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The effect of interface modification on fracture behavior of tungsten fiber reinforced copper matrix composites
Materials Science and Engineering A 536 (2012) 45–48
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of interface modification on fracture behavior of tungsten fiber reinforced copper matrix composites Z. Wu a,b,∗ , P.C. Kang a , G.H. Wu a , Q. Guo a , G.Q. Chen a , L.T. Jiang a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, China
a r t i c l e
i n f o
Article history: Received 28 December 2010 Received in revised form 12 October 2011 Accepted 26 November 2011 Available online 6 December 2011 Keywords: Tungsten fiber reinforced copper matrix composites Three-point bending test Interface strength Dynamic compression test
a b s t r a c t Wf /Cu composites, Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites were fabricated by penetrating casting method. Bending strength and dynamic compressive strength of the composites were tested. Fracture mode and microstructure were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The result showed that the interface strength of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites were higher than Wf /Cu composites. In Wf (Ni)/Cu composites, nickel plated on the surface of tungsten fibers has diffused into the inner of tungsten fibers, and a large number of tungsten grains and Ni4 W intermetallic compounds appeared within tungsten fibers. In Wf /Cu82 Al10 Fe4 Ni4 composites, Fe–Ni solid solutions precipitated on the interface between matrix and tungsten fibers, and tungsten has diffused into the Fe–Ni solid solutions. Dynamic compression test showed that the dynamic compressive strength and plasticity of Wf /Cu82 Al10 Fe4 Ni4 composites were highest among the three kinds of composites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As the development of kinetic energy penetrator to high speed, high strength and high density, the demand to mechanical property of the material used in kinetic energy penetrator also increase, so traditional materials are not adapted to new military requirements [1,2]. Composites are composed of two or more kinds of materials reasonably and possess better performance than single materials, therefore many countries start to research kinetic energy penetrator of the composites in recent years [3–5]. Due to the high melting point, high density and high mechanical property of tungsten fiber, it is selected as reinforcement of the new generation of composites fabricated penetrator [6]. California Institute of Technology [7–9] has developed tungsten fiber reinforced Zr-based bulk metallic glass composites, and many research institutions have done a lot of researches in the preparation and mechanical property of the composites. The result indicates that the addition of tungsten fiber will greatly enhance the mechanical property of the composites and change fracture characteristic of the composites. So we selected tungsten fiber as reinforcement of the composites in this paper. Because copper alloy possesses the properties of high density and high strength and W–Cu composites has been successfully used in electronics, military and aerospace fields, copper and its alloy were selected as the matrix of the composites
∗ Corresponding author. Tel.: +86 451 86412164; fax: +86 451 86412164. E-mail address: [email protected] (Z. Wu). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.088
in the research [10–13]. When the reinforcement and matrix of the composites are determined, the interface will be the key to transfer the load effectively. Appropriate interface reaction can effectively improve the wettability, and enhance the composites’ interface strength and mechanical properties; but excessive interface reaction can weaken the interface, and reduce the composites’ mechanical properties. Because the wettability between tungsten and copper is poor and interface strength is low, the interface of W–Cu composites is modified by means of plating nickel on the surface of tungsten fiber and adding iron and nickel in copper matrix [14,15]. Then the interface strength, dynamic mechanical property and fracture mode of the composites are investigated in this paper. 2. Experimental procedure Pure copper and Cu82 Al10 Fe4 Ni4 alloy were selected as matrix of the composites. The tungsten fibers with the diameter of 0.25 mm and 0.5 mm were selected as reinforcement of the composites at three-point bending test and dynamic compression test respectively. After straightening, tungsten fibers were cut into 100 mm long and immersed in 40% hydrofluoric acid to remove the surface oxide film, then they were cleaned by ultrasonic in acetone and alcohol respectively to get pure surface. The prepared tungsten fibers were put straightly into the clean quartz tube and the master alloy was set above the tungsten fibers, then the composites were fabricated by means of penetrating casting method. Three kinds of composites were prepared in this paper. The first is Wf /Cu
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Fig. 1. Tungsten fibers arrangement of three-point bending samples.
composites which is composed of copper and tungsten fibers. The second is Wf (Ni)/Cu composites which is composed of copper and nickel-plated tungsten fibers, on which 5 m nickel layer was plated. The third is Wf /Cu82 Al10 Fe4 Ni4 composites which is composed of Cu82 Al10 Fe4 Ni4 alloy and tungsten fibers. The volume fraction of tungsten fibers is 80% in the three kinds of composites. The samples with the dimension of 5 mm × 5 mm × 35 mm of three-point bending test were made by linear cutting machine. The arrangement of tungsten fibers in the composites is shown in Fig. 1. The size of impacting samples is ˚4 mm × 4 mm. The dynamic compression test was performed by Split Hopkinson pressure bar (SHPB) at the strain rate of 2000 s−1 . To ensure the reliability of test data, each group test would be done at least three times. The interface reaction of the composites was analyzed by transmission electron microscope (TEM), and the fracture of the composites was analyzed by scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure observation SEM microstructure at interface in Wf /Cu composites is shown in Fig. 2a, from which obvious gaps and micropores can be observed.
