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Infiltrated tungsten-copper composite reinforced with short tungsten fibers
Vacuum 173 (2020) 109123
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Infiltrated tungsten-copper composite reinforced with short tungsten fibers Longchao Zhuo a, b, *, Bin Luo a, Zhao Zhao a, Yiheng Zhang a, Junlu Zhang a, Shuhua Liang a, **, Nan Liu a, c, Qiuyu Chen a a
School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China c School of Materials Science, Northwestern Polytechnical University, Xi’an, 710072, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal and alloys Tungsten fibers Microstructure EBSD Mechanical properties
To achieve an enhancement of mechanical properties, the WCu composite reinforced with short tungsten fibers was advocated. Characteristics including phase constituents, microstructure and mechanical properties (tensile, compressive and frictional behaviors) of the Wf reinforced WCu composite have been systematically investigated. The magnetron sputtered Ti layer on the surface of Wf effectively improved the sinterability of the composite, leading to excellent electrical conductivity of 56.55% IACS and hardness of 182 HB. Cross sectional EBSD results clearly reveals the sharp <110> fiber texture according to the {110} pole figure of the Wf part, indicating a reserved texture structure although after high-temperature sintering and infiltration, which ensures the fiber strengthening effect. Further tests confirmed the enhancement of mechanical properties including high tensile strength of 549.0 MPa and compressive strength of 1047.8 MPa with large plasticity, as well as low friction coefficient of 0.494. A three dimensional model via numerical simulation further revealed that the smaller the angle between the axial direction of the inserted fibers and the loading direction, the easier it was to bear a higher stress, and the middle part of the fiber was more vulnerable to the accumulated damage.
1. Introduction Due to the high melting point and high strength of tungsten, and the high conductivity and ductility of copper, WCu pseudo-alloys [1,2] have been extensively been considered as the ideal materials for plasma-facing heat-sink materials [3], ultrahigh-voltage contact mate rials in electrical transmission system [4] and guide-rail materials for electromagnetic railgun [5]. With the rapid development of the above fields, substantial studies are currently undertaken to improve the per formance of WCu composites through microstructure tailoring and compositing. For instance, grain refining usually leads to improved mechanical properties [6]; therefore, extensive work has been attemp ted on nano-sized W powders by chemical reactions such as spray dry ing, sol-gel procedure and plasma expansion method [7–9]. However, nano-sized powders usually introduces agglomeration and arch bridge effect, leading to obvious formation of porosity and dramatic reduction of electrical conductivity. On the other hand, introducing second phase of one or two dimensional nanoscale materials acting as reinforcement or for special functional purpose [10–12] has attracted extensive
attention in recent years, e.g. for tungsten-based materials, WC strengthening through chemical reaction of W and introduced alterna tive carbon source of acetylene gas (C2H2) [13], phenol-formaldehyde resin powders [14], graphene [15], CNT [16], and so on. It should also be noticed that the introduction of graphene or CNT of high per formance and cost, would react with tungsten powders to be carbides, losing its original structure and excellent properties. On the other hand, commercial tungsten fibers (Wf) exhibit unique high strength of over 3.0 GPa [17] and ductility, which is resulted from the as-drawn microstructure of elongated grains causing strengthening effect under the load along the fiber axis. Therefore, intensive studies have been conducted to circumvent the brittleness of tungsten by inserting Wf as a reinforcement and ensure pseudo-ductility [18,19]. Specifically, in WCu composite system, various type of tungsten fiber networks is reported to assemble into and reinforce the composite [20, 21]. However, the directional alignment of Wf usually causes strong anisotropic mechanical properties between the loading direction par allel to the plane of Wf network and the one perpendicular to the plane of Wf network, which has been recently attenuated through formation of
* Corresponding author. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China. ** Corresponding author. E-mail addresses: [email protected] (L. Zhuo), [email protected] (S. Liang). https://doi.org/10.1016/j.vacuum.2019.109123 Received 31 July 2019; Received in revised form 3 December 2019; Accepted 6 December 2019 Available online 9 December 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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Vacuum 173 (2020) 109123
Fig. 1. SEM morphologies of Ti-coated tungsten fibers (a), the magnified image with inserted EDS data (b) and elemental distribution by EDS mapping (c, d).
Fig. 2. XRD pattern (a) and SEM image (b) of a polished Wf reinforced WCu composite, with corresponding elemental mapping showing distribution of W (c) and Cu (d). 2
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Fig. 3. Band contrast image (a), Euler angle (b), Grain boundary distribution (c), phase map (d), pole figures of Wf part (e), and pole figures of Wp part of the Wf reinforced WCu composite.
torque gradient by twisting the directional fibers [22]. Here in this work, randomly distributed short Wf have been introduced to the WCu com posite, to evaluate the microstructural change and mechanical enhancement.
