Journal of Materials Science & Technology  2019 , 35 (6): 962-971 https://doi.org/10.1016/j.jmst.2018.12.023

Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing

Li Liua, Jian-Tang Jiangab*, Bo Zhanga, Wen-Zhu Shaoab, Liang Zhena*

a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
bNational Key Laboratory of Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China

Corresponding authors:   * Corresponding authors at: School of Materials Science and Engineering, HarbinInstitute of Technology, Harbin 150001, China. E-mail addresses: jjtcy@hit.edu.cn (J.-T. Jiang), lzhen@hit.edu.cn (L. Zhen).* Corresponding authors at: School of Materials Science and Engineering, HarbinInstitute of Technology, Harbin 150001, China. E-mail addresses: jjtcy@hit.edu.cn (J.-T. Jiang), lzhen@hit.edu.cn (L. Zhen).

Received: 2018-10-13

Revised:  2018-11-29

Accepted:  2018-12-10

Online:  2019-06-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Developing heat-resistant conductors with high strength and high electrical conductivity is a key issue in the electrical conductor industries, as the ever-increasing power transmission poses higher requirement on the thermal stability of electrical conductor wires. Dilute Al-Sc-Zr alloys are considered as promising candidates due to the excellent heat resistance and high electrical conductivity, but the low strength always limits their application on electrical wires. Yet, few efforts on process design have been made in dilute Al-Sc-Zr alloys to enhance the strength. Here, various kinds of processing paths via combination of cold drawing, ageing and/or annealing were conducted to improve the strength and electrical conductivity of a dilute Al-Sc-Zr alloy. Results show that enhanced strength and electrical conductivity were obtained after cold drawing + ageing or pre-ageing + cold drawing + annealing treatments processes. Optimal properties (194 MPa in ultimate tensile strength and 61% IACS in electrical conductivity) were obtained through cold drawing followed by ageing. Microstructure evolution which affects strength and electrical conductivity was systematically investigated using TEM and 3DAP. The enhanced strength was mainly attributed to the suitable interactions between strain strengthening and precipitation strengthening. The enhancement in electrical conductivity was caused by precipitation of solute atoms and recovery of defects. These results provide foundations for the processing design of Al-Sc-Zr conducting wires with good properties and push forward their potential application in heat resistant conductor industries.

Keywords: Al-Sc-Zr alloy ; Cold drawing ; Ageing ; Mechanical properties ; Electrical conductivity

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Li Liu, Jian-Tang Jiang, Bo Zhang, Wen-Zhu Shao, Liang Zhen. Enhancement of strength and electrical conductivity for a dilute Al-Sc-Zr alloy via heat treatments and cold drawing[J]. Journal of Materials Science & Technology, 2019, 35(6): 962-971 https://doi.org/10.1016/j.jmst.2018.12.023

1. Introduction

Aluminum wires are widely applied in overhead power lines due to their low weight and excellent corrosion resistance [[1], [2], [3]]. The ever-increasing demand in electric power transmission in recent years requires that conducting wires carry high current, which could inevitably lead to an obvious temperature rise in wires. This temperature rise may result in a significant decrease in the mechanical properties of Al wires and thereby reduce the reliability of grids. The hardness of Al-Mg-Si alloy which is the most widely used Al conductor in overhead power lines decreases by around 74.6% after exposing at 230 °C for 3 h [2]. Because of the poor thermal-resistance property, the service temperature of Al-Mg-Si alloys is limited to 90 °C in long-time operation [4], blocking the feasibility of increasing the current-carrying capacity of conducting wires. The degradation in mechanical properties of Al-Mg-Si alloys is mainly attributed to the coarsening and dissolution of strengthening precipitates (β' or β phase) at elevated temperatures [[5], [6], [7]]. Therefore, it is urgent to develop Al alloys with thermal-stable precipitates to meet the requirement of increased transmission power capability in conducting wires.

The addition of Zr into Al-based alloys has been proved to be effective to improve the thermal stability of Al alloys [[8], [9], [10]]. The Al3Zr particles are very stable upon heating due to the low diffusivity of Zr atoms in α-Al matrix and low interface energy between Al3Zr and matrix [[11], [12], [13]]. No obvious decrease in microhardness is shown in Al-0.25 wt.% Zr alloy even after exposing at 400 °C for 150 h [14]. However, the low strength of Al-Zr alloys prevents its step of replacing Al-Mg-Si alloys to meet the requirement of conducting wires served at elevated temperatures. It is known that Sc can significantly increase the strength of Al alloys through nanoscale coherent Al3Sc precipitates [[15], [16], [17], [18]]. What’s more, the co-addition of Sc and Zr can result in higher mechanical properties to compare with the single addition of Sc or Zr [[17], [18], [19], [20]]. An increment of around 150 MPa is noted in the microhardness of Al-0.1Sc-0.1 Zr (at.%) alloy when Zr and Sc are jointly added [19]. In addition, Al-Sc-Zr alloys also have excellent heat resistant. An aged Al-0.35Sc-0.2 Zr (wt.%) alloy shows sluggish degradation of strength, even after exposing at 400 °C for 100 h [21]. The good thermal stability of Al-Sc-Zr alloys is mainly attributed to the formation of thermal-resistant Al3(Sc,Zr) particles which have slow coarsening kinetics and can strongly pin the grain boundaries and dislocations [20,22].

