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Observation of A15 phase transformation in RHQ-Nb3Al wire by neutron diffraction at high-temperature
Journal of Alloys and Compounds 535 (2012) 124–128
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Observation of A15 phase transformation in RHQ-Nb3Al wire by neutron diffraction at high-temperature Xinzhe Jin a,⇑, Tatsushi Nakamoto a, Stefanus Harjo b, Takayoshi Ito c, Toru Ogitsu a, Kiyosumi Tsuchiya a, Akira Yamamoto a, Akihiro Kikuchi d, Takao Takeuchi d, Tsutomu Hemmi e a
High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan Comprehensive Research Organization for Science and Society, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan d National Institute for Materials Science, Ibaraki 305-0047, Japan e Japan Atomic Energy Agency, Naka-shi, Ibaraki 311-0193, Japan b c
a r t i c l e
i n f o
Article history: Received 12 March 2012 Received in revised form 17 April 2012 Accepted 19 April 2012 Available online 2 May 2012 Keywords: A15 superconductor Neutron diffraction High temperature Phase transition Residual strain Composite materials
a b s t r a c t Nb3Al superconducting wires produced by rapid heating and quenching (RHQ) method have been developed for application to high field accelerator magnet. In an A15-type superconductor, it is known that residual strain in the superconducting phase induced by thermal contraction after heat treatment influences superconducting properties such as the critical current density. After RHQ treatment, a solid solution of NbAly with a bcc structure was formed from a jelly-roll of Nb and Al sheets in the wire. To observe the A15 phase transition in the NbAly and to clarify the mechanism of residual strain generation in the RHQ-Nb3Al wire, neutron diffraction measurements were carried out on the J-PARC ‘‘TAKUMI’’ between room-temperature and 800 °C, in which the Nb3Al superconducting phase is formed. Here, we report measurements on an RHQ-Nb3Al wire with an Nb/Ta composite matrix, using single-peak analysis and multi-peak analysis for peak intensity fitting and peak position fitting, respectively. The phase transition to the A15 was found to occur within a short period about 5 min while the temperature was increasing from 735 to 800 °C. Along the axial direction of the wire, growth of the A15 phase was found to be optimized using a subsequent holding process of 9 h at 800 °C. Following cooling to room temperature, the Nb3Al filaments in the wire exhibited an isotropic tensile residual strain of about 0.07%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Development of Nb3Al superconducting wires for high field accelerator magnets using a rapid heating and quenching (RHQ) process has received considerable attention because of their excellent performance regarding the strain dependence of the critical current at high magnetic fields [1–5]. It has been reported that when a RHQ treatment is applied to Nb and Al raw materials, a solid solution of NbAly with a bcc structure is formed [6–12]. Upon further heat treatment, this undergoes an A15 phase transformation to form Nb3Al. In the heat treatment at 800 °C, the optimum duration for the highest critical current was reported to be 10 h [13]. In the present study, neutron diffraction measurements of RHQNb3Al wires were carried out to examine the A15 phase transition and the generation of residual strain. This is first such observation of gradual changes with time or temperature for RHQ-Nb3Al
⇑ Corresponding author. Tel.: +81 29 864 5460; fax: +81 029 864 3209. E-mail addresses: [email protected], [email protected] (X. Jin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.070
wire, and it is hoped that the results obtained will lead to a better understanding of phase generation and growth, in addition to the residual strain generation mechanism. In-situ observations of the Nb3Al phase transition were carried out during both temperature ramping and holding processes. Strain in a superconducting wire is known to influence superconducting characteristics such as its critical current [14–16]. For Nb3Al filaments subjected to a heat treatment to induce the A15 phase transition, thermal stress occurs during the cooling stage due to the different coefficients of thermal expansion (CTE) between the materials in the composite wire. This leads to the presence of residual strain in the superconducting wire, which can have effects on the strain dependence of critical current. For this reason, it is important to understand the generation mechanism of such strain. The amount of residual strain in the composite wire that is produced depends on the CTE, Young’s modulus and volume fraction of the each material, in addition to the magnitude of the temperature change. For wires of a single-material matrix without Cu stabilizer, it is useful to determine whether the residual strain is compressive or tensile, such as non-Cu RHQ-Nb3Al wires with
