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Effect of Ti–Zr addition on the microstructure and the arc chopping current of melt-spun CuCr ribbon
Vacuum 83 (2009) 980–983
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of Ti–Zr addition on the microstructure and the arc chopping current of melt-spun CuCr ribbon Minyxiang Yu, Youhong Wang*, Yangli Wang Institute of Materials Science and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Received in revised form 23 November 2008 Accepted 24 November 2008
The microstructure and the arc chopping current of melt-spun Cu71Cr29 ribbon added by Ti–Zr are studied in the article. The results reveal that the melt spinning could partly restrain the liquid phase separation of CuCr alloys because it has a high cooling rate (about 106 K/s), the size of the Cr rich phase from liquid phase separation in the Cu71Cr29 microstructure can be decreased from the micron-scale to about 250 nm by using melt spinning. On the melt-spun base, alloying by Ti–Zr could further decrease the size of the Cr rich phase from 250 nm to about 100 nm. For nano-grained CuCr alloys, its lower arc chopping current is advantageous to the use of contact and the circuitry protect, its long arc trace route and high velocity of spot direction motion could mitigate the partial ablate of cathode surface and the lifetime of contact could be prolonged. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Microstructure Arc chopping current CuCr alloys Contact Melt spinning
1. Introduction The CuCr contact material containing 20–50% Cr has been widely investigated because it is a dominating contact material used in medium-voltage vacuum interrupters. To improve its electric properties, especially to decrease its arc chopping current, refining the Cr rich phase in its microstructure is an important subject [1–4]. Melt spinning is a common method of rapid solidification nowadays. It is capable of refining the microstructure, extending the solid solubility limits, forming the metastable phase, etc. A number of studies [5–12] has been undertaken to investigate the microstructures of rapidly solidified metals. However, it has not been well used in the research of refining the microstructure of CuCr alloys containing 20–50% Cr. In these alloys the liquid phase separation of supercooled CuCr melts will occur in rapid solidification process [13]. In this paper, the following convention will be adopted: the abbreviation CuaCrbTicZrd will stand for the composition of an alloy containing a, b, c, and d at percentage of Cu, Cr, Ti, and Zr, respectively. Although the liquid phase separation is disadvantageous to refine the Cr rich phase, the size of the Cr rich phase can be still decreased to about 250 nm in the microstructure of Cu71Cr29 alloys by melt spinning with about 106 K/s cooling rate [13–15]. Besides rapid solidification, alloying is another general
* Corresponding author. Tel.: þ86 0351 699 9329. E-mail address: [email protected] (Y. Wang). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.11.009
way to refine the microstructure of alloys, so a little work has been done on the influence of Ti–Zr addition to the microstructure of melt-spun Cu71Cr29 alloys. 2. Experimental Pure (>99.95%) Cu, Cr, Ti, and Zr were used to prepare the Cu–Cr–Ti–Zr alloys by arc-melting technique. Subsequently, the material with a mass of 10 g was inserted into a quartz tube. When the material was heated by high frequency induction to the required temperature, the ribbon was prepared by liquid quenching on a single roller melt spinning under a pressure of 0.5 atm Ar gas. The velocity of the cooling roller was 33 m/s, the calculated cooling rate of ribbon was about 106 K/s [14]. The dimensions of prepared ribbons were about 3 mm width and 25 w 40 mm in thickness. Under this condition, the maximal supercooling of ribbon is about 400 w 450 K [7,11]. Some melt-spun ribbons were annealed in a vacuum furnace at 600 C for 3 h. The microstructures of samples were analyzed by a Hitachi H-800 transmission electron microscope (TEM). The foil specimen for TEM was prepared by a twin-jet thinning device. The experiment of the arc chopping current of ribbon was carried in a high vacuum chamber, which can reach 1.0 106 Pa using a sputter ion pump. A 20-KV DC voltage was applied to the vacuum gap between the CuCr ribbon cathode and a pure tungsten tip anode. Fig. 1 is the circuit diagram of electric performance test, where, C ¼ 12 mF. A peak current of 10 A was used by changing the value of R4 in the test. The current waveform of discharge was
M. Yu et al. / Vacuum 83 (2009) 980–983
R4
R1
anode R2
20KV DC
981
alloying, see in Table 1. This means that the amounts of solutes in Cu and Cr solid phases are increased simultaneously due to the alloying.