Deformed structure can affect the precipitation and growth kinetics of precipitates in Al-Sc-Zr alloys. It was reported that deformation accelerated Sc precipitation, resulting in an increased strength in Al-Sc-Zr-based alloys [23,24]. A similar behavior of Sc segregation at dislocations was also detected in a hot extruded Al-Mn-Sc-Zr alloy [25]. Moreover, the Sc/Zr-containing particles could affect the dislocation recovery and recrystallization. For example, the Al3(Sc,Zr) precipitates were found to hinder the dislocation movement and improve recrystallization resistance of Sc/Zr-containing alloys [26,27]. In addition, the Al3(Sc,Zr) precipitates acting as obstacles to the movement of grain boundaries effectively refined the grain size in Al-Sc-Zr-based alloys [26,28,29].

Most researches showing good properties in Al-Sc-Zr alloys mainly focus on alloys containing high concentration of Sc [21,30,31], which greatly increases the cost of conductors as Sc is extremely expensive. Few studies have been conducted on dilute Al-Sc-Zr alloys to balance between the cost and properties. As the precipitation strengthening effect from dilute alloys is limited, strengthening methods, such as fine grain strengthening and dislocation strengthening, are thus desired to further improve the mechanical properties of dilute Al-Sc-Zr alloys. The optimization of strength and electrical conductivity is usually mutually exclusive for metals [32], especially when defects (dislocations and vacancies, etc.) are introduced. Cold deformation which brings obvious dislocation strengthening can increase the mechanical properties of Al-Sc-Zr alloys, while the dislocations introduced during cold deformation intensify the electron scattering and consequently lead to a decreased electrical conductivity [21,22]. Therefore, it is important to optimize the process of dilute Al-Sc-Zr alloys to enhance both strength and electrical conductivity.

In the present work, an extruded Al-Sc-Zr alloy with a trace amount of Sc and Zr was prepared. Various processing paths combined cold drawing and heat treatment have been developed to optimize the strength and electrical conductivity of a dilute Al-Sc-Zr alloy cooperatively and to excavate its potential for heat-resistant conducting wires. The evolution of mechanical properties and electrical conductivity under different processing paths (ageing, cold-drawing (CD) and their combination) were systematically studied. A special emphasis was given on the combined effect of CD and ageing on the mechanical properties and electrical conductivity of a dilute Al-Sc-Zr alloy. The dilute Al-Sc-Zr alloy studied exhibits obvious advantages over the Al-Sc-Zr conductors reported so far for the reduced cost and excellent comprehensive properties. This work provides an alternative way for developing heat-resistant Al conductors with enhanced mechanical properties and electrical conductivity.

2. Experimental

A dilute Al-Sc-Zr alloy was prepared by melting highly pure Al (99.999 wt.%), Al-2.24Sc (wt.%) and Al-10.66 Zr (wt.%) master alloys. The melt was poured into a preheated iron mold after stirring for 1 h at 720 °C and cooling naturally to room temperature. Then the cast alloy was extruded into a plate with 6 mm (extrusion ratio $\widetilde{1}$9) in thickness at 400 °C. The chemical compositions measured by inductively coupled plasma (ICP) analysis are 0.06 Sc, 0.23 Zr and Al for balance (wt.%). Minor impurities of Fe and Si (< 0.001 wt.%) were detected in this alloy, which were not considered in this research. The Al-0.6Sc-0.35 Zr (all compositions involved in this work are in wt.% unless specially stated) samples were solution treated at 640 °C for 24 h, followed by water quenching into room temperature immediately. After solution treatment, both one-stage ageing at 300 °C from 20 min to 50 h and two-stage ageing (300 °C/25 h + 400 °C/x h) were conducted.

Cylinder samples with diameter of 5 mm and length of 70 mm were cut from the center of the extruded plate by electrical discharge machining. These samples were solution treated at 640 °C for 24 h then quenched into room temperature oil. Two processing paths were designed for the as-quenched samples: (1) cold drawing + two-stage ageing; (2) two-stage ageing + cold drawing; they were designed as CD-A and A-CD, respectively. These round bars were cold drawn into wires with a final diameter of 1.5 mm through eight passes, accompanied with about 90% cross-sectional area reduction. Annealing treatments were conducted to optimize the combination of strength and electrical conductivity for the A-CD processed samples. Based on the previous study [33], annealing temperature has more pronounced effect on the optimization of microstructure and properties in comparison with the annealing time. Therefore, specific attention was paid to the effect from annealing temperature in the current study. Four temperatures (150, 250, 350 and 400 °C) were conducted on the A-CD processed samples in a furnace with air atmosphere. Regarding the choice of annealing time, through prolonging annealing time, dislocation recovery and vacancy annihilation can be facilitated to enhance the electrical conductivity of the Al-Sc-Zr alloy. However, long time of annealing is prone to give rise to precipitate coarsening for Sc/Zr-containing precipitates especially when high temperature was applied [9,21], which can induce a significant decrement in strength. Therefore, to minimize the effect of defects on electron scattering as well as avoid precipitate coarsening, 5 h is chosen as the annealing time in this study.

