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Fracture toughness of REBaCuO bulk superconductor at liquid nitrogen temperature
Physica C 426–431 (2005) 709–713 www.elsevier.com/locate/physc
Fracture toughness of REBaCuO bulk superconductor at liquid nitrogen temperature K. Katagiri
a,*
, T. Sato a, A. Murakami b, K. Kasaba a, Y. Shoji a, K. Noto c, H. Teshima d, M. Sawamura d
a Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Faculty of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan c Iwate Industrial Promotion Center, 3-52-2 Iioka-shinden, Morioka 020-0852, Japan Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtu 293-8511, Japan b
d
Received 23 November 2004; accepted 17 February 2005 Available online 5 July 2005
Abstract The fracture toughness of Y123 and Gd123(Ag) single-grain bulk samples with 25 mol% RE 211 phase (and 10 wt.% Ag for the latter) prepared by the modified quench and melt-growth technique, was evaluated by the 3-point bending test at liquid nitrogen temperature (LNT) and room temperature (RT). The sharp V-notch was introduced into the specimens such that both of the loading and the crack extension directions are in the c-axis. The data for Y123 scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT. The average value at LNT, 1.6 MPam1/2, was slightly higher than that for RT, 1.5 MPam1/2. For Gd123(Ag) the average fracture toughness at LNT, 1.8 MPam1/2 was higher than that at RT. The temperature dependence of fracture toughness for two kinds of bulks was consistent with that of the bending strength. Further, the order of the magnitude of the fracture toughness almost corresponded with that of bending strength for both bulks. 2005 Elsevier B.V. All rights reserved. PACS: 74.70; 62.20.Mk Keywords: YBa2Cu3Ox; GdBa2Cu3Ox; Bulk superconductors; Fracture toghness; Liquid nitrogen temperature
1. Introduction *
Corresponding author. Tel.: +81 19 621 6412; fax: +81 19 621 6412. E-mail address: [email protected] (K. Katagiri).
Recent advances in melt-processing technique have enabled the fabrication of large single-grain bulk superconductors, which can trap large
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.073
K. Katagiri et al. / Physica C 426–431 (2005) 709–713
magnetic field. It has been recognized that the evaluations of the mechanical properties of the bulk are very important to their practical applications, because they are sometimes fractured by thermal stress on cooling or electromagnetic force during magnetization process. Since bulk superconductors are brittle and micro-cracks are inevitably formed during fabrication process, evaluation of the fracture toughness, KIC, is very important. In order to evaluate the KIC of single-grain bulks at cryogenic temperatures, the Vickers indentation fracture method has been used [1]. This method is useful for the evaluation of the local value in bulks associated with the structure. However, bending tests for notched specimens, which are commonly used to evaluate the KIC of brittle materials, have not been extensively carried out for the bulks [2,3], and no results of the test at cryogenic temperatures have been reported. In this study, 3-point bending tests of V-notched specimens (SEVNB test) at liquid nitrogen temperature (LNT) as well as at room temperature (RT) were carried out. The temperature dependence of the KIC was discussed taking the result tested in other method into account. In addition, in order to investigate the crack propagation behavior, an optical microscope and a scanning electron microscope (SEM) observations on the specimen after the tests were carried out.
2. Experimental An Y123:YBa2Cu3Ox and a Gd123:GdBa2Cu3Ox single-grain bulks with 46 mm in diameter and 18 mm in thickness were fabricated from the precursors with 25.0 mol% (Y or Gd)211:(Y or Gd)2BaCuO5, 10 wt.% Ag2O (only for Gd123 bulk) and 0.5 wt.% Pt by using the modified quench and melt-growth (QMG) technique [4]. The KIC test specimens with the dimensions of 4 · 3 · 18 mm3 were cut from the bulks by using a slow-speed diamond wheel saw as shown in Fig. 1. At the center of the specimens, a V-notch in the c-axis was introduced, the depth and the radius of notch root being about 1.0–2.0 mm and about 20 lm, respectively. Details of the notch are described in Ref. [3].
Load
4
c-axis
710
3 16 18 Fig. 1. Dimensions of KIC test specimen and bending loading method.