X-ray diffraction pattern (XRD) measurements for phase analysis have been executed with a MAXima XRD-7000 with Cu-Ka radiation under continuous scanning at 3� /min. Section microstructure, fractog raphy, X-ray energy dispersive spectroscopy (EDS) analysis and electron backscattering diffraction (EBSD) were carried out on a Zeiss-Merlin field emission scanning electron microscope (SEM) instrument equip ped with an Oxford X-MaxN 50 EDS at the acceleration voltage of 20 kV. Electrical conductivity, Brinell-hardness, density, room temperature tensile and compressive tests, as well as wear resistant properties of the composite have been evaluated. Each test was performed at least three times to ensure the experimental reliability. Samples for tensile test with 20 mm gauge length, 6 mm width and 4 mm thick, and samples for compression with the dimension of 3 mm in diameter and 6 mm in height, were tested under a strain rate of 5 � 10 4 s 1 on Instron-5565 at room temperature. For wear experiment, the test was conducted at the wear time of 120 min, the wear radius of 8 mm, the rotating speed of 80 r/min and the loading of 500 g on a HT-1000 Pin-on-Disk Tester with the wear pin and disk made of the present composite.
2. Materials and methods The initial mixed powders of 85 wt% tungsten (4–6 μm, 99.8%) and 15 wt% copper (50–70 μm, 99.8%) were blended in a V-type mixer for 6 h. Commercial tungsten fibers (Wf) with the diameter of ~10 μm (Fig. 1 (a)) were immersed in 20% HF liquor to remove the surface oxide film. The surface of Wf were magnetron sputtered by Ti target to be ~100 nm thick as a layer to activate the following sinter [23,24] between Wf and its neighboring powders. The content of introduced Wf were 4 wt% and all fibers were cut into the length of 1 mm by an automated cutting machine. To avoid entanglement or winding of these short Wf which was confirmed by mechanical alloying, the method of minus sieve (30 mesh for Wf and 100 mesh for powders) layer by layer was adopted to mix the powders and the short Wf. The resultant mixture was pressed into green compacts with dimensions of ϕ51 � 12 mm under the pressure of 250 MPa for 30 s in a XTM-108-200T Hydraulic Press. After that, the samples were placed in a vacuum furnace with a vacuum degree kept at 5 � 10 3 Pa. Then, hydrogen gas was introduced into the furnace and the samples were sintered at 1350 � C for 2 h, followed by infiltration of Cu-0.5 wt% Cr at 1300 � C for 2 h in hydrogen atmosphere. Finally, the infiltrated samples was conducted under a vacuum degree of 5 � 10 3 Pa by a two stage heat treatment for copper precipitation-hardening. The two stage regime is firstly heating at 1000 � C for 1 h and then aging at 450 � C for 4 h [25]. The temperature precision of the sintering furnace is �5 � C. The metal ingots were machined and polished for later characterization.
3. Results and discussion To provide more detailed information on the short tungsten fibers (Wf) utilized in this work, surface morphologies after magnetron sput tering as well as the EDS results have been illustrated in Fig. 1. The coating layer on the Wf with rugged shape can be clearly observed in Fig. 1(a and b). On spotting on the coated surface of Wf by EDS, the overall statistical composition within the extended micron-sized scale of X-ray [26,27] gives an average value of 99.1%W and 0.9%Ti in weight percentage, as shown in the inset of Fig. 1(b). Further elemental map ping (Fig. 1(c and d)) according to the peaks of Lα1 for W and Kα1 for Ti, 3
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Vacuum 173 (2020) 109123
Fig. 4. Typical tensile stress-strain curve (a), tensile fractography (b, c), compressive stress-strain curve (b), compressive fractography (e, f), friction coefficient curve (h) and worn surface images (i, j) of the Wf reinforced WCu composite.