Vickers hardness (HV) was measured using a load of 100 g and a dwell time of 15 s. Each data presented was an average of at least 15 measurements on polished specimens. Tensile samples, which are 60 mm in full length, 18 mm in gauge length and 1.5 mm × 6 mm in cross-section, were cut from the extruded plate aligning with the extrusion direction by electro-discharge machine. Tensile tests were conducted on the Instron-5965 electron testing machine with a strain rate of about 2 × 10-3 s-1. Electrical conductivity was measured by the Keithley-2420 source meter with a four-point collinear probe method. Each value was obtained from measurements of five different specimens with 40 mm in length and 1 mm × 1 mm in cross-section. The measured conductivity was expressed in %IACS unit (International annealed copper standard, 100% IACS = 58.0 MS/m). All the samples were thoroughly ground up to 2000-grit SiC papers and polished to a mirror-like surface before microhardness measurements were performed. All the cold drawn wires were ground carefully with grinding papers up to 2000-grit before each test to remove the surface layer where oxidation or pollution may occur during preparation procedure (electrical discharge machining and heat treatments).

The specimens for metallographic observation were mechanically ground followed by electro-polishing with 10% perchloric acid in ethanol, then anodic coating was conducted in a mixture solution of 38% sulfuric acid, 43% phosphoric acid and 19% distilled water. Polarized light on the Zeiss HAL-100 microscope was used to observe grain structure. TEM specimens were prepared by jet electron-polishing with 30 vol.% nitric acid in methanol at -30 °C after mechanically thinning and punching. Copper rings (3 mm in diameter) were used to assist the electro-polishing process of longitudinal samples on the cold drawn wires. TEM observations were conducted on the FEI Talos F200x scanning transmission electron microscope (S/TEM) operating at an acceleration voltage of 200 kV. For 3DAP analysis, thin bars (0.5 × 0.5 × 20 mm3) were cut from the aged samples, electron-polished by a two-step process, using an electrolyte of 5% perchloric acid in ethanol. The 3DAP tests were performed on LEAP 3000Si instrument, at a specimen temperature of 25 K. The voltage pulse repetition rate was 200 kHz with 20% pulse voltage fraction and the detection rate was 37%.

3. Results

3.1. Microstructure of hot extruded alloy

Fig. 1 displays the optical photographs of the hot extruded Al-0.06Sc-0.23 Zr alloy on three planes. Some fine recrystallized grains are observed on the ED-TD plane, as shown in Fig. 1(a). The TD-ND and ED-ND planes are mainly composed of elongated grains (Fig. 1(b) and (c)). The TEM micrograph in Fig. 2(a) demonstrates fine equiaxed grains and low density of dislocations in the hot extruded Al-0.06Sc-0.23 Zr alloy, which indicates that recrystallization occurs during hot extrusion. Some secondary Al3(Sc,Zr) particles can be observed in Fig. 2(b).

Fig. 1.   Metallographic photographs of the hot extruded Al-0.06Sc-0.23 Zr alloy: (a) ED-TD plane, (b) TD-ND plane, (c) ED-ND plane. ED, TD and ND stand for extrusion direction, traverse direction and normal direction, respectively.

Fig. 2.   Bright-field TEM micrographs of the hot extruded Al-0.06Sc-0.23 Zr alloy: (a) fine recrystallized grains, (b) Al3(Sc,Zr) precipitates inside grain as indicated by red arrows.

3.2. Ageing treatments

The precipitation behavior of Al-Sc-Zr alloys is diffusion controlled and Sc atoms have larger diffusion coefficient and lower initial precipitation temperature than that of Zr atoms in Al matrix [34]. Therefore, stepped ageing treatments (one-stage and two-stage) were selected in this study to optimize the ageing process. Vickers microhardness and electrical conductivity which are plotted as a function of ageing time are presented in Fig. 3. The hardness increases dramatically from 25 to 60 HV0.1 within the first 10 h when ageing at 300 °C (Fig. 3(a)). The hardness increases slowly during the 10-50 h span. The electrical conductivity increases from about 51 to 55% IACS in the first 10 h and keeps increasing with a decreased rate from 10 h to 50 h before reaching at the peak level of 56% IACS (Fig. 3(b)).

