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Effect of direct current pulses on mechanical and electrical properties of aged Cu–Cr–Zr alloys
Materials and Design 92 (2016) 135–142
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Effect of direct current pulses on mechanical and electrical properties of aged Cu–Cr–Zr alloys Wei Wang a, Rengeng Li a, Cunlei Zou b, Zongning Chen a, Wen Wen c, Tongmin Wang a,⁎, Guomao Yin a a Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China b Laboratory of Special Processing of Raw Materials, Dalian University of Technology, Dalian 116024, PR China c Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Pudong New Area, Shanghai 201204, PR China
a r t i c l e
i n f o
Article history: Received 9 September 2015 Received in revised form 17 November 2015 Accepted 6 December 2015 Available online 8 December 2015 Keywords: Direct current pulses Mechanical properties Electrical conductivity Precipitation Synchrotron X-ray radiation Cu–Cr–Zr alloys
a b s t r a c t In this work, the response of Cu–Cr–Zr alloys to aging under direct current (DC) pulses was investigated. The results demonstrated that well-balanced mechanical properties and electrical conductivity can be achieved in case the alloys were properly treated. The aging behavior of Cu–Cr–Zr alloys is sensitive to DC pulses in the temperature range between 400 and 450 °C, whereas at temperatures out of this range, DC pulses have minor influences. The hardness of aged Cu–Cr–Zr alloy is increased by DC pulses while the electrical conductivity is slightly decreased. The peak hardness is increased by 10 HV after aging at 400 °C for 1 h. With the imposition of DC pulses, the precipitate size of Cr rich precipitates in aged Cu–Cr–Zr alloys is reduced from 238 nm to 178 nm, exhibiting stronger Orowan strengthening. After aging at 450 °C for 2 h with DC pulses, the Cu–Cr–Zr alloy has an average tensile strength of 609 MPa, elongation of 12.8%, and electrical conductivity of 77.6% IACS. A retarding effect of DC pulses on precipitation process is confirmed by synchrotron X-ray diffraction analysis, which is responsible for the slight decrease in electrical conductivity. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Continuous efforts have been made to obtain materials that exhibit both excellent mechanical strength and high electrical conductivity for high performance applications, such as integrated circuit lead frames and high speed railway contact wire [1–3]. As an age-hardenable alloy, the excellent mechanical properties of Cu–Cr–Zr alloys are attributed to the precipitation and particle-dispersion strengthening mechanism [4,5], and the high electrical conductivity is sourced from the very low solubility of Cr and Zr atoms in Cu matrix at room temperature [6,7]. In the early stage of thermomechanical treatment, the dissolution of Cr and Zr atoms into Cu matrix and high density of defects introduced by cold working result in large lattice distortion and increase scattering of the electrons, leading to high resistivity. However, upon subsequent aging, the precipitation of solute atoms from Cu matrix reduces lattice distortion, and thus promotes electrical conductivity. Meanwhile, the dispersion of fine precipitates retards the recovery and recrystallization of grains as well as strengthens the alloy. In addition, the addition of Zr into Cu–Cr alloys can decrease the inner spacing of Cr precipitates
⁎ Corresponding author at: Key Laboratory of Solidification Control and Digital Preparation Technology, School of Material Science and Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning, PR China. E-mail address: [email protected] (T. Wang).
http://dx.doi.org/10.1016/j.matdes.2015.12.013 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
and form Cu5Zr phase on the site of dislocations, hence improving mechanical strength and stress relaxation resistance [8]. In recent decades, much work has been done on the effect of electric current on processing and properties of metals. Silva et al. [9] found reverse effects of Joule heating during annealing of Cu–Co alloys using linearly varying current Joule heating. Wang et al. [10] found that a critical electron current pulse can reduce diffusion activation energy of Pb in Cu–Zn alloys and transform lead inclusions into grain boundaries or defects, forming many dispersed small particles of lead. Zhu et al. [11] conducted electropulsing treatment on cold rolled Cu strips, and found that the evolution of properties, microstructure and texture is strongly influenced by the frequency of electropulsing. Conrad et al. [12,13] reported a concurrent application of electric current pulses during the isochronal annealing of cold-worked copper. The rates of recovery and recrystallization are enhanced and the grain size is refined by pulses. The magnitude of the effect increases with the increase of pulsing frequency but is relatively independent on pulse duration. Wang et al. [14,15] studied the influence of DC current on Cu–Cr–Zr alloy, and found that both microhardness and electrical conductivity can be improved with the imposition of DC current. The morphology of precipitates aged at 400 °C under DC current resembles that of the samples aged at 500 °C, which is attributed to the electronwind force induced by DC current. In summary, the influence of an electric current on the solid state phase transformation in metals is complicated: an electric current can either enhance or retard the precipitation rate, depending
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on the alloy composition, the current density and the frequency of pulsed DC current [16]. As mentioned above, aging process can be promoted by current [14,15], and electric current pulses have a remarkable effect on recovery and recrystallization [12,13]. The effects of electric current pulses on the aging process of deformed copper alloys may be interesting. So far, little work on this subject has been reported. In this paper, we focus on the influence of current pulses on the aging behavior of a hot rolledquenched Cu–Cr–Zr alloy during thermomechanical treatment. The variations in electrical conductivity and hardness of Cu–Cr–Zr alloys were investigated experimentally. Synchrotron X-ray diffraction, scanning electron microscope (SEM), optical microscope (OM) and transmission electron microscope (TEM) were applied to characterize the evolution of the microstructures, upon which detailed discussion of the mechanism was based. 2. Experimental procedure Alloy with a composition of Cu–0.6 wt.% Cr–0.3 wt.% Zr was melted using electrolytic copper, pure chromium and zirconium in a vacuuminduction melting furnace, and then casted in a graphite mold with a size of ϕ40 mm × 200 mm. The ingot was cut into a size of 30 mm × 30 mm × 120 mm before homogenized at 850 °C for 4 h, and hot rolled to a thickness of 15 mm. The hot rolled plate was solution treated at 960 °C for 1 h, followed by quickly quenching in cold water. The solution treated plate was then planed on both sides to remove surface defects and rolled with 85% reduction in thickness at room temperature. The cold rolled plate was cut into small pieces and gripped by two copper holders to apply electrical current. To modify the effect of joule heat, a thermal couple was fitted to the surface of the sample to accurately measure the temperature. The cold rolled pieces were aged with and without DC pulses. In this paper, a single-phase DC current pulse was employed and the frequency was set as 1000 Hz. The density of DC pulses was fixed at 40 A/cm2 as the effect of current is determined by its density. Due to the low frequency of pulses, the influence of skin effect can be ignored. The aging treatment was carried out in a tube furnace in argon atmosphere and current pulses were imposed from the beginning of heating.
