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Microstructure and properties at elevated temperature of a nano-Al2O3 particles dispersion-strengthened copper base composite
Materials Science and Engineering A 435–436 (2006) 705–710
Microstructure and properties at elevated temperature of a nano-Al2O3 particles dispersion-strengthened copper base composite Baohong Tian ∗ , Ping Liu, Kexing Song, Yan Li, Yong Liu, Fengzhang Ren, Juanhua Su School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Received 17 May 2005; received in revised form 24 July 2006; accepted 26 July 2006
Abstract High softening resistance, high strength at elevated temperature and high conductivity is important performances for the high voltage breaker and vacuum interrupter contact materials. A novel nano-Al2 O3 –Cu composite was fabricated for them by using a modified internal oxidation method. The microstructure was investigated by means of a scanning electron microscope and a transmission electron microscopy. The softening temperature, tensile strength at elevated temperature and electrical conductivity of the composite were also determined. The grain size of the Cu–0.5 vol.% Al2 O3 composite is very fine and the annealing temperature greatly affects its microstructure. The electric conductivity reaches 93% IACS and the softening temperature is about 800 ◦ C, while the yield strength reaches 169 MPa and the ultimate tensile strength reaches 172 MPa at the testing temperature of 600 ◦ C. The main contributions of nano-alumina particles is to strengthen the composite, not by Orowan strengthening, but due to the strong pinning effects on the grain boundaries and sub-grain boundaries with high dislocation density to prevent the nuclei formation of recrystallization. © 2006 Elsevier B.V. All rights reserved. Keywords: Internal oxidation; Softening temperature; Strength at elevated temperature; Conductivity
1. Introduction Oxide dispersion strengthened copper base composite (ODSC) has been successfully adopted as key electrical conductive component material in gas insulated switches such as breaker and vacuum interrupter used in high voltage power supply appliances, which are very important for power supply networks to operate safely. The key materials of breaker and vacuum interrupters are the contact terminals and switch lever materials, etc., which are usually made of cold deformed pure copper or precipitates hardened copper alloys such as Cu–Cr, Cu–Ni–Si and Cu–Cr–Zr alloys. The cold deformation hardened copper and precipitates hardened copper alloys are easily softened, or contact tips can be fused and stick together resulting from short circuit operation. Thus, it is urgent to develop a novel material with a high softening temperature, high electrical conductivity and high strength at elevated temperature to overcome these shortcomings and to meet the needs of contact terminals and switch levers. For an ODSC material, the alumina particles have been adopted owing to its superior advantages such as high melting point, high hardness and excellent ther∗
Corresponding author. E-mail address: [email protected] (B. Tian).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.129
mal stability and chemical inertness. Therefore, the dispersion distributed alumina particles can increase the recrystallization temperature and exhibit excellent strength at elevated temperature by pinning down the grain and sub-grain boundaries of the copper matrix and impeding the movement of dislocations [1,2]. Furthermore, the Cu–Al2 O3 composites have almost the same electrical and thermal conductivities as pure copper, but better creep and arc erosion resistance at elevated temperatures [3–6]. The common fabrication processing technology of Cu–Al2 O3 composites is powder metallurgical technology with internal oxidation method, which is to oxidize the Cu–Al alloy powder internally followed by reducing, cold shaping and sintering. In this paper, a modified internal oxidation powder metallurgical technique was adopted to prepare the Cu–Al2 O3 composite. The microstructure, softening temperature, Brinell hardness, strength at elevated temperature and electrical conductivity was investigated compared with a cold deformed pure copper with the same cold deformation value. 2. Experiment The Cu–0.5 vol.% Al2 O3 ODSC was used as the experimental material fabricated by a modified internal oxidation method. The preparation procedures are as follows.
