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Effect of electropulsing treatment on microstructure and tensile fracture behavior of nanocrystalline Ni foil
Materials Science & Engineering A 657 (2016) 347–352
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Short communication
Effect of electropulsing treatment on microstructure and tensile fracture behavior of nanocrystalline Ni foil C. Li a,n, H. Tan b, W.M. Wu b, S. Zhao a, H.B. Zhang a a b
The School of Material Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 25 January 2016 Accepted 25 January 2016 Available online 26 January 2016
Electropulsing treatment (EPT) is utilized to increase the plasticity of nanocrystalline Ni foil. Uniaxial tensile tests at room temperature are adopted to investigate the influence of EPT on the mechanical properties under different peak current density. It is shown that electropulsing with certain strength can improve the elongation significantly. The evolution of the fracture morphology, stacking faults, twinning and dislocation of nanocrystalline Ni under EPT is studied via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the plastic deformation mechanism under EPT is also researched. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline Ni foil Electropulsing Microstructure Deformation and fracture
1. Introduction Microforming technology using nanocrystalline foil is one of the most effective ways to resolve the problem of manufacturing complex shell parts at a low cost and in high efficiency in MEMS, which has attracted worldwide attention [1–4]. Generally speaking, some inferior properties of metallic nanocrystalline material such as poor plasticity and poor formability at room temperature will restrict its application greatly. Therefore, lots of researches on how to improve the plasticity and formability of nanocrystalline foil have been carried out. Matsui et al. [5,6] reported that the orientation and additives are important to improve the tensile ductility of electrodeposited bulk nanocrystalline Ni–W alloys. Lian et al. [7,8] indicated that broad grain size distribution and dislocation activity are suggested as being responsible for the enhanced ductility of nanocrystalline Ni. In addition, from the perspective of forming process, some extra process measures could also be taken to solve the above mentioned issues. Previous work indicated that electropulsing treatment (EPT) has a prominent impact on the microstructure and the mechanical properties of metallic materials [9–14]. Conrad et al. [15] proposed that the EPT can promote the superplasticity of 7474 aluminum alloy. Tang et al. [16,17] showed that EPT was an efficient approach for accelerating dynamic recrystallization and significantly increased the elongation at a high strain rate of AZ31 Mg alloy. n
Corresponding author. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.msea.2016.01.075 0921-5093/& 2016 Elsevier B.V. All rights reserved.
However, the current studies are mostly focusing on macrostructural materials while less work has been done on the effect of EPT on metallic nanocrystalline foil. Therefore, high-density EPT is applied to improve the plasticity of the nanocrystalline foil during this research. The effect of EPT on mechanical properties and microstructure of the nanocrystalline Ni foil at room temperature is studied and the plastic deformation mechanism of nanocrystalline foil under EPT is also analyzed.
2. Material and methods The electrodeposited nanocrystalline Ni foils with a thickness of 50 μm are prepared in this experiment. The electrodeposition bath and conditions are set as follows: A nickel sulfamate bath (the value of pH is 3), containing 15 g/L nickel chloride, 300 g/L nickel sulfate, 30 g/L boric acid and 1 g/L saccharin is used to produce Ni foils. Square waves with a duty cycle 50% and peak current density up to 2 A/dm2 are employed during pulse electro-deposition. The deposition temperature is 323 K. A nickel plate of 99.99% purity is used as the anode and a stainless steel plate is used as the cathode. Fig. 1 shows the TEM bright field image of the electrodeposited nanocrystalline Ni. As shown in Fig. 1, the average grain size of the nanocrystalline Ni is approximately 100 nm. Uniaxial tensile tests are performed on an INSTRON-3340 universal testing machine, using the sample of 3 mm width and 10 mm gauge length. The change of gauge length is experimentally measured and calculated the plastic elongation. The tests are carried out at a rate of
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Fig. 1. TEM bright field image (a) and grain size distribution (b) for electrodeposited nanocrystalline Ni.
Fig. 2. The schematic view of EPT.
