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Al composite at high strain rates
Materials Science and Engineering A 487 (2008) 536–540
Dynamic deformation behavior of a high reinforcement content TiB2/Al composite at high strain rates Dezhi Zhu ∗ , Gaohui Wu, Guoqin Chen, Qiang Zhang School of Materials Science and Engineering, Harbin Institute of Technology, P.O. 3023, Science Park, No. 2 Yikuang Street, Harbin, Heilongjiang 150080, PR China Received 25 June 2007; received in revised form 17 October 2007; accepted 17 October 2007
Abstract Dynamic compressive properties of a 60 vol.% TiB2 /Al composite fabricated by squeeze casting method were measured using split Hopkinson pressure bar. The 60 vol.% TiB2 /Al composite showed significant strain-rate sensitivity compared with the rate insensitive aluminum alloy matrix. The flow stress and the strain-rate sensitivity both showed rise/fall tendency at high strain rates. Moreover, a large plastic strain as 7.8% was obtained at 1850 s−1 . The dynamic compression behavior of high reinforcement content composite was affected mostly by strain-rate strengthening and adiabatic heating softening mechanism at dynamic loading. Adiabatic heating cannot only accelerate the softening and flow of aluminum alloy matrix but also restrain the low-strain failure of high reinforcement content composites. The TiB2 /Al composite exhibited a mixed failure characteristic that consist of cracking of reinforcement particles and ductile fracture of matrix alloy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum matrix composite; Mechanical properties; High strain rate; Damage
1. Introduction Because of high specific strength, specific modulus and excellent dimension stability, the metal matrix composites (MMCs) have attracted more attention than the corresponding alloy matrix for structural applications [1,2]. Furthermore, the MMCs are promising candidate materials for lightweight armors and protective coatings used in defense and aerospace applications [3,4], where the MMCs components may be subjected to collision at a high speed. The dynamic properties of composite materials have been cited as criteria in structural material designs, and the correlated mechanism need to be understood further. The dynamic mechanical properties of metal matrix composites (MMCs) have been studied for about 20 years. The split Hopkinson pressure bar (SHPB), also called the Kolsky bar, has been widely used to investigate the high strain-rate response of MMCs, especially on aluminum matrix composites reinforced with fiber, whisker and particles. Most of the former
∗
Corresponding author. Tel.: +86 451 86402373x4053; fax: +86 451 86412164. E-mail address: [email protected] (D. Zhu). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.037
researchers focused on the composites with reinforcement content lower than 25 vol.%, including the strain-rate sensitivity, micro-damage, dynamic failure mode, and occurrence of adiabatic shear bands [5–9]. But for the application of electronic packaging substrates, armor and the satellite structural components, the composites containing more than 50 vol.% ceramic particles can provide higher specific strength, modulus and hardness and lower coefficient of thermal expansion. Li and Ramesh [10,11] predicted the dependence of the effective rate sensitivity on reinforcement content for particle-reinforced MMCs by numerical modeling. Results showed that the normalized flow stress ratio ranged 2–3 for spherical reinforcement particles with 50% volume fraction at 104 –105 s−1 . Marchi and co-workers [12] also measured the mechanical properties of 40–55% Al2 O3 /Al composites under quasi-static and dynamic loading. They found that the 55 vol.% Al2 O3 /Al composites show significant strain-rate sensitivity and large rate of damage accumulation. However, details on failure mechanism of these high reinforcement content composite materials have not yet been reported. In the present study, a 2024Al matrix composite containing 60% volume fraction of TiB2 was fabricated by squeeze casting method. The dynamic compression properties of the composite at the strain rate ranged 0.0007–1850 s−1 were studied. The
D. Zhu et al. / Materials Science and Engineering A 487 (2008) 536–540
