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Effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheet
Materials Science & Engineering A 772 (2020) 138811
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Effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheet X.B. Li a, b, c, *, G.M. Jiang a, J.P. Di a, Y. Yang a, d, **, C.L. Wang e a
School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology (Zhangjiagang), Suzhou, 215600, Jiangsu province, PR China Institute of Research of Iron and Steel, Jiangsu Sha-steel Group Co., Ltd., Suzhou, 215625, Jiangsu province, PR China c Division of Engineering Steel, Central Iron and Steel Research Institute, Beijing, 100081, PR China d National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, Jiangsu province, PR China e School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu province, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cryogenic rolling Dislocations Recrystallization Mechanical properties Twins
The pure copper sheets were rolled with a small strain in one pass at cryogenic and room temperature, respectively. The total thickness was reduced by 55% through three rolling passes. The microstructural evolution and mechanical properties of Cu sheets were investigated by metallographic characterizations, tensile tests and fracture analysis. It is found that several nanoscale mechanical twins are generated in the cryogenically rolled Cu sample. The restricted atomic motion at ultralow temperature inhibits the dislocation slip and then induces the occurrence of grain kinking and mechanical twining. The yield strength and ultimate tensile strength of cryorolled Cu sample are enhanced by the remarkable accumulation of dislocations and mechanical twins. During annealing treatment, the recrystallization behavior is accelerated by the dramatic strain energy and micro structural defects in cryogenically rolled Cu samples. Compared with the recrystallization concentrated on the surface of room temperature rolled Cu sample, a few of recrystallized grains are formed in the central position of cryogenically rolled Cu sample at 180 � C annealing. The rapid progress of thermal recovery during annealing treatment improves the tensile ductility and strain-hardening capacity of cryo-rolled Cu sample. Several pores are formed on the tensile fracture due to the incompatible deformation between the inhomogeneous grains. The results demonstrate that combining the cryogenic rolling and 200 � C annealing is able to improve the mechanical properties of pure copper sheet.
1. Introduction The contradiction between strength and ductility of pure copper with medium stacking fault energy (SFE) restricts the technological applica tion in industries [1,2]. Nowadays, high strength and high ductility can be simultaneously achieved by the modification of chemical composi tion and microstructure [3]. The introduction of nanoscale twins is a potential approach to optimize the comprehensive properties of bulk materials. The severe plastic deformation (SPD) technologies, such as equal channel angular extrusion, accumulative roll bonding and high-pressure torsion, are increasingly used to produce the bulk metals with ultrafine grains or nanostructures [4–6]. However, the efficiency of grain refinement is sharply reduced by the saturation of dislocations and
other structural defects when plastic strain increases up to 10 [7]. Im purities and cracks are likely to be formed in bulk metals through the repeated operations in SPD processes [2]. Compared with the typical SPD processes operated at room tem perature or elevated temperature, the particular deformation in cryo genic condition could prevent the formation of dynamic recovery in bulk metals [8–11]. Several substructures such as dislocation cells and me chanical twins are formed in the cryogenically deformed metals [12,13]. The accumulation of microstructural defects and strain energy makes the processed materials to be metastable, which will influence the recrystallization and phase transformation in heat-treatment process [14,15]. The cryogenic rolling (CR) with extreme strain has been used to
* Corresponding author. School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology (Zhangjiagang), Suzhou, 215600, Jiangsu province, PR China. ** Corresponding author. E-mail addresses: [email protected] (X.B. Li), [email protected] (Y. Yang). https://doi.org/10.1016/j.msea.2019.138811 Received 6 August 2019; Received in revised form 8 December 2019; Accepted 9 December 2019 Available online 13 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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prepare pure Cu sheet with high strength and high ductility [2]. Several micrometer-sized grains are embedded inside a matrix of nanocrystal line and ultrafine (<300 nm) grains. The microstructure gives rise to a remarkable strain-hardening effect to the rolled sample. The 7075 Al alloy processed by cryogenic rolling and ageing treatment can achieve a simultaneous enhancement of ductility and strength, which can be ascribed to the modified microstructure with very small second-phase precipitations in the nanocrystalline [3]. The further grain refinement and remarkable increase of yield strength have been obtained in the pure vanadium undergoing cryorolling with 60% reduction [16]. The Al-Mg-Si alloy can get ultrafine structure after cryorolling with severe plastic strain [17]. It is also found that the considerable internal stress developed in cryorolling promotes the formation of annealing twins in Cu-Al alloy [18]. The cryogenic impact deformation has been used to prepare the bulk ultrafine grained copper [19]. A great deal of nanoscale mechanical twins exists in the elongated ultrafine grains after annealing at 190 � C for 60 min. Moreover, the dynamic plastic deformation at cryogenic temperature can achieve the nanoscale twins with coherent boundaries in pure copper and simultaneously improve the strength, ductility and electrical conductivity [8]. These studies demonstrate that the microstructural evolution during cryogenic deformation undergoes the diverse mechanisms, such as grain division, kinking, shear bands and twinning. The extreme deformation at ultra-low temperature could induce the distinctive transformation of microstructure. However, the deformation mechanism and formation of mechanical twins is not absolutely clear for pure copper during cryogenic defor mation [12,20]. The material behavior and mechanical properties of pure copper produced by small strain rolling in cryogenic condition need much more attentions. It is favorable for the achievement of nanoscale twins in large-sized pure copper with medium SFE using the relatively simple procedure. In this work, the materials behavior and deformation mechanism are studied based on the microstructural characterization of as-rolled copper and annealed samples. The effect of cryogenic rolling on microstructural evolution and mechanical properties is analyzed to acquire the reasonable approach to optimize the pure copper.
