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Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire
Accepted Manuscript Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire
J.P. Hou, Q. Wang, Z.J. Zhang, Y.Z. Tian, X.M. Wu, H.J. Yang, X.W. Li, Z.F. Zhang PII: DOI: Reference:
S0264-1275(17)30651-2 doi: 10.1016/j.matdes.2017.06.062 JMADE 3180
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
23 February 2017 23 June 2017 27 June 2017
Please cite this article as: J.P. Hou, Q. Wang, Z.J. Zhang, Y.Z. Tian, X.M. Wu, H.J. Yang, X.W. Li, Z.F. Zhang , Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire, Materials & Design (2017), doi: 10.1016/ j.matdes.2017.06.062
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ACCEPTED MANUSCRIPT Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire J. P. Hou a,b, Q. Wang a*, Z. J. Zhang a, Y. Z. Tian a, X. M. Wu c, H. J. Yang a, X. W. Li b and Z. F. Zhang a* a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
b
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Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China Department of Materials Physics and Chemistry, School of Materials Science and Engineering,
Electric Power Research Institute of Liaoning Electric Power Co., Ltd., Liaoning Electric Power
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c
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Northeastern University, Shenyang 110819, P.R. China
Co., Ltd., Shenyang 110006, P.R. China
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Abstract
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Outstanding mechanical and conductive properties are vital to Al alloys used as overhead conductors. However, high strength and high electrical conductivity are
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usually mutually exclusive in metallic materials. In this study, we present a novel
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method to achieve high strength and high conductivity in an Al-Mg-Si conductor. Numerous dispersive nano-scale precipitates were obtained using an artificial aging
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treatment for a 6201RE Al alloy conductor. The precipitation of the alloying elements
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in the form of nano-scale precipitates is determined to play a strengthening role, decreasing the concentration of alloying elements in the matrix and reduce lattice distortion such that high strength (352.3 MPa) and high electrical conductivity (56.0 %IACS) are achieved simultaneously in a 6201RE Al alloy. High strength and enhanced electrical conductivity could be achieved by introducing the nano-scale precipitates in the Al alloy. Finally, the strengthening mechanisms and the electrical
Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1
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conductivity induced by the nano-scale precipitates were discussed. Keywords: Aluminum alloys; Precipitation; Electrical conductivity; Strength 1. Introduction Overhead conductors are important pieces of current conducting hardware. Both
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mechanical and electrical properties are vital in order to use Al alloys as overhead
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conductors. As conductive materials, Al alloys must possess good electrical
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conductivity, reduce the electrical losses caused from electrical resistance, as well as
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have high strength for carrying loads caused by wind, ice and the weight of the conductor. Among a variety of conductive materials, Al-Mg-Si alloys have been widely
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used in the electrical industry as overhead conductors due to their high strength-toweight ratio (specific strength) and outstanding mechanical and electrical properties
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compared to other Al alloys [1-5]. It is well known that artificial aging is a conventional process applied to Al-Mg-Si alloys, which can lead to the formation of second-phase
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particles following the complex phase precipitation sequence as follows: αAl → GP zones → β" → β' [1,5-9]. However, strength and electrical conductivity are usually
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mutually exclusive in most metals [10,11], as a consequence, achieving high strength and high electrical conductivity in Al alloys becomes a significant challenge. In general, solute atoms and defects are widely accepted as strengthening factors in conductors. The role of each strengthening factor can be quantitatively analyzed by well-developed theories corresponding to various strengthening mechanisms, such as grain-boundary strengthening [12-14], solid-solution strengthening [15,16], dislocation 2
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strengthening [17,18] and precipitation strengthening [19-22]. Drude [23] developed a theory for electron conduction in 1900. In his theory, electrons are treated as particles moving through the lattice freely following the Newton’s laws of motion and MaxwellBoltzmann statistics. The interaction between electrons and defects significantly affects
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the electrical properties of Al alloys, as lattice distortion can scatter the electrons,
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resulting in an increase in the electrical resistivity. According to Matthiessen’s rule, the
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electrical resistivity is the sum of two items: a residual part, R , and a thermal part,
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T . The thermal part commonly depends on the ambient temperature, which drives the lattice vibrations, leading to electron scattering. Usually, the electrical resistivity of
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metallic materials increases with the ambient temperature. The residual part is mainly related to the concentration of impurities and defects in metallic materials [23]. In
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general, the solution treatment of Al billets is an essential step in the traditional manufacturing process to strengthen an Al alloy conductor. In previous studies, an
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artificial aging treatment was also applied to Al alloy conductors to modify the combination of materials' strength and electrical conductivity [3,4]. However, almost
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all strengthening factors cause lattice distortions, resulting in electron scattering and an increase in electrical resistivity. Therefore, the mutually exclusive relation between strength and electrical conductivity has for a long time been widely accepted as a tradeoff [10,24]. In this study, a high-strength and high-conductivity Al-Mg-Si conductor with numerous dispersive nano-scale precipitates was obtained using a new method for 3
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manufacturing Al alloy conductors. The mechanisms behind the simultaneous increase in strength and electrical conductivity of the Al-Mg-Si alloy are discussed in view of theoretical models and calculations.
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2. Materials and methods A 6201RE Al alloy conductor was investigated in this study. The chemical
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composition of the 6201RE Al alloy (wt. %) is: Si 0.50, Fe 0.20, Mg 0.67, La-Ce rare
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earth (RE) 0.12, B 0.02, Cu < 0.05, Mn < 0.03, Cr < 0.03, Zn < 0.05 and Al bal.. It
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should be noted that a small amount of RE was added to the 6201 Al alloy in the present study. On the one hand, it has been reported that RE could suppress the formation of
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the plate-like β-AlFeSi particles which are brittle and can result in the poor formability
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of the Al alloys. On the other hand, the addition of RE might also improve the electrical
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conductivity of the Al-Mg-Si alloy by forming phases containing RE and Si, as Si is harmful for the electrical properties of Al-Mg-Si alloy [25-29].
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The manufacturing route of the 6201RE conductor in the present work is shown
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in Fig. 1, which is compared with a traditional route. Al alloy conductors are mainly strengthened by the solid-solution strengthening in a homogenization treatment (560 °C, 4h), which is a necessary process step of the traditional route. Usually, the artificial aging is applied on the cold-drawn conductor [3,4]. However, for the novel route in the present work, precipitation strengthening is induced in the rod by means of artificial aging. Besides, this method introduced in the present work is a new manufacturing route for Al alloy conductors, which has not been reported. The commercially pure Al ingot 4
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(99.6%) was firstly added in the melting furnace; then, Mg, KBF4, La-Ce rare earth and Al-Si master alloy were added in the molten commercially pure Al and stirred for mixing. The molten Al alloy was poured into a metal-mould (preheated to ~150 °C) at ~720 °C. The ingot was a cylinder with a diameter of 100 mm and a height of 250 mm.
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Then, the ingot was forged into an Al alloy bar with a diameter of ~50 mm using a
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forging machine, which was operated at 450 °C. The Al alloy bar was rolled (operated
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at 390 °C ~ 420 °C) into the Al alloy rod (with a diameter of 9.50 mm) following by
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the water quenching. Two different heat-treatment methods were applied to the Al alloy rod: i) an artificial aging treatment at 175 °C for 4 hours and ii) a solid solution
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treatment at 530 °C for 4 hours. After both treatments, the rods were subsequently immersed in cold water until the rod was cooled to room temperature. An as-received
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Al alloy rod that was not heat-treated was used as a reference. The Al alloy rods were cold drawn using 10 passes using a bull block drawing machine to produce an Al alloy
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wire conductor with a diameter of 3.35 mm.
Fig. 1. The flow chart of the manufacturing route for the Al-Mg-Si conductor in the present work 5
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Uniaxial tensile tests of the cold-drawn Al alloy conductors were performed in a Shimadzu AG-X testing machine. The tensile specimens had a gauge length of 150mm and were tested at room temperature at a constant strain rate of 1.0 × 10-3 s-1 with the
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tensile axis parallel to the drawing direction.