0.075 mm/min. The dynamic electropulsing is performed on the sample being tensile deformed. The EPT process is schematically shown in Fig. 2. A pulse power supply is applied to discharge multiple pulses with various current densities. As shown in Fig. 2, the form of the pulse current is DC square wave with very high amplitude (maximum value is 345 A) and low duty ratio (duty ratio D¼ ti/tp, minimum value is 0.1%). Such a waveform could generate very high peak current and relatively low root-meansquare current, which not only help to improve the plasticity of the material, but also avoid the heating of the samples. Multiple electropulses are applied on two electric contactors with a distance of 20 mm. Current parameters, including duty ratio (D ¼0.1%) and peak current density (J¼ 0, 0.6 103, 1.3 103, 1.65 103, 1.85 103, 2.3 103 A/mm2) are controlled by the pulse power supply. Due to the small root-mean-square current density (Jm ¼ 0, 0.6, 1.3, 1.65, 1.85, 2.3 A/mm2) and the short duration time of the tests ( o600 s), the surface temperature of the samples would not increase too much ( o323 K). Six specimen groups are designed and three duplicate specimens are arranged in each group. Tensile fracture is examined by HITACHI S-4800 scanning
electron microscope and microstructure is observed by CM-12 transmission electron microscopy.
3. Results and discussion The results of the EPT and non-EPT samples based on uniaxial tensile tests at room temperature are shown in Fig. 3. As shown in Fig. 3(b), the elongation of the EPT samples has been improved properly comparing with the non-EPT samples, and also with the increasing of electropulsing current density, the sample elongation increases gradually. But with the increasing of pulse current density, the elongation raises nonlinearly while the tensile strength inclines to decrease. As shown in Fig. 3(b), the elongation curve could be roughly divided into three stages. (1) under the conditions of the small current density (0.6 102 A/mm2, 1.3 103 A/mm2), the elongation increases moderately, while the intensity decreases slightly; (2) with the current density increasing (1.65 103 A/mm2), the curve slope becomes smaller, while the intensity of the sample suddenly increases reversely; (3) while
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Fig. 3. Stress–strain curves (a) and effects of pulse peak current density on elongation and tensile strength (b) of tensile samples.
the current density reaches a certain value (1.85 103 A/mm2, 2.3 103 A/mm2), the elongation of the samples has a significant increase, at the same time the intensity decreases significantly. Especially when J ¼2.3 103 A/mm2, the elongation mount up to 11.26% and improved by 66.3% compared with that of the non-EPT samples. From the above results, it is inferred that the electropulsing treatment can significantly improve the plasticity of nanocrystalline Ni foil at room temperature, and this promotion effect is nonlinear. Fig. 4 illustrates the SEM micrographs of fracture characteristics of the tensile samples. While there is no electropulsing current (Fig. 4(a)) or the current density is small (Fig. 4(b)), the fractures consists of numbers of shallow and small dimples, and some cleavage steps, cleavage planes, tearing edges. To the dimple of macrostructural metal, there usually exist second phase particles and inclusions at the bottom of dimple, and the dimples usually extend along the interface of two-phase. But for the nanocrystalline dimple, it is different. The dimples of nanocrystalline grain metal are formed by the way of transgranular fracture, and usually extend along the certain cleavage plane. Therefore, the fracture types of the nanocrystalline grain Ni foil could not be defined as the typical ductile fracture, though numbers of dimple could be observed. To be more precise, the fracture could be classified as a mixture of ductile fracture and brittle cleavage fracture. With the increasing of the current density, fracture morphology changes slightly. The main change could be summarized as, the dimple depth becomes deeper, the dimple size increases, and the proportion of the dimple increases. But for the overall morphology of the fracture, the fracture type has not been changed, and it is still classified as a mixture of ductile fracture and brittle cleavage fracture. Just as the current density increases, the proportion of ductile fracture increases, and the elongation of the sample increases too. These results are in good agreement with the elongation variation trend concluded from the uniaxial tensile test. It is indicated that due to the introduction of pulse current, the plastic deformation capacity is improved as well. TEM micrographs of the samples near the tensile fracture under different conditions are shown as Fig. 5 (vertical direction is the stretching direction). The grain size of sample is not uniform, and the size of a part of grains is only several tens of nanometers. To these small grains, it could be observed that the volume fraction of grain boundaries and triple junctions is larger compare with the coarse-grained material, and little defects could be observed inside the small grains, which inevitably lead to the difficulty of the dislocation nucleation and nucleation. On the other hand, owing to the lack of pace of dislocation motion, the dislocations could hardly move inside the small grains. Therefore, very little dislocations can be observed at the inner of the small grains, and the