Fig. 3.   Microhardness and electrical conductivity evolution of Al-0.06Sc-0.23 Zr as a function of ageing time under different ageing treatments: (a), (c) one-stage ageing at 300 °C, (b), (d) two-stage ageing (first ageing at 300 °C for 25 h, then ageing at 400 °C for different times), the data for the first ageing stage is marked with red circle in (a) and (c).

Ageing at 300 °C for 25 h is chosen as the first stage of the two-stage ageing as it obtains high properties. Fig. 3(c) shows the hardening response of Al-0.06Sc-0.23 Zr over the secondary stage ageing at 400 °C. During the secondary stage ageing, the hardness experiences a slight decrease (about 3 HV0.1) after 2 h, then heads back and increases continually as ageing proceeds before reaching at a peak value of around 66 HV0.1 at 50 h which is 6 HV0.1 higher than the peak hardness after ageing at 300 °C. Over-ageing induced softening doesn’t appear before the ageing time is prolonged to 100 h, suggesting that the Al-0.06Sc-0.23 Zr alloy possesses high thermal stability at temperature up to 400 °C. The electrical conductivity increases quickly after a small drop in the first 2 h, as shown in Fig. 3(d). A peak conductivity of 61% IACS, which is around 5% IACS higher than that achieved after ageing at 300 °C, is obtained after a secondary ageing at 400 °C for 50 h.

Fig. 4 shows the strength variation of Al-0.06Sc-0.23 Zr during the secondary stage ageing. After ageing at 300 °C for 25 h, the yield strength (YS) and ultimate tensile strength (UTS) are around 110 and 135 MPa, respectively. The secondary stage ageing brings an increment of about 20 MPa in YS and 25 MPa in UTS within the first 10 h, indicating a notable secondary ageing hardening in Al-0.06Sc-0.23 Zr. The UTS maintains above 150 MPa and the YS stays around 130 MPa in spite of small fluctuations as the ageing time is prolonged. Enhanced strength and high electrical conductivity are obtained in Al-0.06Sc-0.23 Zr when the two-stage ageing (300 °C/25 h + 400 °C/50 h) was applied. Thus, this optimized two-stage ageing process is applied on the following cold drawn wires.

Fig. 4.   The mechanical properties of Al-0.06Sc-0.23 Zr at different ageing times during the secondary stage ageing at 400 °C after ageing 25 h at 300 °C.

3.3. Cold drawing

Cold drawing (CD) is a common procedure for conducting wires. Both CD and ageing can enhance the mechanical properties. However, CD and ageing affect the electrical conductivity conversely when combined into a manufacturing process. Setting up CD and ageing treatments properly to tailor and balance between mechanical properties and conductivity is then critical for designing manufacturing process. Two paths (ageing before or after CD, namely A-CD or CD-A) were carried out on the Al-0.06Sc-0.23 Zr alloy, aiming to achieve high strength and high electrical conductivity.

Table 1 shows the mechanical properties and electrical conductivity of samples manufactured via each processing path. CD leads to a significant increase in UTS by around 91 MPa but slightly decrease the electrical conductivity of the as-quenched sample, as shown in Table 1. A post-CD ageing (300 °C/25 h + 400 °C/50 h) further increases the strength of Al-0.06Sc-0.23 Zr (by around 35 MPa in YS and 59 MPa in UTS) and simultaneously leads to an increment of 9.3% IACS in electrical conductivity. The YS and UTS of CD-A processed sample reach at 168 and 194 MPa, respectively, which are much higher than those of the peak aged but non-drawn sample. A high electrical conductivity of 61.4% IACS is obtained via the CD-A path.

Table 1   The strengths and electrical conductivity of Al-0.06Sc-0.23 Zr under different processes.

TreatmentsYield strength (MPa)Ultimate tensile strength (MPa)Electrical conductivity (% IACS)
As-quench26.143.951.5
CD130.7135.250.3
Peak ageing125.5154.261.3
CD-A167.6194.361.4
A-CD184.2199.457.6

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High strengths (184 MPa in YS and 199 MPa in UTS) are obtained via A-CD path whereas the electrical conductivity drops apparently from 61.3% IACS to 57.6% IACS, as seen in Table 1. The precipitation of Al3(Sc,Zr) particles after the two-stage ageing (300 °C/25 h + 400 °C/50 h) results in a low supersaturation of Sc and Zr atoms in the solute (Sc and Zr) depleted matrix. In addition, the precipitation of Sc/Zr-containing particles commences at high temperature (around 250 °C for Sc and 400 °C for Zr) due to their low diffusivity [19,20]. The change in the concentration of solute atoms during cold drawing should be negligible. Therefore, the decrement in conductivity of the aged samples could be mainly ascribed to the dislocations and vacancies produced during CD process. Thus, annealing is necessary to improve the electrical conductivity of samples under A-CD path.