The electrical conductivity of the samples was measured using a D60K digital electrical instrument at 20 °C, and presented by International annealed copper standard (IACS). Vickers hardness measurements were performed on a MH-50 type mircrohardness tester with 300 g load for 10 s dwell time. For each sample, the test was repeated 5 times to obtain an arithmetical mean value. The microstructures were observed using a JSM-5600LV type of SEM. Metallographic samples were prepared by the conventional mechanical polishing method and etched in an aqueous solution of 3 g FeCl3 + 95 ml C2H5OH + 2 ml HCI. TEM samples were obtained by double jet electrolytic-polishing at − 30 °C within a bath containing 30% nitric acid and 70% methanol. The resulting foil specimens were observed under Tecnai G2 F30 microscope with an acceleration voltage of 300 kV. The synchrotron X-ray radiation analysis was performed at BL14B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF). The samples were illuminated with a monochromatic X-ray beam at an energy of 18 keV (λ = 0.068878 nm) to acquire an accurate measurement of lattice constants. 3. Results 3.1. Initial microstructures Fig. 1 shows the optical microstructures of Cu–Cr–Zr alloys. In the ascast state (Fig. 1(a)), coarse columnar grains can be observed as a result of continuous grain growth during solidification, which is attributed to the low cooling rate of iron mold. Recrystallized grains were elongated by hot-rolling, forming preferred orientation for following deformation (Fig. 1(b)), and grew into equiaxed grains with typical annealed twins during solution treatment (Fig. 1(c)). However, the grains are not clean and there're still undissolved eutectic Cr particles due to limited solubility of Cr in copper. Fig. 1(d) illustrates microstructures of Cu– Cr–Zr alloy in the cold rolled state. Grains were elongated by cold rolling and exhibited lath-shaped structures. Fig. 2 shows the SEM images of Cu–Cr–Zr alloys. In the as-cast state, a necklace-like distribution of particles is observed along the grain boundaries (Fig. 2(a)). This indicates severe segregation of solute atoms during solidification. In the cold-rolled state, a limited number
Fig. 1. Optical micrographs of Cu–Cr–Zr alloys in different states: (a) as-cast, (b) hot rolled, (c) solution-treated, and (d) cold rolled.
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Fig. 2. SEM images of Cu–Cr–Zr alloys: (a) as-cast and (b) cold rolled sample along the transverse direction.
of particles can be identified within the grains and plate-like particles along the rolling direction can be observed. These coarse particles are residue of primary phase formed during the solidification, as identified by Batra et al. [17]. Due to the limited diffusion rate and solubility of Cr, the coarse particles failed to dissolve into copper matrix completely during hot rolling and solution treatment. These particles remained at the grain boundaries and were aligned along rolling directions during subsequent cold rolling process, thus exhibiting a plate-like morphology (Fig. 2(b)). 3.2. Variation of mechanical and electrical properties with the imposition of DC pulses Cold rolled samples were isochronally aged for 1 h and isothermally aged at 450 °C with and without DC pulses to study its influence on aging behavior of Cu–Cr–Zr alloys. The results are shown in Figs. 3 and 4. Fig. 3 illustrates the hardness and electrical conductivity of the alloy aged with and without current pulses as a function of isothermal aging temperatures. As shown in Fig. 3(a), alloys under both conditions exhibit hardening response during aging treatment. The microhardness reaches its peak at 400 °C, and decreases dramatically with a further increase of temperature due to the occurrence of over-aging at high temperatures. The peak values are 196 and 206 HV without and with DC pulses, respectively. With the imposition of DC pulses, the microhardness increases by 10 HV under most aging temperatures except at 350 °C with an inferior increase, which may be related to the low diffusion rate of atoms at lower temperatures. The electrical conductivity of aged Cu–Cr–Zr alloys is shown in Fig. 3(b). The electrical conductivity under both aging conditions greatly increases with the increase of temperature. However, the electrical conductivity is decreased slightly by imposing of DC pulses.
The decrease is negligible at 350 °C, counts up to a maximum value of 3.6% IACS at 450 °C and ends up with 0.4% IACS at 600 °C. The Vickers hardness and electrical conductivity of the alloys isothermally aged at 450 °C with and without DC pulses are shown in Fig. 4. As shown in Fig. 4(a), the microhardness of the alloy aged without pulses increases from 192 HV at 10 min to a peak value of 197 HV at 0.5 h, and gradually decreases to 177 HV at 6 h. With the imposition of DC pulses, the microhardness increases slightly from 195 HV at 10 min to a peak value of 204 HV at 2 h, and decreases gradually to 184 HV at 6 h. Both plots show consistent trends that can be explained by the reduction of solute atoms from Cu matrix and the nucleation, growth and coarsening of precipitates. For Cu–Cr–Zr alloy aged without pulses, the peak value is obtained through aging for 0.5 h, which is much shorter than previous studies [3,8,18]. This can be attributed to the influence of heavy cold-work before aging process and heating parameters employed in this study. After solution treatment and water quenching, tremendous dislocations are formed. These dislocations can form preferred nucleation sites for precipitates and easy diffusion paths for dissolved solute atoms, thus accelerating precipitation process [19]. Meanwhile, the samples were heated in the furnace from room temperature to the set temperature in the period of 40 min, which may also be responsible for the acceleration of aging process. With the imposition of DC pulses, the hardness is increased and the peak value is obtained through aging for 2 h. The time to obtain peak aging of Cu–Cr–Zr alloys is extended from 0.5 h to 2 h, indicating a retarding effect of DC pulses on the precipitation process, which is similar to the results of Onodera et al. in Al–Cu alloys [20]. The variation in electrical conductivity versus time for Cu–Cr–Zr alloys aged at 450 °C with and without DC pulses is shown in Fig. 4(b), the electrical conductivity increases gradually with the extension of
Fig. 3. Vickers hardness (a) and electrical conductivity (b) versus aging temperature for Cu–Cr–Zr alloys aged for 1 h with and without DC pulses.
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Fig. 4. Vickers hardness (a) and electrical conductivity (b) of Cu–Cr–Zr alloys aged for different time under 450 °C with and without DC pulses.
aging time under both conditions, and the imposition of DC pulses leads to a slight decrease (2 to 4% IACS) in electrical conductivity. The electrical conductivity increases from 72.3% IACS at 0.5 h to 85% IACS at 6 h without pulses, and the values are 68.7% IACS and 83% IACS with pulses, respectively. Data of tensile tests for Cu–Cr–Zr alloy with and without DC pulses is shown in Table 1. The imposition of DC pulses increases the ultimate strength and elongation of Cu–Cr–Zr alloy, while decreases its electrical conductivity. Upon aging at 450 °C for 2 h with DC pulses, the ultimate strength is 609 MPa and the elongation is 12.8%. 3.3. Microstructures of Cu–Cr–Zr alloys aged at 450 °C with the imposition of DC pulses Fig. 5 illustrates the TEM images of Cu–Cr–Zr alloys aged at 450 °C for 4 h with and without current pulses. As shown in Fig. 5(a) and (b), the microstructure is mainly composed of deformed subgrains, dislocations, precipitates and deformation twins. Under both conditions, subgrains with the grain size of several hundred nanometers refined by severe deformation and numerous dislocations tangled at grain boundaries and precipitates are observed. (c) and (d) in Fig. 5present magnified TEM images of aged Cu–Cr–Zr alloys with dislocations, subgrains and shear bands. It can be clearly seen that the deformed microstructure remains and there is no evidence of recrystallization upon aging at 450 °C for 4 h. As for precipitation, when aged at 450 °C for 4 h, ellipsoidal particles with an average size between tens and two hundred nanometers can be identified. With the imposition of DC pulses, spherical particles with the size of tens of nanometers are observed. (e) and (f) in Fig. 5 present the typical morphology of precipitates with indexed selected electron diffraction pattern of Cu–Cr–Zr alloys. Precipitates under both conditions are indexed as BCC (body-centered cubic) Cr particles. When aged at 450 °C for 4 h, ellipsoidal particle with a major axis of 441 nm and a manor axis of 338 nm is observed. With the imposition of DC pulses, spherical particle with the radius of 142 nm is observed along dislocation wall. The imposition of DC pulses significantly reduces the size of precipitates. In addition, deformation twins are also observed and presented in Fig. 6(a) with indexed selected electron diffraction
Table 1 Comprehensive properties of Cu–Cr–Zr alloys aged at 450 °C.