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all-Nb matrix or all-Ta matrix that exhibits all the tensile residual strain in the axial direction with a simple model calculation [17]. For RHQ-Nb3Al wires with composite formed by matrix materials (Ta and Nb) and Cu, the residual strain is necessary to determine precisely in the experiments to examine the strain direction of Nb3Al filaments is tensile or compressive, due to the competitive effects of the matrix tension and the Cu stabilizer compression on the Nb3Al filaments with plastic deformation of Cu. For improving the precision of the residual strain in the experiment, we have previously used multi-peak analysis of neutron diffraction patterns, which allows the average lattice constant to be determined [17]. The strain calculated from the average lattice constant is similar to the macroscopic strain. Multi-peak analysis is a statistical method using multiple lattice planes for a specific phase in the elastic strain regardless of an isotropic or anisotropic strain, and its accuracy is higher than that of single-peak analysis. This analysis method has been demonstrated to be effective for strain evaluation of composite wires containing Nb3Al. In the present study, we applied the multi-peak analysis method to determine the residual strain with thermal contraction during the temperature decreasing, in order to clarify the strain generation mechanism. 2. Experimental 2.1. Sample preparation
Table 1 Specification of sample. Wire diameter (mm) Cu/non-Cu ratio Matrix/NbAly ratio (non-Cu) Thickness of Ta inter-filament (lm) Diameter of Nb3Al filament (lm) Number of Nb3Al filaments
1 1 0.8 10 30–40 294
Fig. 2. The 2D diagram of neutron diffraction measurement for wire sample at high temperature by using infrared heater. The sample was measured in a vacuum.
We prepared Nb3Al wire with a composite matrix of Nb and Ta [17]. The cross section and specifications of the sample are shown in Fig. 1 and Table 1, respectively. The skin, core and center dummy are made from Nb, and the inter-filament material is Ta. To observe the A15 phase transformation, the sample was chosen after the RHQ treatment that solid solution with a composition of NbAly was formed from the raw materials of Nb and Al sheets. For the neutron diffraction measurements at high-temperature, eight wires with each length 20 mm were bundled together using a stainless iron wire at both ends. To determine the residual strain of the Nb3Al filaments in the composite wire, a sample representing a strain-free state was required. Since removal of the Ta or Nb matrix surrounding the Nb3Al filaments is very difficult, the sample was formed into a powder after removing the Cu stabilizer. The powder sample was prepared with a particle size below 20 lm to achieve a strain-free state.
2.2. Neutron diffraction measurements A series of neutron diffraction measurements was carried out between room temperature (23 °C) and 800 °C in the J-PARC ‘‘TAKUMI’’ using the time-of-flight (TOF) method [18,19]. Diffraction patterns could be measured simultaneously in the transverse and axial directions of the wire using two detectors on opposite sides of the sample, with the sample placed at 45° to the incident beam, as shown in Fig. 2. The sample was placed in a non-crystalline glass tube in a vacuum and the temperature was increased using an infrared heater. The neutron beam was 5 mm in width and 6 mm in height at the sample slit. The beam width was just
Fig. 3. Temperature profile during neutron diffraction measurements.
25% of the sample length, in order to avoid neutron diffraction from the stainless iron wires at the ends of the sample. The temperature was measured by using platinum thermocouple in the glass tube. To measure the temperature accurately, thermocouple need to be brought into contact with the part that occurs neutron diffraction in the sample. Therefore, the small peaks associated with the platinum thermocouple appeared in the neutron diffraction patterns. This experiment was carried out with a proton beam power of 200 kW. The temperature profile used during the measurements is shown in Fig. 3. The temperature was first increased from room temperature to 800 °C in 1 h to initiate the A15 phase transition. It was then held at 800 °C for 11 h to observe the growth process of the A15 phase. Finally, the temperature was decreased in steps of 100 °C, and the sample was held at these intermediate temperatures for 30 min to measure the lattice constants at each temperature.
3. Results and discussion 3.1. A15 phase generation
Fig. 1. Cross section of sample. The skin, core and center dummy are all composed of Nb. Only the inter-filament material is composed of Ta.