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dynamic failure modes at different strain rates were observed using scanning electron microscope (SEM) and the corresponding failure mechanism is discussed. 2. Materials and experimental methods An Al–Cu–Mg alloy (2024Al) was selected as the matrix alloy. Mixed-sized TiB2 ceramic particles were chosen as the reinforcement in order to obtain a high reinforcement volume fraction. The size of TiB2 particle is in the range 4–25 m, with the average size of 8 m. The volume fraction of the particles in the composite is 60 vol.%. The TiB2 /2024Al composite was fabricated by squeeze casting technology. The fabrication processes are shown as following: (1) fabrication and pre-heating of TiB2 particles perform; (2) melting of aluminum alloy; (3) infiltration of molten aluminum into the TiB2 perform under pressure; (4) solidification of TiB2 /2024Al composite. T6 heat treatment was performed on TiB2 /Al composites. This involves a solid solution treatment at 768 K for 1 h followed by aging treatment at 433 K for 10 h. The dynamic compression specimens were 8 mm in diameter and 16 mm in length. The interfaces between the specimen and the compression platens were lubricated in order to reduce friction. In order to compare with high strain-rate response, the quasi-static compression tests were conducted on Instron machine with a constant cross-head speed of 0.5 mm/min. The diameter of quasi-static compression specimen is 8 mm and the length is 12 mm. All the compression specimens were observed using S-570 scanning electron microscope (SEM). 3. Results 3.1. Microstructure The microstructure of TiB2 /Al composite is shown in Fig. 1. It can be seen that different sized TiB2 particles are distributed uniformly and homogeneously in the composite. Although the TiB2 particles are pressed into the graphite mold by the machine, the larger TiB2 particles are unbroken and surrounded by many
Fig. 1. Microstructure of TiB2 /2024Al composite.
Fig. 2. Compressive stress–strain curves of the composite under quasi-static and dynamic loading.
small particles. And the composite is free from common cast defects such as porosity and shrinkage cavities. 3.2. Dynamic compression properties The compression stress–strain curves of TiB2 /Al composite under quasi-static and dynamic loading are shown in Fig. 2. The quasi-static compressive specimen exhibits a long elastic deformation process at 0.0007 s−1 with little plastic deformation before failed. The flow stress at 1100 s−1 increases continuously with the increasing strain. When compressed at 1400 s−1 the flow stress of the composite increases firstly then drops at 1.5% strain. In present work, all the specimens exhibited low-strain brittle fracture except the samples deformed at 1850 s−1 with a plastic strain of 7.8%. At high strain rates, it is clear that flow stress of the composite displays a rise/fall tendency with increasing of strain rate. As referred to the flow stress–strain curves at a 1.5% strain, the flow stress of the composite increases with an increase in strain rates (ranged 0.0007–1400 s−1 ), and the flow stress is 211–1132 MPa. We should note that when the strain rate is 1850 s−1 , its compressive strength (at 1.5% strain) drops to 926 MPa, which is lower than those at 1100 s−1 and 1400 s−1 . The composite specimens compressed at 0.0007 s−1 and at 1100 s−1 both show typical brittle characteristic, which are dominated by the cracking of reinforcement particles, as shown in Fig. 3(a) and (b). At 1100 s−1 it exhibits a planar fracture surface and seems to be more brittle than those under quasi-static loading. As revealed in Fig. 3(c), the matrix aluminum alloy was observed to have melted along a certain orientation, pointed as arrows, and the reinforcement particles were covered with a thin layer of melted aluminum alloy. It is consistent with the decreased flow stress at 1.5% strain when compressed at 1400 s−1 . Fig. 3(d) shows a typical ductile fracture characteristic for high reinforcement fraction composite which has never been reported before. Note that the matrix alloy is characterized by ductile tearing and elongating in the raised platform, pointed as arrows A and B. Obviously, as the strain rate increases from 0.0007 s−1 to 1850 s−1 the failure modes of high reinforcement fraction com-
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Fig. 3. SEM micrographs of the fracture surface at varies strain rates. (a) 0.0007 s−1 ; (b) 1100 s−1 ; (c) 1400 s−1 ; (b) 1850 s−1 .