definitely confirms the uniform distribution of surface Ti coatings after sputtered on the Wf. Fig. 2 presents typical XRD pattern (a) and SEM image (b) with corresponding EDS mapping results of the Wf reinforced WCu compos ite. As shown in Fig. 2(a), all diffraction peaks from W and Cu have been confirmed without any other phase. As shown in Fig. 2(b), besides the hierarchical tungsten powders (Wp) embedded in the continuous copper matrix, it can be clearly observed that partially outcropped Wf oriented rather randomly in the composite, without severe entanglement or agglomeration. As the magnetron sputtered Ti as a sintering activator effectively improved the wetting or adhesion of Wf with its neighboring Wp and Cu, the as-fabricated composite exhibits a relatively dense microstructure resulting in excellent electrical conductivity of 56.55% IACS and hardness of 182 HB, which are much higher than the national standard (GB/T8320-2003: 42 %IACS, 175HB). However, it should also be noticed that, due to the introduction of randomly inserted Wf, the particle arrangement of tungsten powders during sintering would be hindered, resulting in a density value of 13.82 g/cm3, which is relatively lower than the value of 13.95 g/cm3 measured from the corresponding composite without inserted Wf. This is also supported by the detected minor pores (black contrast in Fig. 2(b)). Besides, EDS analysis of W72.38Cu26.36Cr0.19O1.07 for the whole area shown in Fig. 2(b) has confirmed that the contamination of oxygen in small quantities were introduced during fabrication processes. The minor pores and the
contaminated oxygen would contribute to a negative effect to the overall properties, which will also be discussed in the following numerical simulation section combined with experimental results. For a more thorough microstructural understanding on the role of Wf, a local region including Wf with its neighboring Wp and Cu in the Wf reinforced WCu composite was taken for EBSD analysis at a scanning step of 0.4 μm. The constituent phases of Cu (face-centered cubic, space group Fm-3m) and W (body-centered cubic, space group Im-3m) were identified by backscattering diffraction without any other phase exist ing. Fig. 3(a and b) presents the band contrast image and Euler angle distribution from the detected region. In Fig. 3(c), the bold lines depict the high angle grain boundaries (�15� ) including blue ones for tungsten and red ones for copper; while the fine lines sketch the low angle grain boundaries (<15� ). Phase map of red for Cu and blue for W is also presented as shown in Fig. 3(d). Fig. 3(e) and (f) illustrate the {110} pole figure corresponding to respective Wf part and Wp part. In comparison with the Wp part (Fig. 3(f)), a very strong <110> fiber texture can be seen with the maximum intensity of 8.49 in the center region of the {110} pole figure in Fig. 3(e) for Wf part. This confirms a preferential grain orientation with the <110> direction parallel to the fiber axis in Wf part, indicating that the texture formed during the fiber drawing process has been reserved although after high temperature sintering and infiltration over 1300 � C. The recrystallization suppression may result from the introduced Ti, playing a similar role as K in the well-developed 4
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Fig. 5. The representative volume element model of the studied WCu composite with randomly inserted short tungsten fibers before (a) and after meshing (b, c), and as-simulated tensile stress-strain curve in comparison with the experimental result (d), as well as Mises stress distribution of the matrix (e) and the short tungsten fibers (f).
potassium-doped tungsten fibers. Instead of the large equiaxed grains after recrystallization leading to severe degradation of mechanical properties [28], the reserved as-drawn texture of Wf here would guar antee the strengthening effect under the load along the fiber axis [29]; besides, the random distribution of the inserted short Wf would result in an isotropic mechanical property for the fabricated composite. As elaborated by Fig. 4(a), the Wf reinforced WCu composite presents a ultimate tensile stress of 549.0 MPa, which is 5.5% higher than pre viously reported WCu composite of 520.5 MPa [20]; besides, a con spicuous plastic deformation stage with the tensile plasticity of ~10.2% can be achieved. In comparison with the slightly improved tensile property, it should be noted that the compressive strength increased dramatically by 89.1% in comparison with the commercial WCu com posite of 554 MPa [13], with obvious work-hardening behavior of the flow stress until a large plasticity of ~41%. The reason can be still resorted to interfacial strength between Wf and its neighboring matrix, or stress concentration due to local aggregated dispersion of Wf, to which the tensile behavior is more sensitive. Trans-granular cleavage of Wf on the fracture surface, as shown in Fig. 4 (b, c, e, f), clearly presents the uniformly finer grain size and elongated shape in comparison with the Wp in the fabricated composite. Besides the dominant cleavage of the Wf due to load transfer, interfacial debonding of Wf and its neighboring matrix can also be seen, as shown in Fig. 4(c), which explains the slight increase under tensile test in comparison with the dramatic change under compression. It is commonly believed that the phase boundary between W and Cu plays a dominant role in determining the perfor mance of WCu composites [30]. For its service evaluation, wear test of the Wf reinforced WCu composite was also conducted. Fig. 4(h) exhibits the friction coefficient curve of the composite. The composite proceeded into the stable stage of wear with a initial wear of 8 min, and kept steady until 120 min. The average friction coefficient is determined to be 0.494, which is superior to the commercial WCu composite of 0.780 [13]. Fig. 4 (i and j) illustrate the worn surfaces of the composite, without serious grooves, which can be ascribed to the inserted Wf, making a reservation of effective contact points and inhibition of pulling out of the tungsten particles between the pin and the disk during wear. To elucidate the role that the tungsten fibers played during
