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High thermal stability and excellent mechanical properties of ultrafine-grained high-purity copper sheets subjected to asymmetric cryorolling
Materials Characterization 153 (2019) 34–45
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High thermal stability and excellent mechanical properties of ultrafinegrained high-purity copper sheets subjected to asymmetric cryorolling ⁎
T
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Hailiang Yua,b, , Lin Wanga,b, Linjiang Chaic,d, , Jintao Lie, Cheng Lue, Ajit Godbolee, Hui Wange, Charlie Kongf a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China c College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China d Department of Mechanical and Materials Engineering, Queen's University, Kingston, ON K7L3N6, Canada e School of Mechanical, Materials, Mechatronics and Biomedical Engineering, University of Wollongong, NSW 2500, Australia f Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: High purity copper Asymmetric rolling Asymmetric cryorolling Low temperature annealing Mechanical property Thermal stability
For most ultrafine-grained metals, the yield stress increases with finer grain size, but the thermal stability reduces. In this study, high purity (99.999%) copper sheets were fabricated using three different techniques: symmetric rolling, asymmetric rolling and asymmetric cryorolling. In each case, the sheets were annealed at a temperature ranging from 50 °C to 125 °C for 1 h. Their mechanical properties were tensile-tested using dog-bone samples, and their microstructure evolution was examined using electron backscatter diffraction and transmission electron microscopy. The results show that the asymmetric-cryorolled copper sheets have finer grains and higher tensile strength, and better thermal stability compared with the copper sheets subjected to symmetric rolling and asymmetric rolling and low-temperature annealing. The finer grains in copper sheets subjected to asymmetric cryorolling result from the additional shear strain and severe plastic deformation at low temperature. The improvement in the thermal stability may be due mainly to the vacancy clusters, small laminate thickness, low-angle grain boundary and high misorientation angle in asymmetric cryorolled samples. These results can provide significant insights into the development of ultrafine-grained metal sheets with both excellent mechanical properties and high thermal stability.
1. Introduction Ultrafine-grained/nanograined (UFG/NG) materials have highly desirable properties such as high strength and high strain-rate superplasticity. These materials have attracted significant attention from scientists and engineers who have developed severe plastic deformation (SPD) techniques such as equal channel angular pressing (ECAP) [1], accumulative roll bonding [2], high pressure torsion (HPT) [3], and special rolling techniques [4]. High-purity copper materials subjected to severe plastic deformation have been used to study the mechanism of grain refinement leading to better mechanical properties [5–11]. Alhajeri et al. [5] found that the initial annealing temperature affects the final grain size of high-purity copper (99.99%) subjected to 10-turn HPT. The final microstructure of samples treated in this way consists of fine homogeneous grains having mean grain sizes of 280 nm and
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340 nm for samples initially annealed at 400 °C and 800 °C, respectively. Bach et al. [6] studied the grain structure of coarse-grained copper with 99.99% and 99.5% purity, subjected to 2, 4 and 8 passes of ECAP. Li et al. [7] studied the microstructure evolution of high-purity (99.99%) copper sheet subjected to 1/4 to 10 turns of HPT. The processing resulted in a homogeneous UFG microstructure with a mean grain size of 250 nm and a simple shear texture. Edalati and Horita [8] studied the softening behavior of pure copper (99.99%) due to a thermal effect by processing and subsequently maintaining the sample at room temperature. Apart from the ECAP and HPT techniques, asymmetric rolling [12–14], cryorolling [15–17], and asymmetric cryorolling [18–20] can be used to produce UFG/NG metal sheets. Asymmetric rolling induces an additional shear strain in the rolling deformation zone, resulting in grain refinement and improvement of mechanical properties of the
Correspondence to: H. Yu, State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China. Correspondence to: L. Chai, College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China. E-mail addresses: [email protected] (H. Yu), [email protected] (L. Chai).
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https://doi.org/10.1016/j.matchar.2019.04.034 Received 8 February 2019; Received in revised form 25 April 2019; Accepted 25 April 2019 Available online 26 April 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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sheets. The formability of sheets can be improved by asymmetric rolling compared to that achieved by conventional symmetric rolling [13]. In addition, the minimum achievable foil thickness by asymmetric rolling can be reduced to 1/3 of that by conventional symmetric rolling [14]. Cryorolling is a technique usually using liquid nitrogen to cool down the samples, which can prevent recrystallization and improve the mechanical properties. A number of studies claim that the cryorolled Cu and alloys have high strength and high ductility [15–31]. The cryorolled Cu alloy has higher strength compared to an alloy rolled at roomtemperature due to smaller grain size, higher dislocation density and deformation twins [23]. Zhang et al. [24] fabricated a Cu-Cr-Zr alloy having high strength and high electrical conductivity using cryorolling and intermediate aging treatment. Wei et al. [29] found that subsequent cryorolling of an ECAP-processed UFG Cu-Al alloy resulted in finer grain size and lamellar thickness of twins. Subramanya Sarma et al. [30] found that the mechanical properties of Cu-Al alloys are much superior to those of Cu-Zn alloys when they are processed by cryorolling and annealing. The higher strength of Cu-Al alloy is due to the solid solution strengthening of Al. Anand et al. [31] found that the twinning and interaction between twins and shear bands play a significant role in grain refinement and strengthening of copper. Asymmetric cryorolling is a technique that combines the features of asymmetric rolling and cryorolling. The grain growth and grain refinement may appear simultaneously in asymmetric cryorolled copper sheets under tensile deformation at room temperature [32]. In addition, Yu et al. [33] found an improvement in the strength of asymmetric cryorolled copper sheets under low-temperature annealing, and a deterioration in the strength of the sheets subjected to asymmetric rolling. However, to date, there have been very few reports [18] documenting the mechanical properties and microstructure evolution of the sheets subjected to symmetric rolling, asymmetric rolling, and asymmetric cryorolling with subsequent annealing. Also, there have been few, if any, reports on the thermal stability of asymmetric cryorolled metal sheets. In this study, we investigated the mechanical properties and microstructure evolution of high-purity copper sheets subjected to symmetric rolling, asymmetric rolling with a rolling speed ratio of 1.2, asymmetric rolling with a rolling speed ratio of 1.4, and asymmetric cryorolling with a rolling speed ratio of 1.4, respectively. In each case, the sheets were subsequently annealed at 50 °C, 75 °C, 100 °C, and 125 °C for 1 h. We found that the asymmetric cryorolled sheets show higher strength and higher thermal stability compared to other processes.
Fig. 1. Optical microscope image of the microstructure of as-received sample.
