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Elongated grains in Ni5W(Ag) RABiTS tapes
Journal of Alloys and Compounds 623 (2015) 132–135
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Elongated grains in Ni5W(Ag) RABiTS tapes Uwe Gaitzsch ⇑, Christian Rodig, Christine Damm, Ludwig Schultz IFW Dresden, Institute for Metallic Materials, PO Box 270116, D-01171 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 9 October 2014 Accepted 21 October 2014 Available online 28 October 2014 Keywords: Texture Recrystallization Rolling EBSD
a b s t r a c t Ni5W is a long known substrate material using the RABiTS approach. Elongated grains in the substrate tape would transfer to the superconductor epitaxially, where these are beneficial according to the brick wall model. 500 ppm and 250 ppm of silver were added to Ni5W and the intermediate and final heat treatments were altered from the standard process to obtain elongated grains. A maximum aspect ratio of 2.4 was realized in the alloy with 500 ppm Ag. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ni–W alloys are known as substrates for superconducting thin films since 2000 [1,2]. These are known as RABiTS, short for Rolling Assisted Biaxially Textured Substrates. The addition of tungsten provides an enhanced temperature stability as well as a decreased magnetization in comparison to pure nickel [3]. Additionally the Zener drag leads to finer grains as it is usually the case when comparing recrystallized grain sizes of pure metals and alloys. Recently, efforts have been made to decrease the magnetization further by adding chromium [4,5], copper [6–8] or more tungsten [9]. All these measures have no beneficial effect on the microstructure of the textured tape. Additionally alloying Ni or Ni–W with copper results in a weaker cube texture and in an increasing amount of recrystallization twins [10,11]. Furthermore copper impedes the standard buffer deposition process. In a substrate for the epitaxial growth of superconducting thin films elongated grains in rolling direction are preferable. Those would cause elongated grains in direction of current flow in the superconducting layer, which is desirable according to the brick wall model by Bulaevskii et al. [12]. The realization of a recrystallized microstructure with elongated grains was successfully demonstrated by Eickemeyer et al. in 2007 [13]. It was a result of alloying nickel with 250 ppm (and also 500 ppm) of silver, named Ni(Ag) in the following. Unpublished research records show elongated grains could not be obtained by alloying nickel–tungsten with silver in previous attempts by Eickemeyer et al. It is therefore the aim of our research work to establish elongated grains in Ni–W alloys. For that, several problems have to be overcome. First the final ⇑ Corresponding author. http://dx.doi.org/10.1016/j.jallcom.2014.10.098 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
annealing temperature has to be raised. For Ni(Ag) an annealing temperature as low as 550 °C was sufficient to fully recrystallize the material. For Ni5at.%W (Ni5W) a high temperature annealing step of about 1000 °C is required to consume unfavorably oriented grains remaining from the primary recrystallization process [14]. This high temperature thermal treatment might cause more equiaxed grains to be formed. Second the deformability might be affected severely, especially high temperature embrittlement is expected due to the segregation of silver at the grain boundaries, as it was observed earlier in Ni(Ag) [15]. 2. Experimental Ni5W with additions of 250 (Ag250), and 500 at. ppm (Ag500) of silver have been induction melted from the elements. The alloys were cast in a 32 38 mm rectangular mold and were subsequently hot rolled at 1075 °C to 22 22 mm. Larger concentrations of silver (1000 ppm) lead to embrittlement of the cast alloy causing fracture in the hot deformation process. After mechanical machining the ingots to 20 20 mm to remove the oxide layer cold rolling to 3 mm was performed at Ag250 and Ag500, where a recrystallization annealing at 775 °C took place. After this treatment TEM investigations were performed to locate the silver in the alloy. TEM foils were prepared by FIB cutting from the center of the samples. From 3 mm the alloys were cold rolled to a thickness of 0.08 mm, accumulating a total deformation degree of 3.6. Additionally up to two intermediate heat treatments were performed at 625 °C at thicknesses of 300 lm and 120 lm. The 80 lm tapes were finally recrystallized to achieve cube texture following a two or three step annealing process. The first step is a so-called nucleation treatment between 600 °C and 650 °C. Afterwards high temperature annealing steps at 800 °C or 1000 °C or both were added. The microstructure of the 80 lm tapes was investigated by EBSD surface maps, of which especially the aspect ratio was evaluated, as well as the cube texture fraction. To avoid the impact of small grains on the aspect ratio statistics, the aspect ratio of the large grains (i. e. an area more than 10 pixels) was evaluated, as well. The step size of the EBSD maps was 3 lm for the tapes annealed at 1000 °C (map size 32,000 points). For the lower annealing temperature (600–650 °C) a step size of 200 nm was chosen (map size 60,000
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U. Gaitzsch et al. / Journal of Alloys and Compounds 623 (2015) 132–135 points). For the intermediate annealing at 800 °C a step size of 1 lm was used with different map sizes. For the grain size determination a critical angle of 2° was defined to separate grains. Quantitative ODF computations from X-ray pole figure measurements were performed using the MTEX toolbox [16].