The specimen together with the 3-point bending jig with 16 mm in fulcrum span was immersed in a liquid nitrogen bath by using a manual jack such that the cooling rate monitored by using a thermocouple placed near the specimen was less than 3 K/min. Bending load was applied at the actuator speed of 0.15 mm/min by using the 3 kN Shimadzu Servopulser testing machine after the holding for 20 min in a liquid nitrogen bath. The KIC was calculated by using the following equations. pffiffiffi 1.5PS K IC ¼ Y ðaÞ a ð1Þ BW 3=2 1.99 að1 aÞð2.15 3.93a þ 2.7a2 Þ Y ðaÞ ¼ ; ð1 þ 2aÞð1 aÞ3=2 a ð2Þ a¼ W where P is the maximum load, a is the V-notch depth, S is the fulcrum span, W and B are the height and the thickness of the specimens, respectively. Polished surfaces of the specimens tested were observed by using an optical and an SEM after etching using Br-ethanol solution.
3. Results and discussion 3.1. Fracture toughness The KIC of the Y123 and the Gd123(Ag) bulks at LNT and RT are shown in Fig. 2. The bending strength, rB, of the both bulks at LNT and RT evaluated by using the specimens with the
K. Katagiri et al. / Physica C 426–431 (2005) 709–713 150
2
100
1
0
50
LNT
RT Y123
RT LNT Gd123(Ag)
0
Fig. 2. KIC of Y123 and Gd123(Ag) bulks at liquid nitrogen and room temperatures. Bending strength of Y123 [5] and Gd123(Ag) [6] bulks are also shown for reference.
dimensions of 4 · 3 · 36 mm3 (Fulcrum span: 30 mm) without notch are also shown for reference [5,6]. The KIC scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT for the Y123 bulk and, for the Gd123(Ag) bulk, from 1.7 to 2.1 MPam1/2 at LNT and 1.3 to 2.1 MPam1/2 at RT. The average KIC of the Y123 and the Gd123(Ag) bulks at LNT, 1.6 and 1.8 MPam1/2, were higher than those at RT, 1.5 and 1.6 MPam1/2, respectively. The average rB of the Y123 and the Gd123(Ag) bulks at LNT, 90 and 96 MPa, were also higher than those at RT, 74 and 71 MPa, respectively. Thus in these bulks, the temperature dependence of the KIC coincides with that of the rB. Comparing two bulks, the average KIC of the Gd123(Ag) bulk was higher than that of the Y123 bulk both at LNT and RT. Although the average rB of the Gd123(Ag) bulk was higher than that Y123 at LNT, it was comparable at RT. The discrepancy appears to be ascribed to the smaller data points. The scattering of the KIC of the Y123 bulks at LNT seems to be larger than that at RT. This coincides with the temperature dependence of the scattering of the bending strength of the Y123 bulk at LNT; the Weibull coefficients were, 6.1 and 17.6, respectively [6]. Some of the present authors have reported that the KIC obtained by tensile tests coincides with those by SEVNB method [2]. The SEVNB KIC ob-
Fracture toughness, MPa x m1/2
2.5
Fracture toughness Bending strength
Bending strength, MPa
Fracture toughness, MPa x m1/2
3
711
LNT RT 2.0
1.5
1.0
0.5
0.0
Y123
Gd123(Ag) Sm17(Ag)[2] Sm29(Ag)[2]
Fig. 3. Comparison of KIC of this study with those in Sm17(Ag) and Sm29(Ag) [2].