For the asymmetric cryorolling, before commencing rolling in each pass, the sheets were cooled by liquid nitrogen for > 8 min. During asymmetric cryorolling, the rolling reduction ratio per pass was strictly controlled and the temperature of the sheet after asymmetric cryorolling was maintained lower than −100 °C. This was suggested by the FE simulations that revealed the relationship between the rolling reduction ratio and temperature rise during cryorolling [36]. After rolling, the sheets were cut into five pieces. Four of these pieces were annealed for 1 h at 50 °C, 75 °C, 100 °C, and 125 °C respectively. The mechanical properties and microstructures were tested for the sheets separately subjected to SR, (SR + annealing), AR, (AR + annealing), ACR, and (ACR + annealing). The copper sheets were machined to strips (length 18 mm × width 3 mm) for tensile tests to generate the engineering strain vs stress curves. The tensile tests were carried out on an INSTRON machine with an initial strain rate of 1.0 × 10−3 s−1. Each test was repeated three times. A Philips CM200 Field Emission Gun Transmission Electron Microscope (FEG-TEM) operating at 200 kV was used to examine the cross-section microstructures in the rolling direction and across the thickness. A ThermoFisher Helios G4 PFIB was used to prepare the TEM specimens with the in-situ liftout technique. Electron backscatter diffraction (EBSD) tests were conducted on a JEOL JSM-7001F field emission gun-scanning electron microscope (FEG-SEM) operating at 15 kV, ~5 nA and 24 mm working distance, and using the Oxford Instruments AZtec acquisition software.
2. Experimental procedure
3. Results
In the present study, 2.0 mm-thick high-purity (99.999%) copper sheets were used. Fig. 1 shows the optical microstructure of the asreceived sample. The rolling experiments were carried out on a multifunction four-high rolling mill with a maximum rolling force of 50 kN and work roll diameter of 50 mm. The work rolls were newly polished to have a smooth surface. The rolling experiments were carried out in a “dry friction” condition. After rolling, the sheets were rolled to 0.1 mm thickness. UFG sheets can be used to offset ‘size effects’ on workpieces during microforming and to improve the structural stability of microparts [34,35]. In this study, four kinds of rolling processes were designed:
Fig. 2a–d show the tensile stress as a function of engineering strain of the Cu sheets subjected to different processing methods. Fig. 2a shows that the yield stress of rolled Cu sheets improves greatly compared to the as-received Cu sheet, increasing from 100 MPa to 405 MPa for SR-processed sheets, to 420 MPa for the sheets subjected to AR1.2, to 465 MPa for the sheets subjected to AR1.4, and to 504 MPa for the sheets subjected to ACR1.4. The yield stress of the ACR-processed sheets is more than five times of that of the as-received sheets. In Fig. 2b, it is seen that the yield stress of the symmetric-rolled sheets reduces with increasing annealing temperature. When the annealing temperature is 50 °C, the yield stress of the copper sheets drops from 405 MPa to 387 MPa, to 372 MPa for 75 °C, to 352 MPa for 100 °C, and to 320 MPa for 125 °C. In addition, the engineering strain rate increases at higher annealing temperatures. Fig. 2c shows that the tensile stress of the copper sheets subjected AR1.4 gradually decreases from 465 MPa to 429 MPa as the annealing temperature increases from 50 °C to 100 °C, to 393 MPa for an annealing temperature of 125 °C. In Fig. 2d, it is seen that the tensile curves corresponding to annealing temperatures lower
(1) Symmetric rolling (SR); (2) Asymmetric rolling with rolling speed ratio between the upper and lower rolls set at 1.2 (AR1.2); (3) Asymmetric rolling with rolling speed ratio between the upper and lower rolls set at 1.4 (AR1.4); (4) Asymmetric cryorolling with rolling speed ratio between the upper and lower rolls set at 1.4 (ACR1.4).
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Fig. 2. Mechanical properties of copper sheets. (a) Engineering stress vs engineering strain curve for rolled sheets and as-received sheet; (b) Engineering stress vs engineering strain curves for symmetric rolled sheets and their annealing; (c) Engineering stress vs engineering strain curves for AR1.4-processed sheets and their annealing; (d) Engineering stress vs engineering strain curves for ACR1.4-processed sheets and their annealing; (e) Tensile stress related to different processes; (f) Ultimate tensile stress vs engineering failure strain.
ACR1.4, after annealing at different temperatures. When the annealing temperature is lower than 100 °C, the tensile stress difference between ACR-processed sheets and symmetric rolled sheets gradually increases, with increasing annealing temperature. When the annealing temperature is 100 °C, the tensile stress difference between ACR-processed sheets and symmetric rolled sheets reaches 130 MPa. However, when the annealing temperature further increases to 125 °C, the tensile stress difference between the ACR-processed sheets and symmetric rolled
than 100 °C are very similar, but when the annealing temperature increases to 125 °C, the curve is markedly different. The reduced mechanical properties may result from the quick growth of nanoscale grains when the annealing temperature is higher than a threshold value. Fig. 2b to d show that the mechanical properties of the copper sheets subjected to asymmetric cryorolling are much more stable when the annealing temperature is lower than 100 °C. Fig. 2e shows the tensile stress of copper sheets subjected to SR, AR1.2, AR1.4 and
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Fig. 3. EBSD orientation imaging maps of (a) symmetric rolled sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 4. EBSD orientation imaging maps of (a) AR1.2-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 5. EBSD orientation imaging maps of (a) AR1.4-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 6. EBSD orientation imaging maps of (a) ACR1.4-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 7. TEM images of the microstructure of copper sheets subjected to (a) SR, (b) AR1.2, (c) AR1.4 and (d) ACR1.4 after annealing at 100 °C for 1 h. In (d), the arrows show vacancy clusters.
with the sheets subjected to the other rolling techniques. For the specimen subjected to conventional SR, recrystallization seems to be initiated after annealing at a temperature as low as 50 °C (Fig. 3b). Inside the recrystallized grains, there exist a few annealing twins with typical lamellar morphologies and straight boundaries, as is often observed in some other annealed Cu alloys [38]. More regions seem to be recrystallized with increasing temperature (Fig. 3c–e), although full recrystallization does not occur at 125 °C (the highest annealing temperature considered in the present work). In the two ARprocessed cases, an annealing temperature of 50 °C is found to be insufficient to initiate any recrystallization at all, and a few new nuclei can only be noticeable after the temperature is increased to 75 °C. Further temperature increase promotes the recrystallization of the deformed structures, similar to the SR case (Fig. 3). Fig. 6 suggests that the ACR-processed structure is relatively stable until the annealing temperature is increased to 100 °C, above which a few recrystallized grains begin to appear. The above microstructural observations suggest that among the deformed structures produced by the four types of rolling, the one produced by ACR has the highest stability upon annealing. These changes are directly reflected in the change in the mechanical properties of the sheets shown in Fig. 2.
sheets reduces. In addition, compared to symmetric rolled sheets, the differences in the tensile stress of sheets subjected to AR1.2, AR1.4 and ACR1.4 become similar when the annealing temperature is higher than 125 °C. Fig. 2f shows the ultimate tensile stress vs engineering failure strain trends for different processing techniques. It shows that the (ACR1.4 + annealing) technique can result in better mechanical properties compared to SR and AR processing techniques. Figs. 3–6 present microstructures (EBSD maps) of various rolled and annealed specimens. These figures show that the laminate structure in the rolled samples gradually changes into a bimodal structure during annealing. In addition, in all four rolled cases, grains are found to be elongated along the rolling direction. Inside the grains, there exist a large number of low-angle boundaries (LABs, 2° < θ < 15°) that could be attributed to proliferation and alignment of deformation-induced dislocations [37]. Compared to conventional SR (Fig. 3a), the LAB density seems to be higher after asymmetric rolling (Fig. 4a). This is also seen after increasing the rolling speed ratio to 1.4 (Fig. 5a). Furthermore, much denser LABs seem to be produced after asymmetric cryorolling (Fig. 6a), suggesting their greater ability to induce microstructural change. The enhanced tensile stress of the ACR-processed sheets seen in Fig. 2a could be due to the smaller grain sizes compared 41
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Fig. 8. Grain size distribution in samples subjected to (a) SR, (b) SR+ annealing at 100 °C for 1 h, (c) AR1.4, (d) AR1.4+ annealing at 100 °C for 1 h, (e) ACR1.4 and (f) ACR1.4 annealing at 100 °C for 1 h.