3. Results TEM investigations revealed that the silver segregated in small precipitations, which were identified as metallic silver by nanodiffraction (see Fig. 1b). The total investigated area was 6 lm 20 lm. 18 Particles were measured to have sizes between 9 nm and 57 nm, with the typical size between 15 nm and 20 nm. Thus the particles are too small to trigger particle induced nucleation, where usually a particle size larger than 1 lm is required [17]. The composition could not be verified via EDX, because the FIB foil is thicker than the silver particles. The following image (Fig. 1a) displays silver particles in the Ni–W Matrix. In the image there are also other dark spots present. However these other spots disappear and reappear as the sample is tilted. These are most likely dislocations or other bending features. The features identified as particles do not disappear during tilting and provide a clear Ag-signal in the EDS analysis. All the silver particles were found in the volume of the grains, in contrast no silver particles were found at the existing grain boundaries. This is valid for both Ag250 and Ag500. The previously (for Ni9W) optimized annealing of 650 °C for 4 h followed by 1000 °C for 2 h was applied to the alloys as a reference. Both Ag250 and Ag500 resulted in a well pronounced cube texture, but the aspect ratio was not enhanced in comparison to other substrate materials. For comparison the previously published Ni9W and Ni9.5W alloys have aspect ratios between 1.6 and 1.8 after our standard annealing regime. The results of the previously mentioned and some other heat treatments are displayed in Table 1. It can be seen that a final annealing temperature of 800 °C is not sufficient to obtain highly cube textured surfaces. Additionally the silver addition does not hamper the cube texture formation.
An intermediate heat treatment was introduced to form recrystallization nuclei, which are deformed during further cold rolling. So an elongated microstructure is provided which may be retained during recrystallization. The heat treatment was performed for 4 h at 625 °C at a thickness of 0.3 mm. The sample will be called Ag500-2 in the following. X-ray pole figure measurements after the intermediate heat treatment revealed a cube texture fraction of 5% indicating recrystallization nuclei to have formed. After rolling to the final thickness the cube texture fraction dropped to 2% indicating the reduction from 0.3 mm to 0.08 mm is enough to at least partly transform cube texture into deformation texture components. The sample Ag500-2 was also subjected to the standard heat treatment of 650 °C/1000 °C. Although that leads to an almost perfect cube texture (99.7% within 10° deviation) the aspect ratio was only 1.74 for the all grains in the map. The aspect ratio was 1.85 for the large grains (both 80% of the area and a cut-off diameter of 10 lm). In the following the first step of the final recrystallization treatment at 80 lm was investigated more closely to determine if elongated grains are formed during the nucleation treatment. The aspect ratio was determined in total as well in the larger grains. These were selected by sorting the grains by size and taking only those grains with at least 10 pixels into account. The results are displayed in Table 2. The nucleation treatment was carried out for 4 h. The aspect ratio of the large grains is always larger especially because 1 pixel grains (with an aspect ratio of 1) do not contribute to the statistic. The results show that the intermediate heat treatment does lead to an increased aspect ratio after the nucleation step of the final heat treatment. Especially the alloy Ag500-2 shows a very large aspect ratio of 5.03 after the nucleation treatment. However it should be noted that after this treatment only 15% of the surface were cube textured. The grains with largest aspect ratio are deformed grains that had recrystallized during the intermediate heat treatment.
Fig. 1. (a) TEM bright field image of Ag500: Ag particles (black dots) in NiW matrix and (b) nanodiffraction pattern of the middle silver particle (and Ni–W matrix, h1 0 0i zone axis).
