- Email: [email protected]
Softening behavior of Al-Zn-Mg alloys with different strengthening mechanisms in a coupled field
Materials Science & Engineering A 771 (2020) 138582
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Softening behavior of Al-Zn-Mg alloys with different strengthening mechanisms in a coupled field Hexiong Zhang, Xinfang Zhang * State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electric pulse Softening behavior Strengthening mechanism Hall-Petch Intragranular Grain boundary
The softening behavior of Al-Zn-Mg alloys in solid solutions and aging states was investigated under an electric pulse by establishing the Hall-Petch relationship. Because intragranular strengthening and grain boundary strengthening are described separately by the Hall-Petch relationship, the difference between the intragranular softening behavior and grain boundary softening behavior under an electric pulse could be discerned. The effect of an electric pulse on the metal was studied, and it was concluded that this effect was caused by the coupled effect of Joule heating and drifting electrons. The decrease in stress (intragranular and grain boundary) caused by Joule heating was significantly greater for the solid solution alloy than that for the aging alloy. On the other hand, the decrease in stress at the grain boundary caused by drifting electrons was greater for the solid solution alloy than that for the aging alloy, while the decrease in intragranular stress caused by drifting electrons was significantly smaller for the solid solution alloy than that for the aging alloy. Based on the principles of electron scattering, interaction between electrons, dislocation, and electromigration, the essential reasons for the observed discrepancy in the softening behavior under different strengthening mechanisms were revealed.
1. Introduction Electric-pulse-assisted forming (EPAF) refers to the application of an electric pulse while processing a material to facilitate its smooth for mation; this technology is thus based on the electroplasticity effect [1]. The high forming quality, energy savings, and environmental protection offered by EPAF has attracted increasing attention toward this technique in the industry [2,3]. EPAF is characterized by a decrease in stress in the machining process, i.e., softening of the processed material, which im proves the formability of the material and prolongs the service life of the tool [4–6]. The reason for the softening of a metal with the application of current is the coupled effect of Joule heating and drifting electrons. The ability of Joule heating to reduce the processing stress of metals has been widely established [7,8]. Metals soften under the influence of drifting electrons due to the interaction between the drifting electrons and dis locations. In other words, drifting electrons can promote the initiation and slipping of dislocations, thereby reducing the deformation stress [9–11]. Recent studies of EPAF have found that electrical pulses can interact with grain boundaries and intragranular structures [12–15], while reducing the dislocation density during deformation [16–18]. This
indicates that a current can soften the defects (such as grain boundaries and precipitates) that hinder dislocation movement and reduce the ability of these defects to block dislocation, thus reducing the dislocation density at which the metal yields. Whether the current promotes dislo cation movement, or the current acts to reduce dislocations by softening the defects in the metal, both viewpoints provide a good explanation of the softening effect of a current on a metal. Owing to the different structures of the intragranular space and grain boundaries, the means by which the dislocations are hindered is different, and the softening effect on the intragranular structures and grain boundaries caused by a current should thus also be different. To reveal the law of electroplasticity in more detail, it is significant to distinguish the softening effect at intra granular structures and grain boundaries caused by a current. Limita tions of the current characterization methods imply that is difficult to observe the softening behavior of intragranular structures and grain boundaries caused by application of a current directly, thereby neces sitating indirect experiments and calculations. The Hall-Petch relationship describes the relationship between the grain size and yield strength. Its most important feature is that the intragranular strengthening and grain boundary strengthening are described separately, as shown in Equation (1) [19,20]:
* Corresponding author. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.msea.2019.138582 Received 13 August 2019; Received in revised form 21 October 2019; Accepted 22 October 2019 Available online 23 October 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
H. Zhang and X. Zhang
σ y ¼ σ o þ kd
0:5
Materials Science & Engineering A 771 (2020) 138582
3. Results
(1)
3.1. Grain size statistics
where σy is the yield strength, σo is the intragranular strengthening term, kd 0.5 is the grain boundary strengthening term, k is the grain boundary strengthening coefficient, and d is the average grain size. A distinction between the softening behavior of the intragranular structures and grain boundaries can be achieved by studying the effect of the current on σo and kd 0.5 in the Hall-Petch relationship. A reduction in σ o caused by the current can be regarded as intragranular softening, while a reduction in kd 0.5 caused by the current can be regarded as the softening behavior at the grain boundary. Another characteristic of the Hall-Petch relationship is that the pa rameters σo and k are greatly influenced by the reinforcement mecha nism. Studies have shown that k differs markedly for low-carbon steel in a solid solution state and an aging state [21,22]. It is speculated that the softening behavior of intragranular structures and grain boundaries caused by the application of a current should also differ when the strengthening mechanism is different. In this study, EPAF experiments were designed to establish the HallPetch relationship under an electric pulse. By analyzing the influence of an electric pulse on σ o and kd 0.5 in the Hall-Petch relationship, the softening behavior of intragranular structures and grain boundaries caused by the current were investigated under various strengthening mechanisms. In addition, the observed experimental phenomena were explained based on the principle of electron scattering. The research focus and methods adopted in this study can play an important role in the improvement of electroplastic theory.
OM images of samples with the three different grain sizes are shown in Fig. 1a–c. The number of grains in Fig. 1a–c was counted with the linear intercept method, and the average equivalent circular area of the grains was calculated. The average grain sizes (grain equivalent circular diameter) of the three samples were 30 μm (Fig. 1a), 38 μm (Fig. 1b), and 103 μm (Fig. 1c). 3.2. Tensile test results To establish the Hall-Petch relationship in the EPAF process, an electric-pulse-assisted tensile test was carried out on the peak aging samples with grain sizes of 30 μm, 38 μm, and 103 μm. The test results are shown in Fig. 2. Fig. 2a shows the results of the tensile tests without an electric pulse. It includes three samples with a grain size of 30 μm (all marked as T1), three samples with a grain size of 38 μm (all marked as T2), and three samples with a grain size of 103 μm (all marked as T3). It can be seen that the yield strength and elongation of the samples increase in the order of T1 > T2 > T3. Fig. 2b shows the results of the electric-pulse-assisted tensile test. The parameters of the electric pulse were 55 A/mm2, 200 Hz, and 185 μs. Under the condition of no air-cooling, two samples with a grain size of 30 μm (all marked as T1-EP), two samples with a grain size of 38 μm (all marked as T2-EP), and two samples with a grain size of 103 μm (all marked as T3-EP) were tested. Under air-cooled conditions, one sample with a grain size of 30 μm (marked as T1-EP-W), one sample with a grain size of 38 μm (marked as T2-EP-W), and one sample with a grain size of 103 μm (marked as T3-EP-W) were tested. It can be seen that the yield strength and elongation of the samples under the two conditions increased in the order of T1-EP > T2-EP > T3-EP and T1-EP-W > T2-EPW > T3-EP-W. The yield strengths of the samples with and without aircooling had the following relationships: T1-EP-W > T1-EP, T2-EPW > T2-EP, and T3-EP-W > T3-EP. The elongation of the samples with and without air-cooling had the following relationships: T1-EP > T1-EPW, T2-EP > T2-EP-W, and T3-EP > T3-EP-W.