dislocations can only be observed at the boundaries of grain and the inner of big grains (usually more than 100 nm). Therefore, it could be surmised that dislocation movement inside the grains would only play a part of role during the plastic deformation under this condition [18,19]. On the contrary, the dislocation move relatively easily at grain boundary, which would result in the sliding of grain boundary, and cause the grain rotation. In addition, some studies [20,21] have indicated that the plastic deformation of nanocrystalline materials could be carried out by the grain rotation and grain boundary movement. Therefore, it is not surprising to infer that grain rotation and grain boundary movement of small grains also play a part of role during the plastic deformation. To sum up, the deformation mechanism belongs to the crossover regime of a continuous transition from dislocation to GB-mediated mechanism owing to the small grains [22]. That is to say that both the dislocation slip and the GB-mediated (grain rotation, grain boundary migration and sliding) play a role during the deformation process. The smaller the grain size and the more the number of small grains the greater the proportion of GB-mediated. While no electropulsing or electropulsing current density is small (as shown in Fig. 5(a) and (b)), in correspondence with the first stage of tensile curve shown in Fig. 3), only a little dislocations could be observed inside the grains even if the grain size is more than 100 nm. This indicates that the density of the electropulsing current is not high enough to promote the activation and motion of dislocations inside the grains. That is the reason of the lower elongation under such deformation condition. In contrast, the dislocation motion may be promoted by the electropulsing current at grains boundary owing to the low motion resistance. In general, it is recognized that grain rotation and grain boundary sliding can be achieved through the atomic diffusion and dislocation movement at the grain boundary. Therefore, the grain boundary sliding and grain rotation could be accelerated by the athermal effect of electropulsing, which could accelerate the atomic diffusion and dislocation movement [23]. What's more, as a matter of fact that the elongation of EPT samples is higher than the non-EPT ones, it can be naturally concluded that the grain rotation and grain boundary movement of the small grains are promoted due to the pulse current, which is one of the mechanisms of plasticity being promoted by pulse current for nanocrystalline Ni foil. With electropulsing current is applied and current density increases, the increasing of deformation twins could be observed in the EPT samples (Fig. 5(b) and (c)). It can be inferred that the formation of the deformation twinning is promoted due to the introduction of pulse current. A certain number of twins inside the samples play a role in the plastic deformation, and on the other hand, they enhance the interaction of dislocation with twinning. More slip systems could be activated and more dislocations could
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Fig. 4. SEM micrographs of fracture of the tensile samples under different conditions: (a) J ¼0; (b) J ¼6 102 A/mm2; (c) J¼ 1.3 103 A/mm2; (d) J¼ 1.65 103 A/mm2; (e) J¼ 1.85 103 A/mm2; (f) J ¼2.3 103 A/mm2.
be accommodated by the reason of that the deformation twinning change a part of grain crystal orientation [24]. Therefore, the plasticity of nanocrystalline Ni foil could be further improved by electropulsing current. With further increasing of the current density, dislocation movement could be further improved under the effect of electron wind caused by pulse current. Therefore, more and more dislocations source can be activated, and then a large number of dislocations can be observed in the relative large grain (Fig. 5(d)). A large number of dislocations tangle with each other and then form the dislocation cells and nets during the motion. Lots of activating and moving dislocation leads to the promotion of the elongation, and dislocations tangle and nets would lead to the increasing of the strength. These results are in good agreement with the elongation and intensity variation trend indicated as the
second stage of the tensile curve as shown in Fig. 3(b) (elongation increasing and intensity increasing). While the current density is further improved, stacking fault could be observed in the sample (Fig. 5(e)). In generally, it is almost impossible that stacking faults could be observed in a metal with such high stacking fault energy as pure nickel. Therefore, it can be inferred that the formation of stacking faults has been promoted by the applying of electropulsing current. For nanocrystalline materials, the stacking fault has an important impact on the dislocation and twinning. This wrong row between atoms can improve the movement ability of dislocations to some extent, and prompt the formation of partial dislocations at the grain boundary, and increase the storage capacity of dislocations, thereby increase the plasticity of the sample [25–27]. While the electropulsing current density is further improved,
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Fig. 5. TEM micrographs of the samples near the tensile fracture under different conditions: (a) J¼ 0; (b) J ¼6 102 A/mm2; (c) J ¼1.3 103 A/mm2; (d) J ¼1.65 103 A/mm2; (e) J¼ 1.85 103 A/mm2; (f) J¼ 2.3 103 A/mm2.