deformation, a three dimensional model with randomly distributed short tungsten fibers was established (Fig. 5 (a)). For the present nu merical simulation, the dynamic explicit algorithm was selected, with an analysis step of 10 s, a linear bulk viscosity parameter of 0.06, and a time scaling factor of 1. The surface of the randomly distributed short tung sten fibers was set as the master surface, and the inner surface of the WCu matrix was used as the slave surface, the interface between which was calculated using the tie constraint. The loading direction was imposed along the positive direction of X-axis according to the experi mental process. The explicit linear solid element of C3D4 was used to mesh the matrix and fiber network (Fig. 5 (b, c)). As shown in Fig. 5(d), in comparison with the experimental data, the simulated tensile stressstrain curve here presents a slight increase in the stress values. The slight deviation can be attributed to the pores and oxygen contamination introduced during the fabrication processes from initial powders. In the experimental samples instead of a simulation model, the minor pores and oxygen contamination confirmed above by SEM and EDS may act as the crack initiation spots, posing a slight negative effect on the overall phenomenal enhancement from Wf. The highly coincidence despite of the slight deviation in stress level demonstrated the feasibility of the established numerical model. From the simulation data specifically, Mises stress maps of the matrix and inserted short tungsten fibers shown in Fig. 5(e and f) at the macroscopic strain value of ~4%, clearly revealed that the smaller the angle between the axial direction of fibers and the loading direction, the easier it was to bear a higher stress. Also, it can be clearly seen that the middle part of the fiber was more likely to fall prey to damage accumulation. Accordingly, it can be predicted that the well aligned tungsten fibers along the loading direction during ser vice may contribute to further improved mechanical properties, providing a new design pattern for improving the properties of WCu composite materials by reinforcing fibers. 4. Conclusions In this study, novel WCu composite reinforced with randomly distributed short tungsten fibers has been fabricated. Characteristics including phase constituents, microstructure, mechanical properties 5
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including tensile, compressive and frictional behaviors of the Wf rein forced WCu composite have been systematically investigated, and the following conclusions can be drawn:
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Tyson, Tensile properties of fibre-reinforced metals: copper/ tungsten and copper/molybdenum, J. Mech. Phys. Solids 13 (1965) 329–350. [20] L.C. Zhuo, Y.H. Zhang, Q.Y. Chen, S.H. Liang, L. Chen, J.T. Zou, Fabrication and properties of the W-Cu composites reinforced with uncoated and nickel-coated tungsten fibers, Int. J. Refract. Metals Hard Mater. 71 (2018) 175–180. [21] L.C. Zhuo, Y.H. Zhang, Z. Zhao, B. Luo, Q.Y. Chen, S.H. Liang, Preparation and properties of ultrafine-grained W-Cu composites reinforced with tungsten fibers, Mater. Lett. 243 (2019) 26–29. [22] W.Q. Guo, H.T. Jiang, S.W. Wang, M.M. Wan, X.F. Fang, S.W. Tian, Self-sharpening ability enhanced by torque gradient in twisted tungsten-fiber-reinforced Cu-Zn matrix composite, J. Alloy. Comp. 794 (2019) 396–401. [23] B. Yuttanant, Effects of low-content activators on low-temperature sintering of tungsten, J. Mater. Process. Technol. 209 (2009) 4084–4087. [24] C. Ren, M. Koopman, Z. Zak Fang, H. Zhang, B. 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(1) The magnetron sputtered Ti layer on the surface of Wf effectively improved the sintering behavior of the composite, leading to excellent electrical conductivity of 56.55% IACS and hardness of 182 HB. (2) Cross sectional EBSD results clearly reveals the sharp <110> fiber texture according to the {110} pole figure of the Wf part, indicating a strong texture structure although after high tem perature sintering and infiltration, which ensures the fiber strengthening effect. (3) Further mechanical tests confirmed the property enhancement including high tensile strength of 549.0 MPa with large tensile plasticity of ~10.2%, compressive strength of 1047.8 MPa with large compressive plasticity of ~41%, as well as low friction coefficient of 0.494, due to the fact that the inserted Wf rein forcement also act as the resistance for pulling out of the tungsten particles during wear. (4) A three dimensional model via numerical simulation revealed in the WCu composite reinforced with randomly distributed short tungsten fibers that, the smaller the angle between the axial di rection of the inserted fibers and the loading direction, the easier it was to bear a higher stress, and the middle part of the fiber was more vulnerable to the accumulated damage. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements The authors would like to acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51604223 and 51631002), Project funded by China Postdoctoral Science Foundation (2018M641006 and 2019T120931), Innovation Capability Support Program of Shaanxi (2019KJXX-052), Science and Technology Project of Shaanxi Province (2019JM-613), and Key R&D Program of Shaanxi Province (2018ZDXM-GY-070). References [1] F. Delannay, J.-M. Missiaen, Assessment of solid state and liquid phase sintering models by comparison of isothermal densification kinetics in W and W-Cu systems, Acta Mater. 106 (2016) 22–31. [2] Q.Y. Chen, S.H. Liang, F. Wang, L.C. Zhuo, Microstructure transformation after arc erosion of W-30wt.%Cu contact material, Vacuum 149 (2018) 256–261. [3] E. Tejado, A.V. Müller, J.-H. You, J.Y. Pastor, The thermo-mechanical behavior of W-Cu metal matrix composites for fusion heat sink applications: the influence of the Cu content, J. Nucl. Mater. 498 (2018) 468–475. [4] L.C. Zhuo, Z. Zhao, Z.C. Qin, Q.Y. Chen, S.H. Liang, X. Yang, F. Wang, Enhanced mechanical and arc erosion resistant properties by homogenously precipitated nanocrystalline fcc-Nb in the hierarchical W-Nb-Cu composite, Compos. B Eng. 161 (2019) 336–343. [5] S.A. Poniaev, B.I. Reznikov, R.O. Kurakin, P.A. Popov, A.I. Sedov, Y.A. Shustrov, B. G. Zhukov, Prospects of use of electromagnetic railgun as plasma thruster for spacecrafts, Acta Astronaut. 150 (2018) 92–96.
