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Cu joints brazed with (Cu-50TiH2) + B composite filler
Fusion Engineering and Design 100 (2015) 152–158
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Microstructure analysis of graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler Yangwu Mao a,∗ , Si Yu a , Yizhong Zhang b , Beibei Guo a , Zhibin Ma a , Quanrong Deng a a b
Key Laboratory of Plasma Chemistry and Advanced Materials of Hubei Province, Wuhan Institute of Technology, Wuhan 430073, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou, Hunan 412007, China
h i g h l i g h t s • TiB whiskers are synthesized in situ in the filler layer of graphite/copper joints. • Boron content has a considerable effect on the strength and microstructure of joints. • TiB whiskers could serve as reinforcements, contributing to the improvement of joints.
a r t i c l e
i n f o
Article history: Received 29 November 2014 Received in revised form 12 April 2015 Accepted 6 May 2015 Available online 21 May 2015 Keywords: Brazing Interfaces Composites Carbon Copper Residual stress
a b s t r a c t Joining of carbon materials to copper will benefit the fabrication of plasma facing components for fusion applications. Graphite/Cu joints have been prepared by brazing with (Cu-50TiH2 ) + B composite filler in a vacuum. The effect of boron content in the composite filler on the mechanical property and microstructure of brazed graphite/Cu joints has been investigated. The average shear strength of joints increases with boron content raising from 0 to 15 vol%. The maximum average shear strength of 19.8 MPa was obtained with boron content of 15 vol%. Then, the strength of joints decreases with boron content higher than 15 vol%. The microstructure analysis of joints brazed with (Cu-50TiH2 ) + 15 vol% B filler indicates that TiB whiskers have been in situ synthesized in the filler layer. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. The distribution of TiB whiskers in the filler layer could serve as reinforcements, contributing to the improvement of graphite/Cu joints. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon materials including graphite and carbon fiber reinforced carbon composites (CFC) have been used for plasma facing materials (PFMs) in fusion applications due to their excellent thermal-mechanical properties. However, the critical issues for carbon PFMs are high erosion rate and pollution of the plasma by heavy particles [1]. Tungsten coatings on carbon materials have been applied as plasma facing components (PFCs) in some fusion equipments since tungsten offers the advantages of low erosion rate and long lifetime [2–5]. In order to dissipate the heat in radiation environment, PFMs need to be joined to copper based heat sink. With regard to the fabrication of carbon PFCs or tungsten coated carbon PFCs, joining of carbon to copper is necessary and critical [6]. Moreover, graphite/Cu joints are essential for the fabrication of
∗ Corresponding author. Tel.: +86 27 8719 5661; fax: +86 27 87195661. E-mail address: [email protected] (Y. Mao). http://dx.doi.org/10.1016/j.fusengdes.2015.05.011 0920-3796/© 2015 Elsevier B.V. All rights reserved.
carbon commutators in automotive industry [7]. The main problems of the carbon materials/Cu alloys joints are the large CTE (coefficient of thermal expansion) mismatch of the components and the high contact angle of molten Cu on carbon materials [8]. With regard to the brazing of carbon materials to Cu alloy, the problem involved in the poor wettability is expected to be solved by the surface modification of carbon materials or by the use of active brazes. Xie et al. [9] studied the pre-metallization of graphite with Cu and then Ni by plating method. Then, brazing of premetallized graphite to oxygen-free copper alloy was realized using CuNiSnP braze and the maximum shear strength of the joints was 5.2 MPa. Song et al. [10] introduced the synthesis of Mo2 C coatings on graphite for joining of graphite to copper alloy. Schedler et al. [11] presented the active metal casting (AMC) process which consists in a casting of pure copper on the Ti-activated laser-machined CFC surface. Robert et al. [12] reported the surface modification of CFC by coating with an organic material carbonized at 1600 ◦ C. Then the coated CFC was joined to Cu with 49Cu-49Ti-2Be braze. Casalegno et al. [13,14] described the surface treatment of CFC with
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Cr powders by the slurry technique to form a carbide layer (Cr23 C6 , Cr7 C3 ). Then brazing of the modified CFC to Cu alloy was realized with a commercial non-reactive brazing alloy (87.75 wt% Cu, 12 wt% Ge and 0.25 wt% Ni). They also proposed a novel method based on the modification of the brazing alloy by sputter depositing with Cr for joining of CFC to Cu [15]. Shen et al. [16] investigated the laser micro-machining of CFC samples to obtain ordered conical holes. Then CFC surface-treated with TiH2 powders by slurry technique was brazed to CuCrZr alloys with Cu-3.5 Si braze. The other way to improve the wettability concerning carbon materials/Cu joints is the use of active brazes which contain Cr or Ti elements [17–24]. The brazes, such as NiCrPCu, AgCuTiSn, CuSiAlTi, and amorphous TiZrNiCu, have been selected to join graphite or CFC to Cu. Singh et al. [24] brazed CFC to copper-clad-molybdenum using two active braze alloys, Cusil-ABA (1.75% Ti) and Ticusil (4.5% Ti). The joints reveal good interfacial bonding and preferential precipitation of Ti at the composite/braze interface. It is reported that the residual stresses of joints could be relaxed by the introduction of interlayers or the reinforcements with low CTE in the brazing layer [25,26]. Zhong et al. [25] introduced Cu or Mo stress relief interlayers to reduce the residual stresses of doped graphite/Cu joints. For the reinforcements introduced, ceramic particles, fiber and whiskers have been selected to add into the brazing alloys for the purpose of stresses alleviation of joints [17–30]. Among them, reinforcements (particles or whiskers) synthesized in situ show the advantages such as, uniform distribution, small size and favorable cohesion with the matrix [31]. He et al. [32] investigated the effect of TiB whiskers in situ synthesized in 73Cu-27Ti (wt%) active brazing filler on the strength of Al2 O3 /Ti-6Al-4V alloy joints. TiB whiskers, serving as effective reinforcements, contribute to the improvements of joints. Lin et al. [33] studied the joining of C/SiC to Ti-alloy by using Ag-Cu-Ti-C mixed powder. In situ synthesized TiCx particles in the brazing layer alleviate the thermal stresses and then reinforce the joints. In order to obtain TiB whiskers synthesized in situ in the filler layer, a composite filler composed of Cu, Ti and boron was designed to braze graphite to Cu in this present work. Taking into account that Ti powders can be oxidized easily during mechanical milling in the preparation of composite filler, TiH2 powders which could be decomposed during heating were applied as the replacement of Ti powders. The effect of boron content in the composite filler on the mechanical property and microstructure of graphite/Cu joints was investigated. It is expected that the TiB reinforcements introduced by in situ synthesis will contribute to the improvement of graphite/Cu joints. The use of composite filler will provide a reference for joining of carbon materials (doped graphite or CFC) to Cu alloy heat sink for the applications in fusion reactor.