recrystallization. The annealing temperature was in the range of 180 � C–220 � C based on the previous study. Annealing treatments were performed in a program-controlled resistance furnace with the deviation of 2 � C. The metallographic observations were taken on the longitudinal section by optical microscope (OM, ZEISS AxioScope A1) to clarify the microstructural evolution. The crystallographic characteristics, such as grain boundary, dislocations and other microstructural defects, were obtained by the transmission electron microscope (TEM, JEM-2100F) operated at 200 kV accelerating voltage. The TEM specimens were cut from the as-rolled sample in the rolling plane. They were prepared through manual grinding and finally thinned by the twin-jet electrolytic polishing. Tensile specimens with five duplications were obtained from the Cu samples using program-controlled spark cutting. The axis of specimen was along the rolling direction. The gauge length was 30 mm according to the national standard GB 228-2010. Tensile tests were conducted on the mechanical testing system UTM5205X at room tem perature. The nominal strain rate was set as 10 3 s 1. Mechanical properties were evaluated from the measurement in tensile tests. The tensile fractures were analyzed through the scanning electron micro scope (SEM, JSM-6510LA). The relationship between microstructure and mechanical performance of the processed Cu samples was analyzed on the basis of experimental results. 3. Experimental results 3.1. Microstructure of the as-rolled Cu sheet Fig. 1 shows the OM metallographic morphology of as-rolled Cu samples. It is found that the samples are composed of the elongated grains and annealing twins. The size and shape of grains are inhomog enous in the thickness direction of rolled sheets. Grains in the sheet surface are evidently rotated to the rolling direction and become much flatter than the grains in central position. Moreover, the grain morphology in the center of CRed-Cu sample is obviously different from that of RTRed-Cu. Some banded grains with zigzag shape are notable in Fig. 1(d). The crystallographic characteristics of as-rolled samples can be ob tained from the TEM micrographs in Fig. 2. The dislocation density of RTRed-Cu sample is found to be less than that of CRed-Cu sample. Dislocation lines are clear and identifiable in Fig. 2(b), but they are highly tangled in CRed-Cu sample. Moreover, the locations indicated by the arrows in Fig. 2(c) are likely to be grain boundaries with the exis tence of dislocation accumulations. Fig. 3(a) demonstrates that the metallographic feature of TEM specimen film of CRed-Cu sample. Banded structures with lamellar spacing in the range of 1 to 2 μm are found in the rolling plane. The boundary between bands is not straight along the rolling direction. The structures can be regarded as the annealing twins with distinct compression after the repeated rolling. The TEM micrograph in Fig. 3(b) presents the existence of parallel stripes with nanoscale spacing in CRedCu sample. High-resolution TEM (HR-TEM) micrograph and selectedarea electron diffraction of the stripes are displayed in Fig. 3(c) and Fig. 3(d), respectively. The atom arrangement proves that the stripes are stacking faults. It is worthy to note that atoms along the marker line in Fig. 3(c) are in accordance with the symmetric relationship. Two sets of lattice spots are found from the electron diffraction pattern in Fig. 3(d). The parallel stripes are proven to be twins [20].
2. Experimental procedure The initial materials were as-received commercial pure copper sheets. After cutting into 30 mm width and 120 mm length, the samples were fully annealed at 600 � C for 120 min. The chemical components and specification were listed in Table 1. The chemical and mechanical methods were used to remove the oxides and contaminates from the surface. The samples with the thickness of 1.5 mm were inserted into the liquid-nitrogen for 20 min and then quickly carried into the roll gap. The rolling processes were conducted in a double-high rolling mill with the diameter of 200 mm. The rolls were also cooled by pouring liquidnitrogen on the surface before rolling. One pass of rolling process was quickly done in about 10 s. The final thickness of sample was 0.68 mm after three rolling passes. The reduction ratio of Cu sample in one pass was 25%, 20% and 20%, respectively. The total reduction was nominally 55%, which was equivalent with the true strain of 0.8. The sample rolled at room temperature was termed as RTRed-Cu, while that rolled at cryogenic condition was CRed-Cu. The rolled samples were then annealed for 30 min to release the residual stress and obtain the Table 1 Chemical components and properties of the initial material with full annealing. Materials
Chemical components, wt. %
Grain size, μm
Yield strength, MPa
Ultimate tensile strength, MPa
Elongation after fracture, %
Commercial pure copper
99.95 0.004 0.003 0.004
45
51.9
216.9
40.3
Cu, Fe, O, Zn
3.2. Microstructure of the rolled Cu sheets during annealing The microstructural evolutions of as-rolled Cu sheets during annealing are shown in Figs. 4–6. It is found that the recrystallization preferentially occurs on the sheet surface rather than the sheet center. The recrystallized grains are generated just near the sheet surface in RTRed-Cu samples when annealing temperature is 180 � C. However, 2
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Fig. 1. OM micrograph on the longitudinal section of as-rolled Cu samples. (Total reduction ratio is 55%) (a) RTRed-Cu sample, (b) magnification of the selected zone by dotted line in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 4(d) indicates that the recrystallization behavior in CRed-Cu sheet is slightly occurred away from the sheet surface. It can be seen from Figs. 5 and Fig. 6 that the recrystallization promptly occurs in the cryogenically-rolled sample with annealing at 200 � C and 220 � C. The deformed grains are completely replaced by the recrystallized grains in the CRed-Cu sample annealed at 220 � C for 30 min. The microstructure is consisted of equiaxed grains with the average size of 9 μm. Never theless, several deformed grains are remained in RTRed-Cu sample due to the incomplete recrystallization. The arrows in Figs. 4 and Fig. 5 reveal that the recrystallization behavior origins from the grain boundaries and twin boundaries.
of the whole plastic deformation of as-rolled sample. However, the tensile ductility and uniform deformation are obviously improved in the Cu samples with annealing treatment. The difference between yield strength and ultimate tensile strength increases with the annealing temperature. The strain-hardening effect of annealed samples is obvious during the tensile loading. The fracture morphologies of Cu samples shown in Fig. 8 to Fig. 11 present the obvious characteristics of micro-porous polymeric fracture. The number of dimples on fracture surface of two kinds of rolled samples increases with the annealing temperature. Nevertheless, as indicated by the arrows in Fig. 8 Figs. 8(c), Fig. 9(c) and Fig. 10(c), some non-uniform pores exist on the fracture surface of Cu samples with cryogenic rolling. It is also found that the dimple size of CRed-Cu sample is larger than that of RTRed-Cu sample with annealing treatment. After annealing at 220 � C, a large number of dimples with fairly uniform size form on the fracture surface of two kinds of Cu samples. However, the dimples un evenly distributes on the fracture of RTRed-Cu sample. Several pores accompanied with smooth area, as shown by the arrows in Fig. 11(b), are formed in the tensile fracture.
3.3. Mechanical properties of the processed Cu samples The experimental results of tensile tests for different Cu samples are listed in Table 2. It is found that the yield strength and ultimate tensile strength evidently increase, while the elongation after fracture sharply decreases after rolling process. The increment of yield strength is much higher than that of ultimate tensile strength. Moreover, the Cu sample with cryogenic rolling are slightly strengthened than that with room temperature rolling. Annealing treatment induces the formation of thermal recovery in asrolled Cu sample. The changes of tensile properties are not significant at 180 � C annealing. When the annealing temperature is at 200 � C, the strengths clearly reduce and the elongation after fracture sharply in creases. Then, the strengths slowly decrease at 220 � C annealing. It can also be found that the CRed-Cu sample and RTRed-Cu sample have different degree of thermal recovery. The annealed sample with cryo genic rolling has lower strengths and higher elongation than that with room temperature rolling. However, the yield ratio of the CRed-Cu sample is obviously smaller than that of another sample when anneal ing temperature is 180 � C and 200 � C. It reveals that the strainhardening capacity is enhanced for the CRed-Cu sample with anneal ing treatment. The typical engineering stress - strain profiles of processed Cu sam ples are shown in Fig. 7. The non-uniform deformation is the main part
4. Discussions 4.1. Effect of cryogenic rolling on the deformation and recrystallization of pure Cu sample The microstructure of as-rolled Cu samples shows that the grains on the sheet surface are clearly elongated along the rolling direction and have the relatively small size. In the present work, there is no lubrication between the work rolls and Cu sheets. The considerable friction induces the stress concentration and then promotes the metal flow on the sheet surface, which results into the inhomogeneous strain distribution in the thickness direction. Moreover, the large deformation on surface zone induces the considerable deformation energy and lattice distortion in the Cu grains. The atomic activities in Cu samples are restricted during cryogenic rolling. The plastic deformation is difficult due to the strongly inhibited 3
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Fig. 2. TEM micrograph of as-rolled Cu samples with total reduction of 55%. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnifiction of the selected zone in (c).