6
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Infiltrated tungsten-copper composite reinforced with short tungsten fibers Longchao Zhuo a, b, *, Bin Luo a, Zhao Zhao a, Yiheng Zhang a, Junlu Zhang a, Shuhua Liang a, **, Nan Liu a, c, Qiuyu Chen a a
School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China National Center for Electron Microscopy in Beijing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China c School of Materials Science, Northwestern Polytechnical University, Xi’an, 710072, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal and alloys Tungsten fibers Microstructure EBSD Mechanical properties
To achieve an enhancement of mechanical properties, the WCu composite reinforced with short tungsten fibers was advocated. Characteristics including phase constituents, microstructure and mechanical properties (tensile, compressive and frictional behaviors) of the Wf reinforced WCu composite have been systematically investigated. The magnetron sputtered Ti layer on the surface of Wf effectively improved the sinterability of the composite, leading to excellent electrical conductivity of 56.55% IACS and hardness of 182 HB. Cross sectional EBSD results clearly reveals the sharp <110> fiber texture according to the {110} pole figure of the Wf part, indicating a reserved texture structure although after high-temperature sintering and infiltration, which ensures the fiber strengthening effect. Further tests confirmed the enhancement of mechanical properties including high tensile strength of 549.0 MPa and compressive strength of 1047.8 MPa with large plasticity, as well as low friction coefficient of 0.494. A three dimensional model via numerical simulation further revealed that the smaller the angle between the axial direction of the inserted fibers and the loading direction, the easier it was to bear a higher stress, and the middle part of the fiber was more vulnerable to the accumulated damage.
1. Introduction Due to the high melting point and high strength of tungsten, and the high conductivity and ductility of copper, WCu pseudo-alloys [1,2] have been extensively been considered as the ideal materials for plasma-facing heat-sink materials [3], ultrahigh-voltage contact mate rials in electrical transmission system [4] and guide-rail materials for electromagnetic railgun [5]. With the rapid development of the above fields, substantial studies are currently undertaken to improve the per formance of WCu composites through microstructure tailoring and compositing. For instance, grain refining usually leads to improved mechanical properties [6]; therefore, extensive work has been attemp ted on nano-sized W powders by chemical reactions such as spray dry ing, sol-gel procedure and plasma expansion method [7–9]. However, nano-sized powders usually introduces agglomeration and arch bridge effect, leading to obvious formation of porosity and dramatic reduction of electrical conductivity. On the other hand, introducing second phase of one or two dimensional nanoscale materials acting as reinforcement or for special functional purpose [10–12] has attracted extensive
attention in recent years, e.g. for tungsten-based materials, WC strengthening through chemical reaction of W and introduced alterna tive carbon source of acetylene gas (C2H2) [13], phenol-formaldehyde resin powders [14], graphene [15], CNT [16], and so on. It should also be noticed that the introduction of graphene or CNT of high per formance and cost, would react with tungsten powders to be carbides, losing its original structure and excellent properties. On the other hand, commercial tungsten fibers (Wf) exhibit unique high strength of over 3.0 GPa [17] and ductility, which is resulted from the as-drawn microstructure of elongated grains causing strengthening effect under the load along the fiber axis. Therefore, intensive studies have been conducted to circumvent the brittleness of tungsten by inserting Wf as a reinforcement and ensure pseudo-ductility [18,19]. Specifically, in WCu composite system, various type of tungsten fiber networks is reported to assemble into and reinforce the composite [20, 21]. However, the directional alignment of Wf usually causes strong anisotropic mechanical properties between the loading direction par allel to the plane of Wf network and the one perpendicular to the plane of Wf network, which has been recently attenuated through formation of
* Corresponding author. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, China. ** Corresponding author. E-mail addresses: [email protected] (L. Zhuo), [email protected] (S. Liang). https://doi.org/10.1016/j.vacuum.2019.109123 Received 31 July 2019; Received in revised form 3 December 2019; Accepted 6 December 2019 Available online 9 December 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
L. Zhuo et al.
Vacuum 173 (2020) 109123
Fig. 1. SEM morphologies of Ti-coated tungsten fibers (a), the magnified image with inserted EDS data (b) and elemental distribution by EDS mapping (c, d).
