- Email: [email protected]
CuCr integral materials
Journal of Alloys and Compounds 686 (2016) 648e655
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of Cr alloying interlayer on the interfacial bond strength of CuW/ CuCr integral materials Xiaohong Yang*, Xuejian Li, Zhe Xiao, Juntao Zou, Shuhua Liang Shaanxi Key Laboratory of Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2015 Received in revised form 28 May 2016 Accepted 4 June 2016 Available online 4 June 2016
The effects of Cr alloying interlayer on the microstructure and mechanical properties of the dissimilar CuW/CuCr joints prepared by sintering-infiltration method were studied. The hardness and electrical conductivity of the CuCr alloy side and the tensile strength of the CuW/CuCr integral materials were tested. The interfacial microstructure and fracture morphology of the CuW/CuCr joints were characterized by X-ray diffraction, optical microscopy, scanning electron microscope equipped with energy dispersive spectrometry. The results show that both the electrical conductivity and hardness of the CuCr alloy side of the aged CuW/CuCr joints are higher than those in the solid solution and infiltration condition. With increase of the Cr content, the regions of eutectic phase are expanded gradually on the CuCr alloy side, and the morphology of Cr particles changes from spot shape to rod-like shape. The highest interfacial bond strength can reach up to 446.66 MPa when Cue15 wt%Cr alloy is introduced into the interfacial region between CuW and CueCr alloy. The interfacial fracture morphology presents more cleavage fractures of W particles, and the Cu/W interphase has a good metallurgical bond. © 2016 Elsevier B.V. All rights reserved.
Keywords: CuW/CuCr dissimilar materials Alloying interlayer Interface Bond strength Hardness
1. Introduction CuW/CuCr integral materials are widely used in high-voltage electric contacts, vacuum load switches and transformer switches [1e4]. These functional-structural integrated materials are usually jointed by CuW pseudo alloy with excellent resistance to arc erosion and CuCr alloy with high electrical and thermal conductivity [3e5]. During switching on & off operations, CuW/CuCr integral materials are subjected to the huge heat generated by arc on the tip of CuW contact, and the mechanical force produced when the moving part and the fixed part are contacted and separated. The CuW/CuCr interface endures high thermal and mechanical stresses when exposed to high heat and mechanical loads. Therefore, the reliable joints between CueW composites and CueCr alloys are of practical importance for electrical contact applications [3]. With development of the ultra-high voltage power transmission, it is required that the larger capacity and high frequency switching for electrical appliances. At this case, the contact materials are subjected to more severe working conditions, giving rise to the possibility of the separation of CuW contact from the integral
* Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.jallcom.2016.06.016 0925-8388/© 2016 Elsevier B.V. All rights reserved.
materials. This poses a challenge for the CuW/CuCr integral contact materials. Higher interfacial bond strength of integral CuW/CuCr electrical contacts is necessary to ensure reliability and longer service life [2,3]. Due to the mutual insolubility between W and Cu, no other phase form under the equilibrium condition, thus leading to the low densification and interfacial bond strength [6e9]. Because of the large difference in their coefficients of thermal expansion and elasticity modulus [10,11], high thermal and mechanical stresses can induce microcracks at the Cu/W interface, and resulting in the failure of the integral material along CuW/CuCr joints. Moreover, the Cr content on the CuCr alloy side is usually below 1 wt% since Cr has a very low solubility in the copper [12,13], whereas only a very small amount of W solid solution can be formed in the CuW/CuCr joint in which Cr is dissolved into W skeletons during the integral infiltration. Subsequently, it is hard to enhance the interfacial bond strength of the CuW/CuCr dissimilar materials. Currently, numerous researches are focused on the fabrication methods and properties of CuW pseudo alloy [14e18]. Ren et al. [14] fabricated Cu90W10 nanocomposites by combination of mechanical alloying and warm pressing, and the nanostructured CueW alloys demonstrate the excellent stability at high temperature. Elsayed et al. [15] reported that the WeCu nanocomposite with electroless Cu coating
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
on the nano-size W particles fabricated by spark plasma sintering, present fine homogeneous microstructure and better mechanical properties. Chen et al. [16,17] found that the small addition of Zn into the CueW alloy can improve mechanical properties and thermal and electrical conductivity. Wang et al. [18] revealed the substitution of W atom by Zr significantly increases the Cu/W interface strength and stability through first principle calculation. So far, few literature have been reported on the interfacial bond strength of CuW/CuCr integral materials whereas the interfacial bond strength of dissimilar materials has been widely studied [19e23]. It is believed that the bond strength can be enhanced by introducing an interlayer into the joints. Brochu et al. [19] jointed the Cu to the CueW composite using an Al foil interlayer through transient liquid phase bonding technique, and the mechanical properties of the interface increase owing to the complete dissolution and diffusion of the Al interlayer into the Cu substrate and composite. Zakipour et al. [20] have investigated transient liquid phase bonding mechanism of two dissimilar alloys stainless steel 316L and Tie6Ale4V using pure Cu interlayer, and found the maximum shear strength of the bond obtains at 900 C. With the rise in bonding temperature to 960 C, a reduction in bond strength occurs attributed to increase in width of joint zone and formation of more brittle intermetallic compounds at the interface. Barrena et al. [21] used AgeCueTi alloy as interlayer to join WCeCo composite and Ti6Al4V alloy by a diffusion bonding process, and found both the hardness and the shear resistance of the joints depend on the intermetallic compounds formed during the diffusion bonding. Jafarlou et al. [22] applied the equal channel angular pressing to join aluminum alloy and steel using an AgeCueSn interlayer at different annealing temperatures, and found that the introduction of an interlayer can lead to higher shear strength and the joint strength exhibits improvement as well with increasing annealing temperature. Wang et al. [23] jointed W/Cu with amorphous FeeW coatings electrodeposited onto the Cu foils by vacuum diffusion bonding, and found that the bonding strength of W/Cu improves and the bonding residual stress decreases. As mentioned above, the introduction of an interlayer can promote the mutual dissolution and diffusion among alloying elements, and, thus, improves the interfacial bond strength of dissimilar materials. In our previous work, Cue5.0 wt%Fe alloy interlayer can enhance the interfacial strength of CuW/CuCr integrated material. However, the W skeletons near the interface are eroded severely if Fe content is beyond 5.0 wt%, giving rise to reduction on the interfacial strength of the CuW/CuCr integrated material [3]. In the present work, the CueCr alloying interlayers with higher Cr contents were introduced into the CuW/CuCr bond interface, and the CuW/CuCr integral materials were prepared by the integral infiltration technology. Since Cr can be entirely dissolved into W at elevated temperatures [12], more WeCr solid solution form on the surface of W skeletons at the CuW/CuCr joints during the infiltration, which is beneficial for the improvement on the bond strength of the CuW/CuCr dissimilar materials. In present work, the effect of Cr addition on the interfacial properties of the CuW/CuCr dissimilar materials was studied. The interfacial microstructures were characterized, and the electrical conductivity and mechanical properties were measured. The purpose is to clarify the microstructural evolutions and the interfacial strengthening mechanisms. The research can also provide references for the manufacture of other dissimilar materials.
