrasse 23, D-58762 Altena, Germany.
ted as a substrate material due to its ability to form a strong cube texture after heavy cold rolling and recrystallisation. In addition its oxidation resistance and the small lattice mismatch allows epitaxial growth of buffer (CeO2 þ YSZ) and YBCO films. In YBCO films on buffered Ni substrates, current densities exceeding 106 A cm2 have been achieved [4]. To achieve a high engineering current density (i.e., the current in the HTS film divided by the cross section of the whole tape) very thin tapes are desirable. But the low tensile strength of Ni limits the possibility of producing very thin tapes by a continuous reel-to-reel deposition process. For this reason the strengthening of substrate material is important. The need for a strong cube texture limits the possibilities of strengthening the substrate. There are two known metallurgical routes of strengthening the substrate material i.e., solid solution and precipitation strengthening. It was shown that the substrate strength could be
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(02)00585-7
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increased by factor 3 with substantial additions of Cr, V ( P 10 at.%, compositions are given in at.% throughout the paper) without losing the strong cube texture [5–7]. The additions (>10%) of Cr and V also suppress the Curie temperature to below 77 K, thus reducing the magnetization losses in the coated conductors in alternating current applications [3]. However, the ease of Cr/V oxide formation on the surface at the buffer and YBCO deposition temperatures (these are made in oxidizing atmospheres) makes the epitaxial growth of these films difficult. Higher strength levels were also achieved by precipitation of Al2 O3 particles through controlled internal oxidation of Ni– 1at.%Al alloy [8]. However, the formation of Al2 O3 particles on the surface degrades its quality rendering it unsuitable for further epitaxial deposition. These surface oxidation problems can be overcome by the design of a composite substrate [9–11]. In the present paper, we report the development of a composite Ni–3%W (outer surface)/Ni–10%Cr– 1.5%Al (core) substrate with strong biaxial texture and high tensile strength. The reasons for the choice of the outer surface and inner core materials are discussed below.
2. Design of the composite The problem of surface oxidation can be overcome by using a non-oxidising outer layer (e.g. Ni) while using a highly alloyed (e.g. Cr, V, Al etc.) inner core. It has been reported that the cube texture in Ni and Ni alloys is significantly improved with an intermediate recrystallisation annealing treatment [12]. Therefore, the problem with using pure Ni in the outer layer is with regard to achieving the required strong cube texture due to the wide differences in the recrystallisation temperatures between Ni (typically around 400 °C) and Ni–Cr, Ni–V alloys (typically 700–800 °C depending on the amount of alloy content). It should be mentioned that a coarse starting grain size has a strong detrimental effect on the final cube texture. Therefore during intermediate annealing of a composite with pure Ni outer layer and highly alloyed core would lead either to fine grain size in Ni and no recrystallisation in the core
or coarse grained outer layer with recrystallised core. These microstructures and all in between are not useful to develop the very strong cube textures needed for the RABiTS application. It is thus necessary to have the outer surface and the inner core materials with similar recrystallisation temperatures. Also of importance during the rolling of composite is the strength difference between the outer half and the inner core. Too strong differences will result in inhomogeneous deformation leading to defects (surface cracks) in the tapes. It was reported that tungsten (W) is a good choice for solid solution strengthening without the problems of oxidation. Ni–W-alloys form a strong cube texture up to 5%W [13]. This is proved by a critical current density of 1.2 MA cm2 achieved in a YBCO film deposited on such a substrate [12,14]. For the core, Ni–Cr and Ni–V alloys are suitable in view of their ability to form strong cube texture (up to 10%) and also become non-magnetic at 77 K [3]. Greater alloying additions (>13% of Cr and 10%V) weaken the recrystallisation cube texture in these alloys though strength increases with increasing alloying content. Since the RABiTS substrate thickness is small and the number of grains in the thickness direction are few (3–4), the presence of a non-cube texture forming alloy in the core of the composite would perhaps lead to the growth of non-cube grains to the surface during recrystallisation. Considering the above points, Ni–3%W and Ni–10%Cr–1.5%Al alloys were chosen as the materials for outer sleeve and inner core. The addition of Al to the core alloy was done with a view to perform controlled internal oxidation to precipitate Al2 O3 particles for improving the strength levels further.