Fig. 4. Adiabatic heating phenomenon on the dynamic fracture surfaces at 1600 s−1 . (a) Band of melted aluminum alloy; (b) corresponding element analysis.
posite changes gradually from brittle fracture to ductile fracture. It is determined essentially by the softening and fluidity of matrix aluminum alloy. In the present work, the melted aluminum alloy band is also observed in the fracture surfaces compressed at 1600 s−1 , as shown in Fig. 4(a). The Al, Cu, Mg and Ti elements are found on the picked point, in Fig. 4(b). The width of melted aluminum alloy band is about 1–50 m. Some small ridges of melted aluminum alloy can also be found in local area.
at high strain rates, the larger particles are prone to rupture from the weakened narrow region, angles and the contact points where several particles interacted. Such failures appear to be predominant in regions where particle clustering exists.
3.3. Damage The broken specimens compressed at 1100 s−1 were sectioned parallel to the loading direction and polished for internal damage observation. As shown in Fig. 5, the micro-damage of high reinforcement content composite is mostly dominated by particle cracking. Because the contact degree of TiB2 particles increases dramatically for high reinforcement content composites, the external load can be transferred directly via the rigid TiB2 particles, not by the soft matrix alloy. When compressed
Fig. 5. Micro-damage of TiB2 /2024Al composite under dynamic loading.
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4. Discussion
4.2. Thermal softening
4.1. Strain-rate sensitivity
Part of the flow stress softening originated from 1.5% strain at 1400 s−1 and the decrease of rate sensitivity at 1850 s−1 can be ascribed to thermal softening. These results are different with the experimental results of low reinforcement fraction (5–25 vol.%) composites researched before [5,11,15]. Thermal softening occurs simultaneously with strain strengthening during the course of high strain-rate deformation. The average adiabatic temperature rise T within the MMC can be calculated as following [16]: εf βσ T = dε (2) ρC m 0
The high reinforcement content composite shows a much higher flow stress at strain rates ranged 0.0007–1850 s−1 than that of quasi-static loading before 1.5% strain. We used the strain-rate sensitivity parameter proposed by Hong and coworkers [13], which is defined as following: R=
1 σd − σ s σs ln(˙εd /˙εs )
(1)
where R is the ratio of strain-rate sensitivity; σ d and σ s are the dynamic and quasi-static flow stresses at a constant strain, respectively. ε˙ d and ε˙ s are the corresponding strain rates for dynamic and quasi-static loading. At a strain of 5%, it can be seen that the quasi-static compression specimens still keep elastic deformation, yet the compression specimens at strain rates of 1100 s−1 and 1400 s−1 have failed. Thus a flow stress at a strain rate of 1850 s−1 is chosen to calculate the ratio of strainrate sensitivity. For the 60 vol.% TiB2 /2024Al composite at 6% strain, a sensitivity of 0.048 is obtained. Some researchers concluded that the strain-rate sensitivity of composite is correlated to the rate sensitivity of matrix alloy [11,12]. While the 2024-T6 alloy was reported to have a low rate sensitivity parameter of about 0.006 at 6% strain in the range 10−2 –103 s−1 [14]. Thus we consider that the high strain-rate sensitivity of the TiB2 /2024Al composite is dominated by high volume content of reinforcement phase. As shown in Fig. 6, the relative strain-rate sensitivity parameter increases with increasing reinforcement volume fraction. And these results agree with the prediction made by Li and Ramesh [10]. In the present work, the reinforcement volume content reaches 60% and it acts as a continuous phase in the composite. The external load can be transferred directly via the TiB2 particles, not by the soft matrix alloy. When compressed at high rates, the composite can afford a high strength because of the supporting of rigid TiB2 framework.