tained in this study and the tensile KIC of Sm17(Ag) (Sm123 with 17 mol% of 211 and 10 wt.% of Ag2O) and Sm29(Ag) (Sm123 with 29 mol% of 211 and 10 wt.% of Ag2O) obtained previously [2] are shown in Fig. 3. From these results, it can be concluded that the KIC at LNT obtained by notched specimen is larger than, or at least comparable to, that at RT. However, it has been reported that the KIC obtained by the indentation method decreases with temperature decrease [1,7,8]. Since there is a difference in the scale of stress field between two method, further study is necessary to clarify the discrepancy. 3.2. Crack propagation behavior The KIC of ceramics is reported to be given by the product of the square root of process zone size, r0, and the local fracture stress, rc [9]. The r0 is a very small damaged region in front of the crack consisted of micro-cracks or nano cracks. The r0 is caluculated by the following formula [9,10]. 2 1 K IC ð3Þ r0 ¼ 2p rc The r0 for the bulks and typical structural ceramics obtained are shown in Table 1 together with the KIC and rB values used for the calculation. In stead of rc the bending strength (rB) is used in the calculation. It can be seen that the r0 of the bulks are significantly larger than those of the ceramics. Although, the KIC increase with
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K. Katagiri et al. / Physica C 426–431 (2005) 709–713
Table 1 Process zone size r0 of Y123, Gd123(Ag) and typical structural ceramics, and the KIC and rB values used for calculation Test temp. KIC (MPa Æ m1/2) rc (MPa) r0 (lm)
Y123
Gd123(Ag)
Al2O3[9]
TZP[10]
SiC[10]
Si3N4[10]
LNT
RT
LNT
RT
RT
RT
RT
RT
1.6 89.6 51.4
1.5 74.0 64.5
1.8 96.2 57.5
1.6 71.2 80.0
3.7 462 10.3
6.0 1200 4.0
4.2 490 11.7
6.0 706 11.5
increases of r0 [9], the KICs of bulks are very low. The very low KIC of the bulks are due to the low rB as compared with rc originated from defects such as cracks and void of the order of 200 lm in diameter to be referred in the following. The discrepancy between rc and rB in the ceramics is rather small, for example 565 MPa and 462 MPa for alumina [9]. Therefore, the r0 for the bulks in the Table 1 appears to be overestimated. In order to improve KIC in the bulks, rB should be increased by reducing the effect of defects. Fig. 4 shows the optical micrograph on the side surface of the Y123 specimen tested. The crack
propagated from the top to the bottom in the caxis direction in the figure. The crack appears to propagate toward the voids in the closest neighbor. It is considered that the voids relax the stress field in front of the crack tip when the crack reaches to them. Fig. 5 shows the SEM micrograph on the side surface of the Y123 specimen tested. It can be observed that the crack (in the circle) deflects along the pre-existing micro-cracks perpendicular to the c-axis. Present authors have reported that the KIC in the c-axis notch was slightly higher than that in the direction perpendicular to it [3]. Therefore it is considered that the pre-existing microcracks obstruct the crack propagation. It can be also observed that the crack (in the circle) deflects along the Y211 particle as shown in Fig. 6. This coincides with the report that the crack initiated from the indentation scar deflected along the Y211 particles [11].
Fig. 4. Optical micrograph on the side surface of the Y123 tested specimen. Crack propagated in the c-axis from the top to the bottom in the figure.
Fig. 5. Deflection of crack along pre-existing micro-cracks perpendicular to the c-axis (in circle) on side surface of tested specimen.
K. Katagiri et al. / Physica C 426–431 (2005) 709–713
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2) The crack in the c-axis direction appears to propagate toward the voids in the closest neighbor. The crack deflected along the pre-existing micro-cracks perpendicular to the c-axis and the secondary phase particles.
Acknowledgement We appreciate helpful assistance in the experiment by K. Sasaki of Iwate University. This work was partly supported by grant-in-aid for scientific research, Japan Society for Promotion of Science.
References Fig. 6. Deflection of crack along Y211 particles (in circle) on side surface of tested specimen.
4. Conclusions We have evaluated the KIC of an Y123 and a Gd123(Ag) single-grain bulks at liquid nitrogen and room temperatures by conducting bending tests of V-notched specimens cut from the bulks. 1) The KIC scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT for the Y123 bulk and, for the Gd123(Ag) bulk scattered from 1.7 to 2.1 MPam1/2 at LNT and 1.3–2.1 MPam1/2 at RT. The average KIC of the Y123 and the Gd123(Ag) bulks at LNT, 1.6 and 1.8 MPam1/2, were higher than those at RT, 1.5 and 1.6 MPam1/2, respectively. The KIC at LNT obtained by notched specimen is larger than, or at least comparable to that at RT, and this temperature dependence coincides with that of the bending strength.