a V-type mixing machine for 6 h. The mix was compacted into an alloying interlayer with a thickness of 2 mm in a steel mold under a pressure of 340 MPa at room temperature. The Cue70 wt%W pseudo alloy and Cue0.89 wt%Cr alloy were machined into cylindrical, followed by washing with acetone. After the alloying interlayers were cleaned with alcohol, the CuW pseudo alloy, the alloying interlayer and CueCr alloy were put in the graphite crucible from bottom to top. All integral samples were placed in an electrical resistivity furnace. The air in the furnace was removed using hydrogen gas for 40 min, and nitrogen atmosphere was subsequently filled. The samples were heated at a rate of 15 C/min till 1350 C, holding for 1 h, and cooled in the furnace. All integral samples were first solutionized at 980 C for 1 h, and subsequently quenched in the water, followed by aging treatment at 450 C for 4 h. The electrical conductivity of CuCr alloy was measured by a FQR-7501 eddy conductivity gauge. The hardness was determined on an HB-3000 hardness at a load of 250 kgf for 30 s, and the results were the average of five measured values on the CuCr alloy side near the interface. X-ray diffractometer (XRD) with a Cu Ka radiation was used to identify the phase constitutions of CuCr alloy side. The CuW/CuCr joint samples with different alloying interlayers were machined by the electrical discharge, and the cross-sections perpendicular to the interface were prepared for metallographic examination. These samples were etched by the erosion solution (FeCl3þHClþH2O), and the interfacial microstructures were observed by optical microscope (OM) and scanning electron microscope (SEM). The compositional definition around the interface was analyzed by the energy dispersive spectrometry (EDS). The interfacial tensile properties of the CuW/CuCr integral materials with different alloying interlayers were determined using an Instron-testing machine HT-2402-100 KN with a cross load speed of 0.5 mm/min. To ensure the fracture occurred at the CuW/CuCr interface and obtain the interfacial tensile strength, the integral materials were machined into a ladder test bar, as shown in Fig. 1. The left side is CueW composite, and the right side is CuCr alloy. The interfacial fracture morphologies were characterized by a JSM6700F scanning electron microscope. 3. Results and discussion 3.1. Effect of Cr content on the electrical conductivity of CuCr alloy side The electrical conductivity of CuCr alloy side for the CuW/CuCr integral materials with the different Cr interlayers after infiltration, solid solution and aging were measured. Fig. 2 shows the variations of electrical conductivity of CuCr alloy side with Cr content. It can be seen that the electrical conductivity after solutionizing decreases with increase of the Cr content ranging from 5 to 15 wt%, and then almost no change occurs above 15 wt%Cr. This can be possibly attributed to the increase of the segregated Cr-rich primary particle
2. Experimental procedure The starting materials are Cu powders (purity > 99.9%), and Cr powders (purity > 99.7%). The Cu powders were mixed with the Cr powders at the Cu/Cr mass ratio of 95:5, 85:15, 75:25, and 65:35 in
649
Fig. 1. Shape and dimensions of ladder test bar (mm).
650
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 2. Variation of the electrical conductivity of CuCr alloy side with different Cr content.
and Cu þ Cr eutectic phase. Except for the diffusion toward the CuW side, element Cr in the alloying interlayer can diffuse into CuCr alloy side during infiltration. If the Cr content in the CuCr alloy is above the solubility limit of Cr into the Cu matrix, more Cu þ Cr eutectic colonies form with increase of Cr content during solidification, thus resulting in the larger electron scattering induced by the internal Cu/Cr interface [24]. The electrical conductivity of CueCr alloy side for the aged CuW/CuCr integral materials increases significantly in comparison with that in the infiltrated and solutionized integral materials. According to the theory of electrical conductivity [25,26], except for the electron scattering induced by phase interfaces, the solid solution of alloying elements in the Cu matrix is another factor to effect the electrical conductivity of the alloys. The solute atoms in matrix cause the lattice imperfections, and, thus, hinder the electron movement [27,28]. During aging treatment, the Cr particles precipitate from the Cu matrix, which decreases the lattice distortion and solute scattering [29]. Though the interface enhance electron scattering during aging, the solute atoms have more deterioration on the electron transmission [30]. Fig. 3 is the XRD patterns of CuCr alloy side with 5% alloying interlayer at different
conditions. It can be seen from Fig. 3 that only Cu diffraction peaks present. Compared with diffraction peaks at the aging condition, the relative peaks move to the left and the intensities decrease at the solid solution condition. In addition, the corresponding interplanar spacing is given in Table 1 calculated by Bragg equation d ¼ nl=2sinq; where l is the wavelength of X-rays, l ¼ 0.154056 nm; q is the diffraction angle. It can be seen that the interplanar spacing of different diffraction peaks at the solid solution are larger than those in the aging condition. And the lattice parameters are determined pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi by the formula, a ¼ d* ðh2 þ k2 þ l2 Þ, where a is the lattice parameter; d is the interplanar crystal spacing; h, k, l are the miller indices [31,32]. The relative deviation can be calculated in accordance with the formula, Da ¼ ða a0 Þ=a0 , where a0 is the lattice parameter of the Cu perfect crystal at room temperature, a0 ¼ 0.36150 nm. Therefore, the lattice parameters and the relative deviations at different conditions are listed in Table 2. It is learnt that the lattice parameters and the relative deviations of copper matrix at the solid solution are also larger than those in the aging condition. As above, the XRD results indicate that the lattice distortion does exist in CuCr alloy side at the solid solution condition, which is derived from the dissolvement of Cr atoms into the Cu matrix. Hence, the increase in electrical conductivity of aged CuCr alloy can be attributed to the decrease of lattice distortion.
3.2. Effect of Cr content on the hardness of CuCr alloy side The interfacial strength of CuW/CuCr integral materials is closely related to the mechanical properties of both side materials. CuCr alloy with less than 1.2 wt% Cr content is a typical agehardening alloy, and the mechanical property of CuCr alloy changes with the Cr content and the heat treatment conditions. When the alloying interlayer is introduced into the CuW/CuCr interface, the composition of CuCr alloy side changes correspondingly during infiltration. The variation of the hardness of CuCr alloy side near the CuW/CuCr interface with Cr content under infiltration, solid solution and aging condition is shown in Fig. 4. In the range of 5 wt%e15 wt% Cr, the hardness increases with Cr content at the aging condition. However, the hardness decreases when the Cr content is above 15 wt%. Furthermore, compared with the solid solution and infiltration condition, the aged CuCr alloy exhibits the highest hardness. The main reason is that the dispersive secondary Cr particles precipitate from the Cu matrix, giving rise to the enhancement on the hardness. However, when the Cr content is above 15 wt%, except for the diffusion of element Cr into the CuW side, Cr concentration can be diluted by the liquid Cue0.89%Cr alloy during infiltrating. If the Cr concentration in the CuCr alloy is beyond the maximum solubility of Cr in the Cu matrix during infiltration, resulting in the formation of the eutectic phase in the consequent solidification process. Generally, the increased Cr content in the CuCr alloy causes the coarsening of the Cu þ Cr eutectic colonies and increase of the amount of eutectic colonies, which isolate the continuous copper matrix, resulting in the decrease in hardness of CuCr alloy.
Table 1 The interplanar spacing of different diffraction peaks at different conditions in Fig. 3. Cu peak
Fig. 3. XRD patterns of CuCr alloy side with 5% Cr alloy interlayer at different conditions.
Cu (111) Cu (200) Cu (220)
Interplanar spacing (Å) Solid condition
Aging condition
2.0990 1.8098 1.2826
2.0891 1.8077 1.2786
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655 Table 2 Lattice parameters and relative deviation of different diffraction peaks at different conditions in Fig. 3. Cu peak
Lattice parameters (Å)
Relative deviation Da (%) Da ¼ (aaa0)/a0
Solid condition
Aging condition
Solid condition
Aging condition
Cu(111) Cu(200) Cu(220)
3.6354 3.6196 3.6272
3.6183 3.6154 3.6159
0.56 0.13 0.34
0.09 0.01 0.02
a
Data from the standard PDF cards, a0 ¼ 3.6150 Å.
Fig. 4. Variation of the hardness of CuCr alloy side with different Cr content.
