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Enhanced strength and electrical conductivity of ultrafine-grained Al-Mg-Si alloy processed by hydrostatic extrusion
Materials Characterization 135 (2018) 104–114
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Enhanced strength and electrical conductivity of ultrafine-grained Al-Mg-Si alloy processed by hydrostatic extrusion
T
⁎
Kamil Majchrowicza, , Zbigniew Pakiełaa, Witold Chrominskia, Mariusz Kulczykb a b
Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics of the Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrafine-grained materials Al-Mg-Si alloys Hydrostatic extrusion Electrical conductivity
The effect of hydrostatic extrusion combined with an artificial aging on microstructure, mechanical and electrical properties of 6101 Al-Mg-Si alloy was investigated. It has been shown that such thermo-mechanical treatment is an effective method for producing of long wires with an ultrafine-grained microstructure (grain size of 300–400 nm) and enhanced ultimate tensile strength (> 330 MPa) and electrical conductivity (up to 58% IACS). The mechanical behavior of 6101 Al-Mg-Si alloy depended strongly on applied strains by hydrostatic extrusion and crystallographic texture. Higher accumulative strain accelerated the precipitation kinetics but decreased the age hardening response. The double fiber ⟨100⟩ and ⟨111⟩ texture was observed for hydrostatically extruded samples. The ⟨001⟩ grains with homogenously distributed needle-like β″ precipitates provided precipitation strengthening of material while ⟨111⟩ grains resulted in more efficient grain boundary strengthening. Quantitative microstructure characterization allowed adjusting physical model to estimate the electrical conductivity and compare it with experimental data. The high conductivity was provided mainly by decomposition of solid solution due to precipitation of needle-like β″ precipitates in the grain interior and spherical β′ or β particles located at grain boundaries.
1. Introduction Al-Mg-Si alloys are currently widely used as conducting materials in overhead power transmission lines and many other applications in electrical engineering [1]. Due to high electrical conductivity, specific strength and corrosion resistance, they have become increasingly popular candidates for industrial usage. Standard conductors are made of Al-Mg-Si alloys containing 0.6–0.9% Mg and 0.5–0.9% Si such as 6101 and 6201 aluminum alloys [2,3]. Higher magnesium and silicon contents significantly deteriorate electrical conductivity due to increased scattering of conduction electrons by solute atoms [4]. High strength and conductivity of conventionally produced Al-Mg-Si alloys are a result of thermo-mechanical processing consisting of solution heat treatment followed by water quenching, cold drawing and artificial aging [2]. Such processing route results in ultimate tensile strength of 255–325 MPa and electrical conductivity in the range of 57.5%–52.5% IACS (International Annealed Copper Standard) [5]. A new strategy has been proposed recently by several authors to enhance strength and electrical conductivity of Al-Mg-Si alloys using severe plastic deformation (SPD) methods combined with an artificial aging [6–10]. An unconventionally high strain applied during SPD
⁎
processing leads to significant grain refinement and obtaining ultrafinegrained (UFG) structures with a grain size in the range of 100 nm to 1 μm [11]. It brings about a strong grain boundary strengthening as described by the Hall-Petch relationship [12,13]. SPD also influences the precipitation kinetics and changes precipitation sequence in agehardenable aluminum alloys as reported in [14–16]. Moreover, pronounced grain boundary segregation has been observed in UFG aluminum alloys [17–19] what may contributes, according to Matthisen's rule [4], to a greater enhancement of electrical conductivity due to reduced solute content in the matrix. SPD processing of Al-Mg-Si alloys, such as equal channel angular pressing with parallel channels (ECAPPC) [6,7] and high pressure torsion (HPT) [8,9], results in increased electrical conductivity up to 58% IACS. However, the main disadvantage of these techniques limiting their industrial usage is the small size of processed billets. To overcome this weakness, M. Murashkin et al. [10] have proposed recently modified continuous ECAP-C technique for fabrication of long billets. In the present work, a new approach has been implemented to produce long wires of UFG Al-Mg-Si alloys by the hydrostatic extrusion (HE) method. In this process, a billet located in a working chamber filled with fluid is extruded through a forming die of specified diameter
Corresponding author at: Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland. E-mail address: [email protected] (K. Majchrowicz).
https://doi.org/10.1016/j.matchar.2017.11.023 Received 21 May 2017; Received in revised form 24 September 2017; Accepted 11 November 2017 Available online 11 November 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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5.5 and 10 m, respectively. In the current study, all HE samples were taken from one batch in order to keep similar deformation conditions what resulted in the sample length of about 0.5, 1.3 and 1.5 m for HE10, HE4 and HE3, respectively. Then, as-HE samples were cut for smaller pieces for further aging treatment. To investigate an age hardening response of the 6101 Al alloy subjected to different HE reductions, Vickers hardness measurements with the load of 200 g were performed using Zwick Roell Z2.5/ZHU0.2. Uniaxial tensile tests were conducted on a servo-hydraulic mechanical testing machine MTS 858 equipped with a 25 kN load cell. Experiments were performed at initial strain rate of 10−3 s−1 using cylindrical test specimens with a diameter of 2.5 mm and gage length of 15 mm. The electrical conductivity was measured at room temperature using an eddy current conductivity meter Sigmascope SMP10 equipped with a 60 kHz probe ES40. The method is standardized according to DIN EN 2004-1 and ASTM E 1004. To avoid conductivity measurement errors resulting from curvature of investigated specimens, the conductivity meter was calibrated before each measurement with reference samples in the form of wires having the exact diameter of measured specimen. The electrical conductivity of reference samples was measured using four-point resistivity measurement method. The microstructure was characterized using Hitachi SU70 scanning electron microscope (SEM) for EBSD mapping and JEOL JEM-1200EX transmission electron microscope (TEM) with an acceleration voltage of 120 kV. EBSD measurements were performed on electrochemically polished surfaces. Maps were taken with an acceleration voltage of 20 kV on a square grid of points separated by 500 nm for general and 100 nm for detailed mapping. Each map had a ratio of successfully indexed Kikuchi maps > 70%, thus data were considered reasonable. Data were analyzed with a dedicated Channel5 software. Thin foils for TEM investigation were prepared from 3 mm diameter discs cut out from the center of final as-HE and aged rods perpendicularly to the extrusion direction. The discs were ground down and electropolished using Struers TenuPol-5 system. The mean grain size was described by the equivalent diameter d2 measured using the MicroMeter software [25] whereas ImageJ software was used for measurements of precipitates' size. The length of precipitates was estimated based on TEM images. Since needle-like precipitates in Al-Mg-Si alloys grow along ⟨001⟩ type directions [26,27] their length can be measured directly from ⟨001⟩ zone axis images. In ⟨111⟩ zone axis images a length taken from the micrograph was divided by a cosine between ⟨001⟩ and ⟨112⟩, which equals 0.81. [12 1] is a perpendicular projection of [010] on (111) plane. The energy dispersive X-ray spectroscopy (EDS) analysis of precipitates located at grain boundaries was carried out on a cold-FEG Hitachi S5500 scanning transmission electron microscope (STEM) using samples prepared for TEM observations. X-ray diffraction (XRD) method was used to estimate dislocation density. Based on measurements conducted on Bruker AXS D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å), the dislocation density ρ was calculated using modified Williamson-Hall formula [28]:
by high hydrostatic pressure of the liquid. The pressure is generated by a piston compressing the liquid in the working chamber. During HE process, the extruded material is severely deformed with a high strain rate (exceeding 102 s−1) resulting in an adiabatic heating of the billet. To reduce heat effects, there is a cooling system placed at the exit of the forming die [20–22]. According to the formal definition of SPD techniques established by R.Z. Valiev et al. [23], SPD processing excludes the hydrostatic extrusion due to the change in the overall dimensions of sample. However, HE process still enables to impose very high strains and requires smaller total deformation to induce significant grain refinement to submicron sizes as compared to SPD methods [20]. The main advantage of HE is the possibility of direct forming of long wires dedicated to many electrical applications. Besides, as shown for 6082 Al-Mg-Si alloy by W. Chrominski et al. [14], the combination of HE and artificial aging leads to higher strength than other SPD techniques combined with different heat treatments. The precipitation process and origins of superior strength of hydroextruded 6082 Al-Mg-Si alloy has been explained in detail in [14,15,24]. The main purpose of this work is to demonstrate the potential of hydrostatic extrusion for fabrication of UFG Al-Mg-Si alloy with both enhanced strength and electrical conductivity in comparison to conventional processing methods as well as SPD techniques. The paper examines the effect of specially designed thermo-mechanical treatment on microstructure of 6101 Al-Mg-Si alloy used as a standard conductor in electrical engineering and correlates its microstructural characteristics with electrical conductivity.
2. Materials and Methods A commercially available 6101 Al alloy with a chemical composition of 0.59Mg-0.54Si-0.07Fe (wt%) was used in this work in the form of ∅ 20 mm extruded rod in the T1 condition (cooled from elevated temperature shaping process and naturally aged). Billets of 6101 Al alloy were solution heat treated at 510 °C for 1 h followed by water quenching and subjected to HE to obtain an ultrafine-grained structure. HE process was applied in the several stages with different reductions of billet's diameter resulted in different accumulative strain. In the first step, the initial diameter was reduced from 20 to 10 mm (total true strain of 1.4) and this condition was represented by sample named as HE10. Subsequently, billets were extruded in the next two stages to final diameter of 4 mm (further referred to as HE4) and 3 mm (HE3) with the intermediate diameter of 6 mm. Hence, the HE4 and HE3 samples were subjected to three-stage HE with a total true strain of 3.2 and 3.8, respectively. Afterwards, as-HE samples were aged at 180 °C for 1–24 h which are standard parameters for conventionally produced 6101 alloy. The aging treatment was also performed for coarse-grained (CG) counterpart to compare the results obtained for HE processed samples with conventional material. All processing stages of 6101 Al alloy are schematically presented in Fig. 1. HE process was performed in CP31 hydroextrusion press with the working chamber diameter of 31 mm operating to maximum pressure of 1.7 GPa which was designed and constructed at the Institute of High Pressure Physics of the Polish Academy of Sciences, Unipress. The maximum length of billets used in this press is 250 mm what enables to obtain long wires, e.g. with diameter of 4 and 3 mm and length of about
1/2
∆K =
0.9 πA2 b2 ⎞ +⎛ D ⎝ 2 ⎠ ⎜
⎟
ρ1/2 (KC 1/2) + O (K 2C )
where: K = 2sinθ/λ, θ is the diffraction angle and λ the X-ray Fig. 1. Processing route of 6101 Al alloy.