SEM microstructure at interface in Wf (Ni)/Cu composites is shown in Fig. 2b, from which recrystallization of tungsten fibers can be observed. The size of tungsten grains generated by recrystallization are about 10 m. Fig. 2c is the TEM microstructure of the area among the tungsten grains shown in Fig. 2b. There exists lots of grains in Fig. 2c, and the selected area electron diffraction pattern (SADP) insert shows the grains are Ni4 W intermetallic compounds. SEM microstructure at interface in Wf /Cu82 Al10 Fe4 Ni4 composites is shown in Fig. 2d, from which interfacial reaction products on the surface of tungsten fibers can be observed, and energy disperse spectroscopy (EDS) insert shows tungsten, iron and nickel are the major elements of interfacial reaction products. Fig. 2e is TEM microstructure of Wf /Cu82 Al10 Fe4 Ni4 composites at interface, the SADP insert indicates that the interfacial reaction products are Fe–Ni solid solutions. The EDS and SADP analysis indicate that tungsten diffuse into Fe–Ni solid solutions on the interface of Wf /Cu82 Al10 Fe4 Ni4 composites. At the same time, there also exists other phase in matrix as shown in Fig. 2f, and the SADP insert indicates that is Al3 Ni intermetallic compound. 3.2. Test of interface strength The interface strength of the composites is tested by means of three-point bending test. Fig. 3 is the bending stress-displacement curves of the three kinds of composites. The result indicates that bending strength of Wf /Cu composites is lowest, while that of Wf /Cu82 Al10 Fe4 Ni4 and Wf (Ni)/Cu composites are obviously higher. As shown in Fig. 4a, copper is completely peeled from tungsten fibers and the surface of tungsten fibers keep integrally, which indicate that the fracture of Wf /Cu composites occurs at the interface between matrix and tungsten fibers. While the fracture of Wf /Cu82 Al10 Fe4 Ni4 and Wf (Ni)/Cu composites occur within tungsten fiber as shown in Fig. 4b and c. The result indicates that
Fig. 2. Microstructure of the three kinds of composites. (a) SEM image at interface in Wf /Cu composites. (b) SEM image at interface in Wf (Ni)/Cu composites. (c) TEM image of the area among the new generation tungsten grains shown in (b). (d) SEM image at interface in Wf /Cu82 Al10 Fe4 Ni4 composites. EDS insert shows the elements of interfacial reaction product. (e) TEM image at interface in Wf /Cu82 Al10 Fe4 Ni4 composites. SADP insert shows interfacial reaction product is Fe–Ni solid solution. (f) TEM image of Wf /Cu82 Al10 Fe4 Ni4 composites in matrix. SADP insert shows Al3 Ni intermetallic compound exists in matrix.
Z. Wu et al. / Materials Science and Engineering A 536 (2012) 45–48
Fig. 3. Bending stress-displacement curves of the composites.
interface strength of Wf /Cu composites is lowest, while interface strength of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites are substantially enhanced by means of plating nickel on tungsten fibers and adding alloy elements in copper matrix. The enhancement of interface strength is due to the chemical reaction between tungsten fibers and nickel in Wf (Ni)/Cu composites, while the enhancement of interface strength is due to the diffusion of tungsten into Fe–Ni solid solutions in Wf /Cu82 Al10 Fe4 Ni4 composites. Fracture morphology of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites are entirely different. As shown in the partial enlarged drawing of Fig. 4b, a lot of tungsten grains appear on the fracture surface of Wf (Ni)/Cu composites, which is consistent with the observation in Fig. 2b. While lots of fibrous substances torn within tungsten fibers appear on the fracture surface of Wf /Cu82 Al10 Fe4 Ni4 composites as shown in Fig. 4c. Because original tungsten fiber is drawn out from block tungsten under high-temperature, the inner structure of original tungsten fiber is fibrous, which makes mechanical property of tungsten fiber excellent. Fracture morphology of Wf /Cu82 Al10 Fe4 Ni4 composites indicates the inner structure of tungsten fibers in Wf /Cu82 Al10 Fe4 Ni4 composites and original tungsten fiber is consistent, and the preparation process of Wf /Cu82 Al10 Fe4 Ni4 composites rarely affects the inner structure of tungsten fibers. Nickel is plated on tungsten fibers in Wf (Ni)/Cu composites, when the heating temperature is above 1000 ◦ C, nickel on the tungsten surface will start to diffuse into tungsten fibers, which causes recrystallization of tungsten fibers [16–18]. The higher the heating
47
Fig. 5. Dynamic compressive stress–strain curves of the composites at the strain rate of 2000 s−1 .