vacuum
C
3.3. The microstructures of melt-spun Cu68.4Cr27.8Ti2.5Zr1.3 ribbons
V
R3
cathode R5
Fig. 1. The circuit for arc experiment. R1 ¼ 100 kU, R2 ¼ 38 MU, R3 ¼ 2 MU, R4 ¼ 40–800 U, and R5 ¼ 1–10 U. The vacuum degree is about 1.5 106 Pa. The CuCr ribbon is used as a cathode and a pure tungsten tip is used as an anode.
recorded by a TDS-2024 of Tektronics oscilloscope, the arc chopping current and the arcing time can be directly measured on the current waveform of discharge. The arc chopping current is an average of 60 times. 3. Results 3.1. The microstructures of melt-spun Cu71Cr29 ribbons The microstructure of melt-spun Cu71Cr29 ribbon is shown in Fig. 2a. The spherical particle marked by an arrow is the Cr rich phase from liquid phase separation [13]. The diameters of the Cr rich phase in Fig. 2a are about 250 nm. By increasing the cooling rate, the size of the Cr rich phase from liquid phase separation in the melt-spun Cu71Cr29 microstructure can be refined from the micron-scale to nano-scale [13]. After annealing at 600 C for 3 h, the Cr rich phase from liquid phase separation did not obviously grow up; the diameters of the Cr rich phase in Fig. 2b are still about 250 nm. The anneal result indicates that the Cr rich phase in the melt-spun Cu71Cr29 microstructure have a good invariance. Even if the material is repeatedly heated by arc in the work process of contact, the melt-spun Cu71Cr29 microstructure would not transform obviously which is advantageous to keep the electric properties of contact.
The microstructure of As-quenched Cu68.4Cr27.8Ti2.5Zr1.3 ribbon is shown in Fig. 4. The most of Cr rich phases in the microstructure in Fig. 4a are refined to about 100 nm by Ti2.5Zr1.3 addition. When the ribbons were annealed at 600 C for 3 h, the Cr rich phase in Fig. 4b did not grow up largely. The X-ray diffraction reveals that the lattice parameters of As-quenched and annealed Cu matrix and Cr phase in Cu68.4Cr27.8Ti2.5Zr1.3 ribbons are continuously increased by the increase of Ti–Zr content, see in Table 1. 3.4. The arc chopping current The current in the circuit is called as the arc chopping current when the arc crushes out completely, which can be read directly on the current waveform of discharge. The average of the arc chopping current for coarse-crystalline Cu71Cr29 alloys made by vacuum induction melting method is 3.2 A, as shown in Fig. 5. For nanocrystalline Cu68.4Cr27.8Ti2.5Zr1.3 ribbon, the average of its arc chopping current is only 1.2 A, see in Fig. 6. 4. Discussion The large supercooling of melt spinning and alloying can make that the size of the Cr rich phase in the microstructure of CuCr alloys was refined from the micron-scale to nano-scale. According to the solidification theory, the critical radius of nucleation (r*) is
2sTm Vs r* ¼ DH DT
Where s is the surface energy; Tm is the liquidus; VS is the mol volume; DH is the difference of enthalpy between liquid and solid; DT is the critical supercooling. We know that r* will be decreased as DT increases by the formula. On the other hand, the rate of nucleation (I) is
3.2. The microstructures of melt-spun Cu69.8Cr28.3Ti1.3Zr0.6 ribbons Fig. 3a shows the microstructure of As-quenched Cu69.8Cr28.3Ti1.3Zr0.6 ribbon. The measured results indicate that the size of Cr rich phase is refined from about 250 nm to about 150 nm by Ti1.3Zr0.6 addition. When the ribbons were annealed at 600 C for 3 h, the size of Cr rich phase in Fig. 3b is almost same with that in Fig. 3a. The X-ray diffraction reveals that the lattice parameters of As-quenched and annealed Cu matrix and Cr phase are increased by 1.3% Ti–0.6% Zr addition comparing with binary Cu71Cr29 without
(1)
I ¼ B1
# " 2 V2 16ps3 Tm DL S exp DLM 3DH2 DT 2 kT
(2)