Neutron diffraction patterns measured during the initial period of increasing temperature are shown in Fig. 4(a) and (b) for the transverse and axial directions of the wire, respectively. Each pattern was measured for 5 min. In the transverse direction, as shown in Fig. 4(a), peaks due to Cu-220, NbAly-211, Nb/Ta-220, and Nb/Ta-211 were observed
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Fig. 4. Neutron diffraction patterns during the temperature increasing stage for (a) transverse and (b) axial directions. Each pattern was measured for 5 min. The up- and down-arrows indicate an increase and decrease in the peak intensities, respectively.
between room temperature and 88 °C. A small Pt-311 peak was also observed due to the thermocouple. With increasing temperature, all of the peaks shifted to a longer TOF, indicating larger lattice spacing. For the patterns in the temperature range 23– 735 °C, no peaks due to the A15 phase were present. Peaks associated with the A15 phase of Nb3Al were observed in the temperature range of 735–800 °C, with an associated decrease in the NbAly-211 peak intensity. This indicates that a phase transition occurred from bcc NbAly to the A15 superconducting phase. In the axial direction, as shown in Fig. 4(b), peaks due to Cu-220, Nb/Ta-220, and Nb/Ta-211 were observed in the range 23–88 °C. In
Fig. 5. Neutron diffraction patterns at the beginning of the 800 °C holding process for the (a) transverse and (b) axial directions. Each pattern was measured for 10 min.
addition, NbAly-220 and Pt-220 peaks were observed, whereas the NbAly-211 peak was absent. These differences between the transverse and axial directions were due to the presence of texture in the sample and in the thermocouple. The same as the result observed in the transverse direction, the phase transition from NbAly to the A15 phase occurred in the temperature range 735–800 °C.
3.2. A15 phase growth During the holding process for 11 h at 800 °C, neutron diffraction patterns were obtained by subdividing the measurement with duration of each 10 min. Fig. 5 shows the results for the first of these measurements. The sample showed a difference in the relative intensity ratio of each hkl peak between the transverse and axial directions. This difference indicates that crystallites in the RHQ-Nb3Al wires have preferred orientation. As an indicator of A15 phase growth, the 321, 320 and 222 peaks of Nb3Al was used. To obtain more accuracy in comparing between the patterns at different holding periods, the neutron beam power per unit of time was normalized. The Cu-220 peak intensities were used to normalize as the peak intensity ratios, i.e. Nb3Al-320/Cu-220. Results for the peak intensity ratios are shown in Fig. 6. The peak intensities were obtained by single-peak fitting to the Gaussian function. As shown in the Fig. 6(a) for the transverse direction, the intensity ratios were almost constant up to a holding time of 11 h. In the axial direction as shown in Fig. 6(b), they were almost constant up to 8 h. These results indicate that the A15 phase transition was almost completed just a several minutes in temperature increasing stage before the sample was held at 800 °C. Following 8 h at 800 °C, the intensity ratios increased strongly for the axial direction pattern. The ratio peaked at about 9 h and then decreased slightly. Considering the preferred orientation between transverse and axial directions as shown in Fig. 5, this increase is probably due to a change of the preferred crystallite orientation in the Nb3Al filaments during heat treatment. Thus, the results indicate that a holding time of 9 h at 800 °C is optimum for growing of peaks that aligned to the axial direction with the preferred orientation.
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Intensity ratio
0.8
1
o
800 C Transverse
0.8
Nb3Al-321/Cu-220 Nb3Al-320/Cu-220 Nb3Al-222/Cu-220
Intensity ratio
1
0.6 0.4
Nb3Al-321/Cu-220 (*Pt-220) Nb3Al-320/Cu-220 Nb3Al-222/Cu-220
0.6 0.4 0.2
0.2
0
o
800 C Axial
2
4
6
8
10
Duration of heat treatment (hour)
(a)
0
2
4
6
8
10
Duration of heat treatment (hour)
(b)
Fig. 6. Intensity ratios of Nb3Al-321/Cu-220, Nb3Al-320/Cu-220 and Nb3Al-222/Cu-220 with duration in the holding process at 800 °C in (a) transverse and (b) axial directions. They are obtained by single-peak fitting to the Gaussian function. In the axial direction, the ⁄Pt-220 in brackets shows that Nb3Al-321 peak was overlapped with the small Pt-220 peak in each pattern as shown in Fig. 5.