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The Cu–Al alloy was atomized with a commercial nitrogen gas followed by drying and sizing. The above Cu–Al alloy powders was internal oxidized at 900 ◦ C for 5 h using flowing nitrogen gas protection followed by hydrogen reducing at 800 ◦ C. Then the internal oxidized and reduced powders were isostaticly cold pressed to a columnar billet followed by sintering at 950 ◦ C. The sintered columnar billet was hot-extruded at 850 ◦ C and cold drawn (60% deformation) to a rod bar of 16 mm in diameter. In order to evaluate the softening resistance at elevated temperature of the Cu–0.5 vol.% Al2 O3 composite, samples were annealed in a muffle electric resistance furnace under a flowing atmosphere of argon with temperature accuracy of ±5 ◦ C at temperature of 300–1050 ◦ C, respectively, accompanied with as-cold drawn pure copper with 60% cold deformation simultaneously. After annealing treatment, the specimens were mechanically ground with waterproof abrasive paper and then polished for electrical conductivity and hardness test. The Brinell hardness of the specimens were measured under a 250 kg load and held for 30 s according to ASTM E10-01e1 Standard Test Method. The tensile properties at room temperature and at elevated temperature were measured according to ASTM E8M-04 and ASTM E21-03a Standard Test Methods with a Shimadzu AGI-250kN universal material test machine and its elevated temperature attachments, respectively. The specimen size is 5 mm in diameter with a 50 mm gauge length. The furnace temperature was varied from room temperature to 300–1050 ◦ C and held for 20 min with the accuracy within ±5 ◦ C. The electrical conductivity was determined by measuring the resistance of sample in 100 mm length using a ZY9987-type standard direct-current four-probe technique. The mean value of three measurements had an estimated accuracy of less than ±0.0002 .
Fig. 1. Back-scattering electron SEM image of Cu–0.5 vol.% Al2 O3 composite (as-hot extruded, un-etched).
The microstructures of the specimens were observed using a transmission electron microscope (TEM). The TEM specimens cut from the as-annealed samples were prepared by conventional double-side electro-polishing method using an electrolyte of HNO3 :CH2 OH = 1:3. The electron microscopy for this study was carried out using a H-800 TEM at 200 kV. The fracture surface observation of the tensile specimen was carried out with a JSM-5610LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure of Al2 O3 –Cu composite The back-scattering electron SEM image of Cu–0.5 vol.% Al2 O3 composite as hot extruded is shown in Fig. 1. It can be seen that there are bright areas and dark areas presenting dif-
Fig. 2. Alumina particles distribution (a, as-extruded) and dislocations in Cu–0.5 vol.% Al2 O3 composite (b, as-drawn).
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Fig. 3. TEM images of Cu–0.5 vol.% Al2 O3 annealed at 600 ◦ C (a) and 1050 ◦ C (b), respectively.
ferent oxygen content. The bright areas indicate relative lower oxygen content, which have the size of 5–10 m and are in relation to the middle parts of internal oxidized Cu–Al alloy particle after sintering. While the dark areas indicate relative higher oxygen content, which are in relation to the original surface layers of Cu–Al alloy particles after internal oxidation and sintering. It can be concluded that the solid state sintering at elevated temperature did not coarsen the fine pre-internally oxidized Cu–Al alloy particles but helped to maintain their original fine size, which is mainly owing to the pinning effect of internal oxidized Al2 O3 particles on the movement of original surface of Cu–Al alloy particles. Fig. 2 shows the TEM images of Al2 O3 –Cu composite fabricated by the internal oxidation method. It can be seen that a mass of internal oxidized Al2 O3 particles distribute homogeneously within the matrix grain and on the grain boundaries. The alumina particles have a size of 20–50 nm and interparticle spacing of 30–120 nm (Fig. 2a). After following strong cold deformation, there is a high dislocation density (Fig. 2b). Fig. 3 shows the TEM images of Cu–0.5 vol.% Al2 O3 composite annealed at 600 and 1050 ◦ C, respectively. It can be inferred that the mixed microstructure of fine equiaxial grains and elongate deformed grains with grain size varying from 0.25 to 0.5 m coexist together after annealing at 600 ◦ C for 1 h. There are large numbers of dislocations tangling around the grain boundaries and sub-grain boundaries, which are contributed to the interaction of the movement of sub-grain boundaries consisting of dislocations and the pinned nano-alumina particles. It is also found that recrystallization occurs in partially deformed grains of the matrix (Fig. 3) and corresponds to the Brinell hardness decreasing. After annealing treatment at 1050 ◦ C for 1 h, the Cu–0.5 vol.% Al2 O3 composite matrix is completely recrystallized having equiaxial grains.