where β is the fraction of the plastic work converts into heat, for example, a value of 0.8 for aluminum alloy; εf is the final plastic strain; ρ is the density and Cm is the specific heat for matrix alloy. For 60 vol.% TiB2 /Al composite under dynamic loading, the average temperature rise calculated would be about 20 K at 5% strain. An adiabatic temperature rise less than 5 K for 55% Al2 O3 /Al composites is also reported by Marchi co-workers [12]. While it was neglected because the author considered that the flow stresses of composites would not be affected. In fact, adiabatic shearing is just happened in local regions with much higher temperature for metals as Al, Cu and Fe. So the average temperature rise as T is not proper for analyzing the effect of thermal softening. Because of a lower thermal conductivity for TiB2 particle than that of 2024Al alloy, abundant heat cannot be disseminated immediately through the TiB2 –Al interfaces. This causes a temperature rise in narrow and local regions. Local temperature rise has no effect on reinforcement particles, while it can lead the low melting point matrix alloy to be softened locally, or even be melted. In the present work, the estimated temperature rise for melted aluminum alloy band is higher than 660 K. Previous researchers also reported a temperature rise ranging 800–1000 K for Fe alloy [16]. Therefore, an improvement on plasticity is obtained for high reinforcement content metal matrix composites at high strain rates. It can resist the impact loading effectively and afford a high compressive strength for engineering application. 5. Conclusion In this paper, the quasi-static and dynamic characteristics of TiB2 /Al composite with different loading rates (0.0007– 1850 s−1 ) under uniaxial compression are experimentally studied by means of an Instron machine and split Hopkinson pressure bar (SHPB). Some important conclusions are listed as follows:
Fig. 6. Relative strain-rate sensitivity R of aluminum alloys and aluminum matrix composites.
1. The 60 vol.% TiB2 /Al composite showed significant strainrate sensitivity at high strain rates, and it also exhibits a high flow stress about 1.2 GPa and a plastic strain of 7.8%. 2. Adiabatic heating decreases the strain-rate sensitivity and compressive strength, and improves the plastic deformation ability for high reinforcement content composite.
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3. The composite fails in a mixed fracture mode, which involves cracking of reinforcement particles and elongating of local matrix alloy. Micro-damage of high reinforcement content composite is mostly dominated by particle cracking. References [1] Q. Zhang, G.Q. Chen, G.H. Wu, Z.Y. Xiu, B.F. Luan, Mater. Lett. 57 (2003) 1453. [2] M. Zhao, G.H. Wu, Z.Y. Dou, L.T. Jiang, Mater. Sci. Eng. A 374 (2004) 303. [3] D.J. Lloyd, Int. Mater. Rev. 39 (1994) 1. [4] T. Christman, A. Needleman, S. Suresh, Acta Metall. 37 (1989) 3029. [5] Q. Wei, Y.M. Wang, K.T. Ramesh, E. Ma, Mater. Sci. Eng. A 381 (2004) 71.
[6] C.C. Perng, J.R. Hwang, J.L. Dong, Scripta Mater. 29 (1993) 311. [7] H. Zhang, K.T. Ramesh, E.S.C. Chin, Mater. Sci. Eng. A 384 (2004) 26. [8] D.R. Chichili, K.T. Ramesh, Int. J. Solids Struct. 32 (1995) 2609. [9] L.H. Dai, Z. Ling, Y.L. Bai, Compos. Sci. Technol. 61 (2001) 1057. [10] Y. Li, K.T. Ramesh, Acta Mater. 46 (1998) 5633. [11] Y. Li, K.T. Ramesh, E.SC. Chin, Acta Mater. 48 (2000) 1563. [12] C.S. Marchi, F.H. Cao, M. Kouzeli, A. Mortensen, Mater. Sci. Eng. A 337 (2002) 202. [13] S.I. Hong, G.T. Gray III, J.J. Lewandowski, Acta Metall. Mater. 41 (1995) 2337. [14] M. Guden, I.W. Hall, Mater. Sci. Eng. A 242 (1998) 141. [15] M.A. Irfan, V. Prakash, Int. J. Solids Struct. 37 (2000) 4477. [16] Q. Wei, L. Kecskes, T. Jiao, K.T. Hartwig, K.T. Ramesh, E. Ma, Acta Mater. 52 (2004) 1859.
