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Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension
Author’s Accepted Manuscript Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension Cong Yan, Ning Li, Huawen Jiang, Duzhen Wang, Lin Liu www.elsevier.com
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S0921-5093(15)00465-7 http://dx.doi.org/10.1016/j.msea.2015.04.058 MSA32282
To appear in: Materials Science & Engineering A Received date: 16 December 2014 Revised date: 19 April 2015 Accepted date: 20 April 2015 Cite this article as: Cong Yan, Ning Li, Huawen Jiang, Duzhen Wang and Lin Liu, Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.04.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension Cong Yan, Ning Li*, Huawen Jiang, Duzhen Wang, Lin Liu School of Materials Science and Engineering and State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, PRC
Authors to whom correspondence should be addressed. Tel.: +86 27 87559606; Fax: +86 27 87554405.
E-mail adress: [email protected] (Ning Li).
Abstract The effect of electropulsing on the deformation behavior, texture and microstructure of 5A02 aluminum alloy was investigated through uniaxial tension, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The Portevin-Le Chatelier (PLC) effect reflected as the serrated characteristic in stress-strain curves, became conspicuous firstly but then disappeared with further increase of electropulsing intensity. The texture analysis exhibited that the electropulsing causes an increase of Cube texture, accompanied with a reduction of S texture. Microstructure characterization revealed a transition of slipping mode from planar slip to wave slip with increasing electropulsing intensity. The temperature rise induced by electropulsing, together with the influence of solute atoms, was proposed to rationalize the present phenomena in detail. Keywords: Aluminum alloy; Electropulsing; Deformation behavior; Microstructure.
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1. Introduction Innovative techniques are attractive in breakthrough the bottleneck of materials processing, improving materials’ ultimate microstructures and properties. For instance, the thermoplastic forming provides a promising technique
in net-shaping metallic glasses [1-6].
Electromagnetic forming exhibits favourable applications in various industries due to its unique characteristics include contactless forming, low springback and enhanced forming limit etc. [7-11] with different deformation mechanism by comparison with that under traditional high speed mechanical processing [12]. Electropulsing treatment (EPT), as an instantaneous high-energy input method, has also attracted enduring attention due to its superiority in the enhancement of plasticity of brittle metallic materials [13-21] and evaluation of microstructure evolution [22-27]. For example, the electropulsing could reduce the forming stress and increase the plastic strain of metallic materials through enhancing the dislocation mobility [20,21] and promoting the homogeneous distribution of dislocations [16,17]. The EPT could also improve the atomic diffusion capacity, such as accelerating the dissolution of β-Mg17Al12 phase (a detrimental phase to the plasticity of AZ91) in Mg-9Al-Zn (AZ91) alloy, facilitating the large elongation [13-15]. The enhancement of atomic diffusion originating from temperature and thermal stress caused by electropulsing [28-30], favors the healing of microcracks and then improves the plasticity of the pre-deformed titanium alloys [18,19]. In addition, the improvement of atomic diffusion induced by electropulsing also increases the dislocation velocity and sub-grains’ growth rate, causing the recrystallized grains with a relatively small size by comparison with those underwent conventional annealing [15,24]. 2
It is notable that the above researches of EPT mainly focused on the improvement of plasticity of brittle metallic materials, but scarcely emphasized on the deformation behavior that is essentially determined by microstructure. Consequently, the microstructure evolution of metallic alloys during EPT should be deeply understood, especially in solid solution alloys, wherein solute atoms usually impede the motion of dislocations, resulting in the occurrence of dynamic strain ageing (DSA) under certain conditions [31,32]. In the case of DSA, dislocations repeatedly break away and capture from solute atoms, leading to the negative strain–rate sensitivity (nSRS) and resulting in serrated features in stress-strain curves, known as the Portevin-Le Chatelier (PLC) effect [31-37]. The reported literatures have shown that the PLC is related to the temperatures and strain rates due to the competition between the diffusion rate of solute atoms and the velocity of dislocations [35-37]. However, whether the electropulsing can affect the interaction between solute atoms and dislocations and result in various PLC effect under different intensities, and what are the possible reasons for this phenomena, are questions still remaining unanswered. In the present study, a typical Al-Mg (2.5 wt. % Mg) solid solution (5A02 aluminum alloy) was selected as the research object. The effect of electropulsing with various intensities on the deformation behavior of the aluminum alloy during uniaxial tension was investigated. The texture and microstructure of the deformed specimens were also characterized by EBSD and TEM, respectively. The results exhibit that the deformation behavior, texture and microstructure of 5A02 aluminum alloy are seriously affected by the intensities of electropulsing. The effect of electropulsing on the interaction between solute atoms and dislocations is understood deeply on the basis of the segregation of solute atoms around 3
dislocations.
2. Experimental Procedure
2.1 Material and uniaxial tensile testing The 5A02-O aluminum alloy [38,39] used in this work was provided by the Southwest Aluminum Company (China) as 2.0 mm thick sheet in a recrystallized condition. The nominal chemical composition (wt. %) of the alloy is shown in Table 1, which reveals that there are some solute atoms such as Mg, Si and Fe. The dog-bone-shaped specimens with thickness of 2.0 mm and gauge length of 50.0 mm were cut from the received aluminum alloy sheet using an electro-spark machining under oil cooling condition. Uniaxial tension was conducted by using a universal testing machine (RGM-4050) at ambient temperature with a constant strain rate of 1×10-3 s-1. During whole tensile testing, electropulsing with various intensities (10A, 20A, 30A, 40A, 50A) was applied, as sketched in Fig. 1, wherein the surface of steel clamp was treated by insulating to avoid the effect of electropulsing on the transducer. The frequency of 90Hz and duty cycle of 25% were selected, corresponding to electropulsing duration of about 2780 s. The temperature rise of specimens during deformation was measured by an infrared thermometer (FLIR A320), and calibrated with a thermocouple mounted on the specimen’s surface, as depicted in Fig. 1.
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2.2 Texture and microstructure characterization The texture of the deformed 5A02 aluminum alloy under various electropulsing intensities was probed by electron backscattered diffraction (EBSD) that is conducted using a field emission scanning electron microscope (FSEM, FEI Sirion 200) equipped with an EBSD detector. Texture characterization was performed on the plane parallel to the tensile direction, with a step size of 0.2 m. The samples for EBSD investigation were first mechanical grinding and subsequently electrolytic polishing in a 5:1 alcohol-perchloric acid solution using a 316 stainless steel as cathode at 20 V for 60 s at room temperature. The microstructure of the deformed aluminum alloy samples was further characterized by transmission electron microscope (TEM, FEI Tecnai G20). TEM thin foils were first mechanically ground to about 50 m thick, followed by a twin-jet polishing method in a solution of 30% nitric acid and 70% methanol at a voltage of 20 V and at the temperature of -20℃.
3. Results Fig. 2(a) illustrates the morphology of grains in the annealed 5A02-O aluminum alloy (before uniaxial tension) obtained via electron backscattered diffraction (EBSD). It can be seen clearly that the grains are uniform, near-equiaxed with diameters in the micrometer (m) range. Fig. 2(b) quantitatively describes the grain size distribution of the annealed 5A02-O aluminum alloy, which reveals an average size of about 9.6m, indicating a fine-grained structure.