the dislocation density begins to decrease in the larger grains of samples, and very little dislocation tangles, cells and nets could be observed (as shown in Fig. 5(f)). It is mainly due to the strong electron wind caused by pulse current not only activate the dislocations source, but also promote the movement of dislocations even more. While the current density reaches a certain value
(1.85 103 A/mm2, 2.3 103 A/mm2), electropulsing helps migration rather than the generation of dislocation. High dislocation mobility will cause a fast annihilation of dislocations and avoid the tangle of dislocations. So then the elongation of the samples has been greatly improved under these conditions. To sum up, the plasticity of nanocrystalline Ni foil could be
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improved by the applying of pulse current effectively. This promotion effect could be attributed to the interaction of the microstructure (dislocation, crystal lattice, atom and so on) with a large number of directional movement electrons. Moreover, it could be also inferred that the main deformation mechanism under EPT is difference while the pulse current density and/or the grain size is difference. Generally, while the current density and/or the grain size are relatively small, the main deformation mechanism of nanocrystalline Ni foil under EPT is grain rotation and grain boundary sliding of small grains promoted by pulse current. While the current density is relatively high and the size of the grain is relatively larger, the main deformation mechanism is the dislocation sliding in bigger grains promoted by the pulse current.
4. Conclusion EPT has an effect on the plasticity of nanocrystalline Ni foil. While peak pulse current density reaches to 2.3 103 A/mm2, the elongation mounts up to 11.26% and is improved by 66.3% compared with that of the non-EPT samples. The deformation mechanisms of the EPT samples including: (1) the grain rotation and grain boundary sliding; (2) the formation of deformation twinning; (3) the dislocation sliding in bigger grains. All of the above mechanisms are reinforced by the introduction of pulse current, which causes the improvement of the plasticity of nanocrystalline Ni foil under EPT.
Acknowledgment This work was supported by “National Natural Science Foundation of China” (51305110) and “The fund of the State Key Laboratory of Solidification Processing in NWPU” (SKLSP201320).
The authors would like to take this opportunity to express their sincere appreciation.
References [1] K.C. Chan, G.F. Wang, C.L. Wang, et al., Mater. Sci. Eng. A 404 (2005) 108–116. [2] S. Ding, H.J. Lu, K.F. Zhang, J. Wuhan Univ. Technol. (Mater. Sci Ed) 01 (2011) 56–60. [3] M.W. Fu, W.L. Chan, Int. J. Adv. Manuf. Technol. 67 (2013) 2411–2437. [4] V.K. Jain, A. Sidpara, R. Balasubramaniam, et al., Proc. Inst. Mech. Eng. 228 (2014) 973–1014. [5] Isao Matsui, Yorinobu Takigawa, Tokuteru Uesugi, et al., Mater. Sci. Eng. A578 (2013) 318–322. [6] Isao Matsui, Yorinobu Takigawa, Tokuteru Uesugi, et al., Mater. Lett. 99 (2013) 65–67. [7] X.X. Shen, J.S. Lian, Z.H. Jiang, et al., Mater. Sci. Eng. A 478 (2008) 410–416. [8] C.D. Gu, J.S. Lian, Z.H. Jiang, et al., Scr. Mater. 54 (2006) 579–584. [9] Y.B. Jiang, G.Y. Tang, C. Shek, Appl. Phys. A 97 (2009) 607–615. [10] Q. Xu, L. Guan, Y.B. Jiang, G.Y. Tang, S.N. Wang, Mater. Lett. 64 (2010) 1085–1087. [11] Y.H. Zhu, T. Suet, X.M. Liu, G.L. Hu, Metall. Mater. Trans. A 42 (2011) 1933–1940. [12] Y. Liu, L. Wang, H.Y. Liu, B.J. Zhang, G.P. Zhao, Trans. Nonferrous Met. Soc. China 09 (2011) 1970–1975. [13] P. Yiu, C.H. Hsueh, C.H. Shek, Mater. Lett. 136 (2014) 353–355. [14] X.X. Ye, Y.Y. Yang, G.L. Song, G.Y. Tang, Appl. Phys. A 117 (2014) 2251–2264. [15] H. Conrad, Scr. Metall. 