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A direct current two-arm bridge was adopted to measure the electrical resistance
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of the Al alloy conductors. The length of the samples was 200.0 mm. The electrical conductivity was calculated by the following equation:
L 100% , R S 5.8 107
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w
(1)
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where w is the electrical conductivity (in %IACS, International Annealed Copper Standard), L is the measured length of the conductors, R is the electrical resistance
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and S is the cross-section area of the samples.
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The cylindrical specimens with a thickness of 1.0 mm for electron backscatter diffraction (EBSD) observations were cut from the conductors. The samples were
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polished using 2000# emery paper and then electrolytically polished for ~90 s at 0 C
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using an etching solution containing 10% perchloric acid and 90% alcohol in volume. The grain size distributions were measured by the EBSD technique integrated in a ZEISS SUPRA 35 scanning electron microscope (SEM). Transmission electron microscopy (TEM) samples were cut from the cross sections of the Al alloy conductors, ground to a thickness of ~0.05 mm and then twin-jet electro-polished at -20 C using a solution of 20% perchloric acid and 80% methanol by volume. The TEM foils were
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examined using an FEI Tecnai F20 microscope operating at 200 kV. 3. Results 3.1 Strength and electrical conductivity of 6201RE Al alloy conductors
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In theory, nano-scale precipitates with a proper radius in an Al alloy can provide
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the conductor with both high strength and high electrical conductivity [10]. In order to
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achieve such a combination, a 6201RE Al alloy conductor was drawn from rods in three different ways: one without a heat treatment, one treated by artificial aging and one
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subjected to a solid solution heat treatment. These specimens were marked as as-
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received, pre-aged and pre-soluted conductors, respectively. The ultimate tensile strength (UTS) and electrical conductivity of the 6201RE Al alloy wires with diameters
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from 3.35 mm to 4.53 mm are shown in Fig. 2. The UTS of the conductors clearly
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increases as the diameter of the wires decreases. In contrast, the electrical conductivity only changes slightly. For example, UTS (~371.0 MPa) of the pre-soluted conductor
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with a diameter of 3.35 mm is clearly higher than that of the as-received conductor
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(~342.2 MPa). However, the electrical conductivity of the pre-soluted conductor displays a decrease from 49.4 %IACS to 47.8 %IACS compared to the as-received conductor. In contrast, the UTS of the pre-aged conductor with a diameter of 3.35 mm improve from 342.2 MPa to 352.3 MPa and the electrical conductivity from 49.4 %IACS to 56.0 %IACS as compared to the as-received conductor. I.e. in comparison to the as-received material, the UTS and the electrical conductivity for the pre-aged material are improved simultaneously, in contrast to that of the pre-soluted 7
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conductor material.
Fig. 2. Variation in the electrical conductivity (EC) as a function of the ultimate tensile strength
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(UTS) at room temperature.
The UTS and the electrical conductivity results for the Al-Mg-Si alloy conductor
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material were compared with data available in the literature [3,4,30,31], as shown in
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Fig. 3. High-strength Al alloys usually possess a low electrical conductivity, and conversely, low-strength Al alloys commonly possess a high electrical conductivity.
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This signifies a trade-off relation between strength and electrical conductivity. In
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addition, the data from the literature are fitted well by the four parallel trend-lines, marked as #1, #2, #3 and #4. The UTS and the electrical conductivity of the pre-soluted conductor fall between the #1 and #2 trend-lines. For the pre-soluted conductor, although the UTS is relatively higher than the other results, the electrical conductivity is still very low. However, for the pre-aged sample, the UTS and the electrical conductivity of the pre-aged conductor lies close to and above trend-line #4, displaying a simultaneous increase in the strength and electrical conductivity. This indicates that 8
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artificial aging is a good way to simultaneously improve the UTS and the electrical conductivity of the Al alloy conductor. In general, the performance of conductive materials in terms of strength and conductivity is closely related to the evolution of their microstructure. Therefore, the microstructures of the as-received, pre-aged and pre-
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soluted conductors were analysed carefully and for which the results will be presented
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below.
Fig. 3. Electrical conductivity (at 20 °C) versus ultimate tensile strength (at ambient room
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temperature) of the Al-Mg-Si ingot and conductors [3,4,30,31]. EHC: Extra high conductivity, HC:
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High conductivity, EN: European Norm, T81: solution treated, cold drawn and precipitation hardened.
3.2 Microstructures 3.2.1 Microstructures of 6201RE rods Fig. 4 shows the grain size distributions of the as-received and pre-aged rods, which were measured by EBSD. The average grain sizes of the as-received (Fig. 4a) and pre-aged rods (Fig. 4b) are 0.86 μm and 1.00 μm, respectively. 9
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Fig. 4. Grain size distributions of (a) the as-received rod and (b) the pre-aged rod as determined by
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EBSD and analyzed by Channel 5 software integrated in a ZEISS SUPRA 35 SEM.
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TEM observations were performed on the cross-sectional area being perpendicular
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to the drawing direction to illustrate the microstructural differences between the asreceived (Fig. 5a, b), pre-aged (Fig. 5c, d) and pre-soluted (Fig. 5e, f) rod. Besides, a
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small amount of dislocations induced by the rolling process can be observed in some grains (Fig. 5). The amount of dislocations in the as-received rod is nearly equal to that
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in the pre-aged rod. For the as-received rod, precipitates appear both in the grains and at the grain boundaries, as shown by yellow arrows in Fig. 5b. It should be emphasized
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that the nano-scale precipitates were mainly obtained in the pre-aged rod. In contrast, nano-scale precipitates were seldom observed in the as-received rod. Moreover, the
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small grains in the Al alloy rod tend to grow into large ones in the solid solution treated Al alloy rod.
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Fig. 5. TEM observations of 6201RE Al alloy rods in the (a, b) as-received, (c, d) artificially aged
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and (e, f) solid solution states.
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3.2.2 Microstructures of 6201RE conductors
As shown in Figs. 6a and b, the average grain size of the as-received conductor is
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0.52 μm, which is almost equal to that of the pre-aged conductor (0.48 μm). However,
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the grain sizes of the pre-soluted conductor are so large that only several grains can be
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observed over a field of view.
Fig. 6. Grain size distributions of (a) the as-received conductor and (b) the pre-aged conductor as determined by EBSD.
In addition, the misorientation angle distributions of the as-received conductor and the pre-aged conductor were also measured and the results are shown in Fig. 7a and 7b, 11
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respectively. The grain boundaries (GBs) with a misorientation angle of 2-15° (including 15°) and exceeding 15° are defined as low-angle grain boundaries (LAGB) and high-angle grain boundaries (HAGB), respectively. The percentage of HAGBs was
NH A G B , NH A GB NL A G B
(2)
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VH A GB
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calculated according to the following relationship:
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where VHAGB is the percentage of HAGBs, NHAGB is the sum of the frequency with
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misorientation angle exceeding 15°, and NLAGB is the sum of the frequency with misorientation angle less than 15°. It can be seen that the VHAGB of the as-received
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conductor (~67.4%) is just slightly higher than that of the pre-aged conductor (~56.4%). As a result, the so minor differences in the types of GBs will not cause any obvious
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changes to the properties of the as-received and the pre-aged conductors.
Fig. 7. Misorientation angle distributions of (a) the as-received conductor and (b) the pre-aged conductor as determined by EBSD.
The microstructures of the 6201RE Al alloy conductors with a diameter of 3.35 mm manufactured from the as-received, pre-aged and pre-soluted rod are shown in Fig. 8. The dislocation pattern and the dislocation density of the as-received conductor (Fig. 8a) are again mainly similar to that of the pre-aged conductor (Fig. 8c). In contrast, the 12
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dislocation pattern in the pre-soluted conductor (Fig. 8e) is clearly different from that of the as-received conductor (Fig. 8a). Additionally, in the pre-soluted conductor, a
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large number of dislocation cells exists in the grains (Fig. 8e, f).