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3.1 Deformation behavior Fig. 3(a) describes the true stress-strain curves of the 5A02 aluminum alloy under various electropulsing intensities. To clearly distinguish each stress-strain curve while presenting all of them in the same plot, some of the curves were shifted along the stress (y) axis. In a glance, the plastic strain (ε) decreases with increasing electropulsing intensity, as depicted in Fig. 3(a), wherein ε is about 0.24 in the electropulsing intensity of 20 A, but ε decreases to 0.22 and 0.21 when the electropulsing intensity increases to 40A and 50A, respectively. It is also noted that all curves show continuous serrated flow from a certain strain, exhibit the so-called Portevin-Le Chatelier (PLC) effect [34-37]. In order to further investigate the effect of electropulsing on the deformation behavior, the stress-strain curves at a certain range of strain (from 0.195 to 0.205, a relative stable state) were magnified, as shown in Fig. 3(b). It is clear that the magnitude of serrations increases with electropulsing intensities ranging from 0 to 40A, while the serrations disappear with further increase of intensity up to 50A. To quantifiably distinguish the difference of PLC effect caused by the variation of electropulsing intensity, the stress drop (Δσ) and waiting time (tw) were analyzed statistically, as depicted in the Fig. 3(c) and (d), respectively. The stress drop is only 2.2 MPa under uniaxial tension without electropulsing, but it almost linearly increases up to about 7.5 MPa when the electropulsing intensity increases up to 40A. The waiting time also shows a similar tendency with increasing electropulsing intensity, as illustrated in Fig. 3(d). These phenomena indicate that the PLC effect is significantly enhanced by the increasing electropulsing intensities. The temperature rise of specimens during uniaxial tension under typical electropulsing intensities (20, 40 and 50A) was measured by thermocouple and infrared thermometer, and 6
the results are described in Fig. 4. By comparing the colors of specimens and surroundings, we can find that the temperature (T) of specimens rises prominently when the electropulsing is applied. The measured temperature on aluminum surface is about 30 ℃ when the electropulsing intensity is 20A (see Fig. 4a), but it increases to about 80℃ (see Fig. 4b) and 119℃ (see Fig. 4c) under the high intensity of 40A and 50A, respectively. To verify the effect of temperature rise on the PLC effect, the comparative uniaxial tensile tests were also carried out under 30 ℃ (corresponding to 20A), 80 ℃ (corresponding to 40A) and 119 ℃ (corresponding to 50A) in a furnace chamber, and the true stress-strain curves are depicted in Figs. 3(a) and (b), from which the serrated characteristics are almost identical as observed under electropulsing. The corresponding stress drops (Δσ) under 30℃ and 80℃ are also plotted in Fig. 3(c), negligible difference can be observed by comparison with those underwent electropulsing. However, ε is ~0.28 at the temperature of 119ºC, larger than that in the electropulsing intensity of 50 A.
3.2 Texture The textures of the deformed aluminum alloy under various electropulsing intensities and temperatures were analyzed by EBSD. Due to the non-prominent difference in both samples, the results of the EPT deformed specimen are selected and described in Fig. 5, in which only φ2=0° section of orientation distribution function (ODF) is selected in order to highlight the Cube texture {100} <100>. From Fig. 5(a), little Cube texture remains in the specimen that was just suffered uniaxial tension without any effect of electropulsing. However, the intensity of Cube texture becomes conspicuous with increasing electropulsing intensity, as indicated by Figs. 5(b) ~ (d), manifesting an electropulsing intensity dependence. 7
In order to quantitatively illustrate the variation of deformation textures (Brass, S and Copper textures) during uniaxial tension under electropulsing, the intensity of deformation textures along the β fiber is obtained and shown in Fig. 6(a), which starts from the Copper orientation {112}<111> through the S orientation {123}<634> and ends at the Brass orientation {110}<112> [40-42]. It is worth noting that the intensities of Copper and S orientations gradually decrease with increasing intensity of electropulsing. In addition, the intensity of S orientation exhibits a more significant reduction. While the intensity of Brass orientation does not significantly weaken when the electropulsing intensities ranging from 0A to 40A, the prominent reduction of Brass orientation occurs under 50A, indicating that Brass orientation owns a good stability due to the low stored energy in Brass oriented grains [43]. In order to quantitatively describe the difference in texture under various electropulsing intensities, the volume fractions of Cube and S textures in the deformed aluminum alloys under various electropulsing intensities are calculated with a tolerance of 15°, as described in Fig. 6(b), from which the volume fraction of Cube texture increases, accompanied with the reduction of S texture with increasing electropulsing intensity. It is notable that these variations become prominent when the electropulsing intensity increases up to 50A. 3.3 Microstructure The microstructures of the deformed 5A02 aluminum alloy under various intensities of electropulsing and were further investigated by TEM, and the bright field images are depicted in Fig. 7. From Fig. 7(a), parallel dislocation bands can be clearly observed, indicating that planar slip prevails in the dislocation motion in the deformed aluminum alloy without any effect of electropulsing [44]. While the elongated dislocation cells distribute in the specimen 8
when electropulsing was applied under a certain intensity, such as 20A (see Fig. 7b) and 40A (see Fig. 7c). However, with further increase of electropulsing intensity to 50A, dislocation configuration is characterized by the irregular dislocation cells mixed with loose dislocation tangles (as indicated by arrows), as described in Fig. 7(d). The above TEM results exhibit a transition of slipping mode under different intensities of electropulsing, consistent with the corresponding effect of temperatures.