2. Experimental methods The commercial graphite with the density of 2.1 g cm−3 and shear strength of 21.7 MPa is supported by Wenzhou Ruizhi Carbon Materials Co. Ltd, China. The commercial pure copper (Cu content: 99.90%) with tensile strength of 295 MPa is supported by Dongguan Shichengjin Co. Ltd, China. Both graphite and copper substrates were machined into small pieces with the dimension of 10 mm × 10 mm × 10 mm. The surfaces for joining were polished with 1.0 m diamond paste. The surface roughness Ra for polished graphite and copper are 0.64 m and 0.16 m, respectively. Then, both the polished graphite and copper samples were cleaned with an ultrasonic bath for 20 min in alcohol. The (Cu-50TiH2 ) + B composite fillers were composed of copper powders (about 50 m), TiH2 powders (about 50 m) and boron powders (1–4 m) supported by Beijing Xingrongyuan Technology Co. Ltd, China. Cu-50TiH2 denotes that the weight ratio of Cu and
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Fig. 1. Sketch of shear strength tests for graphite/Cu joints.
TiH2 powders is 50:50. Boron powders with the volume content of 0, 5 vol%, 10 vol%, 15 vol%, 20 vol% and 25 vol%, respectively, were added into Cu-50TiH2 powders fillers. Then, the (Cu-50TiH2 ) + B composite fillers were prepared by mechanical milling. Before assembling, a few drops of glycerin were added into the (Cu50TiH2 ) + B powder mixtures to make a thick paste with dough-like consistency. The paste was applied to the surfaces of graphite and copper to be joined. The assembly was loaded with a pressure of 50 kPa to make the filler mixtures contact closely with graphite and Cu substrates. The brazing process was carried out in a high temperature furnace under the vacuum of 5 × 10−2 Pa. At the beginning, the assembly was heated to 773 K at a rate of 10 K/min and held for 1 h to volatilize the binder. Then the temperature increased to 1073 K at a rate of 10 K/min to ensure the decomposition of TiH2 powders and later increased to brazing temperature of 1223 K at a rate of 5 K/min. After holding for 10 min at 1223 K, the assembly was cooled to room temperature at a controlled rate (5 K/min from brazing temperature to 773 K, then followed by furnace cooling). The shear strength of the joined samples was measured by an electronic universal materials testing machine (GP-TS2000s) with the jig shown in Fig. 1. Three joining samples brazed with the same parameters were tested to obtain the average shear strength. In order to compare the strength of joints with graphite substrates, graphite samples with size of 10 mm × 10 mm × 20 mm were also tested by the method shown in Fig. 1. Then, the relative strength of graphite/Cu joints to graphite substrates was calculated. In order to character the microstructure and compositions of the interfacial area, the cross-sections of graphite/Cu joints were examined with a field emission scanning electron microscope (FESEM, Hitachi S-4800) equipped with an energy-dispersive X-ray spectrometer (EDS, Bruker Quantax) and an electron probe microanalyzer (EPMA, JEOL JXA-8230) combined with an EDS (Oxford Inca X-Act). The morphology and structure of whiskers in the filler layer of joints were performed using a transmission electron microscopy (TEM, JEOL JEM-2100F). The thinning process of the sample for TEM observation was conducted with a precision ion polishing system (PIPS, Gatan 691). The phase analysis of the interfacial area of joints was determined by an X-ray diffraction spectrometer (XRD, Bruker D8 Advance). 3. Results and discussions 3.1. Microstructure analysis of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler Fig. 2 shows the morphology of interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min. As can be seen from Fig. 2(a), the interfacial area of joints is defect-free and a filler layer with about 100 m thick is
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Fig. 2. Interfacial area of graphite/Cu joints with (Cu-50TiH2 ) + 15 vol% B composite filler (a) and the filler layer with magnification (b).
Table 1 The compositions of points marked in the filler layer shown in Fig.2(b) by EDS. Point
A B C
Compositions, at%
Possible phases
Cu
Ti
Total
95.83 79.04 34.54
4.17 20.96 65.46
100.00 100.00 100.00
Cu based solid solution TiCu4 Ti2 Cu
formed between graphite and Cu substrates. The filler layer can be divided into white area, gray area, dark area and needle-like zone. The point A in the white area, point B in the gray area and point C in the dark area marked in the filler layer shown in Fig. 2(b) are analyzed by means of EPMA. The compositions results are listed in Table 1. The EDS composition profiles at point A show the presence of Cu and trace Ti, indicating the white area in the filler layer is Cu based solid solution containing Ti element. The point B is made up of 79.04 at% Cu and 20.96 at% Ti. The atomic composition of Cu and Ti is nearly 4:1, which is consistent with TiCu4 , inferring the gray area in the filler layer as TiCu4 intermetallic compounds. The EDS composition profiles at point C show the presence of Cu and Ti, and the atomic composition of Ti and Cu is nearly 2:1, suggesting the dark area in the filler layer is Ti2 Cu intermetallic compounds. Furthermore, boron is not detected in the needle-like zone by EPMA in our measurement. The TEM image of whiskers in the filler layer of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler is given in Fig. 3a, showing the sound distribution of
Fig. 3. TEM image of filler layer of graphite/Cu joints brazed with (Cu50TiH2 ) + 15 vol% B composite filler (a) and SAED pattern of TiB whisker (b).