2. Experiments An as-cast Al-5.16Zn-2.2Mg-1.46Cu (wt.%) alloy was successively homogenized (465 � C þ 24 h), warm rolled (200–220 � C), dissolved/ recrystallized (495 � C þ 30 min), and peak aged (160 � C þ 10 h). Thus, peak-aged samples with three different grain sizes were obtained. With the aid of a wire cutting machine, the peak aging alloy specimens were cut into bone-like tensile samples with a gauge length of 27 mm, width of 3 mm, and thickness of 1 mm. The three types of samples with different grain sizes were observed using an optical microscope (OM), and the average grain size of the three samples was obtained statistically using the linear intercept method. Tensile tests were carried out with the help of a tensile testing machine at a strain rate of 2 � 10 4 s 1 to obtain the basic data for the establishment of the Hall-Petch relationship under EPAF. The tensile tests were conducted under two conditions: with no electric pulse (contrast test), and with the assistance of an electric pulse. During the electric-pulse-assisted tensile tests, after turning on the pulse power supply, the sample temperature was allowed to become basically stable (after 300 s) before the tensile test machine was turned on and the tensile test was conducted. Throughout the test process, an electric pulse was supplied by a pulse power supply with a current density of 55 A/ mm2, frequency of 200 Hz, and pulse width of 185 μs (The current density is the ratio of the peak current to the cross-sectional area through which the current passes; the frequency is the number of times of current wave passing through the sample per unit time; the pulse width is the duration of current in each period). For subsequent in-depth analysis, an air-cooling treatment was used to distinguish the effects of drifting electrons and Joule heating on the Hall-Petch relationship. The tem perature of the samples was monitored in real-time using thermocouples with and without the air-cooling treatment. Under each test condition, 2–4 replicate samples were tested. When the tensile strain reached 0.025, stretching was stopped (a small amount of plastic deformation occurred), and one sample was taken under each condition. The gauge length of the tested samples was observed with a transmission electron microscope (TEM) to determine the dislocation configuration. The remaining samples were stretched to failure. Specific test parameters, test numbers, and sample numbers are listed in Table 1.
3.3. Temperature test To distinguish the influence of drifting electrons and Joule heating on the Hall-Petch relationship during the electric-pulse-assisted tensile tests, the temperature of the samples was monitored in real-time during the tests, as shown in Fig. 3. Fig. 3 shows the real-time monitoring data for the sample tempera ture under electric pulse conditions of 55 A/mm2, 200 Hz, and 185 μs without air-cooling (T-EP) and with air-cooling (T-EP-W). For T-EP-W, there is almost no temperature increase in the sample (the ambient temperature is 25 � C). However, for T-EP, the temperature of the sample Table 1 Specific test parameters, test numbers, and sample numbers.
2
Grain Size (μm)
Current Density (A/ mm2)
Air Cooling
Number of Repeated Samples
Test Number
Sample Number
30 38 103 30 38 103 30 38 103
– – – 55 55 55 55 55 55
– – – No No No Yes Yes Yes
4 4 4 3 3 3 2 2 2
T T T T-EP T-EP T-EP T-EP-W T-EP-W T-EP-W
T1 T2 T3 T1-EP T2-EP T3-EP T1-EP-W T2-EP-W T3-EP-W
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 1. OM images of samples with three different grain sizes: a 30 μm; b 38 μm; c 103 μm. Fig. 2. Tensile curves: a tensile tests without an electric pulse for samples with grain sizes of 30 μm (T1, 3 replicates), 38 μm (T2, 3 replicates), and 103 μm (T3, 3 replicates); b electric-pulse-assisted tensile tests with a current density of 55 A/mm2, frequency of 200 Hz, and pulse width of 185 μs for the sample with a grain size of 30 μm grain and no air-cooling (T1-EP, 2 replicates), 30 μm with aircooling (T1-EP-W (-W indicates air-cooling), 1 replicate), 38 μm and no air-cooling (T2-EP, 2 rep licates), 38 μm with air-cooling (T2-EP-W, 1 repli cated), 103 μm and no air-cooling (T3-EP, replicates), and 103 μm with air-cooling (T3-EP-W, 1 replicate).
Fig. 3. Sample temperatures measured during electric-pulse-assisted tensile tests with air-cooling (red curve) and without air-cooling (black curve). (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reaches approximately 52 � C.
Fig. 4a–f shows the dislocation configurations of samples T1, T1-EP, T2, T2-EP, T3, and T3-EP respectively. It can be seen that the dislocation accumulation is very severe when the alloy yields without application of an electric pulse (Fig. 4a, c, and e). However, Fig. 4b, d, and f show that the degree of dislocation accumulation in the pulse-assisted tensile sample is reduced compared with that in Fig. 4a, c, and e, respectively. Dislocation entanglement and matrix distortion will hinder the observation of the precipitates. To observe the effect of the
3.4. Dislocation and precipitate configuration TEM was used to observe the dislocation and precipitate configura tion of the tensile sample gauge section at a strain of 0.025 (plastic section near the yield point). All samples were observed with the <001> crystal band axis. The results are shown in Fig. 4. 3
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 4. Dislocation configurations observed with TEM: a, c, and e are the dislocation configurations for samples T1, T2, and T3, respectively (without an electric pulse); b, d, and f are the dislocation configurations for samples T1-EP, T2-EP, and T3-EP, respectively (with the application of an electric pulse); g and h are enlarged view of the red wireframe areas of e and f, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
electropulsing on the precipitates, the enlarged views of the red wire frame area in Fig. 4e and f are shown in Fig. 4g and h, respectively. A comparison between Fig. 4g and h shows that the characteristics of the precipitates are similar whether or not an electric pulse is applied.