temperature is, the deeper the nickel diffuses into, and the larger grains will be produced by recrystallization [18]. While the diffusion of nickel into tungsten fibers and the recrystallization of tungsten fibers are not found in Wf /Cu82 Al10 Fe4 Ni4 composites. As shown in Fig. 2e and f, nickel exists as the form of Fe–Ni solid solution and Al3 Ni intermetallic compound in Wf /Cu82 Al10 Fe4 Ni4 composites, so the diffusion and recrystallization are restrained. In addition, the addition of aluminum reduces the melting temperature of the copper alloy, which will further restrain the diffusion and recrystallization, so the fibrous structure of tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites as shown in Fig. 4c. 3.3. Fracture properties of the composites after dynamic compression In the dynamic compression test, tungsten fibers in the samples are arranged along the axial direction of the sample, and the impact direction is also along the axial direction of the sample. Fig. 5 shows dynamic compressive stress–strain curves of the three kinds of composites at the strain rate of 2000 s−1 . Flow stress of Wf /Cu composites is linear attenuation under dynamic compression. The dynamic compressive strength of Wf (Ni)/Cu composites is 2000 MPa, the strain at failure is 12%. While the dynamic compressive strength of Wf /Cu82 Al10 Fe4 Ni4 composites is 2500 MPa, plastic strain reaches to 16%. The plastic strain and dynamic compressive strength of Wf /Cu82 Al10 Fe4 Ni4 composites are highest among the three kinds of composites.
Fig. 4. SEM images of three-point bending fracture surfaces of (a) Wf /Cu composites, (b) Wf (Ni)/Cu composites, and (c) Wf /Cu82 Al10 Fe4 Ni4 composites.
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Fig. 6. SEM images of dynamic compressive fracture of (a) Wf /Cu composites, (b) Wf (Ni)/Cu composites, and (c) Wf /Cu82 Al10 Fe4 Ni4 composites.
Fig. 6 shows fracture morphology of the three kinds of composites after dynamic compression. Fig. 6a shows that tungsten fibers and copper matrix disperse during dynamic compression in Wf /Cu composites, and the partial enlarged drawing shows that only a little copper remains on the surface of tungsten fiber, which indicates the fracture occurs on the interface between tungsten fibers and copper matrix. It is noted that the flow stress is high at the beginning of compression, but it decays quickly with the compression progress in Wf /Cu composites as shown in Fig. 5. Because the inner structure of tungsten fibers rarely are influenced during the preparation process and tungsten fibers keep higher strength, Wf /Cu composites owns higher strength at the beginning stage. However, because the interface strength of Wf /Cu composites is poor, and there are some holes and other obvious defects at interface as shown in Fig. 2a, stress concentration will occur at the defects with the compression progress, which causes rapid extension of the cracks along the interface and makes the composites overall failure, so the dynamic compressive curve decays quickly at this phase. Brittle fracture occurs in Wf (Ni)/Cu composites after dynamic compression, which is broken into many pieces, and Fig. 6b is SEM microstructure of one of the pieces. The original tungsten fiber is drawn out from block tungsten under high-temperature, and the tungsten fiber is composed of a large number of small fibers, which are arranged along the axial direction of tungsten fiber vertically and uniaxially, and this kind of structure makes mechanical properties of tungsten fiber excellent. But the reaction between tungsten fibers and nickel, as shown in Fig. 2c, will produce lots of brittle Ni4 W intermetallic compounds, which will increase the brittleness of tungsten fibers, so Wf (Ni)/Cu composites displays feature of brittle fracture after dynamic compression. The partial enlarged drawing of Fig. 6b shows that lots of tungsten grains exist within tungsten fibers, which is consistent with observation in Fig. 2b and 4b. Wf (Ni)/Cu composites possesses higher bending strength and lower dynamic compressive strength compared with Wf /Cu82 Al10 Fe4 Ni4 composites, indicating that the granular structure within tungsten fiber enhances split resistance of tungsten fiber, but obviously reduces the axial compressive strength of tungsten fiber. Fig. 6c shows the fracture morphology of Wf /Cu82 Al10 Fe4 Ni4 composites after dynamic compression. There is obvious plastic deformation and a few of cracks on the surface of the sample. The fibrous structure of tungsten fibers can be observed at the crack in the partial enlarged drawing of Fig. 6c, which indicates that crack occurs and enlarges within the tungsten fiber and the interface strength is high under dynamic compression. The fracture morphology indicates that tungsten fibers play a major role and the split