with the same chemical compositions. For the UFG/NG metals, the yield strength increases with reduced grain size according to the Hall-Petch relationship. Compared to SR, AR induces an additional shear strain, which results in finer grain size in sheets [12,13]. Compared to AR, the low temperature ACR process tends to retard the recrystallization and dislocation movement in sheets [20]. Fig. 8 shows the grain size distribution of samples subjected to different processes. The samples subjected to ACR1.4 and (ACR1.4 + annealing) have much finer grain size compared to these subjected to SR, (SR + annealing), AR1.4 and (AR1.4 + annealing). This explains the strength improvement in ACR-processed copper sheets. Similar mechanical properties have been reported in a Ti-6Al-4V alloy manufactured using cold rolling, AR and ACR [18]. The tensile stress of the Ti-6Al-4V alloy after SR was found to be 1008 MPa, 1046 MPa after AR, and 1113 MPa after ACR [18]. Generally, it is claimed that the thermal stability of UFG/NG metals reduces with finer grain size, higher pressure and strain, and lower processing temperature and stacking fault energy [39]. However, Zhou et al. [40] reported that copper with reduced grain size has higher
Fig. 7 shows TEM images of the copper sheets annealed at 100 °C for 1 h after SR, AR1.2, AR1.4 and ACR1.4 processing, respectively. Fig. 7a shows the appearance of coarse twins in the copper sheets. Fig. 7b–d shows the grains morphologies in the deformed regions of the samples. It is seen that the grain size of ACR-processed sheets is the smallest while it shows the highest thermal stability. This phenomenon is similar to that reported for commercial pure copper sheets subjected to ACR with a rolling speed ratio of 1.3 [31].
4. Discussion As shown in Fig. 2, the mechanical properties in the ACR-processed sheets change slightly during annealing when the annealing temperature is lower than 100 °C. In addition, it is seen in Figs. 3 to 6 that the grain size in the ACR-processed samples is the smallest among all these samples. Fig. 6 shows that the grains have higher thermal stability for ACR-processed samples compared with others. These observations imply that ACR technique results in higher strength, finer grain size as well as better thermal stability for the same high purity copper sheets 42
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Fig. 9. Misorientation angle distribution of copper sheets subjected to (a) SR, (b) AR1.2, (c) AR1.4 and (d) ACR1.4.
the samples are the same copper sheets (99.999% in purity, with negligible fraction of alloys), so that the effect of impurity-drag-effect on the thermal stability can be neglected. The grain size of HPT-processed 99.99% copper changes slightly when the temperature is maintained lower than 150 °C for half hour, while an abnormal grain growth appears in the temperature range between 150 °C and 280 °C [44]. For these samples, the thermal stability is better than that in this study. For the UFG/NG materials with the same chemical composition, the grain growth behavior under isothermal annealing condition can be predicted by Eq. (1) [45,46]:
Q ⎞ d1/ n − d 01/ n = K 0 t exp ⎛− ⎝ RT ⎠
(1)
where d0 and d are the grain size before and after annealing for a given annealing time, n is the grain growth exponent, t is the annealing time, K0 is the kinetic constant, Q the activation energy for grain growth, R the universal gas constant, and T the annealing temperature. From the equation, it can be seen that the improved thermal stability of sheets can be due to a higher activation energy for grain growth with a certain annealing temperature and annealing time. The Q value is affected by the non-equilibrium characteristics of the grain boundaries, vacancies, triple junction as well as purity [45,47]. Anand et al. [31] reported that there are vacancy clusters in cryorolled copper which provides the driving force for grain boundary migration. Such vacancy clusters are seen in Fig. 7d, in samples subjected to (ACR + annealing). In addition, the high fractions of high-angle boundaries may contribute to new crystal nucleation and reduce thermal stability [47]. Liu et al. [48] reported the strongly textured nano-laminated structure with LABs
Fig. 10. Frequency of misorientation angle between 0° and 30° and frequency of misorientation angle between 30° and 60° plotted for different rolling methods.
thermal stability, up to a certain limit. In the present study, we also found that the ACR-processed samples show the best thermal stability although their grain size is smallest as seen in EBSD imaging which has been used to study the thermal stability of UFG copper subjected to eight ECAP passes [41]. Generally, addition of alloying elements can enhance the thermal stability of alloys [42,43]. However, in this study, 43
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among the lamellae in a nickel alloy is ultrahard and ultrastable. We agree with the observation that a smaller laminate thickness results in higher thermal stability. Fig. 6a shows that the laminate thickness is smallest among these samples. Liang et al. [49] reported that the thermal stability of lamellar grains and LABs is higher than that of equiaxed grains with high-angle grain boundaries (HABs, θ > 15°) in ECAP-processed copper due to the lower stored energy in the former case. Wang et al. [50] found that 99.9% pure copper subjected to ECAP with back pressure results in smaller grain size and higher fraction of HABs. This, in turn, results in lower thermal stability compared to a sample subjected to ECAP without back pressure. In Figs. 3a, 4a, 5a and 6a, the density of LABs increases following SR, AR1.2, AR.14 and ACR1.4. Thus, the high density of LABs in samples subjected to ACR1.4 may contribute to the improved thermal stability. Li et al. [51] found that AR-processed copper sheets have higher thermal stability compared to those subjected to SR. This might be due to the induced homogeneous microstructure. Zhao et al. [52] reported that the highly nanotwinned copper has higher thermal stability compared to conventional UFG copper. This may imply that the higher misorientation angle (the misorientation angle of twins is 60°) is, the higher thermal stability of copper is. In the present study, we also found that AR-processed sheets have higher thermal stability compared to those subjected to SR for high purity copper sheets. In addition, we further found that ACR-processed sheets have higher thermal stability compared to ARprocessed sheets. We analysed grain misorientation angle distributions of samples processed in different ways. Fig. 9 reveals that the grain misorientation angles is gradually shifted towards high misorientation angle directions in the sequence SR ➔ AR1.2 ➔ AR1.4 ➔ ACR1.4. The most microstructurally stable sample (ACR1.4) seems to have the largest fraction of high misorientation angles. According to the data in Fig. 9, Fig. 10 shows that the frequency of misorientation angle between 30° and 60° for the samples subjected to SR, AR1.2, AR1.4 and ACR1.4 are 51%, 58.9%, 60.3% and 62.8% respectively, which might also determine the thermal stability of UFG/NG copper sheets.