the whiskers in the filler layer. The selected area electron diffraction (SAED) pattern of the whisker shown in Fig. 3b confirms TiB whiskers synthesized in situ in the filler layer. The identification of needle-like compounds as TiB whiskers is in accord with Refs. [26,34]. For example, Ref. [34] reported that TiB whiskers were synthesized in situ with needle shape through the reaction of boron powders with titanium with regard to Al2 O3 /Al2 O3 joints using Ag-Cu-Ti + B + TiH2 composite filler. Fig. 4 gives the SEM micrograph and the areal distribution of elements C, Ti and Cu at the interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min. Neither cracks nor voids are observed in the interfacial area, indicating a good interfacial bonding for graphite/Cu joints. The infiltration of liquid Cu-Ti filler into the open pores of the graphite substrate can be observed from the SEM image. In addition, a thin gray continuous layer with about 2 m thick is formed between graphite and the filler layer. The areal distribution of elements demonstrates that the filler layer mainly consists of Ti and Cu. Meanwhile, the enrichment of Ti occurs in the thin gray continuous layer near the graphite substrate, suggesting the formation of TiC reaction layer between graphite and the filler layer. The TiC thin continuous layer is developed by the reaction of Ti in the filler with carbon from graphite substrate, which is agreement with the results revealed in Refs. [35,36]. In order to reveal the phase composition of interfacial area of graphite/Cu joints, the joints brazed with (Cu-50TiH2 ) + 15 vol% B filler at 1223 K for 10 min were carried out by XRD measurement. Fig. 5 shows the schematic plot of the planes of joints to be analyzed and their XRD patterns. Firstly, the graphite part of joints was removed by cutting and then grinding until the filler layer exposed. The exposed plane named Plane A was measured with XRD and the results show that the plane is mainly composed of C, TiC, Cu, and Ti2 Cu. The existence of TiC confirms the development of the thin TiC reaction layer between graphite and the filler layer. The presence of carbon is attributed to the residual graphite. The detection of Ti2 Cu and Cu could be resulted from the filler layer. Then, the Plane A was further ground to remove a layer with about 20 m thick, and the revealed plane was named Plane B. The XRD patterns for Plane B show that the new plane mainly consists of Cu, TiCu4 , Ti2 Cu and TiB with TiC, indicating Plane B is located in the filler layer. The presence of TiB confirms the formation of whiskers synthesized by the in situ reaction of Ti with boron powders in the filler. The phases of Cu, TiCu4 , Ti2 Cu may result from the filler layer, since the filler layer is mainly composed of Cu solid solution and Ti-Cu intermetallic compounds with TiB whiskers, according to the previous EPMA analysis. The detection of TiC may be attributed to the residual thin TiC reaction layer. Combined the SEM, EPMA, EDS with XRD results, one can conclude that TiB whiskers are in situ synthesized in the filler layer by the reaction of titanium and boron for the graphite/Cu joints
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Fig. 4. SEM image (a) and EDS areal distribution (b–d) of interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min.
brazed with (Cu-50TiH2 ) + 15 vol% B composite filler. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. Furthermore, a thin TiC reaction layer is developed at interfacial area between the graphite and the filler layer. As presented in Ref. [37], the decomposition of TiH2 started at about 673 K and completed at around 908 K. Thus, TiH2 could be decomposed to Ti during the heating process. When the specimen was heated to brazing temperature, the following reaction may occur, as indicated in Refs. [26,34,38]: Ti + C = TiC
(1)
Ti + B = TiB
(2)
Ti + 2B = TiB2
(3)
Ti + TiB2 = 2TiB
(4)
The values of changes of standard Gibbs free energies, G, can be calculated using the software HSC Chemistry Version 6 (Outokumpu Ra, Oy, Finland). Fig. 6 gives the values of G for reactions (1)–(4) as a function of temperature. The G values of above reactions are negative at joining temperature, indicating the possibility of all the reactions. An interface evolution model for the graphite/Cu joints with (Cu-50TiH2 ) + B composite filler is given in Fig. 7, based on the experimental results and analysis. When the temperature increases (lower than melting point of the filler), the volatilization of glycerin and transformation of TiH2 into Ti could occur. Then the powders fillers contact each other and are dense under the applied pressure, as shown in Fig. 7(a). Liquid Cu-Ti appears in the brazing filler when the temperature reaches the melting point of the filler. At this stage, as shown in Fig. 7(b), Ti and Cu atoms begin to spread from the liquid to graphite and Cu substrates. Some of Ti and Cu liquid phase infiltrates into the open pores of graphite by the capillary attraction. Subsequently, TiC is formed by the reaction of Ti and carbon atom
Fig. 5. Schematic plot to show the planes of graphite/Cu joints analyzed by XRD (a) and the XRD patterns of Plane A and Plane B (b).
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stage, Cu solid solution and Ti-Cu intermetallic compounds including Ti2 Cu and TiCu4 start to nucleate and grow up to the center of brazing filler by the atoms diffusion. The following reactions can take place based on the Ti-Cu phase diagram and thermodynamics formula [38]:
Fig. 6. The values of G for reactions (1)–(4) as a function of temperature.