of tungsten fibers will absorb the impact energy to avoid overall failure of Wf /Cu82 Al10 Fe4 Ni4 composites under dynamic compression. In addition, because the fibrous structure of tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites, the strength and plasticity of Wf /Cu82 Al10 Fe4 Ni4 composites are higher than that of Wf (Ni)/Cu composites under dynamic compression. 4. Conclusions 1. Interface strength of Wf /Cu composites can be enhanced by means of plating nickel on tungsten fiber and adding iron, nickel and aluminum elements in copper matrix. 2. The nickel diffusion will lead to recrystallization of tungsten fiber and generate lots of brittle Ni4 W intermetallic compounds within tungsten fibers in Wf (Ni)/Cu composites, which reduce axial strength and plasticity of tungsten fibers. 3. Tungsten diffuses into Fe–Ni solid solutions precipitated on the surface of tungsten fibers in Wf /Cu82 Al10 Fe4 Ni4 composites, which enhance the interface strength of the composites. Meanwhile, the fibrous structure within tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites, so the composites possesses excellent strength and plasticity under dynamic compression. References [1] S. Pappu, C. Kennedy, L.E. Murr, L.S. Magness, D. Kapoor, Mater. Sci. Eng. A 262 (1999) 115–128. [2] J.X. Liu, S.K. Li, A.L. Fan, H.C. Sun, Mater. Sci. Eng. A 487 (2008) 235–242. [3] R.D. Conner, R.B. Dandliker, W.L. Johnson, Acta Mater. 46 (1998) 6089–6102. [4] H.F. Zhang, H. Li, A.M. Wang, H.M. Fu, B.Z. Ding, Z.Q. Hu, Intermetallics 17 (2009) 1070–1077. [5] H.G. Kim, K.T. Kim, Int. J. Mech. Sci. 42 (2000) 1339–1356. [6] W.F. Ma, H.C. Kou, C.S. Chen, J.S. Li, H. Chang, L. Zhou, H.Z. Fu, Mater. Sci. Eng. A 486 (2008) 308–312. [7] R.D. Conner, R.B. Dandliker, V. Scruggs, W.L. Johnson, Int. J. Impact Eng. 24 (2000) 435–444. [8] D. Dragoi, E. Üstündag, B. Clausen, M.M. Bourke, Scr. Mater. 45 (2001) 245–252. [9] B. Clausen, S.Y. Lee, E. Üstündag, C.C. Aydiner, R.D. Conner, M.A.M. Bourke, Scr. Mater. 2 (2003) 123–128. [10] X.H. Yang, S.H. Liang, X.H. Wang, P. Xiao, Z.K. Fan, Int. J. Refract. Met. Hard Mater. 28 (2010) 305–311. [11] J.G. Cheng, L. Wan, Y.B. Cai, J.C. Zhu, P. Song, J. Dong, J. Mater. Process. Technol. 1 (2010) 137–142. [12] S.H. Hong, B.K. Kim, Z.A. Munir, Mater. Sci. Eng. A 405 (2005) 325–332. [13] A. Ibrahim, M. Abdallah, S.F. Mostafa, A.A Hegazy, Mater. Des. 30 (2009) 1398–1403. [14] F. Doré, S. Lay, N. Eustathopoulos, C.H. Allibert, Scr. Mater. 49 (2003) 237–242. [15] L.P. Zhou, M.P. Wang, R. Wang, Z. Li, J.J. Zhu, K. Peng, D.Y. Li, S.L Li, Trans. Nonferrous Met. Soc. China 18 (2008) 372–377. [16] S.N. Mathaudhu, A.J. deRosset, K.T. Hartwig, L.J. Kecskes, Mater. Sci. Eng. A 503 (2009) 28–31. [17] W. Guo, S.K. Li, F.C. Wang, M. Wang, Scr. Mater. 60 (2009) 329–332. [18] I.H. Moon, K.Y. Kim, S.T. Oh, M.J. Suk, J. Alloys Compd. 201 (1993) 129–137.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of interface modification on fracture behavior of tungsten fiber reinforced copper matrix composites Z. Wu a,b,∗ , P.C. Kang a , G.H. Wu a , Q. Guo a , G.Q. Chen a , L.T. Jiang a a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, China
a r t i c l e
i n f o
Article history: Received 28 December 2010 Received in revised form 12 October 2011 Accepted 26 November 2011 Available online 6 December 2011 Keywords: Tungsten fiber reinforced copper matrix composites Three-point bending test Interface strength Dynamic compression test
a b s t r a c t Wf /Cu composites, Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites were fabricated by penetrating casting method. Bending strength and dynamic compressive strength of the composites were tested. Fracture mode and microstructure were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). The result showed that the interface strength of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites were higher than Wf /Cu composites. In Wf (Ni)/Cu composites, nickel plated on the surface of tungsten fibers has diffused into the inner of tungsten fibers, and a large number of tungsten grains and Ni4 W intermetallic compounds appeared within tungsten fibers. In Wf /Cu82 Al10 Fe4 Ni4 composites, Fe–Ni solid solutions precipitated on the interface between matrix and tungsten fibers, and tungsten has diffused into the Fe–Ni solid solutions. Dynamic compression test showed that the dynamic compressive strength and plasticity of Wf /Cu82 Al10 Fe4 Ni4 composites