motion of dislocations. Under the rolling pressure, the crystal structure receives a strong external loading. The lattice distortion and vacancies are then largely formed in Cu grains. Besides, the effect of deforminduced heating is not to be neglected in the as-rolled samples. The total reduction ratio is 55% through three passes of rolling. The Cu samples after room temperature rolling is detected to be near 60 � C, while the sample after cryogenic rolling is just near zero Celsius degrees. The limited deform-induced heating in the ultra-low temperature con dition is unfavorable for the dynamic recovery. Therefore, the disloca tions are remarkably tangled in the CRed-Cu sample, as displayed by the TEM micrograph in Fig. 2(c). The cryogenic rolling process induces the remarkable accumulation of dislocations in grain boundaries and twin boundaries. The considerable internal stress and strain energy provides the driving force to the annihilation of dislocations. The motions of vacancies greatly formed in the CRed-Cu sample also accelerate the climb of dislocations. Then, the tangled dislocations in Cu grains are likely to transform into dislocation cells [9]. It is favorable for the for mation of recrystallization in heat treatment. Annealing twins occupies very large proportion in the initial samples with full annealing. Twin boundaries play an important role to block the dislocation motions and directly influence the material behavior. The dislocation accumulation and stress concentration are generated in the boundaries of annealing twins. The further mechanical loading could then cause the kinking of twins to maintain the structural integrity in liquid nitrogen condition. The zigzag structures are consequently formed in the as-rolled sample, as shown in Fig. 1(c). When the twin planes are parallel with the rolling plane, annealing twins will be
compressed into the lamellar structure with a small spacing. Thus, the twins present the banded morphology in the specific section, as shown in the selected zone in Fig. 3(a). Zener-HolIomon (Z) parameter is a crucial index to reflect the effect of strain rate and processing temperature on the microstructure and mechanical properties [21]. The parameter can be calculated by the equation as following, Z¼έ exp(Q/RT) Where έ is defined as the strain rate, Q is the activation energy of plastic deformation, and T is the processing temperature. The Z parameter of cryogenic rolling is found to be obviously higher than that of room temperature rolling, which reveals the difficulty of plastic deformation for pure copper in the ultra-low temperature condition. The workhardening effect is strong and promotes a new deformation mecha nism happened in the cryogenically deformed sample. In other words, mechanical twinning and other deformation modes will generate when Z parameter increase to a critical value. For pure copper with medium SFE, the dislocations tend to dissociate into partial dislocations when slip and climb are strongly inhibited during cryogenic rolling. The expansion of partial dislocations induces the formation of stacking faults in crystal lattice. The nucleation of mechanical twins is then promoted in the cryo-rolled Cu sample, as shown in Fig. 3(c). However, the amount and size of mechanical twins are small under the processing with limited plastic deformation. The mechanical twins formed in cryogenically-deformed Cu sample can provide additional sites for the nucleation of recrystallized grains. 4
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Fig. 3. Microstructural characteristics of the CRed-Cu sample. (a) banded structure in OM micrograph, (b) stacking fault in TEM micrograph, (c) HR-TEM graph of the stacking fault in (b), (d) electron diffraction pattern of the stacking fault.
Fig. 4. OM micrograph on the longitudinal section of Cu samples with 180 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c). 5
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Fig. 5. OM micrograph on the longitudinal section of Cu samples with 200 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 6. OM micrograph on the longitudinal section of Cu samples with 220 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Owing to the high storage of strain energy and microstructural defects in CRed-Cu sample, the recrystallization behavior is consequently accel erated at lower annealing temperature. The stress concentration and severe deformation on the sheet surface provide a favorable condition to recrystallization. Thus, the recrystalli zation behavior is accelerated in the surface zone. The phenomenon is obvious in the Cu sample with low temperature annealing because of the
incomplete recrystallization. However, the recrystallization is also found to form in the interior of CRed-Cu sheet with 180 � C annealing. It is ascribed to the reduced gap of strain energy between the surface and central position of as-rolled sample. Therefore, the microstructural uniformity of Cu sheet in thickness direction can be improved by the combination of cryogenic rolling and optimal annealing process [22].
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The as-rolled Cu samples are obviously softened by the thermal re covery during annealing treatment. The strengths quickly decrease due to the elimination of dislocations and other microstructural defects. After annealing at 180 � C for 30 min, the rolled samples receive a low level of recrystallization and are mainly composed of the deformed grains and twins. However, the thermal recovery is obvious in the CRedCu sample due to the prompt activities of microstructural defects. There are few obstacles to pin the dislocations in tensile test. The uncon strained motion of dislocations makes the distinct reduce of the yield strength of CRed-Cu sample. On the other hand, the volume fraction of grain boundaries is not significantly changed due to the limited recrystallization. There is a small difference of ultimate tensile strength between the CRed-Cu sample and RTRed-Cu sample. The recrystallization behavior in CRed-Cu sample is obviously ahead due to the short incubation period of recrystallized nucleus. A lot of recrystallized grains are formed in the center zone away from the sheet surface after 200 � C annealing. The yield strength of CRed-Cu sample is 9.3% lower than the strength of RTRed-Cu sample. Meanwhile, the intense activities of dislocations in strain-free grains bring about the remarkable dislocation accumulation and strain hardening to the CRedCu sample [24]. In addition, a few of equiaxed grains are generated among the elongated grains and increases the volume of grain bound aries in CRed-Cu sample. The inhomogeneous structure induces the asynchrony of plastic deformation and additional internal stress. It is easy for the micro-pores to generate along the grain boundaries. A large number of dimples on the fracture surface mix with several pores, as indicated by the arrows in Fig. 10, suggest the improved ductility and premature micro-cracks. The CRed-Cu sample obtains complete recrystallization at 220 � C annealing. A large number of grain boundaries inhibit the dislocation motions during tensile loading and provide a strengthening to the sample. The yield strength is then slightly reduced for the annealed sample. In addition, some deformed grains are remained in the RTRedCu sample due to the delayed thermal recovery. Thus, the yield strength of CRed-Cu sample is 3.2% lower than that of the RTRed-Cu sample. The inhomogeneous grains in RTRed-Cu sample induce the incompatible deformation between elongated grains and recrystallized grains. The different dimples together with some pores are then formed on the tensile fracture.