Foundation of China (Grant number: 51674303), National Youth Thousand Plan Program of China, Huxiang High-Level Talent Gathering Project of HUNAN Province (Grant number: 2018RS3015), Innovation Driven Program of Central South University (Grant number: 2019CX006), and the Research Fund of the Key Laboratory of High Performance Complex Manufacturing at Central South University. Data availability The processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. References [1] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881–981. [2] N. Tsuji, Y. Saito, S.H. Lee, Y. Minamino, ARB (accumulative roll-bonding) and other new techniques to produce bulk ultrafine grained materials, Adv. Eng. Mater. 5 (2003) 338–344. [3] A.P. Zhilyaev, T.G. Langdon, Using high-pressure torsion for metal processing: fundamentals and applications, Prog. Mater. Sci. 53 (2008) 893–979. [4] H.L. Yu, C. Lu, K. Tieu, H.J. Li, A. Godbole, S.H. Zhang, Special rolling techniques for improvement of mechanical properties of ultrafine-grained metal sheets: a review, Adv. Eng. Mater. 18 (2016) 754–769. [5] S.N. Alhajeri, A.L. Almazrouee, K.J. Al-Fadhalah, T.G. Langdon, Effect of initial annealing temperature on microstructural development and microhardness in highpurity copper processed by high-pressure torsion, Adv. Eng. Mater. 20 (2018) 170503. [6] J. Bach, J.P. Liebig, H.W. Höppel, W. Blum, Influence of grain boundaries on the deformation resistance: insights from an investigation of deformation kinetics and microstructure of copper after predeformation by ECAP, Philoso. Mag. 93 (2013) 4331–4354. [7] J. Li, J. Xu, C.T. Wang, D. Shan, B. Guo, T.G. Langdon, Microstructural evolution and micro-compression in high-purity copper processed by high-pressure torsion, Adv. Eng. Mater. 18 (2016) 241–250. [8] K. Edalati, Z. Horita, Significance of humongous temperature in softening behaviour and grain size of pure metals processed by high-pressure torsion, Mater. Sci. Eng. A 528 (2011) 7514–7523. [9] M.D. Merz, S.D. Dahlgren, Tensile strength and work hardening of ultrafine-grained high-purity copper, J. Appl. Phys. 46 (1975) 3235–3237. [10] J. Horky, G. Khatibi, D. Setman, B. Weiss, M.J. Zehetbauer, Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu, Mech. Mater. 67 (2013) 38–45. [11] L. Collini, Fatigue crack growth resistance of ECAPed ultrafine-grained copper, Eng. Fract. Mech. 77 (2010) 1001–1011. [12] A. Uniwersal, M. Wroński, M. Wróbel, K. Wierzbanowski, A. Baczmański, Texture effects due to asymmetric rolling of polycrystalline copper, Acta Mater. 139 (2017) 30–38. [13] B.H. Cheon, H.W. Kim, J.C. Lee, Asymmetric rolling of strip-cast Al-5.5Mg-0.3Cu alloy sheets: effects on the formability and mechanical properties, Mater. Sci. Eng. A 528 (2011) 5223–5227. [14] D.L. Tang, X.H. Liu, M. Song, H.L. Yu, Experimental and theoretical study on minimum achievable foil thickness during asymmetric rolling, PLoS One 9 (2014) e106637. [15] Y. Wang, E. Ma, R.Z. Valiev, Y.T. Zhu, Tough nanostructured metals at cryogenic temperatures, Adv. Mater. 16 (2004) 328–331. [16] Y. Wang, M. Chen, F. Zhou, E. Ma, High tensile ductility in a nanostructured metal, Nature 419 (2002) 912–915. [17] T. Shanmugasundaram, B.S. Murty, V. Subramanya Sarma, Development of ultrafine grained high strength Al-Cu alloy by cryorolling, Scr. Mater. 54 (2006) 2013–2017. [18] H.L. Yu, M. Yan, J.T. Li, A. Godbole, C. Lu, K. Tieu, H.J. Li, C. Kong, Mechanical properties and microstructure of a Ti-6Al-4V alloy subjected to cold rolling, asymmetric rolling and asymmetric cryorolling, Mater. Sci. Eng. A 710 (2018) 10–16. [19] H.L. Yu, L.H. Su, C. Lu, K. Tieu, H.J. Li, J.T. Li, A. Godbole, C. Kong, Enhanced mechanical properties of ARB-processed aluminum alloy 6061 sheets by subsequent asymmetric cryorolling and ageing, Mater. Sci. Eng. A 674 (2016) 256–261. [20] H.L. Yu, C. Lu, K. Tieu, X.H. Liu, Y. Sun, Q.B. Yu, C. Kong, Asymmetric cryorolling for fabrication of nanostructural aluminum sheets, Sci. Rep. 2 (2012) 772. [21] H. Bahmanpour, et al., Effect of stacking fault energy on deformation behavior of cryo-rolled copper and copper alloys, Mater. Sci. Eng. A 529 (2011) 230–236. [22] P. Wang, J. Jie, C. Liu, L. Guo, T. Li, An effective method to obtain Cu-35Zn alloy with a good combination of strength and ductility through cryogenic rolling, Mater. Sci. Eng. A 715 (2018) 236–242. [23] X. Zhang, X. Yang, W. Chen, J. Qin, J. Fouse, Effects of rolling temperature and subsequent annealing on mechanical properties of ultrafine-grained Cu-Zn-Si alloy, Mater. Charact. 106 (2015) 100–107. [24] S. Zhang, et al., A high strength and high electrical conductivity Cu-Cr-Zr alloy fabricated by cryorolling and intermediate aging treatment, Mater. Sci. Eng. A 680 (2017) 108–114.