[1] Y. Yoshino, A. Iwabuchi, K. Noto, N. Sakai, M. Murakami, Physica C 357–360 (2001) 796. [2] K. Katagiri, A. Murakami, T. Okudera, Y. Yoshino, A. Iwabuchi, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 14 (2004) 1046. [3] T. Okudera, A. Murakami, K. Katagiri, K. Kasaba, Y. Shoji, K. Noto, N. Sakai, M. Murakami, Physica C 392– 396 (2001) 628. [4] M. Morita, M. Sawamura, S. Takebayashi, K. Kimura, H. Teshima, M. Tanaka, K. Miyamoto, Physica C 235– 240 (1994) 209. [5] A. Murakami, K. Katagiri, K. Kasaba, K. Noto, H. Teshima, M. Sawamura, N. Sakai, M. Murakami, Physica C 412–414 (2004) 673. [6] K. Katagiri, A. Murakami, Y. Shoji, H. Teshima, M. Sawamura, A. Iwamoto, T. Mito, M. Murakami, Physica C 412–414 (2004) 633. [7] Y. Yoshino, A. Iwabuchi, K. Katagiri, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 12 (2002) 1755. [8] Y. Yoshino, A. Iwabuchi, N. Takahashi, K. Katagiri, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 14 (2004) 1118. [9] H. Awaji, S.M. Choi, C.H. Chen, N. Kishi, J. Soc. Mater. Sci. Jpn. 53 (2004) 1012, in Japanese. [10] Y. Kimura, S. Akiyama, Proc. 14th Int. Conf. Fatigue and Fatigue Threshold, Honolulu, Hawaii (1996) 769. [11] A. Goyal, W.C. Oliver, P.D. Funkenbusch, D.M. Kroeger, S.J. Burns, Physica C 183 (1991) 221.
Fracture toughness of REBaCuO bulk superconductor at liquid nitrogen temperature K. Katagiri
a,*
, T. Sato a, A. Murakami b, K. Kasaba a, Y. Shoji a, K. Noto c, H. Teshima d, M. Sawamura d
a Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Faculty of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki 036-8561, Japan c Iwate Industrial Promotion Center, 3-52-2 Iioka-shinden, Morioka 020-0852, Japan Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtu 293-8511, Japan b
d
Received 23 November 2004; accepted 17 February 2005 Available online 5 July 2005
Abstract The fracture toughness of Y123 and Gd123(Ag) single-grain bulk samples with 25 mol% RE 211 phase (and 10 wt.% Ag for the latter) prepared by the modified quench and melt-growth technique, was evaluated by the 3-point bending test at liquid nitrogen temperature (LNT) and room temperature (RT). The sharp V-notch was introduced into the specimens such that both of the loading and the crack extension directions are in the c-axis. The data for Y123 scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT. The average value at LNT, 1.6 MPam1/2, was slightly higher than that for RT, 1.5 MPam1/2. For Gd123(Ag) the average fracture toughness at LNT, 1.8 MPam1/2 was higher than that at RT. The temperature dependence of fracture toughness for two kinds of bulks was consistent with that of the bending strength. Further, the order of the magnitude of the fracture toughness almost corresponded with that of bending strength for both bulks. 2005 Elsevier B.V. All rights reserved. PACS: 74.70; 62.20.Mk Keywords: YBa2Cu3Ox; GdBa2Cu3Ox; Bulk superconductors; Fracture toghness; Liquid nitrogen temperature
1. Introduction *
Corresponding author. Tel.: +81 19 621 6412; fax: +81 19 621 6412. E-mail address: [email protected] (K. Katagiri).