Dynamic deformation behavior of a high reinforcement content TiB2/Al composite at high strain rates Dezhi Zhu ∗ , Gaohui Wu, Guoqin Chen, Qiang Zhang School of Materials Science and Engineering, Harbin Institute of Technology, P.O. 3023, Science Park, No. 2 Yikuang Street, Harbin, Heilongjiang 150080, PR China Received 25 June 2007; received in revised form 17 October 2007; accepted 17 October 2007
Abstract Dynamic compressive properties of a 60 vol.% TiB2 /Al composite fabricated by squeeze casting method were measured using split Hopkinson pressure bar. The 60 vol.% TiB2 /Al composite showed significant strain-rate sensitivity compared with the rate insensitive aluminum alloy matrix. The flow stress and the strain-rate sensitivity both showed rise/fall tendency at high strain rates. Moreover, a large plastic strain as 7.8% was obtained at 1850 s−1 . The dynamic compression behavior of high reinforcement content composite was affected mostly by strain-rate strengthening and adiabatic heating softening mechanism at dynamic loading. Adiabatic heating cannot only accelerate the softening and flow of aluminum alloy matrix but also restrain the low-strain failure of high reinforcement content composites. The TiB2 /Al composite exhibited a mixed failure characteristic that consist of cracking of reinforcement particles and ductile fracture of matrix alloy. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum matrix composite; Mechanical properties; High strain rate; Damage
1. Introduction Because of high specific strength, specific modulus and excellent dimension stability, the metal matrix composites (MMCs) have attracted more attention than the corresponding alloy matrix for structural applications [1,2]. Furthermore, the MMCs are promising candidate materials for lightweight armors and protective coatings used in defense and aerospace applications [3,4], where the MMCs components may be subjected to collision at a high speed. The dynamic properties of composite materials have been cited as criteria in structural material designs, and the correlated mechanism need to be understood further. The dynamic mechanical properties of metal matrix composites (MMCs) have been studied for about 20 years. The split Hopkinson pressure bar (SHPB), also called the Kolsky bar, has been widely used to investigate the high strain-rate response of MMCs, especially on aluminum matrix composites reinforced with fiber, whisker and particles. Most of the former
∗
Corresponding author. Tel.: +86 451 86402373x4053; fax: +86 451 86412164. E-mail address: [email protected] (D. Zhu). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.10.037
researchers focused on the composites with reinforcement content lower than 25 vol.%, including the strain-rate sensitivity, micro-damage, dynamic failure mode, and occurrence of adiabatic shear bands [5–9]. But for the application of electronic packaging substrates, armor and the satellite structural components, the composites containing more than 50 vol.% ceramic particles can provide higher specific strength, modulus and hardness and lower coefficient of thermal expansion. Li and Ramesh [10,11] predicted the dependence of the effective rate sensitivity on reinforcement content for particle-reinforced MMCs by numerical modeling. Results showed that the normalized flow stress ratio ranged 2–3 for spherical reinforcement particles with 50% volume fraction at 104 –105 s−1 . Marchi and co-workers [12] also measured the mechanical properties of 40–55% Al2 O3 /Al composites under quasi-static and dynamic loading. They found that the 55 vol.% Al2 O3 /Al composites show significant strain-rate sensitivity and large rate of damage accumulation. However, details on failure mechanism of these high reinforcement content composite materials have not yet been reported. In the present study, a 2024Al matrix composite containing 60% volume fraction of TiB2 was fabricated by squeeze casting method. The dynamic compression properties of the composite at the strain rate ranged 0.0007–1850 s−1 were studied. The
D. Zhu et al. / Materials Science and Engineering A 487 (2008) 536–540
537