pattern in Fig. 6(b). The indexing of pattern shows that the twinning plane is {111}, which is consistent with typical twins in FCC (faceentered cubic) alloys. The features of twinning may be attributed to the lowering of stacking fault energy by the addition of Zr element [21]. Fig. 7 illustrates SEM images and precipitate size distribution of Cu– Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses. (a) and (b) in Fig. 7 show segregated coarse particles in the shape of plates (well over 1 μm) at grain boundaries. These are elongated eutectic Cr particles, which is identical to the elongated Cr particles marked in Fig. 2. For the interior of grains shown in Fig. 7(c) and (d), uniformly distributed fine particles can be observed under both conditions. The pits result from the particles that fell off during sample preparation. With the imposition of DC pulses, both the number and size of precipitates are decreased. (e) and (f) in Fig. 7 present the precipitate size distribution of Cu–Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses respectively. The average particle size of Cu–Cr–Zr alloys aged at 450 °C for 4 h is about 238 nm. With the imposition of DC pulses, the average particle size decreases to about 178 nm. Fig. 8 presents synchrotron X-ray diffraction patterns of Cu–Cr–Zr alloys under three conditions: (a) aged at 450 °C for 4 h without DC pulses, (b) aged at 450 °C for 4 h with DC pulses and (c) cold rolled state. With highly monochromatic bright beam, synchrotron X-ray diffraction experiments provide better counting statistics, hence more well-defined peaks than conventional XRD [22]. As shown in Fig. 8, there're typical peaks of FCC copper and a small peak of BCC Cr (110) at 2θ = 19.43°, and no other peaks can be distinguished. The absence of Cr and Zr peaks can be attributed to the dissolving of Cr and Zr into the copper matrix, which also leads to the shifting of peak positions for the copper matrix. The magnified view of (200)Cu shows that the 2θ angle of copper matrix in the cold rolled state is the largest, aged at 450 °C for 4 h takes the second place and aged at 450 °C for 4 h with DC pulses is the smallest. This relationship is confirmed by the calculated lattice constants in Table 2. The peak area of (110)Cr peak take a minimum value in the cold rolled state and obtains a maximum value when aged at 450 °C for 4 h without DC pulses. The imposition of DC pulses decreases the peak area of (110)Cr peak, indicating a minor amount of Cr precipitates. The value of intensity of (110)Cr peak shows the same variation as peak area, which corresponds with Fig. 7 and further confirms the conclusion that the imposition of DC pulses retards the precipitation process. 4. Discussion
Thermomechanical treatment
Conductivity (%IACS)
Ultimate strength (MPa)
Elongation (%)
2 h with DC pulses 2 h without DC pulses 4 h with DC pulses 4 h without DC pulses
77.6 80.7 78.8 82.8
609 589 583 567
12.8 11.9 13.5 11.1
4.1. Effect of DC pulses on mechanical properties of Cu–Cr–Zr alloys According to Table 1, the imposition of DC pulses leads to an increase of 15 ~ 20 MPa in the ultimate strength of aged Cu–Cr–Zr alloys. For precipitation strengthening alloys going through severe plastic
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Fig. 5. TEM images of Cu–Cr–Zr alloys: (aand c) typical microstructures and (e) precipitate with indexed selected electron diffraction pattern aged at 450 °C for 4 h only; (b and d) typical microstructures and (f) precipitate with indexed selected electron diffraction pattern aged at 450 °C for 4 h with DC pulses.
deformations and corresponding heat treatment, grain boundary strengthening, work hardening, solution strengthening and precipitate strengthening are the dominating strengthening mechanisms. When aged at 450 °C for 4 h, no evidence of recrystallization is observed according to Fig. 5. Thus the variation in strengthening of grain boundaries can be neglected. As for work hardening, the imposition of DC pulses reduces the microstrain from 0.178% to 0.168%. The dislocation can be calculated from microstrain by [23] ρd ¼ 16:1
ε2 2
b
ð1Þ
where ρd is the dislocation density, ε is the microstrain and b is the Burgers vector of copper matrix. The calculated dislocation density
is 7.78 × 1014 m−2 and 6.93 × 1014 m−2 without and with DC pulses. This reduction in dislocation density resulting from the imposition of current is consistent with the occurrence of dislocation recovery at a given current density found by Moon-Jo Kim et al. [24]. The strength increment due to dislocation strengthening Δσd is given by [25] Δσ d ¼ MαGbρ1=2
ð2Þ
where ρd is the dislocation density, α is a constant, which for FCC metals is ~0.2, M = 3.06 is the mean orientation factor for the FCC matrix and G is the shear modulus of the matrix, 41.0 GPa. The results are 179 MPa and 169 MPa without and with DC pulses. The imposition of DC pulses decreases dislocation strengthening by 10 MPa.
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Fig. 6. TEM images of aged Cu–Cr–Zr alloys showing twins (a) and indexed selected electron diffraction pattern (b) (M—matrix; T—twins).
Fig. 7. SEM images showing (a and b) large particles at grain boundaries, (c and d) fine precipitates in the interior of grains and (e and f) precipitate size distribution of Cu–Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses respectively.
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under the assumption of three-dimensional (3-D) growth can be expressed as K¼
Fig. 8. Synchrotron X-ray diffraction patterns of Cu–Cr–Zr alloys (a) cold rolled, (b) aged at 450 °C for 4 h, and (c) aged at 450 °C for 4 h with DC pulses.