4. Discussion 4.1 Mechanisms for the effect of electropulsing on deformation behavior and microstructure The results revealed that there is a prominent effect of electropulsing on deformation behavior and microstructure of 5A02 aluminum alloy during uniaxial tension. In general, the serrated characteristics in strain-stress curve indicate the occurrence of heterogeneous deformation. In this case, the plastic stains are localized in very small regions such as shear bands or called PLC bands, which finally causes the catastrophic fracture of the sample [35-37]. Therefore, the increase of serration’s amplitudes with increasing electropulsing intensity from 0A to 40A results in a reduction of plastic strains, as depicted in Fig. 3(a). Under 50A, the conspicuous serrated feature still exists in small strain below 0.13, indicates that the specimen has underwent the inhomogeneous deformation. While at stain above 0.13, the tensile strength reaches and necking accelerates the catastrophic fracture with small stain, even though the disappearance of serrations caused by necking-induced a relatively high temperature (119℃) as detected (see Fig. 4c). Consequently, under pure heating experiment at 9
119℃, the PLC phenomenon disappears and the aluminum alloy deforms easier than that under 50A, facilities a comparatively large strain. Various Portevin-Le Chatelier (PLC) effects under different electropulsing intensities can be further understood from the interactions between solute atoms and dislocations. Dislocation usually moves at critical yield stress wherein the lattice elastic distortion generates stress field (P) around dislocations [31, 45], which causes a enthalpy change PΔV (ΔV is solute misfit volume), resulting in the diffusion of solute atoms towards dislocations in order to release the stress field and minimize the system energy [31,33,45]. These solute atoms segregated around dislocations are usually called “Cottrell atmosphere” [46] that usually results in the pinning of dislocations. It is notable that the dynamic strain ageing (DSA) can also happen at an appropriate diffusion rate of solute atoms, leading to the serrated characteristics in stress-strain curves, known as the Portevin-Le Chatelier (PLC) effect [34-37]. Considering the negligible difference of stress drop under electropulsing and temperature during uniaxial tension, as described in Fig. 3(c), the effect of electromigration (which is defined as the mass transportation of a flux of atoms driven by the momentum exchange between a high density of moving charge carriers and diffusing atoms [47,48]) is neglected in the present work. The diffusion rate (D) of solute atoms can be well understood based on the Arrhenius equation [49]:
Q D D0 e x p RT
(1)
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in which D0= 6.23×10-4m2/s is the diffusion constant, Q = 1.19eV is the diffusion activation energy, R =8.314J/(mol·K) is the gas constant [49] and T is the absolute temperature. From equation (1), the temperature is the critical parameter that determines the diffusion rate of solute atoms. By applying the measured temperature during uniaxial tension under various electropulsing intensities, the diffusion rate are calculated and listed in Table 2, from which D is only 6.90×10-25 m2/s under uniaxial tension without electropulsing, but it increases monotonously from 1.09×10-24 m2/s to 6.32×10-21 m2/s with increasing electropulsing intensity from 10A to 40A. This tendency becomes more prominent at 50A (D= 3.11×10-19 m2/s), indicating that the diffusion of solute atoms is obviously enhanced when the electropulsing is applied during uniaxial tension. According to the “cross-core diffusion” model proposed by Curtin et al [33], the concentration of solute atoms (c(t)) around the dislocation can be expressed as:
c t c0 c t c 0 c 0t a n h W
2 1e
6 c o s hW
ct2
(2)
wherein c0=2.5% is the bulk solute concentration [38,39], W =0.13eV is the average binding-energy difference, 1 kT (k=1.38×10-23J/K is the Boltzmann constant and T is the absolute temperature), and c v0 e H is the transition rate [33] (v0 = 3.8×1013s-1 [49] is the c
attempt frequency and ΔHc=0.97eV [33] is the average activation enthalpy), which denotes the variation of concentration of solute atoms around dislocations in unit time, arising from Mg atoms diffusion. By applying the data listed in Table 2, the concentration of solute atoms around dislocations can be calculated and plotted in Fig. 8(a). If we make t equal to tw (waiting time), the concentration of solute atoms, c(tw) at the moment when dislocations begin 11
to get away from solute atmosphere can also be obtained, as signed by symbols on concentration-time curves in Fig. 8(a). It is interesting that c(tw) around dislocations increases with increasing electropulsing intensity, especially above 20A due to the longer waiting time (corresponding to a relatively long time to diffuse) and higher diffusion coefficient that facilitate the segregation of solute atoms around dislocations. The variation of solute atoms’ concentration then induces the change of binding energy ( E t ) between solute atoms and dislocations per unit length is expressed as:
E t c t N W wherein Δc(t) is the variation of concentration as expressed in equation (2) and N 2w
(3) 3 b2
is the number of solute atoms around the dislocation with a width w ≈ 21Å [33]. According to equation (3), the variation of binding energy between solute atoms and dislocations with time can be calculated, and the result is depicted in Fig. 8(b). By using the same method, the maximum of variation of binding energy can also be obtained, as signed by the symbols on the curves in Fig. 8(b). The increase of binding energy with electropulsing intensity increasing from 0A to 40A, indicates that the relatively large stress is needed for the dislocations to escape from the “Cottrell atmosphere”, followed by an immediate stress drop once dislocations are free to glide, which is the physical origin of conspicuous serrations with increasing electropulsing intensity in stress-strain curves, as shown in Fig. 3(b). However, the diffusion of solute atoms becomes quite strong (as the diffusion rate is two orders larger that under 40A listed in Table 2) under 50A induced by high temperature, which weakens the stable “Cottrell atmosphere” and the interaction between solute atoms and dislocations [44,50]. Therefore, the dislocations cannot be pinned effectively by solute atoms, leading to 12
the disappearance of PLC effect (corresponding to the smooth stress-strain curve) and large plastic stain. The interaction between solute atoms and mobile dislocations can also affect the slipping mode [44,50,51]. From the perspective of energy, a perfect dislocation usually dissociates into two Shockley partial dislocations, and subsequently the partials can join up to a perfect one when gliding [31]. In general, the joining up of partials of a screw dislocation becomes difficult in solid solutions due to that solute atoms tend to segregate around the partials, which increases the frictional stress [52] and then suppresses the cross slip of screw dislocations. Just as the specimen under uniaxial tension without electropulsing, the concentration of solute atoms around the partials is low and the “solute atmosphere” around the partials cannot overlap (see Fig. 9a), indicating that the partials have to get away from “solute atmosphere” to join up to a perfect one. In this case, the possibility of joining up of the partials is relatively low, and cross slip are strongly inhibited, as what was observed in Fig. 7(a). However, when specimens are deformed with the relatively high electropulsing intensity (such as 40A), lots of solute atoms segregate around the partials (see Fig. 9b). Therefore, the partials can easily join together to form a perfect dislocation without necessary escape from the “solute atmosphere” [51]. Just as Picu et al [53] proposed that the binding energy between solute atoms and screw dislocations could be negligible, so the screw perfect dislocations can easily get away from the “solute atmosphere”, which promotes the occurrence of cross slip, identical to the dislocation cells in Fig. 7(b) and (c). At the intensity of 50A, the rearrangement of solute atoms becomes quite easy and the strong drag force disappears [44,50]. In this case, the partials of screw dislocations are not intensively pinned by solute atoms, suggesting that 13
joining up of partials could easily accomplish under this condition. Therefore, cross slip of screw dislocations could frequently occur, which leads to the formation of dislocation cells and tangles, as what was observed in Fig. 7(d). 4.2 Effect of electropulsing on texture In addition to the variation of deformation behavior and microstructure, the texture also exhibits different characteristics with increasing electropulsing intensity, as described in Figs. 5 and 6. Considering the temperature rise during electropulsing under certain intensity, the effect of electropulsing on texture evolution can be rationalized by the mechanism of oriented nucleation and oriented growth [43,54,55]. In general, grains of cube orientation originate from the Cube bands [43,54-56], which is band-like structures comprising cube oriented subgrains [56]. Daaland et al [54] regarded that the size of these cube oriented subgrains meets requirements of critical diameter for subgrains starting to grow. Therefore, these cube oriented subgrains can act as “nuclei” that grows directly into the surrounding deformed grains without “incubation time”. In addition to the oriented nucleation theory, the nuclei growth should also be considered, which is governed by the grain boundary velocity (vGB) expressed as [57]:
vGB g D , g Rx m g D , g Rx p g D
(4)
wherein m is the mobility of grain boundary and p is the driving force. The mobility m usually relates to the misorientation between the nuclei (with orientation gRx) and the deformed grain (with orientation gD). Cube orientation has a 40° <111> relationship with respect to S orientation in aluminum alloys, which is considered to have a particularly high mobility 14
[43,54-56]. In addition, cube oriented subgrains are capable of growing into all four symmetrically equivalent variants of S orientation due to the high symmetry [55], which further makes sure that the grain boundaries between cube oriented grains and S oriented grains could exhibit the high mobility, indicating a large value of m. On the other hand, the driving force p depends on the stored energy in the deformed grains. Previous literatures have reported that S oriented grains possess a much higher stored energy than grains with other orientations [58,59]. Furthermore, dislocations and other crystal defects increases during plastic deformation, resulting in a relatively higher electrical resistivity of S oriented grains than that of surrounding cube oriented subgrains [60]. The high electrical resistivity causes high Joule heat generation in S oriented grains and this tendency becomes conspicuous with increasing electropulsing intensity, which promotes the increase of stored energy in S oriented grains, acting as the driving force for the motion of grain boundaries. On the basis of above analysis, the velocity of grain boundaries between cube oriented subgrains and S oriented grains becomes large with increasing electropulsing intensity, which facilitates the growth of cube oriented subgrains but weakens S oriented grains. That’s why the volume fraction of Cube texture increases, accompanied with the reduction of S orientation, as shown in Fig. 6(b).