Table 2 Mechanical properties of the pure Cu sheets with various processing parameters. Samples
Yield strength σ0.2, MPa
Ultimate tensile strength σb, MPa
Elongation after fracture, %
Full annealing CR RTR CRþ180 � C annealing RTRþ180 � C annealing CRþ200 � C annealing RTRþ200 � C annealing CRþ220 � C annealing RTRþ220 � C annealing
51.9 354.8 342.1 289.8
217.6 358.6 347.0 310.2
40.3 1.9 2.1 6.7
305.1
320.7
5.9
159.5
247.0
24.8
175.8
254.2
23.2
115.3
228.0
35.9
119.1
237.7
35.6
Fig. 7. Engineering stress - strain curves of the processed Cu samples in ten sile test.
5. Conclusions
4.2. Microstructural effect on the tensile properties and fracture behavior
The effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheets has been investigated by the metallographic characterizations, tensile tests and fracture analysis. The main conclusions are shown as follows.
It is known that the high density of tangled dislocations and me chanical twins in deformed metal strongly hinder the dislocation mo tions during tensile test [23]. The effect of strain hardening induces that the yield strength and ultimate tensile strength of the CRed-Cu sample are higher than that of the RTRed-Cu sample. However, the lattice distortion in cryogenically deformed sample is remarkable and brings about the stress concentration. A large number of micro-pores quickly initiate from the boundaries and develop into micro-cracks under the tensile loading. Owing to the restricted motion of dislocations, plastic deformation at the front end of cracks is difficult to occur. The cracks rapidly propagate in the form of quasi-cleavage, resulting into the pre mature fracture and low toughness of tensile specimens. The pores on fracture surface, as indicated by the arrows in Fig. 8(c), is considered to be the origin site of initial cracks. Compared with the strength value of the RTRed-Cu sample, the yield strength increases 3.7% and the ultimate tensile strength increases 3.3% in the CRed-Cu sample. During tensile test, the effect of dislocation strengthening is enhanced by the strong restriction of dislocation mo tions and induces the obvious increase of yield strength. On the other hand, the high density of microstructural defects aggravates the initia tion of micro-pores in cryo-rolled Cu sample. The formation of prema ture cracks destroys the structural integrity of tensile specimen and weakens the increase of ultimate tensile strength of the CRed-Cu sample.
(1) The repeated cryogenic rolling with small strain in one pass is able to provide an equivalent condition of extreme deformation to pure copper sheet. A number of tangled dislocations are accumulated in grain boundaries and twin boundaries. The difficult atomic activities at ultra-low temperature induce the formation of nanoscale mechanical twins. Some zigzag structures are formed in the sheet center due to the kinking of grains and annealing twins. (2) The dramatic strain energy and microstructural defects provide the driving force and nucleation sites for the recrystallization during annealing treatment. The cryogenically deformed Cu sample obtains an evidently accelerated recrystallization behavior and gets a complete recrystallization at 220 � C anneal ing. Compared to the recrystallization concentrated on the sur face of room temperature rolled Cu sample, a few of recrystallized grains are formed in the central position of cryo-rolled Cu sheet at 180 � C annealing. (3) The yield strength and ultimate tensile strength of cryo-rolled Cu sample are enhanced by the remarkable accumulation of 7
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Fig. 8. Tensile fracture of the as-rolled Cu samples. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 9. Tensile fracture of the Cu samples with 180 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
dislocations and mechanical twins. The rapid progress of thermal recovery during annealing treatment improves the tensile ductility and strain-hardening capacity of CRed-Cu sample. Several pores are formed on the tensile fracture due to the stress concentration and premature cracking between the inhomoge neous grains. Combining the cryogenic rolling and 200 � C annealing is able to improve the mechanical properties of pure copper sheet.
Author contributions section X. B. Li: Conceptualization, Methodology, Experiments, WritingDraft preparation, Reviewing and Editing. G. M. Jiang: Experiments. J. B. Di: Experiments. Y. Yang: Conceptualization, Methodology, Writing- Reviewing and Editing. 8
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Fig. 10. Tensile fracture of the Cu samples with 200 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 11. Tensile fracture of the Cu samples with 220 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
C. L. Wang: Experiments.
Acknowledgements This work was financially supported by the Natural Science Research Project for Higher Education Institutions in Jiangsu Province (Grant No. 16KJB430012), the Youth Science and Technology Innovation Project in Jiangsu University of Science and Technology (Grant No. 17-5), the National Laboratory of Solid State Microstructures of China (Grant No.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 9
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M31024) and the Innovation Training Program for College Students in Jiangsu Province (Grant No. 201810289013Z). One of the authors Dr. Wang would like to express gratitude to Natural Science Foundation for Young of Jiangsu Province (Grant No. BK20190863), Jiangsu “Mass Innovation and Entrepreneurship” Talent Programme (Shuang Chuang Ph.Ds, 2018)
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References [1] W.D. Callister, Fundamentals of Materials Science and Engineering, fifth ed., John Wiley & Sons, New York, 2001. [2] Y. Wang, M. Chen, F. Zhou, E. Ma, High tensile ductility in a nanostructured metal, Nature 419 (2002) 912–915. [3] Y.H. Zhao, X.Z. Liao, S. Cheng, E. Ma, Y.T. Zhu, Simultaneously increasing the ductility and strength of nanostructured alloys, Adv. Mater. 18 (2006) 2280–2283. [4] Y.B. Wang, X.Z. Liao, Y.H. Zhao, E.J. Lavernia, S.P. Ringer, Z. Horita, T.G. Langdon, Y.T. Zhu, The role of stacking faults and twin boundaries in grain refinement of a Cu-Zn alloy processed by high-pressure torsion, Mater. Sci. Eng. A 527 (2010) 4959–4966. [5] W. Wei, S.L. Wang, K.X. Wei, I.V. Alexandrov, Q.B. Du, J. Hu, Microstructure and tensile properties of Cu-Al alloys processed by ECAP and rolling at cryogenic temperature, J. Alloy. Comp. 678 (2016) 506–510. [6] Y.Z. Tian, S. Gao, L.J. Zhao, S. Lu, R. Pippan, Z.F. Zhang, N. Tsuji, Remarkable transitions of yield behavior and Lüders deformation in pure Cu by changing grain sizes, Scr. Mater. 142 (2018) 88–91. [7] T. Konkova, S. Mironov, A. Korznikov, G. Korznikova, M.M. Myshlyaev, S. L. Semiatin, Effect of cryogenic temperature and change of strain path on grain refinement during rolling of Cu-30Zn brass, Mater. Des. 86 (2015) 913–921. [8] N. Tao, K. Lu, Dynamic plastic deformation (DPD): a novel technique for synthesizing bulk nanostructured metals, J. Mater. Sci. Technol. 23 (2007) 771–774. [9] T. Konkova, S. Mironov, A. Korznikov, S.L. Semiatin, Microstructural response of pure copper to cryogenic rolling, Acta Mater. 58 (2010) 5262–5273. [10] L. Hou, M. Liu, X. Wang, L. Zhuang, J. Zhang, Cryogenic processing high-strength 7050 aluminum alloy and controlling of the microstructures and mechanical properties, Acta Metall. Sin. 53 (2017) 1075–1090. [11] J. Shi, L. Hou, J. Zuo, L. Zhuang, J. Zhang, Cryogenic rolling-enhanced mechanical properties and microstructural evolution of 5052 Al-Mg alloy, Mater. Sci. Eng. A 701 (2017) 274–284.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheet X.B. Li a, b, c, *, G.M. Jiang a, J.P. Di a, Y. Yang a, d, **, C.L. Wang e a