Recent advances in melt-processing technique have enabled the fabrication of large single-grain bulk superconductors, which can trap large
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.02.073
K. Katagiri et al. / Physica C 426–431 (2005) 709–713
magnetic field. It has been recognized that the evaluations of the mechanical properties of the bulk are very important to their practical applications, because they are sometimes fractured by thermal stress on cooling or electromagnetic force during magnetization process. Since bulk superconductors are brittle and micro-cracks are inevitably formed during fabrication process, evaluation of the fracture toughness, KIC, is very important. In order to evaluate the KIC of single-grain bulks at cryogenic temperatures, the Vickers indentation fracture method has been used [1]. This method is useful for the evaluation of the local value in bulks associated with the structure. However, bending tests for notched specimens, which are commonly used to evaluate the KIC of brittle materials, have not been extensively carried out for the bulks [2,3], and no results of the test at cryogenic temperatures have been reported. In this study, 3-point bending tests of V-notched specimens (SEVNB test) at liquid nitrogen temperature (LNT) as well as at room temperature (RT) were carried out. The temperature dependence of the KIC was discussed taking the result tested in other method into account. In addition, in order to investigate the crack propagation behavior, an optical microscope and a scanning electron microscope (SEM) observations on the specimen after the tests were carried out.
2. Experimental An Y123:YBa2Cu3Ox and a Gd123:GdBa2Cu3Ox single-grain bulks with 46 mm in diameter and 18 mm in thickness were fabricated from the precursors with 25.0 mol% (Y or Gd)211:(Y or Gd)2BaCuO5, 10 wt.% Ag2O (only for Gd123 bulk) and 0.5 wt.% Pt by using the modified quench and melt-growth (QMG) technique [4]. The KIC test specimens with the dimensions of 4 · 3 · 18 mm3 were cut from the bulks by using a slow-speed diamond wheel saw as shown in Fig. 1. At the center of the specimens, a V-notch in the c-axis was introduced, the depth and the radius of notch root being about 1.0–2.0 mm and about 20 lm, respectively. Details of the notch are described in Ref. [3].
Load
4
c-axis
710
3 16 18 Fig. 1. Dimensions of KIC test specimen and bending loading method.
The specimen together with the 3-point bending jig with 16 mm in fulcrum span was immersed in a liquid nitrogen bath by using a manual jack such that the cooling rate monitored by using a thermocouple placed near the specimen was less than 3 K/min. Bending load was applied at the actuator speed of 0.15 mm/min by using the 3 kN Shimadzu Servopulser testing machine after the holding for 20 min in a liquid nitrogen bath. The KIC was calculated by using the following equations. pffiffiffi 1.5PS K IC ¼ Y ðaÞ a ð1Þ BW 3=2 1.99 að1 aÞð2.15 3.93a þ 2.7a2 Þ Y ðaÞ ¼ ; ð1 þ 2aÞð1 aÞ3=2 a ð2Þ a¼ W where P is the maximum load, a is the V-notch depth, S is the fulcrum span, W and B are the height and the thickness of the specimens, respectively. Polished surfaces of the specimens tested were observed by using an optical and an SEM after etching using Br-ethanol solution.
3. Results and discussion 3.1. Fracture toughness The KIC of the Y123 and the Gd123(Ag) bulks at LNT and RT are shown in Fig. 2. The bending strength, rB, of the both bulks at LNT and RT evaluated by using the specimens with the
K. Katagiri et al. / Physica C 426–431 (2005) 709–713 150
2
100
1
0
50
LNT
RT Y123
RT LNT Gd123(Ag)
0
Fig. 2. KIC of Y123 and Gd123(Ag) bulks at liquid nitrogen and room temperatures. Bending strength of Y123 [5] and Gd123(Ag) [6] bulks are also shown for reference.