5. Conclusions (1) Ultrafine-grained high purity (99.999%) copper sheets have been processed using three techniques: symmetric rolling (SR), asymmetric rolling (AR) and asymmetric cryorolling (ACR). As a result of the different rolling techniques, the yield stress of the rolled Cu sheets improves greatly from 100 MPa (the as-received material) to 405 MPa, 420 MPa, 465 MPa and 504 MPa for the sheets subjected to SR, AR1.2, AR1.4 and ACR1.4, respectively. (2) The rolled sheets were separately heat-treated for 1 h at 50 °C, 75 °C, 100 °C and 125 °C. The tensile strength of symmetric rolled sheets reduces with higher annealing temperature. The tensile strength of ACR-processed sheets changes slightly when the annealing temperature is lower than 100 °C. (3) The recrystallization behavior has occurred in the samples subjected to SR when the annealing temperature is 50 °C. While the recrystallization behavior was observed in the samples subjected to ACR when the annealing temperature reaches 100–125 °C. (4) The ACR-processed sheets have smaller grain size, higher strength and better thermal stability during low-temperature annealing compared to those subjected to AR and SR. This has been shown by results of the tensile tests, and investigations of the microstructure characterization using electron backscatter diffraction and transmission electron microscopy. The smaller the laminate thickness, the more lowangle grain boundary and the higher misorientation angle and the vacancy clusters in the ACR-processed copper sheets may contribute to the enhanced thermal stability. Acknowledgements This research was supported by National Natural Science 44
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
High thermal stability and excellent mechanical properties of ultrafinegrained high-purity copper sheets subjected to asymmetric cryorolling ⁎
T
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Hailiang Yua,b, , Lin Wanga,b, Linjiang Chaic,d, , Jintao Lie, Cheng Lue, Ajit Godbolee, Hui Wange, Charlie Kongf a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China c College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China d Department of Mechanical and Materials Engineering, Queen's University, Kingston, ON K7L3N6, Canada e School of Mechanical, Materials, Mechatronics and Biomedical Engineering, University of Wollongong, NSW 2500, Australia f Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: High purity copper Asymmetric rolling Asymmetric cryorolling Low temperature annealing Mechanical property Thermal stability
For most ultrafine-grained metals, the yield stress increases with finer grain size, but the thermal stability reduces. In this study, high purity (99.999%) copper sheets were fabricated using three different techniques: symmetric rolling, asymmetric rolling and asymmetric cryorolling. In each case, the sheets were annealed at a temperature ranging from 50 °C to 125 °C for 1 h. Their mechanical properties were tensile-tested using dog-bone samples, and their microstructure evolution was examined using electron backscatter diffraction and transmission electron microscopy. The results show that the asymmetric-cryorolled copper sheets have finer grains and higher tensile strength, and better thermal stability compared with the copper sheets subjected to symmetric rolling and asymmetric rolling and low-temperature annealing. The finer grains in copper sheets subjected to asymmetric cryorolling result from the additional shear strain and severe plastic deformation at low temperature. The improvement in the thermal stability may be due mainly to the vacancy clusters, small laminate thickness, low-angle grain boundary and high misorientation angle in asymmetric cryorolled samples. These results can provide significant insights into the development of ultrafine-grained metal sheets with both excellent mechanical properties and high thermal stability.
1. Introduction Ultrafine-grained/nanograined (UFG/NG) materials have highly desirable properties such as high strength and high strain-rate superplasticity. These materials have attracted significant attention from scientists and engineers who have developed severe plastic deformation (SPD) techniques such as equal channel angular pressing (ECAP) [1], accumulative roll bonding [2], high pressure torsion (HPT) [3], and special rolling techniques [4]. High-purity copper materials subjected to severe plastic deformation have been used to study the mechanism of grain refinement leading to better mechanical properties [5–11]. Alhajeri et al. [5] found that the initial annealing temperature affects the final grain size of high-purity copper (99.99%) subjected to 10-turn HPT. The final microstructure of samples treated in this way consists of fine homogeneous grains having mean grain sizes of 280 nm and
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340 nm for samples initially annealed at 400 °C and 800 °C, respectively. Bach et al. [6] studied the grain structure of coarse-grained copper with 99.99% and 99.5% purity, subjected to 2, 4 and 8 passes of ECAP. Li et al. [7] studied the microstructure evolution of high-purity (99.99%) copper sheet subjected to 1/4 to 10 turns of HPT. The processing resulted in a homogeneous UFG microstructure with a mean grain size of 250 nm and a simple shear texture. Edalati and Horita [8] studied the softening behavior of pure copper (99.99%) due to a thermal effect by processing and subsequently maintaining the sample at room temperature. Apart from the ECAP and HPT techniques, asymmetric rolling [12–14], cryorolling [15–17], and asymmetric cryorolling [18–20] can be used to produce UFG/NG metal sheets. Asymmetric rolling induces an additional shear strain in the rolling deformation zone, resulting in grain refinement and improvement of mechanical properties of the
Correspondence to: H. Yu, State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China. Correspondence to: L. Chai, College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China. E-mail addresses: [email protected] (H. Yu), [email protected] (L. Chai).
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https://doi.org/10.1016/j.matchar.2019.04.034 Received 8 February 2019; Received in revised form 25 April 2019; Accepted 25 April 2019 Available online 26 April 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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sheets. The formability of sheets can be improved by asymmetric rolling compared to that achieved by conventional symmetric rolling [13]. In addition, the minimum achievable foil thickness by asymmetric rolling can be reduced to 1/3 of that by conventional symmetric rolling [14]. Cryorolling is a technique usually using liquid nitrogen to cool down the samples, which can prevent recrystallization and improve the mechanical properties. A number of studies claim that the cryorolled Cu and alloys have high strength and high ductility [15–31]. The cryorolled Cu alloy has higher strength compared to an alloy rolled at roomtemperature due to smaller grain size, higher dislocation density and deformation twins [23]. Zhang et al. [24] fabricated a Cu-Cr-Zr alloy having high strength and high electrical conductivity using cryorolling and intermediate aging treatment. Wei et al. [29] found that subsequent cryorolling of an ECAP-processed UFG Cu-Al alloy resulted in finer grain size and lamellar thickness of twins. Subramanya Sarma et al. [30] found that the mechanical properties of Cu-Al alloys are much superior to those of Cu-Zn alloys when they are processed by cryorolling and annealing. The higher strength of Cu-Al alloy is due to the solid solution strengthening of Al. Anand et al. [31] found that the twinning and interaction between twins and shear bands play a significant role in grain refinement and strengthening of copper. Asymmetric cryorolling is a technique that combines the features of asymmetric rolling and cryorolling. The grain growth and grain refinement may appear simultaneously in asymmetric cryorolled copper sheets under tensile deformation at room temperature [32]. In addition, Yu et al. [33] found an improvement in the strength of asymmetric cryorolled copper sheets under low-temperature annealing, and a deterioration in the strength of the sheets subjected to asymmetric rolling. However, to date, there have been very few reports [18] documenting the mechanical properties and microstructure evolution of the sheets subjected to symmetric rolling, asymmetric rolling, and asymmetric cryorolling with subsequent annealing. Also, there have been few, if any, reports on the thermal stability of asymmetric cryorolled metal sheets. In this study, we investigated the mechanical properties and microstructure evolution of high-purity copper sheets subjected to symmetric rolling, asymmetric rolling with a rolling speed ratio of 1.2, asymmetric rolling with a rolling speed ratio of 1.4, and asymmetric cryorolling with a rolling speed ratio of 1.4, respectively. In each case, the sheets were subsequently annealed at 50 °C, 75 °C, 100 °C, and 125 °C for 1 h. We found that the asymmetric cryorolled sheets show higher strength and higher thermal stability compared to other processes.
Fig. 1. Optical microscope image of the microstructure of as-received sample.
For the asymmetric cryorolling, before commencing rolling in each pass, the sheets were cooled by liquid nitrogen for > 8 min. During asymmetric cryorolling, the rolling reduction ratio per pass was strictly controlled and the temperature of the sheet after asymmetric cryorolling was maintained lower than −100 °C. This was suggested by the FE simulations that revealed the relationship between the rolling reduction ratio and temperature rise during cryorolling [36]. After rolling, the sheets were cut into five pieces. Four of these pieces were annealed for 1 h at 50 °C, 75 °C, 100 °C, and 125 °C respectively. The mechanical properties and microstructures were tested for the sheets separately subjected to SR, (SR + annealing), AR, (AR + annealing), ACR, and (ACR + annealing). The copper sheets were machined to strips (length 18 mm × width 3 mm) for tensile tests to generate the engineering strain vs stress curves. The tensile tests were carried out on an INSTRON machine with an initial strain rate of 1.0 × 10−3 s−1. Each test was repeated three times. A Philips CM200 Field Emission Gun Transmission Electron Microscope (FEG-TEM) operating at 200 kV was used to examine the cross-section microstructures in the rolling direction and across the thickness. A ThermoFisher Helios G4 PFIB was used to prepare the TEM specimens with the in-situ liftout technique. Electron backscatter diffraction (EBSD) tests were conducted on a JEOL JSM-7001F field emission gun-scanning electron microscope (FEG-SEM) operating at 15 kV, ~5 nA and 24 mm working distance, and using the Oxford Instruments AZtec acquisition software.