23 (1989) 697–703. [16] Q. Xu, L. Guan, G.Y. Tang, et al., Mater. Lett. 64 (2010) 1085–1087. [17] Q. Xu, G.Y. Tang, Y.B. Jiang, Mater. Sci. Eng. A 528 (2011) 4431–4436. [18] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater. Sci. 51 (2006) 427–556. [19] T. Zhou, U. Erba, K.T. Austa, Scr. Mater. 48 (2003) 825–830. [20] H.V. Swygenhoven, P.M. Derlet, Phys. Rev. B 64 (2001) 224105. [21] F.D. Torrea, H.V. Swygenhovena, M. Victoriab, Acta Mater. 50 (2002) 3957–3970. [22] J.R. Greer, J.T.M.D. Hosson, Prog. Mater. Sci. 56 (2011) 654–724. [23] R.F. Zhu, G.Y. Tang, S.Q. Shi, M.W. Fu, V.E. Gromov, Appl. Phys. A 111 (2013) 1195–1201. [24] X.H. An, S.D. Wu, Z.F. Zhang, Acta Metall. Sin. 50 (2014) 191–201. [25] R. Jamaati, M.R. Toroghinejad, Mater. Sci. Eng. A 606 (2014) 443–450. [26] X.Y. San, X.G. Liang, L.P. Chen, Z.L. Xia, X.K. Zhu, Mater. Sci. Eng. A 528 (2011) 7867–7870. [27] Y.L. Gong, C.E. Wen, Y.C. Li, et al., Sci. Eng. A 569 (2013) 144–149.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Short communication
Effect of electropulsing treatment on microstructure and tensile fracture behavior of nanocrystalline Ni foil C. Li a,n, H. Tan b, W.M. Wu b, S. Zhao a, H.B. Zhang a a b
The School of Material Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 25 January 2016 Accepted 25 January 2016 Available online 26 January 2016
Electropulsing treatment (EPT) is utilized to increase the plasticity of nanocrystalline Ni foil. Uniaxial tensile tests at room temperature are adopted to investigate the influence of EPT on the mechanical properties under different peak current density. It is shown that electropulsing with certain strength can improve the elongation significantly. The evolution of the fracture morphology, stacking faults, twinning and dislocation of nanocrystalline Ni under EPT is studied via scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the plastic deformation mechanism under EPT is also researched. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline Ni foil Electropulsing Microstructure Deformation and fracture
1. Introduction Microforming technology using nanocrystalline foil is one of the most effective ways to resolve the problem of manufacturing complex shell parts at a low cost and in high efficiency in MEMS, which has attracted worldwide attention [1–4]. Generally speaking, some inferior properties of metallic nanocrystalline material such as poor plasticity and poor formability at room temperature will restrict its application greatly. Therefore, lots of researches on how to improve the plasticity and formability of nanocrystalline foil have been carried out. Matsui et al. [5,6] reported that the orientation and additives are important to improve the tensile ductility of electrodeposited bulk nanocrystalline Ni–W alloys. Lian et al. [7,8] indicated that broad grain size distribution and dislocation activity are suggested as being responsible for the enhanced ductility of nanocrystalline Ni. In addition, from the perspective of forming process, some extra process measures could also be taken to solve the above mentioned issues. Previous work indicated that electropulsing treatment (EPT) has a prominent impact on the microstructure and the mechanical properties of metallic materials [9–14]. Conrad et al. [15] proposed that the EPT can promote the superplasticity of 7474 aluminum alloy. Tang et al. [16,17] showed that EPT was an efficient approach for accelerating dynamic recrystallization and significantly increased the elongation at a high strain rate of AZ31 Mg alloy. n
Corresponding author. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.msea.2016.01.075 0921-5093/& 2016 Elsevier B.V. All rights reserved.