Table 1 Cube texture fraction/average aspect ratio of all grains in the map for different recrystallization treatments. No intermediate heat treatment was applied to the tapes. Sample
650 °C, 4 h 1000 °C, 2 h
625 °C, 4 h 1000 °C, 2 h
600 °C, 4 h 1000 °C, 2 h
650 °C, 4 h 800 °C, 2 h
650 °C, 4 h 800 °C, 2 h 1000 °C, 2 h
Ag250
98.3/1.85
96.8/1.84
97.5/1.65
90.9/1.76
98.8/1.8
Ag500
97.6/1.64
95.9/1.89
94.9/1.65
84.0/1.77
96.7/1.70
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U. Gaitzsch et al. / Journal of Alloys and Compounds 623 (2015) 132–135
Table 2 Aspect ratio of the all grains in the map and the large grains (at least 10 pixels) after the nucleation treatment performed at the displayed temperature for 4 h. The cube texture fraction (cube, max. 10° deviation) and grain size (size, lm) are displayed, as well. Temp. (°C)
625
635
645
650
Ag250
Aspect ratio Cube/size
1.97/2.73 0.04/0.88
1.86/2.01 0.36/1.3
1.63/1.81 0.66/1.5
1.73/1.85 0.61/1.24
Ag500
Aspect ratio Cube/size
1.88/2.79 0.06/1.02
1.76/2.22 0.21/1.4
1.52/1.94 0.54/1.5
1.85/2.25 0.64/0.88
Ag500-2
Aspect ratio Cube/size
3.57/5.03 0.15/0.9
1.91/2.87 0.48/1.3
1.56/2.31 0.8/1.7
2.19/2.75 0.83/1.42
Table 3 Aspect ratio of all the grains in the map and the fraction of large grains (min. 10 pixels). The heat treatment comprised of a nucleation treatment for 4 h at the temperature displayed in the column head followed by 800 °C for 2 h and 1000 °C for 2 h. The cube texture fraction (cube, max. 10°. deviation) and grain size (size, lm) are displayed, as well. Temp. (°C)
625
635
645
650
Ag250
Aspect ratio Cube/size
1.69/1.76 0.97/20.0
1.86/1.95 0.99/19.0
1.83/1.97 0.98/19.5
1.8/2.00 0.99/20.0
Ag500
Aspect ratio Cube/size
1.69/1.85 0.96/17.7
1.61/1.73 0.97/17.9
1.64/1.69 0.97/18.3
1.77/1.86 0.97/19.7
Ag500-2
Aspect ratio Cube/size
1.82/2.00 0.98/19.4
1.79/1.94 0.99/21.1
1.93/2.03 0.99/17.8
2.17/2.36 0.996/18.5
Fig. 2. EBSD Map of Ag500-2 after the final heat treatment. Thin lines correspond to grain boundary angles between 2° and 10°. Thick lines correspond to larger grain boundary angles.
Since a two step annealing regime did not result in an enhanced aspect ratio an extra step was included. This was performed at 800 °C for 2 h. The idea was to reduce the stored energy in the system allowing the apparently existing elongated grains to grow slowly without changing their shape too much. The results of this modified treatment are shown in Table 3. The best result with an overall aspect ratio of 2.17 and 2.36 for the large grains was obtained by the treatment starting with 650 °C for 4 h. Interestingly, after this annealing regime the cube texture fraction was largest, as well. For this sample 99.6% of the surface were cube textured within 10° and 88% of the grain boundaries were low angle grain boundaries with a grain misorientation smaller than 10° (see EBSD Map in Fig. 2). To distinguish between the effects of alloying and thermal treatment, the alloy Ag250 was subjected to the intermediate heat treatment at 0.3 mm, as well. The following recrystallization heat treatment at the 80 lm tape (650 °C, 4 h/800 °C, 2 h/1000 °C, 2 h) lead to an aspect ratio of only 1.65 (1.76 for large grains). In an attempt to further optimize the aspect ratio two variants of a second intermediate heat treatment were introduced at
120 lm, consisting of a temperature of 625 °C and an annealing time of 4 h or 1 h, respectively. Unfortunately those attempts did not lead to an enhanced aspect ratio for any final recrystallization annealing treatment.