2. Experimental procedure
3. Results
In the present study, 2.0 mm-thick high-purity (99.999%) copper sheets were used. Fig. 1 shows the optical microstructure of the asreceived sample. The rolling experiments were carried out on a multifunction four-high rolling mill with a maximum rolling force of 50 kN and work roll diameter of 50 mm. The work rolls were newly polished to have a smooth surface. The rolling experiments were carried out in a “dry friction” condition. After rolling, the sheets were rolled to 0.1 mm thickness. UFG sheets can be used to offset ‘size effects’ on workpieces during microforming and to improve the structural stability of microparts [34,35]. In this study, four kinds of rolling processes were designed:
Fig. 2a–d show the tensile stress as a function of engineering strain of the Cu sheets subjected to different processing methods. Fig. 2a shows that the yield stress of rolled Cu sheets improves greatly compared to the as-received Cu sheet, increasing from 100 MPa to 405 MPa for SR-processed sheets, to 420 MPa for the sheets subjected to AR1.2, to 465 MPa for the sheets subjected to AR1.4, and to 504 MPa for the sheets subjected to ACR1.4. The yield stress of the ACR-processed sheets is more than five times of that of the as-received sheets. In Fig. 2b, it is seen that the yield stress of the symmetric-rolled sheets reduces with increasing annealing temperature. When the annealing temperature is 50 °C, the yield stress of the copper sheets drops from 405 MPa to 387 MPa, to 372 MPa for 75 °C, to 352 MPa for 100 °C, and to 320 MPa for 125 °C. In addition, the engineering strain rate increases at higher annealing temperatures. Fig. 2c shows that the tensile stress of the copper sheets subjected AR1.4 gradually decreases from 465 MPa to 429 MPa as the annealing temperature increases from 50 °C to 100 °C, to 393 MPa for an annealing temperature of 125 °C. In Fig. 2d, it is seen that the tensile curves corresponding to annealing temperatures lower
(1) Symmetric rolling (SR); (2) Asymmetric rolling with rolling speed ratio between the upper and lower rolls set at 1.2 (AR1.2); (3) Asymmetric rolling with rolling speed ratio between the upper and lower rolls set at 1.4 (AR1.4); (4) Asymmetric cryorolling with rolling speed ratio between the upper and lower rolls set at 1.4 (ACR1.4).
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Fig. 2. Mechanical properties of copper sheets. (a) Engineering stress vs engineering strain curve for rolled sheets and as-received sheet; (b) Engineering stress vs engineering strain curves for symmetric rolled sheets and their annealing; (c) Engineering stress vs engineering strain curves for AR1.4-processed sheets and their annealing; (d) Engineering stress vs engineering strain curves for ACR1.4-processed sheets and their annealing; (e) Tensile stress related to different processes; (f) Ultimate tensile stress vs engineering failure strain.
ACR1.4, after annealing at different temperatures. When the annealing temperature is lower than 100 °C, the tensile stress difference between ACR-processed sheets and symmetric rolled sheets gradually increases, with increasing annealing temperature. When the annealing temperature is 100 °C, the tensile stress difference between ACR-processed sheets and symmetric rolled sheets reaches 130 MPa. However, when the annealing temperature further increases to 125 °C, the tensile stress difference between the ACR-processed sheets and symmetric rolled
than 100 °C are very similar, but when the annealing temperature increases to 125 °C, the curve is markedly different. The reduced mechanical properties may result from the quick growth of nanoscale grains when the annealing temperature is higher than a threshold value. Fig. 2b to d show that the mechanical properties of the copper sheets subjected to asymmetric cryorolling are much more stable when the annealing temperature is lower than 100 °C. Fig. 2e shows the tensile stress of copper sheets subjected to SR, AR1.2, AR1.4 and
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Fig. 3. EBSD orientation imaging maps of (a) symmetric rolled sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 4. EBSD orientation imaging maps of (a) AR1.2-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 5. EBSD orientation imaging maps of (a) AR1.4-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 6. EBSD orientation imaging maps of (a) ACR1.4-processed sheets (not annealed), and annealed at (b) 50 °C, (c) 75 °C, (d) 100 °C and (e) 125 °C. Black and grey lines represent grain boundaries with θ > 15° and 2° < θ < 15°, respectively.
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Fig. 7. TEM images of the microstructure of copper sheets subjected to (a) SR, (b) AR1.2, (c) AR1.4 and (d) ACR1.4 after annealing at 100 °C for 1 h. In (d), the arrows show vacancy clusters.
with the sheets subjected to the other rolling techniques. For the specimen subjected to conventional SR, recrystallization seems to be initiated after annealing at a temperature as low as 50 °C (Fig. 3b). Inside the recrystallized grains, there exist a few annealing twins with typical lamellar morphologies and straight boundaries, as is often observed in some other annealed Cu alloys [38]. More regions seem to be recrystallized with increasing temperature (Fig. 3c–e), although full recrystallization does not occur at 125 °C (the highest annealing temperature considered in the present work). In the two ARprocessed cases, an annealing temperature of 50 °C is found to be insufficient to initiate any recrystallization at all, and a few new nuclei can only be noticeable after the temperature is increased to 75 °C. Further temperature increase promotes the recrystallization of the deformed structures, similar to the SR case (Fig. 3). Fig. 6 suggests that the ACR-processed structure is relatively stable until the annealing temperature is increased to 100 °C, above which a few recrystallized grains begin to appear. The above microstructural observations suggest that among the deformed structures produced by the four types of rolling, the one produced by ACR has the highest stability upon annealing. These changes are directly reflected in the change in the mechanical properties of the sheets shown in Fig. 2.
sheets reduces. In addition, compared to symmetric rolled sheets, the differences in the tensile stress of sheets subjected to AR1.2, AR1.4 and ACR1.4 become similar when the annealing temperature is higher than 125 °C. Fig. 2f shows the ultimate tensile stress vs engineering failure strain trends for different processing techniques. It shows that the (ACR1.4 + annealing) technique can result in better mechanical properties compared to SR and AR processing techniques. Figs. 3–6 present microstructures (EBSD maps) of various rolled and annealed specimens. These figures show that the laminate structure in the rolled samples gradually changes into a bimodal structure during annealing. In addition, in all four rolled cases, grains are found to be elongated along the rolling direction. Inside the grains, there exist a large number of low-angle boundaries (LABs, 2° < θ < 15°) that could be attributed to proliferation and alignment of deformation-induced dislocations [37]. Compared to conventional SR (Fig. 3a), the LAB density seems to be higher after asymmetric rolling (Fig. 4a). This is also seen after increasing the rolling speed ratio to 1.4 (Fig. 5a). Furthermore, much denser LABs seem to be produced after asymmetric cryorolling (Fig. 6a), suggesting their greater ability to induce microstructural change. The enhanced tensile stress of the ACR-processed sheets seen in Fig. 2a could be due to the smaller grain sizes compared 41
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Fig. 8. Grain size distribution in samples subjected to (a) SR, (b) SR+ annealing at 100 °C for 1 h, (c) AR1.4, (d) AR1.4+ annealing at 100 °C for 1 h, (e) ACR1.4 and (f) ACR1.4 annealing at 100 °C for 1 h.