The softening effect of drifting electrons on the intracrystalline structure is an intrinsic reduction in the dislocation slip resistance caused by drifting electrons, which results in a decrease in the intercept of the Hall-Petch relationship, as shown in Equation (3):
4. Discussion
σ ew
4.1. Distinction of grain boundary and intragranular softening behavior under different strengthening mechanisms It can be seen from Fig. 1 that the grain size of the test materials involved in this study is relatively uniform. According to the previous study [23], it can be concluded that the materials in this study have no texture or deformation structure. Therefore, the influence of grain size distribution (with one maximum, bimodal, etc.) and misorientation angles on the test results can be ignored in this study. According to the relationship between the yield strength and grain size given in Equation (1), the yield strength and grain size data obtained in this study were linearly regressed to obtain the Hall-Petch relation ship for conditions of T, T-EP-W, and T-EP, as shown in Fig. 5. Fig. 5 shows that the slope and intercept of the Hall-Petch line decrease in turn under the T, T-EP-W, and T-EP test conditions, which indicates that both Joule heating and drifting electrons caused by the electric pulse can soften the material grain boundary (slope) and intra granular structures (intercept). To distinguish the softening behavior of the intragranular structures and grain boundaries caused by the electric pulse, the yield strengths of T, T-EP-W, and T-EP were denoted as σ (d), σ EP–W(d), and σ EP(d), respectively. When the grain size is infinite (intercept), the yield strengths of T, T-EP-W and T-EP are denoted as σ (d→∞), σ EP–W(d→∞), and σEP(d→∞), respectively. The softening effect of Joule heating on the intracrystalline structure is essentially a decrease in the dislocation slip resistance caused by Joule heating, which is represented by a decrease in the intercept of the HallPetch relationship, as shown in Equation (2):
σJ
In
¼ σEP
W ðd
→ ∞Þ
σ EP ðd → ∞Þ
In
¼ σ ðd → ∞Þ
σ EP
(3)
W ðd → ∞Þ
The softening effect of Joule heating on the grain boundaries, which is equal to the difference between the total decrease in stress caused by the Joule heating and the softening stress decrease in the intracrystalline structure, can be calculated as shown in Equation (4):
σ J G ðdÞ ¼ σ EP
W ðdÞ
σ EP ðdÞ
σEP
W ðd → ∞Þ
þ σ EP ðd → ∞Þ
(4)
The softening effect of drifting electrons on the grain boundaries, which is equal to the difference between the total decrease in stress caused by drifting electrons and the softening stress decrease in the intracrystalline structure, can be calculated as shown in Equation (5):
σ ew G ðdÞ ¼ σ ðdÞ
σ EP
W ðdÞ
σ ðd → ∞Þ þ σ EP
W ðd
→ ∞Þ
(5)
By calculating and analyzing the reduction in the yield strengths
σ J–In, σew–In, σ J–G(d), and σew–G(d) mentioned above, the softening
behavior of the intragranular structures and grain boundaries caused by the electric pulse can be distinguished. Meanwhile, to investigate the effect of the electric pulse on the grain boundary and intragranular softening behavior under different strengthening mechanisms, the re sults of this study (Al-5.16Zn-2.2Mg-1.46Cu (wt.%), T6 state) were compared with those of a previous study (Al-5.16Zn-2.2Mg-1.46Cu (wt. %), solid solution state) [23]. The four yield strength reductions in this study are recorded as T–σJ–In, T–σew–In, T–σJ–G(d), and T–σ ew–G(d). The corresponding four yield strength reductions given in Ref. [23] are denoted as S–σ J–In, S–σ ew–In, S–σ J–G(d), and S–σew–G(d), respectively, as shown in Fig. 6. The four points marked A, B, C, and D in Fig. 6 are the intersections of T–σew–In and T–σ ew–G(d), T–σJ–In and T–σ J–G(d), S–σew–In and S–σew–G(d), S–σJ–In and S–σJ–G(d), respectively. The grain sizes corresponding to points A, B, C, and D are 65 μm, 58 μm, 128 μm, and 71 μm, respectively.
(2)
Fig. 5. Hall-Petch relationship for three experimental conditions: no pulse assistance (T), electric-pulse-assisted and air-cooled (T-EP-W), and electric-pulse-assisted with no air-cooling (T-EP). 5
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
The results show that there is no obvious tendency for Joule heating to soften either the intragranular structure or grain boundaries in the studied grain size range (18–103 μm). The drifting electrons have no obvious tendency to soften either the intragranular structures or the grain boundaries in the aging state. However, in the solid solution state, the drifting electrons have a greater effect on softening the grain boundaries than the intragranular structures. Overall, in grain size range in this study, the softening effect of an electric pulse with fixed parameters on a sample in the solid solution state is greater than that on a sample in the aging state, as shown in Equation (6): ðT σ J In Þ þ ðT σ ew In Þ þ ðT σJ G ðdÞÞ þ ðT σew G ðdÞÞ < ðS σJ In Þ þ ðS σew In Þ þ ðS σ J G ðdÞÞ þ ðS σew G ðdÞÞ
4.2. Grain boundary softening behavior caused by drifting electrons under different strengthening mechanisms Drifting electrons can interact with defects or structures in metals. Generally, the stronger the scattering effect of the defects is on the drifting electrons, the more marked the softening of the defects by the drifting electrons will be. The intensity of the scattering of drifting electrons in the metal can be characterized by the resistivity of the metal: the larger the resistivity of the metal, the stronger the scattering effect of the defects in the metal will be. As the sample transitions from a solid solution state to an aging state, solid solution atoms are precipitated from the matrix, and thus the grain boundary structure will change, as shown in Fig. 7. Fig. 7a and b shows the grain boundary morphology and elemental analysis perpendicular to the grain boundary in the solid solution state, respectively, indicating that the solid solution state has a good solid solution effect. There are effectively no large precipitate particles pre sent at the grain boundary, and the alloying elements are evenly distributed near the grain boundary. Fig. 7c and d shows the grain boundary morphology and elemental analysis perpendicular to the grain boundary in the aging state, respectively. Fig. 7c shows the occurrence of intermittent precipitates with a size of approximately 20 nm on the grain boundary. A precipitate free zone (PFZ) with a width of approxi mately 50 nm is formed near the grain boundary. Fig. 7d shows that the content of alloying elements in the region between the precipitates on the grain boundary is consistent with that in the PFZ, and the content of alloying elements these two regions is significantly lower than that in the solid solution state. Studies have shown that for Al-Zn-Mg-Cu alloys, the resistivity in the solid solution state is greater than that in the peak aging state [25,26], indicating that the solid solution sample has a greater scattering effect on drifting electrons compared with the aging sample. A region with a width of 25 nm on each side of the grain boundary is examined in greater detail. The mechanisms for the scattering of drifting electrons at the grain boundary and its vicinity under different strengthening states are shown in Fig. 8. From Fig. 4g and h, it can be concluded that the electric pulse under