3.3. Residual strain generation As shown in Fig. 3, following the 11 h holding period at 800 °C, the temperature was decreased in 100 °C steps. Following each step, neutron diffraction patterns were measured for 30 min. With decreasing of the temperature, thermal contraction was induced in the materials of the wire that have different CTE values. To evaluate the strain resulting from the thermal contraction, the lattice constants of the Nb3Al filaments and copper stabilizer were estimated by multi-peak analysis using the Z-Rietveld [20]. As an example, Figs. 7 and 8 show multi-peak analysis results for the powder and wire (transverse direction) samples, respectively, after the sample had been returned to room temperature. The curve labeled Delta represents the difference between the measured and fitted patterns, and it can be seen that a very good fit was obtained. Since Nb and Ta have almost the same lattice constant and a similar CTE, their peaks were never clearly separated and it was not possible to separately distinguish any changes due to residual strain. The composition ratio of Al in the Nb3Al filaments was obtained to be by about 22 at.% by Rietveld analysis, which was close to the stoichiometric proportion of 25 at.%. For Cu and Nb3Al, the temperature dependence of the lattice constant a as a function of decreasing temperature is shown in Fig. 9(a) and (b), respectively. The dashed line represents the
Fig. 7. Multi-peak analysis of the diffraction pattern for the powder sample at room temperature following A15 heat treatment. The lattice constants a were determined by using the Z-Rietveld.
Fig. 8. Multi-peak analysis of transverse diffraction pattern for Nb3Al wire at room temperature following A15 heat treatment. The lattice constants a were determined by using the Z-Rietveld.
strain-free values obtained by calculation with using each CTE [21]. For Cu, the same lattice constants were determined for both the transverse and axial directions. It can be seen that the measured values begin to deviate from the dashed line at temperature between 200 and 300 °C. This temperature range is corresponding to the recrystallization temperature of Cu. It indicates that residual strain due to the phase stress is occurred below this temperature in the Cu stabilizer. For Nb3Al as shown in Fig. 9(b), the lattice constants determined from the transverse and axial directions were roughly equal for each temperature. This indicates that the Nb3Al filaments experience an isotropic residual strain in the cooling process due to the isotropic thermal contraction in each material. On the competitive stress of the composite materials in the wire, the residual strain in the Nb3Al filaments appeared clearly between 200 and 300 °C as shown in the Fig. 9(b). It can be seen that the lattice constants a of Nb3Al are larger than that of the powder at room temperature. The results indicate that the Nb3Al filaments are subject to a tensile residual strain of 0.07%, despite the compressive effect of the Cu on the Nb skin. We previously evaluated the residual tensile strain in Nb3Al filaments in a similar wire without a Cu stabilizer to be about 0.12% at room temperature [17]. The difference of about 0.05% between the two results is therefore thought to be due to the compressive effect of the Cu.
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Fig. 9. Temperature dependence of lattice constant a for (a) Cu and (b) Nb3Al during the decreasing temperature stage. Since the standard deviations were very small, these were overlapped with the obtained statistic values, such as for Cu and for Nb3Al wire at transverse.
4. Conclusion Neutron diffraction measurements on RHQ-Nb3Al wire with Nb/Ta matrix were carried out between room temperature and 800 °C. It was found that an A15 phase transition occurred within 5 min as the temperature was increasing between 735 and 800 °C. Based on the intensities of the 321, 320 and 222 peaks in Nb3Al, a holding time of 9 h at 800 °C was found to optimize growth of the A15 phase at axial direction. Multi-peak analysis was used to measure the lattice constants of Cu and Nb3Al during the subsequent cooling stage, allowing the residual strain to be determined. An isotropic tensile strain of 0.07% was found in the Nb3Al at room temperature, which is thought to be somewhat reduced due to the compressive effect of the Cu stabilizer. The fact that the strain was tensile despite the presence of the Cu indicates that the tensile effect of the matrix is dominant in the RHQ-Nb3Al wire.