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5. Conclusion
The effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy was investigated systemically. The conclusions can be drawn from experimental results and theoretical analysis as follows: (1) The serrated characteristics in stress-strain curves (i.e. PLC effect) became prominent firstly but then disappeared with further increase of the electropulsing intensity. The phenomena are attributed to the increase of binding energy between solute atoms and dislocations with increasing electropulsing intensities (ranging from 0-40A). However, when electropulsing intensity increases up to 50A, the significant enhancement of diffusion of solute atoms obviously weakens “Cottrell atmosphere” and then the binding energy between solute atoms and dislocations, resulting in the disappearance of PLC effect. (2) The increase of electropulsing intensity resulted in transition of slipping mode from planar slip (0A) to wave slip (20A, 40A and 50A). This is due to that the joining up of partials of screw dislocations becomes easier with increasing electropulsing intensity, which leads to the generation of cross slip of screw dislocations. (3) The increasing intensity also resulted in the increase of volume fraction of Cube texture, accompanied with the reduction of S texture, due to that the velocity of grain boundaries between cube oriented subgrains and S oriented grains becomes large with increasing electropulsing intensity, which facilitates the growth of cube oriented subgrains and weakens S oriented grains.
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Acknowledgements
This work was financially supported by the National Fundamental Research Program of China (Grant No.2011CB012806). The authors are grateful to the Analytical and Testing Center, Huazhong University of Science and Technology for technical assistance.
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Table 1 Chemical composition of the 5A02 aluminum alloy used in the experiment
Element wt.%
Mg
Si
Fe
Cu
Mn
Ti
Al
2.5
0.13
0.2
0.08
0.19
<0.10
Balance
19
Table 2 Temperature of specimens’ surface (T) measured by thermo-couple and diffusion coefficient of Mg in Al (DMg/Al) with different intensities of electropulsing Intensity(A)
0
10
20
30
40
50
T(℃)
13
19.33
30.33
61.67
79.67
118.67
DMg/Al(m2/s)
6.903×10-25 1.087×10-24 1.465×10-23 7.701×10-22 6.317×10-21 3.110×10-19
Fig. 1 Sketch of the application of electropulsing during uniaxial tensile testing.
20
(a)
12
(b)
Percentage(%)
9
6
3
0
4
6
8
10
12
14
16
18
Grain Size(m)
Fig. 2 (a) Orientation map of the initial 5A02-O aluminum alloy (before uniaxial tension) obtained by EBSD; (b) the grain size distribution of the aluminum alloy.
21
0A 10A 20A 30A 40A
200
150
250
100
50
200
0A 10A 20A 30℃
200
30A 40A
150
80℃
100
119℃
100 50
0.00
9.0
19℃ 380℃
0.04
0.08
50A 0.00
0.12 0.16 True Strain
0.05
0.10
0.15 0.20 True Strain
0.20
0.25
0.24
0.30
0.196
0.28
0.39
(c)
7.5
80℃ 6.0
30℃
4.5
0.198
0.200 True Strain
0.202
0.204
(d)
0.36
Waiting Time(s)
0
(b)
250
150
0
Stress Drop(MPa)
50A
300
300
True Stress(MPa)
(a)
True Stress(Mpa)
True Stress(MPa)
250
0.33
tw
0.30 0.27 0.24
3.0
Deform under electropulsing Deform under a certain temperature
1.5
0.21 0.18
0
10 20 30 Intensity of electropulsing(A)
40
0
10 20 30 Intensity of electropulsing(A)
40
Fig. 3 (a) True stress-strain curves for specimens with different intensities of electropulsing (The inset is the true stress-strain curves for specimens deformed under different temperatures); (b) Local magnification of (a); (c) Statistical results of stress drop for specimens with different intensities of electropulsing and temperatures; (d) Statistical results of waiting time for specimens with different intensities of electropulsing; (The insets show the definition of stress drop and waiting time, respectively).
22
T=80℃
T=30℃
T=119℃
Fig. 4 Temperature distribution maps of specimens deformed under different intensities of electropulsing (a) 20A; (b) 40A; (c) 50A; The shown temperature on the corresponding map is measured by the thermocouple.
23
(a)
(b)
(c)
(d)
(e) 0°
0°
φ1
Φ
90°
Cube {100} <100> φ2=0°
90°
Fig. 5 φ2=0° section of the orientation distribution function (ODF) for specimens with different intensities of electropulsing (a) 0A; (b) 20A; (c) 40A; (d) 50A; The position of ideal orientation referred to in this paper is shown in (e).
24
S~{123} <634>
C~{112} <111> 7
(a)
B~{110} <112>
0A 20A 40A 50A
6 5
f(g)
4 3 2 1 0 50
Volume Fraction(%)
16
60
70
80
(b)
90
0A 20A 40A 50A
12
8
4
0 Cube
S
Fig. 6 (a) Orientation intensity f(g) along the β fiber for specimens with different intensities of electropulsing; (b) Volume fraction of Cube {100}﹤100﹥and S {123} ﹤ 634 ﹥ components for specimens with different intensities of electropulsing.
25
(a)
1m
(c)
1m
(b)
1m
(d)
1m
Fig. 7 TEM microstructure for specimens with different intensities of electropulsing (a) 0A; (b) 20A; (c) 40A; (d) 50A.
26
5.0
(a)
40A
4.5
30A
Concentration of solute atoms(%)
4.0 3.5 3.0 2.5 2.0 0.0
0.1
0.2
0.3
0.4
0.5
Time(s) 2.60
20A
2.56
10A
2.52
0A 2.48 0.19
10
0.20
0.21
0.22
0.23
(b)
0.24
40A
30A
6
-3
Variation of binding energy(10 eV/Å)
8
4 2 0 0.0
0.1
0.2
Time(s)
0.3
0.4
0.5
0.4 0.2
20A 10A
0.0
0A 0.19
0.20
0.21
0.22
0.23
0.24
Fig. 8 The concentration of solute atoms around the dislocation (a) and variation of binding energy between solute atoms and dislocation per unit length (b) versus time. The concentration of solute atoms and variation of binding energy when t=tw (waiting time) are shown by the symbols on the curves under different intensities of electropulsing, meaning the maximum which could reach to for concentration of solute atoms (a) and variation of binding energy (b) under different intensities of electropulsing on the basis of definition of waiting time. 27
Fig. 9 Hypothetical representations of segregated solute atoms around dissociated partial dislocations. (a) without electropulsing; (b) with a high intensity of electropulsing [39] .