4. Discussion Only the combination of only one intermediate heat treatment, the higher silver concentration of 500 ppm and the three step final annealing regime resulted in an aspect ratio of larger than two for the whole mapped area. The intermediate heat treatment causes recrystallization nuclei to form, some of which are apparently cube textured. The effect of silver is not fully understood, yet. However since the elongated grains are not visible in Ag250 the high silver concentration of 500 ppm is necessary to obtain a large aspect ratio. During the final heat treatment the silver should be completely in solid solution at 1000 °C, presumably enriched at the grain boundaries as proposed by Eickemeyer et al. in pure Nickel [15]. At 800 °C residual precipitates might be present. There were
U. Gaitzsch et al. / Journal of Alloys and Compounds 623 (2015) 132–135
no particles found at existing grain boundaries in the TEM foil. This foil was prepared after a 775 °C recrystallization treatment. We propose that existing silver particles are not fully dissolved at this annealing temperature. It is therefore reasonable to assume that solute silver would migrate to the existing silver particles instead of forming new particles at the grain boundaries, thus avoiding the nucleation barrier. An elongated microstructure is provided by cold rolling. During the intermediate heat treatment at 625 °C partial recrystallization takes place producing equiaxed recrystallized grains as well as grains with a larger aspect ratio. Additionally deformed grains with a larger aspect ratio do still exist. During the final deformation the previously recrystallized grains are deformed to an oblong shape. It is believed, although not proven, that these deformed recrystallized grains are mandatory to form a complete recrystallized microstructure with oblong grains. The silver might be enriched at the grain boundaries and therefore stabilize the grain boundaries of the recrystallized grains formed in the intermediate annealing at 625 °C. The detailed mechanism of silver in the recrystallization process is beyond the scope of this article and will be the subject of further investigation. 5. Conclusions/summary Elongated grains were obtained for the Ag500 alloy after incorporating one intermediate heat treatment, only. Several different final recrystallization treatments lead to sharp cube textures, but the aspect ratio was only little different from silver free substrate tapes. The largest aspect ratio was 2.4 for Ag500-2 with a final heat treatment of 4 h at 650 °C, 2 h at 800 °C and 2 h at 1000 °C. The larger silver concentration produces a larger Zener drag on the grain boundaries in contrast to the Ag250-2 alloy. Therefore the oblong grain shape of the rolled microstructure can be transferred to the recrystallized microstructure. Finally a substrate tape with sharp cube texture and oblong grains is produced which is a
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good candidate to replace existing Ni5W substrates in high current applications. Acknowledgements The authors thank M. Frey for casting, D. Seifert and T. Wolf for rolling and S. Neumann and H.P. Trinks for thermal treatments. T. Sturm is acknowledged for preparing the TEM sample. References [1] J. Eickemeyer, D. Selbmann, R. Opitz, E. Maher, W. Prusseit, Phys. C – Supercond. ITS Appl. 341 (2000) 2425. [2] J. Eickemeyer, D. Selbmann, R. Opitz, B. de Boer, B. Holzapfel, L. Schultz, U. Miller, Supercond. Sci. Technol. 14 (2001) 152. [3] V. Sarma, J. Eickemeyer, L. Schultz, B. Holzapfel, Scr. Mater. 50 (2004) 953. [4] U. Gaitzsch, J. Eickemeyer, C. Rodig, J. Freudenberger, B. Holzapfel, L. Schultz, Scr. Mater. 62 (2010) 512. [5] A. Tuissi, E. Villa, M. Zamboni, J. Evetts, R. Tomov, Phys. C – Supercond. ITS Appl. 372 (2002) 759. [6] A. Vannozzi, G. Celentano, A.A. Armenio, A. Augieri, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, G. Thalmaier, IEEE Trans. Appl. Supercond. 19 (2009) 3283. [7] A. Vannozzi, G. Celentano, A. Angrisani, A. Augieri, L. Ciontea, I. Colantoni, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, G. Thalmaier, Nickel–copper alloy tapes as textured substrates for YBCO coated conductors, in: S. Hoste, M. Ausloos (Eds.), J. Phys. Conf. Ser., vol. 97, Iop Publishing Ltd, Bristol, 2008. [8] A. Vannozzi, G. Thalmaier, A.A. Armenio, A. Augieri, V. Galluzzi, A. Mancini, A. Rufoloni, T. Petrisor, G. Celentano, Acta Mater. 58 (2010) 910. [9] U. Gaitzsch, J. Haenisch, R. Huehne, C. Rodig, J. Freudenberger, B. Holzapfel, L. Schultz, Supercond. Sci. Technol. 26 (2013) 085024. [10] A.C. Wulff, O.V. Mishin, N.H. Andersen, Y. Zhao, J.-C. Grivel, Mater. Lett. 92 (2013) 386. [11] H. Tian, H.L. Suo, A.C. Wulff, J.-C. Grivel, O.V. Mishin, D.J. Jensen, J. Alloys Comp. 601 (2014) 9. [12] L. Bulaevskii, J. Clem, L. Glazman, A. Malozemoff, Phys. Rev. B 45 (1992) 2545. [13] J. Eickemeyer, D. Selbmann, R. Huehne, H. Wendrock, J. Haenisch, A. Gueth, L. Schultz, B. Holzapfel, Appl. Phys. Lett. (2007) 90. [14] A.C. Wulff, O.V. Mishin, J.-C. Grivel, J. Alloys Comp. 539 (2012) 161. [15] J. Eickemeyer, A. Guth, B. Holzapfel, Supercond. Sci. Technol. (2008) 21. [16] R. Hielscher, H. Schaeben, J. Appl. Crystallogr. 41 (2008) 1024. [17] F.J. Humphreys, Acta Mater. 45 (1997) 5031.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Elongated grains in Ni5W(Ag) RABiTS tapes Uwe Gaitzsch ⇑, Christian Rodig, Christine Damm, Ludwig Schultz IFW Dresden, Institute for Metallic Materials, PO Box 270116, D-01171 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 9 October 2014 Accepted 21 October 2014 Available online 28 October 2014 Keywords: Texture Recrystallization Rolling EBSD