3. Experimental Ni–3at.%W and Ni–10at.%Cr–1.5at.%Al alloys were prepared by melting the elements having a purity of 99.98% in an induction furnace and casting them in a cylindrical mould of 32 mm diameter. The Ni–3%W alloy was hot rolled at 1100 °C to square 22 22 mm2 rod. The Ni–10%Cr–1.5%Al alloy was hot forged at 1100 °C to 10 mm diameter rod. The composite preform was machined
V. Subramanya Sarma et al. / Scripta Materialia 48 (2003) 1167–1171
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nique in the scanning electron microscope (JEOL JSM 6400) with Channel 4 (HKL Technology) software. Microtexture measurements on recrystallised samples were made on an area of about 360 750 lm2 with a step size of 3 lm. The yield and tensile strengths of the tapes were measured with INSTRON 8500 testing machine with 25 mm extensometer on samples of 10 cm length. 4. Results and discussion 4.1. Texture in the composite
Fig. 1. A schematic drawing of the Ni–3%W/Ni–10%Cr– 1.5%Al composite preform.
with the geometry and dimensions shown Fig. 1. This preform was cold rolled to long tapes of 80 lm thickness and 10 mm width with an intermediate annealing treatment (800 °C for 40 min) at 3 mm thickness stage. Samples cut from the long tape were recrystallised at 900 and 950 °C for 30 min in high vacuum. The microtextures and misorientation distributions were investigated by electron back scattered diffraction (EBSD) tech-
The microstructure of the composite (surface and transverse section) is shown in Fig. 2. The EBSD maps of the surface of recrystallised (at 900 and 950 °C) composite tapes are shown in Fig. 3a and b. The orientation map of the transverse section (recrystallised at 950 °C) is given in Fig. 3c. The misorientation distributions for the recrystallised tapes are given in Fig. 4. It is clear from Figs. 3 and 4 that a strong cube texture is obtained on the surface and through the thickness of the composite tape after recrystallisation at 900 and 950 °C. The small fraction of twin boundaries (misoriented by 60°) which are present after recrystallisation at 900 °C are eliminated after annealing at 950 °C due to grain growth which occurs at higher temperatures. At both recrystallisation temperatures, the majority of the grains are misoriented below 10° with the maximum in the misorientation
Fig. 2. Microstructure of the recrystallised (at 900 °C for 30 min in high vacuum) Ni–3%W/Ni–10%Cr–1.5%Al composite: (a) plan view; and (b) transverse sectional view.
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Fig. 3. EBSD orientation maps of Ni–3%W/Ni–10%Cr–1.5%Al composite: (a) surface after recrystallisation at 900 °C for 30 min; (b) surface after recrystallisation at 950 °C for 30 min (thick lines indicate grain boundaries with misorientation angle above 10°) and (c) transverse section after recrystallisation at 950 °C for 30 min (the black lines are drawn to indicate the location of interfaces).
Fig. 4. Grain boundary mis-orientation distributions of the Ni– 3%W/Ni–10%Cr–1.5%Al composite after recrystallisation at 900 and 950 °C for 30 min.
distribution being close to 7° (Fig. 4). Also the composite interface is continuous and growth of bi-axially textured grains occurred through the interface (Figs. 2b and 3c). Thus the quality of the substrate after recrystallisation at 950 °C is excellent from the texture point of view. 4.2. Tensile properties Since the superconducting coating does not withstand a strain above 0.5% in compression and
Fig. 5. Stress–strain curves of the recrystallised substrates.
0.2% in tension without degradation, the stress at low strains (e.g. the 0.2% yield strength) is more critical for the application as a substrate material [15]. The room temperature stress–strain response of the composite recrystallised at 900 °C is given in Fig. 5. Stress–strain data of pure Ni and Ni– 10%Cr– 1.5%Al tapes of 80 lm thickness are also included for comparison. It can be seen that the composite exhibits increased yield strength (175 MPa) i.e., by a factor of 4 when compared to pure Ni and about 40 MPa more than that of Ni– 10%Cr–1.5%Al tape (Fig. 5). It should be noted that the yield stress of thin films is strongly
V. Subramanya Sarma et al. / Scripta Materialia 48 (2003) 1167–1171
influenced by dimensional constraint on dislocation motion (due to increased contribution of image forces on dislocations), which results in a pronounced size (inverse thickness) effect [16]. In the present case the increased strength of the composite comes from solid solution strengthening of the matrix and also the constraint on dislocation motion by the presence of interfaces though texturally the interfaces are invisible. This probably contributes to the increased strength (by 40 MPa) of the composite substrate when compared with the Ni–10%Cr–1.5%Al substrate of 80 lm thickness while considering the simple rule of mixtures should have resulted in lower strength level of the composite. However these aspects need to be investigated further to understand the reasons for the improved strength level.