3.4. Annealing treatments

Post-annealing was carried out at four temperatures for 5 h on A-CD processed samples to explore the feasibility of achieving desired properties for the Al-0.06Sc-0.23 Zr wires. The mechanical properties and electrical conductivity as a function of annealing temperatures are presented in Fig. 5. The strength of samples processed via A-CD path decreases slightly with the increasing of annealing temperature when the temperature is below 350 °C (Fig. 5(a)). Annealing for 5 h at 350 °C leads to a decrement of around 24 and 10 MPa in YS and UTS, respectively. While the decrement in strength suspends and a slight increment is observed in YS and UTS when the annealing temperature is further elevated from 350 °C to 400 °C. UTS of 192 MPa is achieved after annealing at 400 °C for 5 h, which is comparable to that of Al-Mg-Si alloys [2,6]. On the other hand, the electrical conductivity increases as the annealing temperature increases, as it shown in Fig. 5(b). After annealing at 150 °C for 5 h, the electrical conductivity experiences a rapid increase and remains at around 59.0% IACS before temperature increases to 350 °C A high conductivity of about 59.7% IACS is obtained after annealing at 400 °C for 5 h.

Fig. 5.   Mechanical strengths and electrical conductivity of Al-0.06Sc-0.23 Zr (with A-CD path) annealed at different temperatures for 5 h.

3.5. Microstructure evolution

Fig. 6 shows the microstructure of Al-0.06Sc-0.23 Zr after one-stage and two-stage ageing. A large number of nano-sized precipitates with diameters less than 5 nm uniformly distribute throughout the matrix after ageing at 300 °C for 25 h (Fig. 6(a)). After a secondary ageing at 400 °C for 50 h, the size of precipitates obviously increases (Fig. 6(c)), resulting in an increased volume fraction of precipitates. Fig. 6(b) and (d) display the selected-area-electron-diffraction (SAED) patterns taken along [011] orientation of Al matrix. The characteristics of superlattice diffraction can be clearly seen in Fig. 6(d), indicating the presence of precipitates with L12 structure (Al3Sc or Al3(Sc,Zr)). The diffraction characteristic of precipitates after two-stage ageing is more obvious than that after one-stage ageing. To better illustrate the structure of Al3(Sc,Zr) precipitates, the HRTEM image is shown in Fig. 6(e). It is obvious that the precipitate is well coherent with the Al matrix. The lattice fringes with interplanar spacing of 0.408 nm which is almost twice the interplanar spacing of {002}Al correspond to the {001} planes of the Al3(Sc,Zr) precipitate. The diffraction spots of the Al3(Sc,Zr) precipitate have been indexed in the corresponding FFT image in Fig. 6(f).

Fig. 6.   TEM micrographs of Al-0.06Sc-0.23 Zr: (a) under one-stage ageing at 300 °C for 25 h, (c) under two-stage ageing at 300 °C for 25 h, then at 400 °C for 50 h, (b) and (d) the SAED patterns taken at [011] direction of Al matrix for (a) and (c), respectively; (e) HRTEM of Al3(Sc,Zr) precipitate taken at [011] direction of Al matrix after two-stage ageing, (f) its corresponding FFT image.

To provide additional insights into the composition of precipitates, 3DAP experiments of the aged samples were conducted. Fig. 7 displays the results of sample after one-stage ageing at 300 °C for 25 h. Sc-containing particles can be clearly seen in the matrix while Zr atoms are uniformly distributed (Fig. 7(a) and (b)). An iso-concentration interface with 7 at.% Sc atoms is chosen to designate the precipitates and the concentration distribution from matrix to precipitates is shown in Fig. 7(c). The concentration of Sc atoms increases progressively as deepening from the matrix into the precipitates, fluctuating between 20 and 30 at.% inside the precipitates, as is evident in Fig. 7(c). Conversely, the Al concentration decreases gradually and keeps at a level of 70$\widetilde{8}$0 at.% within the precipitates. The compositions inside the precipitates are around 25 at.% Sc and 75 at.% Al, corresponding well to the Al3Sc stoichiometry. That is to say, only Sc atoms precipitate when ageing at 300 °C, forming Al3Sc precipitates in the matrix. The results of sample after the two-staged ageing (300 °C for 25 h, then at 400 °C for 50 h) are demonstrated in Fig. 8. Segregations of Sc and Zr atoms are observed in the matrix (Fig. 8(a) and (b)). Obviously, the segregated Sc atoms are enveloped by Zr atoms, as shown in Fig. 8(c). In Fig. 8(d), the concentration of Sc shows an increased trend as it approaches into the precipitates, with a maximum value of about 25 at.% in the center, which is similar to the Sc distribution after one-stage ageing (Fig. 7(c)). While the Zr enrichment with a maximum concentration of about 20 at.% is visible close to the edge of precipitates, forming a Zr-rich shell around precipitate. Therefore, Zr atoms segregate at the outside of the Sc-containing precipitates formed during first-stage ageing, forming Al3(Sc, Zr) precipitates with core-shell structure during the two-stage ageing.

Fig. 7.   (a) and (b) three-dimensional atom reconstruction of Al-0.06Sc-0.23 Zr after ageing at 300 °C for 25 h, displaying (a) Sc atoms, (b) Zr atoms, (c) proxigram of Al, Sc and Zr concentration in precipitates.