28
PII: DOI: Reference:
S0921-5093(15)00465-7 http://dx.doi.org/10.1016/j.msea.2015.04.058 MSA32282
To appear in: Materials Science & Engineering A Received date: 16 December 2014 Revised date: 19 April 2015 Accepted date: 20 April 2015 Cite this article as: Cong Yan, Ning Li, Huawen Jiang, Duzhen Wang and Lin Liu, Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.04.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy during uniaxial tension Cong Yan, Ning Li*, Huawen Jiang, Duzhen Wang, Lin Liu School of Materials Science and Engineering and State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, 430074 Wuhan, PRC
Authors to whom correspondence should be addressed. Tel.: +86 27 87559606; Fax: +86 27 87554405.
E-mail adress: [email protected] (Ning Li).
Abstract The effect of electropulsing on the deformation behavior, texture and microstructure of 5A02 aluminum alloy was investigated through uniaxial tension, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The Portevin-Le Chatelier (PLC) effect reflected as the serrated characteristic in stress-strain curves, became conspicuous firstly but then disappeared with further increase of electropulsing intensity. The texture analysis exhibited that the electropulsing causes an increase of Cube texture, accompanied with a reduction of S texture. Microstructure characterization revealed a transition of slipping mode from planar slip to wave slip with increasing electropulsing intensity. The temperature rise induced by electropulsing, together with the influence of solute atoms, was proposed to rationalize the present phenomena in detail. Keywords: Aluminum alloy; Electropulsing; Deformation behavior; Microstructure.
1
1. Introduction Innovative techniques are attractive in breakthrough the bottleneck of materials processing, improving materials’ ultimate microstructures and properties. For instance, the thermoplastic forming provides a promising technique
in net-shaping metallic glasses [1-6].
Electromagnetic forming exhibits favourable applications in various industries due to its unique characteristics include contactless forming, low springback and enhanced forming limit etc. [7-11] with different deformation mechanism by comparison with that under traditional high speed mechanical processing [12]. Electropulsing treatment (EPT), as an instantaneous high-energy input method, has also attracted enduring attention due to its superiority in the enhancement of plasticity of brittle metallic materials [13-21] and evaluation of microstructure evolution [22-27]. For example, the electropulsing could reduce the forming stress and increase the plastic strain of metallic materials through enhancing the dislocation mobility [20,21] and promoting the homogeneous distribution of dislocations [16,17]. The EPT could also improve the atomic diffusion capacity, such as accelerating the dissolution of β-Mg17Al12 phase (a detrimental phase to the plasticity of AZ91) in Mg-9Al-Zn (AZ91) alloy, facilitating the large elongation [13-15]. The enhancement of atomic diffusion originating from temperature and thermal stress caused by electropulsing [28-30], favors the healing of microcracks and then improves the plasticity of the pre-deformed titanium alloys [18,19]. In addition, the improvement of atomic diffusion induced by electropulsing also increases the dislocation velocity and sub-grains’ growth rate, causing the recrystallized grains with a relatively small size by comparison with those underwent conventional annealing [15,24]. 2
It is notable that the above researches of EPT mainly focused on the improvement of plasticity of brittle metallic materials, but scarcely emphasized on the deformation behavior that is essentially determined by microstructure. Consequently, the microstructure evolution of metallic alloys during EPT should be deeply understood, especially in solid solution alloys, wherein solute atoms usually impede the motion of dislocations, resulting in the occurrence of dynamic strain ageing (DSA) under certain conditions [31,32]. In the case of DSA, dislocations repeatedly break away and capture from solute atoms, leading to the negative strain–rate sensitivity (nSRS) and resulting in serrated features in stress-strain curves, known as the Portevin-Le Chatelier (PLC) effect [31-37]. The reported literatures have shown that the PLC is related to the temperatures and strain rates due to the competition between the diffusion rate of solute atoms and the velocity of dislocations [35-37]. However, whether the electropulsing can affect the interaction between solute atoms and dislocations and result in various PLC effect under different intensities, and what are the possible reasons for this phenomena, are questions still remaining unanswered. In the present study, a typical Al-Mg (2.5 wt. % Mg) solid solution (5A02 aluminum alloy) was selected as the research object. The effect of electropulsing with various intensities on the deformation behavior of the aluminum alloy during uniaxial tension was investigated. The texture and microstructure of the deformed specimens were also characterized by EBSD and TEM, respectively. The results exhibit that the deformation behavior, texture and microstructure of 5A02 aluminum alloy are seriously affected by the intensities of electropulsing. The effect of electropulsing on the interaction between solute atoms and dislocations is understood deeply on the basis of the segregation of solute atoms around 3
dislocations.
2. Experimental Procedure
2.1 Material and uniaxial tensile testing The 5A02-O aluminum alloy [38,39] used in this work was provided by the Southwest Aluminum Company (China) as 2.0 mm thick sheet in a recrystallized condition. The nominal chemical composition (wt. %) of the alloy is shown in Table 1, which reveals that there are some solute atoms such as Mg, Si and Fe. The dog-bone-shaped specimens with thickness of 2.0 mm and gauge length of 50.0 mm were cut from the received aluminum alloy sheet using an electro-spark machining under oil cooling condition. Uniaxial tension was conducted by using a universal testing machine (RGM-4050) at ambient temperature with a constant strain rate of 1×10-3 s-1. During whole tensile testing, electropulsing with various intensities (10A, 20A, 30A, 40A, 50A) was applied, as sketched in Fig. 1, wherein the surface of steel clamp was treated by insulating to avoid the effect of electropulsing on the transducer. The frequency of 90Hz and duty cycle of 25% were selected, corresponding to electropulsing duration of about 2780 s. The temperature rise of specimens during deformation was measured by an infrared thermometer (FLIR A320), and calibrated with a thermocouple mounted on the specimen’s surface, as depicted in Fig. 1.
4
2.2 Texture and microstructure characterization The texture of the deformed 5A02 aluminum alloy under various electropulsing intensities was probed by electron backscattered diffraction (EBSD) that is conducted using a field emission scanning electron microscope (FSEM, FEI Sirion 200) equipped with an EBSD detector. Texture characterization was performed on the plane parallel to the tensile direction, with a step size of 0.2 m. The samples for EBSD investigation were first mechanical grinding and subsequently electrolytic polishing in a 5:1 alcohol-perchloric acid solution using a 316 stainless steel as cathode at 20 V for 60 s at room temperature. The microstructure of the deformed aluminum alloy samples was further characterized by transmission electron microscope (TEM, FEI Tecnai G20). TEM thin foils were first mechanically ground to about 50 m thick, followed by a twin-jet polishing method in a solution of 30% nitric acid and 70% methanol at a voltage of 20 V and at the temperature of -20℃.
3. Results Fig. 2(a) illustrates the morphology of grains in the annealed 5A02-O aluminum alloy (before uniaxial tension) obtained via electron backscattered diffraction (EBSD). It can be seen clearly that the grains are uniform, near-equiaxed with diameters in the micrometer (m) range. Fig. 2(b) quantitatively describes the grain size distribution of the annealed 5A02-O aluminum alloy, which reveals an average size of about 9.6m, indicating a fine-grained structure.