651
3.3. Microstructure of CuW/CuCr interface and CuCr alloy side Fig. 5 shows the optical micrographs of the CuW/CuCr interface with different alloying interlayers. It can be seen that the CueCr alloying interlayers introduced into the CuW/CuCr interface completely dissolve into the both sides, and no residue Cr powder particles occur. The microstructure of CuCr alloy side is composed of the Cu matrix and the Cu þ Cr eutectics. With increase of the Cr content in the alloying interlayer, the distributed zone of Cu þ Cr eutectics increases, leading to the decrease in the zone of copper matrix, see Fig. 5(a)e(d). To further examine the morphology of element Cr in the eutectic phase, the microstructure of the CuCr alloy side near the CuW/CuCr interface were magnified as shown in Fig. 6. It can be seen from Fig. 6(a) that small amounts of dotted Cr particles are distributed among the Cu dendritic arms. When the Cr content is 15 wt%, the zone of Cu þ Cr eutectic colonies is widen, the spherical Cr particles present in the eutectics, and the amount of Cr particles increases as well, see Fig. 6(b). However, as shown in Fig. 6(c), with increase of Cr content, the zone of eutectics become larger, and the morphology of Cr particles evolves from sphere to rod-like shape. At 35 wt%Cr, the Cr particles in the eutectics become coarsening with the presence of longer rod, and the zone of eutectics are expanded significantly, see Fig. 6(d). To clarify the elemental distributions at the CuW/CuCr interface, the CuW/CuCr integral material with Cue25 wt%Cr alloy interlayer was selected, and the line scanning was conducted by the electron probe micro analyzer (EPMA) attached to SEM. The results are presented in Fig. 7. As seen from Fig. 7(a), the interface between CuW and CuCr alloys is clean and no pores generate, a dark transition layer appears at the boundaries of bright gray W particles because the element Cr is dissolved into the W skeletons, as marked by the white arrow. From Fig. 7(b), it is found that the phenomenon of mutual diffusion and dissolution among various elements occurs at the interface, element Cr is diffused into both sides. Moreover,
Fig. 5. Interface microstructure of CuW/CuCr materials introduced different alloying interlayers at the aging condition (a) Cue5wt%Cr (b) Cue15 wt%Cr (c) Cue25 wt%Cr (d) Cue35 wt%Cr.
652
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 6. Microstructure of CuCr alloy with different alloying interlayers at the aging condition (a) Cue5wt%Cr (b) Cue15 wt%Cr (c) Cue25 wt%Cr (d) Cue35 wt%Cr.
Fig. 7. Line scanning of CuW/CuCr interfacial regions with the Cue25 wt%Cr alloy interlayer (a) secondary electron image (b) distribution of the elements.
the Cr content in the dark W solid solution layer at the interface is larger than that in other regions. It indicates that the element Cr is easier to be dissolved into W phase than that into the Cu matrix. 3.4. The interfacial tensile strength of CuW/CuCr integral materials and fracture morphology The tensile bars of CuW/CuCr integral materials with different alloying interlayers were fabricated by the same sinteringinfiltration method. After the same solution and aging treatment, the interfacial tensile strength of CuW/CuCr integral materials was tested, and the stress-strain curves are shown in Fig. 8. It can be seen that the interfacial bond strength of the CuW/CuCr integral material enhances when the element Cr is introduced into the CuW/CuCr interface. When the Cr content in the alloying
interlayers are below 15 wt%, the interfacial tensile strength of CuW/CuCr materials increases with increase of Cr content. However, the tensile strength decreases when the Cr content is above 15 wt%. The interfacial strength of the CuW/CuCr materials has the maximum value of 446.66 MPa at 15 wt%Cr, which is increased by 31.74% compared with that without alloying interlayer. Combining with the hardness of CuCr alloy side in Fig. 4, it is inferred that the diffusion and dissolution of the Cr alloying interlayer into both sides are beneficial for the improvement on the mechanical properties of CuW/CuCr interface. Especially, both the interfacial strength and hardness of CuCr alloy side reach the maximum values when the Cr content is 15 wt%. Compared with our previous results [3], the interfacial bond strength of CuW/CuCr obtained at the present work is above 110 MPa than that of CuW/CuCr with CueFe alloying interlayer. It suggests that the element Cr has more
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 8. Tensile stress-strain curves of integral materials with different Cr alloying interlayers.
contribution on the enhancement of the interfacial mechanical properties than element Fe. Fig. 9 shows the tensile fracture morphologies of the CuW/CuCr integral materials with different alloying interlayers. It can be seen from Fig. 9(a) that the fracture is consisted of the ductile fracture area and the larger flat area. A small amount of copper phase in both sides exhibits the ductile fracture, and the reticulated ductile
653
tearing ridges generate. The formation of smooth flat area reveals that the CuW and CuCr alloys are separated from each other easily at the interface, and the interfacial bond is weak. Nevertheless, when the 15 wt%Cr alloying interlayer was introduced into the CuW/CuCr interface, there is few flat characteristic area, and a few of smaller tungsten particles are pulled out and left in the deeper dimples due to debonding of Cu/W interphase as shown in Fig. 9(b). A number of tungsten particles exhibit the transgranular cleavage fracture. It indicates that the fracture tends to move toward the CuW side, and the interfacial bond strength of CuW/CuCr is improved. For the CuW/CuCr integral material with Cue25 wt%Cr interlayer, it is found that a few cleavage fractures occur on small amounts of larger tungsten particles, and numberous fine tungsten particles segregate from W skeletons and fall into dimples as shown in Fig. 9(c). It reveals that the W skeletons at the CuW/CuCr interface are dissolved and eroded by excessive element Cr during infiltration, and, thus, some W particles fragment and fall from the tungsten skeletons. Subsequently, the discontinuous W skeletons cannot bear the tensile load effectively, thus resulting in the decrease of interfacial bond strength. To get more insights of the strengthening and failure mechanisms of the CuW/CuCr interface after introducing the different alloying interlayers, as illustrated in Fig. 10, the chemical compositions of the ductile tearing ridges and the transgranular cleavage of W particles in the fracture morphologies were determined by EDS, and the results are listed in Table 3. It can be seen that the content of element Cr dissolved into W particles increases with increase of the Cr content in the alloying interlayer, and the content of Cr in the tearing ridges of Cu phase increases as well. It can be inferred that element Cr in the alloying interlayers diffuses and
Fig. 9. Fracture morphologies of the CuW/CuCr integral materials with different Cr contents (a) Without alloying interlayer (b) 15 wt%Cr (c) 25 wt%Cr.
654
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
fraction of cleavage fracture increase, and the fracture moves toward CuW side with increase of Cr content ranging from 5 wt% to 15 wt%. Moreover, both the Cr contents of the CreW solid solution in which the transgranular cleavage occurs and at the ductile copper phase tearing ridges increase, and the metallurgical bond is achieved at the Cu/W interphase. Acknowledgements This work was supported by the National Natural Science Foundation of China (51201132), National High-Tech Program (863) of China (No. 2015AA034304), the Science and Technique Innovation Program of Shaanxi Province for Key Laboratory (2014SZS08K02), the Research of Science and Technology of the Shaanxi Provincial Education Department for Key Laboratory (14JS065), and the Shaanxi Provincial Project of the Special Foundation of Key Disciplines. References Fig. 10. EDS analysis points in the SEM fracture image.
Table 3 EDS analysis results in the CuW/CuCr interfacial tensile fractures of CuW/CuCr materials introduced different Cr alloying interlayers. Different regions
Cr contents (wt%) Without interlayer
15 wt%Cr
25 wt%Cr
Ductile tearing ridges W particles
0.24 0.65
0.61 10.27
3.53 27.45
dissolves across the CuW/CuCr interface during infiltration, and the CreW solid solution forms at the CuW/CuCr interface. Consequently, more Cu/W interphases at the CuW/CuCr interface achieve the metallurgical bond by introducing higher content Cr. Compared with the initial mechanical bond, the bonding strength of the Cu/W interphase is enhanced remarkably. Therefore, when the tensile load apply at the CuW/CuCr interface, the load transfers from the weaker copper phase to the stronger W phase with higher elastic modulus via Cu/W interphase. In addition, the micro-cracks cannot propagate along the Cu/W interphase, leading to the better interfacial bond strength of the CuW/CuCr materials.