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Fig. 2. (a) Hardness and (b) electrical conductivity of the 6101 Al alloy subjected to different HE reductions and aged at 180 °C up to 24 h.
wavelength; ΔK = 2cosθ(Δθ)/λ, Δθ is the full width at half maximum (FWHM) of a peak; b is the Burgers vector of dislocation (b = 0.286 nm [29]) and D corresponds to the average crystallite size. A is a constant depending on the effective outer cut-off radius of dislocations and it was estimated for Al-Mg-Si alloy by W. Woo et al. [30] (A2 = 0.63). The dislocation contrast factor C was determined by the elastic anisotropy factor, dislocation type and single-crystal elastic constants according to [31]. For calculations of dislocation contrast factor, the elastic constants were used from [32] and the edge/screw dislocation ratio was assumed as 75 to 25% according to [33]. Besides, texture analysis was performed by means of X-ray diffractometer Bruker AXS D8 Discover. Three incomplete pole figures, i.e. (111), (200), (220), were used to calculate the orientation distribution function and subsequently recalculate complete pole figures.
the electrical conductivity of HE processed samples stabilizes and exhibits slightly rising tendency with increasing aging time. Noteworthy is also the fact that the 6101 Al alloy subjected to the smallest HE reduction (HE10) shows lower conductivity than HE4 and HE3 for all aging times. The initial conductivity for as-HE samples is 49.67 ± 0.11, 50.94 ± 0.43 and 50.69 ± 0.64% IACS for HE10, HE4 and HE3, respectively. This difference of around 1% IACS between HE10 and both HE4 and HE3 maintains for longer aging times. The HE4 and HE3 samples reach similar conductivity what indicates that higher level of deformation applied to the 6101 Al alloy will not provide further improvement of electrical properties. The initial conductivity for CG sample is 49.63 ± 0.02% IACS what is close to HE10. However, further aging treatment of CG 6101 alloy do not cause as high improvement of electrical conductivity as observed for material subjected to HE process. The conductivity for peak aged CG sample reaches 52.54 ± 0.17% IACS. As shown in Fig. 2, the level of applied deformation by HE processing has a significant effect on age hardening response and electrical conductivity of the 6101 Al alloy. To explain obtained results, as-HE samples without aging and HE10 and HE3 peak-aged samples have been chosen for further mechanical and microstructural investigations. The HE4 sample aged at 180 °C for 7 h has been chosen as the material with the best combination of high electrical conductivity and moderate hardness. Mechanical properties assessed in the uniaxial tensile tests are summarized in Table 1. Among as-HE samples, the HE10 exhibits the lowest YS and UTS of 214 ± 5 and 227 ± 6 MPa, respectively. UTS reaches the maximum value for HE4 (305 ± 3 MPa) and the higher level of applied deformation do not provide any significant changes. YS and UTS for HE3 is 288 ± 4 and 302 ± 4 MPa, respectively. Besides, it is noted that strength improvement comes at the expense of ductility
3. Results 3.1. Mechanical Properties and Electrical Conductivity Age hardening response of the 6101 Al alloy subjected to different HE reductions is presented in Fig. 2(a) in the form of hardness change as a function of aging time at 180 °C. The initial hardness for as-HE samples increases with higher billet's diameter reductions and equals to 73 ± 4, 87 ± 4 and 89 ± 4 HV for HE10, HE4 and HE3, respectively. This implies that higher accumulative strain induced by HE improves the hardness more effectively due to higher density of generated microstructural defects. Three-stage age hardening response including under-aging, peak-aging and over-aging is noted successively with extending aging time for all samples. Increasing accumulative strain shifts hardness peak to a shorter aging time of 7 h for HE10 and ~ 2 h for both HE4 and HE3. The maximum value of hardness reaches 94 HV for HE10 and 95 HV for HE4 and HE3. It suggests that increasing strain alters materials response to post-deformation annealing. As suggested by other authors [34–36] this may be related to increasing density of microstructural defects which serve as potential nucleation sites for strengthening precipitates. For longer aging times, the overaging stage is observed for all HE processed samples as the continuous hardness decrease down to 76–78 HV. The CG 6101 alloy shows much higher age hardening response in comparison to as-HE samples. Its initial hardness equals to 52 ± 2 HV and increases up to peak value of 94 ± 3 HV for 7 h of aging. The electrical conductivity of the 6101 Al alloy after aging at 180 °C for different times is plotted in Fig. 2(b). It is clear that the conductivity is extremely sensitive to the aging time. It sharply increases at the early stage of aging (up to 4 h) due to decreasing solute concentration in the matrix caused by rapid precipitates nucleation and growth. Afterwards,
Table 1 Mechanical properties and electrical conductivity of the 6101 Al alloy under different processing conditions (YS – yield strength; UTS – ultimate tensile strength; A – elongation to failure; IACS – electrical conductivity expressed as a relative value of International Annealed Copper Standard).
106
State
YS (MPa)
UTS (MPa)
A (%)
IACS (%)
T1 (as-received) T6 (180 °C/7 h) HE10 HE10 + 180 °C/7 h HE4 HE4 + 180 °C/7 h HE3 HE3 + 180 °C/2 h
116 216 214 298 294 329 288 345
181 228 227 309 305 332 302 354
19 9.0 ± 2.5 8.9 ± 1.4 11.8 ± 0.8 7.8 ± 2.5 8.8 ± 1.2 6.1 ± 0.9 5.5 ± 0.9
51.12 52.54 49.67 57.12 50.94 58.04 50.69 55.88
± ± ± ± ± ± ±
5 5 2 3 2 4 6
± ± ± ± ± ± ±
9 6 2 3 2 4 6
± ± ± ± ± ± ± ±
0.07 0.17 0.11 0.10 0.43 0.66 0.64 0.26
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Fig. 3. EBSD orientation maps of the 6101 Al alloy in the state (a) HE10 and (b) HE4 with color code for orientation maps and (d) misorientation angle distributions.
peak close to 55°. Based upon quantification of obtained EBSD results the LAGB fraction is 90% in the former and 73% in the latter. TEM observations after HE process were performed for HE4 and HE3 samples. Since they exhibit similar microstructure characteristics (described in detail in further paragraphs) and age hardening response (Fig. 2(a)), following samples were selected for a detailed precipitation analysis: HE3 + 180 °C/2 h corresponding to the peak-aging condition and HE4 + 180 °C/7 h corresponding to slightly over-aged condition but still showing enhanced electrical conductivity and strength in comparison with as-HE samples. A typical ultrafine-grained microstructure for hydrostatically extruded samples, as characterized in [14,15,24], is observed for the 6101 Al alloy. It consists of two types of grains: fine, equiaxed grains with a size of 200–300 nm oriented with ⟨111⟩ parallel to extrusion direction (named as ⟨111⟩ grains) and relatively large (at least 1 μm in size) grains with a dislocation substructure having an orientation with ⟨001⟩ parallel to extrusion direction (named as ⟨001⟩ grains) (Fig. 4(a)). Dislocation boundaries with a low misorientation angle are arranged within ⟨001⟩ grains as well as dense network of LAGBs is observed within regions of ⟨111⟩ oriented grains. HAGBs are present mainly between ⟨001⟩ and ⟨111⟩ grains or around some ⟨001⟩ and ⟨111⟩ grains (Fig. 4(b)). Considering precipitation condition in 6101 Al alloy after HE process, TEM inspection reveals some fine precipitates in the interior of ⟨001⟩ grains (Fig. 5(a)) which were recognized by HRTEM observations in [15,24] as GP zones. Besides, there is no precipitates at grain boundaries of ⟨001⟩ (Fig. 5(b)) and ⟨111⟩ grains (Fig. 5(c)). After HE process, homogenously distributed GP zones were observed only in ⟨001⟩ grains. Aging for 2 h brings about their
which decreases with increasing HE reduction. The aging treatment of HE10 gives significant enhancement of YS and UTS to 298 ± 2 and 309 ± 2 MPa. Despite of similar hardness for peak-aged HE10 and HE3, the latter one reaches the highest YS and UTS of 345 ± 6 and 354 ± 6 MPa, respectively. The results for HE4 aged at 180 °C for 7 h confirm that it is the best combination of high electrical conductivity (58.04 ± 0.66% IACS) and strength (YS of 329 ± 2 and UTS of 332 ± 2 MPa). Additionally, all samples subjected to HE and aging are much stronger and have higher conductivity as compared to their conventionally produced counterpart in state T1 and T6 (aged for 7 h at 180 °C).
3.2. Microstructure After HE Process and Aging EBSD orientation maps presented in Fig. 3(a) and (b) show development of the grain refinement process in two chosen steps – HE10 and HE4 studied on a cross section of extrusions. The most striking feature is domination of two orientations, namely ⟨001⟩ and ⟨111⟩ parallel to extrusion direction, red and blue respectively as can be noticed from color code Fig. 3(c). HE10 sample exhibit primary grains separated by high angle grain boundaries (HAGBs, represented by black lines) with low angle boundaries network inside (LAGBs, white lines). As the applied strain increases the boundaries of primary grains become blurrier. The network of LAGBs in the interior of ⟨001⟩ and ⟨111⟩ oriented areas become denser for HE4 as well as the number of HAGBs between ⟨001⟩ and ⟨111⟩ oriented grains. Misorientation angle distributions, Fig. 3(d) shows a difference between two considered materials condition. HE10 features higher fraction of LAGBs than HE4, which exhibits a slight 107
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Fig. 4. Microstructure of the 6101 Al alloy processed by HE in the state HE4: (a) TEM image, (b) EBSD orientation map (color code as in Fig. 3(c)).