(6)
Comparing the softening behavior caused by Joule heating under two strengthening mechanisms reveals that within the range of grain sizes studied, the degree of intragranular and grain boundary softening caused by Joule heating in the solution state is higher than that in the aging state. The reason for this phenomenon is that the precipitate has a higher thermal stability than the atoms in solid solution at the test temperature. The Al-Zn-Mg alloy has a tendency to precipitate in the range of 25–150 � C [24]. The test temperature is approximately 52 � C, which is within that precipitation range. Thus, it is concluded that the precipitate has a higher thermal stability than the atoms in solid solution at the test temperature. Within the range of grain sizes studied, the degree of intragranular softening caused by drifting electrons in the solid solution state is lower than that in the aging state; however, the degree of grain boundary softening caused by drifting electrons in the solid solution state is higher than that in the aging state. The reasons for these phenomena lie in the different scattering effects of the precipitate and solid solution atoms on electrons and the different grain boundary structures present in alloys in the solid solution and aging states. A more detailed analysis of this is given below.
Fig. 6. Decrease in stress caused by the electrical pulse in solid solution and aging sample states: S–σew–In and T–σ ew–In are the decrease in stress caused by drifting elec trons softening the intragranular structures; S–σJ–In and T–σ J–In are the decrease in stress caused by Joule heating softening the intra and granular structures; S–σew–G(d) T–σ ew–G(d) are the decrease in stress caused by drifting electrons softening the grain boundaries; S–σ J–G(d) and T–σJ–G(d) are the decrease in stress caused by Joule heating softening the grain boundaries.
6
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 7. Grain boundary morphology and elemental distribution obtained with TEM: a and b are the grain boundary morphology and elemental analysis (using energy-dispersive spectroscopy (EDS) attachment with the trajectory shown by the yellow arrow), respectively, in the solid solution sample; c and d are the grain boundary morphology and elemental analysis (using EDS with the trajectory shown by the yellow arrow), respectively, in the aging sample; in c, the trajectory of the elemental analysis crosses vertically between two adjacent precipitates on the boundary without passing through the precipitate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the parameter in this study have no significant effect on the precipitation and dissolution behavior in the aging state. Previous studies have shown that the electric pulse under this parameter has no significant effect on the precipitation and dissolution behavior in the solid solution state [23]. So the physical model in Fig. 8 is reasonable. For solid solution samples, the drifting electrons are scattered by the solid solution atoms around the grain boundary, as shown in Fig. 8a. With aging and the precipitation of solid solution atoms, the content of solid solution atoms within the 50 nm width around the grain boundary decreases sharply. A “channel” with a low content of solid solution atoms (i.e., low resistivity) is formed between the precipitates. The drifting electrons thus detour from the precipitate to this “channel,” which reduces the scattering effect of the grain boundary structure, as shown in Fig. 8b. The effect of scattering by the grain boundary on drifting electrons in the aging state should thus be less than that in the solid solution state, which is consistent with the conclusions in Refs. [25, 26]. After aging, the scattering of drifting electrons by the grain boundary decreases, resulting in a degree of grain boundary softening caused by drifting electrons in the solid solution state that is greater than that in the aging state.
The direction of the electron wind force is the same as that of the drifting electrons [27]. The electron wind force can be calculated as shown in Equation (7): few ¼ Kew lJ
(7)
where Kew is electron wind force constant, l is the dislocation length, J is the current density, and few is the electron wind force on a dislocation. In this study, the sample matrix is aluminum in both the solid solu tion state and the aging state. According to the literature [28], the effective atomic valence for electromigration of aluminum atoms is ZAl* < 0, i.e., the direction of the electromigration of aluminum atoms is the same as that of the drifting electrons. In both the solid solution state and the aging state, the effect of the electron wind force on dislocation can be considered consistent. The drifting electrons strike the matrix atoms near the vacancies below the dislocations, causing the matrix atoms to migrate toward the vacancies along the direction of the drifting elec trons, eventually causing dislocations to occur and a reverse movement of the drifting electrons, as shown in Fig. 9 (gray arrow and gray rectangle). In the solid solution state, the drifting electrons interact with dislo cations as well as with the solute atoms pinning dislocations, resulting in the electromigration of solute atoms. Previous studies [28] have re ported that the effective atomic valences of zinc and copper atoms are ZZn* < 0 and ZCu* < 0, and the migration direction is the same as that of the drifting electrons, as shown in Fig. 9 (blue arrow and blue rectangle). The calculation of the electromigration force is given by Equation (8):
4.3. Intragranular softening behavior caused by drifting electrons under different strengthening mechanisms In the aging state, the intragranular softening caused by drifting electrons can be considered equivalent to the effect of the electron wind force (between drifting electrons and dislocations) on the dislocations. 7
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 8. a Schematic diagram of the scattering of electrons by solid solution atoms in the 50 nm width around the grain boundary in the solid solution sample; b schematic diagram of the scattering of electrons by precipitates on the grain boundary in the aging sample; in b, the region between the precipitates and the PFZ together form a low-resistivity “channel,” which will induce the current to detour from the precipitate to this “channel".