Fig. 2. XRD pattern (a) and SEM image (b) of a polished Wf reinforced WCu composite, with corresponding elemental mapping showing distribution of W (c) and Cu (d). 2
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Fig. 3. Band contrast image (a), Euler angle (b), Grain boundary distribution (c), phase map (d), pole figures of Wf part (e), and pole figures of Wp part of the Wf reinforced WCu composite.
torque gradient by twisting the directional fibers [22]. Here in this work, randomly distributed short Wf have been introduced to the WCu com posite, to evaluate the microstructural change and mechanical enhancement.
X-ray diffraction pattern (XRD) measurements for phase analysis have been executed with a MAXima XRD-7000 with Cu-Ka radiation under continuous scanning at 3� /min. Section microstructure, fractog raphy, X-ray energy dispersive spectroscopy (EDS) analysis and electron backscattering diffraction (EBSD) were carried out on a Zeiss-Merlin field emission scanning electron microscope (SEM) instrument equip ped with an Oxford X-MaxN 50 EDS at the acceleration voltage of 20 kV. Electrical conductivity, Brinell-hardness, density, room temperature tensile and compressive tests, as well as wear resistant properties of the composite have been evaluated. Each test was performed at least three times to ensure the experimental reliability. Samples for tensile test with 20 mm gauge length, 6 mm width and 4 mm thick, and samples for compression with the dimension of 3 mm in diameter and 6 mm in height, were tested under a strain rate of 5 � 10 4 s 1 on Instron-5565 at room temperature. For wear experiment, the test was conducted at the wear time of 120 min, the wear radius of 8 mm, the rotating speed of 80 r/min and the loading of 500 g on a HT-1000 Pin-on-Disk Tester with the wear pin and disk made of the present composite.
2. Materials and methods The initial mixed powders of 85 wt% tungsten (4–6 μm, 99.8%) and 15 wt% copper (50–70 μm, 99.8%) were blended in a V-type mixer for 6 h. Commercial tungsten fibers (Wf) with the diameter of ~10 μm (Fig. 1 (a)) were immersed in 20% HF liquor to remove the surface oxide film. The surface of Wf were magnetron sputtered by Ti target to be ~100 nm thick as a layer to activate the following sinter [23,24] between Wf and its neighboring powders. The content of introduced Wf were 4 wt% and all fibers were cut into the length of 1 mm by an automated cutting machine. To avoid entanglement or winding of these short Wf which was confirmed by mechanical alloying, the method of minus sieve (30 mesh for Wf and 100 mesh for powders) layer by layer was adopted to mix the powders and the short Wf. The resultant mixture was pressed into green compacts with dimensions of ϕ51 � 12 mm under the pressure of 250 MPa for 30 s in a XTM-108-200T Hydraulic Press. After that, the samples were placed in a vacuum furnace with a vacuum degree kept at 5 � 10 3 Pa. Then, hydrogen gas was introduced into the furnace and the samples were sintered at 1350 � C for 2 h, followed by infiltration of Cu-0.5 wt% Cr at 1300 � C for 2 h in hydrogen atmosphere. Finally, the infiltrated samples was conducted under a vacuum degree of 5 � 10 3 Pa by a two stage heat treatment for copper precipitation-hardening. The two stage regime is firstly heating at 1000 � C for 1 h and then aging at 450 � C for 4 h [25]. The temperature precision of the sintering furnace is �5 � C. The metal ingots were machined and polished for later characterization.
3. Results and discussion To provide more detailed information on the short tungsten fibers (Wf) utilized in this work, surface morphologies after magnetron sput tering as well as the EDS results have been illustrated in Fig. 1. The coating layer on the Wf with rugged shape can be clearly observed in Fig. 1(a and b). On spotting on the coated surface of Wf by EDS, the overall statistical composition within the extended micron-sized scale of X-ray [26,27] gives an average value of 99.1%W and 0.9%Ti in weight percentage, as shown in the inset of Fig. 1(b). Further elemental map ping (Fig. 1(c and d)) according to the peaks of Lα1 for W and Kα1 for Ti, 3
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Fig. 4. Typical tensile stress-strain curve (a), tensile fractography (b, c), compressive stress-strain curve (b), compressive fractography (e, f), friction coefficient curve (h) and worn surface images (i, j) of the Wf reinforced WCu composite.