where B1 is a coefficient that is decided by the critical radius of nucleation and the surface energy; DL is the diffusivity in liquid; DLM is the diffusivity in liquid at melting point; k is Boltzmann constant; T is the temperature of supercooling liquid. The relation between I and DT assumes the Gaussian distribution. For melt spinning, as DT increases, I will be increased. The increase of r*and I
Fig. 2. The microstructure of melt-spun Cu71Cr29 ribbons (a. As-quenched, b. annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
982
M. Yu et al. / Vacuum 83 (2009) 980–983
Fig. 3. The microstructure of melt-spun Cu69.8Cr28.3Ti1.3Zr0.6 ribbons (a-As-quenched, b-annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
Table 1 The lattice parameters of melt-spun Cu–Cr–Ti–Zr ribbons. Composition, at % Cu
71.0 69.8 68.4 a
Lattice parameter, Å Cr
Ti
29 28.3 27.8
Zr
– 1.3 2.5
Fcc Cu phase
– 0.6 1.3
Bcc Cr phase
As-quenched
Annealeda
As-quenched
Annealed
3.6177 3.6197 3.6338
3.6116 3.6193 3.6226
2.8817 2.8842 2.8967
2.8793 2.8830 2.8892
Annealed at 600 C for 3 h.
Fig. 4. The microstructure of melt-spun Cu68.4Cr27.8Ti2.5Zr1.3 ribbons (a-As-quench, b-annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
10
8
I, A
6
4
2
0 0
50
100
150
200
250
Time, µs Fig. 5. The current waveform of discharge for coarse-crystalline Cu71Cr29 alloys.
will result that the number of nucleus will be increased, so the microstructure is refined. The arc chopping current of CuCr contact is an important behavior in service. The arc between electrodes will gradually crushes out and the current in a circuit will become smaller and smaller when a breaker is turned off, but the current will not descend to zero. The arc chopping current will arise an overvoltage by the inductance in a circuit. The overvoltage is very disadvantageous to the electric system. In generally, the higher the vapor tension of contact materials, the lower the arc chopping current. The interfacial energy in nano-grained CuCr alloys is higher than that in coarse-grained CuCr alloys, so the vapor tension of nanograined CuCr alloys is also higher than that of coarse-grained CuCr alloys, which results that the nano-grained CuCr alloys has a lower arc chopping current. The arc spot stays in a small area with a diameter about 1 mm on the surface of coarse-grained Cu71Cr29 cathode. For nano-grained CuCr alloys, the moving way of arc spot is in sub-direction which is different with the random walk pattern of coarse-grained Cu71Cr29 cathode, the arc trace route is about 3 mm length and the velocity of spot direction motion is about 60 m/s [2], which could mitigate the partial ablate of cathode surface and the lifetime of contact
M. Yu et al. / Vacuum 83 (2009) 980–983
10
For nano-grained CuCr alloys, its lower arc chopping current is advantageous to the use of contact and the circuitry protect, its long arc trace route and high velocity of spot direction motion could mitigate the partial ablate of cathode surface and the lifetime of contact could be prolonged.
8 6
I, A
983
4
Acknowledgement
2
Supported by the National Science Foundation of China (50371066).
0 0
50
100
150
200
250
Time, µs Fig. 6. The current waveform of discharge for nanocrystalline Cu68.4Cr27.8Ti2.5Zr1.3 ribbon.
could be prolonged. About the moving way of arc spot and its effects on the contact characteristics had been studied in detail by Mr. Zhimao Yang et al. [2]. In a word, the size decrease of the Cr rich phase in the microstructure of Cu71Cr29 alloys could improve the electric properties of its contact. 5. Conclusions By using melt spinning, the size of the Cr rich phase from liquid phase separation in the Cu71Cr29 microstructure can be decreased from the micron-scale to about 250 nm, which reveals that increasing the cooling rate of solidification process, the microstructure of Cu71Cr29 alloys can be markedly refined. On the meltspun base, alloying by Ti–Zr could further decrease the size of the Cr rich phase from about 250 nm to about 100 nm.