References
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[1] T. Takeuchi, M. Kosuge, N. Banno, A. Kikuchi, Y. Iijima, Supercond. Sci. Technol. 18 (2005) 985. [2] T. Takeuchi, A. Kikuchi, N. Banno, H. Kitaguchi, Y. Iijima, K. Tagawa, K. Nakagawa, K. Tsuchiya, C. Mitsuda, N. Koizumi, K. Okuno, Cryogenics 48 (2008) 371. [3] A. Kikuchi, R. Yamada, E. Barzi, M. Lamm, T. Takeuchi, D. Turrioni, A.V. Zlobin, IEEE Trans. Appl. Supercond. 18 (2008) 1026. [4] A. Kikuchi, R. Yamada, G. Ambrosio, N. Andreev, E. Barzi, C. Cooper, Y. Iijima, M. Kobayashi, H. Kitaguchi, S. Nimori, M. Lamm, K. Tagawa, T. Takeuchi, K.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Observation of A15 phase transformation in RHQ-Nb3Al wire by neutron diffraction at high-temperature Xinzhe Jin a,⇑, Tatsushi Nakamoto a, Stefanus Harjo b, Takayoshi Ito c, Toru Ogitsu a, Kiyosumi Tsuchiya a, Akira Yamamoto a, Akihiro Kikuchi d, Takao Takeuchi d, Tsutomu Hemmi e a
High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan Comprehensive Research Organization for Science and Society, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan d National Institute for Materials Science, Ibaraki 305-0047, Japan e Japan Atomic Energy Agency, Naka-shi, Ibaraki 311-0193, Japan b c
a r t i c l e
i n f o
Article history: Received 12 March 2012 Received in revised form 17 April 2012 Accepted 19 April 2012 Available online 2 May 2012 Keywords: A15 superconductor Neutron diffraction High temperature Phase transition Residual strain Composite materials
a b s t r a c t Nb3Al superconducting wires produced by rapid heating and quenching (RHQ) method have been developed for application to high field accelerator magnet. In an A15-type superconductor, it is known that residual strain in the superconducting phase induced by thermal contraction after heat treatment influences superconducting properties such as the critical current density. After RHQ treatment, a solid solution of NbAly with a bcc structure was formed from a jelly-roll of Nb and Al sheets in the wire. To observe the A15 phase transition in the NbAly and to clarify the mechanism of residual strain generation in the RHQ-Nb3Al wire, neutron diffraction measurements were carried out on the J-PARC ‘‘TAKUMI’’ between room-temperature and 800 °C, in which the Nb3Al superconducting phase is formed. Here, we report measurements on an RHQ-Nb3Al wire with an Nb/Ta composite matrix, using single-peak analysis and multi-peak analysis for peak intensity fitting and peak position fitting, respectively. The phase transition to the A15 was found to occur within a short period about 5 min while the temperature was increasing from 735 to 800 °C. Along the axial direction of the wire, growth of the A15 phase was found to be optimized using a subsequent holding process of 9 h at 800 °C. Following cooling to room temperature, the Nb3Al filaments in the wire exhibited an isotropic tensile residual strain of about 0.07%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Development of Nb3Al superconducting wires for high field accelerator magnets using a rapid heating and quenching (RHQ) process has received considerable attention because of their excellent performance regarding the strain dependence of the critical current at high magnetic fields [1–5]. It has been reported that when a RHQ treatment is applied to Nb and Al raw materials, a solid solution of NbAly with a bcc structure is formed [6–12]. Upon further heat treatment, this undergoes an A15 phase transformation to form Nb3Al. In the heat treatment at 800 °C, the optimum duration for the highest critical current was reported to be 10 h [13]. In the present study, neutron diffraction measurements of RHQNb3Al wires were carried out to examine the A15 phase transition and the generation of residual strain. This is first such observation of gradual changes with time or temperature for RHQ-Nb3Al
⇑ Corresponding author. Tel.: +81 29 864 5460; fax: +81 029 864 3209. E-mail addresses: [email protected], [email protected] (X. Jin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.070
wire, and it is hoped that the results obtained will lead to a better understanding of phase generation and growth, in addition to the residual strain generation mechanism. In-situ observations of the Nb3Al phase transition were carried out during both temperature ramping and holding processes. Strain in a superconducting wire is known to influence superconducting characteristics such as its critical current [14–16]. For Nb3Al filaments subjected to a heat treatment to induce the A15 phase transition, thermal stress occurs during the cooling stage due to the different coefficients of thermal expansion (CTE) between the materials in the composite wire. This leads to the presence of residual strain in the superconducting wire, which can have effects on the strain dependence of critical current. For this reason, it is important to understand the generation mechanism of such strain. The amount of residual strain in the composite wire that is produced depends on the CTE, Young’s modulus and volume fraction of the each material, in addition to the magnitude of the temperature change. For wires of a single-material matrix without Cu stabilizer, it is useful to determine whether the residual strain is compressive or tensile, such as non-Cu RHQ-Nb3Al wires with