4. Conclusions (1) With the introduction of alloying interlayer into the CuW/ CuCr interface, the electrical conductivity and hardness of the CuCr alloy side at aging condition are higher than those at the infiltration and solid solution condition. The increased Cr content in the alloying interlayer expands the region of Cu þ Cr eutectics in the CuCr alloy sides near the interface, and the morphology of Cr particles in eutectics evolves from sphere to the rod-like shape. Furthermore, a solid solution layer forms at the boundaries of W skeletons at the CuW/ CuCr interface, and it is much easier for element Cr to dissolve into W than that into Cu. (2) The interfacial strength of CuW/CuCr integral materials can reach up to 446.66 MPa when the Cr content is 15 wt% in the alloying interlayer. Compared with the CuW/CuCr material without alloying interlayer, the interfacial strength is increased by 31.74%. The fracture morphologies of the CuW/ CuCr integral materials are consisted of the ductile fracture of Cu phase and the cleavage fracture of W particles. The
[1] S. Liang, L. Chen, Z. Yuan, Y. Li, J. Zou, P. Xiao, Infiltrated W-Cu composites with combined architecture of hierarchical particulate tungsten and tungsten fibers, Mater. Charact. 110 (2015) 33e38. [2] Q. Zhang, S. Liang, X. Yang, Z. Li, Y. Li, Y. Zhang, Failure analysis of CuW/CuCrZr contact materials in capacitor bank switch, Eng. Fail. Anal. 62 (2016) 156e163. [3] X. Yang, J. Zou, S. Liang, P. Xiao, Z. Fan, Effect of alloying interlayer on interfacial bond strength of CuW/CuCr integral materials, Mater. Sci. Eng. A 527 (2010) 5631e5636. [4] X. Yang, J. Zou, P. Xiao, X. Wang, Effects of Zr addition on properties and vacuum arc characteristics of Cu-W alloy, Vacuum 106 (2014) 16e20. [5] Q. Zhang, S. Liang, J. Zou, Q. Yang, Interfacial microstructure of CuCr/ 1Cr18Ni9Ti bi-metal materials and its effect on bonding strength, Sci. China Tech. Sci. 58 (2015) 825e831. [6] L. Xu, M. Yan, Y. Xia, J. Peng, W. Li, L. Zhang, C. Liu, G. Chen, Influence of copper content on the property of Cu-W alloy prepared by microwave vacuum infiltration sintering, J. Alloys Compd. 592 (2014) 202e206. [7] X. Wei, J. Tang, N. Ye, H. Zhuo, A novel preparation method for W-Cu composite powders, J. Alloys Compd. 661 (2016) 471e475. [8] Y. Kong, J. Pi, S. Xiao, Y. Du, Asymmetric mixing behavior and stability of the predicted phases in the W-Cu system, CALPHAD 53 (2016) 116e121. [9] Q. Zhou, P. Chen, Fabrication of W-Cu composite by shock consolidation of Cucoated W powders, J. Alloys Compd. 657 (2016) 215e223. [10] P. Zhao, S.B. Guo, G.H. Liu, Y.X. Chen, J.T. Li, Fabrication of WeCu functionally graded material with improved mechanical strength, J. Alloys Compd. 601 (2014) 289e292. [11] Y.F. Meng, J.P. Zhang, C.Y. Duan, C. Chen, X.M. Feng, Y.F. Shen, Microstructures and properties of WeCu functionally graded composite coatings on copper substrate via high-energy mechanical alloying method, Adv. Powder Technol. 26 (2015) 392e400. [12] T.B. Massal, P.R. Subramanian, H. Okamoto, L. kacprzak, Alloy phase diagrams, ASM Int. 3 (1992). [13] M. Bizjak, B. Karpe, G. Jaksa, J. Kova c, Surface precipitation of chromium in rapidly solidified Cu-Cr alloys, Appl. Surf. Sci. 277 (2013) 83e87. [14] F.Z. Ren, W.W. Zhu, K.J. Chu, C.C. Zhao, Tribological and corrosion behaviors of bulk Cu-W nanocomposites fabricated by mechanical alloying and warm pressing, J. Alloys Compd. 676 (2016) 164e172. [15] A. Elsayed, W. Li, O.A. El Kady, Experimental investigations on the synthesis of W-Cu nanocomposite through spark plasma sintering, J. Alloys Compd. 639 (2015) 373e380. [16] P. Chen, G. Luo, M. Li, Q. Shen, L. Zhang, Effects of Zn additions on the solidstate sintering of W-Cu composites, Mater. Des. 36 (2012) 108e112. [17] P. Chen, G. Luo, Q. Shen, M. Li, L. Zhang, Thermal and electrical properties of W-Cu composite produced by activated sintering, Mater. Des. 46 (2013) 101e105. [18] J. Wang, J. Fan, H. Gong, Effects of Zr alloying on cohesion properties of Cu/W interfaces, J. Alloys Compd. 661 (2016) 553e556. [19] M. Brochu, P. Wanjara, Transient liquid phase bonding of Cu to Cu-W composite using an electron beam energy source, Int. J. Refract. Met. Hard Mater 25 (2007) 67e71. [20] S. Zakipour, A. Halvaee, A. Amadeh, M. Samavatian, A. Khodabandeh, An investigation on microstructure evolution and mechanical properties during transient liquid phase bonding of stainless steel 316L to Ti-6Al-4V, J. Alloys Compd. 626 (2015) 269e276. mez de Salazar, N. Merino, L. Matesanz, Characterization [21] M.I. Barrena, J.M. Go of WC-Co/Ti6Al4V diffusion bonding joints using Ag as interlayer, Mater. Charact. 59 (2008) 1407e1411. [22] D.M. Jafarlou, E. Zalnezhad, M.A. Ezazi, N.A. Mardi, M.A. Hassan, The application of equal channel angular pressing to join dissimilar metals, aluminium alloy and steel, using an AgeCueSn interlayer, Mater. Des. 87 (2015)
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655 553e566. [23] S. Wang, Y. Ling, J. Wang, G. Xu, Microstructure and mechanical properties of W/Cu vacuum diffusion bonding joints using amorphous Fe-W alloy as interlayer, Vacuum 114 (2015) 58e65. [24] L. Zhang, L. Meng, Evolution of microstructure and electrical resistivity of Cu12wt%Ag filamentary microcomposite with drawing deformation, Scr. Mater. 52 (2005) 1187e1191. [25] Y. Sakai, K. Inoue, T. Asano, H. Wada, H. Maeda, Development of high-strength, high-conductivity Cu-Ag alloys for high-field pulsed magnet use, Appl. Phys. Lett. 59 (1991) 2965e2967. [26] L. Zhang, L. Meng, J.B. Liu, Effects of Cr addition on the microstructural, mechanical and electrical characteristics of Cu-6wt%Ag microcomposite, Scr. Mater. 52 (2005) 587e592. [27] S.G. Jia, M.S. Zheng, P. Liu, F.Z. Ren, B.H. Tian, G.S. Zhou, H.F. Lou, Aging
655
properties studies in a Cu-Ag-Cr Alloy, Mater. Sci. Eng. A 419 (2006) 8e11. [28] J.H. Su, H.J. Li, P. Liu, Q.M. Dong, A.J. Li, Aging process optimization for a copper alloy considering hardness and electrical conductivity, Comput. Mater. Sci. 38 (2007) 697e701. [29] S.G. Jia, P. Liu, F.Z. Ren, M.S. Zheng, Strengthening mechanism in high-strength and high-conductivity Cu-0.1Ag-0.11Cr alloy, Chin. J. Nonferr. Met. 14 (2004) 1144e1148. [30] J.B. Liu, L. Meng, Phase orientation, interface structure, and properties of aged Cu-6 wt.% Ag, J. Mater. Sci. 43 (2008) 2006e2011. r, Microstructural parameters from X-ray diffraction peak broadening, [31] T. Unga Scr. Mater. 51 (2004) 777e781. [32] R. Delhez, T.H. de Keijser, E.J. Mittemeijer, Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis, Fresenius Z. Anal. Chem. 312 (1982) 1e16.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of Cr alloying interlayer on the interfacial bond strength of CuW/ CuCr integral materials Xiaohong Yang*, Xuejian Li, Zhe Xiao, Juntao Zou, Shuhua Liang Shaanxi Key Laboratory of Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi’an University of Technology, Xi’an, 710048, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2015 Received in revised form 28 May 2016 Accepted 4 June 2016 Available online 4 June 2016
The effects of Cr alloying interlayer on the microstructure and mechanical properties of the dissimilar CuW/CuCr joints prepared by sintering-infiltration method were studied. The hardness and electrical conductivity of the CuCr alloy side and the tensile strength of the CuW/CuCr integral materials were tested. The interfacial microstructure and fracture morphology of the CuW/CuCr joints were characterized by X-ray diffraction, optical microscopy, scanning electron microscope equipped with energy dispersive spectrometry. The results show that both the electrical conductivity and hardness of the CuCr alloy side of the aged CuW/CuCr joints are higher than those in the solid solution and infiltration condition. With increase of the Cr content, the regions of eutectic phase are expanded gradually on the CuCr alloy side, and the morphology of Cr particles changes from spot shape to rod-like shape. The highest interfacial bond strength can reach up to 446.66 MPa when Cue15 wt%Cr alloy is introduced into the interfacial region between CuW and CueCr alloy. The interfacial fracture morphology presents more cleavage fractures of W particles, and the Cu/W interphase has a good metallurgical bond. © 2016 Elsevier B.V. All rights reserved.