Fig. 8. TEM images of the conductor in the (a, b) as-received, (c, d) pre-aged and (e, f) pre-soluted
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states. Numerous nano-scale precipitates formed in the pre-aged conductor.
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The microstructures of the longitudinal section of the as-received and pre-aged conductors are shown in Figs. 9a and b, respectively. The grains observed in the
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longitudinal section are elongated due to the cold-drawing deformation. Fig. 9c, for
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which the incident beam direction is parallel with the [114] zone axis, shows that the nano-scale precipitates in the longitudinal section are circular (Fig. 9d). Besides, the nano-scale precipitates are also circular as observed from the cross section (Fig. 9e). Fig. 9f shows that a strong <111> texture in the cross section. In addition, the angle between the [114] zone axis and the [111] zone axis is ~ 57°, that is to say, the nanoprecipitates can be proved to be spherical as they are circular when observed from two inclined directions. It follows that a representative value for size of the precipitate radii 13
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(r) is obtained from the high-magnification images acquired by TEM. From this assumption, the volume fraction of precipitates can be estimated by the following equation: 3
N 2 4 f r 3 , S 3
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(3)
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where N is the number of precipitates, S is the area of the measured region. Based
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on this analysis, the mean radius and the volume fraction of the nano-scale precipitates in the pre-aged conductor were estimated to be ~1.3 nm and ~0.068%, respectively.
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Both the size and the shape of the precipitates in the present work are similar to those
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be considered as the GP-I zone.
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of the GP-I zone in Al-Mg-Si alloy [32-34]; as a result, the nano-scale precipitates can
Fig. 9. The longitudinal section observations of the (a) as-received and (b) pre-aged conductors. The high-magnification images of the nano-scale precipitates in the pre-aged conductor observed from the (c) longitudinal section and (d) its selected area electron diffraction (SAED) pattern. (e) The high-magnification images of the nano-scale precipitates in the pre-aged conductor observed from the cross section. (f) The orientation distribution map of the pre-aged conductor observed from the 14
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4. Discussion As a conductive material, an Al alloy conductor is required to combine with high strength and high electrical conductivity. The grain boundaries and the dislocations
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could lead to a strengthening effect at the expense of electrical conductivity. In the
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present work, both the grain size and the number of dislocations in the as-received rod
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and conductor are nearly equal to those of the pre-aged rod and conductor, respectively.
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RE was found in the phases (Al-Mg-Si-Cu-RE) and intermetallics (Al-Cu-RE) both in the as-received and the pre-aged rods, which form during the casting process (seen in
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the supplementary material). Besides, the newly formed nano-scale precipitate in the pre-aged rod is more likely to be GP-I zone. As a result, the added RE was not
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concerned to play a significant role in the simultaneous increase of strength and electrical conductivity in the Al alloy conductor in the present work. However, the
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nano-scale precipitates can only be observed in the pre-aged rod and conductor. As a result, it is deduced that the nano-scale precipitates in the pre-aged conductor play a
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key role in the simultaneous increase in the strength and electrical conductivity. 4.1 The relationship between the microstructure and the UTS For mechanical strength, the UTS of an Al alloy conductor can be estimated from the following relation assuming that the different strengthening mechanisms are independent of each other:
UTS 0 gb d ss p ,
(4) 15
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0 is the Peierls Nabarro stress, gb is the grain-boundary strengthening, d
is the dislocation strengthening,
ss is the solid-solution strengthening and p is the
precipitation strengthening. One of the significant results from the cold-drawing process is the elongation of
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the grain, which introduces a large number of grain boundaries that effectively impede
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the dislocation motion [35,36]. The UTS of Al alloy conductors generally increases
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during the cold-drawing process according to the Hall-Petch effect or grain-boundary
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strengthening [12-14]. Additionally, there must be a rapid increase in the dislocation density during plastic deformation [37], as dislocations can interact with each other and
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impede their own movement. As a result, an increase in the dislocation density within the grains always leads to an increase in the UTS. In addition, the solid-solution
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strengthening mechanism and the precipitation strengthening mechanism are two main mechanisms that dominate the strengthening of Al alloys. However, high strength and
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high electrical conductivity are in most cases mutually exclusive in metallic materials. That is to say, the factors that contribute to the tensile strength always affect the
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electrical conductivity [10,23]. 4.2 The relationship between the microstructure and the electrical resistivity The electrical resistivity of metallic materials consists mainly of two items: a thermal part, T that is correlated to temperature, and a residual part, R that is closely related to the microstructure. Therefore, according to Matthiessen’s rule, the resistivity can be represented as [23,38]: 16
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(5)
where R is the sum of the contributions from various defects or atoms leading to the electron scattering. Thus, R can be expressed as follows:
R gb d ss p ,
ss and p are the resistivity components due to the solute atoms
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by dislocations,
d is electron scattering
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where gb is the resistivity induced by the grain boundary,
(6)
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and precipitates, respectively. As a result, the grain boundaries, the dislocations, the
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solute atoms and the precipitates induced by the cold-drawing and the heat-treat process may all lead to an increase in the electrical resistivity.
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4.3 The mechanisms of high strength and high conductivity 4.3.1 The key factor of the nano-scale precipitates in a high strength conductor
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From a traditional perspective, alloying is a popular method that may be used to obtain excellent mechanical properties, for example, high strength. Unfortunately, the
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solid-solution of solute atoms in Al alloys usually causes a serious lattice distortion which increases the probability of electron scattering. It is reported that the resistivity
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of Al alloys increases linearly with the amount of solute atoms dissolved in the matrix [4,11]. Thus, although precipitation of solute atoms can effectively decrease the electrical resistivity, it may also cause a softening of the Al alloy. The contradiction lies in the state of the alloying atoms: either solutionizing or precipitating? In addition, the size and the distribution of the precipitates have a significant effect on the strength and electrical conductivity of the Al alloy conductor. 17
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It is well known that precipitation strengthening is also a good way to strengthen Al alloys [17]. More importantly, compared to precipitates, solute atoms may lead to a more significant decrease in the electrical conductivity. Therefore, one method to minimize the effect of alloying elements have on the electrical conductivity is to
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transform the solute atoms in solid solution into precipitates. Furthermore, precipitation
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strengthening is governed by either the Orowan dislocation bypassing or dislocation
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shearing mechanisms. The one causing a smaller strength increment is the dominant
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mechanism [17,39]. The Orowan bypassing mechanism can be described by the Orowan-Ashby equation [39]:
0.13 G b r ln , L b
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or
where or is the change in the tensile strength,
G is the shear modulus of the
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b is the Burgers vector of the matrix, r is the mean radius of the precipitate
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matrix,
(7)
and L is the inter-precipitate distance which can be expressed as [40]: (8)
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3 2 L 2 r , 3 4 f
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where f is the volume fraction of the precipitates. Substituting L for r in Eq. (8) results in Eq. (7) becoming:
or
0.13 G b 1 r ln . 3 2 r b 2 4 f 3
(9)
However, it is well known that the change in strength induced by the shear mechanism consists of coherency strengthening ( cs ), modulus mismatch strengthening ( ms ) and order strengthening ( os ). The larger of ( cs + ms ) or 18
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r f 2 , 0.5 G b
(10)
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cs M E G
1
3 c 2
3m
2 f r 2 ms M 0.0055 G G b
2 b
,
3 f , 8
(11)
(12)
M 3.06 is the Taylor factor, E 2.6 is a constant, c is the constrained
the matrix,
G is the modulus mismatch between the precipitate and
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lattice parameter mismatch,
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where
APB
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os M 0.81
1
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3 2
m 0.85 and APB is the anti-phase boundary free energy of the
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precipitate. It is reported that the Orowan bypassing mechanism is one of the
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mechanisms that dislocations interact with nano-scale precipitates during deformation processes in metals [17,41].