from graphite through the reaction (1). Meanwhile, boron powders in the composite filler start to dissolve into the liquid system and then react with Ti element to form TiB with the needle-like shape by the reaction (2) and TiB2 with a block shape by the reaction (3) [34]. With further increasing the temperature, TiC compounds increase in size and develop a continuous layer near graphite (shown in Fig. 7c). Boron powders keep in reacting with Ti element to form TiB or TiB2 . Although the G of TiB2 formation is more negative than that of TiB formation, the TiB formation can take place due to the reaction of TiB2 with the excessive Ti in the liquid at brazing temperature through the reaction (4). On the other hand, although TiB2 is easier to be formed than TiB, both the growth rate of TiB whisker and the estimated diffusion coefficient for boron in TiB along the needle directions are much higher than that of TiB2 particles [39]. Thus, TiB whiskers rather than TiB2 particles will eventually be synthesized in the filler layer provided the proper proportion of boron content in the liquid Cu-Ti filler. After holding at brazing temperature, the graphite/Cu joints were cooled to room temperature at a controlled rate. At this
2Ti + Cu = Ti2 CuG(J/mol) = −36393 + 14.06 T
(5)
Ti + 4Cu = TiCu4 G(J/mol) = −30055 + 11.70 T
(6)
Therefore, the filler layer is mainly composed of continuous Cu solid solution, TiCu4 and Ti2 Cu intermetallic compounds with TiB whiskers distributed inside, as shown in Fig. 7d). The distribution of TiB whiskers in the filler layer could serve as effective reinforcements, contributing to the improvement of joints. On the other hand, considering that the CTE of TiB, 7.15 × 10−6 K−1 , is less than that of Cu (16.5 × 10−6 K−1 ) and most metallic brazes (∼15–19 × 10−6 K−1 ) [40,41], it is possible to decrease the CTE mismatch between graphite and the brazing filler by introduction of TiB whiskers in the filler layer, which may also benefit the improvement of graphite/Cu joints. 3.2. Effect of boron content in composite filler on the mechanical property and microstructure of graphite/Cu joints Fig. 8 shows the average shear strength of graphite/Cu joints obtained at 1223 K for 10 min as a function of boron content in the (Cu-50TiH2 ) + B composite filler. The joints brazed without boron additives in the filler give an average shear strength of 10.8 MPa. The strength of joints increases with boron content raising from 0 to 15 vol% in the (Cu-50TiH2 ) + B composite filler. The joints obtained with (Cu-50TiH2 ) + 15 vol% B composite filler show the maximum shear strength, 19.8 MPa, which is 91.2% of the strength of graphite substrate. The fracture is located in the graphite substrate near the brazing seam, which implies the strong bonding between the filler and the substrates. Meanwhile, the fracture position also indicates the residual stresses caused by the CTE mismatch existing in the graphite substrate near the brazing seam, though it can be reduced slightly by the introduction of the composite filler. The shear strength of joints decreases with boron content higher than
Fig. 7. An interface evolution model for the graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler. (a) The stage before the melting of filler; (b) the initial stage after the melting of filler; (c) the final stage after the melting of filler; and (d) the cooling stage.
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Fig. 10. XRD patterns of the filler layer of graphite/Cu joints brazed with (Cu50TiH2 ) + 20 vol% B composite filler at 1223 K for 10 min. Fig. 8. The average shear strength of joints obtained at 1223 K for 10 min as a function of boron content in the (Cu-50TiH2 ) + B composite filler.
15 vol% and the joining fails with boron content of 25 vol% in the composite filler. Fig. 9 shows SEM images of interfacial area between graphite and filler layer of graphite/Cu joints brazed with boron content of 5 vol%, 10 vol%, 15 vol% and 20 vol% in the composite filler. As can be seen from the images, the filler layer in the right side is mainly composed of gray area with dark particles and whiskers inside. Meanwhile, it can be seen clearly that the amount of whiskers in the filler layer increase with boron content ranging from 5 vol% to 15 vol% in the composite filler. Since the whiskers synthesized in situ distribute uniformly and serve as reinforcements in the filler layer, the increasing of the amount of whiskers can result in the reinforcing of graphite/Cu joints, leading to the increasing of shear strength of joints. However, the amount of whiskers begin to decrease for the joints brazed with boron content of 20 vol% in the composite filler, as shown in Fig. 9(d). This is because Ti in the
filler is insufficient for the formation of TiB whiskers through the reaction (4), when boron content is too high in the composite filler. Thus, large amounts of TiB2 with block shape are formed, causing the deterioration of the joints. The XRD patterns of the filler layer of joints with (Cu-50TiH2 ) + 20 vol% B composite filler, as shown in Fig. 10, confirm the formation of TiB2 in the filler layer. The joining was proved to be unsuccessful when boron content further increasing to 25 vol% in the composite filler.
4. Conclusion Brazing of graphite to oxygen-free copper has been realized successfully with (Cu-50TiH2 ) + B composite filler. The effect of boron content in the composite filler on the mechanical property and microstructure of joints has been investigated. The obtained conclusions are as follows:
Fig. 9. SEM images of interfacial area between graphite and filler layer of graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler in which the boron content is (a) 5 vol%; (b) 10 vol%; (c) 15 vol% and (d) 20 vol%, respectively.
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1) TiB whiskers are in situ synthesized in the filler layer by the reaction of titanium and boron. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. The distribution of TiB whiskers in the filler layer could serve as reinforcements, contributing to the improvement of graphite/Cu joints. Furthermore, a thin TiC reaction layer is formed at interfacial area between the graphite and the filler layer. 2) The boron content in the composite filler has a considerable effect on the strength of graphite/Cu joints. The shear strength of joints increases with boron content raising from 0 to 15 vol%. The joints brazed with (Cu-50TiH2 ) 15 vol% B composite filler show the maximum shear strength, 19.8 MPa, which is 91.2% of the strength of graphite substrate. However, the strength of joints decreases with boron content higher than 15 vol%. 3) The amount of TiB whiskers synthesized in situ in the filler layer increase with boron content ranging from 5 vol% to 15 vol%. Then, TiB2 with block shape rather than TiB whiskers are formed in the filler layer with boron content higher than 15 vol%, leading to deterioration of the joints. The next step for our research will contain the preparation of tungsten coating on carbon materials and the fabrication of mockups composed of carbon/copper or tungsten coated carbon/copper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51304148) and the Scientific Research Project under Hubei Provincial Department of Education (No. D20131504). References [1] S. Roccella, E. Cacciotti, D. Candura, A. Mancini, A. Pizzuto, A. Reale, A. Tatì, E. Visca, Fusion Eng. Des. 88 (2013) 1802–1807. [2] N. Sun, Y. Zhang, F. Jiang, S. Lang, M. Xia, Fusion Eng. Des. 89 (2014) 2529–2533. [3] N. Sun, Y. Zhang, S. Lang, F. Jiang, L. Wang, J. Nucl. Mater. 455 (2014) 450–453. [4] N. Ashikawa, N. Asakura, M. Fukumoto, T. Hayashi, Y. Ueda, T. Muroga, J. Nucl. Mater. 438 (Suppl.) (2013) S664–S667. [5] C. Ruset, E. Grigore, I. Munteanu, H. Maier, H. Greuner, C. Hopf, V. Phylipps, G. Matthews, JET-EFDA Contributors, Fusion Eng. Des. 84 (2009) 1662–1665.