average softening rate is about 5HB/100 ◦ C, which is attributed to its relative lower volume fraction of alumina particle and the difference of pinning effects of various located alumina particles in the composite matrix. It can be testified by the microstructure features of coexisting of fine equiaxial grains and elongate deformed grains after annealed at 600 ◦ C in Fig. 3a. The softening temperature is defined as the annealing temperature in relation to the sharp decrease of the composite hardness. Thus, it is inferred from Fig. 4 that the softening temperature of cold deformed pure copper is about 400 ◦ C and that of Cu–0.5 vol.% Al2 O3 composite is about 800 ◦ C. In this experiment, the composite hardness is higher than that of cold deformed pure copper. The composite hardness after annealed at 900 ◦ C is equivalent to that of as-cold deformed pure copper. While the surface temperature of contact terminals for high voltage power supply is lower than 600 ◦ C in operation [3]. The Cu–0.5 vol.% Al2 O3 composite Brinell hardness is 96 after annealed at 600 ◦ C, which is higher than that of as cold deformed pure copper and is suitable for switches material for high voltage power supply. The mechanism responsible for the composite softening with increasing of annealing temperature is probably attributed to
3.2. Softening resistance Fig. 4 shows the relationship of Brinell hardness of Al2 O3 –Cu composite after annealing treatment with annealing temperature. With increasing the annealing temperature, the hardness slowly decreases from Brinell hardness value of 126 at room temperature without annealing to 87 after annealing at 800 ◦ C. The
Fig. 4. Brinell hardness of Cu–0.5 vol.% Al2 O3 composite and pure copper with annealing temperature.
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the effects of the following: (a) the distinguished differences of crystal structure, lattice parameters, specific volume, Young’s module between alumina particles and copper matrix result in higher phase interface energy, which in return need to be overcome the increased resistance as the phase interface bending into a curve when applied stress is loaded. (b) Although the volume fraction of internal oxidized Al2 O3 is so small and is only about 0.5%, the nano-alumina particles distribute within grains and pinned on the grain boundaries and sub-grain boundaries to counteract the movement of these boundaries, as a result, the recrystallization process was restrained due to the prolonged nuclei formation process by means of sub-grain boundaries protruding mechanism and adjacent sub-grains merging mechanism. With increasing the annealing temperature above 600 ◦ C, the more recrystallization proceeding, and the lower the composite hardness decreasing. As soon as the annealing temperature approaching the melting point of copper, the pinning effects of nano-alumina particles lose due to the Oswald ripening [6] and complete recrystallization occurs, which results in the rapid decreasing of the composite hardness. 3.3. Electrical conductivity The electrical conductivity of Cu–0.5 vol.% Al2 O3 composite is 93%IACS, which is similar to that of Glidcop series ODS composite [7,8]. The electrical conductivity of the composite is mainly decided by the conductivity of the matrix copper and the volume fraction of hard ceramic particles. As above-mentioned, the volume fraction of internal oxidized alumina particles is so small that they have little effect on the composite’s electrical conductivity. In addition, the substitute aluminum atoms in copper crystal lattice usually cannot be oxidized completely in terms of oxygen diffusion kinetics during the practical process. Thus, the retained trace aluminum and other soluble impurities reduce the electrical conductivity of matrix. 3.4. Strength at elevated temperature Fig. 5 shows the yield strength (YS) and ultimate tensile strength (UTS) of the Cu–0.5 vol.% Al2 O3 composite with testing temperature compared with cold deformed pure copper. The yield strength of the Cu–0.5 vol.% Al2 O3 composite is much higher than that of the compared material at testing temperature range. With increasing the test temperature, the composite yield strength decreases slowly at the rate of 43–46 MPa/100 ◦ C. The yield strength reaches 169 MPa at the test temperature of 600 ◦ C. And the difference between the yield strength and the ultimate tensile strength decrease simultaneously. When the test temperature reaches 700 ◦ C and above, the UTS and YS converge, which is attributed to a change in dislocation movement mechanisms. At lower temperatures, the dislocation movement mechanism of the composite is mainly sliding mechanism and the strengthening effect is due to Orowan mechanism, interface strengthening and cold deformation hardening mechanisms. As a result, the yield strength and flow stress is inverse proportion to the interparticle spacing of nano-alumina [7]. With increasing
Fig. 5. Yield strength of Cu–0.5 vol.% Al2 O3 composite and compare materials at elevated temperature.
the test temperature, the plastic deformation occurs inhomogeneously in the whole composite matrix, in which the strain localizes gradually. At the same time, thermal activation is helpfully to the tangled dislocation around the nano-alumina particle climbing up and in turn resulting the stress relaxation around the alumina–copper matrix interfaces. Thus the Orowan strengthening effect becomes weak. The test temperature has the similar effect on the ultimate tensile strength. With increasing test temperature, the ultimate tensile strength decreases at a rate of 45–46 MPa/100 ◦ C, which is equivalent to that of the yield strength. According to the Orowan strengthening mechanism [9] and Nadkarni [10], the mean interparticle spacing λ of dispersion distributed alumina particles is decided by the volume fraction f and average diameter r, i.e.: 1/2 2π λ= r (1) 3f It can be inferred that the mean interparticle spacing λ decreases with increasing f and decreasing r. So, adjusting the chemical composition, changing internal oxidizing parameters and sintering parameters of the composite can obtain the optimum strengthening effect. The increase of critical resolved shear stress τ 0 produced by the interaction of moving dislocation and dispersion distributed nano-alumina particles is as follows [11]: τ0 = 0.84
2T bλ
(2)
where T denotes the line tension of the dislocation line and b the modulus of its Burgers vector. From Eqs. (1) and (2), the τ 0 is rewritten as: 1
Tf 2 τ0 = 1.16 br
(3)
Since the residual concentration of aluminum solid solution atoms is considered to be very low, the line tension values are those of pure copper. According to Mughrabi [12] the values are different for edge and screw dislocations with 1.0 × 10−9 and 2.5 × 10−9 N, respectively. With these two figures, and with
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Fig. 6. Tensile fractgraphs of cold-drawn Cu–0.5 vol.% Al2 O3 composite at (a) 300 ◦ C, (b) 500 ◦ C and (c and d) 600 ◦ C, respectively.