Fig. 8.   Three-dimensional atom reconstruction of Al-0.06Sc-0.23 Zr under two-stage ageing (300 °C/25 h + 400 °C/50 h): (a) Sc atoms are displayed in red, (b) Zr atoms are in blue, (c) both Sc and Zr atoms are displayed and Al atoms are omitted for clarity, (d) proxigram of Al, Sc and Zr concentration in Al3(Sc,Zr) precipitates.

Fig. 9 shows TEM micrographs of CD and CD-A processed samples. Elongated grains along drawing direction are obviously seen in the CD processed sample (Fig. 9(a) and (b)). Dislocation cells composed of tangled dislocations unevenly distribute within the elongated grains. Only the diffraction spots of Al matrix can be recognized in the SAED pattern after CD processing, as shown in Fig. 9(c). After the post-CD ageing treatment, recrystallization occurs and fine equiaxed grains with a diameter of around 1 μm can be seen in Fig. 9(d). The SAED pattern in Fig. 9(f) indicates that existence of Al3(Sc,Zr) precipitates. These fine precipitates impede the migration of grain boundaries and dislocation movement, as seen in the arrowed regions in Fig. 9(e).

Fig. 9.   Bright-field TEM micrographs of Al-0.06Sc-0.23 Zr under different processing paths: (a), (b) CD, (d), (e) CD-A path, recrystallized grains are marked with blue dashed lines in (d) and precipitates on dislocations are indicated by blue arrows in (e), (c), (f) the SAED patterns taken from [011] of Al matrix for CD and CD-A processed samples, respectively.

Fig. 10 shows the microstructure of A-CD processed samples before and after annealing at different temperatures. Cold drawing results in lamellar structure in which grains are 0.5-2 μm in width and tens of microns in length (Fig. 10(a)). A large number of dislocations and Al3(Sc,Zr) precipitates can be seen in the magnified image in Fig. 10(d). After annealing at 250 °C, elongated grains distribute along the drawing direction and no evidences of recrystallization are observed, as shown in Fig. 10(b). Dislocation rearrangement however can be observed in Fig. 10(e), indicating that dislocation recovery occurs. After annealing at 400 °C, fine recrystallized grains are observed between the elongated lamellar grains, as marked by blue dashed lines in Fig. 10(c). On the other hand, the dislocation density decreases and dislocation walls can be seen. The pining effect of Al3(Sc,Zr) precipitates on dislocations or sub-grains can be observed in Fig. 10(f), which inhibit the dislocation recovery and recrystallization of the cold drawn samples. Similar phenomenon has been observed in a deformed Al-Mg-Sc-Zr alloy [26].

Fig. 10.   Bright-field TEM micrographs of the A-CD processed Al-0.06Sc-0.23 Zr alloy under different treatments: (a), (d) before annealing, the drawing direction is indicated by red arrows, (b), (e) anneal at 250 °C for 5 h, (c), (f) anneal at 400 °C for 5 h, some recrystallized grains are marked by blue dashed lines in (c) and the pinning effect of precipitates are indicated by red arrows in (f).

4. Discussion

4.1. Mechanical properties

Assuming the contributions from each strength mechanism are independent, the total strength of the Al-Sc-Zr alloy σtotal can be presented as [20]:

σtotal0gbsspredis (1)

where σ0 is the strength of pure Al, σgb is the strength due to grain boundaries, σss is the contribution from solid solution strengthening, σpre is the strength due to the Al3(Sc,Zr) precipitates, and σdis is the contribution of dislocation strengthening.

The mechanical strength of the Al-0.06Sc-0.23 Zr alloy is significantly improved after ageing due to σpre that originates from the pining effect of nano-sized precipitates formed in the matrix (Fig. 6). The precipitates formed at 300 °C (as seen in Fig. 7(a)) are Al3Sc precipitates which contribute to the quick hardening response of Al-0.06Sc-0.23 Zr. Zr atoms mainly contribute to the solid solution strengthening σss during ageing at 300 °C. The precipitation of Sc initiates at around 280 °C which is lower than that of Zr atoms (375 °C) [18,19]. After a secondary ageing at 400 °C, Zr atoms precipitate at the outer shell of Al3Sc particles, forming Al3(Sc,Zr) precipitates with core-shell structure (as seen in Fig. 8). These Al3(Sc,Zr) precipitates are coherent with the matrix and the precipitation of Zr on the out shell increase the volume fraction of precipitates, thus contributed to an enhanced precipitation strengthening (σpre) after the second-stage ageing at 400 °C.