force on the solute atoms. In the solid solution state, because part of the electron wind force on the dislocation is absorbed by the solid solution atoms pinning the dislocation, the dislocation de-pinning force caused by drifting electrons is the difference between the electron wind force and the electro migration force, as shown in Equation (9): fS ¼ few
fem ¼ ðKew l
Z * eρÞJ
(9)
where fS is the dislocation de-pinning force caused by drifting electrons in the solid solution state. Equations (7) and (9) show that under the same current parameters, the dislocation de-pinning force provided by drifting electrons in aging samples is greater than that in solid solution samples, which may be the reason that the degree of intracrystalline softening caused by drifting electrons is greater in aging samples than in solid solution samples. 5. Conclusions (1) Over the range of grain sizes (18–103 μm), both with or without air-cooling, the electric pulse has obvious softening effect under the solid solution state in our previously observed reference [23] and the aging state in this study. The softening effect caused by Joule heating is greater than that of drifting electrons. (2) Combined the results of this study with reference [23], softening effect of drift electrons on grain boundary in the solid solution state is greater than that in the aging state. However, softening effect of drift electrons on intragranular in the aging state is greater than that in the solid solution state. (3) A “channel” with low solid solution atom content is formed at the grain boundary during the aging process. The drifting electrons thus detour from the precipitate to this “channel,” which reduces the scattering of drifting electrons by the grain boundary in the original solid solution state. As a result, the degree of grain
Fig. 9. Mechanisms of the electron wind force (between drifting electrons and dislocations) and electromigration force (between drifting electrons and the solid solution atoms pinning dislocations); in solid solution samples, the elec tron wind force acts on dislocations by interacting with matrix atoms near the vacancies below dislocations; the electron wind force on the dislocations is in the same direction as that on the solid solution atoms pinning dislocations, and part of the electron wind force on the dislocations will be absorbed by the solid solution atoms pinning dislocations; in other words, the solid solution atoms pinning dislocations reduce the effect of drifting electrons on the dislocations.
fem ¼ Z * eρJ
(8)
where Z* is the effective atomic valence for the electromigration of solid solution atoms, e is the electronic charge, ρ is the resistivity caused by the solute atoms pinning dislocations, and fem is the electromigration 8
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
boundary softening caused by drifting electrons in the solid so lution state is greater than that in the aging state. (4) Drifting electrons provide a greater dislocation de-pinning force in aging samples than in solid solution samples. As a result, the degree of intracrystalline softening caused by drifting electrons in aging samples is greater than that in solid solution samples.
[8] G. Chen, J.T. Li, Z.K. Yin, G.M. Xu, Improvement of microstructure and properties in twin-roll casting 7075 sheet by lower casting speed and compound field, Mater. Char. 127 (2017) 325–332. [9] V.Y. Kravchenko, Effect of directed electron beam on moving dislocations, Sov. Phys. JETP 24 (1967) 1135–1142. [10] K. Okazaki, M. Kagawa, H. Conrad, An evaluation of the contributions of skin, pinch and heating effects to the electroplastic effect in titatnium, Mater. Sci. Eng. 45 (1980) 109–116. [11] G.V. Stepanov, A.I. Babutskii, Effect of electric current on stress relaxation in metal, Strength Mater. 28 (1996) 125–128. [12] X. Li, X. Li, Y. Ye, R. Zhang, S.Z. Kure-Chu, G. Tang, Deformation mechanisms and recrystallization behavior of Mg-3Al-1Zn and Mg-1Gd alloys deformed by electroplastic-asymmetric rolling, Mater. Sci. Eng. A 742 (2019) 722–733. [13] M. Li, D. Guo, J. Li, S. Zhu, C. Xu, K. Li, Y. Zhao, B. Wei, Q. Zhang, X. Zhang, Achieving heterogeneous structure in hcp Zr via electroplastic rolling, Mater. Sci. Eng. A 722 (2018) 93–98. [14] K. Li, M. Li, Y. Zhao, W. Shan, Y. Cao, D. Guo, Achieving ultralow elastic modulus in TiNi alloy by controlling nanoscale martensite phase, Mater. Lett. 233 (2018) 282–285. [15] X. Li, F. Wang, X. Li, J. Zhu, G. Tang, Mg-3Al-1Zn alloy strips processed by electroplastic differential speed rolling, Mater. Sci. Technol. 33 (2017) 215–219. [16] H.J. Jeong, J.W. Park, K.J. Jeong, N.M. Hwang, S.T. Hong, H.N. Han, Effect of pulsed electric current on TRIP-aided steel, Int. J. Precis. Eng. Manuf-Green Technol. (2018) 1–13. [17] A. Ghiotti, S. Bruschi, E. Simonetto, C. Gennari, I. Calliari, P. Bariani, Electroplastic effect on AA1050 aluminium alloy formability, CIRP Annals 67 (2018) 289–292. [18] H. Krishnaswamy, M.J. Kim, S.T. Hong, D. Kim, J.H. Song, M.G. Lee, H.N. Han, Electroplastic behaviour in an aluminium alloy and dislocation density based modeling, Mater. Des. 124 (2017) 131–142. [19] E.O. Hall, The deformation and ageing of mild steel: III discussion of results, Proc. Phys. Soc. Sect. B 64 (1951) 747–753. [20] N.J. Petch, The cleavage strength of polycrystals, Journal of the Iron and Steel Institute 174 (1953) 25–28. [21] S. Takaki, D. Akama, N. Nakada, T. Tsuchiyama, Effect of grain boundary segregation of interstitial elements on Hall-Petch coefficient in steels, Mater. Trans. 55 (2014) 28–34. [22] S. Araki, K. Fujii, D. Akama, T. Tsuchiyama, S. Takaki, T. Ohmura, J. Takahashi, Effect of low temperature aging on Hall-Petch coefficient in ferritic steels containing a small amount of carbon and nitrogen, ISIJ Int. 58 (2018) 1920–1926. [23] H. Zhang and X. Zhang, Hall-Petch relationship in electrically pulsed Al-Zn-Mg alloys, Adv. Eng. Mater., https://doi.org/10.1002/adem.2019006381900638; 1 to 13. [24] P.K. Rout, M.M. Ghosh, K.S. Ghosh, Microstructural, mechanical and electrochemical behaviour of a 7017 Al-Zn-Mg alloy of different tempers, Mater. Char. 104 (2015) 49–60. [25] X. Li, Q. Cai, B. Zhao, B. Liu, W. Li, Precipitation behaviors and properties of solution-aging Al-Zn-Mg-Cu alloy refined with TiN nanoparticles, J. Alloy. Comp. 746 (2018) 462–470. [26] K. Wen, Y. Fan, G. Wang, L. Jin, X. Li, Z. Li, Y. Zhang, B. Xiong, Aging behavior and fatigue crack propagation of high Zn-containing Al-Zn-Mg-Cu alloys with zinc variation, Prog. Nat. Sci.: Materials International 27 (2017) 217–227. [27] S.W. Nam, H.S. Chung, Y.C. Lo, L. Qi, J. Li, Y. Lu, A.T. Charlie Johnson, Y. Jung, P. Nukala, R. Agarwal, Electrical wind force-driven and dislocation-templated amorphization in phase-change nanowires, Science 336 (2012) 1561–1566. [28] P.S. Ho, T. Kwok, Electromigration in metals, Rep. Prog. Phys. 52 (1989) 301–348.