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Microstructure analysis of graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler Yangwu Mao a,∗ , Si Yu a , Yizhong Zhang b , Beibei Guo a , Zhibin Ma a , Quanrong Deng a a b
Key Laboratory of Plasma Chemistry and Advanced Materials of Hubei Province, Wuhan Institute of Technology, Wuhan 430073, China Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou, Hunan 412007, China
h i g h l i g h t s • TiB whiskers are synthesized in situ in the filler layer of graphite/copper joints. • Boron content has a considerable effect on the strength and microstructure of joints. • TiB whiskers could serve as reinforcements, contributing to the improvement of joints.
a r t i c l e
i n f o
Article history: Received 29 November 2014 Received in revised form 12 April 2015 Accepted 6 May 2015 Available online 21 May 2015 Keywords: Brazing Interfaces Composites Carbon Copper Residual stress
a b s t r a c t Joining of carbon materials to copper will benefit the fabrication of plasma facing components for fusion applications. Graphite/Cu joints have been prepared by brazing with (Cu-50TiH2 ) + B composite filler in a vacuum. The effect of boron content in the composite filler on the mechanical property and microstructure of brazed graphite/Cu joints has been investigated. The average shear strength of joints increases with boron content raising from 0 to 15 vol%. The maximum average shear strength of 19.8 MPa was obtained with boron content of 15 vol%. Then, the strength of joints decreases with boron content higher than 15 vol%. The microstructure analysis of joints brazed with (Cu-50TiH2 ) + 15 vol% B filler indicates that TiB whiskers have been in situ synthesized in the filler layer. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. The distribution of TiB whiskers in the filler layer could serve as reinforcements, contributing to the improvement of graphite/Cu joints. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon materials including graphite and carbon fiber reinforced carbon composites (CFC) have been used for plasma facing materials (PFMs) in fusion applications due to their excellent thermal-mechanical properties. However, the critical issues for carbon PFMs are high erosion rate and pollution of the plasma by heavy particles [1]. Tungsten coatings on carbon materials have been applied as plasma facing components (PFCs) in some fusion equipments since tungsten offers the advantages of low erosion rate and long lifetime [2–5]. In order to dissipate the heat in radiation environment, PFMs need to be joined to copper based heat sink. With regard to the fabrication of carbon PFCs or tungsten coated carbon PFCs, joining of carbon to copper is necessary and critical [6]. Moreover, graphite/Cu joints are essential for the fabrication of
∗ Corresponding author. Tel.: +86 27 8719 5661; fax: +86 27 87195661. E-mail address: [email protected] (Y. Mao). http://dx.doi.org/10.1016/j.fusengdes.2015.05.011 0920-3796/© 2015 Elsevier B.V. All rights reserved.
carbon commutators in automotive industry [7]. The main problems of the carbon materials/Cu alloys joints are the large CTE (coefficient of thermal expansion) mismatch of the components and the high contact angle of molten Cu on carbon materials [8]. With regard to the brazing of carbon materials to Cu alloy, the problem involved in the poor wettability is expected to be solved by the surface modification of carbon materials or by the use of active brazes. Xie et al. [9] studied the pre-metallization of graphite with Cu and then Ni by plating method. Then, brazing of premetallized graphite to oxygen-free copper alloy was realized using CuNiSnP braze and the maximum shear strength of the joints was 5.2 MPa. Song et al. [10] introduced the synthesis of Mo2 C coatings on graphite for joining of graphite to copper alloy. Schedler et al. [11] presented the active metal casting (AMC) process which consists in a casting of pure copper on the Ti-activated laser-machined CFC surface. Robert et al. [12] reported the surface modification of CFC by coating with an organic material carbonized at 1600 ◦ C. Then the coated CFC was joined to Cu with 49Cu-49Ti-2Be braze. Casalegno et al. [13,14] described the surface treatment of CFC with
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Cr powders by the slurry technique to form a carbide layer (Cr23 C6 , Cr7 C3 ). Then brazing of the modified CFC to Cu alloy was realized with a commercial non-reactive brazing alloy (87.75 wt% Cu, 12 wt% Ge and 0.25 wt% Ni). They also proposed a novel method based on the modification of the brazing alloy by sputter depositing with Cr for joining of CFC to Cu [15]. Shen et al. [16] investigated the laser micro-machining of CFC samples to obtain ordered conical holes. Then CFC surface-treated with TiH2 powders by slurry technique was brazed to CuCrZr alloys with Cu-3.5 Si braze. The other way to improve the wettability concerning carbon materials/Cu joints is the use of active brazes which contain Cr or Ti elements [17–24]. The brazes, such as NiCrPCu, AgCuTiSn, CuSiAlTi, and amorphous TiZrNiCu, have been selected to join graphite or CFC to Cu. Singh et al. [24] brazed CFC to copper-clad-molybdenum using two active braze alloys, Cusil-ABA (1.75% Ti) and Ticusil (4.5% Ti). The joints reveal good interfacial bonding and preferential precipitation of Ti at the composite/braze interface. It is reported that the residual stresses of joints could be relaxed by the introduction of interlayers or the reinforcements with low CTE in the brazing layer [25,26]. Zhong et al. [25] introduced Cu or Mo stress relief interlayers to reduce the residual stresses of doped graphite/Cu joints. For the reinforcements introduced, ceramic particles, fiber and whiskers have been selected to add into the brazing alloys for the purpose of stresses alleviation of joints [17–30]. Among them, reinforcements (particles or whiskers) synthesized in situ show the advantages such as, uniform distribution, small size and favorable cohesion with the matrix [31]. He et al. [32] investigated the effect of TiB whiskers in situ synthesized in 73Cu-27Ti (wt%) active brazing filler on the strength of Al2 O3 /Ti-6Al-4V alloy joints. TiB whiskers, serving as effective reinforcements, contribute to the improvements of joints. Lin et al. [33] studied the joining of C/SiC to Ti-alloy by using Ag-Cu-Ti-C mixed powder. In situ synthesized TiCx particles in the brazing layer alleviate the thermal stresses and then reinforce the joints. In order to obtain TiB whiskers synthesized in situ in the filler layer, a composite filler composed of Cu, Ti and boron was designed to braze graphite to Cu in this present work. Taking into account that Ti powders can be oxidized easily during mechanical milling in the preparation of composite filler, TiH2 powders which could be decomposed during heating were applied as the replacement of Ti powders. The effect of boron content in the composite filler on the mechanical property and microstructure of graphite/Cu joints was investigated. It is expected that the TiB reinforcements introduced by in situ synthesis will contribute to the improvement of graphite/Cu joints. The use of composite filler will provide a reference for joining of carbon materials (doped graphite or CFC) to Cu alloy heat sink for the applications in fusion reactor.