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According to Eq. (9), for given values of
G 26.9 GPa, b 0.268 nm [17],
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f 0.00068 and r 1.3 nm, the change in the yield strength induced by the Orowan bypassing mechanism is ~20.4 MPa. If the value of the strength change induced by the coherency strengthening is greater than 20.4 MPa, the precipitation strengthening will be controlled by the Orowan bypassing mechanism. The constrained lattice parameter mismatch can be calculated as [43,44]:
2 G 1 2 v , c e f f 1 Bc 1 v
(13)
19
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eff
33 are defined as: bp ap c 1, 33 p 1, 1 , 22 2 am am 1.5 2 am
(15)
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11
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where 11, 22 and
1 2 11 222 22 332 33 112 2 , (14) 3
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where am = 0.405 nm is the lattice parameter of the Al matrix, ap =0.808 nm, bp
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=0.874 nm and c p = 0.405 nm are the lattice parameters for the nano-scale precipitate [46]. GP-I zone is a face center cubic based orthorhombic super cell with the alternate
011
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stacking of two rows of Mg and one row of Si (same stacking as MoPt2 structure) on Al-matrix planes along [100] [46]. It is a pity that the value of Bc is
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unavailable. However, the value of Bc should range from the shear modulus of the matrix (26.9 GPa) to the shear modulus of the β" precipitates (60 GPa) [45]. The value
c calculated from Eq. (13) is ~0.022. Thus, the change in the yield strength caused
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of
by coherency strengthening is in the range of 39.9 MPa to 54.3 MPa, which is always
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greater than the ~20.4 MPa change induced by the Orowan bypassing mechanism. As a result, the precipitation strengthening of the 6201RE Al alloy conductor in this study should be governed primarily by the Orowan bypassing mechanism. It should be noted that artificial aging may lead to a decrease in the strength derived from solid-solution strengthening, and thus the strength change calculated by the Orowan-Ashby equation (20.4 MPa) is slightly larger than the actual improvement to the UTS of the pre-aged 20
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conductor compared to the as-received conductor. 4.3.2 The key factor of the nano-scale precipitates in high conductivity conductor Adding solute atoms into the Al matrix may lead to an increase in the electrical resistivity. This change can be expressed by Nordheim’s rule [47,48]:
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A C 1 D C,
(16)
C is the matrix solute concentration. Meanwhile, the Gibbs-
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empirical constants and
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where is the change in the resistivity compared with the pure Al, A and D are
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Thomson equation was adopted to set up the relationship between the electrical resistivity and the mean radius of the precipitates [49,50]. The Gibbs-Thomson
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equation (Eq. (17)) describes the solubility limit of B atoms in the matrix in equilibrium with the a phase that is precipitated as spherical precipitates with a
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radius of r :
2 VM Cr Ceq exp , R T r
(17)
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where Cr and Ceq are the matrix solubility corresponding to the precipitate size r
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and the precipitate with infinite radius, respectively, is the surface energy, VM is the molar volume, R the molar gas constant and T the temperature. The nano-scale precipitates in the Al-Mg-Si alloy may be assumed to be the solute atoms. As a result, the nano-scale precipitates and the Al matrix are assumed to be a binary system. Thus, combining Eq. (16) and Eq. (17), one can get,
r C 1 D Cr r . eq Ceq 1 D Ceq
(18)
21
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r Cr . eq Ceq
(19)
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Eq. (20) may be achieved by combining Eq. (17) and Eq. (19): (20)
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2 VM r r Al eq exp . R T r
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Considering that the electrical conductivity ( w ) is the reciprocal of the resistivity ( r ),
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the relationship between electrical conductivity and the precipitate radius can be expressed as
2 VM Al eq exp R T r
.
(21)
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Al , eq , , VM , R and T can be approximately assumed to be
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The values of
1
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w
fixed, and the electron scattering induced by precipitates can be ignored [10]. As a result,
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the relationship between the electrical conductivity and the mean radius of the precipitates should follow the curve shown in Fig. 10.
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The changes of strength and electrical conductivity as a function of the mean radius of the precipitates are drawn based on the precipitation strengthening mechanism including dislocation shearing mechanism and dislocation bypassing mechanism, Nordheim’s rule and the Gibbs-Thomson equation (Fig. 10). The electrical conductivity exhibits a continuously increasing trend with increasing the mean radius of the precipitates. Furthermore, the change of strength also increases with the precipitate 22
ACCEPTED MANUSCRIPT radius when r is smaller than the critical size rc based on the dislocation shearing mechanism. If r is greater than rc , the dislocation bypassing mechanism is the operating mechanism in precipitation strengthening, resulting in a decreased strength change with the increasing mean radius of the nano-scale precipitates. Consequently,
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the increment of UTS firstly increase with the increasing r, following by a decrease of
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the increment of UTS with the increasing r, as r is larger than rc. Therefore, the nano-
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scale precipitates play a decisive role in determining the UTS and the electrical
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conductivity of Al alloy conductors. The nucleation of the nano-scale precipitate in an Al alloy usually leads to the purification of the alloy matrix and the electron scattering
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caused by the presence of solute atoms decreases. As a result, the electrical conductivity of the pre-aged conductor increases compared to that of the as-received conductor.
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However, the nano-precipitates can strongly impede dislocation movement, which enhances the tensile strength of the pre-aged conductor based on the Orowan bypassing
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mechanism. Finally, the simultaneous increase in the UTS and the electrical conductivity of the Al-Mg-Si alloy can be attributed to the formation of a large amount
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of nano-scale precipitates that result from the artificial aging heat treatment of the Al alloy rod prior to the drawing process.
23
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Fig. 10. A theoretical sketch of the relationships between the changes in the increment of ultimate
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tensile strength (UTS) and the electrical conductivity as precipitate radius increases. High strength and enhance electrical conductivity can be achieved as r is close to rc.
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5. Conclusion
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In this study, a 6201RE Al alloy conductor with both high strength and high electrical conductivity was successfully fabricated by an artificial aging heat treatment,
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which was carried out on Al alloy rod. Three states of 6201RE Al alloy conductors were drawn from the as-received, artificially aged and solid-solution rods using the same
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manufacturing processes. Considering both the UTS and the electrical conductivity, the pre-aged conductor has excellent properties (UTS: ~352.3 MPa and electrical
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conductivity: ~56.0 %IACS) compared to the as-received conductor (UTS: ~342.2 MPa and electrical conductivity: ~49.3 %IACS) and the pre-soluted conductor (UTS: ~371.0 MPa and electrical conductivity: ~47.8 %IACS), achieving a record for Al-Mg-Si conductors. The nano-scale precipitates are considered to play the key role in breaking the mutually exclusive relationship between the strength and electrical conductivity. In addition, the correlations between the mean radius of the precipitates and the strength 24
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change, as well as the electrical conductivity, is established based on the precipitation strengthening mechanism including dislocation shearing mechanism and dislocation bypassing mechanism, Nordheim’s rule and the Gibbs-Thomson equation. The electrical conductivity can be improved by increasing the mean radius of the
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precipitates, which explains the simultaneous increase in the strength and electrical
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conductivity in the pre-aged conductor and offers a rule for designing Al alloy
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conductors with high strength and high electrical conductivity.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (NSFC) [grant number 51331007] and State Grid Corporation of China [grant
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number 52110416001z].
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Graphical Abstract
T P
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C S U
N A
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C C
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Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1
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Highlights 1. Simultaneous increase in strength and electrical conductivity of an AlMg-Si alloy conductor is achieved by a novel method.
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2. Numerous dispersive nano-scale (radius: ~1.3 nm) precipitates are
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formed in the Al alloy conductor.
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3. The properties of the conductor in the present work are outstanding
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compared with those of the other Al-Mg-Si conductors.
Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1
J.P. Hou, Q. Wang, Z.J. Zhang, Y.Z. Tian, X.M. Wu, H.J. Yang, X.W. Li, Z.F. Zhang PII: DOI: Reference:
S0264-1275(17)30651-2 doi: 10.1016/j.matdes.2017.06.062 JMADE 3180
To appear in:
Materials & Design
Received date: Revised date: Accepted date:
23 February 2017 23 June 2017 27 June 2017
Please cite this article as: J.P. Hou, Q. Wang, Z.J. Zhang, Y.Z. Tian, X.M. Wu, H.J. Yang, X.W. Li, Z.F. Zhang , Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire, Materials & Design (2017), doi: 10.1016/ j.matdes.2017.06.062
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ACCEPTED MANUSCRIPT Nano-scale precipitates: The key to high strength and high conductivity in Al alloy wire J. P. Hou a,b, Q. Wang a*, Z. J. Zhang a, Y. Z. Tian a, X. M. Wu c, H. J. Yang a, X. W. Li b and Z. F. Zhang a* a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
b
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Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P.R. China Department of Materials Physics and Chemistry, School of Materials Science and Engineering,
Electric Power Research Institute of Liaoning Electric Power Co., Ltd., Liaoning Electric Power
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c
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Northeastern University, Shenyang 110819, P.R. China
Co., Ltd., Shenyang 110006, P.R. China
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Abstract
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Outstanding mechanical and conductive properties are vital to Al alloys used as overhead conductors. However, high strength and high electrical conductivity are
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usually mutually exclusive in metallic materials. In this study, we present a novel
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method to achieve high strength and high conductivity in an Al-Mg-Si conductor. Numerous dispersive nano-scale precipitates were obtained using an artificial aging
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treatment for a 6201RE Al alloy conductor. The precipitation of the alloying elements
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in the form of nano-scale precipitates is determined to play a strengthening role, decreasing the concentration of alloying elements in the matrix and reduce lattice distortion such that high strength (352.3 MPa) and high electrical conductivity (56.0 %IACS) are achieved simultaneously in a 6201RE Al alloy. High strength and enhanced electrical conductivity could be achieved by introducing the nano-scale precipitates in the Al alloy. Finally, the strengthening mechanisms and the electrical
Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1
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conductivity induced by the nano-scale precipitates were discussed. Keywords: Aluminum alloys; Precipitation; Electrical conductivity; Strength 1. Introduction Overhead conductors are important pieces of current conducting hardware. Both
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mechanical and electrical properties are vital in order to use Al alloys as overhead
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conductors. As conductive materials, Al alloys must possess good electrical
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conductivity, reduce the electrical losses caused from electrical resistance, as well as
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have high strength for carrying loads caused by wind, ice and the weight of the conductor. Among a variety of conductive materials, Al-Mg-Si alloys have been widely
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used in the electrical industry as overhead conductors due to their high strength-toweight ratio (specific strength) and outstanding mechanical and electrical properties
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compared to other Al alloys [1-5]. It is well known that artificial aging is a conventional process applied to Al-Mg-Si alloys, which can lead to the formation of second-phase
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particles following the complex phase precipitation sequence as follows: αAl → GP zones → β" → β' [1,5-9]. However, strength and electrical conductivity are usually
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mutually exclusive in most metals [10,11], as a consequence, achieving high strength and high electrical conductivity in Al alloys becomes a significant challenge. In general, solute atoms and defects are widely accepted as strengthening factors in conductors. The role of each strengthening factor can be quantitatively analyzed by well-developed theories corresponding to various strengthening mechanisms, such as grain-boundary strengthening [12-14], solid-solution strengthening [15,16], dislocation 2
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strengthening [17,18] and precipitation strengthening [19-22]. Drude [23] developed a theory for electron conduction in 1900. In his theory, electrons are treated as particles moving through the lattice freely following the Newton’s laws of motion and MaxwellBoltzmann statistics. The interaction between electrons and defects significantly affects
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the electrical properties of Al alloys, as lattice distortion can scatter the electrons,
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resulting in an increase in the electrical resistivity. According to Matthiessen’s rule, the
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electrical resistivity is the sum of two items: a residual part, R , and a thermal part,
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T . The thermal part commonly depends on the ambient temperature, which drives the lattice vibrations, leading to electron scattering. Usually, the electrical resistivity of
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metallic materials increases with the ambient temperature. The residual part is mainly related to the concentration of impurities and defects in metallic materials [23]. In
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general, the solution treatment of Al billets is an essential step in the traditional manufacturing process to strengthen an Al alloy conductor. In previous studies, an
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artificial aging treatment was also applied to Al alloy conductors to modify the combination of materials' strength and electrical conductivity [3,4]. However, almost
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all strengthening factors cause lattice distortions, resulting in electron scattering and an increase in electrical resistivity. Therefore, the mutually exclusive relation between strength and electrical conductivity has for a long time been widely accepted as a tradeoff [10,24]. In this study, a high-strength and high-conductivity Al-Mg-Si conductor with numerous dispersive nano-scale precipitates was obtained using a new method for 3
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manufacturing Al alloy conductors. The mechanisms behind the simultaneous increase in strength and electrical conductivity of the Al-Mg-Si alloy are discussed in view of theoretical models and calculations.
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2. Materials and methods A 6201RE Al alloy conductor was investigated in this study. The chemical
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composition of the 6201RE Al alloy (wt. %) is: Si 0.50, Fe 0.20, Mg 0.67, La-Ce rare
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earth (RE) 0.12, B 0.02, Cu < 0.05, Mn < 0.03, Cr < 0.03, Zn < 0.05 and Al bal.. It
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should be noted that a small amount of RE was added to the 6201 Al alloy in the present study. On the one hand, it has been reported that RE could suppress the formation of
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the plate-like β-AlFeSi particles which are brittle and can result in the poor formability
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of the Al alloys. On the other hand, the addition of RE might also improve the electrical
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conductivity of the Al-Mg-Si alloy by forming phases containing RE and Si, as Si is harmful for the electrical properties of Al-Mg-Si alloy [25-29].
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The manufacturing route of the 6201RE conductor in the present work is shown
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in Fig. 1, which is compared with a traditional route. Al alloy conductors are mainly strengthened by the solid-solution strengthening in a homogenization treatment (560 °C, 4h), which is a necessary process step of the traditional route. Usually, the artificial aging is applied on the cold-drawn conductor [3,4]. However, for the novel route in the present work, precipitation strengthening is induced in the rod by means of artificial aging. Besides, this method introduced in the present work is a new manufacturing route for Al alloy conductors, which has not been reported. The commercially pure Al ingot 4
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(99.6%) was firstly added in the melting furnace; then, Mg, KBF4, La-Ce rare earth and Al-Si master alloy were added in the molten commercially pure Al and stirred for mixing. The molten Al alloy was poured into a metal-mould (preheated to ~150 °C) at ~720 °C. The ingot was a cylinder with a diameter of 100 mm and a height of 250 mm.
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Then, the ingot was forged into an Al alloy bar with a diameter of ~50 mm using a
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forging machine, which was operated at 450 °C. The Al alloy bar was rolled (operated
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at 390 °C ~ 420 °C) into the Al alloy rod (with a diameter of 9.50 mm) following by
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the water quenching. Two different heat-treatment methods were applied to the Al alloy rod: i) an artificial aging treatment at 175 °C for 4 hours and ii) a solid solution
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treatment at 530 °C for 4 hours. After both treatments, the rods were subsequently immersed in cold water until the rod was cooled to room temperature. An as-received
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Al alloy rod that was not heat-treated was used as a reference. The Al alloy rods were cold drawn using 10 passes using a bull block drawing machine to produce an Al alloy
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wire conductor with a diameter of 3.35 mm.
Fig. 1. The flow chart of the manufacturing route for the Al-Mg-Si conductor in the present work 5
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Uniaxial tensile tests of the cold-drawn Al alloy conductors were performed in a Shimadzu AG-X testing machine. The tensile specimens had a gauge length of 150mm and were tested at room temperature at a constant strain rate of 1.0 × 10-3 s-1 with the
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tensile axis parallel to the drawing direction.