Data availability The data that support the finding of this study are available from the corresponding author upon request. Declaration of competing interest There are no conflicts of interest for the publication of “Softening Behavior of Al-Zn-Mg Alloys with Different Strengthening Mechanisms in a Coupled Field”. Acknowledgements The work was financially supported by National Natural Science Foundation of China (51874023, 51601011, U1860206), Fundamental Research Funds for the Central Universities (FRF-TP-18-003B1), Recruitment Program of Global Experts. References [1] O.A. Troitskii, V.I. Likhtman, Anisotropy of action of gamma-electronic emission and gamma rays on deformation of single zinc crystals in the brittle state, Dokl. Akad. Nauk SSSR 148 (1963) 332–334. [2] H.D. Nguyen-Tran, H.S. Oh, S.T. Hong, H.N. Han, J. Cao, S.H. Ahn, D.M. Chun, A review of electrically-assisted manufacturing, Int. J. Precis. Eng. Manuf-Green Technol. 2 (2015) 365–376. [3] X. Zhang, H. Li, S. Yan, N. Zhang, Experimental study and analysis on the electrically-assisted tensile behaviors of Inconel 718 alloy, Procedia Eng. 207 (2017) 365–370. [4] C. Montilla Monta~ na, V. Kallewaard, H.A. Gonz� alez Rojas, Effect of electropulses on the machinability of a C45E steel, Dyna Ingeniería E Industria 94 (2019) 94–99. [5] B.J. Ruszkiewicz, E. Gendreau, F.A. Niaki, L. Mears, Electroplastic drilling of lowand high-strength steels, J. Manuf. Sci. Eng. 140 (2018), 061017. [6] B.J. Ruszkiewicz, T. Grimm, I. Ragai, L. Mears, J.T. Roth, A review of electricallyassisted manufacturing with emphasis on modeling and understanding of the electroplastic effect, J. Manuf. Sci. Eng. 139 (2017) 110801. [7] L.M. Yan, J. Shen, D. An, Z.X. Du, J.B. Zhang, Recrystallization characterizations of an Al-Zn-Mg-Cu-Zr alloy during multi-pass hot rolling simulation, Rare Metal Mater. Eng. 46 (2017) 3233–3238.
9
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Softening behavior of Al-Zn-Mg alloys with different strengthening mechanisms in a coupled field Hexiong Zhang, Xinfang Zhang * State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electric pulse Softening behavior Strengthening mechanism Hall-Petch Intragranular Grain boundary
The softening behavior of Al-Zn-Mg alloys in solid solutions and aging states was investigated under an electric pulse by establishing the Hall-Petch relationship. Because intragranular strengthening and grain boundary strengthening are described separately by the Hall-Petch relationship, the difference between the intragranular softening behavior and grain boundary softening behavior under an electric pulse could be discerned. The effect of an electric pulse on the metal was studied, and it was concluded that this effect was caused by the coupled effect of Joule heating and drifting electrons. The decrease in stress (intragranular and grain boundary) caused by Joule heating was significantly greater for the solid solution alloy than that for the aging alloy. On the other hand, the decrease in stress at the grain boundary caused by drifting electrons was greater for the solid solution alloy than that for the aging alloy, while the decrease in intragranular stress caused by drifting electrons was significantly smaller for the solid solution alloy than that for the aging alloy. Based on the principles of electron scattering, interaction between electrons, dislocation, and electromigration, the essential reasons for the observed discrepancy in the softening behavior under different strengthening mechanisms were revealed.
1. Introduction Electric-pulse-assisted forming (EPAF) refers to the application of an electric pulse while processing a material to facilitate its smooth for mation; this technology is thus based on the electroplasticity effect [1]. The high forming quality, energy savings, and environmental protection offered by EPAF has attracted increasing attention toward this technique in the industry [2,3]. EPAF is characterized by a decrease in stress in the machining process, i.e., softening of the processed material, which im proves the formability of the material and prolongs the service life of the tool [4–6]. The reason for the softening of a metal with the application of current is the coupled effect of Joule heating and drifting electrons. The ability of Joule heating to reduce the processing stress of metals has been widely established [7,8]. Metals soften under the influence of drifting electrons due to the interaction between the drifting electrons and dis locations. In other words, drifting electrons can promote the initiation and slipping of dislocations, thereby reducing the deformation stress [9–11]. Recent studies of EPAF have found that electrical pulses can interact with grain boundaries and intragranular structures [12–15], while reducing the dislocation density during deformation [16–18]. This
indicates that a current can soften the defects (such as grain boundaries and precipitates) that hinder dislocation movement and reduce the ability of these defects to block dislocation, thus reducing the dislocation density at which the metal yields. Whether the current promotes dislo cation movement, or the current acts to reduce dislocations by softening the defects in the metal, both viewpoints provide a good explanation of the softening effect of a current on a metal. Owing to the different structures of the intragranular space and grain boundaries, the means by which the dislocations are hindered is different, and the softening effect on the intragranular structures and grain boundaries caused by a current should thus also be different. To reveal the law of electroplasticity in more detail, it is significant to distinguish the softening effect at intra granular structures and grain boundaries caused by a current. Limita tions of the current characterization methods imply that is difficult to observe the softening behavior of intragranular structures and grain boundaries caused by application of a current directly, thereby neces sitating indirect experiments and calculations. The Hall-Petch relationship describes the relationship between the grain size and yield strength. Its most important feature is that the intragranular strengthening and grain boundary strengthening are described separately, as shown in Equation (1) [19,20]:
* Corresponding author. E-mail address: [email protected] (X. Zhang). https://doi.org/10.1016/j.msea.2019.138582 Received 13 August 2019; Received in revised form 21 October 2019; Accepted 22 October 2019 Available online 23 October 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
H. Zhang and X. Zhang
σ y ¼ σ o þ kd
0:5
Materials Science & Engineering A 771 (2020) 138582
3. Results
(1)
3.1. Grain size statistics
where σy is the yield strength, σo is the intragranular strengthening term, kd 0.5 is the grain boundary strengthening term, k is the grain boundary strengthening coefficient, and d is the average grain size. A distinction between the softening behavior of the intragranular structures and grain boundaries can be achieved by studying the effect of the current on σo and kd 0.5 in the Hall-Petch relationship. A reduction in σ o caused by the current can be regarded as intragranular softening, while a reduction in kd 0.5 caused by the current can be regarded as the softening behavior at the grain boundary. Another characteristic of the Hall-Petch relationship is that the pa rameters σo and k are greatly influenced by the reinforcement mecha nism. Studies have shown that k differs markedly for low-carbon steel in a solid solution state and an aging state [21,22]. It is speculated that the softening behavior of intragranular structures and grain boundaries caused by the application of a current should also differ when the strengthening mechanism is different. In this study, EPAF experiments were designed to establish the HallPetch relationship under an electric pulse. By analyzing the influence of an electric pulse on σ o and kd 0.5 in the Hall-Petch relationship, the softening behavior of intragranular structures and grain boundaries caused by the current were investigated under various strengthening mechanisms. In addition, the observed experimental phenomena were explained based on the principle of electron scattering. The research focus and methods adopted in this study can play an important role in the improvement of electroplastic theory.