5. Conclusions and outlook Ni–3at.%W/Ni–10at.%Cr–1.5at.%Al composite tape of 80 lm thickness was developed with strong recrystallisation cube texture and high tensile yield strength (four times that of pure Ni) as a substrate material for coated conductor application. The point of interest is the importance of cube texture forming alloy in the inner core of the composite. Also of importance is the matching of strength levels between the outer surface and the core materials. It is believed that theses parameters are likely to limit the highest achievable strength levels of the composite substrates. Currently these issues are being investigated. The present composite strength can possibly be further enhanced by increasing the core thickness (presently 30 lm) and by the controlled internal oxidation to precipitate Al2 O3 particles in the inner core. Also the alloy compositions can further be optimized to achieve higher strength levels.
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Acknowledgements The authors thank Mr. Frey, Mr. Kuszinski, Mr. Opitz, Mr. Seifert, Mr. Wolf, Mr. Neumann, Mr. Trinks, Mr. Klauss and Mrs. Grundlich for their help in the experimental work. This work is supported by the Federal Ministry of Education, Science and Technology, Germany under contract no. 13N7267.
References [1] Goyal A, Norton DP, Budai JD, Paranthaman M, Specht ED, Kroeger DM, et al. Appl Phys Lett 1996;69:1795. [2] Goyal A, Budai JD, Kroeger DM, Norton DP, Specht ED, Christen DK. US Patent no. 5,739,086, 1998. [3] de Boer B, Eickemeyer J, Reger N, Fernandez L, Richter J, Holzapfel B, et al. Acta Mater 2001;49:1421. [4] Goyal A, List FA, Mathis J, Paranthaman M, Specht ED, Norton DP, et al. J Supercond 1998;11:481. [5] Goyal A, Specht ED, Kroeger DM, Paranthaman M. US Patent no. 5,964,966, 1999. [6] Goyal A, Specht ED, Kroeger DM, Paranthaman M. US Patent no. 6,106,615, 2000. [7] Sarma VS, de Boer B, Reger N, Eickemeyer J, Opitz R, Holzapfel B. Mater Sci Forum 2002;408–412:1561. [8] deBoer B, Sarma VS, Reger N, Eickemeyer J, Holzapfel B. Physica C 2002;372–376:798. [9] Goyal A. US Patent no. 6,180,570, 2001. [10] Goyal A. US Patent no. 6,375,768, 2002. [11] Watanabe T, Ohashi Y, Ozaki M, Yamamoto K, Maeda T, Wada K, et al. HTS Conductors, Processing and Applications. The 2002 International Workshop on Superconductivity (The 5th joint ISTEC/MRS Workshop), June 24–27, 2001, Honolulu, Hawai, USA. p. 21–4. [12] Eickemeyer J, Selbmann D, Opitz R, Wendrock H, Maher E, Miller U, et al. Physica C 2002;372–376:814. [13] Eickemeyer J, Selbmann D, Opitz R, Holzapfel B. German Patent no. DE 100 05861 C2, 2002. [14] Eickemeyer J, Selbmann D, Opitz R, de Boer B, Holzapfel B, Schultz L, et al. Supercond Sci Technol 2001;14:152. [15] Goyal A, Norton DP, Kroeger DM, Christen DK, Paranthaman M, Specht ED, et al. J Mater Res 1997;12:2924. [16] Arzt E. Acta Mater 1998;46:5611.