were highest among the three kinds of composites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As the development of kinetic energy penetrator to high speed, high strength and high density, the demand to mechanical property of the material used in kinetic energy penetrator also increase, so traditional materials are not adapted to new military requirements [1,2]. Composites are composed of two or more kinds of materials reasonably and possess better performance than single materials, therefore many countries start to research kinetic energy penetrator of the composites in recent years [3–5]. Due to the high melting point, high density and high mechanical property of tungsten fiber, it is selected as reinforcement of the new generation of composites fabricated penetrator [6]. California Institute of Technology [7–9] has developed tungsten fiber reinforced Zr-based bulk metallic glass composites, and many research institutions have done a lot of researches in the preparation and mechanical property of the composites. The result indicates that the addition of tungsten fiber will greatly enhance the mechanical property of the composites and change fracture characteristic of the composites. So we selected tungsten fiber as reinforcement of the composites in this paper. Because copper alloy possesses the properties of high density and high strength and W–Cu composites has been successfully used in electronics, military and aerospace fields, copper and its alloy were selected as the matrix of the composites
∗ Corresponding author. Tel.: +86 451 86412164; fax: +86 451 86412164. E-mail address: [email protected] (Z. Wu). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.088
in the research [10–13]. When the reinforcement and matrix of the composites are determined, the interface will be the key to transfer the load effectively. Appropriate interface reaction can effectively improve the wettability, and enhance the composites’ interface strength and mechanical properties; but excessive interface reaction can weaken the interface, and reduce the composites’ mechanical properties. Because the wettability between tungsten and copper is poor and interface strength is low, the interface of W–Cu composites is modified by means of plating nickel on the surface of tungsten fiber and adding iron and nickel in copper matrix [14,15]. Then the interface strength, dynamic mechanical property and fracture mode of the composites are investigated in this paper. 2. Experimental procedure Pure copper and Cu82 Al10 Fe4 Ni4 alloy were selected as matrix of the composites. The tungsten fibers with the diameter of 0.25 mm and 0.5 mm were selected as reinforcement of the composites at three-point bending test and dynamic compression test respectively. After straightening, tungsten fibers were cut into 100 mm long and immersed in 40% hydrofluoric acid to remove the surface oxide film, then they were cleaned by ultrasonic in acetone and alcohol respectively to get pure surface. The prepared tungsten fibers were put straightly into the clean quartz tube and the master alloy was set above the tungsten fibers, then the composites were fabricated by means of penetrating casting method. Three kinds of composites were prepared in this paper. The first is Wf /Cu
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Z. Wu et al. / Materials Science and Engineering A 536 (2012) 45–48
Fig. 1. Tungsten fibers arrangement of three-point bending samples.
composites which is composed of copper and tungsten fibers. The second is Wf (Ni)/Cu composites which is composed of copper and nickel-plated tungsten fibers, on which 5 m nickel layer was plated. The third is Wf /Cu82 Al10 Fe4 Ni4 composites which is composed of Cu82 Al10 Fe4 Ni4 alloy and tungsten fibers. The volume fraction of tungsten fibers is 80% in the three kinds of composites. The samples with the dimension of 5 mm × 5 mm × 35 mm of three-point bending test were made by linear cutting machine. The arrangement of tungsten fibers in the composites is shown in Fig. 1. The size of impacting samples is ˚4 mm × 4 mm. The dynamic compression test was performed by Split Hopkinson pressure bar (SHPB) at the strain rate of 2000 s−1 . To ensure the reliability of test data, each group test would be done at least three times. The interface reaction of the composites was analyzed by transmission electron microscope (TEM), and the fracture of the composites was analyzed by scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure observation SEM microstructure at interface in Wf /Cu composites is shown in Fig. 2a, from which obvious gaps and micropores can be observed.