transformation into fine needle-like β″ precipitates aligned with ⟨001⟩ directions (Fig. 6(a)). These precipitates with a length of 16 ± 8 nm are homogenously distributed in the grain interior and gives strong strain contrast suggesting their coherency with the matrix. They are present in ⟨001⟩ and ⟨111⟩ grains but their density seems to be smaller in ⟨111⟩ grains. For ⟨111⟩ grains, needle-like β″ particles with a length of 17 ± 6 nm are observed as well as precipitation free zone (PFZ) of about 20 nm (Fig. 6(b)). It suggests that grain boundaries surrounding ⟨111⟩ grains effectively attract solute atoms. However, a relatively large (up to 20 nm in diameter) spherical precipitates are located only at grain boundaries of ⟨001⟩ grains (Fig. 7). During further aging for 7 h, the length of needle-like β″ precipitates grows to 22 ± 14 and 22 ± 11 nm for ⟨001⟩ and ⟨111⟩ grains, respectively. Longer aging time does not change significantly their overall appearance in comparison to 2 h of aging (Fig. 8). They are aligned with three mutually perpendicular ⟨001⟩ type directions as shown in Fig. 8(b) for ⟨111⟩ grains. For ⟨001⟩ grains, needle-like β″ precipitates along [100] and [010] directions are clearly visible while precipitates
along [001] direction are seen as dark black spots (Fig. 8(a)). Apart from them, fine spherical precipitates similar to GP zones found in asHE samples (Fig. 5(a)) are also localized in ⟨001⟩ grains of aged sample (visible as fine grey spots in Fig. 8(a)). These fine precipitates are also present after 2 h of aging. Longer aging times favors pronounced precipitation at HAGBs. The spherical precipitates with a diameter up to 20 nm are located at grain boundaries of both ⟨001⟩ and ⟨111⟩ grains as depicted in Fig. 9. The EDS spectrums collected from these particles suggest that they are β′ (Mg1.8Si) or β (Mg2Si) phase due to Mg:Si atomic ratio close to 2 [37]. Such precipitates located at grain boundaries in UFG Al-Mg-Si alloys have been also found elsewhere [9,14]. The as-HE samples, i.e. HE4 and HE3, are characterized by ultrafinegrained microstructure with an average grain size of 315 ± 134 and 293 ± 126 nm, respectively. The dislocation density determined by modified Williamson-Hall method reaches 5.1 ± 1.1 and 6.7 ± 1.2 × 1014 m−2 for HE4 and HE3, respectively, and is higher by one order in comparison to as-received state (Table 2). Annealing for 2 Fig. 5. Precipitation condition in the 6101 Al alloy processed by HE (in the state HE4): (a) fine precipitates (GP zones) in the interior of ⟨001⟩ grains; (b) lack of precipitates at grain boundaries of ⟨001⟩ and (c) ⟨111⟩ grains.
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Fig. 6. Precipitation condition in the 6101 Al alloy in the state HE3 + 180 °C/2 h: (a) GP zones and needle-like β″ precipitates in ⟨001⟩ and (b) needle-like β″ precipitates in ⟨111⟩ grains with a precipitation free zone of about 20 nm.
Fig. 7. Precipitation condition in the 6101 Al alloy in the state HE3 + 180 °C/2 h: (a) spherical precipitates at grain boundaries of ⟨001⟩ grains with (b) STEM image of precipitate and EDS spectra; (c) lack of precipitates at grain boundaries of ⟨111⟩ grains.
⟨100⟩ fiber. HE4 features a much stronger ⟨111⟩ fiber texture with a weak ⟨100⟩ component. The maximum intensity of (111) pole figure is 17.5 MRD. Texture analysis with respect to EBSD studies (Fig. 3) confirms that HE4 consists high content of ⟨111⟩ grains and small fraction of ⟨001⟩ grains whereas HE10 is composed of higher number of ⟨001⟩ grains with a dislocation substructure.
and 7 h causes minor grain growth to 389 ± 135 nm and 408 ± 159 nm, respectively, and a significant reduction of dislocation density due to recovery process. The microstructural parameters measured in this work are summarized in Table 2. 3.3. Texture Fig. 10 shows texture analysis results of as-HE samples, i.e. HE10 and HE4, in the form of recalculated pole figures (111) and (200) with its reflecting surface normal to extrusion direction. HE10 exhibits a double fiber texture with ⟨100⟩ and ⟨111⟩ fiber axes with a maximum intensity of 6.3 MRD (multiples of random distribution) observed for 109
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Fig. 8. Precipitation condition in the 6101 Al alloy in the state HE4 + 180 °C/7 h: (a) GP zones and needle-like β″ precipitates in ⟨001⟩ and (b) needle-like β″ precipitates in ⟨111⟩ grains.
4. Discussion
results in the accelerated precipitation kinetics. The time required to reach peak hardness is shorter for higher total cumulative strains (Fig. 2(a)) due to increased density of microstructural defects which provide privileged sites for precipitates nucleation. It should be also noted that early stage precipitates (GP zones) are already present in the microstructure of as-HE samples (Fig. 5). As shown in [14], the adiabatic heating of a billet with a temperature rise up to 150 °C for a few seconds is observed in every stage of HE. Therefore, the precipitation phenomena is likely to occur during HE processing. The observed GP zones combined with ultrafine-grained microstructure and high dislocation density (Table 2) result in a significant enhancement of YS and
4.1. Effect of Processing Conditions on Precipitation and Mechanical Properties In conventional CG Al-Mg-Si alloys, the precipitation sequence is well described and proceeds in the following order: supersaturated solid solution → Mg/Si co-clusters → GP zones → β″ → β′ → β [26]. As it has been already mentioned, SPD processing may facilitate the precipitation phenomena and influence the aforementioned sequence [14–16]. High strain applied during HE processing of the 6101 Al alloy also
Fig. 9. Precipitation condition in the 6101 Al alloy in the state HE4 + 180 °C/7 h: (a) spherical precipitates at grain boundaries of ⟨001⟩ and (c) ⟨111⟩ grains with (b, d) STEM image of precipitate and EDS spectra.
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Table 1) since it is commonly accepted that needle-like β″ precipitates provide the highest strengthening effect [26,27]. It should be noted that needles formed in the UFG 6101 Al alloy are much shorter than in peakaged coarse grained Al-Mg-Si alloys which are typically characterized by the length of 50–100 nm [14,26]. This implies that time needed to form such precipitates in UFG alloys is shorter than in CG alloys and thus the peak hardness is achieved faster (Fig. 2(a)). After aging for 2 h, spherical precipitates with a diameter up to 20 nm are also observed at grain boundaries of ⟨001⟩ grains (Fig. 7) but they do not strengthen the material. Further aging for 7 h does not change significantly precipitates size and morphology and their arrangement in the microstructure (Fig. 8). The pronounced precipitation at HAGBs is observed at both ⟨001⟩ and ⟨111⟩ grains (Fig. 9). However, these factors do not contribute to further improvement of material's strength. They are accompanied by grain growth and recovery process (Table 2) which cause decrease of hardness, YS and UTS (Fig. 2(a) and Table 1). Similar behavior of 6082 Al-Mg-Si alloy processed by HE was observed and explained in detail by W. Chrominski et al. [14,15,24]. Noteworthy is also the fact that HE10 exhibits significantly different age hardening response in comparison to HE4 and HE3 (Fig. 2(a)). The strengthening effect provided by precipitates is much stronger for HE10. The peak-aging treatment gives the hardness enhancement of about 30% for HE10 whereas the hardness increase for HE4 and HE3 does not reach even 10%. This is also confirmed by results of tensile test (Table 1) where UTS for peak-aged samples is improved by 82 and 52 MPa for HE10 and HE3, respectively. It is caused probably by higher number of ⟨001⟩ grains in the microstructure of HE10 as shown during EBSD and texture analysis (Figs. 3 and 10). The ⟨001⟩ grains with homogenously distributed needle-like β″ precipitates provide mainly precipitation strengthening of material while ⟨111⟩ grains, characterized by lower density of needle-like precipitates and the presence of PFZ (Fig. 6), results in more efficient grain boundary strengthening. Such
Table 2 Microstructural characteristics of the 6101 Al alloy under different processing conditions (d2 – mean equivalent grain diameter; ρ – dislocation density). State
d2 (nm)
Precipitates characteristic
ρ (1014 m−2)
T1 (as-received) HE4 HE4 + 180 °C/7 h
110 ± 60 (μm) 315 ± 134 408 ± 159
0.64 ± 0.10 5.1 ± 1.1 3.9 ± 0.3
HE3 HE3 + 180 °C/2 h
293 ± 126 389 ± 135
– - GP zones in ⟨001⟩ grains - GP zones in ⟨001⟩ grains - Needle-like β″ precipitates in ⟨001⟩ and ⟨111⟩ grains - Spherical β′ or β precipitates at GB of ⟨001⟩ and ⟨111⟩ grains - GP zones in ⟨001⟩ grains - GP zones in ⟨001⟩ grains - Needle-like β″ precipitates in ⟨001⟩ and ⟨111⟩ grains - Spherical β′ or β precipitates at GB of ⟨001⟩ grains
6.7 ± 1.2 3.6 ± 0.3
UTS by HE processing in comparison to conventionally processed 6101 Al alloy in T1 state (Table 1). In contrary to HPT processed Al-Mg-Si alloys [9], the artificial aging of as-HE samples brings about further improvement of mechanical properties. As shown in [9], the aging treatment performed after HPT of 6101 Al alloy strongly reduces the hardness by 20–30% in the first few hours of aging due to significant grain growth. For HE processed materials, it is still possible to enhance their mechanical properties by precipitation strengthening. The short-time aging (2 h) at 180 °C brings about the transformation of fine spherical GP zones into needle-like β″ precipitates aligned with ⟨001⟩ type directions (Fig. 6). Such processing conditions results in the peak hardness, YS and UTS (Fig. 2(a) and
Fig. 10. Pole figures of the 6101 Al alloy processed by HE: (a) HE10; (b) HE4 (the reflecting surface is normal to extrusion direction).