dynamic failure modes at different strain rates were observed using scanning electron microscope (SEM) and the corresponding failure mechanism is discussed. 2. Materials and experimental methods An Al–Cu–Mg alloy (2024Al) was selected as the matrix alloy. Mixed-sized TiB2 ceramic particles were chosen as the reinforcement in order to obtain a high reinforcement volume fraction. The size of TiB2 particle is in the range 4–25 m, with the average size of 8 m. The volume fraction of the particles in the composite is 60 vol.%. The TiB2 /2024Al composite was fabricated by squeeze casting technology. The fabrication processes are shown as following: (1) fabrication and pre-heating of TiB2 particles perform; (2) melting of aluminum alloy; (3) infiltration of molten aluminum into the TiB2 perform under pressure; (4) solidification of TiB2 /2024Al composite. T6 heat treatment was performed on TiB2 /Al composites. This involves a solid solution treatment at 768 K for 1 h followed by aging treatment at 433 K for 10 h. The dynamic compression specimens were 8 mm in diameter and 16 mm in length. The interfaces between the specimen and the compression platens were lubricated in order to reduce friction. In order to compare with high strain-rate response, the quasi-static compression tests were conducted on Instron machine with a constant cross-head speed of 0.5 mm/min. The diameter of quasi-static compression specimen is 8 mm and the length is 12 mm. All the compression specimens were observed using S-570 scanning electron microscope (SEM). 3. Results 3.1. Microstructure The microstructure of TiB2 /Al composite is shown in Fig. 1. It can be seen that different sized TiB2 particles are distributed uniformly and homogeneously in the composite. Although the TiB2 particles are pressed into the graphite mold by the machine, the larger TiB2 particles are unbroken and surrounded by many
Fig. 1. Microstructure of TiB2 /2024Al composite.
Fig. 2. Compressive stress–strain curves of the composite under quasi-static and dynamic loading.
small particles. And the composite is free from common cast defects such as porosity and shrinkage cavities. 3.2. Dynamic compression properties The compression stress–strain curves of TiB2 /Al composite under quasi-static and dynamic loading are shown in Fig. 2. The quasi-static compressive specimen exhibits a long elastic deformation process at 0.0007 s−1 with little plastic deformation before failed. The flow stress at 1100 s−1 increases continuously with the increasing strain. When compressed at 1400 s−1 the flow stress of the composite increases firstly then drops at 1.5% strain. In present work, all the specimens exhibited low-strain brittle fracture except the samples deformed at 1850 s−1 with a plastic strain of 7.8%. At high strain rates, it is clear that flow stress of the composite displays a rise/fall tendency with increasing of strain rate. As referred to the flow stress–strain curves at a 1.5% strain, the flow stress of the composite increases with an increase in strain rates (ranged 0.0007–1400 s−1 ), and the flow stress is 211–1132 MPa. We should note that when the strain rate is 1850 s−1 , its compressive strength (at 1.5% strain) drops to 926 MPa, which is lower than those at 1100 s−1 and 1400 s−1 . The composite specimens compressed at 0.0007 s−1 and at 1100 s−1 both show typical brittle characteristic, which are dominated by the cracking of reinforcement particles, as shown in Fig. 3(a) and (b). At 1100 s−1 it exhibits a planar fracture surface and seems to be more brittle than those under quasi-static loading. As revealed in Fig. 3(c), the matrix aluminum alloy was observed to have melted along a certain orientation, pointed as arrows, and the reinforcement particles were covered with a thin layer of melted aluminum alloy. It is consistent with the decreased flow stress at 1.5% strain when compressed at 1400 s−1 . Fig. 3(d) shows a typical ductile fracture characteristic for high reinforcement fraction composite which has never been reported before. Note that the matrix alloy is characterized by ductile tearing and elongating in the raised platform, pointed as arrows A and B. Obviously, as the strain rate increases from 0.0007 s−1 to 1850 s−1 the failure modes of high reinforcement fraction com-
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Fig. 3. SEM micrographs of the fracture surface at varies strain rates. (a) 0.0007 s−1 ; (b) 1100 s−1 ; (c) 1400 s−1 ; (b) 1850 s−1 .