SEM microstructure at interface in Wf (Ni)/Cu composites is shown in Fig. 2b, from which recrystallization of tungsten fibers can be observed. The size of tungsten grains generated by recrystallization are about 10 m. Fig. 2c is the TEM microstructure of the area among the tungsten grains shown in Fig. 2b. There exists lots of grains in Fig. 2c, and the selected area electron diffraction pattern (SADP) insert shows the grains are Ni4 W intermetallic compounds. SEM microstructure at interface in Wf /Cu82 Al10 Fe4 Ni4 composites is shown in Fig. 2d, from which interfacial reaction products on the surface of tungsten fibers can be observed, and energy disperse spectroscopy (EDS) insert shows tungsten, iron and nickel are the major elements of interfacial reaction products. Fig. 2e is TEM microstructure of Wf /Cu82 Al10 Fe4 Ni4 composites at interface, the SADP insert indicates that the interfacial reaction products are Fe–Ni solid solutions. The EDS and SADP analysis indicate that tungsten diffuse into Fe–Ni solid solutions on the interface of Wf /Cu82 Al10 Fe4 Ni4 composites. At the same time, there also exists other phase in matrix as shown in Fig. 2f, and the SADP insert indicates that is Al3 Ni intermetallic compound. 3.2. Test of interface strength The interface strength of the composites is tested by means of three-point bending test. Fig. 3 is the bending stress-displacement curves of the three kinds of composites. The result indicates that bending strength of Wf /Cu composites is lowest, while that of Wf /Cu82 Al10 Fe4 Ni4 and Wf (Ni)/Cu composites are obviously higher. As shown in Fig. 4a, copper is completely peeled from tungsten fibers and the surface of tungsten fibers keep integrally, which indicate that the fracture of Wf /Cu composites occurs at the interface between matrix and tungsten fibers. While the fracture of Wf /Cu82 Al10 Fe4 Ni4 and Wf (Ni)/Cu composites occur within tungsten fiber as shown in Fig. 4b and c. The result indicates that
Fig. 2. Microstructure of the three kinds of composites. (a) SEM image at interface in Wf /Cu composites. (b) SEM image at interface in Wf (Ni)/Cu composites. (c) TEM image of the area among the new generation tungsten grains shown in (b). (d) SEM image at interface in Wf /Cu82 Al10 Fe4 Ni4 composites. EDS insert shows the elements of interfacial reaction product. (e) TEM image at interface in Wf /Cu82 Al10 Fe4 Ni4 composites. SADP insert shows interfacial reaction product is Fe–Ni solid solution. (f) TEM image of Wf /Cu82 Al10 Fe4 Ni4 composites in matrix. SADP insert shows Al3 Ni intermetallic compound exists in matrix.
Z. Wu et al. / Materials Science and Engineering A 536 (2012) 45–48
Fig. 3. Bending stress-displacement curves of the composites.
interface strength of Wf /Cu composites is lowest, while interface strength of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites are substantially enhanced by means of plating nickel on tungsten fibers and adding alloy elements in copper matrix. The enhancement of interface strength is due to the chemical reaction between tungsten fibers and nickel in Wf (Ni)/Cu composites, while the enhancement of interface strength is due to the diffusion of tungsten into Fe–Ni solid solutions in Wf /Cu82 Al10 Fe4 Ni4 composites. Fracture morphology of Wf (Ni)/Cu composites and Wf /Cu82 Al10 Fe4 Ni4 composites are entirely different. As shown in the partial enlarged drawing of Fig. 4b, a lot of tungsten grains appear on the fracture surface of Wf (Ni)/Cu composites, which is consistent with the observation in Fig. 2b. While lots of fibrous substances torn within tungsten fibers appear on the fracture surface of Wf /Cu82 Al10 Fe4 Ni4 composites as shown in Fig. 4c. Because original tungsten fiber is drawn out from block tungsten under high-temperature, the inner structure of original tungsten fiber is fibrous, which makes mechanical property of tungsten fiber excellent. Fracture morphology of Wf /Cu82 Al10 Fe4 Ni4 composites indicates the inner structure of tungsten fibers in Wf /Cu82 Al10 Fe4 Ni4 composites and original tungsten fiber is consistent, and the preparation process of Wf /Cu82 Al10 Fe4 Ni4 composites rarely affects the inner structure of tungsten fibers. Nickel is plated on tungsten fibers in Wf (Ni)/Cu composites, when the heating temperature is above 1000 ◦ C, nickel on the tungsten surface will start to diffuse into tungsten fibers, which causes recrystallization of tungsten fibers [16–18]. The higher the heating
47
Fig. 5. Dynamic compressive stress–strain curves of the composites at the strain rate of 2000 s−1 .