with the same chemical compositions. For the UFG/NG metals, the yield strength increases with reduced grain size according to the Hall-Petch relationship. Compared to SR, AR induces an additional shear strain, which results in finer grain size in sheets [12,13]. Compared to AR, the low temperature ACR process tends to retard the recrystallization and dislocation movement in sheets [20]. Fig. 8 shows the grain size distribution of samples subjected to different processes. The samples subjected to ACR1.4 and (ACR1.4 + annealing) have much finer grain size compared to these subjected to SR, (SR + annealing), AR1.4 and (AR1.4 + annealing). This explains the strength improvement in ACR-processed copper sheets. Similar mechanical properties have been reported in a Ti-6Al-4V alloy manufactured using cold rolling, AR and ACR [18]. The tensile stress of the Ti-6Al-4V alloy after SR was found to be 1008 MPa, 1046 MPa after AR, and 1113 MPa after ACR [18]. Generally, it is claimed that the thermal stability of UFG/NG metals reduces with finer grain size, higher pressure and strain, and lower processing temperature and stacking fault energy [39]. However, Zhou et al. [40] reported that copper with reduced grain size has higher
Fig. 7 shows TEM images of the copper sheets annealed at 100 °C for 1 h after SR, AR1.2, AR1.4 and ACR1.4 processing, respectively. Fig. 7a shows the appearance of coarse twins in the copper sheets. Fig. 7b–d shows the grains morphologies in the deformed regions of the samples. It is seen that the grain size of ACR-processed sheets is the smallest while it shows the highest thermal stability. This phenomenon is similar to that reported for commercial pure copper sheets subjected to ACR with a rolling speed ratio of 1.3 [31].
4. Discussion As shown in Fig. 2, the mechanical properties in the ACR-processed sheets change slightly during annealing when the annealing temperature is lower than 100 °C. In addition, it is seen in Figs. 3 to 6 that the grain size in the ACR-processed samples is the smallest among all these samples. Fig. 6 shows that the grains have higher thermal stability for ACR-processed samples compared with others. These observations imply that ACR technique results in higher strength, finer grain size as well as better thermal stability for the same high purity copper sheets 42
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Fig. 9. Misorientation angle distribution of copper sheets subjected to (a) SR, (b) AR1.2, (c) AR1.4 and (d) ACR1.4.
the samples are the same copper sheets (99.999% in purity, with negligible fraction of alloys), so that the effect of impurity-drag-effect on the thermal stability can be neglected. The grain size of HPT-processed 99.99% copper changes slightly when the temperature is maintained lower than 150 °C for half hour, while an abnormal grain growth appears in the temperature range between 150 °C and 280 °C [44]. For these samples, the thermal stability is better than that in this study. For the UFG/NG materials with the same chemical composition, the grain growth behavior under isothermal annealing condition can be predicted by Eq. (1) [45,46]:
Q ⎞ d1/ n − d 01/ n = K 0 t exp ⎛− ⎝ RT ⎠
(1)
where d0 and d are the grain size before and after annealing for a given annealing time, n is the grain growth exponent, t is the annealing time, K0 is the kinetic constant, Q the activation energy for grain growth, R the universal gas constant, and T the annealing temperature. From the equation, it can be seen that the improved thermal stability of sheets can be due to a higher activation energy for grain growth with a certain annealing temperature and annealing time. The Q value is affected by the non-equilibrium characteristics of the grain boundaries, vacancies, triple junction as well as purity [45,47]. Anand et al. [31] reported that there are vacancy clusters in cryorolled copper which provides the driving force for grain boundary migration. Such vacancy clusters are seen in Fig. 7d, in samples subjected to (ACR + annealing). In addition, the high fractions of high-angle boundaries may contribute to new crystal nucleation and reduce thermal stability [47]. Liu et al. [48] reported the strongly textured nano-laminated structure with LABs
Fig. 10. Frequency of misorientation angle between 0° and 30° and frequency of misorientation angle between 30° and 60° plotted for different rolling methods.
thermal stability, up to a certain limit. In the present study, we also found that the ACR-processed samples show the best thermal stability although their grain size is smallest as seen in EBSD imaging which has been used to study the thermal stability of UFG copper subjected to eight ECAP passes [41]. Generally, addition of alloying elements can enhance the thermal stability of alloys [42,43]. However, in this study, 43
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among the lamellae in a nickel alloy is ultrahard and ultrastable. We agree with the observation that a smaller laminate thickness results in higher thermal stability. Fig. 6a shows that the laminate thickness is smallest among these samples. Liang et al. [49] reported that the thermal stability of lamellar grains and LABs is higher than that of equiaxed grains with high-angle grain boundaries (HABs, θ > 15°) in ECAP-processed copper due to the lower stored energy in the former case. Wang et al. [50] found that 99.9% pure copper subjected to ECAP with back pressure results in smaller grain size and higher fraction of HABs. This, in turn, results in lower thermal stability compared to a sample subjected to ECAP without back pressure. In Figs. 3a, 4a, 5a and 6a, the density of LABs increases following SR, AR1.2, AR.14 and ACR1.4. Thus, the high density of LABs in samples subjected to ACR1.4 may contribute to the improved thermal stability. Li et al. [51] found that AR-processed copper sheets have higher thermal stability compared to those subjected to SR. This might be due to the induced homogeneous microstructure. Zhao et al. [52] reported that the highly nanotwinned copper has higher thermal stability compared to conventional UFG copper. This may imply that the higher misorientation angle (the misorientation angle of twins is 60°) is, the higher thermal stability of copper is. In the present study, we also found that AR-processed sheets have higher thermal stability compared to those subjected to SR for high purity copper sheets. In addition, we further found that ACR-processed sheets have higher thermal stability compared to ARprocessed sheets. We analysed grain misorientation angle distributions of samples processed in different ways. Fig. 9 reveals that the grain misorientation angles is gradually shifted towards high misorientation angle directions in the sequence SR ➔ AR1.2 ➔ AR1.4 ➔ ACR1.4. The most microstructurally stable sample (ACR1.4) seems to have the largest fraction of high misorientation angles. According to the data in Fig. 9, Fig. 10 shows that the frequency of misorientation angle between 30° and 60° for the samples subjected to SR, AR1.2, AR1.4 and ACR1.4 are 51%, 58.9%, 60.3% and 62.8% respectively, which might also determine the thermal stability of UFG/NG copper sheets.