a b s t r a c t Ni5W is a long known substrate material using the RABiTS approach. Elongated grains in the substrate tape would transfer to the superconductor epitaxially, where these are beneficial according to the brick wall model. 500 ppm and 250 ppm of silver were added to Ni5W and the intermediate and final heat treatments were altered from the standard process to obtain elongated grains. A maximum aspect ratio of 2.4 was realized in the alloy with 500 ppm Ag. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Ni–W alloys are known as substrates for superconducting thin films since 2000 [1,2]. These are known as RABiTS, short for Rolling Assisted Biaxially Textured Substrates. The addition of tungsten provides an enhanced temperature stability as well as a decreased magnetization in comparison to pure nickel [3]. Additionally the Zener drag leads to finer grains as it is usually the case when comparing recrystallized grain sizes of pure metals and alloys. Recently, efforts have been made to decrease the magnetization further by adding chromium [4,5], copper [6–8] or more tungsten [9]. All these measures have no beneficial effect on the microstructure of the textured tape. Additionally alloying Ni or Ni–W with copper results in a weaker cube texture and in an increasing amount of recrystallization twins [10,11]. Furthermore copper impedes the standard buffer deposition process. In a substrate for the epitaxial growth of superconducting thin films elongated grains in rolling direction are preferable. Those would cause elongated grains in direction of current flow in the superconducting layer, which is desirable according to the brick wall model by Bulaevskii et al. [12]. The realization of a recrystallized microstructure with elongated grains was successfully demonstrated by Eickemeyer et al. in 2007 [13]. It was a result of alloying nickel with 250 ppm (and also 500 ppm) of silver, named Ni(Ag) in the following. Unpublished research records show elongated grains could not be obtained by alloying nickel–tungsten with silver in previous attempts by Eickemeyer et al. It is therefore the aim of our research work to establish elongated grains in Ni–W alloys. For that, several problems have to be overcome. First the final ⇑ Corresponding author. http://dx.doi.org/10.1016/j.jallcom.2014.10.098 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
annealing temperature has to be raised. For Ni(Ag) an annealing temperature as low as 550 °C was sufficient to fully recrystallize the material. For Ni5at.%W (Ni5W) a high temperature annealing step of about 1000 °C is required to consume unfavorably oriented grains remaining from the primary recrystallization process [14]. This high temperature thermal treatment might cause more equiaxed grains to be formed. Second the deformability might be affected severely, especially high temperature embrittlement is expected due to the segregation of silver at the grain boundaries, as it was observed earlier in Ni(Ag) [15]. 2. Experimental Ni5W with additions of 250 (Ag250), and 500 at. ppm (Ag500) of silver have been induction melted from the elements. The alloys were cast in a 32 38 mm rectangular mold and were subsequently hot rolled at 1075 °C to 22 22 mm. Larger concentrations of silver (1000 ppm) lead to embrittlement of the cast alloy causing fracture in the hot deformation process. After mechanical machining the ingots to 20 20 mm to remove the oxide layer cold rolling to 3 mm was performed at Ag250 and Ag500, where a recrystallization annealing at 775 °C took place. After this treatment TEM investigations were performed to locate the silver in the alloy. TEM foils were prepared by FIB cutting from the center of the samples. From 3 mm the alloys were cold rolled to a thickness of 0.08 mm, accumulating a total deformation degree of 3.6. Additionally up to two intermediate heat treatments were performed at 625 °C at thicknesses of 300 lm and 120 lm. The 80 lm tapes were finally recrystallized to achieve cube texture following a two or three step annealing process. The first step is a so-called nucleation treatment between 600 °C and 650 °C. Afterwards high temperature annealing steps at 800 °C or 1000 °C or both were added. The microstructure of the 80 lm tapes was investigated by EBSD surface maps, of which especially the aspect ratio was evaluated, as well as the cube texture fraction. To avoid the impact of small grains on the aspect ratio statistics, the aspect ratio of the large grains (i. e. an area more than 10 pixels) was evaluated, as well. The step size of the EBSD maps was 3 lm for the tapes annealed at 1000 °C (map size 32,000 points). For the lower annealing temperature (600–650 °C) a step size of 200 nm was chosen (map size 60,000
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U. Gaitzsch et al. / Journal of Alloys and Compounds 623 (2015) 132–135 points). For the intermediate annealing at 800 °C a step size of 1 lm was used with different map sizes. For the grain size determination a critical angle of 2° was defined to separate grains. Quantitative ODF computations from X-ray pole figure measurements were performed using the MTEX toolbox [16].