temperature is, the deeper the nickel diffuses into, and the larger grains will be produced by recrystallization [18]. While the diffusion of nickel into tungsten fibers and the recrystallization of tungsten fibers are not found in Wf /Cu82 Al10 Fe4 Ni4 composites. As shown in Fig. 2e and f, nickel exists as the form of Fe–Ni solid solution and Al3 Ni intermetallic compound in Wf /Cu82 Al10 Fe4 Ni4 composites, so the diffusion and recrystallization are restrained. In addition, the addition of aluminum reduces the melting temperature of the copper alloy, which will further restrain the diffusion and recrystallization, so the fibrous structure of tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites as shown in Fig. 4c. 3.3. Fracture properties of the composites after dynamic compression In the dynamic compression test, tungsten fibers in the samples are arranged along the axial direction of the sample, and the impact direction is also along the axial direction of the sample. Fig. 5 shows dynamic compressive stress–strain curves of the three kinds of composites at the strain rate of 2000 s−1 . Flow stress of Wf /Cu composites is linear attenuation under dynamic compression. The dynamic compressive strength of Wf (Ni)/Cu composites is 2000 MPa, the strain at failure is 12%. While the dynamic compressive strength of Wf /Cu82 Al10 Fe4 Ni4 composites is 2500 MPa, plastic strain reaches to 16%. The plastic strain and dynamic compressive strength of Wf /Cu82 Al10 Fe4 Ni4 composites are highest among the three kinds of composites.
Fig. 4. SEM images of three-point bending fracture surfaces of (a) Wf /Cu composites, (b) Wf (Ni)/Cu composites, and (c) Wf /Cu82 Al10 Fe4 Ni4 composites.
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Fig. 6. SEM images of dynamic compressive fracture of (a) Wf /Cu composites, (b) Wf (Ni)/Cu composites, and (c) Wf /Cu82 Al10 Fe4 Ni4 composites.
Fig. 6 shows fracture morphology of the three kinds of composites after dynamic compression. Fig. 6a shows that tungsten fibers and copper matrix disperse during dynamic compression in Wf /Cu composites, and the partial enlarged drawing shows that only a little copper remains on the surface of tungsten fiber, which indicates the fracture occurs on the interface between tungsten fibers and copper matrix. It is noted that the flow stress is high at the beginning of compression, but it decays quickly with the compression progress in Wf /Cu composites as shown in Fig. 5. Because the inner structure of tungsten fibers rarely are influenced during the preparation process and tungsten fibers keep higher strength, Wf /Cu composites owns higher strength at the beginning stage. However, because the interface strength of Wf /Cu composites is poor, and there are some holes and other obvious defects at interface as shown in Fig. 2a, stress concentration will occur at the defects with the compression progress, which causes rapid extension of the cracks along the interface and makes the composites overall failure, so the dynamic compressive curve decays quickly at this phase. Brittle fracture occurs in Wf (Ni)/Cu composites after dynamic compression, which is broken into many pieces, and Fig. 6b is SEM microstructure of one of the pieces. The original tungsten fiber is drawn out from block tungsten under high-temperature, and the tungsten fiber is composed of a large number of small fibers, which are arranged along the axial direction of tungsten fiber vertically and uniaxially, and this kind of structure makes mechanical properties of tungsten fiber excellent. But the reaction between tungsten fibers and nickel, as shown in Fig. 2c, will produce lots of brittle Ni4 W intermetallic compounds, which will increase the brittleness of tungsten fibers, so Wf (Ni)/Cu composites displays feature of brittle fracture after dynamic compression. The partial