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A direct current two-arm bridge was adopted to measure the electrical resistance
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of the Al alloy conductors. The length of the samples was 200.0 mm. The electrical conductivity was calculated by the following equation:
L 100% , R S 5.8 107
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w
(1)
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where w is the electrical conductivity (in %IACS, International Annealed Copper Standard), L is the measured length of the conductors, R is the electrical resistance
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and S is the cross-section area of the samples.
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The cylindrical specimens with a thickness of 1.0 mm for electron backscatter diffraction (EBSD) observations were cut from the conductors. The samples were
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polished using 2000# emery paper and then electrolytically polished for ~90 s at 0 C
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using an etching solution containing 10% perchloric acid and 90% alcohol in volume. The grain size distributions were measured by the EBSD technique integrated in a ZEISS SUPRA 35 scanning electron microscope (SEM). Transmission electron microscopy (TEM) samples were cut from the cross sections of the Al alloy conductors, ground to a thickness of ~0.05 mm and then twin-jet electro-polished at -20 C using a solution of 20% perchloric acid and 80% methanol by volume. The TEM foils were
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examined using an FEI Tecnai F20 microscope operating at 200 kV. 3. Results 3.1 Strength and electrical conductivity of 6201RE Al alloy conductors
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In theory, nano-scale precipitates with a proper radius in an Al alloy can provide
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the conductor with both high strength and high electrical conductivity [10]. In order to
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achieve such a combination, a 6201RE Al alloy conductor was drawn from rods in three different ways: one without a heat treatment, one treated by artificial aging and one
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subjected to a solid solution heat treatment. These specimens were marked as as-
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received, pre-aged and pre-soluted conductors, respectively. The ultimate tensile strength (UTS) and electrical conductivity of the 6201RE Al alloy wires with diameters
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from 3.35 mm to 4.53 mm are shown in Fig. 2. The UTS of the conductors clearly
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increases as the diameter of the wires decreases. In contrast, the electrical conductivity only changes slightly. For example, UTS (~371.0 MPa) of the pre-soluted conductor
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with a diameter of 3.35 mm is clearly higher than that of the as-received conductor
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(~342.2 MPa). However, the electrical conductivity of the pre-soluted conductor displays a decrease from 49.4 %IACS to 47.8 %IACS compared to the as-received conductor. In contrast, the UTS of the pre-aged conductor with a diameter of 3.35 mm improve from 342.2 MPa to 352.3 MPa and the electrical conductivity from 49.4 %IACS to 56.0 %IACS as compared to the as-received conductor. I.e. in comparison to the as-received material, the UTS and the electrical conductivity for the pre-aged material are improved simultaneously, in contrast to that of the pre-soluted 7
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conductor material.
Fig. 2. Variation in the electrical conductivity (EC) as a function of the ultimate tensile strength
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(UTS) at room temperature.
The UTS and the electrical conductivity results for the Al-Mg-Si alloy conductor
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material were compared with data available in the literature [3,4,30,31], as shown in
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Fig. 3. High-strength Al alloys usually possess a low electrical conductivity, and conversely, low-strength Al alloys commonly possess a high electrical conductivity.
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This signifies a trade-off relation between strength and electrical conductivity. In
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addition, the data from the literature are fitted well by the four parallel trend-lines, marked as #1, #2, #3 and #4. The UTS and the electrical conductivity of the pre-soluted conductor fall between the #1 and #2 trend-lines. For the pre-soluted conductor, although the UTS is relatively higher than the other results, the electrical conductivity is still very low. However, for the pre-aged sample, the UTS and the electrical conductivity of the pre-aged conductor lies close to and above trend-line #4, displaying a simultaneous increase in the strength and electrical conductivity. This indicates that 8
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artificial aging is a good way to simultaneously improve the UTS and the electrical conductivity of the Al alloy conductor. In general, the performance of conductive materials in terms of strength and conductivity is closely related to the evolution of their microstructure. Therefore, the microstructures of the as-received, pre-aged and pre-
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soluted conductors were analysed carefully and for which the results will be presented
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below.
Fig. 3. Electrical conductivity (at 20 °C) versus ultimate tensile strength (at ambient room
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temperature) of the Al-Mg-Si ingot and conductors [3,4,30,31]. EHC: Extra high conductivity, HC:
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High conductivity, EN: European Norm, T81: solution treated, cold drawn and precipitation hardened.
3.2 Microstructures 3.2.1 Microstructures of 6201RE rods Fig. 4 shows the grain size distributions of the as-received and pre-aged rods, which were measured by EBSD. The average grain sizes of the as-received (Fig. 4a) and pre-aged rods (Fig. 4b) are 0.86 μm and 1.00 μm, respectively. 9
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Fig. 4. Grain size distributions of (a) the as-received rod and (b) the pre-aged rod as determined by
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EBSD and analyzed by Channel 5 software integrated in a ZEISS SUPRA 35 SEM.
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TEM observations were performed on the cross-sectional area being perpendicular
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to the drawing direction to illustrate the microstructural differences between the asreceived (Fig. 5a, b), pre-aged (Fig. 5c, d) and pre-soluted (Fig. 5e, f) rod. Besides, a
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small amount of dislocations induced by the rolling process can be observed in some grains (Fig. 5). The amount of dislocations in the as-received rod is nearly equal to that
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in the pre-aged rod. For the as-received rod, precipitates appear both in the grains and at the grain boundaries, as shown by yellow arrows in Fig. 5b. It should be emphasized
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that the nano-scale precipitates were mainly obtained in the pre-aged rod. In contrast, nano-scale precipitates were seldom observed in the as-received rod. Moreover, the
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small grains in the Al alloy rod tend to grow into large ones in the solid solution treated Al alloy rod.
10
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Fig. 5. TEM observations of 6201RE Al alloy rods in the (a, b) as-received, (c, d) artificially aged
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and (e, f) solid solution states.
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3.2.2 Microstructures of 6201RE conductors
As shown in Figs. 6a and b, the average grain size of the as-received conductor is
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0.52 μm, which is almost equal to that of the pre-aged conductor (0.48 μm). However,
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the grain sizes of the pre-soluted conductor are so large that only several grains can be
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observed over a field of view.
Fig. 6. Grain size distributions of (a) the as-received conductor and (b) the pre-aged conductor as determined by EBSD.
In addition, the misorientation angle distributions of the as-received conductor and the pre-aged conductor were also measured and the results are shown in Fig. 7a and 7b, 11
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respectively. The grain boundaries (GBs) with a misorientation angle of 2-15° (including 15°) and exceeding 15° are defined as low-angle grain boundaries (LAGB) and high-angle grain boundaries (HAGB), respectively. The percentage of HAGBs was
NH A G B , NH A GB NL A G B
(2)
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VH A GB
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calculated according to the following relationship:
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where VHAGB is the percentage of HAGBs, NHAGB is the sum of the frequency with
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misorientation angle exceeding 15°, and NLAGB is the sum of the frequency with misorientation angle less than 15°. It can be seen that the VHAGB of the as-received
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conductor (~67.4%) is just slightly higher than that of the pre-aged conductor (~56.4%). As a result, the so minor differences in the types of GBs will not cause any obvious
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changes to the properties of the as-received and the pre-aged conductors.
Fig. 7. Misorientation angle distributions of (a) the as-received conductor and (b) the pre-aged conductor as determined by EBSD.
The microstructures of the 6201RE Al alloy conductors with a diameter of 3.35 mm manufactured from the as-received, pre-aged and pre-soluted rod are shown in Fig. 8. The dislocation pattern and the dislocation density of the as-received conductor (Fig. 8a) are again mainly similar to that of the pre-aged conductor (Fig. 8c). In contrast, the 12
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dislocation pattern in the pre-soluted conductor (Fig. 8e) is clearly different from that of the as-received conductor (Fig. 8a). Additionally, in the pre-soluted conductor, a
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large number of dislocation cells exists in the grains (Fig. 8e, f).
Fig. 8. TEM images of the conductor in the (a, b) as-received, (c, d) pre-aged and (e, f) pre-soluted
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states. Numerous nano-scale precipitates formed in the pre-aged conductor.