Fig. 4. Adiabatic heating phenomenon on the dynamic fracture surfaces at 1600 s−1 . (a) Band of melted aluminum alloy; (b) corresponding element analysis.
posite changes gradually from brittle fracture to ductile fracture. It is determined essentially by the softening and fluidity of matrix aluminum alloy. In the present work, the melted aluminum alloy band is also observed in the fracture surfaces compressed at 1600 s−1 , as shown in Fig. 4(a). The Al, Cu, Mg and Ti elements are found on the picked point, in Fig. 4(b). The width of melted aluminum alloy band is about 1–50 m. Some small ridges of melted aluminum alloy can also be found in local area.
at high strain rates, the larger particles are prone to rupture from the weakened narrow region, angles and the contact points where several particles interacted. Such failures appear to be predominant in regions where particle clustering exists.
3.3. Damage The broken specimens compressed at 1100 s−1 were sectioned parallel to the loading direction and polished for internal damage observation. As shown in Fig. 5, the micro-damage of high reinforcement content composite is mostly dominated by particle cracking. Because the contact degree of TiB2 particles increases dramatically for high reinforcement content composites, the external load can be transferred directly via the rigid TiB2 particles, not by the soft matrix alloy. When compressed
Fig. 5. Micro-damage of TiB2 /2024Al composite under dynamic loading.
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4. Discussion
4.2. Thermal softening
4.1. Strain-rate sensitivity
Part of the flow stress softening originated from 1.5% strain at 1400 s−1 and the decrease of rate sensitivity at 1850 s−1 can be ascribed to thermal softening. These results are different with the experimental results of low reinforcement fraction (5–25 vol.%) composites researched before [5,11,15]. Thermal softening occurs simultaneously with strain strengthening during the course of high strain-rate deformation. The average adiabatic temperature rise T within the MMC can be calculated as following [16]: εf βσ T = dε (2) ρC m 0
The high reinforcement content composite shows a much higher flow stress at strain rates ranged 0.0007–1850 s−1 than that of quasi-static loading before 1.5% strain. We used the strain-rate sensitivity parameter proposed by Hong and coworkers [13], which is defined as following: R=
1 σd − σ s σs ln(˙εd /˙εs )
(1)
where R is the ratio of strain-rate sensitivity; σ d and σ s are the dynamic and quasi-static flow stresses at a constant strain, respectively. ε˙ d and ε˙ s are the corresponding strain rates for dynamic and quasi-static loading. At a strain of 5%, it can be seen that the quasi-static compression specimens still keep elastic deformation, yet the compression specimens at strain rates of 1100 s−1 and 1400 s−1 have failed. Thus a flow stress at a strain rate of 1850 s−1 is chosen to calculate the ratio of strainrate sensitivity. For the 60 vol.% TiB2 /2024Al composite at 6% strain, a sensitivity of 0.048 is obtained. Some researchers concluded that the strain-rate sensitivity of composite is correlated to the rate sensitivity of matrix alloy [11,12]. While the 2024-T6 alloy was reported to have a low rate sensitivity parameter of about 0.006 at 6% strain in the range 10−2 –103 s−1 [14]. Thus we consider that the high strain-rate sensitivity of the TiB2 /2024Al composite is dominated by high volume content of reinforcement phase. As shown in Fig. 6, the relative strain-rate sensitivity parameter increases with increasing reinforcement volume fraction. And these results agree with the prediction made by Li and Ramesh [10]. In the present work, the reinforcement volume content reaches 60% and it acts as a continuous phase in the composite. The external load can be transferred directly via the TiB2 particles, not by the soft matrix alloy. When compressed at high rates, the composite can afford a high strength because of the supporting of rigid TiB2 framework.