Cold drawing can greatly affect the mechanical properties of alloys. The strength of the as-quench sample is mainly caused by solid solution strengthening σss from Sc and Zr solute atoms in α-Al matrix. High density of dislocations was introduced in grains after cold drawing (Fig. 9(a)). Thus, apart from the solid solution strengthening σss, the 91 MPa increment in strength of cold drawn sample is mainly attributed to the dislocation strengthening σdis. For CD-A processing path, most dislocations introduced by CD recovered after ageing (see Fig. 9(d) and (e)), thus the dislocation strengthening σdis is inconspicuous for the CD-A processed sample. The high strength of the CD-A processed sample is mainly attributed to the enhanced precipitation strengthening σpre from fine Al3(Sc,Zr) precipitates.

For the A-CD processed sample, Al3(Sc,Zr) precipitates definitely provide remarkable precipitate strengthening σpre. Meanwhile, the dislocation strengthening σdis caused by the post cold deformation can also make a great contribution to the strength. Therefore, the A-CD processed samples show high strength (Table 1). During annealing at a low temperature range (≤250 °C), dislocation recovery was not obvious and only a slight decrement in strength can be seen. Increasing the annealing temperature, dislocation recovery can reduce the mechanical properties, including yield strength and tensile strength. For the Al-Sc-Zr alloy, when increasing the temperature from 250 to 400 °C, though the dislocation recovery leads to a decreased strength, the formation of fine recrystallized grains (around 500 nm in diameter) at high annealing temperature can partly compensate for the drop in strength, thus only a slight decrease was observed in yield strength. The strengths of samples under the A-CD plus annealing path are comparable with the CD-A processed samples, and both are higher than those under the CD or ageing process.

4.2. Electrical conductivity

Electrical conductivity is determined by the electrons scattering caused by lattice distortion which is mainly caused by defects, impurities, solid solute atoms, etc [32]. According to the Matthiessen’s rule [35], the total electrical resistivity can ρtotal be addressed as:

ρtotal0gbsspredisvac (2)

where ρ0 is the lattice resistivity of Al matrix, ρgb is the resistivity due to the grain boundaries, ρss is the resistivity caused by solute atoms dissolved into matrix which are effective lattice defects scattering electrons for metals [32,36,37], ρpre ρpre is the resistivity attributed to precipitates. Al3Sc and Al3(Sc,Zr) precipitates are coherent with the matrix, which have negligible effect on the resistivity [18,21], so the ρpre will not be considered in the following discussion, ρdis stands for the resistivity caused by dislocations and ρvac represents the resistivity caused by vacancies.

The crystal lattice is highly distorted due to the supersaturated solid solution of Sc and Zr atoms in α-Al, that’s why the electrical conductivity of the as-quench sample is low (Fig. 3(b) and Table 1). The increment in electrical conductivity after ageing is mainly attributed to the continuously precipitation of Sc and/or Zr atoms. The concentration of solutes in α-Al decreases quickly during the early ageing stage and then precipitation slows down gradually for the decreased supersaturation of solute atoms as ageing proceeds. Precipitates continue to form during the later ageing stage, resulting in a continuously decrease in solute contents. That’s why the electrical conductivity keeps increasing after 10 h at 300 °C. Moreover, the secondary stage ageing at 400 °C leads to a continuous precipitation of Zr atoms, thus providing an additional increase in electrical conductivity.

High density of dislocations and numerous vacancies were introduced in the grain/subgrain interior during cold drawing [38,39]. The high density of dislocations and vacancies could significantly reduce the electrical conductivity of the Al-0.06Sc-0.23 Zr alloy as it leads to intensified electron scattering. For CD-A processed sample, on one hand, the precipitation of Sc and Zr atoms relieves the lattice distortion thus decreases the resistivity caused by solute atoms ρss. On the other hand, most of the dislocations recovered and fine recrystallized grains formed during the subsequent ageing (Fig. 9(d) and (e)), which weaken the influence of dislocations on the electrical conductivity (ρdis). Grain refinement down to sub-micro can increase the strength without sacrificing the electrical conductivity, which has been observed in Al alloys [3,6,40,41] and Cu alloys [[42], [43], [44], [45]], the effect of recrystallized grain boundaries on electrical conductivity is thus ignorable. The defects introduced by cold drawing during the CD-A path inevitably produce detrimental effect on the electrical conductivity, while the electrical conductivity of CD-A processed sample is still as high as the aged one. This phenomenon can be explained as a result of more solute atoms precipitating out in the CD-A processed sample in comparison with the aged samples. It implies that the precipitation of Sc and Zr can be effectively promoted by pre-cold drawing. A similar behavior has been observed in Al-Sc-Zr-based alloys when cold rolling was applied [23,24]. Dislocations and vacancies lead to lattice distortions at the terminal stage of the A-CD path, resulting in a drop in electrical conductivity of A-CD processed sample. Annealing is thus necessary to improve the electrical conductivity. As reported in previous studies [38,46,47], the vacancy annihilation was benefited to improve the electrical conductivity. After annealing in a low temperature range (150 °C $\widetilde{2}$50 °C), numerous vacancies in the cold-draw Al-Sc-Zr alloy are eliminated, contributing to a remarkable increment in the electrical conductivity from around 57.4 to 59.0% IACS. When increasing the annealing temperature to 400 °C, a large number of dislocations recover and recrystallized grains gradually form (Fig. 10c and f), which increase the electrical conductivity by reducing the lattice distortion in Al matrix. A dramatic increase of electrical conductivity thus can be seen during annealing (Fig. 5b).