111
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in solid solution. M. H. Mulazimoglu et al. [43] have found that the resistivity of the Al-Mg-Si alloys increases linearly with increasing Mg and Si content at a rate of ΔρMg = 5.8 nΩm/wt% and ΔρSi = 7.0 nΩm/ wt%, respectively [43]. Assuming that the total content of Mg and Si has been dissolved in the matrix during solution heat treatment performed before HE, it gives a contribution of solute atoms to resistivity of about 7.20 nΩm. The estimated resistivity of solute atoms and precipitates for HE4 and HE3 is significantly smaller that suggests the process of decomposition of supersaturated solid solution during HE process. TEM observations revealed that fine GP zones are formed in the interior of ⟨001⟩ grains (Fig. 5). However, it is well-known that atom clusters and very fine precipitates like GP zones are very effective in electron scattering and further decreasing electrical conductivity [4]. This implies that solute atoms may segregate to grain boundaries what has been already observed in UFG aluminum alloys [17–19]. HAGBs are known to be preferential sites for solute atoms segregation and nucleation of precipitates [15,44]. The presence of PFZs (Fig. 6(b)) and spherical precipitates located at HAGBs of ⟨001⟩ and ⟨111⟩ grains (Fig. 9) after artificial aging confirms the presence of grain boundary segregation during HE processing. Besides, atom probe tomography (APT) analysis performed on HPT processed 6101 Al alloy by X. Sauvage et al. [9] showed that fractions of Mg and Si that may segregate at grain boundaries during SPD processing are in the range of 0.01–0.04 and 0.04–0.15 at.%, respectively. The electrical conductivity measurements have shown that HE10 sample exhibits lower conductivity than HE4 and HE3 for all aging times (Fig. 2(b)). It could be explained by aforementioned grain boundary segregation. HE10 contains significantly lower number of fine ⟨111⟩ grains in the microstructure in comparison to HE4 (Figs. 3 and 10) and thus lower surface area of grain boundaries, specially HAGBs observed mainly between ⟨001⟩ and ⟨111⟩ grains. Moreover, the higher fraction of HAGBs, which attract solute atoms effectively [15,44], is observed for HE4 (Fig. 3(d)) what results in an enhanced grain boundary segregation as compared to HE10. The aging treatment at 180 °C results in a significant reduction of electrical resistivity of HE processed 6101 Al alloy (Table 3). It is caused by annihilation of dislocations, grain growth and decrease of solute content in the matrix due to the precipitation of Mg- and Si-rich phases. It should be noted that the latter one is the most effective in terms of lowering the electrical resistivity of material since the contribution of solute atoms and precipitates is significantly reduced from 5.51 to 2.87 and 1.78 nΩm for aged HE3 and HE4 samples, respectively. TEM observations of HE3 and HE4 after aging revealed the presence of fine spherical GP zones, needle-like β″ precipitates located in grain interiors together with spherical β′ or β phase precipitates located at grain boundaries (Fig. 6-9). Longer aging periods of 7 h of HE4 resulted in the higher number of precipitates, especially spherical particles located at grain boundaries, what explains the lower contribution of solute atoms to total resistivity calculated for this sample. The most frequent precipitates in aged HE4 sample are needle-like β″ (Mg5Si6 [37]) and spherical β′ (Mg1.8Si) or β (Mg2Si) phases with Mg:Si weight ratio of 0.72:1, 1.56:1 and 1.73:1, respectively. If we assume, to simplify the calculations, that for investigated precipitation condition Mg:Si weight ratio is 1:1, the difference between solute atoms contribution to resistivity of aged HE4 (1.78 nΩm) and as-HE sample (5.51 nΩm) results from Mg and Si depletion of the matrix of about 0.3 wt%. Similar values of solute atoms depletion due to precipitation in the range of 0–0.3 at.% were obtained by X. Sauvage et al. [9] during APT measurements of HPT processed 6101 Al alloy. It should be noted that the presence of precipitates in the material also increases its resistivity. B. Raeisinia et al. [45] have proposed a relationship between the resistivity contribution from precipitates and their spacing. However, it has been neglected in this discussion due to impossibility of precise measurements of TEM foils thickness and thus the density of precipitates in the microstructure.
behavior of hydroextruded Al-Mg-Si alloy has been already quantitatively analyzed and described in [24]. 4.2. Effect of Microstructure on Electrical Conductivity The observed microstructural changes during HE process and artificial aging of the 6101 Al alloy affect the electrical conductivity significantly. According to Matthisen's rule, the total resistivity of metal alloys is influenced by parent metal (ρAl), lattice defects such as vacancies (ρvac), dislocations (ρdisl) and grain boundaries (ρGB) and solute atoms (ρss) or precipitates (ρppt) and can be expressed as a sum of all contributions in the following form [4,38]:
ρtotal = ρAl + Nvac ∆ρvac + Ldisl ∆ρdisl + SGB ∆ρGB +
∑ Ci ∆ρi + ρppt i
where ρAl is the resistivity of pure aluminum with no lattice defects at room temperature (ρAl = 26.55 nΩm [39]). Nvac (at.%), Ldisl (m−2), SGB (m−1), Ci (wt%) are the concentration or density of vacancies, dislocations, grain boundaries and i-th solute atoms, respectively. Δρvac (Ωm/at.%), Δρdisl (Ωm3), ΔρGB (Ωm2) and Δρi (Ωm/wt%) are constants accounting for the contribution of unit concentration or density of vacancies, dislocations, grain boundaries and i-th solute atom, respectively. To determine the contribution of lattice defects to the resistivity of aluminum, the following values of constants were used in this study: Δρvac = 26 nΩm/at.% [38], Δρdisl = 2.7 × 10−25 Ωm3 and ΔρGB = 2.6 × 10−16 Ωm2 [40]. The vacancy concentration has not been investigated in this work. However, it generally does not exceed 0.001% for SPD processed materials [41]. Precipitation phenomena confirmed by TEM observations are related with a bulk diffusion with the most likely vacancy-solute mechanism. Thus, the concentration of vacancies may be reduced with aging time. Then, it can be concluded that vacancies contribution to the resistivity would not be significant. It can be neglected since it will not exceed 2.6 × 10−2 nΩm in the case of no vacancy depletion by aging. The contributions of dislocations and grain boundaries were estimated based on experimentally measured values of dislocation density and mean equivalent grain diameter d2 (Table 2) which is related to the surface area of grain boundaries per unit volume SGB in the following way: SGB = 2/d2 [42]. The values of electrical conductivity were recalculated to total resistivity using the copper conductivity value of 58.0 MS/m. The influence of solute atoms and precipitates was calculated as a difference between total resistivity and contributions of pure aluminum and other microstructural features. All estimated contributions of different microstructural features are summarized in Table 3. The high density of dislocations and grain boundaries generated by HE processing increases the resistivity of as-HE samples of the 6101 Al alloy in comparison to its conventional counterpart (Table 3). However, their contribution to total resistivity is still significantly smaller than the influence of solute atoms in the matrix. The resistivity of dislocations and grain boundaries does not exceed 1% and 6–7% of the electrical resistivity of pure aluminum, respectively. The electrical conductivity is extremely sensitive to the concentration of Mg and Si atoms Table 3 Estimation of contribution of different microstructural features to total electrical resistivity ρtotal of the 6101 Al alloy under different processing conditions (ρAl – resistivity of pure aluminum; ρdisl – dislocation contribution; ρGB – grain boundaries contribution; ρss – solute atoms contribution; ρppt – precipitates contribution). State
ρtotal (nΩm)
ρAl (nΩm)
ρdisl (nΩm)
ρGB (nΩm)
ρss + ρppt (nΩm)
T1 (as-received) HE4 HE4 + 180 °C/7 h HE3 HE3 + 180 °C/2 h
33.73 33.85 29.71 34.01 30.86
26.55
0.02 0.14 0.11 0.18 0.10
< 0.01 1.65 1.27 1.77 1.34
7.16 5.51 1.78 5.51 2.87
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in the matrix seems to be the most important. The high conductivity is provided mainly by decomposition of solid solution due to precipitation of needle-like β″ precipitates in the grain interior and spherical β′ or β particles located at grain boundaries. Moreover, the presence of pronounced grain boundary segregation has been suggested for hydrostatically extruded Al-Mg-Si alloy. - It is demonstrated that the best combination of high strength (332 MPa) and high electrical conductivity (58.04% IACS) can be obtained by the three stage hydrostatic extrusion with a total true strain of 3.2 followed by artificial aging at 180 °C for 7 h. Acknowledgements This work was supported by The National Centre for Research and Development within the ERA.NET-RUS program (contract no. NCBR/ ERA NET RUS/02/2012). References Fig. 11. Ultimate tensile strength (UTS) and electrical conductivity of the 6101 Al alloy in the state HE4 + 180 °C/7 h in comparison to conventional Al-Mg-Si alloys (AL2-AL7) and 6101 Al alloy processed by SPD methods.
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4.3. Comparison of HE and SPD Methods Fig. 11 presents the comparison of UTS and electrical conductivity of the 6101 Al alloy processed by HE with SPD methods [7,10] and conventional Al-Mg-Si alloys used in the electrical engineering [5]. It is clear that UFG 6101 Al alloy processed by various SPD methods exhibits enhanced strength and conductivity when compared to its CG counterparts. Among heavily deformed materials, the HE4 sample subjected to artificial aging (AA) at 180 °C for 7 h possess optimized properties, i.e. the highest electrical conductivity of 58.04% IACS and UTS of 332 MPa. The higher strength of 364 MPa is achieved for the 6101 Al alloy processed by continuous ECAP-C combined with aging at 170 °C for 12 h and final drawing to a diameter of 3.2 mm but it comes at the expense of electrical conductivity reduced to 56.4% IACS [10]. It could be stated that hydrostatic extrusion combined with aging treatment provides significant improvement of mechanical and electrical properties which is comparable to various SPD methods. However, it possess the technological advantage of the easier processing of long wires. 5. Conclusions It has been shown that the hydrostatic extrusion combined with an artificial aging is an effective method for producing of long wires of ultrafine-grained Al-Mg-Si alloy with enhanced strength and electrical conductivity. The performed investigation allows to conclude: - The hydrostatic extrusion of the 6101 Al alloy results in the formation of ultrafine-grained microstructure with grain size of about 300 nm and fine GP zones precipitated in grains with ⟨001⟩ parallel to extrusion direction. - Mechanical strength of Al-Mg-Si alloy processed by hydrostatic extrusion can be further increased by artificial aging. Higher applied strains during HE processing accelerates the precipitation kinetics but decreases the age hardening response of the 6101 Al alloy. The mechanical behavior depend on crystallographic texture of hydrostatically extruded Al-Mg-Si alloy. The grains with ⟨001⟩ parallel to extrusion direction with homogenously distributed needle-like β″ precipitates provide mainly precipitation strengthening of material while grains with ⟨111⟩ parallel to extrusion direction results in more efficient grain boundary strengthening. - Electrical conductivity of the 6101 Al alloy processed by hydrostatic extrusion is sensitive to microstructural defects such as dislocations and grain boundaries but the resistivity contribution of solute atoms 113
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Enhanced strength and electrical conductivity of ultrafine-grained Al-Mg-Si alloy processed by hydrostatic extrusion
T
⁎
Kamil Majchrowicza, , Zbigniew Pakiełaa, Witold Chrominskia, Mariusz Kulczykb a b
Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics of the Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrafine-grained materials Al-Mg-Si alloys Hydrostatic extrusion Electrical conductivity
The effect of hydrostatic extrusion combined with an artificial aging on microstructure, mechanical and electrical properties of 6101 Al-Mg-Si alloy was investigated. It has been shown that such thermo-mechanical treatment is an effective method for producing of long wires with an ultrafine-grained microstructure (grain size of 300–400 nm) and enhanced ultimate tensile strength (> 330 MPa) and electrical conductivity (up to 58% IACS). The mechanical behavior of 6101 Al-Mg-Si alloy depended strongly on applied strains by hydrostatic extrusion and crystallographic texture. Higher accumulative strain accelerated the precipitation kinetics but decreased the age hardening response. The double fiber ⟨100⟩ and ⟨111⟩ texture was observed for hydrostatically extruded samples. The ⟨001⟩ grains with homogenously distributed needle-like β″ precipitates provided precipitation strengthening of material while ⟨111⟩ grains resulted in more efficient grain boundary strengthening. Quantitative microstructure characterization allowed adjusting physical model to estimate the electrical conductivity and compare it with experimental data. The high conductivity was provided mainly by decomposition of solid solution due to precipitation of needle-like β″ precipitates in the grain interior and spherical β′ or β particles located at grain boundaries.