OM images of samples with the three different grain sizes are shown in Fig. 1a–c. The number of grains in Fig. 1a–c was counted with the linear intercept method, and the average equivalent circular area of the grains was calculated. The average grain sizes (grain equivalent circular diameter) of the three samples were 30 μm (Fig. 1a), 38 μm (Fig. 1b), and 103 μm (Fig. 1c). 3.2. Tensile test results To establish the Hall-Petch relationship in the EPAF process, an electric-pulse-assisted tensile test was carried out on the peak aging samples with grain sizes of 30 μm, 38 μm, and 103 μm. The test results are shown in Fig. 2. Fig. 2a shows the results of the tensile tests without an electric pulse. It includes three samples with a grain size of 30 μm (all marked as T1), three samples with a grain size of 38 μm (all marked as T2), and three samples with a grain size of 103 μm (all marked as T3). It can be seen that the yield strength and elongation of the samples increase in the order of T1 > T2 > T3. Fig. 2b shows the results of the electric-pulse-assisted tensile test. The parameters of the electric pulse were 55 A/mm2, 200 Hz, and 185 μs. Under the condition of no air-cooling, two samples with a grain size of 30 μm (all marked as T1-EP), two samples with a grain size of 38 μm (all marked as T2-EP), and two samples with a grain size of 103 μm (all marked as T3-EP) were tested. Under air-cooled conditions, one sample with a grain size of 30 μm (marked as T1-EP-W), one sample with a grain size of 38 μm (marked as T2-EP-W), and one sample with a grain size of 103 μm (marked as T3-EP-W) were tested. It can be seen that the yield strength and elongation of the samples under the two conditions increased in the order of T1-EP > T2-EP > T3-EP and T1-EP-W > T2-EPW > T3-EP-W. The yield strengths of the samples with and without aircooling had the following relationships: T1-EP-W > T1-EP, T2-EPW > T2-EP, and T3-EP-W > T3-EP. The elongation of the samples with and without air-cooling had the following relationships: T1-EP > T1-EPW, T2-EP > T2-EP-W, and T3-EP > T3-EP-W.
2. Experiments An as-cast Al-5.16Zn-2.2Mg-1.46Cu (wt.%) alloy was successively homogenized (465 � C þ 24 h), warm rolled (200–220 � C), dissolved/ recrystallized (495 � C þ 30 min), and peak aged (160 � C þ 10 h). Thus, peak-aged samples with three different grain sizes were obtained. With the aid of a wire cutting machine, the peak aging alloy specimens were cut into bone-like tensile samples with a gauge length of 27 mm, width of 3 mm, and thickness of 1 mm. The three types of samples with different grain sizes were observed using an optical microscope (OM), and the average grain size of the three samples was obtained statistically using the linear intercept method. Tensile tests were carried out with the help of a tensile testing machine at a strain rate of 2 � 10 4 s 1 to obtain the basic data for the establishment of the Hall-Petch relationship under EPAF. The tensile tests were conducted under two conditions: with no electric pulse (contrast test), and with the assistance of an electric pulse. During the electric-pulse-assisted tensile tests, after turning on the pulse power supply, the sample temperature was allowed to become basically stable (after 300 s) before the tensile test machine was turned on and the tensile test was conducted. Throughout the test process, an electric pulse was supplied by a pulse power supply with a current density of 55 A/ mm2, frequency of 200 Hz, and pulse width of 185 μs (The current density is the ratio of the peak current to the cross-sectional area through which the current passes; the frequency is the number of times of current wave passing through the sample per unit time; the pulse width is the duration of current in each period). For subsequent in-depth analysis, an air-cooling treatment was used to distinguish the effects of drifting electrons and Joule heating on the Hall-Petch relationship. The tem perature of the samples was monitored in real-time using thermocouples with and without the air-cooling treatment. Under each test condition, 2–4 replicate samples were tested. When the tensile strain reached 0.025, stretching was stopped (a small amount of plastic deformation occurred), and one sample was taken under each condition. The gauge length of the tested samples was observed with a transmission electron microscope (TEM) to determine the dislocation configuration. The remaining samples were stretched to failure. Specific test parameters, test numbers, and sample numbers are listed in Table 1.
3.3. Temperature test To distinguish the influence of drifting electrons and Joule heating on the Hall-Petch relationship during the electric-pulse-assisted tensile tests, the temperature of the samples was monitored in real-time during the tests, as shown in Fig. 3. Fig. 3 shows the real-time monitoring data for the sample tempera ture under electric pulse conditions of 55 A/mm2, 200 Hz, and 185 μs without air-cooling (T-EP) and with air-cooling (T-EP-W). For T-EP-W, there is almost no temperature increase in the sample (the ambient temperature is 25 � C). However, for T-EP, the temperature of the sample Table 1 Specific test parameters, test numbers, and sample numbers.