Keywords: CuW/CuCr dissimilar materials Alloying interlayer Interface Bond strength Hardness
1. Introduction CuW/CuCr integral materials are widely used in high-voltage electric contacts, vacuum load switches and transformer switches [1e4]. These functional-structural integrated materials are usually jointed by CuW pseudo alloy with excellent resistance to arc erosion and CuCr alloy with high electrical and thermal conductivity [3e5]. During switching on & off operations, CuW/CuCr integral materials are subjected to the huge heat generated by arc on the tip of CuW contact, and the mechanical force produced when the moving part and the fixed part are contacted and separated. The CuW/CuCr interface endures high thermal and mechanical stresses when exposed to high heat and mechanical loads. Therefore, the reliable joints between CueW composites and CueCr alloys are of practical importance for electrical contact applications [3]. With development of the ultra-high voltage power transmission, it is required that the larger capacity and high frequency switching for electrical appliances. At this case, the contact materials are subjected to more severe working conditions, giving rise to the possibility of the separation of CuW contact from the integral
* Corresponding author. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.jallcom.2016.06.016 0925-8388/© 2016 Elsevier B.V. All rights reserved.
materials. This poses a challenge for the CuW/CuCr integral contact materials. Higher interfacial bond strength of integral CuW/CuCr electrical contacts is necessary to ensure reliability and longer service life [2,3]. Due to the mutual insolubility between W and Cu, no other phase form under the equilibrium condition, thus leading to the low densification and interfacial bond strength [6e9]. Because of the large difference in their coefficients of thermal expansion and elasticity modulus [10,11], high thermal and mechanical stresses can induce microcracks at the Cu/W interface, and resulting in the failure of the integral material along CuW/CuCr joints. Moreover, the Cr content on the CuCr alloy side is usually below 1 wt% since Cr has a very low solubility in the copper [12,13], whereas only a very small amount of W solid solution can be formed in the CuW/CuCr joint in which Cr is dissolved into W skeletons during the integral infiltration. Subsequently, it is hard to enhance the interfacial bond strength of the CuW/CuCr dissimilar materials. Currently, numerous researches are focused on the fabrication methods and properties of CuW pseudo alloy [14e18]. Ren et al. [14] fabricated Cu90W10 nanocomposites by combination of mechanical alloying and warm pressing, and the nanostructured CueW alloys demonstrate the excellent stability at high temperature. Elsayed et al. [15] reported that the WeCu nanocomposite with electroless Cu coating
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
on the nano-size W particles fabricated by spark plasma sintering, present fine homogeneous microstructure and better mechanical properties. Chen et al. [16,17] found that the small addition of Zn into the CueW alloy can improve mechanical properties and thermal and electrical conductivity. Wang et al. [18] revealed the substitution of W atom by Zr significantly increases the Cu/W interface strength and stability through first principle calculation. So far, few literature have been reported on the interfacial bond strength of CuW/CuCr integral materials whereas the interfacial bond strength of dissimilar materials has been widely studied [19e23]. It is believed that the bond strength can be enhanced by introducing an interlayer into the joints. Brochu et al. [19] jointed the Cu to the CueW composite using an Al foil interlayer through transient liquid phase bonding technique, and the mechanical properties of the interface increase owing to the complete dissolution and diffusion of the Al interlayer into the Cu substrate and composite. Zakipour et al. [20] have investigated transient liquid phase bonding mechanism of two dissimilar alloys stainless steel 316L and Tie6Ale4V using pure Cu interlayer, and found the maximum shear strength of the bond obtains at 900 C. With the rise in bonding temperature to 960 C, a reduction in bond strength occurs attributed to increase in width of joint zone and formation of more brittle intermetallic compounds at the interface. Barrena et al. [21] used AgeCueTi alloy as interlayer to join WCeCo composite and Ti6Al4V alloy by a diffusion bonding process, and found both the hardness and the shear resistance of the joints depend on the intermetallic compounds formed during the diffusion bonding. Jafarlou et al. [22] applied the equal channel angular pressing to join aluminum alloy and steel using an AgeCueSn interlayer at different annealing temperatures, and found that the introduction of an interlayer can lead to higher shear strength and the joint strength exhibits improvement as well with increasing annealing temperature. Wang et al. [23] jointed W/Cu with amorphous FeeW coatings electrodeposited onto the Cu foils by vacuum diffusion bonding, and found that the bonding strength of W/Cu improves and the bonding residual stress decreases. As mentioned above, the introduction of an interlayer can promote the mutual dissolution and diffusion among alloying elements, and, thus, improves the interfacial bond strength of dissimilar materials. In our previous work, Cue5.0 wt%Fe alloy interlayer can enhance the interfacial strength of CuW/CuCr integrated material. However, the W skeletons near the interface are eroded severely if Fe content is beyond 5.0 wt%, giving rise to reduction on the interfacial strength of the CuW/CuCr integrated material [3]. In the present work, the CueCr alloying interlayers with higher Cr contents were introduced into the CuW/CuCr bond interface, and the CuW/CuCr integral materials were prepared by the integral infiltration technology. Since Cr can be entirely dissolved into W at elevated temperatures [12], more WeCr solid solution form on the surface of W skeletons at the CuW/CuCr joints during the infiltration, which is beneficial for the improvement on the bond strength of the CuW/CuCr dissimilar materials. In present work, the effect of Cr addition on the interfacial properties of the CuW/CuCr dissimilar materials was studied. The interfacial microstructures were characterized, and the electrical conductivity and mechanical properties were measured. The purpose is to clarify the microstructural evolutions and the interfacial strengthening mechanisms. The research can also provide references for the manufacture of other dissimilar materials.