the average diameter of nano-alumina r = 35 nm, the volume fraction f = 0.5% and b = 2.56 × 10−10 m for unit dislocation in copper matrix, A range for the increase of the critical resolved shear stress of τ 0 = 9–22 MPa is deduced, which is about 2–5% of the composite yield strength at room temperature. In another word, the nano-alumina particle cannot be attributed to the increase of the yield strength by blocking the movement of dislocations, which is far from the coherency strengthening produced by aging precipitates in copper alloy [13,14]. It is to be verified that the high strength of Al2 O3 dispersion strengthened copper may mainly attribute to the strong cold deformation strengthening and interface strengthening. Fig. 6 shows the tensile fractographs of Cu–0.5 vol.% Al2 O3 composite testing at 300, 500 and 700 ◦ C, respectively. The tensile fractograph for testing at 300 ◦ C is relative flat with some under developed dimples on the fracture surface presenting localized strain congregation before the final fracture (Fig. 6a). The fractograph of 500 ◦ C is much more flatter than that of testing at 300 ◦ C. The fracture surface of the specimen is covered by coarse fracture ridges presenting heavy localized strain congregation before the final fracture (Fig. 6b). The fractograph of 700 ◦ C presents wavy features covered with oxide films, which is owing to the large localized strain congregation at partial weak areas where micro-cracks formed and propagated resulting in the composite final fracture (Fig. 6c). Some coarse alumina particles with about 2 m in diameter were found by further magnifying on the fracture surface (Fig. 6d), which are so large that they are not the internal oxidized alumina particles and may attribute to the nucleation of micro-crack on the alumina–copper interface owing to incoherence plastic deformation [3].
4. Conclusions (1) The annealing temperature has great effect on the microstructure of Cu–0.5 vol.% Al2 O3 composite. For annealing at the 600 ◦ C, the mixed microstructure of fine equiaxial grains and elongate deformed grains presents. There are large numbers of dislocations tangling around the grain boundaries and sub-grain boundaries. After annealing treatment at 1050 ◦ C, the composite matrix is completely recrystallization. (2) The softening temperature of the Cu–0.5 vol.% Al2 O3 composite is about 800 ◦ C and the electrical conductivity reaches 93%IACS. The composite presents excellent strength at elevated temperature. The yield strength reaches 169 MPa and the ultimate tensile strength reaches 173 MPa at the temperature of 600 ◦ C. (3) The main contributions of nano-alumina particles to the composite strength at elevated temperature are not the Orowan strengthening, but the strong pinning effects on the grain boundaries and sub-grain boundaries with high density dislocations, and preventing the nuclei formation of recrystallization process from relaxing the cold deformation strengthening effect, which result in the excellent softening resistance and high strength at elevated temperature. Acknowledgements This work is sponsored by the Foundation for Outstanding Youth Scientists of Henan Province (0512002700), and the Henan Natural Science Foundation (0411051400).
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G. Plascencia, T.A. Utigard, Corros. Sci. 47 (2005) 1149. S. Liang, Z. Fan, L. Xu, et al., Compos.: Part A 35 (2004) 1441. B. Tian, P. Liu, S. Han, et al., Key Eng. Mater. 274–276 (2004) 367. K. Song, J. Xing, Q. Dong, et al., Mater. Des. 26 (2005) 337. P. Liu, B. Tian, et al., Mater. Sci. Forum 475–479 (2005) 993. K. Song, P. Liu, B. Tian, et al., Mater. Sci. Forum 475–479 (2005) 993. S. Chen, J.Y. Liu, B.A. Chin, J. Nucl. Mater. 212–215 (1994) 1600. K. Song, J. Xing, Q. Dong, et al., Mater. Sci. Eng. A 380 (2004) 117.