definitely confirms the uniform distribution of surface Ti coatings after sputtered on the Wf. Fig. 2 presents typical XRD pattern (a) and SEM image (b) with corresponding EDS mapping results of the Wf reinforced WCu compos ite. As shown in Fig. 2(a), all diffraction peaks from W and Cu have been confirmed without any other phase. As shown in Fig. 2(b), besides the hierarchical tungsten powders (Wp) embedded in the continuous copper matrix, it can be clearly observed that partially outcropped Wf oriented rather randomly in the composite, without severe entanglement or agglomeration. As the magnetron sputtered Ti as a sintering activator effectively improved the wetting or adhesion of Wf with its neighboring Wp and Cu, the as-fabricated composite exhibits a relatively dense microstructure resulting in excellent electrical conductivity of 56.55% IACS and hardness of 182 HB, which are much higher than the national standard (GB/T8320-2003: 42 %IACS, 175HB). However, it should also be noticed that, due to the introduction of randomly inserted Wf, the particle arrangement of tungsten powders during sintering would be hindered, resulting in a density value of 13.82 g/cm3, which is relatively lower than the value of 13.95 g/cm3 measured from the corresponding composite without inserted Wf. This is also supported by the detected minor pores (black contrast in Fig. 2(b)). Besides, EDS analysis of W72.38Cu26.36Cr0.19O1.07 for the whole area shown in Fig. 2(b) has confirmed that the contamination of oxygen in small quantities were introduced during fabrication processes. The minor pores and the
contaminated oxygen would contribute to a negative effect to the overall properties, which will also be discussed in the following numerical simulation section combined with experimental results. For a more thorough microstructural understanding on the role of Wf, a local region including Wf with its neighboring Wp and Cu in the Wf reinforced WCu composite was taken for EBSD analysis at a scanning step of 0.4 μm. The constituent phases of Cu (face-centered cubic, space group Fm-3m) and W (body-centered cubic, space group Im-3m) were identified by backscattering diffraction without any other phase exist ing. Fig. 3(a and b) presents the band contrast image and Euler angle distribution from the detected region. In Fig. 3(c), the bold lines depict the high angle grain boundaries (�15� ) including blue ones for tungsten and red ones for copper; while the fine lines sketch the low angle grain boundaries (<15� ). Phase map of red for Cu and blue for W is also presented as shown in Fig. 3(d). Fig. 3(e) and (f) illustrate the {110} pole figure corresponding to respective Wf part and Wp part. In comparison with the Wp part (Fig. 3(f)), a very strong <110> fiber texture can be seen with the maximum intensity of 8.49 in the center region of the {110} pole figure in Fig. 3(e) for Wf part. This confirms a preferential grain orientation with the <110> direction parallel to the fiber axis in Wf part, indicating that the texture formed during the fiber drawing process has been reserved although after high temperature sintering and infiltration over 1300 � C. The recrystallization suppression may result from the introduced Ti, playing a similar role as K in the well-developed 4
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Fig. 5. The representative volume element model of the studied WCu composite with randomly inserted short tungsten fibers before (a) and after meshing (b, c), and as-simulated tensile stress-strain curve in comparison with the experimental result (d), as well as Mises stress distribution of the matrix (e) and the short tungsten fibers (f).
potassium-doped tungsten fibers. Instead of the large equiaxed grains after recrystallization leading to severe degradation of mechanical properties [28], the reserved as-drawn texture of Wf here would guar antee the strengthening effect under the load along the fiber axis [29]; besides, the random distribution of the inserted short Wf would result in an isotropic mechanical property for the fabricated composite. As elaborated by Fig. 4(a), the Wf reinforced WCu composite presents a ultimate tensile stress of 549.0 MPa, which is 5.5% higher than pre viously reported WCu composite of 520.5 MPa [20]; besides, a con spicuous plastic deformation stage with the tensile plasticity of ~10.2% can be achieved. In comparison with the slightly improved tensile property, it should be noted that the compressive strength increased dramatically by 89.1% in comparison with the commercial WCu com posite of 554 MPa [13], with obvious work-hardening behavior of the flow stress until a large plasticity of ~41%. The reason can be still resorted to interfacial strength between Wf and its neighboring matrix, or stress concentration due to local aggregated dispersion of Wf, to which the tensile behavior is more sensitive. Trans-granular cleavage of Wf on the fracture surface, as shown in Fig. 4 (b, c, e, f), clearly presents the uniformly finer grain size and elongated shape in comparison with the Wp in the fabricated composite. Besides the dominant cleavage of the Wf due to load transfer, interfacial debonding of Wf and its neighboring matrix can also be seen, as shown in Fig. 4(c), which explains the slight increase under tensile test in comparison with the dramatic change under compression. It is commonly believed that the phase boundary between W and Cu plays a dominant role in determining the perfor mance of WCu composites [30]. For its service evaluation, wear test of the Wf reinforced WCu composite was also conducted. Fig. 4(h) exhibits the friction coefficient curve of the composite. The composite proceeded into the stable stage of wear with a initial wear of 8 min, and kept steady until 120 min. The average friction coefficient is determined to be 0.494, which is superior to the commercial WCu composite of 0.780 [13]. Fig. 4 (i and j) illustrate the worn surfaces of the composite, without serious grooves, which can be ascribed to the inserted Wf, making a reservation of effective contact points and inhibition of pulling out of the tungsten particles between the pin and the disk during wear. To elucidate the role that the tungsten fibers played during