However, the current studies are mostly focusing on macrostructural materials while less work has been done on the effect of EPT on metallic nanocrystalline foil. Therefore, high-density EPT is applied to improve the plasticity of the nanocrystalline foil during this research. The effect of EPT on mechanical properties and microstructure of the nanocrystalline Ni foil at room temperature is studied and the plastic deformation mechanism of nanocrystalline foil under EPT is also analyzed.
2. Material and methods The electrodeposited nanocrystalline Ni foils with a thickness of 50 μm are prepared in this experiment. The electrodeposition bath and conditions are set as follows: A nickel sulfamate bath (the value of pH is 3), containing 15 g/L nickel chloride, 300 g/L nickel sulfate, 30 g/L boric acid and 1 g/L saccharin is used to produce Ni foils. Square waves with a duty cycle 50% and peak current density up to 2 A/dm2 are employed during pulse electro-deposition. The deposition temperature is 323 K. A nickel plate of 99.99% purity is used as the anode and a stainless steel plate is used as the cathode. Fig. 1 shows the TEM bright field image of the electrodeposited nanocrystalline Ni. As shown in Fig. 1, the average grain size of the nanocrystalline Ni is approximately 100 nm. Uniaxial tensile tests are performed on an INSTRON-3340 universal testing machine, using the sample of 3 mm width and 10 mm gauge length. The change of gauge length is experimentally measured and calculated the plastic elongation. The tests are carried out at a rate of
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Fig. 1. TEM bright field image (a) and grain size distribution (b) for electrodeposited nanocrystalline Ni.
Fig. 2. The schematic view of EPT.
0.075 mm/min. The dynamic electropulsing is performed on the sample being tensile deformed. The EPT process is schematically shown in Fig. 2. A pulse power supply is applied to discharge multiple pulses with various current densities. As shown in Fig. 2, the form of the pulse current is DC square wave with very high amplitude (maximum value is 345 A) and low duty ratio (duty ratio D¼ ti/tp, minimum value is 0.1%). Such a waveform could generate very high peak current and relatively low root-meansquare current, which not only help to improve the plasticity of the material, but also avoid the heating of the samples. Multiple electropulses are applied on two electric contactors with a distance of 20 mm. Current parameters, including duty ratio (D ¼0.1%) and peak current density (J¼ 0, 0.6 103, 1.3 103, 1.65 103, 1.85 103, 2.3 103 A/mm2) are controlled by the pulse power supply. Due to the small root-mean-square current density (Jm ¼ 0, 0.6, 1.3, 1.65, 1.85, 2.3 A/mm2) and the short duration time of the tests ( o600 s), the surface temperature of the samples would not increase too much ( o323 K). Six specimen groups are designed and three duplicate specimens are arranged in each group. Tensile fracture is examined by HITACHI S-4800 scanning
electron microscope and microstructure is observed by CM-12 transmission electron microscopy.
3. Results and discussion The results of the EPT and non-EPT samples based on uniaxial tensile tests at room temperature are shown in Fig. 3. As shown in Fig. 3(b), the elongation of the EPT samples has been improved properly comparing with the non-EPT samples, and also with the increasing of electropulsing current density, the sample elongation increases gradually. But with the increasing of pulse current density, the elongation raises nonlinearly while the tensile strength inclines to decrease. As shown in Fig. 3(b), the elongation curve could be roughly divided into three stages. (1) under the conditions of the small current density (0.6 102 A/mm2, 1.3 103 A/mm2), the elongation increases moderately, while the intensity decreases slightly; (2) with the current density increasing (1.65 103 A/mm2), the curve slope becomes smaller, while the intensity of the sample suddenly increases reversely; (3) while
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Fig. 3. Stress–strain curves (a) and effects of pulse peak current density on elongation and tensile strength (b) of tensile samples.