On the development of high strength and bi-axially textured Ni–3%W/Ni–10%Cr–1.5%Al composite substrate for coated conductor application V. Subramanya Sarma *, B. de Boer 1, J. Eickemeyer, B. Holzapfel Institute for Solid State and Materials Research (IFW), Helmholtz Street 20, 01069 Dresden, Germany Received 24 July 2002; received in revised form 4 November 2002; accepted 27 November 2002
Abstract Ni and Ni base alloys are good candidate substrate materials in the development of coated conductors. The present paper reports the development of a high strength (Ni–3%W/Ni–10%Cr–1.5%Al) composite tape of 80 lm thickness with strong cube texture. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Coated conductors; Recrystallisation; Cube texture; Ni-alloys; Composite
1. Introduction A very promising approach for the profitable production of long lengths of high temperature superconducting (HTS) YBa2 Cu3 O7d (YBCO) tapes capable of carrying high currents in magnetic fields is the rolling assisted biaxially textured substrates (RABiTSe) method developed at the Oak Ridge National Laboratory [1–3]. In this method, the desired strong biaxial texture in the superconducting YBCO film is achieved by epitaxial growth of a buffer and YBCO film on a highly textured substrate. It has been shown that Ni is ideally sui* Corresponding author. Tel.: +49-351-4659203; fax: +49351-4659320. E-mail address: [email protected] (V. Subramanya Sarma). 1 Present address: ThyssenKrupp VDM GmbH, Kleffstrasse 23, D-58762 Altena, Germany.
ted as a substrate material due to its ability to form a strong cube texture after heavy cold rolling and recrystallisation. In addition its oxidation resistance and the small lattice mismatch allows epitaxial growth of buffer (CeO2 þ YSZ) and YBCO films. In YBCO films on buffered Ni substrates, current densities exceeding 106 A cm2 have been achieved [4]. To achieve a high engineering current density (i.e., the current in the HTS film divided by the cross section of the whole tape) very thin tapes are desirable. But the low tensile strength of Ni limits the possibility of producing very thin tapes by a continuous reel-to-reel deposition process. For this reason the strengthening of substrate material is important. The need for a strong cube texture limits the possibilities of strengthening the substrate. There are two known metallurgical routes of strengthening the substrate material i.e., solid solution and precipitation strengthening. It was shown that the substrate strength could be
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(02)00585-7
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increased by factor 3 with substantial additions of Cr, V ( P 10 at.%, compositions are given in at.% throughout the paper) without losing the strong cube texture [5–7]. The additions (>10%) of Cr and V also suppress the Curie temperature to below 77 K, thus reducing the magnetization losses in the coated conductors in alternating current applications [3]. However, the ease of Cr/V oxide formation on the surface at the buffer and YBCO deposition temperatures (these are made in oxidizing atmospheres) makes the epitaxial growth of these films difficult. Higher strength levels were also achieved by precipitation of Al2 O3 particles through controlled internal oxidation of Ni– 1at.%Al alloy [8]. However, the formation of Al2 O3 particles on the surface degrades its quality rendering it unsuitable for further epitaxial deposition. These surface oxidation problems can be overcome by the design of a composite substrate [9–11]. In the present paper, we report the development of a composite Ni–3%W (outer surface)/Ni–10%Cr– 1.5%Al (core) substrate with strong biaxial texture and high tensile strength. The reasons for the choice of the outer surface and inner core materials are discussed below.
2. Design of the composite The problem of surface oxidation can be overcome by using a non-oxidising outer layer (e.g. Ni) while using a highly alloyed (e.g. Cr, V, Al etc.) inner core. It has been reported that the cube texture in Ni and Ni alloys is significantly improved with an intermediate recrystallisation annealing treatment [12]. Therefore, the problem with using pure Ni in the outer layer is with regard to achieving the required strong cube texture due to the wide differences in the recrystallisation temperatures between Ni (typically around 400 °C) and Ni–Cr, Ni–V alloys (typically 700–800 °C depending on the amount of alloy content). It should be mentioned that a coarse starting grain size has a strong detrimental effect on the final cube texture. Therefore during intermediate annealing of a composite with pure Ni outer layer and highly alloyed core would lead either to fine grain size in Ni and no recrystallisation in the core
or coarse grained outer layer with recrystallised core. These microstructures and all in between are not useful to develop the very strong cube textures needed for the RABiTS application. It is thus necessary to have the outer surface and the inner core materials with similar recrystallisation temperatures. Also of importance during the rolling of composite is the strength difference between the outer half and the inner core. Too strong differences will result in inhomogeneous deformation leading to defects (surface cracks) in the tapes. It was reported that tungsten (W) is a good choice for solid solution strengthening without the problems of oxidation. Ni–W-alloys form a strong cube texture up to 5%W [13]. This is proved by a critical current density of 1.2 MA cm2 achieved in a YBCO film deposited on such a substrate [12,14]. For the core, Ni–Cr and Ni–V alloys are suitable in view of their ability to form strong cube texture (up to 10%) and also become non-magnetic at 77 K [3]. Greater alloying additions (>13% of Cr and 10%V) weaken the recrystallisation cube texture in these alloys though strength increases with increasing alloying content. Since the RABiTS substrate thickness is small and the number of grains in the thickness direction are few (3–4), the presence of a non-cube texture forming alloy in the core of the composite would perhaps lead to the growth of non-cube grains to the surface during recrystallisation. Considering the above points, Ni–3%W and Ni–10%Cr–1.5%Al alloys were chosen as the materials for outer sleeve and inner core. The addition of Al to the core alloy was done with a view to perform controlled internal oxidation to precipitate Al2 O3 particles for improving the strength levels further.