5
3.1 Deformation behavior Fig. 3(a) describes the true stress-strain curves of the 5A02 aluminum alloy under various electropulsing intensities. To clearly distinguish each stress-strain curve while presenting all of them in the same plot, some of the curves were shifted along the stress (y) axis. In a glance, the plastic strain (ε) decreases with increasing electropulsing intensity, as depicted in Fig. 3(a), wherein ε is about 0.24 in the electropulsing intensity of 20 A, but ε decreases to 0.22 and 0.21 when the electropulsing intensity increases to 40A and 50A, respectively. It is also noted that all curves show continuous serrated flow from a certain strain, exhibit the so-called Portevin-Le Chatelier (PLC) effect [34-37]. In order to further investigate the effect of electropulsing on the deformation behavior, the stress-strain curves at a certain range of strain (from 0.195 to 0.205, a relative stable state) were magnified, as shown in Fig. 3(b). It is clear that the magnitude of serrations increases with electropulsing intensities ranging from 0 to 40A, while the serrations disappear with further increase of intensity up to 50A. To quantifiably distinguish the difference of PLC effect caused by the variation of electropulsing intensity, the stress drop (Δσ) and waiting time (tw) were analyzed statistically, as depicted in the Fig. 3(c) and (d), respectively. The stress drop is only 2.2 MPa under uniaxial tension without electropulsing, but it almost linearly increases up to about 7.5 MPa when the electropulsing intensity increases up to 40A. The waiting time also shows a similar tendency with increasing electropulsing intensity, as illustrated in Fig. 3(d). These phenomena indicate that the PLC effect is significantly enhanced by the increasing electropulsing intensities. The temperature rise of specimens during uniaxial tension under typical electropulsing intensities (20, 40 and 50A) was measured by thermocouple and infrared thermometer, and 6
the results are described in Fig. 4. By comparing the colors of specimens and surroundings, we can find that the temperature (T) of specimens rises prominently when the electropulsing is applied. The measured temperature on aluminum surface is about 30 ℃ when the electropulsing intensity is 20A (see Fig. 4a), but it increases to about 80℃ (see Fig. 4b) and 119℃ (see Fig. 4c) under the high intensity of 40A and 50A, respectively. To verify the effect of temperature rise on the PLC effect, the comparative uniaxial tensile tests were also carried out under 30 ℃ (corresponding to 20A), 80 ℃ (corresponding to 40A) and 119 ℃ (corresponding to 50A) in a furnace chamber, and the true stress-strain curves are depicted in Figs. 3(a) and (b), from which the serrated characteristics are almost identical as observed under electropulsing. The corresponding stress drops (Δσ) under 30℃ and 80℃ are also plotted in Fig. 3(c), negligible difference can be observed by comparison with those underwent electropulsing. However, ε is ~0.28 at the temperature of 119ºC, larger than that in the electropulsing intensity of 50 A.
3.2 Texture The textures of the deformed aluminum alloy under various electropulsing intensities and temperatures were analyzed by EBSD. Due to the non-prominent difference in both samples, the results of the EPT deformed specimen are selected and described in Fig. 5, in which only φ2=0° section of orientation distribution function (ODF) is selected in order to highlight the Cube texture {100} <100>. From Fig. 5(a), little Cube texture remains in the specimen that was just suffered uniaxial tension without any effect of electropulsing. However, the intensity of Cube texture becomes conspicuous with increasing electropulsing intensity, as indicated by Figs. 5(b) ~ (d), manifesting an electropulsing intensity dependence. 7
In order to quantitatively illustrate the variation of deformation textures (Brass, S and Copper textures) during uniaxial tension under electropulsing, the intensity of deformation textures along the β fiber is obtained and shown in Fig. 6(a), which starts from the Copper orientation {112}<111> through the S orientation {123}<634> and ends at the Brass orientation {110}<112> [40-42]. It is worth noting that the intensities of Copper and S orientations gradually decrease with increasing intensity of electropulsing. In addition, the intensity of S orientation exhibits a more significant reduction. While the intensity of Brass orientation does not significantly weaken when the electropulsing intensities ranging from 0A to 40A, the prominent reduction of Brass orientation occurs under 50A, indicating that Brass orientation owns a good stability due to the low stored energy in Brass oriented grains [43]. In order to quantitatively describe the difference in texture under various electropulsing intensities, the volume fractions of Cube and S textures in the deformed aluminum alloys under various electropulsing intensities are calculated with a tolerance of 15°, as described in Fig. 6(b), from which the volume fraction of Cube texture increases, accompanied with the reduction of S texture with increasing electropulsing intensity. It is notable that these variations become prominent when the electropulsing intensity increases up to 50A. 3.3 Microstructure The microstructures of the deformed 5A02 aluminum alloy under various intensities of electropulsing and were further investigated by TEM, and the bright field images are depicted in Fig. 7. From Fig. 7(a), parallel dislocation bands can be clearly observed, indicating that planar slip prevails in the dislocation motion in the deformed aluminum alloy without any effect of electropulsing [44]. While the elongated dislocation cells distribute in the specimen 8
when electropulsing was applied under a certain intensity, such as 20A (see Fig. 7b) and 40A (see Fig. 7c). However, with further increase of electropulsing intensity to 50A, dislocation configuration is characterized by the irregular dislocation cells mixed with loose dislocation tangles (as indicated by arrows), as described in Fig. 7(d). The above TEM results exhibit a transition of slipping mode under different intensities of electropulsing, consistent with the corresponding effect of temperatures.