1. Introduction Al-Mg-Si alloys are currently widely used as conducting materials in overhead power transmission lines and many other applications in electrical engineering [1]. Due to high electrical conductivity, specific strength and corrosion resistance, they have become increasingly popular candidates for industrial usage. Standard conductors are made of Al-Mg-Si alloys containing 0.6–0.9% Mg and 0.5–0.9% Si such as 6101 and 6201 aluminum alloys [2,3]. Higher magnesium and silicon contents significantly deteriorate electrical conductivity due to increased scattering of conduction electrons by solute atoms [4]. High strength and conductivity of conventionally produced Al-Mg-Si alloys are a result of thermo-mechanical processing consisting of solution heat treatment followed by water quenching, cold drawing and artificial aging [2]. Such processing route results in ultimate tensile strength of 255–325 MPa and electrical conductivity in the range of 57.5%–52.5% IACS (International Annealed Copper Standard) [5]. A new strategy has been proposed recently by several authors to enhance strength and electrical conductivity of Al-Mg-Si alloys using severe plastic deformation (SPD) methods combined with an artificial aging [6–10]. An unconventionally high strain applied during SPD
⁎
processing leads to significant grain refinement and obtaining ultrafinegrained (UFG) structures with a grain size in the range of 100 nm to 1 μm [11]. It brings about a strong grain boundary strengthening as described by the Hall-Petch relationship [12,13]. SPD also influences the precipitation kinetics and changes precipitation sequence in agehardenable aluminum alloys as reported in [14–16]. Moreover, pronounced grain boundary segregation has been observed in UFG aluminum alloys [17–19] what may contributes, according to Matthisen's rule [4], to a greater enhancement of electrical conductivity due to reduced solute content in the matrix. SPD processing of Al-Mg-Si alloys, such as equal channel angular pressing with parallel channels (ECAPPC) [6,7] and high pressure torsion (HPT) [8,9], results in increased electrical conductivity up to 58% IACS. However, the main disadvantage of these techniques limiting their industrial usage is the small size of processed billets. To overcome this weakness, M. Murashkin et al. [10] have proposed recently modified continuous ECAP-C technique for fabrication of long billets. In the present work, a new approach has been implemented to produce long wires of UFG Al-Mg-Si alloys by the hydrostatic extrusion (HE) method. In this process, a billet located in a working chamber filled with fluid is extruded through a forming die of specified diameter
Corresponding author at: Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland. E-mail address: [email protected] (K. Majchrowicz).
https://doi.org/10.1016/j.matchar.2017.11.023 Received 21 May 2017; Received in revised form 24 September 2017; Accepted 11 November 2017 Available online 11 November 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.
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5.5 and 10 m, respectively. In the current study, all HE samples were taken from one batch in order to keep similar deformation conditions what resulted in the sample length of about 0.5, 1.3 and 1.5 m for HE10, HE4 and HE3, respectively. Then, as-HE samples were cut for smaller pieces for further aging treatment. To investigate an age hardening response of the 6101 Al alloy subjected to different HE reductions, Vickers hardness measurements with the load of 200 g were performed using Zwick Roell Z2.5/ZHU0.2. Uniaxial tensile tests were conducted on a servo-hydraulic mechanical testing machine MTS 858 equipped with a 25 kN load cell. Experiments were performed at initial strain rate of 10−3 s−1 using cylindrical test specimens with a diameter of 2.5 mm and gage length of 15 mm. The electrical conductivity was measured at room temperature using an eddy current conductivity meter Sigmascope SMP10 equipped with a 60 kHz probe ES40. The method is standardized according to DIN EN 2004-1 and ASTM E 1004. To avoid conductivity measurement errors resulting from curvature of investigated specimens, the conductivity meter was calibrated before each measurement with reference samples in the form of wires having the exact diameter of measured specimen. The electrical conductivity of reference samples was measured using four-point resistivity measurement method. The microstructure was characterized using Hitachi SU70 scanning electron microscope (SEM) for EBSD mapping and JEOL JEM-1200EX transmission electron microscope (TEM) with an acceleration voltage of 120 kV. EBSD measurements were performed on electrochemically polished surfaces. Maps were taken with an acceleration voltage of 20 kV on a square grid of points separated by 500 nm for general and 100 nm for detailed mapping. Each map had a ratio of successfully indexed Kikuchi maps > 70%, thus data were considered reasonable. Data were analyzed with a dedicated Channel5 software. Thin foils for TEM investigation were prepared from 3 mm diameter discs cut out from the center of final as-HE and aged rods perpendicularly to the extrusion direction. The discs were ground down and electropolished using Struers TenuPol-5 system. The mean grain size was described by the equivalent diameter d2 measured using the MicroMeter software [25] whereas ImageJ software was used for measurements of precipitates' size. The length of precipitates was estimated based on TEM images. Since needle-like precipitates in Al-Mg-Si alloys grow along ⟨001⟩ type directions [26,27] their length can be measured directly from ⟨001⟩ zone axis images. In ⟨111⟩ zone axis images a length taken from the micrograph was divided by a cosine between ⟨001⟩ and ⟨112⟩, which equals 0.81. [12 1] is a perpendicular projection of [010] on (111) plane. The energy dispersive X-ray spectroscopy (EDS) analysis of precipitates located at grain boundaries was carried out on a cold-FEG Hitachi S5500 scanning transmission electron microscope (STEM) using samples prepared for TEM observations. X-ray diffraction (XRD) method was used to estimate dislocation density. Based on measurements conducted on Bruker AXS D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å), the dislocation density ρ was calculated using modified Williamson-Hall formula [28]:
by high hydrostatic pressure of the liquid. The pressure is generated by a piston compressing the liquid in the working chamber. During HE process, the extruded material is severely deformed with a high strain rate (exceeding 102 s−1) resulting in an adiabatic heating of the billet. To reduce heat effects, there is a cooling system placed at the exit of the forming die [20–22]. According to the formal definition of SPD techniques established by R.Z. Valiev et al. [23], SPD processing excludes the hydrostatic extrusion due to the change in the overall dimensions of sample. However, HE process still enables to impose very high strains and requires smaller total deformation to induce significant grain refinement to submicron sizes as compared to SPD methods [20]. The main advantage of HE is the possibility of direct forming of long wires dedicated to many electrical applications. Besides, as shown for 6082 Al-Mg-Si alloy by W. Chrominski et al. [14], the combination of HE and artificial aging leads to higher strength than other SPD techniques combined with different heat treatments. The precipitation process and origins of superior strength of hydroextruded 6082 Al-Mg-Si alloy has been explained in detail in [14,15,24]. The main purpose of this work is to demonstrate the potential of hydrostatic extrusion for fabrication of UFG Al-Mg-Si alloy with both enhanced strength and electrical conductivity in comparison to conventional processing methods as well as SPD techniques. The paper examines the effect of specially designed thermo-mechanical treatment on microstructure of 6101 Al-Mg-Si alloy used as a standard conductor in electrical engineering and correlates its microstructural characteristics with electrical conductivity.
2. Materials and Methods A commercially available 6101 Al alloy with a chemical composition of 0.59Mg-0.54Si-0.07Fe (wt%) was used in this work in the form of ∅ 20 mm extruded rod in the T1 condition (cooled from elevated temperature shaping process and naturally aged). Billets of 6101 Al alloy were solution heat treated at 510 °C for 1 h followed by water quenching and subjected to HE to obtain an ultrafine-grained structure. HE process was applied in the several stages with different reductions of billet's diameter resulted in different accumulative strain. In the first step, the initial diameter was reduced from 20 to 10 mm (total true strain of 1.4) and this condition was represented by sample named as HE10. Subsequently, billets were extruded in the next two stages to final diameter of 4 mm (further referred to as HE4) and 3 mm (HE3) with the intermediate diameter of 6 mm. Hence, the HE4 and HE3 samples were subjected to three-stage HE with a total true strain of 3.2 and 3.8, respectively. Afterwards, as-HE samples were aged at 180 °C for 1–24 h which are standard parameters for conventionally produced 6101 alloy. The aging treatment was also performed for coarse-grained (CG) counterpart to compare the results obtained for HE processed samples with conventional material. All processing stages of 6101 Al alloy are schematically presented in Fig. 1. HE process was performed in CP31 hydroextrusion press with the working chamber diameter of 31 mm operating to maximum pressure of 1.7 GPa which was designed and constructed at the Institute of High Pressure Physics of the Polish Academy of Sciences, Unipress. The maximum length of billets used in this press is 250 mm what enables to obtain long wires, e.g. with diameter of 4 and 3 mm and length of about
1/2
∆K =
0.9 πA2 b2 ⎞ +⎛ D ⎝ 2 ⎠ ⎜
⎟
ρ1/2 (KC 1/2) + O (K 2C )
where: K = 2sinθ/λ, θ is the diffraction angle and λ the X-ray Fig. 1. Processing route of 6101 Al alloy.