where β is the fraction of the plastic work converts into heat, for example, a value of 0.8 for aluminum alloy; εf is the final plastic strain; ρ is the density and Cm is the specific heat for matrix alloy. For 60 vol.% TiB2 /Al composite under dynamic loading, the average temperature rise calculated would be about 20 K at 5% strain. An adiabatic temperature rise less than 5 K for 55% Al2 O3 /Al composites is also reported by Marchi co-workers [12]. While it was neglected because the author considered that the flow stresses of composites would not be affected. In fact, adiabatic shearing is just happened in local regions with much higher temperature for metals as Al, Cu and Fe. So the average temperature rise as T is not proper for analyzing the effect of thermal softening. Because of a lower thermal conductivity for TiB2 particle than that of 2024Al alloy, abundant heat cannot be disseminated immediately through the TiB2 –Al interfaces. This causes a temperature rise in narrow and local regions. Local temperature rise has no effect on reinforcement particles, while it can lead the low melting point matrix alloy to be softened locally, or even be melted. In the present work, the estimated temperature rise for melted aluminum alloy band is higher than 660 K. Previous researchers also reported a temperature rise ranging 800–1000 K for Fe alloy [16]. Therefore, an improvement on plasticity is obtained for high reinforcement content metal matrix composites at high strain rates. It can resist the impact loading effectively and afford a high compressive strength for engineering application. 5. Conclusion In this paper, the quasi-static and dynamic characteristics of TiB2 /Al composite with different loading rates (0.0007– 1850 s−1 ) under uniaxial compression are experimentally studied by means of an Instron machine and split Hopkinson pressure bar (SHPB). Some important conclusions are listed as follows:
Fig. 6. Relative strain-rate sensitivity R of aluminum alloys and aluminum matrix composites.
1. The 60 vol.% TiB2 /Al composite showed significant strainrate sensitivity at high strain rates, and it also exhibits a high flow stress about 1.2 GPa and a plastic strain of 7.8%. 2. Adiabatic heating decreases the strain-rate sensitivity and compressive strength, and improves the plastic deformation ability for high reinforcement content composite.
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3. The composite fails in a mixed fracture mode, which involves cracking of reinforcement particles and elongating of local matrix alloy. Micro-damage of high reinforcement content composite is mostly dominated by particle cracking. References [1] Q. Zhang, G.Q. Chen, G.H. Wu, Z.Y. Xiu, B.F. Luan, Mater. Lett. 57 (2003) 1453. [2] M. Zhao, G.H. Wu, Z.Y. Dou, L.T. Jiang, Mater. Sci. Eng. A 374 (2004) 303. [3] D.J. Lloyd, Int. Mater. Rev. 39 (1994) 1. [4] T. Christman, A. Needleman, S. Suresh, Acta Metall. 37 (1989) 3029. [5] Q. Wei, Y.M. Wang, K.T. Ramesh, E. Ma, Mater. Sci. Eng. A 381 (2004) 71.
[6] C.C. Perng, J.R. Hwang, J.L. Dong, Scripta Mater. 29 (1993) 311. [7] H. Zhang, K.T. Ramesh, E.S.C. Chin, Mater. Sci. Eng. A 384 (2004) 26. [8] D.R. Chichili, K.T. Ramesh, Int. J. Solids Struct. 32 (1995) 2609. [9] L.H. Dai, Z. Ling, Y.L. Bai, Compos. Sci. Technol. 61 (2001) 1057. [10] Y. Li, K.T. Ramesh, Acta Mater. 46 (1998) 5633. [11] Y. Li, K.T. Ramesh, E.SC. Chin, Acta Mater. 48 (2000) 1563. [12] C.S. Marchi, F.H. Cao, M. Kouzeli, A. Mortensen, Mater. Sci. Eng. A 337 (2002) 202. [13] S.I. Hong, G.T. Gray III, J.J. Lewandowski, Acta Metall. Mater. 41 (1995) 2337. [14] M. Guden, I.W. Hall, Mater. Sci. Eng. A 242 (1998) 141. [15] M.A. Irfan, V. Prakash, Int. J. Solids Struct. 37 (2000) 4477. [16] Q. Wei, L. Kecskes, T. Jiao, K.T. Hartwig, K.T. Ramesh, E. Ma, Acta Mater. 52 (2004) 1859.