4. Discussion 4.1 Mechanisms for the effect of electropulsing on deformation behavior and microstructure The results revealed that there is a prominent effect of electropulsing on deformation behavior and microstructure of 5A02 aluminum alloy during uniaxial tension. In general, the serrated characteristics in strain-stress curve indicate the occurrence of heterogeneous deformation. In this case, the plastic stains are localized in very small regions such as shear bands or called PLC bands, which finally causes the catastrophic fracture of the sample [35-37]. Therefore, the increase of serration’s amplitudes with increasing electropulsing intensity from 0A to 40A results in a reduction of plastic strains, as depicted in Fig. 3(a). Under 50A, the conspicuous serrated feature still exists in small strain below 0.13, indicates that the specimen has underwent the inhomogeneous deformation. While at stain above 0.13, the tensile strength reaches and necking accelerates the catastrophic fracture with small stain, even though the disappearance of serrations caused by necking-induced a relatively high temperature (119℃) as detected (see Fig. 4c). Consequently, under pure heating experiment at 9
119℃, the PLC phenomenon disappears and the aluminum alloy deforms easier than that under 50A, facilities a comparatively large strain. Various Portevin-Le Chatelier (PLC) effects under different electropulsing intensities can be further understood from the interactions between solute atoms and dislocations. Dislocation usually moves at critical yield stress wherein the lattice elastic distortion generates stress field (P) around dislocations [31, 45], which causes a enthalpy change PΔV (ΔV is solute misfit volume), resulting in the diffusion of solute atoms towards dislocations in order to release the stress field and minimize the system energy [31,33,45]. These solute atoms segregated around dislocations are usually called “Cottrell atmosphere” [46] that usually results in the pinning of dislocations. It is notable that the dynamic strain ageing (DSA) can also happen at an appropriate diffusion rate of solute atoms, leading to the serrated characteristics in stress-strain curves, known as the Portevin-Le Chatelier (PLC) effect [34-37]. Considering the negligible difference of stress drop under electropulsing and temperature during uniaxial tension, as described in Fig. 3(c), the effect of electromigration (which is defined as the mass transportation of a flux of atoms driven by the momentum exchange between a high density of moving charge carriers and diffusing atoms [47,48]) is neglected in the present work. The diffusion rate (D) of solute atoms can be well understood based on the Arrhenius equation [49]:
Q D D0 e x p RT
(1)
10
in which D0= 6.23×10-4m2/s is the diffusion constant, Q = 1.19eV is the diffusion activation energy, R =8.314J/(mol·K) is the gas constant [49] and T is the absolute temperature. From equation (1), the temperature is the critical parameter that determines the diffusion rate of solute atoms. By applying the measured temperature during uniaxial tension under various electropulsing intensities, the diffusion rate are calculated and listed in Table 2, from which D is only 6.90×10-25 m2/s under uniaxial tension without electropulsing, but it increases monotonously from 1.09×10-24 m2/s to 6.32×10-21 m2/s with increasing electropulsing intensity from 10A to 40A. This tendency becomes more prominent at 50A (D= 3.11×10-19 m2/s), indicating that the diffusion of solute atoms is obviously enhanced when the electropulsing is applied during uniaxial tension. According to the “cross-core diffusion” model proposed by Curtin et al [33], the concentration of solute atoms (c(t)) around the dislocation can be expressed as:
c t c0 c t c 0 c 0t a n h W
2 1e
6 c o s hW
ct2
(2)
wherein c0=2.5% is the bulk solute concentration [38,39], W =0.13eV is the average binding-energy difference, 1 kT (k=1.38×10-23J/K is the Boltzmann constant and T is the absolute temperature), and c v0 e H is the transition rate [33] (v0 = 3.8×1013s-1 [49] is the c
attempt frequency and ΔHc=0.97eV [33] is the average activation enthalpy), which denotes the variation of concentration of solute atoms around dislocations in unit time, arising from Mg atoms diffusion. By applying the data listed in Table 2, the concentration of solute atoms around dislocations can be calculated and plotted in Fig. 8(a). If we make t equal to tw (waiting time), the concentration of solute atoms, c(tw) at the moment when dislocations begin 11
to get away from solute atmosphere can also be obtained, as signed by symbols on concentration-time curves in Fig. 8(a). It is interesting that c(tw) around dislocations increases with increasing electropulsing intensity, especially above 20A due to the longer waiting time (corresponding to a relatively long time to diffuse) and higher diffusion coefficient that facilitate the segregation of solute atoms around dislocations. The variation of solute atoms’ concentration then induces the change of binding energy ( E t ) between solute atoms and dislocations per unit length is expressed as:
E t c t N W wherein Δc(t) is the variation of concentration as expressed in equation (2) and N 2w
(3) 3 b2
is the number of solute atoms around the dislocation with a width w ≈ 21Å [33]. According to equation (3), the variation of binding energy between solute atoms and dislocations with time can be calculated, and the result is depicted in Fig. 8(b). By using the same method, the maximum of variation of binding energy can also be obtained, as signed by the symbols on the curves in Fig. 8(b). The increase of binding energy with electropulsing intensity increasing from 0A to 40A, indicates that the relatively large stress is needed for the dislocations to escape from the “Cottrell atmosphere”, followed by an immediate stress drop once dislocations are free to glide, which is the physical origin of conspicuous serrations with increasing electropulsing intensity in stress-strain curves, as shown in Fig. 3(b). However, the diffusion of solute atoms becomes quite strong (as the diffusion rate is two orders larger that under 40A listed in Table 2) under 50A induced by high temperature, which weakens the stable “Cottrell atmosphere” and the interaction between solute atoms and dislocations [44,50]. Therefore, the dislocations cannot be pinned effectively by solute atoms, leading to 12
the disappearance of PLC effect (corresponding to the smooth stress-strain curve) and large plastic stain. The interaction between solute atoms and mobile dislocations can also affect the slipping mode [44,50,51]. From the perspective of energy, a perfect dislocation usually dissociates into two Shockley partial dislocations, and subsequently the partials can join up to a perfect one when gliding [31]. In general, the joining up of partials of a screw dislocation becomes difficult in solid solutions due to that solute atoms tend to segregate around the partials, which increases the frictional stress [52] and then suppresses the cross slip of screw dislocations. Just as the specimen under uniaxial tension without electropulsing, the concentration of solute atoms around the partials is low and the “solute atmosphere” around the partials cannot overlap (see Fig. 9a), indicating that the partials have to get away from “solute atmosphere” to join up to a perfect one. In this case, the possibility of joining up of the partials is relatively low, and cross slip are strongly inhibited, as what was observed in Fig. 7(a). However, when specimens are deformed with the relatively high electropulsing intensity (such as 40A), lots of solute atoms segregate around the partials (see Fig. 9b). Therefore, the partials can easily join together to form a perfect dislocation without necessary escape from the “solute atmosphere” [51]. Just as Picu et al [53] proposed that the binding energy between solute atoms and screw dislocations could be negligible, so the screw perfect dislocations can easily get away from the “solute atmosphere”, which promotes the occurrence of cross slip, identical to the dislocation cells in Fig. 7(b) and (c). At the intensity of 50A, the rearrangement of solute atoms becomes quite easy and the strong drag force disappears [44,50]. In this case, the partials of screw dislocations are not intensively pinned by solute atoms, suggesting that 13
joining up of partials could easily accomplish under this condition. Therefore, cross slip of screw dislocations could frequently occur, which leads to the formation of dislocation cells and tangles, as what was observed in Fig. 7(d). 4.2 Effect of electropulsing on texture In addition to the variation of deformation behavior and microstructure, the texture also exhibits different characteristics with increasing electropulsing intensity, as described in Figs. 5 and 6. Considering the temperature rise during electropulsing under certain intensity, the effect of electropulsing on texture evolution can be rationalized by the mechanism of oriented nucleation and oriented growth [43,54,55]. In general, grains of cube orientation originate from the Cube bands [43,54-56], which is band-like structures comprising cube oriented subgrains [56]. Daaland et al [54] regarded that the size of these cube oriented subgrains meets requirements of critical diameter for subgrains starting to grow. Therefore, these cube oriented subgrains can act as “nuclei” that grows directly into the surrounding deformed grains without “incubation time”. In addition to the oriented nucleation theory, the nuclei growth should also be considered, which is governed by the grain boundary velocity (vGB) expressed as [57]:
vGB g D , g Rx m g D , g Rx p g D
(4)
wherein m is the mobility of grain boundary and p is the driving force. The mobility m usually relates to the misorientation between the nuclei (with orientation gRx) and the deformed grain (with orientation gD). Cube orientation has a 40° <111> relationship with respect to S orientation in aluminum alloys, which is considered to have a particularly high mobility 14
[43,54-56]. In addition, cube oriented subgrains are capable of growing into all four symmetrically equivalent variants of S orientation due to the high symmetry [55], which further makes sure that the grain boundaries between cube oriented grains and S oriented grains could exhibit the high mobility, indicating a large value of m. On the other hand, the driving force p depends on the stored energy in the deformed grains. Previous literatures have reported that S oriented grains possess a much higher stored energy than grains with other orientations [58,59]. Furthermore, dislocations and other crystal defects increases during plastic deformation, resulting in a relatively higher electrical resistivity of S oriented grains than that of surrounding cube oriented subgrains [60]. The high electrical resistivity causes high Joule heat generation in S oriented grains and this tendency becomes conspicuous with increasing electropulsing intensity, which promotes the increase of stored energy in S oriented grains, acting as the driving force for the motion of grain boundaries. On the basis of above analysis, the velocity of grain boundaries between cube oriented subgrains and S oriented grains becomes large with increasing electropulsing intensity, which facilitates the growth of cube oriented subgrains but weakens S oriented grains. That’s why the volume fraction of Cube texture increases, accompanied with the reduction of S orientation, as shown in Fig. 6(b).