deformation, a three dimensional model with randomly distributed short tungsten fibers was established (Fig. 5 (a)). For the present nu merical simulation, the dynamic explicit algorithm was selected, with an analysis step of 10 s, a linear bulk viscosity parameter of 0.06, and a time scaling factor of 1. The surface of the randomly distributed short tung sten fibers was set as the master surface, and the inner surface of the WCu matrix was used as the slave surface, the interface between which was calculated using the tie constraint. The loading direction was imposed along the positive direction of X-axis according to the experi mental process. The explicit linear solid element of C3D4 was used to mesh the matrix and fiber network (Fig. 5 (b, c)). As shown in Fig. 5(d), in comparison with the experimental data, the simulated tensile stressstrain curve here presents a slight increase in the stress values. The slight deviation can be attributed to the pores and oxygen contamination introduced during the fabrication processes from initial powders. In the experimental samples instead of a simulation model, the minor pores and oxygen contamination confirmed above by SEM and EDS may act as the crack initiation spots, posing a slight negative effect on the overall phenomenal enhancement from Wf. The highly coincidence despite of the slight deviation in stress level demonstrated the feasibility of the established numerical model. From the simulation data specifically, Mises stress maps of the matrix and inserted short tungsten fibers shown in Fig. 5(e and f) at the macroscopic strain value of ~4%, clearly revealed that the smaller the angle between the axial direction of fibers and the loading direction, the easier it was to bear a higher stress. Also, it can be clearly seen that the middle part of the fiber was more likely to fall prey to damage accumulation. Accordingly, it can be predicted that the well aligned tungsten fibers along the loading direction during ser vice may contribute to further improved mechanical properties, providing a new design pattern for improving the properties of WCu composite materials by reinforcing fibers. 4. Conclusions In this study, novel WCu composite reinforced with randomly distributed short tungsten fibers has been fabricated. Characteristics including phase constituents, microstructure, mechanical properties 5
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including tensile, compressive and frictional behaviors of the Wf rein forced WCu composite have been systematically investigated, and the following conclusions can be drawn:
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(1) The magnetron sputtered Ti layer on the surface of Wf effectively improved the sintering behavior of the composite, leading to excellent electrical conductivity of 56.55% IACS and hardness of 182 HB. (2) Cross sectional EBSD results clearly reveals the sharp <110> fiber texture according to the {110} pole figure of the Wf part, indicating a strong texture structure although after high tem perature sintering and infiltration, which ensures the fiber strengthening effect. (3) Further mechanical tests confirmed the property enhancement including high tensile strength of 549.0 MPa with large tensile plasticity of ~10.2%, compressive strength of 1047.8 MPa with large compressive plasticity of ~41%, as well as low friction coefficient of 0.494, due to the fact that the inserted Wf rein forcement also act as the resistance for pulling out of the tungsten particles during wear. (4) A three dimensional model via numerical simulation revealed in the WCu composite reinforced with randomly distributed short tungsten fibers that, the smaller the angle between the axial di rection of the inserted fibers and the loading direction, the easier it was to bear a higher stress, and the middle part of the fiber was more vulnerable to the accumulated damage. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements The authors would like to acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51604223 and 51631002), Project funded by China Postdoctoral Science Foundation (2018M641006 and 2019T120931), Innovation Capability Support Program of Shaanxi (2019KJXX-052), Science and Technology Project of Shaanxi Province (2019JM-613), and Key R&D Program of Shaanxi Province (2018ZDXM-GY-070). References [1] F. Delannay, J.-M. Missiaen, Assessment of solid state and liquid phase sintering models by comparison of isothermal densification kinetics in W and W-Cu systems, Acta Mater. 106 (2016) 22–31. [2] Q.Y. Chen, S.H. Liang, F. Wang, L.C. Zhuo, Microstructure transformation after arc erosion of W-30wt.%Cu contact material, Vacuum 149 (2018) 256–261. [3] E. Tejado, A.V. Müller, J.-H. You, J.Y. Pastor, The thermo-mechanical behavior of W-Cu metal matrix composites for fusion heat sink applications: the influence of the Cu content, J. Nucl. Mater. 498 (2018) 468–475. [4] L.C. Zhuo, Z. Zhao, Z.C. Qin, Q.Y. Chen, S.H. Liang, X. Yang, F. Wang, Enhanced mechanical and arc erosion resistant properties by homogenously precipitated nanocrystalline fcc-Nb in the hierarchical W-Nb-Cu composite, Compos. B Eng. 161 (2019) 336–343. [5] S.A. Poniaev, B.I. Reznikov, R.O. Kurakin, P.A. Popov, A.I. Sedov, Y.A. Shustrov, B. G. Zhukov, Prospects of use of electromagnetic railgun as plasma thruster for spacecrafts, Acta Astronaut. 150 (2018) 92–96.
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