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The microstructures of the longitudinal section of the as-received and pre-aged conductors are shown in Figs. 9a and b, respectively. The grains observed in the
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longitudinal section are elongated due to the cold-drawing deformation. Fig. 9c, for
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which the incident beam direction is parallel with the [114] zone axis, shows that the nano-scale precipitates in the longitudinal section are circular (Fig. 9d). Besides, the nano-scale precipitates are also circular as observed from the cross section (Fig. 9e). Fig. 9f shows that a strong <111> texture in the cross section. In addition, the angle between the [114] zone axis and the [111] zone axis is ~ 57°, that is to say, the nanoprecipitates can be proved to be spherical as they are circular when observed from two inclined directions. It follows that a representative value for size of the precipitate radii 13
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(r) is obtained from the high-magnification images acquired by TEM. From this assumption, the volume fraction of precipitates can be estimated by the following equation: 3
N 2 4 f r 3 , S 3
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(3)
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where N is the number of precipitates, S is the area of the measured region. Based
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on this analysis, the mean radius and the volume fraction of the nano-scale precipitates in the pre-aged conductor were estimated to be ~1.3 nm and ~0.068%, respectively.
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Both the size and the shape of the precipitates in the present work are similar to those
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be considered as the GP-I zone.
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of the GP-I zone in Al-Mg-Si alloy [32-34]; as a result, the nano-scale precipitates can
Fig. 9. The longitudinal section observations of the (a) as-received and (b) pre-aged conductors. The high-magnification images of the nano-scale precipitates in the pre-aged conductor observed from the (c) longitudinal section and (d) its selected area electron diffraction (SAED) pattern. (e) The high-magnification images of the nano-scale precipitates in the pre-aged conductor observed from the cross section. (f) The orientation distribution map of the pre-aged conductor observed from the 14
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4. Discussion As a conductive material, an Al alloy conductor is required to combine with high strength and high electrical conductivity. The grain boundaries and the dislocations
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could lead to a strengthening effect at the expense of electrical conductivity. In the
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present work, both the grain size and the number of dislocations in the as-received rod
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and conductor are nearly equal to those of the pre-aged rod and conductor, respectively.
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RE was found in the phases (Al-Mg-Si-Cu-RE) and intermetallics (Al-Cu-RE) both in the as-received and the pre-aged rods, which form during the casting process (seen in
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the supplementary material). Besides, the newly formed nano-scale precipitate in the pre-aged rod is more likely to be GP-I zone. As a result, the added RE was not
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concerned to play a significant role in the simultaneous increase of strength and electrical conductivity in the Al alloy conductor in the present work. However, the
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nano-scale precipitates can only be observed in the pre-aged rod and conductor. As a result, it is deduced that the nano-scale precipitates in the pre-aged conductor play a
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key role in the simultaneous increase in the strength and electrical conductivity. 4.1 The relationship between the microstructure and the UTS For mechanical strength, the UTS of an Al alloy conductor can be estimated from the following relation assuming that the different strengthening mechanisms are independent of each other:
UTS 0 gb d ss p ,
(4) 15
ACCEPTED MANUSCRIPT where
0 is the Peierls Nabarro stress, gb is the grain-boundary strengthening, d
is the dislocation strengthening,
ss is the solid-solution strengthening and p is the
precipitation strengthening. One of the significant results from the cold-drawing process is the elongation of
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the grain, which introduces a large number of grain boundaries that effectively impede
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the dislocation motion [35,36]. The UTS of Al alloy conductors generally increases
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during the cold-drawing process according to the Hall-Petch effect or grain-boundary
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strengthening [12-14]. Additionally, there must be a rapid increase in the dislocation density during plastic deformation [37], as dislocations can interact with each other and
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impede their own movement. As a result, an increase in the dislocation density within the grains always leads to an increase in the UTS. In addition, the solid-solution
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strengthening mechanism and the precipitation strengthening mechanism are two main mechanisms that dominate the strengthening of Al alloys. However, high strength and
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high electrical conductivity are in most cases mutually exclusive in metallic materials. That is to say, the factors that contribute to the tensile strength always affect the
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electrical conductivity [10,23]. 4.2 The relationship between the microstructure and the electrical resistivity The electrical resistivity of metallic materials consists mainly of two items: a thermal part, T that is correlated to temperature, and a residual part, R that is closely related to the microstructure. Therefore, according to Matthiessen’s rule, the resistivity can be represented as [23,38]: 16
ACCEPTED MANUSCRIPT T R ,
(5)
where R is the sum of the contributions from various defects or atoms leading to the electron scattering. Thus, R can be expressed as follows:
R gb d ss p ,
ss and p are the resistivity components due to the solute atoms
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by dislocations,
d is electron scattering
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where gb is the resistivity induced by the grain boundary,
(6)
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and precipitates, respectively. As a result, the grain boundaries, the dislocations, the
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solute atoms and the precipitates induced by the cold-drawing and the heat-treat process may all lead to an increase in the electrical resistivity.
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4.3 The mechanisms of high strength and high conductivity 4.3.1 The key factor of the nano-scale precipitates in a high strength conductor
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From a traditional perspective, alloying is a popular method that may be used to obtain excellent mechanical properties, for example, high strength. Unfortunately, the
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solid-solution of solute atoms in Al alloys usually causes a serious lattice distortion which increases the probability of electron scattering. It is reported that the resistivity
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of Al alloys increases linearly with the amount of solute atoms dissolved in the matrix [4,11]. Thus, although precipitation of solute atoms can effectively decrease the electrical resistivity, it may also cause a softening of the Al alloy. The contradiction lies in the state of the alloying atoms: either solutionizing or precipitating? In addition, the size and the distribution of the precipitates have a significant effect on the strength and electrical conductivity of the Al alloy conductor. 17
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It is well known that precipitation strengthening is also a good way to strengthen Al alloys [17]. More importantly, compared to precipitates, solute atoms may lead to a more significant decrease in the electrical conductivity. Therefore, one method to minimize the effect of alloying elements have on the electrical conductivity is to
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transform the solute atoms in solid solution into precipitates. Furthermore, precipitation
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strengthening is governed by either the Orowan dislocation bypassing or dislocation
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shearing mechanisms. The one causing a smaller strength increment is the dominant
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mechanism [17,39]. The Orowan bypassing mechanism can be described by the Orowan-Ashby equation [39]:
0.13 G b r ln , L b
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or
where or is the change in the tensile strength,
G is the shear modulus of the
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b is the Burgers vector of the matrix, r is the mean radius of the precipitate
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matrix,
(7)
and L is the inter-precipitate distance which can be expressed as [40]: (8)
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3 2 L 2 r , 3 4 f
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where f is the volume fraction of the precipitates. Substituting L for r in Eq. (8) results in Eq. (7) becoming:
or
0.13 G b 1 r ln . 3 2 r b 2 4 f 3
(9)
However, it is well known that the change in strength induced by the shear mechanism consists of coherency strengthening ( cs ), modulus mismatch strengthening ( ms ) and order strengthening ( os ). The larger of ( cs + ms ) or 18
ACCEPTED MANUSCRIPT os is the total strength changes that results from the dislocation shear mechanism. The three equations for coherency strengthening, modulus mismatch strengthening and order strengthening are [19,41,42]:
r f 2 , 0.5 G b
(10)
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cs M E G
1
3 c 2
3m
2 f r 2 ms M 0.0055 G G b
2 b
,
3 f , 8
(11)
(12)
M 3.06 is the Taylor factor, E 2.6 is a constant, c is the constrained
the matrix,
G is the modulus mismatch between the precipitate and
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lattice parameter mismatch,
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where
APB
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os M 0.81
1
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3 2
m 0.85 and APB is the anti-phase boundary free energy of the
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precipitate. It is reported that the Orowan bypassing mechanism is one of the
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mechanisms that dislocations interact with nano-scale precipitates during deformation processes in metals [17,41].