[9] V. Gerold, in: F.R.N. Nabarro (Ed.), Dislocations in Solids, vol. 4, NorthHolland, Amsterdam, 1979, p. 219. [10] A.V. Nadkarni, Dispersion strengthened copper properties and applications, in: E. Ling, P.W. Taubenblat (Eds.), High Conductivity Copper and Aluminum Alloys, Metall. Soc. AIME, Warrendale, 1984, pp. 77–101. [11] U. Holzwarth, H. Stamm, J. Nucl. Mater. 279 (2000) 31. [12] H. Mughrabi, in: O. Brulin, R.K.T. Hsieh (Eds.), Continuum Models of Discrete Systems, North-Holland, Amsterdam, 1981, p. 241. [13] J. Su, Q. Dong, P. Liu, et al., Mater. Sci. Eng. A 392 (2005) 422. [14] D. Zhao, Q. Dong, P. Liu, et al., Mater. Chem. Phys. 79 (2003) 81.
Microstructure and properties at elevated temperature of a nano-Al2O3 particles dispersion-strengthened copper base composite Baohong Tian ∗ , Ping Liu, Kexing Song, Yan Li, Yong Liu, Fengzhang Ren, Juanhua Su School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Received 17 May 2005; received in revised form 24 July 2006; accepted 26 July 2006
Abstract High softening resistance, high strength at elevated temperature and high conductivity is important performances for the high voltage breaker and vacuum interrupter contact materials. A novel nano-Al2 O3 –Cu composite was fabricated for them by using a modified internal oxidation method. The microstructure was investigated by means of a scanning electron microscope and a transmission electron microscopy. The softening temperature, tensile strength at elevated temperature and electrical conductivity of the composite were also determined. The grain size of the Cu–0.5 vol.% Al2 O3 composite is very fine and the annealing temperature greatly affects its microstructure. The electric conductivity reaches 93% IACS and the softening temperature is about 800 ◦ C, while the yield strength reaches 169 MPa and the ultimate tensile strength reaches 172 MPa at the testing temperature of 600 ◦ C. The main contributions of nano-alumina particles is to strengthen the composite, not by Orowan strengthening, but due to the strong pinning effects on the grain boundaries and sub-grain boundaries with high dislocation density to prevent the nuclei formation of recrystallization. © 2006 Elsevier B.V. All rights reserved. Keywords: Internal oxidation; Softening temperature; Strength at elevated temperature; Conductivity
1. Introduction Oxide dispersion strengthened copper base composite (ODSC) has been successfully adopted as key electrical conductive component material in gas insulated switches such as breaker and vacuum interrupter used in high voltage power supply appliances, which are very important for power supply networks to operate safely. The key materials of breaker and vacuum interrupters are the contact terminals and switch lever materials, etc., which are usually made of cold deformed pure copper or precipitates hardened copper alloys such as Cu–Cr, Cu–Ni–Si and Cu–Cr–Zr alloys. The cold deformation hardened copper and precipitates hardened copper alloys are easily softened, or contact tips can be fused and stick together resulting from short circuit operation. Thus, it is urgent to develop a novel material with a high softening temperature, high electrical conductivity and high strength at elevated temperature to overcome these shortcomings and to meet the needs of contact terminals and switch levers. For an ODSC material, the alumina particles have been adopted owing to its superior advantages such as high melting point, high hardness and excellent ther∗
Corresponding author. E-mail address: [email protected] (B. Tian).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.129
mal stability and chemical inertness. Therefore, the dispersion distributed alumina particles can increase the recrystallization temperature and exhibit excellent strength at elevated temperature by pinning down the grain and sub-grain boundaries of the copper matrix and impeding the movement of dislocations [1,2]. Furthermore, the Cu–Al2 O3 composites have almost the same electrical and thermal conductivities as pure copper, but better creep and arc erosion resistance at elevated temperatures [3–6]. The common fabrication processing technology of Cu–Al2 O3 composites is powder metallurgical technology with internal oxidation method, which is to oxidize the Cu–Al alloy powder internally followed by reducing, cold shaping and sintering. In this paper, a modified internal oxidation powder metallurgical technique was adopted to prepare the Cu–Al2 O3 composite. The microstructure, softening temperature, Brinell hardness, strength at elevated temperature and electrical conductivity was investigated compared with a cold deformed pure copper with the same cold deformation value. 2. Experiment The Cu–0.5 vol.% Al2 O3 ODSC was used as the experimental material fabricated by a modified internal oxidation method. The preparation procedures are as follows.