References [1] Yang Zhimao, Zhang Qiuli, Wang Qingfeng, Zhang Chengyu, Ding Bingjun. Vacuum 2006;81:545–9. [2] Yang Zhimao, Zhang Qiuli, Zhang Chengyu, Sun Yue, Ding Bingjun. Physics Letters A 2006;353:98–100. [3] Rieder WF, Schussek M, Glatzle W, Kny E. IEEE Transactions on Components Hybrids and Manufacturing Technology 1989;12:273–8. [4] Wang Y, Ding B. IEEE Transactions on Component Packaging and Manufacturing Technology 1999;22:467–72. [5] Tenwick MJ, Davies HA. Materials Science and Technology 1988;98:543–6. [6] Zhang Zhonghua, Bian Xiufang, Wang Yan, Liu Xiangfa. Journal of Materials Science 2002;37:4473–80. [7] Maslov VV, Nosenko VK. Journal of Materials Science 2002;37:4663–8. [8] Cheng Tianyi, Lo¨ser W, Leonhardt M. Journal of Materials Science 1998; 33:4365–74. [9] Lin Zhang, Youshi Wu, Xiufang Bian, Zhina Xing. Journal of Materials Science Letters 1999;18:1969–72. [10] Nagarajan R, Manjini S, Chattopadhyay K, Aoki K. Journal of Materials Science 1997;32:6021–7. [11] Liu Feng, Yang Gencang, Guo Xuefeng. Journal of Materials Science 2001; 36:3607–15. [12] Cooper KP, Jones HN. Journal of Materials Science 2001;36:5315–23. [13] Wang Youhong, Song Xiaoping, Sun Zhanbo, Zhou Xuan, Guo Juan. Materials Science Poland 2007;25:199–207. [14] Wang Youhong, Sun Zhanbo, Zhou Xuan, Xiaoping Song. Rare Metal Materials and Engineering 2006;35:1289–93. [15] Sun Zhanbo, Wang Youhong, Guo Juan. Material Science Engineering A 2007;452–453:411–6.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of Ti–Zr addition on the microstructure and the arc chopping current of melt-spun CuCr ribbon Minyxiang Yu, Youhong Wang*, Yangli Wang Institute of Materials Science and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Received in revised form 23 November 2008 Accepted 24 November 2008
The microstructure and the arc chopping current of melt-spun Cu71Cr29 ribbon added by Ti–Zr are studied in the article. The results reveal that the melt spinning could partly restrain the liquid phase separation of CuCr alloys because it has a high cooling rate (about 106 K/s), the size of the Cr rich phase from liquid phase separation in the Cu71Cr29 microstructure can be decreased from the micron-scale to about 250 nm by using melt spinning. On the melt-spun base, alloying by Ti–Zr could further decrease the size of the Cr rich phase from 250 nm to about 100 nm. For nano-grained CuCr alloys, its lower arc chopping current is advantageous to the use of contact and the circuitry protect, its long arc trace route and high velocity of spot direction motion could mitigate the partial ablate of cathode surface and the lifetime of contact could be prolonged. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Microstructure Arc chopping current CuCr alloys Contact Melt spinning
1. Introduction The CuCr contact material containing 20–50% Cr has been widely investigated because it is a dominating contact material used in medium-voltage vacuum interrupters. To improve its electric properties, especially to decrease its arc chopping current, refining the Cr rich phase in its microstructure is an important subject [1–4]. Melt spinning is a common method of rapid solidification nowadays. It is capable of refining the microstructure, extending the solid solubility limits, forming the metastable phase, etc. A number of studies [5–12] has been undertaken to investigate the microstructures of rapidly solidified metals. However, it has not been well used in the research of refining the microstructure of CuCr alloys containing 20–50% Cr. In these alloys the liquid phase separation of supercooled CuCr melts will occur in rapid solidification process [13]. In this paper, the following convention will be adopted: the abbreviation CuaCrbTicZrd will stand for the composition of an alloy containing a, b, c, and d at percentage of Cu, Cr, Ti, and Zr, respectively. Although the liquid phase separation is disadvantageous to refine the Cr rich phase, the size of the Cr rich phase can be still decreased to about 250 nm in the microstructure of Cu71Cr29 alloys by melt spinning with about 106 K/s cooling rate [13–15]. Besides rapid solidification, alloying is another general