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all-Nb matrix or all-Ta matrix that exhibits all the tensile residual strain in the axial direction with a simple model calculation [17]. For RHQ-Nb3Al wires with composite formed by matrix materials (Ta and Nb) and Cu, the residual strain is necessary to determine precisely in the experiments to examine the strain direction of Nb3Al filaments is tensile or compressive, due to the competitive effects of the matrix tension and the Cu stabilizer compression on the Nb3Al filaments with plastic deformation of Cu. For improving the precision of the residual strain in the experiment, we have previously used multi-peak analysis of neutron diffraction patterns, which allows the average lattice constant to be determined [17]. The strain calculated from the average lattice constant is similar to the macroscopic strain. Multi-peak analysis is a statistical method using multiple lattice planes for a specific phase in the elastic strain regardless of an isotropic or anisotropic strain, and its accuracy is higher than that of single-peak analysis. This analysis method has been demonstrated to be effective for strain evaluation of composite wires containing Nb3Al. In the present study, we applied the multi-peak analysis method to determine the residual strain with thermal contraction during the temperature decreasing, in order to clarify the strain generation mechanism. 2. Experimental 2.1. Sample preparation
Table 1 Specification of sample. Wire diameter (mm) Cu/non-Cu ratio Matrix/NbAly ratio (non-Cu) Thickness of Ta inter-filament (lm) Diameter of Nb3Al filament (lm) Number of Nb3Al filaments
1 1 0.8 10 30–40 294
Fig. 2. The 2D diagram of neutron diffraction measurement for wire sample at high temperature by using infrared heater. The sample was measured in a vacuum.
We prepared Nb3Al wire with a composite matrix of Nb and Ta [17]. The cross section and specifications of the sample are shown in Fig. 1 and Table 1, respectively. The skin, core and center dummy are made from Nb, and the inter-filament material is Ta. To observe the A15 phase transformation, the sample was chosen after the RHQ treatment that solid solution with a composition of NbAly was formed from the raw materials of Nb and Al sheets. For the neutron diffraction measurements at high-temperature, eight wires with each length 20 mm were bundled together using a stainless iron wire at both ends. To determine the residual strain of the Nb3Al filaments in the composite wire, a sample representing a strain-free state was required. Since removal of the Ta or Nb matrix surrounding the Nb3Al filaments is very difficult, the sample was formed into a powder after removing the Cu stabilizer. The powder sample was prepared with a particle size below 20 lm to achieve a strain-free state.
2.2. Neutron diffraction measurements A series of neutron diffraction measurements was carried out between room temperature (23 °C) and 800 °C in the J-PARC ‘‘TAKUMI’’ using the time-of-flight (TOF) method [18,19]. Diffraction patterns could be measured simultaneously in the transverse and axial directions of the wire using two detectors on opposite sides of the sample, with the sample placed at 45° to the incident beam, as shown in Fig. 2. The sample was placed in a non-crystalline glass tube in a vacuum and the temperature was increased using an infrared heater. The neutron beam was 5 mm in width and 6 mm in height at the sample slit. The beam width was just
Fig. 3. Temperature profile during neutron diffraction measurements.
25% of the sample length, in order to avoid neutron diffraction from the stainless iron wires at the ends of the sample. The temperature was measured by using platinum thermocouple in the glass tube. To measure the temperature accurately, thermocouple need to be brought into contact with the part that occurs neutron diffraction in the sample. Therefore, the small peaks associated with the platinum thermocouple appeared in the neutron diffraction patterns. This experiment was carried out with a proton beam power of 200 kW. The temperature profile used during the measurements is shown in Fig. 3. The temperature was first increased from room temperature to 800 °C in 1 h to initiate the A15 phase transition. It was then held at 800 °C for 11 h to observe the growth process of the A15 phase. Finally, the temperature was decreased in steps of 100 °C, and the sample was held at these intermediate temperatures for 30 min to measure the lattice constants at each temperature.