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The Cu–Al alloy was atomized with a commercial nitrogen gas followed by drying and sizing. The above Cu–Al alloy powders was internal oxidized at 900 ◦ C for 5 h using flowing nitrogen gas protection followed by hydrogen reducing at 800 ◦ C. Then the internal oxidized and reduced powders were isostaticly cold pressed to a columnar billet followed by sintering at 950 ◦ C. The sintered columnar billet was hot-extruded at 850 ◦ C and cold drawn (60% deformation) to a rod bar of 16 mm in diameter. In order to evaluate the softening resistance at elevated temperature of the Cu–0.5 vol.% Al2 O3 composite, samples were annealed in a muffle electric resistance furnace under a flowing atmosphere of argon with temperature accuracy of ±5 ◦ C at temperature of 300–1050 ◦ C, respectively, accompanied with as-cold drawn pure copper with 60% cold deformation simultaneously. After annealing treatment, the specimens were mechanically ground with waterproof abrasive paper and then polished for electrical conductivity and hardness test. The Brinell hardness of the specimens were measured under a 250 kg load and held for 30 s according to ASTM E10-01e1 Standard Test Method. The tensile properties at room temperature and at elevated temperature were measured according to ASTM E8M-04 and ASTM E21-03a Standard Test Methods with a Shimadzu AGI-250kN universal material test machine and its elevated temperature attachments, respectively. The specimen size is 5 mm in diameter with a 50 mm gauge length. The furnace temperature was varied from room temperature to 300–1050 ◦ C and held for 20 min with the accuracy within ±5 ◦ C. The electrical conductivity was determined by measuring the resistance of sample in 100 mm length using a ZY9987-type standard direct-current four-probe technique. The mean value of three measurements had an estimated accuracy of less than ±0.0002 .
Fig. 1. Back-scattering electron SEM image of Cu–0.5 vol.% Al2 O3 composite (as-hot extruded, un-etched).
The microstructures of the specimens were observed using a transmission electron microscope (TEM). The TEM specimens cut from the as-annealed samples were prepared by conventional double-side electro-polishing method using an electrolyte of HNO3 :CH2 OH = 1:3. The electron microscopy for this study was carried out using a H-800 TEM at 200 kV. The fracture surface observation of the tensile specimen was carried out with a JSM-5610LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Microstructure of Al2 O3 –Cu composite The back-scattering electron SEM image of Cu–0.5 vol.% Al2 O3 composite as hot extruded is shown in Fig. 1. It can be seen that there are bright areas and dark areas presenting dif-
Fig. 2. Alumina particles distribution (a, as-extruded) and dislocations in Cu–0.5 vol.% Al2 O3 composite (b, as-drawn).
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Fig. 3. TEM images of Cu–0.5 vol.% Al2 O3 annealed at 600 ◦ C (a) and 1050 ◦ C (b), respectively.
ferent oxygen content. The bright areas indicate relative lower oxygen content, which have the size of 5–10 m and are in relation to the middle parts of internal oxidized Cu–Al alloy particle after sintering. While the dark areas indicate relative higher oxygen content, which are in relation to the original surface layers of Cu–Al alloy particles after internal oxidation and sintering. It can be concluded that the solid state sintering at elevated temperature did not coarsen the fine pre-internally oxidized Cu–Al alloy particles but helped to maintain their original fine size, which is mainly owing to the pinning effect of internal oxidized Al2 O3 particles on the movement of original surface of Cu–Al alloy particles. Fig. 2 shows the TEM images of Al2 O3 –Cu composite fabricated by the internal oxidation method. It can be seen that a mass of internal oxidized Al2 O3 particles distribute homogeneously within the matrix grain and on the grain boundaries. The alumina particles have a size of 20–50 nm and interparticle spacing of 30–120 nm (Fig. 2a). After following strong cold deformation, there is a high dislocation density (Fig. 2b). Fig. 3 shows the TEM images of Cu–0.5 vol.% Al2 O3 composite annealed at 600 and 1050 ◦ C, respectively. It can be inferred that the mixed microstructure of fine equiaxial grains and elongate deformed grains with grain size varying from 0.25 to 0.5 m coexist together after annealing at 600 ◦ C for 1 h. There are large numbers of dislocations tangling around the grain boundaries and sub-grain boundaries, which are contributed to the interaction of the movement of sub-grain boundaries consisting of dislocations and the pinned nano-alumina particles. It is also found that recrystallization occurs in partially deformed grains of the matrix (Fig. 3) and corresponds to the Brinell hardness decreasing. After annealing treatment at 1050 ◦ C for 1 h, the Cu–0.5 vol.% Al2 O3 composite matrix is completely recrystallized having equiaxial grains.