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According to Eq. (9), for given values of
G 26.9 GPa, b 0.268 nm [17],
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f 0.00068 and r 1.3 nm, the change in the yield strength induced by the Orowan bypassing mechanism is ~20.4 MPa. If the value of the strength change induced by the coherency strengthening is greater than 20.4 MPa, the precipitation strengthening will be controlled by the Orowan bypassing mechanism. The constrained lattice parameter mismatch can be calculated as [43,44]:
2 G 1 2 v , c e f f 1 Bc 1 v
(13)
19
ACCEPTED MANUSCRIPT where = 0.33 is the Poisson ratio of the matrix, Bc is the bulk modulus of the nanoscale precipitates observed in this study and eff is defined as [45]:
eff
33 are defined as: bp ap c 1, 33 p 1, 1 , 22 2 am am 1.5 2 am
(15)
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11
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where 11, 22 and
1 2 11 222 22 332 33 112 2 , (14) 3
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where am = 0.405 nm is the lattice parameter of the Al matrix, ap =0.808 nm, bp
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=0.874 nm and c p = 0.405 nm are the lattice parameters for the nano-scale precipitate [46]. GP-I zone is a face center cubic based orthorhombic super cell with the alternate
011
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stacking of two rows of Mg and one row of Si (same stacking as MoPt2 structure) on Al-matrix planes along [100] [46]. It is a pity that the value of Bc is
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unavailable. However, the value of Bc should range from the shear modulus of the matrix (26.9 GPa) to the shear modulus of the β" precipitates (60 GPa) [45]. The value
c calculated from Eq. (13) is ~0.022. Thus, the change in the yield strength caused
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of
by coherency strengthening is in the range of 39.9 MPa to 54.3 MPa, which is always
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greater than the ~20.4 MPa change induced by the Orowan bypassing mechanism. As a result, the precipitation strengthening of the 6201RE Al alloy conductor in this study should be governed primarily by the Orowan bypassing mechanism. It should be noted that artificial aging may lead to a decrease in the strength derived from solid-solution strengthening, and thus the strength change calculated by the Orowan-Ashby equation (20.4 MPa) is slightly larger than the actual improvement to the UTS of the pre-aged 20
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conductor compared to the as-received conductor. 4.3.2 The key factor of the nano-scale precipitates in high conductivity conductor Adding solute atoms into the Al matrix may lead to an increase in the electrical resistivity. This change can be expressed by Nordheim’s rule [47,48]:
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A C 1 D C,
(16)
C is the matrix solute concentration. Meanwhile, the Gibbs-
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empirical constants and
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where is the change in the resistivity compared with the pure Al, A and D are
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Thomson equation was adopted to set up the relationship between the electrical resistivity and the mean radius of the precipitates [49,50]. The Gibbs-Thomson
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equation (Eq. (17)) describes the solubility limit of B atoms in the matrix in equilibrium with the a phase that is precipitated as spherical precipitates with a
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radius of r :
2 VM Cr Ceq exp , R T r
(17)
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where Cr and Ceq are the matrix solubility corresponding to the precipitate size r
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and the precipitate with infinite radius, respectively, is the surface energy, VM is the molar volume, R the molar gas constant and T the temperature. The nano-scale precipitates in the Al-Mg-Si alloy may be assumed to be the solute atoms. As a result, the nano-scale precipitates and the Al matrix are assumed to be a binary system. Thus, combining Eq. (16) and Eq. (17), one can get,
r C 1 D Cr r . eq Ceq 1 D Ceq
(18)
21
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r Cr . eq Ceq
(19)
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Eq. (20) may be achieved by combining Eq. (17) and Eq. (19): (20)
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2 VM r r Al eq exp . R T r
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Considering that the electrical conductivity ( w ) is the reciprocal of the resistivity ( r ),
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the relationship between electrical conductivity and the precipitate radius can be expressed as
2 VM Al eq exp R T r
.
(21)
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Al , eq , , VM , R and T can be approximately assumed to be
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The values of
1
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w
fixed, and the electron scattering induced by precipitates can be ignored [10]. As a result,
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the relationship between the electrical conductivity and the mean radius of the precipitates should follow the curve shown in Fig. 10.
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The changes of strength and electrical conductivity as a function of the mean radius of the precipitates are drawn based on the precipitation strengthening mechanism including dislocation shearing mechanism and dislocation bypassing mechanism, Nordheim’s rule and the Gibbs-Thomson equation (Fig. 10). The electrical conductivity exhibits a continuously increasing trend with increasing the mean radius of the precipitates. Furthermore, the change of strength also increases with the precipitate 22
ACCEPTED MANUSCRIPT radius when r is smaller than the critical size rc based on the dislocation shearing mechanism. If r is greater than rc , the dislocation bypassing mechanism is the operating mechanism in precipitation strengthening, resulting in a decreased strength change with the increasing mean radius of the nano-scale precipitates. Consequently,
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the increment of UTS firstly increase with the increasing r, following by a decrease of
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the increment of UTS with the increasing r, as r is larger than rc. Therefore, the nano-
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scale precipitates play a decisive role in determining the UTS and the electrical
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conductivity of Al alloy conductors. The nucleation of the nano-scale precipitate in an Al alloy usually leads to the purification of the alloy matrix and the electron scattering
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caused by the presence of solute atoms decreases. As a result, the electrical conductivity of the pre-aged conductor increases compared to that of the as-received conductor.
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However, the nano-precipitates can strongly impede dislocation movement, which enhances the tensile strength of the pre-aged conductor based on the Orowan bypassing
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mechanism. Finally, the simultaneous increase in the UTS and the electrical conductivity of the Al-Mg-Si alloy can be attributed to the formation of a large amount
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of nano-scale precipitates that result from the artificial aging heat treatment of the Al alloy rod prior to the drawing process.
23
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Fig. 10. A theoretical sketch of the relationships between the changes in the increment of ultimate
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tensile strength (UTS) and the electrical conductivity as precipitate radius increases. High strength and enhance electrical conductivity can be achieved as r is close to rc.
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5. Conclusion
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In this study, a 6201RE Al alloy conductor with both high strength and high electrical conductivity was successfully fabricated by an artificial aging heat treatment,
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which was carried out on Al alloy rod. Three states of 6201RE Al alloy conductors were drawn from the as-received, artificially aged and solid-solution rods using the same
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manufacturing processes. Considering both the UTS and the electrical conductivity, the pre-aged conductor has excellent properties (UTS: ~352.3 MPa and electrical
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conductivity: ~56.0 %IACS) compared to the as-received conductor (UTS: ~342.2 MPa and electrical conductivity: ~49.3 %IACS) and the pre-soluted conductor (UTS: ~371.0 MPa and electrical conductivity: ~47.8 %IACS), achieving a record for Al-Mg-Si conductors. The nano-scale precipitates are considered to play the key role in breaking the mutually exclusive relationship between the strength and electrical conductivity. In addition, the correlations between the mean radius of the precipitates and the strength 24
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change, as well as the electrical conductivity, is established based on the precipitation strengthening mechanism including dislocation shearing mechanism and dislocation bypassing mechanism, Nordheim’s rule and the Gibbs-Thomson equation. The electrical conductivity can be improved by increasing the mean radius of the
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precipitates, which explains the simultaneous increase in the strength and electrical
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conductivity in the pre-aged conductor and offers a rule for designing Al alloy
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conductors with high strength and high electrical conductivity.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (NSFC) [grant number 51331007] and State Grid Corporation of China [grant
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CE
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number 52110416001z].
25
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Graphical Abstract
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Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1
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Highlights 1. Simultaneous increase in strength and electrical conductivity of an AlMg-Si alloy conductor is achieved by a novel method.
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2. Numerous dispersive nano-scale (radius: ~1.3 nm) precipitates are
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formed in the Al alloy conductor.
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3. The properties of the conductor in the present work are outstanding
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compared with those of the other Al-Mg-Si conductors.
Correspondence authors, Q. Wang and Z. F. Zhang, Tel: 0086-24-23971043, E-mail: [email protected] and [email protected] 1