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Fig. 2. (a) Hardness and (b) electrical conductivity of the 6101 Al alloy subjected to different HE reductions and aged at 180 °C up to 24 h.
wavelength; ΔK = 2cosθ(Δθ)/λ, Δθ is the full width at half maximum (FWHM) of a peak; b is the Burgers vector of dislocation (b = 0.286 nm [29]) and D corresponds to the average crystallite size. A is a constant depending on the effective outer cut-off radius of dislocations and it was estimated for Al-Mg-Si alloy by W. Woo et al. [30] (A2 = 0.63). The dislocation contrast factor C was determined by the elastic anisotropy factor, dislocation type and single-crystal elastic constants according to [31]. For calculations of dislocation contrast factor, the elastic constants were used from [32] and the edge/screw dislocation ratio was assumed as 75 to 25% according to [33]. Besides, texture analysis was performed by means of X-ray diffractometer Bruker AXS D8 Discover. Three incomplete pole figures, i.e. (111), (200), (220), were used to calculate the orientation distribution function and subsequently recalculate complete pole figures.
the electrical conductivity of HE processed samples stabilizes and exhibits slightly rising tendency with increasing aging time. Noteworthy is also the fact that the 6101 Al alloy subjected to the smallest HE reduction (HE10) shows lower conductivity than HE4 and HE3 for all aging times. The initial conductivity for as-HE samples is 49.67 ± 0.11, 50.94 ± 0.43 and 50.69 ± 0.64% IACS for HE10, HE4 and HE3, respectively. This difference of around 1% IACS between HE10 and both HE4 and HE3 maintains for longer aging times. The HE4 and HE3 samples reach similar conductivity what indicates that higher level of deformation applied to the 6101 Al alloy will not provide further improvement of electrical properties. The initial conductivity for CG sample is 49.63 ± 0.02% IACS what is close to HE10. However, further aging treatment of CG 6101 alloy do not cause as high improvement of electrical conductivity as observed for material subjected to HE process. The conductivity for peak aged CG sample reaches 52.54 ± 0.17% IACS. As shown in Fig. 2, the level of applied deformation by HE processing has a significant effect on age hardening response and electrical conductivity of the 6101 Al alloy. To explain obtained results, as-HE samples without aging and HE10 and HE3 peak-aged samples have been chosen for further mechanical and microstructural investigations. The HE4 sample aged at 180 °C for 7 h has been chosen as the material with the best combination of high electrical conductivity and moderate hardness. Mechanical properties assessed in the uniaxial tensile tests are summarized in Table 1. Among as-HE samples, the HE10 exhibits the lowest YS and UTS of 214 ± 5 and 227 ± 6 MPa, respectively. UTS reaches the maximum value for HE4 (305 ± 3 MPa) and the higher level of applied deformation do not provide any significant changes. YS and UTS for HE3 is 288 ± 4 and 302 ± 4 MPa, respectively. Besides, it is noted that strength improvement comes at the expense of ductility
3. Results 3.1. Mechanical Properties and Electrical Conductivity Age hardening response of the 6101 Al alloy subjected to different HE reductions is presented in Fig. 2(a) in the form of hardness change as a function of aging time at 180 °C. The initial hardness for as-HE samples increases with higher billet's diameter reductions and equals to 73 ± 4, 87 ± 4 and 89 ± 4 HV for HE10, HE4 and HE3, respectively. This implies that higher accumulative strain induced by HE improves the hardness more effectively due to higher density of generated microstructural defects. Three-stage age hardening response including under-aging, peak-aging and over-aging is noted successively with extending aging time for all samples. Increasing accumulative strain shifts hardness peak to a shorter aging time of 7 h for HE10 and ~ 2 h for both HE4 and HE3. The maximum value of hardness reaches 94 HV for HE10 and 95 HV for HE4 and HE3. It suggests that increasing strain alters materials response to post-deformation annealing. As suggested by other authors [34–36] this may be related to increasing density of microstructural defects which serve as potential nucleation sites for strengthening precipitates. For longer aging times, the overaging stage is observed for all HE processed samples as the continuous hardness decrease down to 76–78 HV. The CG 6101 alloy shows much higher age hardening response in comparison to as-HE samples. Its initial hardness equals to 52 ± 2 HV and increases up to peak value of 94 ± 3 HV for 7 h of aging. The electrical conductivity of the 6101 Al alloy after aging at 180 °C for different times is plotted in Fig. 2(b). It is clear that the conductivity is extremely sensitive to the aging time. It sharply increases at the early stage of aging (up to 4 h) due to decreasing solute concentration in the matrix caused by rapid precipitates nucleation and growth. Afterwards,
Table 1 Mechanical properties and electrical conductivity of the 6101 Al alloy under different processing conditions (YS – yield strength; UTS – ultimate tensile strength; A – elongation to failure; IACS – electrical conductivity expressed as a relative value of International Annealed Copper Standard).
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State
YS (MPa)
UTS (MPa)
A (%)
IACS (%)
T1 (as-received) T6 (180 °C/7 h) HE10 HE10 + 180 °C/7 h HE4 HE4 + 180 °C/7 h HE3 HE3 + 180 °C/2 h
116 216 214 298 294 329 288 345
181 228 227 309 305 332 302 354
19 9.0 ± 2.5 8.9 ± 1.4 11.8 ± 0.8 7.8 ± 2.5 8.8 ± 1.2 6.1 ± 0.9 5.5 ± 0.9
51.12 52.54 49.67 57.12 50.94 58.04 50.69 55.88
± ± ± ± ± ± ±
5 5 2 3 2 4 6
± ± ± ± ± ± ±
9 6 2 3 2 4 6
± ± ± ± ± ± ± ±
0.07 0.17 0.11 0.10 0.43 0.66 0.64 0.26
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Fig. 3. EBSD orientation maps of the 6101 Al alloy in the state (a) HE10 and (b) HE4 with color code for orientation maps and (d) misorientation angle distributions.
peak close to 55°. Based upon quantification of obtained EBSD results the LAGB fraction is 90% in the former and 73% in the latter. TEM observations after HE process were performed for HE4 and HE3 samples. Since they exhibit similar microstructure characteristics (described in detail in further paragraphs) and age hardening response (Fig. 2(a)), following samples were selected for a detailed precipitation analysis: HE3 + 180 °C/2 h corresponding to the peak-aging condition and HE4 + 180 °C/7 h corresponding to slightly over-aged condition but still showing enhanced electrical conductivity and strength in comparison with as-HE samples. A typical ultrafine-grained microstructure for hydrostatically extruded samples, as characterized in [14,15,24], is observed for the 6101 Al alloy. It consists of two types of grains: fine, equiaxed grains with a size of 200–300 nm oriented with ⟨111⟩ parallel to extrusion direction (named as ⟨111⟩ grains) and relatively large (at least 1 μm in size) grains with a dislocation substructure having an orientation with ⟨001⟩ parallel to extrusion direction (named as ⟨001⟩ grains) (Fig. 4(a)). Dislocation boundaries with a low misorientation angle are arranged within ⟨001⟩ grains as well as dense network of LAGBs is observed within regions of ⟨111⟩ oriented grains. HAGBs are present mainly between ⟨001⟩ and ⟨111⟩ grains or around some ⟨001⟩ and ⟨111⟩ grains (Fig. 4(b)). Considering precipitation condition in 6101 Al alloy after HE process, TEM inspection reveals some fine precipitates in the interior of ⟨001⟩ grains (Fig. 5(a)) which were recognized by HRTEM observations in [15,24] as GP zones. Besides, there is no precipitates at grain boundaries of ⟨001⟩ (Fig. 5(b)) and ⟨111⟩ grains (Fig. 5(c)). After HE process, homogenously distributed GP zones were observed only in ⟨001⟩ grains. Aging for 2 h brings about their
which decreases with increasing HE reduction. The aging treatment of HE10 gives significant enhancement of YS and UTS to 298 ± 2 and 309 ± 2 MPa. Despite of similar hardness for peak-aged HE10 and HE3, the latter one reaches the highest YS and UTS of 345 ± 6 and 354 ± 6 MPa, respectively. The results for HE4 aged at 180 °C for 7 h confirm that it is the best combination of high electrical conductivity (58.04 ± 0.66% IACS) and strength (YS of 329 ± 2 and UTS of 332 ± 2 MPa). Additionally, all samples subjected to HE and aging are much stronger and have higher conductivity as compared to their conventionally produced counterpart in state T1 and T6 (aged for 7 h at 180 °C).
3.2. Microstructure After HE Process and Aging EBSD orientation maps presented in Fig. 3(a) and (b) show development of the grain refinement process in two chosen steps – HE10 and HE4 studied on a cross section of extrusions. The most striking feature is domination of two orientations, namely ⟨001⟩ and ⟨111⟩ parallel to extrusion direction, red and blue respectively as can be noticed from color code Fig. 3(c). HE10 sample exhibit primary grains separated by high angle grain boundaries (HAGBs, represented by black lines) with low angle boundaries network inside (LAGBs, white lines). As the applied strain increases the boundaries of primary grains become blurrier. The network of LAGBs in the interior of ⟨001⟩ and ⟨111⟩ oriented areas become denser for HE4 as well as the number of HAGBs between ⟨001⟩ and ⟨111⟩ oriented grains. Misorientation angle distributions, Fig. 3(d) shows a difference between two considered materials condition. HE10 features higher fraction of LAGBs than HE4, which exhibits a slight 107
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Fig. 4. Microstructure of the 6101 Al alloy processed by HE in the state HE4: (a) TEM image, (b) EBSD orientation map (color code as in Fig. 3(c)).