School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology (Zhangjiagang), Suzhou, 215600, Jiangsu province, PR China Institute of Research of Iron and Steel, Jiangsu Sha-steel Group Co., Ltd., Suzhou, 215625, Jiangsu province, PR China c Division of Engineering Steel, Central Iron and Steel Research Institute, Beijing, 100081, PR China d National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, Jiangsu province, PR China e School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu province, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Cryogenic rolling Dislocations Recrystallization Mechanical properties Twins
The pure copper sheets were rolled with a small strain in one pass at cryogenic and room temperature, respectively. The total thickness was reduced by 55% through three rolling passes. The microstructural evolution and mechanical properties of Cu sheets were investigated by metallographic characterizations, tensile tests and fracture analysis. It is found that several nanoscale mechanical twins are generated in the cryogenically rolled Cu sample. The restricted atomic motion at ultralow temperature inhibits the dislocation slip and then induces the occurrence of grain kinking and mechanical twining. The yield strength and ultimate tensile strength of cryorolled Cu sample are enhanced by the remarkable accumulation of dislocations and mechanical twins. During annealing treatment, the recrystallization behavior is accelerated by the dramatic strain energy and micro structural defects in cryogenically rolled Cu samples. Compared with the recrystallization concentrated on the surface of room temperature rolled Cu sample, a few of recrystallized grains are formed in the central position of cryogenically rolled Cu sample at 180 � C annealing. The rapid progress of thermal recovery during annealing treatment improves the tensile ductility and strain-hardening capacity of cryo-rolled Cu sample. Several pores are formed on the tensile fracture due to the incompatible deformation between the inhomogeneous grains. The results demonstrate that combining the cryogenic rolling and 200 � C annealing is able to improve the mechanical properties of pure copper sheet.
1. Introduction The contradiction between strength and ductility of pure copper with medium stacking fault energy (SFE) restricts the technological applica tion in industries [1,2]. Nowadays, high strength and high ductility can be simultaneously achieved by the modification of chemical composi tion and microstructure [3]. The introduction of nanoscale twins is a potential approach to optimize the comprehensive properties of bulk materials. The severe plastic deformation (SPD) technologies, such as equal channel angular extrusion, accumulative roll bonding and high-pressure torsion, are increasingly used to produce the bulk metals with ultrafine grains or nanostructures [4–6]. However, the efficiency of grain refinement is sharply reduced by the saturation of dislocations and
other structural defects when plastic strain increases up to 10 [7]. Im purities and cracks are likely to be formed in bulk metals through the repeated operations in SPD processes [2]. Compared with the typical SPD processes operated at room tem perature or elevated temperature, the particular deformation in cryo genic condition could prevent the formation of dynamic recovery in bulk metals [8–11]. Several substructures such as dislocation cells and me chanical twins are formed in the cryogenically deformed metals [12,13]. The accumulation of microstructural defects and strain energy makes the processed materials to be metastable, which will influence the recrystallization and phase transformation in heat-treatment process [14,15]. The cryogenic rolling (CR) with extreme strain has been used to
* Corresponding author. School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology (Zhangjiagang), Suzhou, 215600, Jiangsu province, PR China. ** Corresponding author. E-mail addresses: [email protected] (X.B. Li), [email protected] (Y. Yang). https://doi.org/10.1016/j.msea.2019.138811 Received 6 August 2019; Received in revised form 8 December 2019; Accepted 9 December 2019 Available online 13 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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prepare pure Cu sheet with high strength and high ductility [2]. Several micrometer-sized grains are embedded inside a matrix of nanocrystal line and ultrafine (<300 nm) grains. The microstructure gives rise to a remarkable strain-hardening effect to the rolled sample. The 7075 Al alloy processed by cryogenic rolling and ageing treatment can achieve a simultaneous enhancement of ductility and strength, which can be ascribed to the modified microstructure with very small second-phase precipitations in the nanocrystalline [3]. The further grain refinement and remarkable increase of yield strength have been obtained in the pure vanadium undergoing cryorolling with 60% reduction [16]. The Al-Mg-Si alloy can get ultrafine structure after cryorolling with severe plastic strain [17]. It is also found that the considerable internal stress developed in cryorolling promotes the formation of annealing twins in Cu-Al alloy [18]. The cryogenic impact deformation has been used to prepare the bulk ultrafine grained copper [19]. A great deal of nanoscale mechanical twins exists in the elongated ultrafine grains after annealing at 190 � C for 60 min. Moreover, the dynamic plastic deformation at cryogenic temperature can achieve the nanoscale twins with coherent boundaries in pure copper and simultaneously improve the strength, ductility and electrical conductivity [8]. These studies demonstrate that the microstructural evolution during cryogenic deformation undergoes the diverse mechanisms, such as grain division, kinking, shear bands and twinning. The extreme deformation at ultra-low temperature could induce the distinctive transformation of microstructure. However, the deformation mechanism and formation of mechanical twins is not absolutely clear for pure copper during cryogenic defor mation [12,20]. The material behavior and mechanical properties of pure copper produced by small strain rolling in cryogenic condition need much more attentions. It is favorable for the achievement of nanoscale twins in large-sized pure copper with medium SFE using the relatively simple procedure. In this work, the materials behavior and deformation mechanism are studied based on the microstructural characterization of as-rolled copper and annealed samples. The effect of cryogenic rolling on microstructural evolution and mechanical properties is analyzed to acquire the reasonable approach to optimize the pure copper.