average softening rate is about 5HB/100 ◦ C, which is attributed to its relative lower volume fraction of alumina particle and the difference of pinning effects of various located alumina particles in the composite matrix. It can be testified by the microstructure features of coexisting of fine equiaxial grains and elongate deformed grains after annealed at 600 ◦ C in Fig. 3a. The softening temperature is defined as the annealing temperature in relation to the sharp decrease of the composite hardness. Thus, it is inferred from Fig. 4 that the softening temperature of cold deformed pure copper is about 400 ◦ C and that of Cu–0.5 vol.% Al2 O3 composite is about 800 ◦ C. In this experiment, the composite hardness is higher than that of cold deformed pure copper. The composite hardness after annealed at 900 ◦ C is equivalent to that of as-cold deformed pure copper. While the surface temperature of contact terminals for high voltage power supply is lower than 600 ◦ C in operation [3]. The Cu–0.5 vol.% Al2 O3 composite Brinell hardness is 96 after annealed at 600 ◦ C, which is higher than that of as cold deformed pure copper and is suitable for switches material for high voltage power supply. The mechanism responsible for the composite softening with increasing of annealing temperature is probably attributed to
3.2. Softening resistance Fig. 4 shows the relationship of Brinell hardness of Al2 O3 –Cu composite after annealing treatment with annealing temperature. With increasing the annealing temperature, the hardness slowly decreases from Brinell hardness value of 126 at room temperature without annealing to 87 after annealing at 800 ◦ C. The
Fig. 4. Brinell hardness of Cu–0.5 vol.% Al2 O3 composite and pure copper with annealing temperature.
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the effects of the following: (a) the distinguished differences of crystal structure, lattice parameters, specific volume, Young’s module between alumina particles and copper matrix result in higher phase interface energy, which in return need to be overcome the increased resistance as the phase interface bending into a curve when applied stress is loaded. (b) Although the volume fraction of internal oxidized Al2 O3 is so small and is only about 0.5%, the nano-alumina particles distribute within grains and pinned on the grain boundaries and sub-grain boundaries to counteract the movement of these boundaries, as a result, the recrystallization process was restrained due to the prolonged nuclei formation process by means of sub-grain boundaries protruding mechanism and adjacent sub-grains merging mechanism. With increasing the annealing temperature above 600 ◦ C, the more recrystallization proceeding, and the lower the composite hardness decreasing. As soon as the annealing temperature approaching the melting point of copper, the pinning effects of nano-alumina particles lose due to the Oswald ripening [6] and complete recrystallization occurs, which results in the rapid decreasing of the composite hardness. 3.3. Electrical conductivity The electrical conductivity of Cu–0.5 vol.% Al2 O3 composite is 93%IACS, which is similar to that of Glidcop series ODS composite [7,8]. The electrical conductivity of the composite is mainly decided by the conductivity of the matrix copper and the volume fraction of hard ceramic particles. As above-mentioned, the volume fraction of internal oxidized alumina particles is so small that they have little effect on the composite’s electrical conductivity. In addition, the substitute aluminum atoms in copper crystal lattice usually cannot be oxidized completely in terms of oxygen diffusion kinetics during the practical process. Thus, the retained trace aluminum and other soluble impurities reduce the electrical conductivity of matrix. 3.4. Strength at elevated temperature Fig. 5 shows the yield strength (YS) and ultimate tensile strength (UTS) of the Cu–0.5 vol.% Al2 O3 composite with testing temperature compared with cold deformed pure copper. The yield strength of the Cu–0.5 vol.% Al2 O3 composite is much higher than that of the compared material at testing temperature range. With increasing the test temperature, the composite yield strength decreases slowly at the rate of 43–46 MPa/100 ◦ C. The yield strength reaches 169 MPa at the test temperature of 600 ◦ C. And the difference between the yield strength and the ultimate tensile strength decrease simultaneously. When the test temperature reaches 700 ◦ C and above, the UTS and YS converge, which is attributed to a change in dislocation movement mechanisms. At lower temperatures, the dislocation movement mechanism of the composite is mainly sliding mechanism and the strengthening effect is due to Orowan mechanism, interface strengthening and cold deformation hardening mechanisms. As a result, the yield strength and flow stress is inverse proportion to the interparticle spacing of nano-alumina [7]. With increasing
Fig. 5. Yield strength of Cu–0.5 vol.% Al2 O3 composite and compare materials at elevated temperature.