3. Experimental Ni–3at.%W and Ni–10at.%Cr–1.5at.%Al alloys were prepared by melting the elements having a purity of 99.98% in an induction furnace and casting them in a cylindrical mould of 32 mm diameter. The Ni–3%W alloy was hot rolled at 1100 °C to square 22 22 mm2 rod. The Ni–10%Cr–1.5%Al alloy was hot forged at 1100 °C to 10 mm diameter rod. The composite preform was machined
V. Subramanya Sarma et al. / Scripta Materialia 48 (2003) 1167–1171
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nique in the scanning electron microscope (JEOL JSM 6400) with Channel 4 (HKL Technology) software. Microtexture measurements on recrystallised samples were made on an area of about 360 750 lm2 with a step size of 3 lm. The yield and tensile strengths of the tapes were measured with INSTRON 8500 testing machine with 25 mm extensometer on samples of 10 cm length. 4. Results and discussion 4.1. Texture in the composite
Fig. 1. A schematic drawing of the Ni–3%W/Ni–10%Cr– 1.5%Al composite preform.
with the geometry and dimensions shown Fig. 1. This preform was cold rolled to long tapes of 80 lm thickness and 10 mm width with an intermediate annealing treatment (800 °C for 40 min) at 3 mm thickness stage. Samples cut from the long tape were recrystallised at 900 and 950 °C for 30 min in high vacuum. The microtextures and misorientation distributions were investigated by electron back scattered diffraction (EBSD) tech-
The microstructure of the composite (surface and transverse section) is shown in Fig. 2. The EBSD maps of the surface of recrystallised (at 900 and 950 °C) composite tapes are shown in Fig. 3a and b. The orientation map of the transverse section (recrystallised at 950 °C) is given in Fig. 3c. The misorientation distributions for the recrystallised tapes are given in Fig. 4. It is clear from Figs. 3 and 4 that a strong cube texture is obtained on the surface and through the thickness of the composite tape after recrystallisation at 900 and 950 °C. The small fraction of twin boundaries (misoriented by 60°) which are present after recrystallisation at 900 °C are eliminated after annealing at 950 °C due to grain growth which occurs at higher temperatures. At both recrystallisation temperatures, the majority of the grains are misoriented below 10° with the maximum in the misorientation
Fig. 2. Microstructure of the recrystallised (at 900 °C for 30 min in high vacuum) Ni–3%W/Ni–10%Cr–1.5%Al composite: (a) plan view; and (b) transverse sectional view.
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Fig. 3. EBSD orientation maps of Ni–3%W/Ni–10%Cr–1.5%Al composite: (a) surface after recrystallisation at 900 °C for 30 min; (b) surface after recrystallisation at 950 °C for 30 min (thick lines indicate grain boundaries with misorientation angle above 10°) and (c) transverse section after recrystallisation at 950 °C for 30 min (the black lines are drawn to indicate the location of interfaces).
Fig. 4. Grain boundary mis-orientation distributions of the Ni– 3%W/Ni–10%Cr–1.5%Al composite after recrystallisation at 900 and 950 °C for 30 min.
distribution being close to 7° (Fig. 4). Also the composite interface is continuous and growth of bi-axially textured grains occurred through the interface (Figs. 2b and 3c). Thus the quality of the substrate after recrystallisation at 950 °C is excellent from the texture point of view. 4.2. Tensile properties Since the superconducting coating does not withstand a strain above 0.5% in compression and
Fig. 5. Stress–strain curves of the recrystallised substrates.