dimensions of 4 · 3 · 36 mm3 (Fulcrum span: 30 mm) without notch are also shown for reference [5,6]. The KIC scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT for the Y123 bulk and, for the Gd123(Ag) bulk, from 1.7 to 2.1 MPam1/2 at LNT and 1.3 to 2.1 MPam1/2 at RT. The average KIC of the Y123 and the Gd123(Ag) bulks at LNT, 1.6 and 1.8 MPam1/2, were higher than those at RT, 1.5 and 1.6 MPam1/2, respectively. The average rB of the Y123 and the Gd123(Ag) bulks at LNT, 90 and 96 MPa, were also higher than those at RT, 74 and 71 MPa, respectively. Thus in these bulks, the temperature dependence of the KIC coincides with that of the rB. Comparing two bulks, the average KIC of the Gd123(Ag) bulk was higher than that of the Y123 bulk both at LNT and RT. Although the average rB of the Gd123(Ag) bulk was higher than that Y123 at LNT, it was comparable at RT. The discrepancy appears to be ascribed to the smaller data points. The scattering of the KIC of the Y123 bulks at LNT seems to be larger than that at RT. This coincides with the temperature dependence of the scattering of the bending strength of the Y123 bulk at LNT; the Weibull coefficients were, 6.1 and 17.6, respectively [6]. Some of the present authors have reported that the KIC obtained by tensile tests coincides with those by SEVNB method [2]. The SEVNB KIC ob-
Fracture toughness, MPa x m1/2
2.5
Fracture toughness Bending strength
Bending strength, MPa
Fracture toughness, MPa x m1/2
3
711
LNT RT 2.0
1.5
1.0
0.5
0.0
Y123
Gd123(Ag) Sm17(Ag)[2] Sm29(Ag)[2]
Fig. 3. Comparison of KIC of this study with those in Sm17(Ag) and Sm29(Ag) [2].
tained in this study and the tensile KIC of Sm17(Ag) (Sm123 with 17 mol% of 211 and 10 wt.% of Ag2O) and Sm29(Ag) (Sm123 with 29 mol% of 211 and 10 wt.% of Ag2O) obtained previously [2] are shown in Fig. 3. From these results, it can be concluded that the KIC at LNT obtained by notched specimen is larger than, or at least comparable to, that at RT. However, it has been reported that the KIC obtained by the indentation method decreases with temperature decrease [1,7,8]. Since there is a difference in the scale of stress field between two method, further study is necessary to clarify the discrepancy. 3.2. Crack propagation behavior The KIC of ceramics is reported to be given by the product of the square root of process zone size, r0, and the local fracture stress, rc [9]. The r0 is a very small damaged region in front of the crack consisted of micro-cracks or nano cracks. The r0 is caluculated by the following formula [9,10]. 2 1 K IC ð3Þ r0 ¼ 2p rc The r0 for the bulks and typical structural ceramics obtained are shown in Table 1 together with the KIC and rB values used for the calculation. In stead of rc the bending strength (rB) is used in the calculation. It can be seen that the r0 of the bulks are significantly larger than those of the ceramics. Although, the KIC increase with
712
K. Katagiri et al. / Physica C 426–431 (2005) 709–713
Table 1 Process zone size r0 of Y123, Gd123(Ag) and typical structural ceramics, and the KIC and rB values used for calculation Test temp. KIC (MPa Æ m1/2) rc (MPa) r0 (lm)
Y123
Gd123(Ag)
Al2O3[9]
TZP[10]
SiC[10]
Si3N4[10]
LNT
RT
LNT
RT
RT
RT
RT
RT
1.6 89.6 51.4
1.5 74.0 64.5
1.8 96.2 57.5
1.6 71.2 80.0
3.7 462 10.3
6.0 1200 4.0
4.2 490 11.7
6.0 706 11.5
increases of r0 [9], the KICs of bulks are very low. The very low KIC of the bulks are due to the low rB as compared with rc originated from defects such as cracks and void of the order of 200 lm in diameter to be referred in the following. The discrepancy between rc and rB in the ceramics is rather small, for example 565 MPa and 462 MPa for alumina [9]. Therefore, the r0 for the bulks in the Table 1 appears to be overestimated. In order to improve KIC in the bulks, rB should be increased by reducing the effect of defects. Fig. 4 shows the optical micrograph on the side surface of the Y123 specimen tested. The crack
propagated from the top to the bottom in the caxis direction in the figure. The crack appears to propagate toward the voids in the closest neighbor. It is considered that the voids relax the stress field in front of the crack tip when the crack reaches to them. Fig. 5 shows the SEM micrograph on the side surface of the Y123 specimen tested. It can be observed that the crack (in the circle) deflects along the pre-existing micro-cracks perpendicular to the c-axis. Present authors have reported that the KIC in the c-axis notch was slightly higher than that in the direction perpendicular to it [3]. Therefore it is considered that the pre-existing microcracks obstruct the crack propagation. It can be also observed that the crack (in the circle) deflects along the Y211 particle as shown in Fig. 6. This coincides with the report that the crack initiated from the indentation scar deflected along the Y211 particles [11].