For Cu–Cr system alloys under peak aging and over aging conditions, the strengthening is dominated by Orowan mechanism [21], the Orowan stress (TOS) can be expressed as T OS ¼
0:8Gb 2r ln 2πLð1−υÞ r0
ð3Þ
where G is the shear modulus of matrix, b is the Burgers vector of dislocations, L is the inter-precipitate spacing, υ is the Poisson's ratio, r is radius of precipitates and r0 is the inner cutoff radius of dislocations, the value of which can be taken as equal to b. It can be inferred from Eq. (1) that the Orowan stress is mainly controlled by the radius of precipitates r and the inter-precipitate spacing L. As shown in Figs. 5 and 7, the precipitates are clearly refined with the imposition of DC pulses, thus the radius of precipitates r is reduced. Finer precipitates would result in higher density of precipitates in the matrix, consequently reduce the inter-precipitate spacing L. Therefore, strengthening of Orowan mechanism is enhanced. In addition, the imposition of DC pulses retards precipitation and consequently leads to more solute atoms in the Cu matrix, exhibiting stronger solution strengthening. 4.2. Mechanism of refining effect of DC pulses on precipitates For substitutional solutes like Cu–Cr–Zr alloys, aging process is a diffusion-controlled phase transformation, in which the coarsening of precipitates requires the diffusion of solute atoms from far away to the phase interface [21]. The precipitate radius r can be given as [26] r ¼ K ðDtÞ1=2
ð4Þ
where K refers to the growth coefficient, D refers to the effective diffusion efficient and t refers to time. The growth coefficient K derived
Table 2 Calculated parameters from XRD patterns of Cu–Cr–Zr under different conditions. Sample conditions
Lattice constant (Ả)
Peak area (1 1 1)Cr
Intensity (1 1 0)Cr
As cold rolled Aged at 450 °C for 4 h without pulses Aged at 450 °C for 4 h with pulses
3.6221 3.6193 3.6197
4655 12,312 5669
127 438 360
2 C M −C I rs −1 C P −C I
1=2 ð5Þ
where rs is the ratio of the solute depleted zone and the precipitate, CM is the content of solute elements in the matrix, CI is the solute content at the interface of precipitates and Cp is the precipitate content. In this work, the samples have gone through a series of thermomechanical treatments to obtain uniform microstructures. Moreover, the thermocouple was placed on the sample during heating to avoid the effect of Joule heating by current, thus the term K can be taken as a constant value regardless of current treatment. In the present study, a huge amount of dislocations were produced by cold rolling, which can act as paths for pipe diffusion of solutes. Considering the contribution of dislocations, the effective diffusivity of deformed Cu–Cr–Zr alloys can be given as [27] D ¼ D0 1 þ πc2 ρd ðDd =D0 −1Þ
ð6Þ
where Dd is the diffusivity in the dislocation pipe, D0 is the diffusivity outside the pipe and c is the radius of cylindrical dislocation pipes, the value of which is usually taken as 0.5 nm, and ρd is the dislocation density. The diffusivity in the dislocation pipe, Dd, is larger than the lattice diffusivity, D0, outside the pipe. As discussed in Section 4.1, the dislocation density of aged Cu–Cr–Zr alloys was reduced by DC pulses, thus decreasing the value of diffusivity D. In summary, when DC pulses are employed during aging, the growth coefficient α remains constant and the diffusivity D is decreased. At a given time t, the precipitate radius r is thereby decreased according to Eq. (4). The imposition of DC pulses reduces the dislocation density of deformed Cu–Cr–Zr alloys during aging, hindering diffusion of solutes from matrix to precipitates, and consequently refines precipitates. 4.3. Effect of DC pulses on the electrical conductivity of Cu–Cr–Zr alloys As shown in Figs. 3 and 4, the imposition of DC pulses deteriorates the electrical conductivity of Cu–Cr–Zr alloys upon aging. According to Matthiessen's rule, the electrical resistivity of metals is given by ρ ¼ ρ0 þ Δρs þ ΔρP þ ΔρD þ ΔρV þ þΔρB
ð7Þ
where ρ is the real resistivity, ρ0 is the resistivity of ideal pure metal, ΔρS, Δρp, ΔρD, ΔρV and ΔρB are the resistivity sources from solute atoms, precipitates, dislocations, vacancies and grain boundaries respectively. In this study, the samples have gone through a series of thermomechanical treatments to obtain uniform microstructures. So the influence of composition segregation is negligible, and an identical value of ρ0 can be confirmed. Dislocations, vacancies and grain boundaries can act as obstacles of electron scattering and deteriorate electrical conductivity. However, compared with the influence of solutes, the resistivity sourcing from dislocations, vacancies and grain boundaries (ΔρD, ΔρV and ΔρB) can be neglected [28]. Thus the electrical conductivity of aged Cu–Cr–Zr alloys is dominated by the resistivity sourcing from solute atoms and precipitates (ΔρS and Δρp). For solid solutions, the lattice constant can be given empirically by Vegard's Law [29]: aMð1−xÞ SX ¼ ð1−xÞaM þ xaS
ð8Þ
where a refers to the lattice constant, M refers to solvent element of matrix and S refers to solute elements. For Cu–Cr–Zr alloys, the solution of larger atoms into copper matrix (rCu = 1.57 Ả, rCr = 1.85 Ả and rZr = 2.16 Ả) inevitably increases the lattice constant. As shown in Table 2, the cold rolled sample has the largest lattice constant of 3.6221 Ả, which is 0.2% larger than that of pure Cu (3.6147 Ả). Upon aging, solute
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atoms precipitate from copper matrix and in turn decrease lattice constant. With the imposition of DC pulses, the decrease of lattice constant of Cu–Cr–Zr alloys during aging process is lessened, indicating a higher concentration of solutes in the matrix, which results in a higher value of ΔρS. According to Fig. 7 and Table 2, the imposition of DC pulses retards precipitation process. However, the magnitude of Δρp is minor compared to ΔρS and the overall resistivity sourcing from solute atoms and precipitates (ΔρS and Δρp) is still increased. In summary, the deterioration of electrical conductivity caused by DC pulses is mainly attributed to the retarding effect on precipitation. 5. Conclusions 1. The hardness of aged Cu–Cr–Zr alloy is increased by DC pulses while the electrical conductivity is slightly decreased. The peak hardness is increased by 10 HV (from 196 HV to 206 HV) after aging at 400 °C for 1 h. 2. With the imposition of DC pulses, the precipitate size of Cr rich precipitates in aged Cu–Cr–Zr alloys is reduced from 238 nm to 178 nm, exhibiting stronger Orowan strengthening, which results in the improvement of mechanical strength. 3. After aging at 450 °C for 2 h with DC pulses, a superior comprehensive properties of Cu–Cr–Zr alloy are achieved: tensile strength (609 MPa), elongation (12.8%) and electrical conductivity (77.63% IACS). 4. The retarding effect of DC pulses on precipitation process is confirmed by synchrotron X-ray diffraction analysis, which is responsible for the slight decrease in electrical conductivity. Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Nos. 51274054, U1332115), the Key Grant Project of the Chinese Ministry of Education (No. 313011), and the Science and Technology Planning Project of Dalian (Grant No. 2013A16GX110). The authors wish to thank all the staff members of the BL14B1 beamline of SSRF and 4B9A beamline of Beijing Synchrotron Radiation Facility (BSRF) for corresponding experiments and discussion. References [1] S. Jia, P. Liu, F. Ren, B. Tian, M. Zheng, G. Zhou, Sliding wear behavior of copper alloy contact wire against copper-based strip for high-speed electrified railways, Wear 262 (2007) 772–777. [2] M. Kermajani, S. Raygan, K. Hanayi, H. Ghaffari, Influence of thermomechanical treatment on microstructure and properties of electroslag remelted Cu–Cr–Zr alloy, Mater. Des. 51 (2013) 688–694. [3] H.T. Zhou, J.W. Zhong, X. Zhou, Z.K. Zhao, Q.B. Li, Microstructure and properties of Cu–1.0Cr–0.2Zr–0.03Fe alloy, Mater. Sci. Eng. A 498 (2008) 225–230. [4] K.V. León, M.A. Muñoz-Morris, D.G. Morris, Optimisation of strength and ductility of Cu–Cr–Zr by combining severe plastic deformation and precipitation, Mater. Sci. Eng. A 536 (2012) 181–189.