the test temperature, the plastic deformation occurs inhomogeneously in the whole composite matrix, in which the strain localizes gradually. At the same time, thermal activation is helpfully to the tangled dislocation around the nano-alumina particle climbing up and in turn resulting the stress relaxation around the alumina–copper matrix interfaces. Thus the Orowan strengthening effect becomes weak. The test temperature has the similar effect on the ultimate tensile strength. With increasing test temperature, the ultimate tensile strength decreases at a rate of 45–46 MPa/100 ◦ C, which is equivalent to that of the yield strength. According to the Orowan strengthening mechanism [9] and Nadkarni [10], the mean interparticle spacing λ of dispersion distributed alumina particles is decided by the volume fraction f and average diameter r, i.e.: 1/2 2π λ= r (1) 3f It can be inferred that the mean interparticle spacing λ decreases with increasing f and decreasing r. So, adjusting the chemical composition, changing internal oxidizing parameters and sintering parameters of the composite can obtain the optimum strengthening effect. The increase of critical resolved shear stress τ 0 produced by the interaction of moving dislocation and dispersion distributed nano-alumina particles is as follows [11]: τ0 = 0.84
2T bλ
(2)
where T denotes the line tension of the dislocation line and b the modulus of its Burgers vector. From Eqs. (1) and (2), the τ 0 is rewritten as: 1
Tf 2 τ0 = 1.16 br
(3)
Since the residual concentration of aluminum solid solution atoms is considered to be very low, the line tension values are those of pure copper. According to Mughrabi [12] the values are different for edge and screw dislocations with 1.0 × 10−9 and 2.5 × 10−9 N, respectively. With these two figures, and with
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Fig. 6. Tensile fractgraphs of cold-drawn Cu–0.5 vol.% Al2 O3 composite at (a) 300 ◦ C, (b) 500 ◦ C and (c and d) 600 ◦ C, respectively.
the average diameter of nano-alumina r = 35 nm, the volume fraction f = 0.5% and b = 2.56 × 10−10 m for unit dislocation in copper matrix, A range for the increase of the critical resolved shear stress of τ 0 = 9–22 MPa is deduced, which is about 2–5% of the composite yield strength at room temperature. In another word, the nano-alumina particle cannot be attributed to the increase of the yield strength by blocking the movement of dislocations, which is far from the coherency strengthening produced by aging precipitates in copper alloy [13,14]. It is to be verified that the high strength of Al2 O3 dispersion strengthened copper may mainly attribute to the strong cold deformation strengthening and interface strengthening. Fig. 6 shows the tensile fractographs of Cu–0.5 vol.% Al2 O3 composite testing at 300, 500 and 700 ◦ C, respectively. The tensile fractograph for testing at 300 ◦ C is relative flat with some under developed dimples on the fracture surface presenting localized strain congregation before the final fracture (Fig. 6a). The fractograph of 500 ◦ C is much more flatter than that of testing at 300 ◦ C. The fracture surface of the specimen is covered by coarse fracture ridges presenting heavy localized strain congregation before the final fracture (Fig. 6b). The fractograph of 700 ◦ C presents wavy features covered with oxide films, which is owing to the large localized strain congregation at partial weak areas where micro-cracks formed and propagated resulting in the composite final fracture (Fig. 6c). Some coarse alumina particles with about 2 m in diameter were found by further magnifying on the fracture surface (Fig. 6d), which are so large that they are not the internal oxidized alumina particles and may attribute to the nucleation of micro-crack on the alumina–copper interface owing to incoherence plastic deformation [3].
4. Conclusions (1) The annealing temperature has great effect on the microstructure of Cu–0.5 vol.% Al2 O3 composite. For annealing at the 600 ◦ C, the mixed microstructure of fine equiaxial grains and elongate deformed grains presents. There are large numbers of dislocations tangling around the grain boundaries and sub-grain boundaries. After annealing treatment at 1050 ◦ C, the composite matrix is completely recrystallization. (2) The softening temperature of the Cu–0.5 vol.% Al2 O3 composite is about 800 ◦ C and the electrical conductivity reaches 93%IACS. The composite presents excellent strength at elevated temperature. The yield strength reaches 169 MPa and the ultimate tensile strength reaches 173 MPa at the temperature of 600 ◦ C. (3) The main contributions of nano-alumina particles to the composite strength at elevated temperature are not the Orowan strengthening, but the strong pinning effects on the grain boundaries and sub-grain boundaries with high density dislocations, and preventing the nuclei formation of recrystallization process from relaxing the cold deformation strengthening effect, which result in the excellent softening resistance and high strength at elevated temperature. Acknowledgements This work is sponsored by the Foundation for Outstanding Youth Scientists of Henan Province (0512002700), and the Henan Natural Science Foundation (0411051400).
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