4.3. Property comparisons under different processes

The electrical conductivity vs. corresponding ultimate tensile strength of Al-0.06Sc-0.23 Zr under different processes is plotted in Fig. 11. Ageing can improve both the strength and electrical conductivity of the Al-Sc-Zr alloy, however, the contribution of ageing to strength is insufficient for dilute Al-Sc-Zr alloys. Cold drawing can also enhance strength, while it leads to a decrease in electrical conductivity reversely. Proper process via a combination of cold drawing and ageing can achieve good properties with high strength and high electrical conductivity for the dilute Al-Sc-Zr alloy.

Fig. 11.   Electrical conductivity of Al-0.06Sc-0.23 Zr under various processes is plotted vs. their corresponding ultimate tensile strengths.

CD-A path yields the best comprehensive properties with high strength and high electrical conductivity for the dilute Al-0.06Sc-0.23 Zr alloy among all these processes due to a good coordination of advantages from cold deformation, precipitation and defect elimination. The high strength of CD-A processed sample is mainly attributed to the precipitation strengthening and grain boundary strengthening, the high electrical conductivity is mainly caused by precipitation of solute atoms and recovery of defects. Additionally, the CD-A path endues with good thermal stability for samples because high strength still maintains even though a heat treatment at 400 °C for 50 h is applied on the samples. The good thermal stability of Al3(Sc,Zr) precipitates and their hinder effect on the grain growth are main reasons for the strength and microstructure stability [19,22,33].

The A-CD path, the last step of which is cold deformation, endues the highest strength among all processes as the deformed microstructure is well preserved. The dislocation strengthening and precipitation strengthening are dominant in this condition. while the electrical conductivity is degraded due to the introduced defects. The subsequent annealing process can compensate for the decreased electrical conductivity to some extent, but it is at a slight expense of strength. A-CD path combined with a post-annealing can provide excellent properties and exert the potential of the Al-0.06Sc-0.23 Zr alloy. This path, however, is still not as convenient and efficient as the CD-A one.

The processes combined cold drawing and heat treatments properly can provide enhancement in both strength and electrical conductivity of the dilute Al-0.06Sc-0.23 Zr alloy. Both paths, CD-A and A-CD, can obtain good comprehensive performances, which provide flexible options for the processing design for dilute Al-Sc-Zr alloys to potentially applied in Al conductors. In comparison, CD-A path can provide a thermal stable Al-0.06Sc-0.23 Zr wire with the best comprehensive properties: 194 MPa in UTS and 61% IACS in electrical conductivity, which are obviously higher than those in IEC standard 32004-2007 (162 MPa in tensile strength and 59.9% IACS in electrical conductivity). Moreover, compared with the reported Al-Sc-Zr alloys, the Sc composition in this studied alloy is only 17% of that in Al-0.35Sc-0.2 Zr [21] and 30% of that in Al-0.2Sc-0.04Zr [22], whereas similar electrical conductivity is achieved and the strength is only reduced by 8% and 9%, respectively. The low concentration of Sc can greatly reduce the cost.

5. Conclusions

Different processing paths (ageing, CD-A and A-CD plus annealing) were designed to enhance the strength and electrical conductivity of a dilute Al-0.06Sc-0.23 Zr alloy in this work. The following conclusions were drawn:

(1) Two-stage ageing (300 °C/ 25 h + 400 °C/ 50 h) yields higher strength and electrical conductivity for the hot extruded Al-0.06Sc-0.23 Zr alloy since the stepped ageing can achieve more precipitation of alloying elements and form core-shell precipitates in the matrix.

(2)A-CD path (ageing followed by cold drawing) endues high strength ($\widetilde{2}$00 MPa) for the dilute Al-0.06Sc-0.23 Zr alloy due to the significant strain strengthening and precipitation strengthening. And the accompanied drop in electrical conductivity can be improved by the subsequent annealing treatment. High electrical conductivity of 59.7% IACS can be obtained and the strength is still as high as 191 MPa after annealing at 400 °C for 5 h.

(3)The best combined properties of 194 MPa in tensile strength and 61% IACS in electrical conductivity can be obtained by CD-A path for the dilute Al-0.06Sc-0.23 Zr alloy. These excellent properties are mainly attributed to suitable interactions among cold deformation, precipitation behavior, recrystallization and recovery of defects.

Acknowledgments

This work was financially supported by Defense Industrial Technology Development Program and Natural Science Foundation of China (Grant No. 51474195 and No. U1737206). We would like to appreciate Prof. Wenqing Liu and Dr. Hui Li at Shanghai University for the help on 3DAP experiments.

The authors have declared that no competing interests exist.


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