a V-type mixing machine for 6 h. The mix was compacted into an alloying interlayer with a thickness of 2 mm in a steel mold under a pressure of 340 MPa at room temperature. The Cue70 wt%W pseudo alloy and Cue0.89 wt%Cr alloy were machined into cylindrical, followed by washing with acetone. After the alloying interlayers were cleaned with alcohol, the CuW pseudo alloy, the alloying interlayer and CueCr alloy were put in the graphite crucible from bottom to top. All integral samples were placed in an electrical resistivity furnace. The air in the furnace was removed using hydrogen gas for 40 min, and nitrogen atmosphere was subsequently filled. The samples were heated at a rate of 15 C/min till 1350 C, holding for 1 h, and cooled in the furnace. All integral samples were first solutionized at 980 C for 1 h, and subsequently quenched in the water, followed by aging treatment at 450 C for 4 h. The electrical conductivity of CuCr alloy was measured by a FQR-7501 eddy conductivity gauge. The hardness was determined on an HB-3000 hardness at a load of 250 kgf for 30 s, and the results were the average of five measured values on the CuCr alloy side near the interface. X-ray diffractometer (XRD) with a Cu Ka radiation was used to identify the phase constitutions of CuCr alloy side. The CuW/CuCr joint samples with different alloying interlayers were machined by the electrical discharge, and the cross-sections perpendicular to the interface were prepared for metallographic examination. These samples were etched by the erosion solution (FeCl3þHClþH2O), and the interfacial microstructures were observed by optical microscope (OM) and scanning electron microscope (SEM). The compositional definition around the interface was analyzed by the energy dispersive spectrometry (EDS). The interfacial tensile properties of the CuW/CuCr integral materials with different alloying interlayers were determined using an Instron-testing machine HT-2402-100 KN with a cross load speed of 0.5 mm/min. To ensure the fracture occurred at the CuW/CuCr interface and obtain the interfacial tensile strength, the integral materials were machined into a ladder test bar, as shown in Fig. 1. The left side is CueW composite, and the right side is CuCr alloy. The interfacial fracture morphologies were characterized by a JSM6700F scanning electron microscope. 3. Results and discussion 3.1. Effect of Cr content on the electrical conductivity of CuCr alloy side The electrical conductivity of CuCr alloy side for the CuW/CuCr integral materials with the different Cr interlayers after infiltration, solid solution and aging were measured. Fig. 2 shows the variations of electrical conductivity of CuCr alloy side with Cr content. It can be seen that the electrical conductivity after solutionizing decreases with increase of the Cr content ranging from 5 to 15 wt%, and then almost no change occurs above 15 wt%Cr. This can be possibly attributed to the increase of the segregated Cr-rich primary particle
2. Experimental procedure The starting materials are Cu powders (purity > 99.9%), and Cr powders (purity > 99.7%). The Cu powders were mixed with the Cr powders at the Cu/Cr mass ratio of 95:5, 85:15, 75:25, and 65:35 in
649
Fig. 1. Shape and dimensions of ladder test bar (mm).
650
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 2. Variation of the electrical conductivity of CuCr alloy side with different Cr content.
and Cu þ Cr eutectic phase. Except for the diffusion toward the CuW side, element Cr in the alloying interlayer can diffuse into CuCr alloy side during infiltration. If the Cr content in the CuCr alloy is above the solubility limit of Cr into the Cu matrix, more Cu þ Cr eutectic colonies form with increase of Cr content during solidification, thus resulting in the larger electron scattering induced by the internal Cu/Cr interface [24]. The electrical conductivity of CueCr alloy side for the aged CuW/CuCr integral materials increases significantly in comparison with that in the infiltrated and solutionized integral materials. According to the theory of electrical conductivity [25,26], except for the electron scattering induced by phase interfaces, the solid solution of alloying elements in the Cu matrix is another factor to effect the electrical conductivity of the alloys. The solute atoms in matrix cause the lattice imperfections, and, thus, hinder the electron movement [27,28]. During aging treatment, the Cr particles precipitate from the Cu matrix, which decreases the lattice distortion and solute scattering [29]. Though the interface enhance electron scattering during aging, the solute atoms have more deterioration on the electron transmission [30]. Fig. 3 is the XRD patterns of CuCr alloy side with 5% alloying interlayer at different
conditions. It can be seen from Fig. 3 that only Cu diffraction peaks present. Compared with diffraction peaks at the aging condition, the relative peaks move to the left and the intensities decrease at the solid solution condition. In addition, the corresponding interplanar spacing is given in Table 1 calculated by Bragg equation d ¼ nl=2sinq; where l is the wavelength of X-rays, l ¼ 0.154056 nm; q is the diffraction angle. It can be seen that the interplanar spacing of different diffraction peaks at the solid solution are larger than those in the aging condition. And the lattice parameters are determined pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi by the formula, a ¼ d* ðh2 þ k2 þ l2 Þ, where a is the lattice parameter; d is the interplanar crystal spacing; h, k, l are the miller indices [31,32]. The relative deviation can be calculated in accordance with the formula, Da ¼ ða a0 Þ=a0 , where a0 is the lattice parameter of the Cu perfect crystal at room temperature, a0 ¼ 0.36150 nm. Therefore, the lattice parameters and the relative deviations at different conditions are listed in Table 2. It is learnt that the lattice parameters and the relative deviations of copper matrix at the solid solution are also larger than those in the aging condition. As above, the XRD results indicate that the lattice distortion does exist in CuCr alloy side at the solid solution condition, which is derived from the dissolvement of Cr atoms into the Cu matrix. Hence, the increase in electrical conductivity of aged CuCr alloy can be attributed to the decrease of lattice distortion.
3.2. Effect of Cr content on the hardness of CuCr alloy side The interfacial strength of CuW/CuCr integral materials is closely related to the mechanical properties of both side materials. CuCr alloy with less than 1.2 wt% Cr content is a typical agehardening alloy, and the mechanical property of CuCr alloy changes with the Cr content and the heat treatment conditions. When the alloying interlayer is introduced into the CuW/CuCr interface, the composition of CuCr alloy side changes correspondingly during infiltration. The variation of the hardness of CuCr alloy side near the CuW/CuCr interface with Cr content under infiltration, solid solution and aging condition is shown in Fig. 4. In the range of 5 wt%e15 wt% Cr, the hardness increases with Cr content at the aging condition. However, the hardness decreases when the Cr content is above 15 wt%. Furthermore, compared with the solid solution and infiltration condition, the aged CuCr alloy exhibits the highest hardness. The main reason is that the dispersive secondary Cr particles precipitate from the Cu matrix, giving rise to the enhancement on the hardness. However, when the Cr content is above 15 wt%, except for the diffusion of element Cr into the CuW side, Cr concentration can be diluted by the liquid Cue0.89%Cr alloy during infiltrating. If the Cr concentration in the CuCr alloy is beyond the maximum solubility of Cr in the Cu matrix during infiltration, resulting in the formation of the eutectic phase in the consequent solidification process. Generally, the increased Cr content in the CuCr alloy causes the coarsening of the Cu þ Cr eutectic colonies and increase of the amount of eutectic colonies, which isolate the continuous copper matrix, resulting in the decrease in hardness of CuCr alloy.
Table 1 The interplanar spacing of different diffraction peaks at different conditions in Fig. 3. Cu peak
Fig. 3. XRD patterns of CuCr alloy side with 5% Cr alloy interlayer at different conditions.
Cu (111) Cu (200) Cu (220)
Interplanar spacing (Å) Solid condition
Aging condition
2.0990 1.8098 1.2826
2.0891 1.8077 1.2786
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655 Table 2 Lattice parameters and relative deviation of different diffraction peaks at different conditions in Fig. 3. Cu peak
Lattice parameters (Å)
Relative deviation Da (%) Da ¼ (aaa0)/a0
Solid condition
Aging condition
Solid condition
Aging condition
Cu(111) Cu(200) Cu(220)
3.6354 3.6196 3.6272
3.6183 3.6154 3.6159
0.56 0.13 0.34
0.09 0.01 0.02
a
Data from the standard PDF cards, a0 ¼ 3.6150 Å.
Fig. 4. Variation of the hardness of CuCr alloy side with different Cr content.