3. Results and discussion 3.1. A15 phase generation
Fig. 1. Cross section of sample. The skin, core and center dummy are all composed of Nb. Only the inter-filament material is composed of Ta.
Neutron diffraction patterns measured during the initial period of increasing temperature are shown in Fig. 4(a) and (b) for the transverse and axial directions of the wire, respectively. Each pattern was measured for 5 min. In the transverse direction, as shown in Fig. 4(a), peaks due to Cu-220, NbAly-211, Nb/Ta-220, and Nb/Ta-211 were observed
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Fig. 4. Neutron diffraction patterns during the temperature increasing stage for (a) transverse and (b) axial directions. Each pattern was measured for 5 min. The up- and down-arrows indicate an increase and decrease in the peak intensities, respectively.
between room temperature and 88 °C. A small Pt-311 peak was also observed due to the thermocouple. With increasing temperature, all of the peaks shifted to a longer TOF, indicating larger lattice spacing. For the patterns in the temperature range 23– 735 °C, no peaks due to the A15 phase were present. Peaks associated with the A15 phase of Nb3Al were observed in the temperature range of 735–800 °C, with an associated decrease in the NbAly-211 peak intensity. This indicates that a phase transition occurred from bcc NbAly to the A15 superconducting phase. In the axial direction, as shown in Fig. 4(b), peaks due to Cu-220, Nb/Ta-220, and Nb/Ta-211 were observed in the range 23–88 °C. In
Fig. 5. Neutron diffraction patterns at the beginning of the 800 °C holding process for the (a) transverse and (b) axial directions. Each pattern was measured for 10 min.
addition, NbAly-220 and Pt-220 peaks were observed, whereas the NbAly-211 peak was absent. These differences between the transverse and axial directions were due to the presence of texture in the sample and in the thermocouple. The same as the result observed in the transverse direction, the phase transition from NbAly to the A15 phase occurred in the temperature range 735–800 °C.
3.2. A15 phase growth During the holding process for 11 h at 800 °C, neutron diffraction patterns were obtained by subdividing the measurement with duration of each 10 min. Fig. 5 shows the results for the first of these measurements. The sample showed a difference in the relative intensity ratio of each hkl peak between the transverse and axial directions. This difference indicates that crystallites in the RHQ-Nb3Al wires have preferred orientation. As an indicator of A15 phase growth, the 321, 320 and 222 peaks of Nb3Al was used. To obtain more accuracy in comparing between the patterns at different holding periods, the neutron beam power per unit of time was normalized. The Cu-220 peak intensities were used to normalize as the peak intensity ratios, i.e. Nb3Al-320/Cu-220. Results for the peak intensity ratios are shown in Fig. 6. The peak intensities were obtained by single-peak fitting to the Gaussian function. As shown in the Fig. 6(a) for the transverse direction, the intensity ratios were almost constant up to a holding time of 11 h. In the axial direction as shown in Fig. 6(b), they were almost constant up to 8 h. These results indicate that the A15 phase transition was almost completed just a several minutes in temperature increasing stage before the sample was held at 800 °C. Following 8 h at 800 °C, the intensity ratios increased strongly for the axial direction pattern. The ratio peaked at about 9 h and then decreased slightly. Considering the preferred orientation between transverse and axial directions as shown in Fig. 5, this increase is probably due to a change of the preferred crystallite orientation in the Nb3Al filaments during heat treatment. Thus, the results indicate that a holding time of 9 h at 800 °C is optimum for growing of peaks that aligned to the axial direction with the preferred orientation.
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Intensity ratio
0.8
1
o
800 C Transverse
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Nb3Al-321/Cu-220 Nb3Al-320/Cu-220 Nb3Al-222/Cu-220
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0.6 0.4
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2
4
6
8
10
Duration of heat treatment (hour)
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0
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6
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Duration of heat treatment (hour)
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Fig. 6. Intensity ratios of Nb3Al-321/Cu-220, Nb3Al-320/Cu-220 and Nb3Al-222/Cu-220 with duration in the holding process at 800 °C in (a) transverse and (b) axial directions. They are obtained by single-peak fitting to the Gaussian function. In the axial direction, the ⁄Pt-220 in brackets shows that Nb3Al-321 peak was overlapped with the small Pt-220 peak in each pattern as shown in Fig. 5.