* Corresponding author. Tel.: þ86 0351 699 9329. E-mail address: [email protected] (Y. Wang). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.11.009
way to refine the microstructure of alloys, so a little work has been done on the influence of Ti–Zr addition to the microstructure of melt-spun Cu71Cr29 alloys. 2. Experimental Pure (>99.95%) Cu, Cr, Ti, and Zr were used to prepare the Cu–Cr–Ti–Zr alloys by arc-melting technique. Subsequently, the material with a mass of 10 g was inserted into a quartz tube. When the material was heated by high frequency induction to the required temperature, the ribbon was prepared by liquid quenching on a single roller melt spinning under a pressure of 0.5 atm Ar gas. The velocity of the cooling roller was 33 m/s, the calculated cooling rate of ribbon was about 106 K/s [14]. The dimensions of prepared ribbons were about 3 mm width and 25 w 40 mm in thickness. Under this condition, the maximal supercooling of ribbon is about 400 w 450 K [7,11]. Some melt-spun ribbons were annealed in a vacuum furnace at 600 C for 3 h. The microstructures of samples were analyzed by a Hitachi H-800 transmission electron microscope (TEM). The foil specimen for TEM was prepared by a twin-jet thinning device. The experiment of the arc chopping current of ribbon was carried in a high vacuum chamber, which can reach 1.0 106 Pa using a sputter ion pump. A 20-KV DC voltage was applied to the vacuum gap between the CuCr ribbon cathode and a pure tungsten tip anode. Fig. 1 is the circuit diagram of electric performance test, where, C ¼ 12 mF. A peak current of 10 A was used by changing the value of R4 in the test. The current waveform of discharge was
M. Yu et al. / Vacuum 83 (2009) 980–983
R4
R1
anode R2
20KV DC
981
alloying, see in Table 1. This means that the amounts of solutes in Cu and Cr solid phases are increased simultaneously due to the alloying.
vacuum
C
3.3. The microstructures of melt-spun Cu68.4Cr27.8Ti2.5Zr1.3 ribbons
V
R3
cathode R5
Fig. 1. The circuit for arc experiment. R1 ¼ 100 kU, R2 ¼ 38 MU, R3 ¼ 2 MU, R4 ¼ 40–800 U, and R5 ¼ 1–10 U. The vacuum degree is about 1.5 106 Pa. The CuCr ribbon is used as a cathode and a pure tungsten tip is used as an anode.
recorded by a TDS-2024 of Tektronics oscilloscope, the arc chopping current and the arcing time can be directly measured on the current waveform of discharge. The arc chopping current is an average of 60 times. 3. Results 3.1. The microstructures of melt-spun Cu71Cr29 ribbons The microstructure of melt-spun Cu71Cr29 ribbon is shown in Fig. 2a. The spherical particle marked by an arrow is the Cr rich phase from liquid phase separation [13]. The diameters of the Cr rich phase in Fig. 2a are about 250 nm. By increasing the cooling rate, the size of the Cr rich phase from liquid phase separation in the melt-spun Cu71Cr29 microstructure can be refined from the micron-scale to nano-scale [13]. After annealing at 600 C for 3 h, the Cr rich phase from liquid phase separation did not obviously grow up; the diameters of the Cr rich phase in Fig. 2b are still about 250 nm. The anneal result indicates that the Cr rich phase in the melt-spun Cu71Cr29 microstructure have a good invariance. Even if the material is repeatedly heated by arc in the work process of contact, the melt-spun Cu71Cr29 microstructure would not transform obviously which is advantageous to keep the electric properties of contact.