2. Experimental methods The commercial graphite with the density of 2.1 g cm−3 and shear strength of 21.7 MPa is supported by Wenzhou Ruizhi Carbon Materials Co. Ltd, China. The commercial pure copper (Cu content: 99.90%) with tensile strength of 295 MPa is supported by Dongguan Shichengjin Co. Ltd, China. Both graphite and copper substrates were machined into small pieces with the dimension of 10 mm × 10 mm × 10 mm. The surfaces for joining were polished with 1.0 m diamond paste. The surface roughness Ra for polished graphite and copper are 0.64 m and 0.16 m, respectively. Then, both the polished graphite and copper samples were cleaned with an ultrasonic bath for 20 min in alcohol. The (Cu-50TiH2 ) + B composite fillers were composed of copper powders (about 50 m), TiH2 powders (about 50 m) and boron powders (1–4 m) supported by Beijing Xingrongyuan Technology Co. Ltd, China. Cu-50TiH2 denotes that the weight ratio of Cu and
153
Fig. 1. Sketch of shear strength tests for graphite/Cu joints.
TiH2 powders is 50:50. Boron powders with the volume content of 0, 5 vol%, 10 vol%, 15 vol%, 20 vol% and 25 vol%, respectively, were added into Cu-50TiH2 powders fillers. Then, the (Cu-50TiH2 ) + B composite fillers were prepared by mechanical milling. Before assembling, a few drops of glycerin were added into the (Cu50TiH2 ) + B powder mixtures to make a thick paste with dough-like consistency. The paste was applied to the surfaces of graphite and copper to be joined. The assembly was loaded with a pressure of 50 kPa to make the filler mixtures contact closely with graphite and Cu substrates. The brazing process was carried out in a high temperature furnace under the vacuum of 5 × 10−2 Pa. At the beginning, the assembly was heated to 773 K at a rate of 10 K/min and held for 1 h to volatilize the binder. Then the temperature increased to 1073 K at a rate of 10 K/min to ensure the decomposition of TiH2 powders and later increased to brazing temperature of 1223 K at a rate of 5 K/min. After holding for 10 min at 1223 K, the assembly was cooled to room temperature at a controlled rate (5 K/min from brazing temperature to 773 K, then followed by furnace cooling). The shear strength of the joined samples was measured by an electronic universal materials testing machine (GP-TS2000s) with the jig shown in Fig. 1. Three joining samples brazed with the same parameters were tested to obtain the average shear strength. In order to compare the strength of joints with graphite substrates, graphite samples with size of 10 mm × 10 mm × 20 mm were also tested by the method shown in Fig. 1. Then, the relative strength of graphite/Cu joints to graphite substrates was calculated. In order to character the microstructure and compositions of the interfacial area, the cross-sections of graphite/Cu joints were examined with a field emission scanning electron microscope (FESEM, Hitachi S-4800) equipped with an energy-dispersive X-ray spectrometer (EDS, Bruker Quantax) and an electron probe microanalyzer (EPMA, JEOL JXA-8230) combined with an EDS (Oxford Inca X-Act). The morphology and structure of whiskers in the filler layer of joints were performed using a transmission electron microscopy (TEM, JEOL JEM-2100F). The thinning process of the sample for TEM observation was conducted with a precision ion polishing system (PIPS, Gatan 691). The phase analysis of the interfacial area of joints was determined by an X-ray diffraction spectrometer (XRD, Bruker D8 Advance). 3. Results and discussions 3.1. Microstructure analysis of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler Fig. 2 shows the morphology of interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min. As can be seen from Fig. 2(a), the interfacial area of joints is defect-free and a filler layer with about 100 m thick is
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Fig. 2. Interfacial area of graphite/Cu joints with (Cu-50TiH2 ) + 15 vol% B composite filler (a) and the filler layer with magnification (b).
Table 1 The compositions of points marked in the filler layer shown in Fig.2(b) by EDS. Point
A B C
Compositions, at%
Possible phases
Cu
Ti
Total
95.83 79.04 34.54
4.17 20.96 65.46
100.00 100.00 100.00
Cu based solid solution TiCu4 Ti2 Cu
formed between graphite and Cu substrates. The filler layer can be divided into white area, gray area, dark area and needle-like zone. The point A in the white area, point B in the gray area and point C in the dark area marked in the filler layer shown in Fig. 2(b) are analyzed by means of EPMA. The compositions results are listed in Table 1. The EDS composition profiles at point A show the presence of Cu and trace Ti, indicating the white area in the filler layer is Cu based solid solution containing Ti element. The point B is made up of 79.04 at% Cu and 20.96 at% Ti. The atomic composition of Cu and Ti is nearly 4:1, which is consistent with TiCu4 , inferring the gray area in the filler layer as TiCu4 intermetallic compounds. The EDS composition profiles at point C show the presence of Cu and Ti, and the atomic composition of Ti and Cu is nearly 2:1, suggesting the dark area in the filler layer is Ti2 Cu intermetallic compounds. Furthermore, boron is not detected in the needle-like zone by EPMA in our measurement. The TEM image of whiskers in the filler layer of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler is given in Fig. 3a, showing the sound distribution of
Fig. 3. TEM image of filler layer of graphite/Cu joints brazed with (Cu50TiH2 ) + 15 vol% B composite filler (a) and SAED pattern of TiB whisker (b).