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Effect of direct current pulses on mechanical and electrical properties of aged Cu–Cr–Zr alloys Wei Wang a, Rengeng Li a, Cunlei Zou b, Zongning Chen a, Wen Wen c, Tongmin Wang a,⁎, Guomao Yin a a Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China b Laboratory of Special Processing of Raw Materials, Dalian University of Technology, Dalian 116024, PR China c Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Pudong New Area, Shanghai 201204, PR China
a r t i c l e
i n f o
Article history: Received 9 September 2015 Received in revised form 17 November 2015 Accepted 6 December 2015 Available online 8 December 2015 Keywords: Direct current pulses Mechanical properties Electrical conductivity Precipitation Synchrotron X-ray radiation Cu–Cr–Zr alloys
a b s t r a c t In this work, the response of Cu–Cr–Zr alloys to aging under direct current (DC) pulses was investigated. The results demonstrated that well-balanced mechanical properties and electrical conductivity can be achieved in case the alloys were properly treated. The aging behavior of Cu–Cr–Zr alloys is sensitive to DC pulses in the temperature range between 400 and 450 °C, whereas at temperatures out of this range, DC pulses have minor influences. The hardness of aged Cu–Cr–Zr alloy is increased by DC pulses while the electrical conductivity is slightly decreased. The peak hardness is increased by 10 HV after aging at 400 °C for 1 h. With the imposition of DC pulses, the precipitate size of Cr rich precipitates in aged Cu–Cr–Zr alloys is reduced from 238 nm to 178 nm, exhibiting stronger Orowan strengthening. After aging at 450 °C for 2 h with DC pulses, the Cu–Cr–Zr alloy has an average tensile strength of 609 MPa, elongation of 12.8%, and electrical conductivity of 77.6% IACS. A retarding effect of DC pulses on precipitation process is confirmed by synchrotron X-ray diffraction analysis, which is responsible for the slight decrease in electrical conductivity. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Continuous efforts have been made to obtain materials that exhibit both excellent mechanical strength and high electrical conductivity for high performance applications, such as integrated circuit lead frames and high speed railway contact wire [1–3]. As an age-hardenable alloy, the excellent mechanical properties of Cu–Cr–Zr alloys are attributed to the precipitation and particle-dispersion strengthening mechanism [4,5], and the high electrical conductivity is sourced from the very low solubility of Cr and Zr atoms in Cu matrix at room temperature [6,7]. In the early stage of thermomechanical treatment, the dissolution of Cr and Zr atoms into Cu matrix and high density of defects introduced by cold working result in large lattice distortion and increase scattering of the electrons, leading to high resistivity. However, upon subsequent aging, the precipitation of solute atoms from Cu matrix reduces lattice distortion, and thus promotes electrical conductivity. Meanwhile, the dispersion of fine precipitates retards the recovery and recrystallization of grains as well as strengthens the alloy. In addition, the addition of Zr into Cu–Cr alloys can decrease the inner spacing of Cr precipitates
⁎ Corresponding author at: Key Laboratory of Solidification Control and Digital Preparation Technology, School of Material Science and Engineering, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning, PR China. E-mail address: [email protected] (T. Wang).
http://dx.doi.org/10.1016/j.matdes.2015.12.013 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
and form Cu5Zr phase on the site of dislocations, hence improving mechanical strength and stress relaxation resistance [8]. In recent decades, much work has been done on the effect of electric current on processing and properties of metals. Silva et al. [9] found reverse effects of Joule heating during annealing of Cu–Co alloys using linearly varying current Joule heating. Wang et al. [10] found that a critical electron current pulse can reduce diffusion activation energy of Pb in Cu–Zn alloys and transform lead inclusions into grain boundaries or defects, forming many dispersed small particles of lead. Zhu et al. [11] conducted electropulsing treatment on cold rolled Cu strips, and found that the evolution of properties, microstructure and texture is strongly influenced by the frequency of electropulsing. Conrad et al. [12,13] reported a concurrent application of electric current pulses during the isochronal annealing of cold-worked copper. The rates of recovery and recrystallization are enhanced and the grain size is refined by pulses. The magnitude of the effect increases with the increase of pulsing frequency but is relatively independent on pulse duration. Wang et al. [14,15] studied the influence of DC current on Cu–Cr–Zr alloy, and found that both microhardness and electrical conductivity can be improved with the imposition of DC current. The morphology of precipitates aged at 400 °C under DC current resembles that of the samples aged at 500 °C, which is attributed to the electronwind force induced by DC current. In summary, the influence of an electric current on the solid state phase transformation in metals is complicated: an electric current can either enhance or retard the precipitation rate, depending
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on the alloy composition, the current density and the frequency of pulsed DC current [16]. As mentioned above, aging process can be promoted by current [14,15], and electric current pulses have a remarkable effect on recovery and recrystallization [12,13]. The effects of electric current pulses on the aging process of deformed copper alloys may be interesting. So far, little work on this subject has been reported. In this paper, we focus on the influence of current pulses on the aging behavior of a hot rolledquenched Cu–Cr–Zr alloy during thermomechanical treatment. The variations in electrical conductivity and hardness of Cu–Cr–Zr alloys were investigated experimentally. Synchrotron X-ray diffraction, scanning electron microscope (SEM), optical microscope (OM) and transmission electron microscope (TEM) were applied to characterize the evolution of the microstructures, upon which detailed discussion of the mechanism was based. 2. Experimental procedure Alloy with a composition of Cu–0.6 wt.% Cr–0.3 wt.% Zr was melted using electrolytic copper, pure chromium and zirconium in a vacuuminduction melting furnace, and then casted in a graphite mold with a size of ϕ40 mm × 200 mm. The ingot was cut into a size of 30 mm × 30 mm × 120 mm before homogenized at 850 °C for 4 h, and hot rolled to a thickness of 15 mm. The hot rolled plate was solution treated at 960 °C for 1 h, followed by quickly quenching in cold water. The solution treated plate was then planed on both sides to remove surface defects and rolled with 85% reduction in thickness at room temperature. The cold rolled plate was cut into small pieces and gripped by two copper holders to apply electrical current. To modify the effect of joule heat, a thermal couple was fitted to the surface of the sample to accurately measure the temperature. The cold rolled pieces were aged with and without DC pulses. In this paper, a single-phase DC current pulse was employed and the frequency was set as 1000 Hz. The density of DC pulses was fixed at 40 A/cm2 as the effect of current is determined by its density. Due to the low frequency of pulses, the influence of skin effect can be ignored. The aging treatment was carried out in a tube furnace in argon atmosphere and current pulses were imposed from the beginning of heating.