recrystallization. The annealing temperature was in the range of 180 � C–220 � C based on the previous study. Annealing treatments were performed in a program-controlled resistance furnace with the deviation of 2 � C. The metallographic observations were taken on the longitudinal section by optical microscope (OM, ZEISS AxioScope A1) to clarify the microstructural evolution. The crystallographic characteristics, such as grain boundary, dislocations and other microstructural defects, were obtained by the transmission electron microscope (TEM, JEM-2100F) operated at 200 kV accelerating voltage. The TEM specimens were cut from the as-rolled sample in the rolling plane. They were prepared through manual grinding and finally thinned by the twin-jet electrolytic polishing. Tensile specimens with five duplications were obtained from the Cu samples using program-controlled spark cutting. The axis of specimen was along the rolling direction. The gauge length was 30 mm according to the national standard GB 228-2010. Tensile tests were conducted on the mechanical testing system UTM5205X at room tem perature. The nominal strain rate was set as 10 3 s 1. Mechanical properties were evaluated from the measurement in tensile tests. The tensile fractures were analyzed through the scanning electron micro scope (SEM, JSM-6510LA). The relationship between microstructure and mechanical performance of the processed Cu samples was analyzed on the basis of experimental results. 3. Experimental results 3.1. Microstructure of the as-rolled Cu sheet Fig. 1 shows the OM metallographic morphology of as-rolled Cu samples. It is found that the samples are composed of the elongated grains and annealing twins. The size and shape of grains are inhomog enous in the thickness direction of rolled sheets. Grains in the sheet surface are evidently rotated to the rolling direction and become much flatter than the grains in central position. Moreover, the grain morphology in the center of CRed-Cu sample is obviously different from that of RTRed-Cu. Some banded grains with zigzag shape are notable in Fig. 1(d). The crystallographic characteristics of as-rolled samples can be ob tained from the TEM micrographs in Fig. 2. The dislocation density of RTRed-Cu sample is found to be less than that of CRed-Cu sample. Dislocation lines are clear and identifiable in Fig. 2(b), but they are highly tangled in CRed-Cu sample. Moreover, the locations indicated by the arrows in Fig. 2(c) are likely to be grain boundaries with the exis tence of dislocation accumulations. Fig. 3(a) demonstrates that the metallographic feature of TEM specimen film of CRed-Cu sample. Banded structures with lamellar spacing in the range of 1 to 2 μm are found in the rolling plane. The boundary between bands is not straight along the rolling direction. The structures can be regarded as the annealing twins with distinct compression after the repeated rolling. The TEM micrograph in Fig. 3(b) presents the existence of parallel stripes with nanoscale spacing in CRedCu sample. High-resolution TEM (HR-TEM) micrograph and selectedarea electron diffraction of the stripes are displayed in Fig. 3(c) and Fig. 3(d), respectively. The atom arrangement proves that the stripes are stacking faults. It is worthy to note that atoms along the marker line in Fig. 3(c) are in accordance with the symmetric relationship. Two sets of lattice spots are found from the electron diffraction pattern in Fig. 3(d). The parallel stripes are proven to be twins [20].
2. Experimental procedure The initial materials were as-received commercial pure copper sheets. After cutting into 30 mm width and 120 mm length, the samples were fully annealed at 600 � C for 120 min. The chemical components and specification were listed in Table 1. The chemical and mechanical methods were used to remove the oxides and contaminates from the surface. The samples with the thickness of 1.5 mm were inserted into the liquid-nitrogen for 20 min and then quickly carried into the roll gap. The rolling processes were conducted in a double-high rolling mill with the diameter of 200 mm. The rolls were also cooled by pouring liquidnitrogen on the surface before rolling. One pass of rolling process was quickly done in about 10 s. The final thickness of sample was 0.68 mm after three rolling passes. The reduction ratio of Cu sample in one pass was 25%, 20% and 20%, respectively. The total reduction was nominally 55%, which was equivalent with the true strain of 0.8. The sample rolled at room temperature was termed as RTRed-Cu, while that rolled at cryogenic condition was CRed-Cu. The rolled samples were then annealed for 30 min to release the residual stress and obtain the Table 1 Chemical components and properties of the initial material with full annealing. Materials
Chemical components, wt. %
Grain size, μm
Yield strength, MPa
Ultimate tensile strength, MPa
Elongation after fracture, %
Commercial pure copper
99.95 0.004 0.003 0.004
45
51.9
216.9
40.3
Cu, Fe, O, Zn
3.2. Microstructure of the rolled Cu sheets during annealing The microstructural evolutions of as-rolled Cu sheets during annealing are shown in Figs. 4–6. It is found that the recrystallization preferentially occurs on the sheet surface rather than the sheet center. The recrystallized grains are generated just near the sheet surface in RTRed-Cu samples when annealing temperature is 180 � C. However, 2
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Fig. 1. OM micrograph on the longitudinal section of as-rolled Cu samples. (Total reduction ratio is 55%) (a) RTRed-Cu sample, (b) magnification of the selected zone by dotted line in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 4(d) indicates that the recrystallization behavior in CRed-Cu sheet is slightly occurred away from the sheet surface. It can be seen from Figs. 5 and Fig. 6 that the recrystallization promptly occurs in the cryogenically-rolled sample with annealing at 200 � C and 220 � C. The deformed grains are completely replaced by the recrystallized grains in the CRed-Cu sample annealed at 220 � C for 30 min. The microstructure is consisted of equiaxed grains with the average size of 9 μm. Never theless, several deformed grains are remained in RTRed-Cu sample due to the incomplete recrystallization. The arrows in Figs. 4 and Fig. 5 reveal that the recrystallization behavior origins from the grain boundaries and twin boundaries.
of the whole plastic deformation of as-rolled sample. However, the tensile ductility and uniform deformation are obviously improved in the Cu samples with annealing treatment. The difference between yield strength and ultimate tensile strength increases with the annealing temperature. The strain-hardening effect of annealed samples is obvious during the tensile loading. The fracture morphologies of Cu samples shown in Fig. 8 to Fig. 11 present the obvious characteristics of micro-porous polymeric fracture. The number of dimples on fracture surface of two kinds of rolled samples increases with the annealing temperature. Nevertheless, as indicated by the arrows in Fig. 8 Figs. 8(c), Fig. 9(c) and Fig. 10(c), some non-uniform pores exist on the fracture surface of Cu samples with cryogenic rolling. It is also found that the dimple size of CRed-Cu sample is larger than that of RTRed-Cu sample with annealing treatment. After annealing at 220 � C, a large number of dimples with fairly uniform size form on the fracture surface of two kinds of Cu samples. However, the dimples un evenly distributes on the fracture of RTRed-Cu sample. Several pores accompanied with smooth area, as shown by the arrows in Fig. 11(b), are formed in the tensile fracture.