The microstructure of As-quenched Cu68.4Cr27.8Ti2.5Zr1.3 ribbon is shown in Fig. 4. The most of Cr rich phases in the microstructure in Fig. 4a are refined to about 100 nm by Ti2.5Zr1.3 addition. When the ribbons were annealed at 600 C for 3 h, the Cr rich phase in Fig. 4b did not grow up largely. The X-ray diffraction reveals that the lattice parameters of As-quenched and annealed Cu matrix and Cr phase in Cu68.4Cr27.8Ti2.5Zr1.3 ribbons are continuously increased by the increase of Ti–Zr content, see in Table 1. 3.4. The arc chopping current The current in the circuit is called as the arc chopping current when the arc crushes out completely, which can be read directly on the current waveform of discharge. The average of the arc chopping current for coarse-crystalline Cu71Cr29 alloys made by vacuum induction melting method is 3.2 A, as shown in Fig. 5. For nanocrystalline Cu68.4Cr27.8Ti2.5Zr1.3 ribbon, the average of its arc chopping current is only 1.2 A, see in Fig. 6. 4. Discussion The large supercooling of melt spinning and alloying can make that the size of the Cr rich phase in the microstructure of CuCr alloys was refined from the micron-scale to nano-scale. According to the solidification theory, the critical radius of nucleation (r*) is
2sTm Vs r* ¼ DH DT
Where s is the surface energy; Tm is the liquidus; VS is the mol volume; DH is the difference of enthalpy between liquid and solid; DT is the critical supercooling. We know that r* will be decreased as DT increases by the formula. On the other hand, the rate of nucleation (I) is
3.2. The microstructures of melt-spun Cu69.8Cr28.3Ti1.3Zr0.6 ribbons Fig. 3a shows the microstructure of As-quenched Cu69.8Cr28.3Ti1.3Zr0.6 ribbon. The measured results indicate that the size of Cr rich phase is refined from about 250 nm to about 150 nm by Ti1.3Zr0.6 addition. When the ribbons were annealed at 600 C for 3 h, the size of Cr rich phase in Fig. 3b is almost same with that in Fig. 3a. The X-ray diffraction reveals that the lattice parameters of As-quenched and annealed Cu matrix and Cr phase are increased by 1.3% Ti–0.6% Zr addition comparing with binary Cu71Cr29 without
(1)
I ¼ B1
# " 2 V2 16ps3 Tm DL S exp DLM 3DH2 DT 2 kT
(2)
where B1 is a coefficient that is decided by the critical radius of nucleation and the surface energy; DL is the diffusivity in liquid; DLM is the diffusivity in liquid at melting point; k is Boltzmann constant; T is the temperature of supercooling liquid. The relation between I and DT assumes the Gaussian distribution. For melt spinning, as DT increases, I will be increased. The increase of r*and I
Fig. 2. The microstructure of melt-spun Cu71Cr29 ribbons (a. As-quenched, b. annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
982
M. Yu et al. / Vacuum 83 (2009) 980–983
Fig. 3. The microstructure of melt-spun Cu69.8Cr28.3Ti1.3Zr0.6 ribbons (a-As-quenched, b-annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
Table 1 The lattice parameters of melt-spun Cu–Cr–Ti–Zr ribbons. Composition, at % Cu
71.0 69.8 68.4 a
Lattice parameter, Å Cr
Ti
29 28.3 27.8
Zr
– 1.3 2.5
Fcc Cu phase
– 0.6 1.3
Bcc Cr phase
As-quenched
Annealeda
As-quenched
Annealed
3.6177 3.6197 3.6338
3.6116 3.6193 3.6226
2.8817 2.8842 2.8967
2.8793 2.8830 2.8892
Annealed at 600 C for 3 h.
Fig. 4. The microstructure of melt-spun Cu68.4Cr27.8Ti2.5Zr1.3 ribbons (a-As-quench, b-annealed at 600 C for 3 h. The arrows note the Cr rich phase from liquid phase separation).