651
3.3. Microstructure of CuW/CuCr interface and CuCr alloy side Fig. 5 shows the optical micrographs of the CuW/CuCr interface with different alloying interlayers. It can be seen that the CueCr alloying interlayers introduced into the CuW/CuCr interface completely dissolve into the both sides, and no residue Cr powder particles occur. The microstructure of CuCr alloy side is composed of the Cu matrix and the Cu þ Cr eutectics. With increase of the Cr content in the alloying interlayer, the distributed zone of Cu þ Cr eutectics increases, leading to the decrease in the zone of copper matrix, see Fig. 5(a)e(d). To further examine the morphology of element Cr in the eutectic phase, the microstructure of the CuCr alloy side near the CuW/CuCr interface were magnified as shown in Fig. 6. It can be seen from Fig. 6(a) that small amounts of dotted Cr particles are distributed among the Cu dendritic arms. When the Cr content is 15 wt%, the zone of Cu þ Cr eutectic colonies is widen, the spherical Cr particles present in the eutectics, and the amount of Cr particles increases as well, see Fig. 6(b). However, as shown in Fig. 6(c), with increase of Cr content, the zone of eutectics become larger, and the morphology of Cr particles evolves from sphere to rod-like shape. At 35 wt%Cr, the Cr particles in the eutectics become coarsening with the presence of longer rod, and the zone of eutectics are expanded significantly, see Fig. 6(d). To clarify the elemental distributions at the CuW/CuCr interface, the CuW/CuCr integral material with Cue25 wt%Cr alloy interlayer was selected, and the line scanning was conducted by the electron probe micro analyzer (EPMA) attached to SEM. The results are presented in Fig. 7. As seen from Fig. 7(a), the interface between CuW and CuCr alloys is clean and no pores generate, a dark transition layer appears at the boundaries of bright gray W particles because the element Cr is dissolved into the W skeletons, as marked by the white arrow. From Fig. 7(b), it is found that the phenomenon of mutual diffusion and dissolution among various elements occurs at the interface, element Cr is diffused into both sides. Moreover,
Fig. 5. Interface microstructure of CuW/CuCr materials introduced different alloying interlayers at the aging condition (a) Cue5wt%Cr (b) Cue15 wt%Cr (c) Cue25 wt%Cr (d) Cue35 wt%Cr.
652
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 6. Microstructure of CuCr alloy with different alloying interlayers at the aging condition (a) Cue5wt%Cr (b) Cue15 wt%Cr (c) Cue25 wt%Cr (d) Cue35 wt%Cr.
Fig. 7. Line scanning of CuW/CuCr interfacial regions with the Cue25 wt%Cr alloy interlayer (a) secondary electron image (b) distribution of the elements.
the Cr content in the dark W solid solution layer at the interface is larger than that in other regions. It indicates that the element Cr is easier to be dissolved into W phase than that into the Cu matrix. 3.4. The interfacial tensile strength of CuW/CuCr integral materials and fracture morphology The tensile bars of CuW/CuCr integral materials with different alloying interlayers were fabricated by the same sinteringinfiltration method. After the same solution and aging treatment, the interfacial tensile strength of CuW/CuCr integral materials was tested, and the stress-strain curves are shown in Fig. 8. It can be seen that the interfacial bond strength of the CuW/CuCr integral material enhances when the element Cr is introduced into the CuW/CuCr interface. When the Cr content in the alloying
interlayers are below 15 wt%, the interfacial tensile strength of CuW/CuCr materials increases with increase of Cr content. However, the tensile strength decreases when the Cr content is above 15 wt%. The interfacial strength of the CuW/CuCr materials has the maximum value of 446.66 MPa at 15 wt%Cr, which is increased by 31.74% compared with that without alloying interlayer. Combining with the hardness of CuCr alloy side in Fig. 4, it is inferred that the diffusion and dissolution of the Cr alloying interlayer into both sides are beneficial for the improvement on the mechanical properties of CuW/CuCr interface. Especially, both the interfacial strength and hardness of CuCr alloy side reach the maximum values when the Cr content is 15 wt%. Compared with our previous results [3], the interfacial bond strength of CuW/CuCr obtained at the present work is above 110 MPa than that of CuW/CuCr with CueFe alloying interlayer. It suggests that the element Cr has more
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
Fig. 8. Tensile stress-strain curves of integral materials with different Cr alloying interlayers.
contribution on the enhancement of the interfacial mechanical properties than element Fe. Fig. 9 shows the tensile fracture morphologies of the CuW/CuCr integral materials with different alloying interlayers. It can be seen from Fig. 9(a) that the fracture is consisted of the ductile fracture area and the larger flat area. A small amount of copper phase in both sides exhibits the ductile fracture, and the reticulated ductile
653
tearing ridges generate. The formation of smooth flat area reveals that the CuW and CuCr alloys are separated from each other easily at the interface, and the interfacial bond is weak. Nevertheless, when the 15 wt%Cr alloying interlayer was introduced into the CuW/CuCr interface, there is few flat characteristic area, and a few of smaller tungsten particles are pulled out and left in the deeper dimples due to debonding of Cu/W interphase as shown in Fig. 9(b). A number of tungsten particles exhibit the transgranular cleavage fracture. It indicates that the fracture tends to move toward the CuW side, and the interfacial bond strength of CuW/CuCr is improved. For the CuW/CuCr integral material with Cue25 wt%Cr interlayer, it is found that a few cleavage fractures occur on small amounts of larger tungsten particles, and numberous fine tungsten particles segregate from W skeletons and fall into dimples as shown in Fig. 9(c). It reveals that the W skeletons at the CuW/CuCr interface are dissolved and eroded by excessive element Cr during infiltration, and, thus, some W particles fragment and fall from the tungsten skeletons. Subsequently, the discontinuous W skeletons cannot bear the tensile load effectively, thus resulting in the decrease of interfacial bond strength. To get more insights of the strengthening and failure mechanisms of the CuW/CuCr interface after introducing the different alloying interlayers, as illustrated in Fig. 10, the chemical compositions of the ductile tearing ridges and the transgranular cleavage of W particles in the fracture morphologies were determined by EDS, and the results are listed in Table 3. It can be seen that the content of element Cr dissolved into W particles increases with increase of the Cr content in the alloying interlayer, and the content of Cr in the tearing ridges of Cu phase increases as well. It can be inferred that element Cr in the alloying interlayers diffuses and
Fig. 9. Fracture morphologies of the CuW/CuCr integral materials with different Cr contents (a) Without alloying interlayer (b) 15 wt%Cr (c) 25 wt%Cr.
654
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655
fraction of cleavage fracture increase, and the fracture moves toward CuW side with increase of Cr content ranging from 5 wt% to 15 wt%. Moreover, both the Cr contents of the CreW solid solution in which the transgranular cleavage occurs and at the ductile copper phase tearing ridges increase, and the metallurgical bond is achieved at the Cu/W interphase. Acknowledgements This work was supported by the National Natural Science Foundation of China (51201132), National High-Tech Program (863) of China (No. 2015AA034304), the Science and Technique Innovation Program of Shaanxi Province for Key Laboratory (2014SZS08K02), the Research of Science and Technology of the Shaanxi Provincial Education Department for Key Laboratory (14JS065), and the Shaanxi Provincial Project of the Special Foundation of Key Disciplines. References Fig. 10. EDS analysis points in the SEM fracture image.
Table 3 EDS analysis results in the CuW/CuCr interfacial tensile fractures of CuW/CuCr materials introduced different Cr alloying interlayers. Different regions
Cr contents (wt%) Without interlayer
15 wt%Cr
25 wt%Cr
Ductile tearing ridges W particles
0.24 0.65
0.61 10.27
3.53 27.45
dissolves across the CuW/CuCr interface during infiltration, and the CreW solid solution forms at the CuW/CuCr interface. Consequently, more Cu/W interphases at the CuW/CuCr interface achieve the metallurgical bond by introducing higher content Cr. Compared with the initial mechanical bond, the bonding strength of the Cu/W interphase is enhanced remarkably. Therefore, when the tensile load apply at the CuW/CuCr interface, the load transfers from the weaker copper phase to the stronger W phase with higher elastic modulus via Cu/W interphase. In addition, the micro-cracks cannot propagate along the Cu/W interphase, leading to the better interfacial bond strength of the CuW/CuCr materials.