3.3. Mechanical properties of the processed Cu samples The experimental results of tensile tests for different Cu samples are listed in Table 2. It is found that the yield strength and ultimate tensile strength evidently increase, while the elongation after fracture sharply decreases after rolling process. The increment of yield strength is much higher than that of ultimate tensile strength. Moreover, the Cu sample with cryogenic rolling are slightly strengthened than that with room temperature rolling. Annealing treatment induces the formation of thermal recovery in asrolled Cu sample. The changes of tensile properties are not significant at 180 � C annealing. When the annealing temperature is at 200 � C, the strengths clearly reduce and the elongation after fracture sharply in creases. Then, the strengths slowly decrease at 220 � C annealing. It can also be found that the CRed-Cu sample and RTRed-Cu sample have different degree of thermal recovery. The annealed sample with cryo genic rolling has lower strengths and higher elongation than that with room temperature rolling. However, the yield ratio of the CRed-Cu sample is obviously smaller than that of another sample when anneal ing temperature is 180 � C and 200 � C. It reveals that the strainhardening capacity is enhanced for the CRed-Cu sample with anneal ing treatment. The typical engineering stress - strain profiles of processed Cu sam ples are shown in Fig. 7. The non-uniform deformation is the main part
4. Discussions 4.1. Effect of cryogenic rolling on the deformation and recrystallization of pure Cu sample The microstructure of as-rolled Cu samples shows that the grains on the sheet surface are clearly elongated along the rolling direction and have the relatively small size. In the present work, there is no lubrication between the work rolls and Cu sheets. The considerable friction induces the stress concentration and then promotes the metal flow on the sheet surface, which results into the inhomogeneous strain distribution in the thickness direction. Moreover, the large deformation on surface zone induces the considerable deformation energy and lattice distortion in the Cu grains. The atomic activities in Cu samples are restricted during cryogenic rolling. The plastic deformation is difficult due to the strongly inhibited 3
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Fig. 2. TEM micrograph of as-rolled Cu samples with total reduction of 55%. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnifiction of the selected zone in (c).
motion of dislocations. Under the rolling pressure, the crystal structure receives a strong external loading. The lattice distortion and vacancies are then largely formed in Cu grains. Besides, the effect of deforminduced heating is not to be neglected in the as-rolled samples. The total reduction ratio is 55% through three passes of rolling. The Cu samples after room temperature rolling is detected to be near 60 � C, while the sample after cryogenic rolling is just near zero Celsius degrees. The limited deform-induced heating in the ultra-low temperature con dition is unfavorable for the dynamic recovery. Therefore, the disloca tions are remarkably tangled in the CRed-Cu sample, as displayed by the TEM micrograph in Fig. 2(c). The cryogenic rolling process induces the remarkable accumulation of dislocations in grain boundaries and twin boundaries. The considerable internal stress and strain energy provides the driving force to the annihilation of dislocations. The motions of vacancies greatly formed in the CRed-Cu sample also accelerate the climb of dislocations. Then, the tangled dislocations in Cu grains are likely to transform into dislocation cells [9]. It is favorable for the for mation of recrystallization in heat treatment. Annealing twins occupies very large proportion in the initial samples with full annealing. Twin boundaries play an important role to block the dislocation motions and directly influence the material behavior. The dislocation accumulation and stress concentration are generated in the boundaries of annealing twins. The further mechanical loading could then cause the kinking of twins to maintain the structural integrity in liquid nitrogen condition. The zigzag structures are consequently formed in the as-rolled sample, as shown in Fig. 1(c). When the twin planes are parallel with the rolling plane, annealing twins will be
compressed into the lamellar structure with a small spacing. Thus, the twins present the banded morphology in the specific section, as shown in the selected zone in Fig. 3(a). Zener-HolIomon (Z) parameter is a crucial index to reflect the effect of strain rate and processing temperature on the microstructure and mechanical properties [21]. The parameter can be calculated by the equation as following, Z¼έ exp(Q/RT) Where έ is defined as the strain rate, Q is the activation energy of plastic deformation, and T is the processing temperature. The Z parameter of cryogenic rolling is found to be obviously higher than that of room temperature rolling, which reveals the difficulty of plastic deformation for pure copper in the ultra-low temperature condition. The workhardening effect is strong and promotes a new deformation mecha nism happened in the cryogenically deformed sample. In other words, mechanical twinning and other deformation modes will generate when Z parameter increase to a critical value. For pure copper with medium SFE, the dislocations tend to dissociate into partial dislocations when slip and climb are strongly inhibited during cryogenic rolling. The expansion of partial dislocations induces the formation of stacking faults in crystal lattice. The nucleation of mechanical twins is then promoted in the cryo-rolled Cu sample, as shown in Fig. 3(c). However, the amount and size of mechanical twins are small under the processing with limited plastic deformation. The mechanical twins formed in cryogenically-deformed Cu sample can provide additional sites for the nucleation of recrystallized grains. 4
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Fig. 3. Microstructural characteristics of the CRed-Cu sample. (a) banded structure in OM micrograph, (b) stacking fault in TEM micrograph, (c) HR-TEM graph of the stacking fault in (b), (d) electron diffraction pattern of the stacking fault.
Fig. 4. OM micrograph on the longitudinal section of Cu samples with 180 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c). 5
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Fig. 5. OM micrograph on the longitudinal section of Cu samples with 200 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 6. OM micrograph on the longitudinal section of Cu samples with 220 � C annealing. (a) RTRed-Cu sample, (b) magnification of the selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Owing to the high storage of strain energy and microstructural defects in CRed-Cu sample, the recrystallization behavior is consequently accel erated at lower annealing temperature. The stress concentration and severe deformation on the sheet surface provide a favorable condition to recrystallization. Thus, the recrystalli zation behavior is accelerated in the surface zone. The phenomenon is obvious in the Cu sample with low temperature annealing because of the
incomplete recrystallization. However, the recrystallization is also found to form in the interior of CRed-Cu sheet with 180 � C annealing. It is ascribed to the reduced gap of strain energy between the surface and central position of as-rolled sample. Therefore, the microstructural uniformity of Cu sheet in thickness direction can be improved by the combination of cryogenic rolling and optimal annealing process [22].