10
8
I, A
6
4
2
0 0
50
100
150
200
250
Time, µs Fig. 5. The current waveform of discharge for coarse-crystalline Cu71Cr29 alloys.
will result that the number of nucleus will be increased, so the microstructure is refined. The arc chopping current of CuCr contact is an important behavior in service. The arc between electrodes will gradually crushes out and the current in a circuit will become smaller and smaller when a breaker is turned off, but the current will not descend to zero. The arc chopping current will arise an overvoltage by the inductance in a circuit. The overvoltage is very disadvantageous to the electric system. In generally, the higher the vapor tension of contact materials, the lower the arc chopping current. The interfacial energy in nano-grained CuCr alloys is higher than that in coarse-grained CuCr alloys, so the vapor tension of nanograined CuCr alloys is also higher than that of coarse-grained CuCr alloys, which results that the nano-grained CuCr alloys has a lower arc chopping current. The arc spot stays in a small area with a diameter about 1 mm on the surface of coarse-grained Cu71Cr29 cathode. For nano-grained CuCr alloys, the moving way of arc spot is in sub-direction which is different with the random walk pattern of coarse-grained Cu71Cr29 cathode, the arc trace route is about 3 mm length and the velocity of spot direction motion is about 60 m/s [2], which could mitigate the partial ablate of cathode surface and the lifetime of contact
M. Yu et al. / Vacuum 83 (2009) 980–983
10
For nano-grained CuCr alloys, its lower arc chopping current is advantageous to the use of contact and the circuitry protect, its long arc trace route and high velocity of spot direction motion could mitigate the partial ablate of cathode surface and the lifetime of contact could be prolonged.
8 6
I, A
983
4
Acknowledgement
2
Supported by the National Science Foundation of China (50371066).
0 0
50
100
150
200
250
Time, µs Fig. 6. The current waveform of discharge for nanocrystalline Cu68.4Cr27.8Ti2.5Zr1.3 ribbon.
could be prolonged. About the moving way of arc spot and its effects on the contact characteristics had been studied in detail by Mr. Zhimao Yang et al. [2]. In a word, the size decrease of the Cr rich phase in the microstructure of Cu71Cr29 alloys could improve the electric properties of its contact. 5. Conclusions By using melt spinning, the size of the Cr rich phase from liquid phase separation in the Cu71Cr29 microstructure can be decreased from the micron-scale to about 250 nm, which reveals that increasing the cooling rate of solidification process, the microstructure of Cu71Cr29 alloys can be markedly refined. On the meltspun base, alloying by Ti–Zr could further decrease the size of the Cr rich phase from about 250 nm to about 100 nm.
References [1] Yang Zhimao, Zhang Qiuli, Wang Qingfeng, Zhang Chengyu, Ding Bingjun. Vacuum 2006;81:545–9. [2] Yang Zhimao, Zhang Qiuli, Zhang Chengyu, Sun Yue, Ding Bingjun. Physics Letters A 2006;353:98–100. [3] Rieder WF, Schussek M, Glatzle W, Kny E. IEEE Transactions on Components Hybrids and Manufacturing Technology 1989;12:273–8. [4] Wang Y, Ding B. IEEE Transactions on Component Packaging and Manufacturing Technology 1999;22:467–72. [5] Tenwick MJ, Davies HA. Materials Science and Technology 1988;98:543–6. [6] Zhang Zhonghua, Bian Xiufang, Wang Yan, Liu Xiangfa. Journal of Materials Science 2002;37:4473–80. [7] Maslov VV, Nosenko VK. Journal of Materials Science 2002;37:4663–8. [8] Cheng Tianyi, Lo¨ser W, Leonhardt M. Journal of Materials Science 1998; 33:4365–74. [9] Lin Zhang, Youshi Wu, Xiufang Bian, Zhina Xing. Journal of Materials Science Letters 1999;18:1969–72. [10] Nagarajan R, Manjini S, Chattopadhyay K, Aoki K. Journal of Materials Science 1997;32:6021–7. [11] Liu Feng, Yang Gencang, Guo Xuefeng. Journal of Materials Science 2001; 36:3607–15. [12] Cooper KP, Jones HN. Journal of Materials Science 2001;36:5315–23. [13] Wang Youhong, Song Xiaoping, Sun Zhanbo, Zhou Xuan, Guo Juan. Materials Science Poland 2007;25:199–207. [14] Wang Youhong, Sun Zhanbo, Zhou Xuan, Xiaoping Song. Rare Metal Materials and Engineering 2006;35:1289–93. [15] Sun Zhanbo, Wang Youhong, Guo Juan. Material Science Engineering A 2007;452–453:411–6.