3. Results TEM investigations revealed that the silver segregated in small precipitations, which were identified as metallic silver by nanodiffraction (see Fig. 1b). The total investigated area was 6 lm 20 lm. 18 Particles were measured to have sizes between 9 nm and 57 nm, with the typical size between 15 nm and 20 nm. Thus the particles are too small to trigger particle induced nucleation, where usually a particle size larger than 1 lm is required [17]. The composition could not be verified via EDX, because the FIB foil is thicker than the silver particles. The following image (Fig. 1a) displays silver particles in the Ni–W Matrix. In the image there are also other dark spots present. However these other spots disappear and reappear as the sample is tilted. These are most likely dislocations or other bending features. The features identified as particles do not disappear during tilting and provide a clear Ag-signal in the EDS analysis. All the silver particles were found in the volume of the grains, in contrast no silver particles were found at the existing grain boundaries. This is valid for both Ag250 and Ag500. The previously (for Ni9W) optimized annealing of 650 °C for 4 h followed by 1000 °C for 2 h was applied to the alloys as a reference. Both Ag250 and Ag500 resulted in a well pronounced cube texture, but the aspect ratio was not enhanced in comparison to other substrate materials. For comparison the previously published Ni9W and Ni9.5W alloys have aspect ratios between 1.6 and 1.8 after our standard annealing regime. The results of the previously mentioned and some other heat treatments are displayed in Table 1. It can be seen that a final annealing temperature of 800 °C is not sufficient to obtain highly cube textured surfaces. Additionally the silver addition does not hamper the cube texture formation.
An intermediate heat treatment was introduced to form recrystallization nuclei, which are deformed during further cold rolling. So an elongated microstructure is provided which may be retained during recrystallization. The heat treatment was performed for 4 h at 625 °C at a thickness of 0.3 mm. The sample will be called Ag500-2 in the following. X-ray pole figure measurements after the intermediate heat treatment revealed a cube texture fraction of 5% indicating recrystallization nuclei to have formed. After rolling to the final thickness the cube texture fraction dropped to 2% indicating the reduction from 0.3 mm to 0.08 mm is enough to at least partly transform cube texture into deformation texture components. The sample Ag500-2 was also subjected to the standard heat treatment of 650 °C/1000 °C. Although that leads to an almost perfect cube texture (99.7% within 10° deviation) the aspect ratio was only 1.74 for the all grains in the map. The aspect ratio was 1.85 for the large grains (both 80% of the area and a cut-off diameter of 10 lm). In the following the first step of the final recrystallization treatment at 80 lm was investigated more closely to determine if elongated grains are formed during the nucleation treatment. The aspect ratio was determined in total as well in the larger grains. These were selected by sorting the grains by size and taking only those grains with at least 10 pixels into account. The results are displayed in Table 2. The nucleation treatment was carried out for 4 h. The aspect ratio of the large grains is always larger especially because 1 pixel grains (with an aspect ratio of 1) do not contribute to the statistic. The results show that the intermediate heat treatment does lead to an increased aspect ratio after the nucleation step of the final heat treatment. Especially the alloy Ag500-2 shows a very large aspect ratio of 5.03 after the nucleation treatment. However it should be noted that after this treatment only 15% of the surface were cube textured. The grains with largest aspect ratio are deformed grains that had recrystallized during the intermediate heat treatment.
Fig. 1. (a) TEM bright field image of Ag500: Ag particles (black dots) in NiW matrix and (b) nanodiffraction pattern of the middle silver particle (and Ni–W matrix, h1 0 0i zone axis).