The electrical conductivity of the samples was measured using a D60K digital electrical instrument at 20 °C, and presented by International annealed copper standard (IACS). Vickers hardness measurements were performed on a MH-50 type mircrohardness tester with 300 g load for 10 s dwell time. For each sample, the test was repeated 5 times to obtain an arithmetical mean value. The microstructures were observed using a JSM-5600LV type of SEM. Metallographic samples were prepared by the conventional mechanical polishing method and etched in an aqueous solution of 3 g FeCl3 + 95 ml C2H5OH + 2 ml HCI. TEM samples were obtained by double jet electrolytic-polishing at − 30 °C within a bath containing 30% nitric acid and 70% methanol. The resulting foil specimens were observed under Tecnai G2 F30 microscope with an acceleration voltage of 300 kV. The synchrotron X-ray radiation analysis was performed at BL14B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF). The samples were illuminated with a monochromatic X-ray beam at an energy of 18 keV (λ = 0.068878 nm) to acquire an accurate measurement of lattice constants. 3. Results 3.1. Initial microstructures Fig. 1 shows the optical microstructures of Cu–Cr–Zr alloys. In the ascast state (Fig. 1(a)), coarse columnar grains can be observed as a result of continuous grain growth during solidification, which is attributed to the low cooling rate of iron mold. Recrystallized grains were elongated by hot-rolling, forming preferred orientation for following deformation (Fig. 1(b)), and grew into equiaxed grains with typical annealed twins during solution treatment (Fig. 1(c)). However, the grains are not clean and there're still undissolved eutectic Cr particles due to limited solubility of Cr in copper. Fig. 1(d) illustrates microstructures of Cu– Cr–Zr alloy in the cold rolled state. Grains were elongated by cold rolling and exhibited lath-shaped structures. Fig. 2 shows the SEM images of Cu–Cr–Zr alloys. In the as-cast state, a necklace-like distribution of particles is observed along the grain boundaries (Fig. 2(a)). This indicates severe segregation of solute atoms during solidification. In the cold-rolled state, a limited number
Fig. 1. Optical micrographs of Cu–Cr–Zr alloys in different states: (a) as-cast, (b) hot rolled, (c) solution-treated, and (d) cold rolled.
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Fig. 2. SEM images of Cu–Cr–Zr alloys: (a) as-cast and (b) cold rolled sample along the transverse direction.
of particles can be identified within the grains and plate-like particles along the rolling direction can be observed. These coarse particles are residue of primary phase formed during the solidification, as identified by Batra et al. [17]. Due to the limited diffusion rate and solubility of Cr, the coarse particles failed to dissolve into copper matrix completely during hot rolling and solution treatment. These particles remained at the grain boundaries and were aligned along rolling directions during subsequent cold rolling process, thus exhibiting a plate-like morphology (Fig. 2(b)). 3.2. Variation of mechanical and electrical properties with the imposition of DC pulses Cold rolled samples were isochronally aged for 1 h and isothermally aged at 450 °C with and without DC pulses to study its influence on aging behavior of Cu–Cr–Zr alloys. The results are shown in Figs. 3 and 4. Fig. 3 illustrates the hardness and electrical conductivity of the alloy aged with and without current pulses as a function of isothermal aging temperatures. As shown in Fig. 3(a), alloys under both conditions exhibit hardening response during aging treatment. The microhardness reaches its peak at 400 °C, and decreases dramatically with a further increase of temperature due to the occurrence of over-aging at high temperatures. The peak values are 196 and 206 HV without and with DC pulses, respectively. With the imposition of DC pulses, the microhardness increases by 10 HV under most aging temperatures except at 350 °C with an inferior increase, which may be related to the low diffusion rate of atoms at lower temperatures. The electrical conductivity of aged Cu–Cr–Zr alloys is shown in Fig. 3(b). The electrical conductivity under both aging conditions greatly increases with the increase of temperature. However, the electrical conductivity is decreased slightly by imposing of DC pulses.
The decrease is negligible at 350 °C, counts up to a maximum value of 3.6% IACS at 450 °C and ends up with 0.4% IACS at 600 °C. The Vickers hardness and electrical conductivity of the alloys isothermally aged at 450 °C with and without DC pulses are shown in Fig. 4. As shown in Fig. 4(a), the microhardness of the alloy aged without pulses increases from 192 HV at 10 min to a peak value of 197 HV at 0.5 h, and gradually decreases to 177 HV at 6 h. With the imposition of DC pulses, the microhardness increases slightly from 195 HV at 10 min to a peak value of 204 HV at 2 h, and decreases gradually to 184 HV at 6 h. Both plots show consistent trends that can be explained by the reduction of solute atoms from Cu matrix and the nucleation, growth and coarsening of precipitates. For Cu–Cr–Zr alloy aged without pulses, the peak value is obtained through aging for 0.5 h, which is much shorter than previous studies [3,8,18]. This can be attributed to the influence of heavy cold-work before aging process and heating parameters employed in this study. After solution treatment and water quenching, tremendous dislocations are formed. These dislocations can form preferred nucleation sites for precipitates and easy diffusion paths for dissolved solute atoms, thus accelerating precipitation process [19]. Meanwhile, the samples were heated in the furnace from room temperature to the set temperature in the period of 40 min, which may also be responsible for the acceleration of aging process. With the imposition of DC pulses, the hardness is increased and the peak value is obtained through aging for 2 h. The time to obtain peak aging of Cu–Cr–Zr alloys is extended from 0.5 h to 2 h, indicating a retarding effect of DC pulses on the precipitation process, which is similar to the results of Onodera et al. in Al–Cu alloys [20]. The variation in electrical conductivity versus time for Cu–Cr–Zr alloys aged at 450 °C with and without DC pulses is shown in Fig. 4(b), the electrical conductivity increases gradually with the extension of
Fig. 3. Vickers hardness (a) and electrical conductivity (b) versus aging temperature for Cu–Cr–Zr alloys aged for 1 h with and without DC pulses.
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Fig. 4. Vickers hardness (a) and electrical conductivity (b) of Cu–Cr–Zr alloys aged for different time under 450 °C with and without DC pulses.
aging time under both conditions, and the imposition of DC pulses leads to a slight decrease (2 to 4% IACS) in electrical conductivity. The electrical conductivity increases from 72.3% IACS at 0.5 h to 85% IACS at 6 h without pulses, and the values are 68.7% IACS and 83% IACS with pulses, respectively. Data of tensile tests for Cu–Cr–Zr alloy with and without DC pulses is shown in Table 1. The imposition of DC pulses increases the ultimate strength and elongation of Cu–Cr–Zr alloy, while decreases its electrical conductivity. Upon aging at 450 °C for 2 h with DC pulses, the ultimate strength is 609 MPa and the elongation is 12.8%. 3.3. Microstructures of Cu–Cr–Zr alloys aged at 450 °C with the imposition of DC pulses Fig. 5 illustrates the TEM images of Cu–Cr–Zr alloys aged at 450 °C for 4 h with and without current pulses. As shown in Fig. 5(a) and (b), the microstructure is mainly composed of deformed subgrains, dislocations, precipitates and deformation twins. Under both conditions, subgrains with the grain size of several hundred nanometers refined by severe deformation and numerous dislocations tangled at grain boundaries and precipitates are observed. (c) and (d) in Fig. 5present magnified TEM images of aged Cu–Cr–Zr alloys with dislocations, subgrains and shear bands. It can be clearly seen that the deformed microstructure remains and there is no evidence of recrystallization upon aging at 450 °C for 4 h. As for precipitation, when aged at 450 °C for 4 h, ellipsoidal particles with an average size between tens and two hundred nanometers can be identified. With the imposition of DC pulses, spherical particles with the size of tens of nanometers are observed. (e) and (f) in Fig. 5 present the typical morphology of precipitates with indexed selected electron diffraction pattern of Cu–Cr–Zr alloys. Precipitates under both conditions are indexed as BCC (body-centered cubic) Cr particles. When aged at 450 °C for 4 h, ellipsoidal particle with a major axis of 441 nm and a manor axis of 338 nm is observed. With the imposition of DC pulses, spherical particle with the radius of 142 nm is observed along dislocation wall. The imposition of DC pulses significantly reduces the size of precipitates. In addition, deformation twins are also observed and presented in Fig. 6(a) with indexed selected electron diffraction
Table 1 Comprehensive properties of Cu–Cr–Zr alloys aged at 450 °C.