the current density reaches a certain value (1.85 103 A/mm2, 2.3 103 A/mm2), the elongation of the samples has a significant increase, at the same time the intensity decreases significantly. Especially when J ¼2.3 103 A/mm2, the elongation mount up to 11.26% and improved by 66.3% compared with that of the non-EPT samples. From the above results, it is inferred that the electropulsing treatment can significantly improve the plasticity of nanocrystalline Ni foil at room temperature, and this promotion effect is nonlinear. Fig. 4 illustrates the SEM micrographs of fracture characteristics of the tensile samples. While there is no electropulsing current (Fig. 4(a)) or the current density is small (Fig. 4(b)), the fractures consists of numbers of shallow and small dimples, and some cleavage steps, cleavage planes, tearing edges. To the dimple of macrostructural metal, there usually exist second phase particles and inclusions at the bottom of dimple, and the dimples usually extend along the interface of two-phase. But for the nanocrystalline dimple, it is different. The dimples of nanocrystalline grain metal are formed by the way of transgranular fracture, and usually extend along the certain cleavage plane. Therefore, the fracture types of the nanocrystalline grain Ni foil could not be defined as the typical ductile fracture, though numbers of dimple could be observed. To be more precise, the fracture could be classified as a mixture of ductile fracture and brittle cleavage fracture. With the increasing of the current density, fracture morphology changes slightly. The main change could be summarized as, the dimple depth becomes deeper, the dimple size increases, and the proportion of the dimple increases. But for the overall morphology of the fracture, the fracture type has not been changed, and it is still classified as a mixture of ductile fracture and brittle cleavage fracture. Just as the current density increases, the proportion of ductile fracture increases, and the elongation of the sample increases too. These results are in good agreement with the elongation variation trend concluded from the uniaxial tensile test. It is indicated that due to the introduction of pulse current, the plastic deformation capacity is improved as well. TEM micrographs of the samples near the tensile fracture under different conditions are shown as Fig. 5 (vertical direction is the stretching direction). The grain size of sample is not uniform, and the size of a part of grains is only several tens of nanometers. To these small grains, it could be observed that the volume fraction of grain boundaries and triple junctions is larger compare with the coarse-grained material, and little defects could be observed inside the small grains, which inevitably lead to the difficulty of the dislocation nucleation and nucleation. On the other hand, owing to the lack of pace of dislocation motion, the dislocations could hardly move inside the small grains. Therefore, very little dislocations can be observed at the inner of the small grains, and the
dislocations can only be observed at the boundaries of grain and the inner of big grains (usually more than 100 nm). Therefore, it could be surmised that dislocation movement inside the grains would only play a part of role during the plastic deformation under this condition [18,19]. On the contrary, the dislocation move relatively easily at grain boundary, which would result in the sliding of grain boundary, and cause the grain rotation. In addition, some studies [20,21] have indicated that the plastic deformation of nanocrystalline materials could be carried out by the grain rotation and grain boundary movement. Therefore, it is not surprising to infer that grain rotation and grain boundary movement of small grains also play a part of role during the plastic deformation. To sum up, the deformation mechanism belongs to the crossover regime of a continuous transition from dislocation to GB-mediated mechanism owing to the small grains [22]. That is to say that both the dislocation slip and the GB-mediated (grain rotation, grain boundary migration and sliding) play a role during the deformation process. The smaller the grain size and the more the number of small grains the greater the proportion of GB-mediated. While no electropulsing or electropulsing current density is small (as shown in Fig. 5(a) and (b)), in correspondence with the first stage of tensile curve shown in Fig. 3), only a little dislocations could be observed inside the grains even if the grain size is more than 100 nm. This indicates that the density of the electropulsing current is not high enough to promote the activation and motion of dislocations inside the grains. That is the reason of the lower elongation under such deformation condition. In contrast, the dislocation motion may be promoted by the electropulsing current at grains boundary owing to the low motion resistance. In general, it is recognized that grain rotation and grain boundary sliding can be achieved through the atomic diffusion and dislocation movement at the grain boundary. Therefore, the grain boundary sliding and grain rotation could be accelerated by the athermal effect of electropulsing, which could accelerate the atomic diffusion and dislocation movement [23]. What's more, as a matter of fact that the elongation of EPT samples is higher than the non-EPT ones, it can be naturally concluded that the grain rotation and grain boundary movement of the small grains are promoted due to the pulse current, which is one of the mechanisms of plasticity being promoted by pulse current for nanocrystalline Ni foil. With electropulsing current is applied and current density increases, the increasing of deformation twins could be observed in the EPT samples (Fig. 5(b) and (c)). It can be inferred that the formation of the deformation twinning is promoted due to the introduction of pulse current. A certain number of twins inside the samples play a role in the plastic deformation, and on the other hand, they enhance the interaction of dislocation with twinning. More slip systems could be activated and more dislocations could
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Fig. 4. SEM micrographs of fracture of the tensile samples under different conditions: (a) J ¼0; (b) J ¼6 102 A/mm2; (c) J¼ 1.3 103 A/mm2; (d) J¼ 1.65 103 A/mm2; (e) J¼ 1.85 103 A/mm2; (f) J ¼2.3 103 A/mm2.