Table 1 Cube texture fraction/average aspect ratio of all grains in the map for different recrystallization treatments. No intermediate heat treatment was applied to the tapes. Sample
650 °C, 4 h 1000 °C, 2 h
625 °C, 4 h 1000 °C, 2 h
600 °C, 4 h 1000 °C, 2 h
650 °C, 4 h 800 °C, 2 h
650 °C, 4 h 800 °C, 2 h 1000 °C, 2 h
Ag250
98.3/1.85
96.8/1.84
97.5/1.65
90.9/1.76
98.8/1.8
Ag500
97.6/1.64
95.9/1.89
94.9/1.65
84.0/1.77
96.7/1.70
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U. Gaitzsch et al. / Journal of Alloys and Compounds 623 (2015) 132–135
Table 2 Aspect ratio of the all grains in the map and the large grains (at least 10 pixels) after the nucleation treatment performed at the displayed temperature for 4 h. The cube texture fraction (cube, max. 10° deviation) and grain size (size, lm) are displayed, as well. Temp. (°C)
625
635
645
650
Ag250
Aspect ratio Cube/size
1.97/2.73 0.04/0.88
1.86/2.01 0.36/1.3
1.63/1.81 0.66/1.5
1.73/1.85 0.61/1.24
Ag500
Aspect ratio Cube/size
1.88/2.79 0.06/1.02
1.76/2.22 0.21/1.4
1.52/1.94 0.54/1.5
1.85/2.25 0.64/0.88
Ag500-2
Aspect ratio Cube/size
3.57/5.03 0.15/0.9
1.91/2.87 0.48/1.3
1.56/2.31 0.8/1.7
2.19/2.75 0.83/1.42
Table 3 Aspect ratio of all the grains in the map and the fraction of large grains (min. 10 pixels). The heat treatment comprised of a nucleation treatment for 4 h at the temperature displayed in the column head followed by 800 °C for 2 h and 1000 °C for 2 h. The cube texture fraction (cube, max. 10°. deviation) and grain size (size, lm) are displayed, as well. Temp. (°C)
625
635
645
650
Ag250
Aspect ratio Cube/size
1.69/1.76 0.97/20.0
1.86/1.95 0.99/19.0
1.83/1.97 0.98/19.5
1.8/2.00 0.99/20.0
Ag500
Aspect ratio Cube/size
1.69/1.85 0.96/17.7
1.61/1.73 0.97/17.9
1.64/1.69 0.97/18.3
1.77/1.86 0.97/19.7
Ag500-2
Aspect ratio Cube/size
1.82/2.00 0.98/19.4
1.79/1.94 0.99/21.1
1.93/2.03 0.99/17.8
2.17/2.36 0.996/18.5
Fig. 2. EBSD Map of Ag500-2 after the final heat treatment. Thin lines correspond to grain boundary angles between 2° and 10°. Thick lines correspond to larger grain boundary angles.
Since a two step annealing regime did not result in an enhanced aspect ratio an extra step was included. This was performed at 800 °C for 2 h. The idea was to reduce the stored energy in the system allowing the apparently existing elongated grains to grow slowly without changing their shape too much. The results of this modified treatment are shown in Table 3. The best result with an overall aspect ratio of 2.17 and 2.36 for the large grains was obtained by the treatment starting with 650 °C for 4 h. Interestingly, after this annealing regime the cube texture fraction was largest, as well. For this sample 99.6% of the surface were cube textured within 10° and 88% of the grain boundaries were low angle grain boundaries with a grain misorientation smaller than 10° (see EBSD Map in Fig. 2). To distinguish between the effects of alloying and thermal treatment, the alloy Ag250 was subjected to the intermediate heat treatment at 0.3 mm, as well. The following recrystallization heat treatment at the 80 lm tape (650 °C, 4 h/800 °C, 2 h/1000 °C, 2 h) lead to an aspect ratio of only 1.65 (1.76 for large grains). In an attempt to further optimize the aspect ratio two variants of a second intermediate heat treatment were introduced at
120 lm, consisting of a temperature of 625 °C and an annealing time of 4 h or 1 h, respectively. Unfortunately those attempts did not lead to an enhanced aspect ratio for any final recrystallization annealing treatment.