enlarged drawing of Fig. 6b shows that lots of tungsten grains exist within tungsten fibers, which is consistent with observation in Fig. 2b and 4b. Wf (Ni)/Cu composites possesses higher bending strength and lower dynamic compressive strength compared with Wf /Cu82 Al10 Fe4 Ni4 composites, indicating that the granular structure within tungsten fiber enhances split resistance of tungsten fiber, but obviously reduces the axial compressive strength of tungsten fiber. Fig. 6c shows the fracture morphology of Wf /Cu82 Al10 Fe4 Ni4 composites after dynamic compression. There is obvious plastic deformation and a few of cracks on the surface of the sample. The fibrous structure of tungsten fibers can be observed at the crack in the partial enlarged drawing of Fig. 6c, which indicates that crack occurs and enlarges within the tungsten fiber and the interface strength is high under dynamic compression. The fracture morphology indicates that tungsten fibers play a major role and the split
of tungsten fibers will absorb the impact energy to avoid overall failure of Wf /Cu82 Al10 Fe4 Ni4 composites under dynamic compression. In addition, because the fibrous structure of tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites, the strength and plasticity of Wf /Cu82 Al10 Fe4 Ni4 composites are higher than that of Wf (Ni)/Cu composites under dynamic compression. 4. Conclusions 1. Interface strength of Wf /Cu composites can be enhanced by means of plating nickel on tungsten fiber and adding iron, nickel and aluminum elements in copper matrix. 2. The nickel diffusion will lead to recrystallization of tungsten fiber and generate lots of brittle Ni4 W intermetallic compounds within tungsten fibers in Wf (Ni)/Cu composites, which reduce axial strength and plasticity of tungsten fibers. 3. Tungsten diffuses into Fe–Ni solid solutions precipitated on the surface of tungsten fibers in Wf /Cu82 Al10 Fe4 Ni4 composites, which enhance the interface strength of the composites. Meanwhile, the fibrous structure within tungsten fibers is kept in Wf /Cu82 Al10 Fe4 Ni4 composites, so the composites possesses excellent strength and plasticity under dynamic compression. References [1] S. Pappu, C. Kennedy, L.E. Murr, L.S. Magness, D. Kapoor, Mater. Sci. Eng. A 262 (1999) 115–128. [2] J.X. Liu, S.K. Li, A.L. Fan, H.C. Sun, Mater. Sci. Eng. A 487 (2008) 235–242. [3] R.D. Conner, R.B. Dandliker, W.L. Johnson, Acta Mater. 46 (1998) 6089–6102. [4] H.F. Zhang, H. Li, A.M. Wang, H.M. Fu, B.Z. Ding, Z.Q. Hu, Intermetallics 17 (2009) 1070–1077. [5] H.G. Kim, K.T. Kim, Int. J. Mech. Sci. 42 (2000) 1339–1356. [6] W.F. Ma, H.C. Kou, C.S. Chen, J.S. Li, H. Chang, L. Zhou, H.Z. Fu, Mater. Sci. Eng. A 486 (2008) 308–312. [7] R.D. Conner, R.B. Dandliker, V. Scruggs, W.L. Johnson, Int. J. Impact Eng. 24 (2000) 435–444. [8] D. Dragoi, E. Üstündag, B. Clausen, M.M. Bourke, Scr. Mater. 45 (2001) 245–252. [9] B. Clausen, S.Y. Lee, E. Üstündag, C.C. Aydiner, R.D. Conner, M.A.M. Bourke, Scr. Mater. 2 (2003) 123–128. [10] X.H. Yang, S.H. Liang, X.H. Wang, P. Xiao, Z.K. Fan, Int. J. Refract. Met. Hard Mater. 28 (2010) 305–311. [11] J.G. Cheng, L. Wan, Y.B. Cai, J.C. Zhu, P. Song, J. Dong, J. Mater. Process. Technol. 1 (2010) 137–142. [12] S.H. Hong, B.K. Kim, Z.A. Munir, Mater. Sci. Eng. A 405 (2005) 325–332. [13] A. Ibrahim, M. Abdallah, S.F. Mostafa, A.A Hegazy, Mater. Des. 30 (2009) 1398–1403. [14] F. Doré, S. Lay, N. Eustathopoulos, C.H. Allibert, Scr. Mater. 49 (2003) 237–242. [15] L.P. Zhou, M.P. Wang, R. Wang, Z. Li, J.J. Zhu, K. Peng, D.Y. Li, S.L Li, Trans. Nonferrous Met. Soc. China 18 (2008) 372–377. [16] S.N. Mathaudhu, A.J. deRosset, K.T. Hartwig, L.J. Kecskes, Mater. Sci. Eng. A 503 (2009) 28–31. [17] W. Guo, S.K. Li, F.C. Wang, M. Wang, Scr. Mater. 60 (2009) 329–332. [18] I.H. Moon, K.Y. Kim, S.T. Oh, M.J. Suk, J. Alloys Compd. 201 (1993) 129–137.