the whiskers in the filler layer. The selected area electron diffraction (SAED) pattern of the whisker shown in Fig. 3b confirms TiB whiskers synthesized in situ in the filler layer. The identification of needle-like compounds as TiB whiskers is in accord with Refs. [26,34]. For example, Ref. [34] reported that TiB whiskers were synthesized in situ with needle shape through the reaction of boron powders with titanium with regard to Al2 O3 /Al2 O3 joints using Ag-Cu-Ti + B + TiH2 composite filler. Fig. 4 gives the SEM micrograph and the areal distribution of elements C, Ti and Cu at the interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min. Neither cracks nor voids are observed in the interfacial area, indicating a good interfacial bonding for graphite/Cu joints. The infiltration of liquid Cu-Ti filler into the open pores of the graphite substrate can be observed from the SEM image. In addition, a thin gray continuous layer with about 2 m thick is formed between graphite and the filler layer. The areal distribution of elements demonstrates that the filler layer mainly consists of Ti and Cu. Meanwhile, the enrichment of Ti occurs in the thin gray continuous layer near the graphite substrate, suggesting the formation of TiC reaction layer between graphite and the filler layer. The TiC thin continuous layer is developed by the reaction of Ti in the filler with carbon from graphite substrate, which is agreement with the results revealed in Refs. [35,36]. In order to reveal the phase composition of interfacial area of graphite/Cu joints, the joints brazed with (Cu-50TiH2 ) + 15 vol% B filler at 1223 K for 10 min were carried out by XRD measurement. Fig. 5 shows the schematic plot of the planes of joints to be analyzed and their XRD patterns. Firstly, the graphite part of joints was removed by cutting and then grinding until the filler layer exposed. The exposed plane named Plane A was measured with XRD and the results show that the plane is mainly composed of C, TiC, Cu, and Ti2 Cu. The existence of TiC confirms the development of the thin TiC reaction layer between graphite and the filler layer. The presence of carbon is attributed to the residual graphite. The detection of Ti2 Cu and Cu could be resulted from the filler layer. Then, the Plane A was further ground to remove a layer with about 20 m thick, and the revealed plane was named Plane B. The XRD patterns for Plane B show that the new plane mainly consists of Cu, TiCu4 , Ti2 Cu and TiB with TiC, indicating Plane B is located in the filler layer. The presence of TiB confirms the formation of whiskers synthesized by the in situ reaction of Ti with boron powders in the filler. The phases of Cu, TiCu4 , Ti2 Cu may result from the filler layer, since the filler layer is mainly composed of Cu solid solution and Ti-Cu intermetallic compounds with TiB whiskers, according to the previous EPMA analysis. The detection of TiC may be attributed to the residual thin TiC reaction layer. Combined the SEM, EPMA, EDS with XRD results, one can conclude that TiB whiskers are in situ synthesized in the filler layer by the reaction of titanium and boron for the graphite/Cu joints
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Fig. 4. SEM image (a) and EDS areal distribution (b–d) of interfacial area of graphite/Cu joints brazed with (Cu-50TiH2 ) + 15 vol% B composite filler at 1223 K for 10 min.
brazed with (Cu-50TiH2 ) + 15 vol% B composite filler. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. Furthermore, a thin TiC reaction layer is developed at interfacial area between the graphite and the filler layer. As presented in Ref. [37], the decomposition of TiH2 started at about 673 K and completed at around 908 K. Thus, TiH2 could be decomposed to Ti during the heating process. When the specimen was heated to brazing temperature, the following reaction may occur, as indicated in Refs. [26,34,38]: Ti + C = TiC
(1)
Ti + B = TiB
(2)
Ti + 2B = TiB2
(3)
Ti + TiB2 = 2TiB
(4)
The values of changes of standard Gibbs free energies, G, can be calculated using the software HSC Chemistry Version 6 (Outokumpu Ra, Oy, Finland). Fig. 6 gives the values of G for reactions (1)–(4) as a function of temperature. The G values of above reactions are negative at joining temperature, indicating the possibility of all the reactions. An interface evolution model for the graphite/Cu joints with (Cu-50TiH2 ) + B composite filler is given in Fig. 7, based on the experimental results and analysis. When the temperature increases (lower than melting point of the filler), the volatilization of glycerin and transformation of TiH2 into Ti could occur. Then the powders fillers contact each other and are dense under the applied pressure, as shown in Fig. 7(a). Liquid Cu-Ti appears in the brazing filler when the temperature reaches the melting point of the filler. At this stage, as shown in Fig. 7(b), Ti and Cu atoms begin to spread from the liquid to graphite and Cu substrates. Some of Ti and Cu liquid phase infiltrates into the open pores of graphite by the capillary attraction. Subsequently, TiC is formed by the reaction of Ti and carbon atom
Fig. 5. Schematic plot to show the planes of graphite/Cu joints analyzed by XRD (a) and the XRD patterns of Plane A and Plane B (b).
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stage, Cu solid solution and Ti-Cu intermetallic compounds including Ti2 Cu and TiCu4 start to nucleate and grow up to the center of brazing filler by the atoms diffusion. The following reactions can take place based on the Ti-Cu phase diagram and thermodynamics formula [38]:
Fig. 6. The values of G for reactions (1)–(4) as a function of temperature.