4. Discussion Only the combination of only one intermediate heat treatment, the higher silver concentration of 500 ppm and the three step final annealing regime resulted in an aspect ratio of larger than two for the whole mapped area. The intermediate heat treatment causes recrystallization nuclei to form, some of which are apparently cube textured. The effect of silver is not fully understood, yet. However since the elongated grains are not visible in Ag250 the high silver concentration of 500 ppm is necessary to obtain a large aspect ratio. During the final heat treatment the silver should be completely in solid solution at 1000 °C, presumably enriched at the grain boundaries as proposed by Eickemeyer et al. in pure Nickel [15]. At 800 °C residual precipitates might be present. There were
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no particles found at existing grain boundaries in the TEM foil. This foil was prepared after a 775 °C recrystallization treatment. We propose that existing silver particles are not fully dissolved at this annealing temperature. It is therefore reasonable to assume that solute silver would migrate to the existing silver particles instead of forming new particles at the grain boundaries, thus avoiding the nucleation barrier. An elongated microstructure is provided by cold rolling. During the intermediate heat treatment at 625 °C partial recrystallization takes place producing equiaxed recrystallized grains as well as grains with a larger aspect ratio. Additionally deformed grains with a larger aspect ratio do still exist. During the final deformation the previously recrystallized grains are deformed to an oblong shape. It is believed, although not proven, that these deformed recrystallized grains are mandatory to form a complete recrystallized microstructure with oblong grains. The silver might be enriched at the grain boundaries and therefore stabilize the grain boundaries of the recrystallized grains formed in the intermediate annealing at 625 °C. The detailed mechanism of silver in the recrystallization process is beyond the scope of this article and will be the subject of further investigation. 5. Conclusions/summary Elongated grains were obtained for the Ag500 alloy after incorporating one intermediate heat treatment, only. Several different final recrystallization treatments lead to sharp cube textures, but the aspect ratio was only little different from silver free substrate tapes. The largest aspect ratio was 2.4 for Ag500-2 with a final heat treatment of 4 h at 650 °C, 2 h at 800 °C and 2 h at 1000 °C. The larger silver concentration produces a larger Zener drag on the grain boundaries in contrast to the Ag250-2 alloy. Therefore the oblong grain shape of the rolled microstructure can be transferred to the recrystallized microstructure. Finally a substrate tape with sharp cube texture and oblong grains is produced which is a
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good candidate to replace existing Ni5W substrates in high current applications. Acknowledgements The authors thank M. Frey for casting, D. Seifert and T. Wolf for rolling and S. Neumann and H.P. Trinks for thermal treatments. T. Sturm is acknowledged for preparing the TEM sample. References [1] J. Eickemeyer, D. Selbmann, R. Opitz, E. Maher, W. Prusseit, Phys. C – Supercond. ITS Appl. 341 (2000) 2425. [2] J. Eickemeyer, D. Selbmann, R. Opitz, B. de Boer, B. Holzapfel, L. Schultz, U. Miller, Supercond. Sci. Technol. 14 (2001) 152. [3] V. Sarma, J. Eickemeyer, L. Schultz, B. Holzapfel, Scr. Mater. 50 (2004) 953. [4] U. Gaitzsch, J. Eickemeyer, C. Rodig, J. Freudenberger, B. Holzapfel, L. Schultz, Scr. Mater. 62 (2010) 512. [5] A. Tuissi, E. Villa, M. Zamboni, J. Evetts, R. Tomov, Phys. C – Supercond. ITS Appl. 372 (2002) 759. [6] A. Vannozzi, G. Celentano, A.A. Armenio, A. Augieri, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, G. Thalmaier, IEEE Trans. Appl. Supercond. 19 (2009) 3283. [7] A. Vannozzi, G. Celentano, A. Angrisani, A. Augieri, L. Ciontea, I. Colantoni, V. Galluzzi, U. Gambardella, A. Mancini, T. Petrisor, A. Rufoloni, G. Thalmaier, Nickel–copper alloy tapes as textured substrates for YBCO coated conductors, in: S. Hoste, M. Ausloos (Eds.), J. Phys. Conf. Ser., vol. 97, Iop Publishing Ltd, Bristol, 2008. [8] A. Vannozzi, G. Thalmaier, A.A. Armenio, A. Augieri, V. Galluzzi, A. Mancini, A. Rufoloni, T. Petrisor, G. Celentano, Acta Mater. 58 (2010) 910. [9] U. Gaitzsch, J. Haenisch, R. Huehne, C. Rodig, J. Freudenberger, B. Holzapfel, L. Schultz, Supercond. Sci. Technol. 26 (2013) 085024. [10] A.C. Wulff, O.V. Mishin, N.H. Andersen, Y. Zhao, J.-C. Grivel, Mater. Lett. 92 (2013) 386. [11] H. Tian, H.L. Suo, A.C. Wulff, J.-C. Grivel, O.V. Mishin, D.J. Jensen, J. Alloys Comp. 601 (2014) 9. [12] L. Bulaevskii, J. Clem, L. Glazman, A. Malozemoff, Phys. Rev. B 45 (1992) 2545. [13] J. Eickemeyer, D. Selbmann, R. Huehne, H. Wendrock, J. Haenisch, A. Gueth, L. Schultz, B. Holzapfel, Appl. Phys. Lett. (2007) 90. [14] A.C. Wulff, O.V. Mishin, J.-C. Grivel, J. Alloys Comp. 539 (2012) 161. [15] J. Eickemeyer, A. Guth, B. Holzapfel, Supercond. Sci. Technol. (2008) 21. [16] R. Hielscher, H. Schaeben, J. Appl. Crystallogr. 41 (2008) 1024. [17] F.J. Humphreys, Acta Mater. 45 (1997) 5031.