transformation into fine needle-like β″ precipitates aligned with ⟨001⟩ directions (Fig. 6(a)). These precipitates with a length of 16 ± 8 nm are homogenously distributed in the grain interior and gives strong strain contrast suggesting their coherency with the matrix. They are present in ⟨001⟩ and ⟨111⟩ grains but their density seems to be smaller in ⟨111⟩ grains. For ⟨111⟩ grains, needle-like β″ particles with a length of 17 ± 6 nm are observed as well as precipitation free zone (PFZ) of about 20 nm (Fig. 6(b)). It suggests that grain boundaries surrounding ⟨111⟩ grains effectively attract solute atoms. However, a relatively large (up to 20 nm in diameter) spherical precipitates are located only at grain boundaries of ⟨001⟩ grains (Fig. 7). During further aging for 7 h, the length of needle-like β″ precipitates grows to 22 ± 14 and 22 ± 11 nm for ⟨001⟩ and ⟨111⟩ grains, respectively. Longer aging time does not change significantly their overall appearance in comparison to 2 h of aging (Fig. 8). They are aligned with three mutually perpendicular ⟨001⟩ type directions as shown in Fig. 8(b) for ⟨111⟩ grains. For ⟨001⟩ grains, needle-like β″ precipitates along [100] and [010] directions are clearly visible while precipitates
along [001] direction are seen as dark black spots (Fig. 8(a)). Apart from them, fine spherical precipitates similar to GP zones found in asHE samples (Fig. 5(a)) are also localized in ⟨001⟩ grains of aged sample (visible as fine grey spots in Fig. 8(a)). These fine precipitates are also present after 2 h of aging. Longer aging times favors pronounced precipitation at HAGBs. The spherical precipitates with a diameter up to 20 nm are located at grain boundaries of both ⟨001⟩ and ⟨111⟩ grains as depicted in Fig. 9. The EDS spectrums collected from these particles suggest that they are β′ (Mg1.8Si) or β (Mg2Si) phase due to Mg:Si atomic ratio close to 2 [37]. Such precipitates located at grain boundaries in UFG Al-Mg-Si alloys have been also found elsewhere [9,14]. The as-HE samples, i.e. HE4 and HE3, are characterized by ultrafinegrained microstructure with an average grain size of 315 ± 134 and 293 ± 126 nm, respectively. The dislocation density determined by modified Williamson-Hall method reaches 5.1 ± 1.1 and 6.7 ± 1.2 × 1014 m−2 for HE4 and HE3, respectively, and is higher by one order in comparison to as-received state (Table 2). Annealing for 2 Fig. 5. Precipitation condition in the 6101 Al alloy processed by HE (in the state HE4): (a) fine precipitates (GP zones) in the interior of ⟨001⟩ grains; (b) lack of precipitates at grain boundaries of ⟨001⟩ and (c) ⟨111⟩ grains.
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Fig. 6. Precipitation condition in the 6101 Al alloy in the state HE3 + 180 °C/2 h: (a) GP zones and needle-like β″ precipitates in ⟨001⟩ and (b) needle-like β″ precipitates in ⟨111⟩ grains with a precipitation free zone of about 20 nm.
Fig. 7. Precipitation condition in the 6101 Al alloy in the state HE3 + 180 °C/2 h: (a) spherical precipitates at grain boundaries of ⟨001⟩ grains with (b) STEM image of precipitate and EDS spectra; (c) lack of precipitates at grain boundaries of ⟨111⟩ grains.
⟨100⟩ fiber. HE4 features a much stronger ⟨111⟩ fiber texture with a weak ⟨100⟩ component. The maximum intensity of (111) pole figure is 17.5 MRD. Texture analysis with respect to EBSD studies (Fig. 3) confirms that HE4 consists high content of ⟨111⟩ grains and small fraction of ⟨001⟩ grains whereas HE10 is composed of higher number of ⟨001⟩ grains with a dislocation substructure.
and 7 h causes minor grain growth to 389 ± 135 nm and 408 ± 159 nm, respectively, and a significant reduction of dislocation density due to recovery process. The microstructural parameters measured in this work are summarized in Table 2. 3.3. Texture Fig. 10 shows texture analysis results of as-HE samples, i.e. HE10 and HE4, in the form of recalculated pole figures (111) and (200) with its reflecting surface normal to extrusion direction. HE10 exhibits a double fiber texture with ⟨100⟩ and ⟨111⟩ fiber axes with a maximum intensity of 6.3 MRD (multiples of random distribution) observed for 109
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Fig. 8. Precipitation condition in the 6101 Al alloy in the state HE4 + 180 °C/7 h: (a) GP zones and needle-like β″ precipitates in ⟨001⟩ and (b) needle-like β″ precipitates in ⟨111⟩ grains.
4. Discussion
results in the accelerated precipitation kinetics. The time required to reach peak hardness is shorter for higher total cumulative strains (Fig. 2(a)) due to increased density of microstructural defects which provide privileged sites for precipitates nucleation. It should be also noted that early stage precipitates (GP zones) are already present in the microstructure of as-HE samples (Fig. 5). As shown in [14], the adiabatic heating of a billet with a temperature rise up to 150 °C for a few seconds is observed in every stage of HE. Therefore, the precipitation phenomena is likely to occur during HE processing. The observed GP zones combined with ultrafine-grained microstructure and high dislocation density (Table 2) result in a significant enhancement of YS and
4.1. Effect of Processing Conditions on Precipitation and Mechanical Properties In conventional CG Al-Mg-Si alloys, the precipitation sequence is well described and proceeds in the following order: supersaturated solid solution → Mg/Si co-clusters → GP zones → β″ → β′ → β [26]. As it has been already mentioned, SPD processing may facilitate the precipitation phenomena and influence the aforementioned sequence [14–16]. High strain applied during HE processing of the 6101 Al alloy also
Fig. 9. Precipitation condition in the 6101 Al alloy in the state HE4 + 180 °C/7 h: (a) spherical precipitates at grain boundaries of ⟨001⟩ and (c) ⟨111⟩ grains with (b, d) STEM image of precipitate and EDS spectra.
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Table 1) since it is commonly accepted that needle-like β″ precipitates provide the highest strengthening effect [26,27]. It should be noted that needles formed in the UFG 6101 Al alloy are much shorter than in peakaged coarse grained Al-Mg-Si alloys which are typically characterized by the length of 50–100 nm [14,26]. This implies that time needed to form such precipitates in UFG alloys is shorter than in CG alloys and thus the peak hardness is achieved faster (Fig. 2(a)). After aging for 2 h, spherical precipitates with a diameter up to 20 nm are also observed at grain boundaries of ⟨001⟩ grains (Fig. 7) but they do not strengthen the material. Further aging for 7 h does not change significantly precipitates size and morphology and their arrangement in the microstructure (Fig. 8). The pronounced precipitation at HAGBs is observed at both ⟨001⟩ and ⟨111⟩ grains (Fig. 9). However, these factors do not contribute to further improvement of material's strength. They are accompanied by grain growth and recovery process (Table 2) which cause decrease of hardness, YS and UTS (Fig. 2(a) and Table 1). Similar behavior of 6082 Al-Mg-Si alloy processed by HE was observed and explained in detail by W. Chrominski et al. [14,15,24]. Noteworthy is also the fact that HE10 exhibits significantly different age hardening response in comparison to HE4 and HE3 (Fig. 2(a)). The strengthening effect provided by precipitates is much stronger for HE10. The peak-aging treatment gives the hardness enhancement of about 30% for HE10 whereas the hardness increase for HE4 and HE3 does not reach even 10%. This is also confirmed by results of tensile test (Table 1) where UTS for peak-aged samples is improved by 82 and 52 MPa for HE10 and HE3, respectively. It is caused probably by higher number of ⟨001⟩ grains in the microstructure of HE10 as shown during EBSD and texture analysis (Figs. 3 and 10). The ⟨001⟩ grains with homogenously distributed needle-like β″ precipitates provide mainly precipitation strengthening of material while ⟨111⟩ grains, characterized by lower density of needle-like precipitates and the presence of PFZ (Fig. 6), results in more efficient grain boundary strengthening. Such
Table 2 Microstructural characteristics of the 6101 Al alloy under different processing conditions (d2 – mean equivalent grain diameter; ρ – dislocation density). State
d2 (nm)
Precipitates characteristic
ρ (1014 m−2)
T1 (as-received) HE4 HE4 + 180 °C/7 h
110 ± 60 (μm) 315 ± 134 408 ± 159
0.64 ± 0.10 5.1 ± 1.1 3.9 ± 0.3
HE3 HE3 + 180 °C/2 h
293 ± 126 389 ± 135
– - GP zones in ⟨001⟩ grains - GP zones in ⟨001⟩ grains - Needle-like β″ precipitates in ⟨001⟩ and ⟨111⟩ grains - Spherical β′ or β precipitates at GB of ⟨001⟩ and ⟨111⟩ grains - GP zones in ⟨001⟩ grains - GP zones in ⟨001⟩ grains - Needle-like β″ precipitates in ⟨001⟩ and ⟨111⟩ grains - Spherical β′ or β precipitates at GB of ⟨001⟩ grains
6.7 ± 1.2 3.6 ± 0.3
UTS by HE processing in comparison to conventionally processed 6101 Al alloy in T1 state (Table 1). In contrary to HPT processed Al-Mg-Si alloys [9], the artificial aging of as-HE samples brings about further improvement of mechanical properties. As shown in [9], the aging treatment performed after HPT of 6101 Al alloy strongly reduces the hardness by 20–30% in the first few hours of aging due to significant grain growth. For HE processed materials, it is still possible to enhance their mechanical properties by precipitation strengthening. The short-time aging (2 h) at 180 °C brings about the transformation of fine spherical GP zones into needle-like β″ precipitates aligned with ⟨001⟩ type directions (Fig. 6). Such processing conditions results in the peak hardness, YS and UTS (Fig. 2(a) and
Fig. 10. Pole figures of the 6101 Al alloy processed by HE: (a) HE10; (b) HE4 (the reflecting surface is normal to extrusion direction).