4. Conclusions (1) With the introduction of alloying interlayer into the CuW/ CuCr interface, the electrical conductivity and hardness of the CuCr alloy side at aging condition are higher than those at the infiltration and solid solution condition. The increased Cr content in the alloying interlayer expands the region of Cu þ Cr eutectics in the CuCr alloy sides near the interface, and the morphology of Cr particles in eutectics evolves from sphere to the rod-like shape. Furthermore, a solid solution layer forms at the boundaries of W skeletons at the CuW/ CuCr interface, and it is much easier for element Cr to dissolve into W than that into Cu. (2) The interfacial strength of CuW/CuCr integral materials can reach up to 446.66 MPa when the Cr content is 15 wt% in the alloying interlayer. Compared with the CuW/CuCr material without alloying interlayer, the interfacial strength is increased by 31.74%. The fracture morphologies of the CuW/ CuCr integral materials are consisted of the ductile fracture of Cu phase and the cleavage fracture of W particles. The
[1] S. Liang, L. Chen, Z. Yuan, Y. Li, J. Zou, P. Xiao, Infiltrated W-Cu composites with combined architecture of hierarchical particulate tungsten and tungsten fibers, Mater. Charact. 110 (2015) 33e38. [2] Q. Zhang, S. Liang, X. Yang, Z. Li, Y. Li, Y. Zhang, Failure analysis of CuW/CuCrZr contact materials in capacitor bank switch, Eng. Fail. Anal. 62 (2016) 156e163. [3] X. Yang, J. Zou, S. Liang, P. Xiao, Z. Fan, Effect of alloying interlayer on interfacial bond strength of CuW/CuCr integral materials, Mater. Sci. Eng. A 527 (2010) 5631e5636. [4] X. Yang, J. Zou, P. Xiao, X. Wang, Effects of Zr addition on properties and vacuum arc characteristics of Cu-W alloy, Vacuum 106 (2014) 16e20. [5] Q. Zhang, S. Liang, J. Zou, Q. Yang, Interfacial microstructure of CuCr/ 1Cr18Ni9Ti bi-metal materials and its effect on bonding strength, Sci. China Tech. Sci. 58 (2015) 825e831. [6] L. Xu, M. Yan, Y. Xia, J. Peng, W. Li, L. Zhang, C. Liu, G. Chen, Influence of copper content on the property of Cu-W alloy prepared by microwave vacuum infiltration sintering, J. Alloys Compd. 592 (2014) 202e206. [7] X. Wei, J. Tang, N. Ye, H. Zhuo, A novel preparation method for W-Cu composite powders, J. Alloys Compd. 661 (2016) 471e475. [8] Y. Kong, J. Pi, S. Xiao, Y. Du, Asymmetric mixing behavior and stability of the predicted phases in the W-Cu system, CALPHAD 53 (2016) 116e121. [9] Q. Zhou, P. Chen, Fabrication of W-Cu composite by shock consolidation of Cucoated W powders, J. Alloys Compd. 657 (2016) 215e223. [10] P. Zhao, S.B. Guo, G.H. Liu, Y.X. Chen, J.T. Li, Fabrication of WeCu functionally graded material with improved mechanical strength, J. Alloys Compd. 601 (2014) 289e292. [11] Y.F. Meng, J.P. Zhang, C.Y. Duan, C. Chen, X.M. Feng, Y.F. Shen, Microstructures and properties of WeCu functionally graded composite coatings on copper substrate via high-energy mechanical alloying method, Adv. Powder Technol. 26 (2015) 392e400. [12] T.B. Massal, P.R. Subramanian, H. Okamoto, L. kacprzak, Alloy phase diagrams, ASM Int. 3 (1992). [13] M. Bizjak, B. Karpe, G. Jaksa, J. Kova c, Surface precipitation of chromium in rapidly solidified Cu-Cr alloys, Appl. Surf. Sci. 277 (2013) 83e87. [14] F.Z. Ren, W.W. Zhu, K.J. Chu, C.C. Zhao, Tribological and corrosion behaviors of bulk Cu-W nanocomposites fabricated by mechanical alloying and warm pressing, J. Alloys Compd. 676 (2016) 164e172. [15] A. Elsayed, W. Li, O.A. El Kady, Experimental investigations on the synthesis of W-Cu nanocomposite through spark plasma sintering, J. Alloys Compd. 639 (2015) 373e380. [16] P. Chen, G. Luo, M. Li, Q. Shen, L. Zhang, Effects of Zn additions on the solidstate sintering of W-Cu composites, Mater. Des. 36 (2012) 108e112. [17] P. Chen, G. Luo, Q. Shen, M. Li, L. Zhang, Thermal and electrical properties of W-Cu composite produced by activated sintering, Mater. Des. 46 (2013) 101e105. [18] J. Wang, J. Fan, H. Gong, Effects of Zr alloying on cohesion properties of Cu/W interfaces, J. Alloys Compd. 661 (2016) 553e556. [19] M. Brochu, P. Wanjara, Transient liquid phase bonding of Cu to Cu-W composite using an electron beam energy source, Int. J. Refract. Met. Hard Mater 25 (2007) 67e71. [20] S. Zakipour, A. Halvaee, A. Amadeh, M. Samavatian, A. Khodabandeh, An investigation on microstructure evolution and mechanical properties during transient liquid phase bonding of stainless steel 316L to Ti-6Al-4V, J. Alloys Compd. 626 (2015) 269e276. mez de Salazar, N. Merino, L. Matesanz, Characterization [21] M.I. Barrena, J.M. Go of WC-Co/Ti6Al4V diffusion bonding joints using Ag as interlayer, Mater. Charact. 59 (2008) 1407e1411. [22] D.M. Jafarlou, E. Zalnezhad, M.A. Ezazi, N.A. Mardi, M.A. Hassan, The application of equal channel angular pressing to join dissimilar metals, aluminium alloy and steel, using an AgeCueSn interlayer, Mater. Des. 87 (2015)
X. Yang et al. / Journal of Alloys and Compounds 686 (2016) 648e655 553e566. [23] S. Wang, Y. Ling, J. Wang, G. Xu, Microstructure and mechanical properties of W/Cu vacuum diffusion bonding joints using amorphous Fe-W alloy as interlayer, Vacuum 114 (2015) 58e65. [24] L. Zhang, L. Meng, Evolution of microstructure and electrical resistivity of Cu12wt%Ag filamentary microcomposite with drawing deformation, Scr. Mater. 52 (2005) 1187e1191. [25] Y. Sakai, K. Inoue, T. Asano, H. Wada, H. Maeda, Development of high-strength, high-conductivity Cu-Ag alloys for high-field pulsed magnet use, Appl. Phys. Lett. 59 (1991) 2965e2967. [26] L. Zhang, L. Meng, J.B. Liu, Effects of Cr addition on the microstructural, mechanical and electrical characteristics of Cu-6wt%Ag microcomposite, Scr. Mater. 52 (2005) 587e592. [27] S.G. Jia, M.S. Zheng, P. Liu, F.Z. Ren, B.H. Tian, G.S. Zhou, H.F. Lou, Aging
655
properties studies in a Cu-Ag-Cr Alloy, Mater. Sci. Eng. A 419 (2006) 8e11. [28] J.H. Su, H.J. Li, P. Liu, Q.M. Dong, A.J. Li, Aging process optimization for a copper alloy considering hardness and electrical conductivity, Comput. Mater. Sci. 38 (2007) 697e701. [29] S.G. Jia, P. Liu, F.Z. Ren, M.S. Zheng, Strengthening mechanism in high-strength and high-conductivity Cu-0.1Ag-0.11Cr alloy, Chin. J. Nonferr. Met. 14 (2004) 1144e1148. [30] J.B. Liu, L. Meng, Phase orientation, interface structure, and properties of aged Cu-6 wt.% Ag, J. Mater. Sci. 43 (2008) 2006e2011. r, Microstructural parameters from X-ray diffraction peak broadening, [31] T. Unga Scr. Mater. 51 (2004) 777e781. [32] R. Delhez, T.H. de Keijser, E.J. Mittemeijer, Determination of crystallite size and lattice distortions through X-ray diffraction line profile analysis, Fresenius Z. Anal. Chem. 312 (1982) 1e16.