be accommodated by the reason of that the deformation twinning change a part of grain crystal orientation [24]. Therefore, the plasticity of nanocrystalline Ni foil could be further improved by electropulsing current. With further increasing of the current density, dislocation movement could be further improved under the effect of electron wind caused by pulse current. Therefore, more and more dislocations source can be activated, and then a large number of dislocations can be observed in the relative large grain (Fig. 5(d)). A large number of dislocations tangle with each other and then form the dislocation cells and nets during the motion. Lots of activating and moving dislocation leads to the promotion of the elongation, and dislocations tangle and nets would lead to the increasing of the strength. These results are in good agreement with the elongation and intensity variation trend indicated as the
second stage of the tensile curve as shown in Fig. 3(b) (elongation increasing and intensity increasing). While the current density is further improved, stacking fault could be observed in the sample (Fig. 5(e)). In generally, it is almost impossible that stacking faults could be observed in a metal with such high stacking fault energy as pure nickel. Therefore, it can be inferred that the formation of stacking faults has been promoted by the applying of electropulsing current. For nanocrystalline materials, the stacking fault has an important impact on the dislocation and twinning. This wrong row between atoms can improve the movement ability of dislocations to some extent, and prompt the formation of partial dislocations at the grain boundary, and increase the storage capacity of dislocations, thereby increase the plasticity of the sample [25–27]. While the electropulsing current density is further improved,
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Fig. 5. TEM micrographs of the samples near the tensile fracture under different conditions: (a) J¼ 0; (b) J ¼6 102 A/mm2; (c) J ¼1.3 103 A/mm2; (d) J ¼1.65 103 A/mm2; (e) J¼ 1.85 103 A/mm2; (f) J¼ 2.3 103 A/mm2.
the dislocation density begins to decrease in the larger grains of samples, and very little dislocation tangles, cells and nets could be observed (as shown in Fig. 5(f)). It is mainly due to the strong electron wind caused by pulse current not only activate the dislocations source, but also promote the movement of dislocations even more. While the current density reaches a certain value
(1.85 103 A/mm2, 2.3 103 A/mm2), electropulsing helps migration rather than the generation of dislocation. High dislocation mobility will cause a fast annihilation of dislocations and avoid the tangle of dislocations. So then the elongation of the samples has been greatly improved under these conditions. To sum up, the plasticity of nanocrystalline Ni foil could be
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improved by the applying of pulse current effectively. This promotion effect could be attributed to the interaction of the microstructure (dislocation, crystal lattice, atom and so on) with a large number of directional movement electrons. Moreover, it could be also inferred that the main deformation mechanism under EPT is difference while the pulse current density and/or the grain size is difference. Generally, while the current density and/or the grain size are relatively small, the main deformation mechanism of nanocrystalline Ni foil under EPT is grain rotation and grain boundary sliding of small grains promoted by pulse current. While the current density is relatively high and the size of the grain is relatively larger, the main deformation mechanism is the dislocation sliding in bigger grains promoted by the pulse current.
4. Conclusion EPT has an effect on the plasticity of nanocrystalline Ni foil. While peak pulse current density reaches to 2.3 103 A/mm2, the elongation mounts up to 11.26% and is improved by 66.3% compared with that of the non-EPT samples. The deformation mechanisms of the EPT samples including: (1) the grain rotation and grain boundary sliding; (2) the formation of deformation twinning; (3) the dislocation sliding in bigger grains. All of the above mechanisms are reinforced by the introduction of pulse current, which causes the improvement of the plasticity of nanocrystalline Ni foil under EPT.
Acknowledgment This work was supported by “National Natural Science Foundation of China” (51305110) and “The fund of the State Key Laboratory of Solidification Processing in NWPU” (SKLSP201320).
The authors would like to take this opportunity to express their sincere appreciation.
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