111
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in solid solution. M. H. Mulazimoglu et al. [43] have found that the resistivity of the Al-Mg-Si alloys increases linearly with increasing Mg and Si content at a rate of ΔρMg = 5.8 nΩm/wt% and ΔρSi = 7.0 nΩm/ wt%, respectively [43]. Assuming that the total content of Mg and Si has been dissolved in the matrix during solution heat treatment performed before HE, it gives a contribution of solute atoms to resistivity of about 7.20 nΩm. The estimated resistivity of solute atoms and precipitates for HE4 and HE3 is significantly smaller that suggests the process of decomposition of supersaturated solid solution during HE process. TEM observations revealed that fine GP zones are formed in the interior of ⟨001⟩ grains (Fig. 5). However, it is well-known that atom clusters and very fine precipitates like GP zones are very effective in electron scattering and further decreasing electrical conductivity [4]. This implies that solute atoms may segregate to grain boundaries what has been already observed in UFG aluminum alloys [17–19]. HAGBs are known to be preferential sites for solute atoms segregation and nucleation of precipitates [15,44]. The presence of PFZs (Fig. 6(b)) and spherical precipitates located at HAGBs of ⟨001⟩ and ⟨111⟩ grains (Fig. 9) after artificial aging confirms the presence of grain boundary segregation during HE processing. Besides, atom probe tomography (APT) analysis performed on HPT processed 6101 Al alloy by X. Sauvage et al. [9] showed that fractions of Mg and Si that may segregate at grain boundaries during SPD processing are in the range of 0.01–0.04 and 0.04–0.15 at.%, respectively. The electrical conductivity measurements have shown that HE10 sample exhibits lower conductivity than HE4 and HE3 for all aging times (Fig. 2(b)). It could be explained by aforementioned grain boundary segregation. HE10 contains significantly lower number of fine ⟨111⟩ grains in the microstructure in comparison to HE4 (Figs. 3 and 10) and thus lower surface area of grain boundaries, specially HAGBs observed mainly between ⟨001⟩ and ⟨111⟩ grains. Moreover, the higher fraction of HAGBs, which attract solute atoms effectively [15,44], is observed for HE4 (Fig. 3(d)) what results in an enhanced grain boundary segregation as compared to HE10. The aging treatment at 180 °C results in a significant reduction of electrical resistivity of HE processed 6101 Al alloy (Table 3). It is caused by annihilation of dislocations, grain growth and decrease of solute content in the matrix due to the precipitation of Mg- and Si-rich phases. It should be noted that the latter one is the most effective in terms of lowering the electrical resistivity of material since the contribution of solute atoms and precipitates is significantly reduced from 5.51 to 2.87 and 1.78 nΩm for aged HE3 and HE4 samples, respectively. TEM observations of HE3 and HE4 after aging revealed the presence of fine spherical GP zones, needle-like β″ precipitates located in grain interiors together with spherical β′ or β phase precipitates located at grain boundaries (Fig. 6-9). Longer aging periods of 7 h of HE4 resulted in the higher number of precipitates, especially spherical particles located at grain boundaries, what explains the lower contribution of solute atoms to total resistivity calculated for this sample. The most frequent precipitates in aged HE4 sample are needle-like β″ (Mg5Si6 [37]) and spherical β′ (Mg1.8Si) or β (Mg2Si) phases with Mg:Si weight ratio of 0.72:1, 1.56:1 and 1.73:1, respectively. If we assume, to simplify the calculations, that for investigated precipitation condition Mg:Si weight ratio is 1:1, the difference between solute atoms contribution to resistivity of aged HE4 (1.78 nΩm) and as-HE sample (5.51 nΩm) results from Mg and Si depletion of the matrix of about 0.3 wt%. Similar values of solute atoms depletion due to precipitation in the range of 0–0.3 at.% were obtained by X. Sauvage et al. [9] during APT measurements of HPT processed 6101 Al alloy. It should be noted that the presence of precipitates in the material also increases its resistivity. B. Raeisinia et al. [45] have proposed a relationship between the resistivity contribution from precipitates and their spacing. However, it has been neglected in this discussion due to impossibility of precise measurements of TEM foils thickness and thus the density of precipitates in the microstructure.
behavior of hydroextruded Al-Mg-Si alloy has been already quantitatively analyzed and described in [24]. 4.2. Effect of Microstructure on Electrical Conductivity The observed microstructural changes during HE process and artificial aging of the 6101 Al alloy affect the electrical conductivity significantly. According to Matthisen's rule, the total resistivity of metal alloys is influenced by parent metal (ρAl), lattice defects such as vacancies (ρvac), dislocations (ρdisl) and grain boundaries (ρGB) and solute atoms (ρss) or precipitates (ρppt) and can be expressed as a sum of all contributions in the following form [4,38]:
ρtotal = ρAl + Nvac ∆ρvac + Ldisl ∆ρdisl + SGB ∆ρGB +
∑ Ci ∆ρi + ρppt i
where ρAl is the resistivity of pure aluminum with no lattice defects at room temperature (ρAl = 26.55 nΩm [39]). Nvac (at.%), Ldisl (m−2), SGB (m−1), Ci (wt%) are the concentration or density of vacancies, dislocations, grain boundaries and i-th solute atoms, respectively. Δρvac (Ωm/at.%), Δρdisl (Ωm3), ΔρGB (Ωm2) and Δρi (Ωm/wt%) are constants accounting for the contribution of unit concentration or density of vacancies, dislocations, grain boundaries and i-th solute atom, respectively. To determine the contribution of lattice defects to the resistivity of aluminum, the following values of constants were used in this study: Δρvac = 26 nΩm/at.% [38], Δρdisl = 2.7 × 10−25 Ωm3 and ΔρGB = 2.6 × 10−16 Ωm2 [40]. The vacancy concentration has not been investigated in this work. However, it generally does not exceed 0.001% for SPD processed materials [41]. Precipitation phenomena confirmed by TEM observations are related with a bulk diffusion with the most likely vacancy-solute mechanism. Thus, the concentration of vacancies may be reduced with aging time. Then, it can be concluded that vacancies contribution to the resistivity would not be significant. It can be neglected since it will not exceed 2.6 × 10−2 nΩm in the case of no vacancy depletion by aging. The contributions of dislocations and grain boundaries were estimated based on experimentally measured values of dislocation density and mean equivalent grain diameter d2 (Table 2) which is related to the surface area of grain boundaries per unit volume SGB in the following way: SGB = 2/d2 [42]. The values of electrical conductivity were recalculated to total resistivity using the copper conductivity value of 58.0 MS/m. The influence of solute atoms and precipitates was calculated as a difference between total resistivity and contributions of pure aluminum and other microstructural features. All estimated contributions of different microstructural features are summarized in Table 3. The high density of dislocations and grain boundaries generated by HE processing increases the resistivity of as-HE samples of the 6101 Al alloy in comparison to its conventional counterpart (Table 3). However, their contribution to total resistivity is still significantly smaller than the influence of solute atoms in the matrix. The resistivity of dislocations and grain boundaries does not exceed 1% and 6–7% of the electrical resistivity of pure aluminum, respectively. The electrical conductivity is extremely sensitive to the concentration of Mg and Si atoms Table 3 Estimation of contribution of different microstructural features to total electrical resistivity ρtotal of the 6101 Al alloy under different processing conditions (ρAl – resistivity of pure aluminum; ρdisl – dislocation contribution; ρGB – grain boundaries contribution; ρss – solute atoms contribution; ρppt – precipitates contribution). State
ρtotal (nΩm)
ρAl (nΩm)
ρdisl (nΩm)
ρGB (nΩm)
ρss + ρppt (nΩm)
T1 (as-received) HE4 HE4 + 180 °C/7 h HE3 HE3 + 180 °C/2 h
33.73 33.85 29.71 34.01 30.86
26.55
0.02 0.14 0.11 0.18 0.10
< 0.01 1.65 1.27 1.77 1.34
7.16 5.51 1.78 5.51 2.87
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in the matrix seems to be the most important. The high conductivity is provided mainly by decomposition of solid solution due to precipitation of needle-like β″ precipitates in the grain interior and spherical β′ or β particles located at grain boundaries. Moreover, the presence of pronounced grain boundary segregation has been suggested for hydrostatically extruded Al-Mg-Si alloy. - It is demonstrated that the best combination of high strength (332 MPa) and high electrical conductivity (58.04% IACS) can be obtained by the three stage hydrostatic extrusion with a total true strain of 3.2 followed by artificial aging at 180 °C for 7 h. Acknowledgements This work was supported by The National Centre for Research and Development within the ERA.NET-RUS program (contract no. NCBR/ ERA NET RUS/02/2012). References Fig. 11. Ultimate tensile strength (UTS) and electrical conductivity of the 6101 Al alloy in the state HE4 + 180 °C/7 h in comparison to conventional Al-Mg-Si alloys (AL2-AL7) and 6101 Al alloy processed by SPD methods.
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4.3. Comparison of HE and SPD Methods Fig. 11 presents the comparison of UTS and electrical conductivity of the 6101 Al alloy processed by HE with SPD methods [7,10] and conventional Al-Mg-Si alloys used in the electrical engineering [5]. It is clear that UFG 6101 Al alloy processed by various SPD methods exhibits enhanced strength and conductivity when compared to its CG counterparts. Among heavily deformed materials, the HE4 sample subjected to artificial aging (AA) at 180 °C for 7 h possess optimized properties, i.e. the highest electrical conductivity of 58.04% IACS and UTS of 332 MPa. The higher strength of 364 MPa is achieved for the 6101 Al alloy processed by continuous ECAP-C combined with aging at 170 °C for 12 h and final drawing to a diameter of 3.2 mm but it comes at the expense of electrical conductivity reduced to 56.4% IACS [10]. It could be stated that hydrostatic extrusion combined with aging treatment provides significant improvement of mechanical and electrical properties which is comparable to various SPD methods. However, it possess the technological advantage of the easier processing of long wires. 5. Conclusions It has been shown that the hydrostatic extrusion combined with an artificial aging is an effective method for producing of long wires of ultrafine-grained Al-Mg-Si alloy with enhanced strength and electrical conductivity. The performed investigation allows to conclude: - The hydrostatic extrusion of the 6101 Al alloy results in the formation of ultrafine-grained microstructure with grain size of about 300 nm and fine GP zones precipitated in grains with ⟨001⟩ parallel to extrusion direction. - Mechanical strength of Al-Mg-Si alloy processed by hydrostatic extrusion can be further increased by artificial aging. Higher applied strains during HE processing accelerates the precipitation kinetics but decreases the age hardening response of the 6101 Al alloy. The mechanical behavior depend on crystallographic texture of hydrostatically extruded Al-Mg-Si alloy. The grains with ⟨001⟩ parallel to extrusion direction with homogenously distributed needle-like β″ precipitates provide mainly precipitation strengthening of material while grains with ⟨111⟩ parallel to extrusion direction results in more efficient grain boundary strengthening. - Electrical conductivity of the 6101 Al alloy processed by hydrostatic extrusion is sensitive to microstructural defects such as dislocations and grain boundaries but the resistivity contribution of solute atoms 113
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