0.2% in tension without degradation, the stress at low strains (e.g. the 0.2% yield strength) is more critical for the application as a substrate material [15]. The room temperature stress–strain response of the composite recrystallised at 900 °C is given in Fig. 5. Stress–strain data of pure Ni and Ni– 10%Cr– 1.5%Al tapes of 80 lm thickness are also included for comparison. It can be seen that the composite exhibits increased yield strength (175 MPa) i.e., by a factor of 4 when compared to pure Ni and about 40 MPa more than that of Ni– 10%Cr–1.5%Al tape (Fig. 5). It should be noted that the yield stress of thin films is strongly
V. Subramanya Sarma et al. / Scripta Materialia 48 (2003) 1167–1171
influenced by dimensional constraint on dislocation motion (due to increased contribution of image forces on dislocations), which results in a pronounced size (inverse thickness) effect [16]. In the present case the increased strength of the composite comes from solid solution strengthening of the matrix and also the constraint on dislocation motion by the presence of interfaces though texturally the interfaces are invisible. This probably contributes to the increased strength (by 40 MPa) of the composite substrate when compared with the Ni–10%Cr–1.5%Al substrate of 80 lm thickness while considering the simple rule of mixtures should have resulted in lower strength level of the composite. However these aspects need to be investigated further to understand the reasons for the improved strength level.
5. Conclusions and outlook Ni–3at.%W/Ni–10at.%Cr–1.5at.%Al composite tape of 80 lm thickness was developed with strong recrystallisation cube texture and high tensile yield strength (four times that of pure Ni) as a substrate material for coated conductor application. The point of interest is the importance of cube texture forming alloy in the inner core of the composite. Also of importance is the matching of strength levels between the outer surface and the core materials. It is believed that theses parameters are likely to limit the highest achievable strength levels of the composite substrates. Currently these issues are being investigated. The present composite strength can possibly be further enhanced by increasing the core thickness (presently 30 lm) and by the controlled internal oxidation to precipitate Al2 O3 particles in the inner core. Also the alloy compositions can further be optimized to achieve higher strength levels.
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Acknowledgements The authors thank Mr. Frey, Mr. Kuszinski, Mr. Opitz, Mr. Seifert, Mr. Wolf, Mr. Neumann, Mr. Trinks, Mr. Klauss and Mrs. Grundlich for their help in the experimental work. This work is supported by the Federal Ministry of Education, Science and Technology, Germany under contract no. 13N7267.
References [1] Goyal A, Norton DP, Budai JD, Paranthaman M, Specht ED, Kroeger DM, et al. Appl Phys Lett 1996;69:1795. [2] Goyal A, Budai JD, Kroeger DM, Norton DP, Specht ED, Christen DK. US Patent no. 5,739,086, 1998. [3] de Boer B, Eickemeyer J, Reger N, Fernandez L, Richter J, Holzapfel B, et al. Acta Mater 2001;49:1421. [4] Goyal A, List FA, Mathis J, Paranthaman M, Specht ED, Norton DP, et al. J Supercond 1998;11:481. [5] Goyal A, Specht ED, Kroeger DM, Paranthaman M. US Patent no. 5,964,966, 1999. [6] Goyal A, Specht ED, Kroeger DM, Paranthaman M. US Patent no. 6,106,615, 2000. [7] Sarma VS, de Boer B, Reger N, Eickemeyer J, Opitz R, Holzapfel B. Mater Sci Forum 2002;408–412:1561. [8] deBoer B, Sarma VS, Reger N, Eickemeyer J, Holzapfel B. Physica C 2002;372–376:798. [9] Goyal A. US Patent no. 6,180,570, 2001. [10] Goyal A. US Patent no. 6,375,768, 2002. [11] Watanabe T, Ohashi Y, Ozaki M, Yamamoto K, Maeda T, Wada K, et al. HTS Conductors, Processing and Applications. The 2002 International Workshop on Superconductivity (The 5th joint ISTEC/MRS Workshop), June 24–27, 2001, Honolulu, Hawai, USA. p. 21–4. [12] Eickemeyer J, Selbmann D, Opitz R, Wendrock H, Maher E, Miller U, et al. Physica C 2002;372–376:814. [13] Eickemeyer J, Selbmann D, Opitz R, Holzapfel B. German Patent no. DE 100 05861 C2, 2002. [14] Eickemeyer J, Selbmann D, Opitz R, de Boer B, Holzapfel B, Schultz L, et al. Supercond Sci Technol 2001;14:152. [15] Goyal A, Norton DP, Kroeger DM, Christen DK, Paranthaman M, Specht ED, et al. J Mater Res 1997;12:2924. [16] Arzt E. Acta Mater 1998;46:5611.