Fig. 4. Optical micrograph on the side surface of the Y123 tested specimen. Crack propagated in the c-axis from the top to the bottom in the figure.
Fig. 5. Deflection of crack along pre-existing micro-cracks perpendicular to the c-axis (in circle) on side surface of tested specimen.
K. Katagiri et al. / Physica C 426–431 (2005) 709–713
713
2) The crack in the c-axis direction appears to propagate toward the voids in the closest neighbor. The crack deflected along the pre-existing micro-cracks perpendicular to the c-axis and the secondary phase particles.
Acknowledgement We appreciate helpful assistance in the experiment by K. Sasaki of Iwate University. This work was partly supported by grant-in-aid for scientific research, Japan Society for Promotion of Science.
References Fig. 6. Deflection of crack along Y211 particles (in circle) on side surface of tested specimen.
4. Conclusions We have evaluated the KIC of an Y123 and a Gd123(Ag) single-grain bulks at liquid nitrogen and room temperatures by conducting bending tests of V-notched specimens cut from the bulks. 1) The KIC scattered from 1.2 to 2.0 MPam1/2 at LNT and 1.4 to 1.7 MPam1/2 at RT for the Y123 bulk and, for the Gd123(Ag) bulk scattered from 1.7 to 2.1 MPam1/2 at LNT and 1.3–2.1 MPam1/2 at RT. The average KIC of the Y123 and the Gd123(Ag) bulks at LNT, 1.6 and 1.8 MPam1/2, were higher than those at RT, 1.5 and 1.6 MPam1/2, respectively. The KIC at LNT obtained by notched specimen is larger than, or at least comparable to that at RT, and this temperature dependence coincides with that of the bending strength.
[1] Y. Yoshino, A. Iwabuchi, K. Noto, N. Sakai, M. Murakami, Physica C 357–360 (2001) 796. [2] K. Katagiri, A. Murakami, T. Okudera, Y. Yoshino, A. Iwabuchi, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 14 (2004) 1046. [3] T. Okudera, A. Murakami, K. Katagiri, K. Kasaba, Y. Shoji, K. Noto, N. Sakai, M. Murakami, Physica C 392– 396 (2001) 628. [4] M. Morita, M. Sawamura, S. Takebayashi, K. Kimura, H. Teshima, M. Tanaka, K. Miyamoto, Physica C 235– 240 (1994) 209. [5] A. Murakami, K. Katagiri, K. Kasaba, K. Noto, H. Teshima, M. Sawamura, N. Sakai, M. Murakami, Physica C 412–414 (2004) 673. [6] K. Katagiri, A. Murakami, Y. Shoji, H. Teshima, M. Sawamura, A. Iwamoto, T. Mito, M. Murakami, Physica C 412–414 (2004) 633. [7] Y. Yoshino, A. Iwabuchi, K. Katagiri, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 12 (2002) 1755. [8] Y. Yoshino, A. Iwabuchi, N. Takahashi, K. Katagiri, K. Noto, N. Sakai, M. Murakami, IEEE Trans. Appl. Supercond. 14 (2004) 1118. [9] H. Awaji, S.M. Choi, C.H. Chen, N. Kishi, J. Soc. Mater. Sci. Jpn. 53 (2004) 1012, in Japanese. [10] Y. Kimura, S. Akiyama, Proc. 14th Int. Conf. Fatigue and Fatigue Threshold, Honolulu, Hawaii (1996) 769. [11] A. Goyal, W.C. Oliver, P.D. Funkenbusch, D.M. Kroeger, S.J. Burns, Physica C 183 (1991) 221.