pattern in Fig. 6(b). The indexing of pattern shows that the twinning plane is {111}, which is consistent with typical twins in FCC (faceentered cubic) alloys. The features of twinning may be attributed to the lowering of stacking fault energy by the addition of Zr element [21]. Fig. 7 illustrates SEM images and precipitate size distribution of Cu– Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses. (a) and (b) in Fig. 7 show segregated coarse particles in the shape of plates (well over 1 μm) at grain boundaries. These are elongated eutectic Cr particles, which is identical to the elongated Cr particles marked in Fig. 2. For the interior of grains shown in Fig. 7(c) and (d), uniformly distributed fine particles can be observed under both conditions. The pits result from the particles that fell off during sample preparation. With the imposition of DC pulses, both the number and size of precipitates are decreased. (e) and (f) in Fig. 7 present the precipitate size distribution of Cu–Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses respectively. The average particle size of Cu–Cr–Zr alloys aged at 450 °C for 4 h is about 238 nm. With the imposition of DC pulses, the average particle size decreases to about 178 nm. Fig. 8 presents synchrotron X-ray diffraction patterns of Cu–Cr–Zr alloys under three conditions: (a) aged at 450 °C for 4 h without DC pulses, (b) aged at 450 °C for 4 h with DC pulses and (c) cold rolled state. With highly monochromatic bright beam, synchrotron X-ray diffraction experiments provide better counting statistics, hence more well-defined peaks than conventional XRD [22]. As shown in Fig. 8, there're typical peaks of FCC copper and a small peak of BCC Cr (110) at 2θ = 19.43°, and no other peaks can be distinguished. The absence of Cr and Zr peaks can be attributed to the dissolving of Cr and Zr into the copper matrix, which also leads to the shifting of peak positions for the copper matrix. The magnified view of (200)Cu shows that the 2θ angle of copper matrix in the cold rolled state is the largest, aged at 450 °C for 4 h takes the second place and aged at 450 °C for 4 h with DC pulses is the smallest. This relationship is confirmed by the calculated lattice constants in Table 2. The peak area of (110)Cr peak take a minimum value in the cold rolled state and obtains a maximum value when aged at 450 °C for 4 h without DC pulses. The imposition of DC pulses decreases the peak area of (110)Cr peak, indicating a minor amount of Cr precipitates. The value of intensity of (110)Cr peak shows the same variation as peak area, which corresponds with Fig. 7 and further confirms the conclusion that the imposition of DC pulses retards the precipitation process. 4. Discussion
Thermomechanical treatment
Conductivity (%IACS)
Ultimate strength (MPa)
Elongation (%)
2 h with DC pulses 2 h without DC pulses 4 h with DC pulses 4 h without DC pulses
77.6 80.7 78.8 82.8
609 589 583 567
12.8 11.9 13.5 11.1
4.1. Effect of DC pulses on mechanical properties of Cu–Cr–Zr alloys According to Table 1, the imposition of DC pulses leads to an increase of 15 ~ 20 MPa in the ultimate strength of aged Cu–Cr–Zr alloys. For precipitation strengthening alloys going through severe plastic
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Fig. 5. TEM images of Cu–Cr–Zr alloys: (aand c) typical microstructures and (e) precipitate with indexed selected electron diffraction pattern aged at 450 °C for 4 h only; (b and d) typical microstructures and (f) precipitate with indexed selected electron diffraction pattern aged at 450 °C for 4 h with DC pulses.
deformations and corresponding heat treatment, grain boundary strengthening, work hardening, solution strengthening and precipitate strengthening are the dominating strengthening mechanisms. When aged at 450 °C for 4 h, no evidence of recrystallization is observed according to Fig. 5. Thus the variation in strengthening of grain boundaries can be neglected. As for work hardening, the imposition of DC pulses reduces the microstrain from 0.178% to 0.168%. The dislocation can be calculated from microstrain by [23] ρd ¼ 16:1
ε2 2
b
ð1Þ
where ρd is the dislocation density, ε is the microstrain and b is the Burgers vector of copper matrix. The calculated dislocation density
is 7.78 × 1014 m−2 and 6.93 × 1014 m−2 without and with DC pulses. This reduction in dislocation density resulting from the imposition of current is consistent with the occurrence of dislocation recovery at a given current density found by Moon-Jo Kim et al. [24]. The strength increment due to dislocation strengthening Δσd is given by [25] Δσ d ¼ MαGbρ1=2
ð2Þ
where ρd is the dislocation density, α is a constant, which for FCC metals is ~0.2, M = 3.06 is the mean orientation factor for the FCC matrix and G is the shear modulus of the matrix, 41.0 GPa. The results are 179 MPa and 169 MPa without and with DC pulses. The imposition of DC pulses decreases dislocation strengthening by 10 MPa.
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Fig. 6. TEM images of aged Cu–Cr–Zr alloys showing twins (a) and indexed selected electron diffraction pattern (b) (M—matrix; T—twins).
Fig. 7. SEM images showing (a and b) large particles at grain boundaries, (c and d) fine precipitates in the interior of grains and (e and f) precipitate size distribution of Cu–Cr–Zr alloys aged at 450 °C for 4 h without and with DC pulses respectively.
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under the assumption of three-dimensional (3-D) growth can be expressed as K¼
Fig. 8. Synchrotron X-ray diffraction patterns of Cu–Cr–Zr alloys (a) cold rolled, (b) aged at 450 °C for 4 h, and (c) aged at 450 °C for 4 h with DC pulses.