3.3. Residual strain generation As shown in Fig. 3, following the 11 h holding period at 800 °C, the temperature was decreased in 100 °C steps. Following each step, neutron diffraction patterns were measured for 30 min. With decreasing of the temperature, thermal contraction was induced in the materials of the wire that have different CTE values. To evaluate the strain resulting from the thermal contraction, the lattice constants of the Nb3Al filaments and copper stabilizer were estimated by multi-peak analysis using the Z-Rietveld [20]. As an example, Figs. 7 and 8 show multi-peak analysis results for the powder and wire (transverse direction) samples, respectively, after the sample had been returned to room temperature. The curve labeled Delta represents the difference between the measured and fitted patterns, and it can be seen that a very good fit was obtained. Since Nb and Ta have almost the same lattice constant and a similar CTE, their peaks were never clearly separated and it was not possible to separately distinguish any changes due to residual strain. The composition ratio of Al in the Nb3Al filaments was obtained to be by about 22 at.% by Rietveld analysis, which was close to the stoichiometric proportion of 25 at.%. For Cu and Nb3Al, the temperature dependence of the lattice constant a as a function of decreasing temperature is shown in Fig. 9(a) and (b), respectively. The dashed line represents the
Fig. 7. Multi-peak analysis of the diffraction pattern for the powder sample at room temperature following A15 heat treatment. The lattice constants a were determined by using the Z-Rietveld.
Fig. 8. Multi-peak analysis of transverse diffraction pattern for Nb3Al wire at room temperature following A15 heat treatment. The lattice constants a were determined by using the Z-Rietveld.
strain-free values obtained by calculation with using each CTE [21]. For Cu, the same lattice constants were determined for both the transverse and axial directions. It can be seen that the measured values begin to deviate from the dashed line at temperature between 200 and 300 °C. This temperature range is corresponding to the recrystallization temperature of Cu. It indicates that residual strain due to the phase stress is occurred below this temperature in the Cu stabilizer. For Nb3Al as shown in Fig. 9(b), the lattice constants determined from the transverse and axial directions were roughly equal for each temperature. This indicates that the Nb3Al filaments experience an isotropic residual strain in the cooling process due to the isotropic thermal contraction in each material. On the competitive stress of the composite materials in the wire, the residual strain in the Nb3Al filaments appeared clearly between 200 and 300 °C as shown in the Fig. 9(b). It can be seen that the lattice constants a of Nb3Al are larger than that of the powder at room temperature. The results indicate that the Nb3Al filaments are subject to a tensile residual strain of 0.07%, despite the compressive effect of the Cu on the Nb skin. We previously evaluated the residual tensile strain in Nb3Al filaments in a similar wire without a Cu stabilizer to be about 0.12% at room temperature [17]. The difference of about 0.05% between the two results is therefore thought to be due to the compressive effect of the Cu.
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Fig. 9. Temperature dependence of lattice constant a for (a) Cu and (b) Nb3Al during the decreasing temperature stage. Since the standard deviations were very small, these were overlapped with the obtained statistic values, such as for Cu and for Nb3Al wire at transverse.
4. Conclusion Neutron diffraction measurements on RHQ-Nb3Al wire with Nb/Ta matrix were carried out between room temperature and 800 °C. It was found that an A15 phase transition occurred within 5 min as the temperature was increasing between 735 and 800 °C. Based on the intensities of the 321, 320 and 222 peaks in Nb3Al, a holding time of 9 h at 800 °C was found to optimize growth of the A15 phase at axial direction. Multi-peak analysis was used to measure the lattice constants of Cu and Nb3Al during the subsequent cooling stage, allowing the residual strain to be determined. An isotropic tensile strain of 0.07% was found in the Nb3Al at room temperature, which is thought to be somewhat reduced due to the compressive effect of the Cu stabilizer. The fact that the strain was tensile despite the presence of the Cu indicates that the tensile effect of the matrix is dominant in the RHQ-Nb3Al wire.
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