from graphite through the reaction (1). Meanwhile, boron powders in the composite filler start to dissolve into the liquid system and then react with Ti element to form TiB with the needle-like shape by the reaction (2) and TiB2 with a block shape by the reaction (3) [34]. With further increasing the temperature, TiC compounds increase in size and develop a continuous layer near graphite (shown in Fig. 7c). Boron powders keep in reacting with Ti element to form TiB or TiB2 . Although the G of TiB2 formation is more negative than that of TiB formation, the TiB formation can take place due to the reaction of TiB2 with the excessive Ti in the liquid at brazing temperature through the reaction (4). On the other hand, although TiB2 is easier to be formed than TiB, both the growth rate of TiB whisker and the estimated diffusion coefficient for boron in TiB along the needle directions are much higher than that of TiB2 particles [39]. Thus, TiB whiskers rather than TiB2 particles will eventually be synthesized in the filler layer provided the proper proportion of boron content in the liquid Cu-Ti filler. After holding at brazing temperature, the graphite/Cu joints were cooled to room temperature at a controlled rate. At this
2Ti + Cu = Ti2 CuG(J/mol) = −36393 + 14.06 T
(5)
Ti + 4Cu = TiCu4 G(J/mol) = −30055 + 11.70 T
(6)
Therefore, the filler layer is mainly composed of continuous Cu solid solution, TiCu4 and Ti2 Cu intermetallic compounds with TiB whiskers distributed inside, as shown in Fig. 7d). The distribution of TiB whiskers in the filler layer could serve as effective reinforcements, contributing to the improvement of joints. On the other hand, considering that the CTE of TiB, 7.15 × 10−6 K−1 , is less than that of Cu (16.5 × 10−6 K−1 ) and most metallic brazes (∼15–19 × 10−6 K−1 ) [40,41], it is possible to decrease the CTE mismatch between graphite and the brazing filler by introduction of TiB whiskers in the filler layer, which may also benefit the improvement of graphite/Cu joints. 3.2. Effect of boron content in composite filler on the mechanical property and microstructure of graphite/Cu joints Fig. 8 shows the average shear strength of graphite/Cu joints obtained at 1223 K for 10 min as a function of boron content in the (Cu-50TiH2 ) + B composite filler. The joints brazed without boron additives in the filler give an average shear strength of 10.8 MPa. The strength of joints increases with boron content raising from 0 to 15 vol% in the (Cu-50TiH2 ) + B composite filler. The joints obtained with (Cu-50TiH2 ) + 15 vol% B composite filler show the maximum shear strength, 19.8 MPa, which is 91.2% of the strength of graphite substrate. The fracture is located in the graphite substrate near the brazing seam, which implies the strong bonding between the filler and the substrates. Meanwhile, the fracture position also indicates the residual stresses caused by the CTE mismatch existing in the graphite substrate near the brazing seam, though it can be reduced slightly by the introduction of the composite filler. The shear strength of joints decreases with boron content higher than
Fig. 7. An interface evolution model for the graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler. (a) The stage before the melting of filler; (b) the initial stage after the melting of filler; (c) the final stage after the melting of filler; and (d) the cooling stage.
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157
Fig. 10. XRD patterns of the filler layer of graphite/Cu joints brazed with (Cu50TiH2 ) + 20 vol% B composite filler at 1223 K for 10 min. Fig. 8. The average shear strength of joints obtained at 1223 K for 10 min as a function of boron content in the (Cu-50TiH2 ) + B composite filler.
15 vol% and the joining fails with boron content of 25 vol% in the composite filler. Fig. 9 shows SEM images of interfacial area between graphite and filler layer of graphite/Cu joints brazed with boron content of 5 vol%, 10 vol%, 15 vol% and 20 vol% in the composite filler. As can be seen from the images, the filler layer in the right side is mainly composed of gray area with dark particles and whiskers inside. Meanwhile, it can be seen clearly that the amount of whiskers in the filler layer increase with boron content ranging from 5 vol% to 15 vol% in the composite filler. Since the whiskers synthesized in situ distribute uniformly and serve as reinforcements in the filler layer, the increasing of the amount of whiskers can result in the reinforcing of graphite/Cu joints, leading to the increasing of shear strength of joints. However, the amount of whiskers begin to decrease for the joints brazed with boron content of 20 vol% in the composite filler, as shown in Fig. 9(d). This is because Ti in the
filler is insufficient for the formation of TiB whiskers through the reaction (4), when boron content is too high in the composite filler. Thus, large amounts of TiB2 with block shape are formed, causing the deterioration of the joints. The XRD patterns of the filler layer of joints with (Cu-50TiH2 ) + 20 vol% B composite filler, as shown in Fig. 10, confirm the formation of TiB2 in the filler layer. The joining was proved to be unsuccessful when boron content further increasing to 25 vol% in the composite filler.
4. Conclusion Brazing of graphite to oxygen-free copper has been realized successfully with (Cu-50TiH2 ) + B composite filler. The effect of boron content in the composite filler on the mechanical property and microstructure of joints has been investigated. The obtained conclusions are as follows:
Fig. 9. SEM images of interfacial area between graphite and filler layer of graphite/Cu joints brazed with (Cu-50TiH2 ) + B composite filler in which the boron content is (a) 5 vol%; (b) 10 vol%; (c) 15 vol% and (d) 20 vol%, respectively.
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1) TiB whiskers are in situ synthesized in the filler layer by the reaction of titanium and boron. The filler layer is mainly composed of Cu based solid solution and Ti-Cu intermetallic compounds with TiB whiskers distributed inside. The distribution of TiB whiskers in the filler layer could serve as reinforcements, contributing to the improvement of graphite/Cu joints. Furthermore, a thin TiC reaction layer is formed at interfacial area between the graphite and the filler layer. 2) The boron content in the composite filler has a considerable effect on the strength of graphite/Cu joints. The shear strength of joints increases with boron content raising from 0 to 15 vol%. The joints brazed with (Cu-50TiH2 ) 15 vol% B composite filler show the maximum shear strength, 19.8 MPa, which is 91.2% of the strength of graphite substrate. However, the strength of joints decreases with boron content higher than 15 vol%. 3) The amount of TiB whiskers synthesized in situ in the filler layer increase with boron content ranging from 5 vol% to 15 vol%. Then, TiB2 with block shape rather than TiB whiskers are formed in the filler layer with boron content higher than 15 vol%, leading to deterioration of the joints. The next step for our research will contain the preparation of tungsten coating on carbon materials and the fabrication of mockups composed of carbon/copper or tungsten coated carbon/copper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 51304148) and the Scientific Research Project under Hubei Provincial Department of Education (No. D20131504). References [1] S. Roccella, E. Cacciotti, D. Candura, A. Mancini, A. Pizzuto, A. Reale, A. Tatì, E. Visca, Fusion Eng. Des. 88 (2013) 1802–1807. [2] N. Sun, Y. Zhang, F. Jiang, S. Lang, M. Xia, Fusion Eng. Des. 89 (2014) 2529–2533. [3] N. Sun, Y. Zhang, S. Lang, F. Jiang, L. Wang, J. Nucl. Mater. 455 (2014) 450–453. [4] N. Ashikawa, N. Asakura, M. Fukumoto, T. Hayashi, Y. Ueda, T. Muroga, J. Nucl. Mater. 438 (Suppl.) (2013) S664–S667. [5] C. Ruset, E. Grigore, I. Munteanu, H. Maier, H. Greuner, C. Hopf, V. Phylipps, G. Matthews, JET-EFDA Contributors, Fusion Eng. Des. 84 (2009) 1662–1665.
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