For Cu–Cr system alloys under peak aging and over aging conditions, the strengthening is dominated by Orowan mechanism [21], the Orowan stress (TOS) can be expressed as T OS ¼
0:8Gb 2r ln 2πLð1−υÞ r0
ð3Þ
where G is the shear modulus of matrix, b is the Burgers vector of dislocations, L is the inter-precipitate spacing, υ is the Poisson's ratio, r is radius of precipitates and r0 is the inner cutoff radius of dislocations, the value of which can be taken as equal to b. It can be inferred from Eq. (1) that the Orowan stress is mainly controlled by the radius of precipitates r and the inter-precipitate spacing L. As shown in Figs. 5 and 7, the precipitates are clearly refined with the imposition of DC pulses, thus the radius of precipitates r is reduced. Finer precipitates would result in higher density of precipitates in the matrix, consequently reduce the inter-precipitate spacing L. Therefore, strengthening of Orowan mechanism is enhanced. In addition, the imposition of DC pulses retards precipitation and consequently leads to more solute atoms in the Cu matrix, exhibiting stronger solution strengthening. 4.2. Mechanism of refining effect of DC pulses on precipitates For substitutional solutes like Cu–Cr–Zr alloys, aging process is a diffusion-controlled phase transformation, in which the coarsening of precipitates requires the diffusion of solute atoms from far away to the phase interface [21]. The precipitate radius r can be given as [26] r ¼ K ðDtÞ1=2
ð4Þ
where K refers to the growth coefficient, D refers to the effective diffusion efficient and t refers to time. The growth coefficient K derived
Table 2 Calculated parameters from XRD patterns of Cu–Cr–Zr under different conditions. Sample conditions
Lattice constant (Ả)
Peak area (1 1 1)Cr
Intensity (1 1 0)Cr
As cold rolled Aged at 450 °C for 4 h without pulses Aged at 450 °C for 4 h with pulses
3.6221 3.6193 3.6197
4655 12,312 5669
127 438 360
2 C M −C I rs −1 C P −C I
1=2 ð5Þ
where rs is the ratio of the solute depleted zone and the precipitate, CM is the content of solute elements in the matrix, CI is the solute content at the interface of precipitates and Cp is the precipitate content. In this work, the samples have gone through a series of thermomechanical treatments to obtain uniform microstructures. Moreover, the thermocouple was placed on the sample during heating to avoid the effect of Joule heating by current, thus the term K can be taken as a constant value regardless of current treatment. In the present study, a huge amount of dislocations were produced by cold rolling, which can act as paths for pipe diffusion of solutes. Considering the contribution of dislocations, the effective diffusivity of deformed Cu–Cr–Zr alloys can be given as [27] D ¼ D0 1 þ πc2 ρd ðDd =D0 −1Þ
ð6Þ
where Dd is the diffusivity in the dislocation pipe, D0 is the diffusivity outside the pipe and c is the radius of cylindrical dislocation pipes, the value of which is usually taken as 0.5 nm, and ρd is the dislocation density. The diffusivity in the dislocation pipe, Dd, is larger than the lattice diffusivity, D0, outside the pipe. As discussed in Section 4.1, the dislocation density of aged Cu–Cr–Zr alloys was reduced by DC pulses, thus decreasing the value of diffusivity D. In summary, when DC pulses are employed during aging, the growth coefficient α remains constant and the diffusivity D is decreased. At a given time t, the precipitate radius r is thereby decreased according to Eq. (4). The imposition of DC pulses reduces the dislocation density of deformed Cu–Cr–Zr alloys during aging, hindering diffusion of solutes from matrix to precipitates, and consequently refines precipitates. 4.3. Effect of DC pulses on the electrical conductivity of Cu–Cr–Zr alloys As shown in Figs. 3 and 4, the imposition of DC pulses deteriorates the electrical conductivity of Cu–Cr–Zr alloys upon aging. According to Matthiessen's rule, the electrical resistivity of metals is given by ρ ¼ ρ0 þ Δρs þ ΔρP þ ΔρD þ ΔρV þ þΔρB
ð7Þ
where ρ is the real resistivity, ρ0 is the resistivity of ideal pure metal, ΔρS, Δρp, ΔρD, ΔρV and ΔρB are the resistivity sources from solute atoms, precipitates, dislocations, vacancies and grain boundaries respectively. In this study, the samples have gone through a series of thermomechanical treatments to obtain uniform microstructures. So the influence of composition segregation is negligible, and an identical value of ρ0 can be confirmed. Dislocations, vacancies and grain boundaries can act as obstacles of electron scattering and deteriorate electrical conductivity. However, compared with the influence of solutes, the resistivity sourcing from dislocations, vacancies and grain boundaries (ΔρD, ΔρV and ΔρB) can be neglected [28]. Thus the electrical conductivity of aged Cu–Cr–Zr alloys is dominated by the resistivity sourcing from solute atoms and precipitates (ΔρS and Δρp). For solid solutions, the lattice constant can be given empirically by Vegard's Law [29]: aMð1−xÞ SX ¼ ð1−xÞaM þ xaS
ð8Þ
where a refers to the lattice constant, M refers to solvent element of matrix and S refers to solute elements. For Cu–Cr–Zr alloys, the solution of larger atoms into copper matrix (rCu = 1.57 Ả, rCr = 1.85 Ả and rZr = 2.16 Ả) inevitably increases the lattice constant. As shown in Table 2, the cold rolled sample has the largest lattice constant of 3.6221 Ả, which is 0.2% larger than that of pure Cu (3.6147 Ả). Upon aging, solute
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atoms precipitate from copper matrix and in turn decrease lattice constant. With the imposition of DC pulses, the decrease of lattice constant of Cu–Cr–Zr alloys during aging process is lessened, indicating a higher concentration of solutes in the matrix, which results in a higher value of ΔρS. According to Fig. 7 and Table 2, the imposition of DC pulses retards precipitation process. However, the magnitude of Δρp is minor compared to ΔρS and the overall resistivity sourcing from solute atoms and precipitates (ΔρS and Δρp) is still increased. In summary, the deterioration of electrical conductivity caused by DC pulses is mainly attributed to the retarding effect on precipitation. 5. Conclusions 1. The hardness of aged Cu–Cr–Zr alloy is increased by DC pulses while the electrical conductivity is slightly decreased. The peak hardness is increased by 10 HV (from 196 HV to 206 HV) after aging at 400 °C for 1 h. 2. With the imposition of DC pulses, the precipitate size of Cr rich precipitates in aged Cu–Cr–Zr alloys is reduced from 238 nm to 178 nm, exhibiting stronger Orowan strengthening, which results in the improvement of mechanical strength. 3. After aging at 450 °C for 2 h with DC pulses, a superior comprehensive properties of Cu–Cr–Zr alloy are achieved: tensile strength (609 MPa), elongation (12.8%) and electrical conductivity (77.63% IACS). 4. The retarding effect of DC pulses on precipitation process is confirmed by synchrotron X-ray diffraction analysis, which is responsible for the slight decrease in electrical conductivity. Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Nos. 51274054, U1332115), the Key Grant Project of the Chinese Ministry of Education (No. 313011), and the Science and Technology Planning Project of Dalian (Grant No. 2013A16GX110). The authors wish to thank all the staff members of the BL14B1 beamline of SSRF and 4B9A beamline of Beijing Synchrotron Radiation Facility (BSRF) for corresponding experiments and discussion. References [1] S. Jia, P. Liu, F. Ren, B. Tian, M. Zheng, G. Zhou, Sliding wear behavior of copper alloy contact wire against copper-based strip for high-speed electrified railways, Wear 262 (2007) 772–777. [2] M. Kermajani, S. Raygan, K. Hanayi, H. Ghaffari, Influence of thermomechanical treatment on microstructure and properties of electroslag remelted Cu–Cr–Zr alloy, Mater. Des. 51 (2013) 688–694. [3] H.T. Zhou, J.W. Zhong, X. Zhou, Z.K. Zhao, Q.B. Li, Microstructure and properties of Cu–1.0Cr–0.2Zr–0.03Fe alloy, Mater. Sci. Eng. A 498 (2008) 225–230. [4] K.V. León, M.A. Muñoz-Morris, D.G. Morris, Optimisation of strength and ductility of Cu–Cr–Zr by combining severe plastic deformation and precipitation, Mater. Sci. Eng. A 536 (2012) 181–189.
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