Foundation of China (Grant number: 51674303), National Youth Thousand Plan Program of China, Huxiang High-Level Talent Gathering Project of HUNAN Province (Grant number: 2018RS3015), Innovation Driven Program of Central South University (Grant number: 2019CX006), and the Research Fund of the Key Laboratory of High Performance Complex Manufacturing at Central South University. Data availability The processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. References [1] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881–981. [2] N. Tsuji, Y. Saito, S.H. Lee, Y. Minamino, ARB (accumulative roll-bonding) and other new techniques to produce bulk ultrafine grained materials, Adv. Eng. Mater. 5 (2003) 338–344. [3] A.P. Zhilyaev, T.G. Langdon, Using high-pressure torsion for metal processing: fundamentals and applications, Prog. Mater. Sci. 53 (2008) 893–979. [4] H.L. Yu, C. Lu, K. Tieu, H.J. Li, A. Godbole, S.H. Zhang, Special rolling techniques for improvement of mechanical properties of ultrafine-grained metal sheets: a review, Adv. Eng. Mater. 18 (2016) 754–769. [5] S.N. Alhajeri, A.L. Almazrouee, K.J. Al-Fadhalah, T.G. Langdon, Effect of initial annealing temperature on microstructural development and microhardness in highpurity copper processed by high-pressure torsion, Adv. Eng. Mater. 20 (2018) 170503. [6] J. Bach, J.P. Liebig, H.W. Höppel, W. Blum, Influence of grain boundaries on the deformation resistance: insights from an investigation of deformation kinetics and microstructure of copper after predeformation by ECAP, Philoso. Mag. 93 (2013) 4331–4354. [7] J. Li, J. Xu, C.T. Wang, D. Shan, B. Guo, T.G. Langdon, Microstructural evolution and micro-compression in high-purity copper processed by high-pressure torsion, Adv. Eng. Mater. 18 (2016) 241–250. [8] K. Edalati, Z. Horita, Significance of humongous temperature in softening behaviour and grain size of pure metals processed by high-pressure torsion, Mater. Sci. Eng. A 528 (2011) 7514–7523. [9] M.D. Merz, S.D. Dahlgren, Tensile strength and work hardening of ultrafine-grained high-purity copper, J. Appl. Phys. 46 (1975) 3235–3237. [10] J. Horky, G. Khatibi, D. Setman, B. Weiss, M.J. Zehetbauer, Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu, Mech. Mater. 67 (2013) 38–45. [11] L. Collini, Fatigue crack growth resistance of ECAPed ultrafine-grained copper, Eng. Fract. Mech. 77 (2010) 1001–1011. [12] A. Uniwersal, M. Wroński, M. Wróbel, K. Wierzbanowski, A. Baczmański, Texture effects due to asymmetric rolling of polycrystalline copper, Acta Mater. 139 (2017) 30–38. [13] B.H. Cheon, H.W. Kim, J.C. Lee, Asymmetric rolling of strip-cast Al-5.5Mg-0.3Cu alloy sheets: effects on the formability and mechanical properties, Mater. Sci. Eng. A 528 (2011) 5223–5227. [14] D.L. Tang, X.H. Liu, M. Song, H.L. Yu, Experimental and theoretical study on minimum achievable foil thickness during asymmetric rolling, PLoS One 9 (2014) e106637. [15] Y. Wang, E. Ma, R.Z. Valiev, Y.T. Zhu, Tough nanostructured metals at cryogenic temperatures, Adv. Mater. 16 (2004) 328–331. [16] Y. Wang, M. Chen, F. Zhou, E. Ma, High tensile ductility in a nanostructured metal, Nature 419 (2002) 912–915. [17] T. Shanmugasundaram, B.S. Murty, V. Subramanya Sarma, Development of ultrafine grained high strength Al-Cu alloy by cryorolling, Scr. Mater. 54 (2006) 2013–2017. [18] H.L. Yu, M. Yan, J.T. Li, A. Godbole, C. Lu, K. Tieu, H.J. Li, C. Kong, Mechanical properties and microstructure of a Ti-6Al-4V alloy subjected to cold rolling, asymmetric rolling and asymmetric cryorolling, Mater. Sci. Eng. A 710 (2018) 10–16. [19] H.L. Yu, L.H. Su, C. Lu, K. Tieu, H.J. Li, J.T. Li, A. Godbole, C. Kong, Enhanced mechanical properties of ARB-processed aluminum alloy 6061 sheets by subsequent asymmetric cryorolling and ageing, Mater. Sci. Eng. A 674 (2016) 256–261. [20] H.L. Yu, C. Lu, K. Tieu, X.H. Liu, Y. Sun, Q.B. Yu, C. Kong, Asymmetric cryorolling for fabrication of nanostructural aluminum sheets, Sci. Rep. 2 (2012) 772. [21] H. Bahmanpour, et al., Effect of stacking fault energy on deformation behavior of cryo-rolled copper and copper alloys, Mater. Sci. Eng. A 529 (2011) 230–236. [22] P. Wang, J. Jie, C. Liu, L. Guo, T. Li, An effective method to obtain Cu-35Zn alloy with a good combination of strength and ductility through cryogenic rolling, Mater. Sci. Eng. A 715 (2018) 236–242. [23] X. Zhang, X. Yang, W. Chen, J. Qin, J. Fouse, Effects of rolling temperature and subsequent annealing on mechanical properties of ultrafine-grained Cu-Zn-Si alloy, Mater. Charact. 106 (2015) 100–107. [24] S. Zhang, et al., A high strength and high electrical conductivity Cu-Cr-Zr alloy fabricated by cryorolling and intermediate aging treatment, Mater. Sci. Eng. A 680 (2017) 108–114.
5. Conclusions (1) Ultrafine-grained high purity (99.999%) copper sheets have been processed using three techniques: symmetric rolling (SR), asymmetric rolling (AR) and asymmetric cryorolling (ACR). As a result of the different rolling techniques, the yield stress of the rolled Cu sheets improves greatly from 100 MPa (the as-received material) to 405 MPa, 420 MPa, 465 MPa and 504 MPa for the sheets subjected to SR, AR1.2, AR1.4 and ACR1.4, respectively. (2) The rolled sheets were separately heat-treated for 1 h at 50 °C, 75 °C, 100 °C and 125 °C. The tensile strength of symmetric rolled sheets reduces with higher annealing temperature. The tensile strength of ACR-processed sheets changes slightly when the annealing temperature is lower than 100 °C. (3) The recrystallization behavior has occurred in the samples subjected to SR when the annealing temperature is 50 °C. While the recrystallization behavior was observed in the samples subjected to ACR when the annealing temperature reaches 100–125 °C. (4) The ACR-processed sheets have smaller grain size, higher strength and better thermal stability during low-temperature annealing compared to those subjected to AR and SR. This has been shown by results of the tensile tests, and investigations of the microstructure characterization using electron backscatter diffraction and transmission electron microscopy. The smaller the laminate thickness, the more lowangle grain boundary and the higher misorientation angle and the vacancy clusters in the ACR-processed copper sheets may contribute to the enhanced thermal stability. Acknowledgements This research was supported by National Natural Science 44
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