15
5. Conclusion
The effect of electropulsing on deformation behavior, texture and microstructure of 5A02 aluminum alloy was investigated systemically. The conclusions can be drawn from experimental results and theoretical analysis as follows: (1) The serrated characteristics in stress-strain curves (i.e. PLC effect) became prominent firstly but then disappeared with further increase of the electropulsing intensity. The phenomena are attributed to the increase of binding energy between solute atoms and dislocations with increasing electropulsing intensities (ranging from 0-40A). However, when electropulsing intensity increases up to 50A, the significant enhancement of diffusion of solute atoms obviously weakens “Cottrell atmosphere” and then the binding energy between solute atoms and dislocations, resulting in the disappearance of PLC effect. (2) The increase of electropulsing intensity resulted in transition of slipping mode from planar slip (0A) to wave slip (20A, 40A and 50A). This is due to that the joining up of partials of screw dislocations becomes easier with increasing electropulsing intensity, which leads to the generation of cross slip of screw dislocations. (3) The increasing intensity also resulted in the increase of volume fraction of Cube texture, accompanied with the reduction of S texture, due to that the velocity of grain boundaries between cube oriented subgrains and S oriented grains becomes large with increasing electropulsing intensity, which facilitates the growth of cube oriented subgrains and weakens S oriented grains.
16
Acknowledgements
This work was financially supported by the National Fundamental Research Program of China (Grant No.2011CB012806). The authors are grateful to the Analytical and Testing Center, Huazhong University of Science and Technology for technical assistance.
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Table 1 Chemical composition of the 5A02 aluminum alloy used in the experiment
Element wt.%
Mg
Si
Fe
Cu
Mn
Ti
Al
2.5
0.13
0.2
0.08
0.19
<0.10
Balance
19
Table 2 Temperature of specimens’ surface (T) measured by thermo-couple and diffusion coefficient of Mg in Al (DMg/Al) with different intensities of electropulsing Intensity(A)
0
10
20
30
40
50
T(℃)
13
19.33
30.33
61.67
79.67
118.67
DMg/Al(m2/s)
6.903×10-25 1.087×10-24 1.465×10-23 7.701×10-22 6.317×10-21 3.110×10-19
Fig. 1 Sketch of the application of electropulsing during uniaxial tensile testing.
20
(a)
12
(b)
Percentage(%)
9
6
3
0
4
6
8
10
12
14
16
18
Grain Size(m)
Fig. 2 (a) Orientation map of the initial 5A02-O aluminum alloy (before uniaxial tension) obtained by EBSD; (b) the grain size distribution of the aluminum alloy.
21
0A 10A 20A 30A 40A
200
150
250
100
50
200
0A 10A 20A 30℃
200
30A 40A
150
80℃
100
119℃
100 50
0.00
9.0
19℃ 380℃
0.04
0.08
50A 0.00
0.12 0.16 True Strain
0.05
0.10
0.15 0.20 True Strain
0.20
0.25
0.24
0.30
0.196
0.28
0.39
(c)
7.5
80℃ 6.0
30℃
4.5
0.198
0.200 True Strain
0.202
0.204
(d)
0.36
Waiting Time(s)
0
(b)
250
150
0
Stress Drop(MPa)
50A
300
300
True Stress(MPa)
(a)
True Stress(Mpa)
True Stress(MPa)
250
0.33
tw
0.30 0.27 0.24
3.0
Deform under electropulsing Deform under a certain temperature
1.5
0.21 0.18
0
10 20 30 Intensity of electropulsing(A)
40
0
10 20 30 Intensity of electropulsing(A)
40
Fig. 3 (a) True stress-strain curves for specimens with different intensities of electropulsing (The inset is the true stress-strain curves for specimens deformed under different temperatures); (b) Local magnification of (a); (c) Statistical results of stress drop for specimens with different intensities of electropulsing and temperatures; (d) Statistical results of waiting time for specimens with different intensities of electropulsing; (The insets show the definition of stress drop and waiting time, respectively).
22
T=80℃
T=30℃
T=119℃
Fig. 4 Temperature distribution maps of specimens deformed under different intensities of electropulsing (a) 20A; (b) 40A; (c) 50A; The shown temperature on the corresponding map is measured by the thermocouple.
23
(a)
(b)
(c)
(d)
(e) 0°
0°
φ1
Φ
90°
Cube {100} <100> φ2=0°
90°
Fig. 5 φ2=0° section of the orientation distribution function (ODF) for specimens with different intensities of electropulsing (a) 0A; (b) 20A; (c) 40A; (d) 50A; The position of ideal orientation referred to in this paper is shown in (e).
24
S~{123} <634>
C~{112} <111> 7
(a)
B~{110} <112>
0A 20A 40A 50A
6 5
f(g)
4 3 2 1 0 50
Volume Fraction(%)
16
60
70
80
(b)
90
0A 20A 40A 50A
12
8
4
0 Cube
S
Fig. 6 (a) Orientation intensity f(g) along the β fiber for specimens with different intensities of electropulsing; (b) Volume fraction of Cube {100}﹤100﹥and S {123} ﹤ 634 ﹥ components for specimens with different intensities of electropulsing.
25
(a)
1m
(c)
1m
(b)
1m
(d)
1m
Fig. 7 TEM microstructure for specimens with different intensities of electropulsing (a) 0A; (b) 20A; (c) 40A; (d) 50A.
26
5.0
(a)
40A
4.5
30A
Concentration of solute atoms(%)
4.0 3.5 3.0 2.5 2.0 0.0
0.1
0.2
0.3
0.4
0.5
Time(s) 2.60
20A
2.56
10A
2.52
0A 2.48 0.19
10
0.20
0.21
0.22
0.23
(b)
0.24
40A
30A
6
-3
Variation of binding energy(10 eV/Å)
8
4 2 0 0.0
0.1
0.2
Time(s)
0.3
0.4
0.5
0.4 0.2
20A 10A
0.0
0A 0.19
0.20
0.21
0.22
0.23
0.24
Fig. 8 The concentration of solute atoms around the dislocation (a) and variation of binding energy between solute atoms and dislocation per unit length (b) versus time. The concentration of solute atoms and variation of binding energy when t=tw (waiting time) are shown by the symbols on the curves under different intensities of electropulsing, meaning the maximum which could reach to for concentration of solute atoms (a) and variation of binding energy (b) under different intensities of electropulsing on the basis of definition of waiting time. 27
Fig. 9 Hypothetical representations of segregated solute atoms around dissociated partial dislocations. (a) without electropulsing; (b) with a high intensity of electropulsing [39] .
28