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The as-rolled Cu samples are obviously softened by the thermal re covery during annealing treatment. The strengths quickly decrease due to the elimination of dislocations and other microstructural defects. After annealing at 180 � C for 30 min, the rolled samples receive a low level of recrystallization and are mainly composed of the deformed grains and twins. However, the thermal recovery is obvious in the CRedCu sample due to the prompt activities of microstructural defects. There are few obstacles to pin the dislocations in tensile test. The uncon strained motion of dislocations makes the distinct reduce of the yield strength of CRed-Cu sample. On the other hand, the volume fraction of grain boundaries is not significantly changed due to the limited recrystallization. There is a small difference of ultimate tensile strength between the CRed-Cu sample and RTRed-Cu sample. The recrystallization behavior in CRed-Cu sample is obviously ahead due to the short incubation period of recrystallized nucleus. A lot of recrystallized grains are formed in the center zone away from the sheet surface after 200 � C annealing. The yield strength of CRed-Cu sample is 9.3% lower than the strength of RTRed-Cu sample. Meanwhile, the intense activities of dislocations in strain-free grains bring about the remarkable dislocation accumulation and strain hardening to the CRedCu sample [24]. In addition, a few of equiaxed grains are generated among the elongated grains and increases the volume of grain bound aries in CRed-Cu sample. The inhomogeneous structure induces the asynchrony of plastic deformation and additional internal stress. It is easy for the micro-pores to generate along the grain boundaries. A large number of dimples on the fracture surface mix with several pores, as indicated by the arrows in Fig. 10, suggest the improved ductility and premature micro-cracks. The CRed-Cu sample obtains complete recrystallization at 220 � C annealing. A large number of grain boundaries inhibit the dislocation motions during tensile loading and provide a strengthening to the sample. The yield strength is then slightly reduced for the annealed sample. In addition, some deformed grains are remained in the RTRedCu sample due to the delayed thermal recovery. Thus, the yield strength of CRed-Cu sample is 3.2% lower than that of the RTRed-Cu sample. The inhomogeneous grains in RTRed-Cu sample induce the incompatible deformation between elongated grains and recrystallized grains. The different dimples together with some pores are then formed on the tensile fracture.
Table 2 Mechanical properties of the pure Cu sheets with various processing parameters. Samples
Yield strength σ0.2, MPa
Ultimate tensile strength σb, MPa
Elongation after fracture, %
Full annealing CR RTR CRþ180 � C annealing RTRþ180 � C annealing CRþ200 � C annealing RTRþ200 � C annealing CRþ220 � C annealing RTRþ220 � C annealing
51.9 354.8 342.1 289.8
217.6 358.6 347.0 310.2
40.3 1.9 2.1 6.7
305.1
320.7
5.9
159.5
247.0
24.8
175.8
254.2
23.2
115.3
228.0
35.9
119.1
237.7
35.6
Fig. 7. Engineering stress - strain curves of the processed Cu samples in ten sile test.
5. Conclusions
4.2. Microstructural effect on the tensile properties and fracture behavior
The effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheets has been investigated by the metallographic characterizations, tensile tests and fracture analysis. The main conclusions are shown as follows.
It is known that the high density of tangled dislocations and me chanical twins in deformed metal strongly hinder the dislocation mo tions during tensile test [23]. The effect of strain hardening induces that the yield strength and ultimate tensile strength of the CRed-Cu sample are higher than that of the RTRed-Cu sample. However, the lattice distortion in cryogenically deformed sample is remarkable and brings about the stress concentration. A large number of micro-pores quickly initiate from the boundaries and develop into micro-cracks under the tensile loading. Owing to the restricted motion of dislocations, plastic deformation at the front end of cracks is difficult to occur. The cracks rapidly propagate in the form of quasi-cleavage, resulting into the pre mature fracture and low toughness of tensile specimens. The pores on fracture surface, as indicated by the arrows in Fig. 8(c), is considered to be the origin site of initial cracks. Compared with the strength value of the RTRed-Cu sample, the yield strength increases 3.7% and the ultimate tensile strength increases 3.3% in the CRed-Cu sample. During tensile test, the effect of dislocation strengthening is enhanced by the strong restriction of dislocation mo tions and induces the obvious increase of yield strength. On the other hand, the high density of microstructural defects aggravates the initia tion of micro-pores in cryo-rolled Cu sample. The formation of prema ture cracks destroys the structural integrity of tensile specimen and weakens the increase of ultimate tensile strength of the CRed-Cu sample.
(1) The repeated cryogenic rolling with small strain in one pass is able to provide an equivalent condition of extreme deformation to pure copper sheet. A number of tangled dislocations are accumulated in grain boundaries and twin boundaries. The difficult atomic activities at ultra-low temperature induce the formation of nanoscale mechanical twins. Some zigzag structures are formed in the sheet center due to the kinking of grains and annealing twins. (2) The dramatic strain energy and microstructural defects provide the driving force and nucleation sites for the recrystallization during annealing treatment. The cryogenically deformed Cu sample obtains an evidently accelerated recrystallization behavior and gets a complete recrystallization at 220 � C anneal ing. Compared to the recrystallization concentrated on the sur face of room temperature rolled Cu sample, a few of recrystallized grains are formed in the central position of cryo-rolled Cu sheet at 180 � C annealing. (3) The yield strength and ultimate tensile strength of cryo-rolled Cu sample are enhanced by the remarkable accumulation of 7
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Fig. 8. Tensile fracture of the as-rolled Cu samples. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 9. Tensile fracture of the Cu samples with 180 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
dislocations and mechanical twins. The rapid progress of thermal recovery during annealing treatment improves the tensile ductility and strain-hardening capacity of CRed-Cu sample. Several pores are formed on the tensile fracture due to the stress concentration and premature cracking between the inhomoge neous grains. Combining the cryogenic rolling and 200 � C annealing is able to improve the mechanical properties of pure copper sheet.
Author contributions section X. B. Li: Conceptualization, Methodology, Experiments, WritingDraft preparation, Reviewing and Editing. G. M. Jiang: Experiments. J. B. Di: Experiments. Y. Yang: Conceptualization, Methodology, Writing- Reviewing and Editing. 8
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Fig. 10. Tensile fracture of the Cu samples with 200 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
Fig. 11. Tensile fracture of the Cu samples with 220 � C annealing. (a) RTRed-Cu sample, (b) magnification of selected zone in (a), (c) CRed-Cu sample, (d) magnification of (c).
C. L. Wang: Experiments.
Acknowledgements This work was financially supported by the Natural Science Research Project for Higher Education Institutions in Jiangsu Province (Grant No. 16KJB430012), the Youth Science and Technology Innovation Project in Jiangsu University of Science and Technology (Grant No. 17-5), the National Laboratory of Solid State Microstructures of China (Grant No.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 9
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M31024) and the Innovation Training Program for College Students in Jiangsu Province (Grant No. 201810289013Z). One of the authors Dr. Wang would like to express gratitude to Natural Science Foundation for Young of Jiangsu Province (Grant No. BK20190863), Jiangsu “Mass Innovation and Entrepreneurship” Talent Programme (Shuang Chuang Ph.Ds, 2018)
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