2
Grain Size (μm)
Current Density (A/ mm2)
Air Cooling
Number of Repeated Samples
Test Number
Sample Number
30 38 103 30 38 103 30 38 103
– – – 55 55 55 55 55 55
– – – No No No Yes Yes Yes
4 4 4 3 3 3 2 2 2
T T T T-EP T-EP T-EP T-EP-W T-EP-W T-EP-W
T1 T2 T3 T1-EP T2-EP T3-EP T1-EP-W T2-EP-W T3-EP-W
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 1. OM images of samples with three different grain sizes: a 30 μm; b 38 μm; c 103 μm. Fig. 2. Tensile curves: a tensile tests without an electric pulse for samples with grain sizes of 30 μm (T1, 3 replicates), 38 μm (T2, 3 replicates), and 103 μm (T3, 3 replicates); b electric-pulse-assisted tensile tests with a current density of 55 A/mm2, frequency of 200 Hz, and pulse width of 185 μs for the sample with a grain size of 30 μm grain and no air-cooling (T1-EP, 2 replicates), 30 μm with aircooling (T1-EP-W (-W indicates air-cooling), 1 replicate), 38 μm and no air-cooling (T2-EP, 2 rep licates), 38 μm with air-cooling (T2-EP-W, 1 repli cated), 103 μm and no air-cooling (T3-EP, replicates), and 103 μm with air-cooling (T3-EP-W, 1 replicate).
Fig. 3. Sample temperatures measured during electric-pulse-assisted tensile tests with air-cooling (red curve) and without air-cooling (black curve). (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reaches approximately 52 � C.
Fig. 4a–f shows the dislocation configurations of samples T1, T1-EP, T2, T2-EP, T3, and T3-EP respectively. It can be seen that the dislocation accumulation is very severe when the alloy yields without application of an electric pulse (Fig. 4a, c, and e). However, Fig. 4b, d, and f show that the degree of dislocation accumulation in the pulse-assisted tensile sample is reduced compared with that in Fig. 4a, c, and e, respectively. Dislocation entanglement and matrix distortion will hinder the observation of the precipitates. To observe the effect of the
3.4. Dislocation and precipitate configuration TEM was used to observe the dislocation and precipitate configura tion of the tensile sample gauge section at a strain of 0.025 (plastic section near the yield point). All samples were observed with the <001> crystal band axis. The results are shown in Fig. 4. 3
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 4. Dislocation configurations observed with TEM: a, c, and e are the dislocation configurations for samples T1, T2, and T3, respectively (without an electric pulse); b, d, and f are the dislocation configurations for samples T1-EP, T2-EP, and T3-EP, respectively (with the application of an electric pulse); g and h are enlarged view of the red wireframe areas of e and f, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
electropulsing on the precipitates, the enlarged views of the red wire frame area in Fig. 4e and f are shown in Fig. 4g and h, respectively. A comparison between Fig. 4g and h shows that the characteristics of the precipitates are similar whether or not an electric pulse is applied.
The softening effect of drifting electrons on the intracrystalline structure is an intrinsic reduction in the dislocation slip resistance caused by drifting electrons, which results in a decrease in the intercept of the Hall-Petch relationship, as shown in Equation (3):
4. Discussion
σ ew
4.1. Distinction of grain boundary and intragranular softening behavior under different strengthening mechanisms It can be seen from Fig. 1 that the grain size of the test materials involved in this study is relatively uniform. According to the previous study [23], it can be concluded that the materials in this study have no texture or deformation structure. Therefore, the influence of grain size distribution (with one maximum, bimodal, etc.) and misorientation angles on the test results can be ignored in this study. According to the relationship between the yield strength and grain size given in Equation (1), the yield strength and grain size data obtained in this study were linearly regressed to obtain the Hall-Petch relation ship for conditions of T, T-EP-W, and T-EP, as shown in Fig. 5. Fig. 5 shows that the slope and intercept of the Hall-Petch line decrease in turn under the T, T-EP-W, and T-EP test conditions, which indicates that both Joule heating and drifting electrons caused by the electric pulse can soften the material grain boundary (slope) and intra granular structures (intercept). To distinguish the softening behavior of the intragranular structures and grain boundaries caused by the electric pulse, the yield strengths of T, T-EP-W, and T-EP were denoted as σ (d), σ EP–W(d), and σ EP(d), respectively. When the grain size is infinite (intercept), the yield strengths of T, T-EP-W and T-EP are denoted as σ (d→∞), σ EP–W(d→∞), and σEP(d→∞), respectively. The softening effect of Joule heating on the intracrystalline structure is essentially a decrease in the dislocation slip resistance caused by Joule heating, which is represented by a decrease in the intercept of the HallPetch relationship, as shown in Equation (2):
σJ
In
¼ σEP
W ðd
→ ∞Þ
σ EP ðd → ∞Þ
In
¼ σ ðd → ∞Þ
σ EP
(3)
W ðd → ∞Þ
The softening effect of Joule heating on the grain boundaries, which is equal to the difference between the total decrease in stress caused by the Joule heating and the softening stress decrease in the intracrystalline structure, can be calculated as shown in Equation (4):
σ J G ðdÞ ¼ σ EP
W ðdÞ
σ EP ðdÞ
σEP
W ðd → ∞Þ
þ σ EP ðd → ∞Þ
(4)
The softening effect of drifting electrons on the grain boundaries, which is equal to the difference between the total decrease in stress caused by drifting electrons and the softening stress decrease in the intracrystalline structure, can be calculated as shown in Equation (5):
σ ew G ðdÞ ¼ σ ðdÞ
σ EP
W ðdÞ
σ ðd → ∞Þ þ σ EP
W ðd
→ ∞Þ
(5)
By calculating and analyzing the reduction in the yield strengths
σ J–In, σew–In, σ J–G(d), and σew–G(d) mentioned above, the softening
behavior of the intragranular structures and grain boundaries caused by the electric pulse can be distinguished. Meanwhile, to investigate the effect of the electric pulse on the grain boundary and intragranular softening behavior under different strengthening mechanisms, the re sults of this study (Al-5.16Zn-2.2Mg-1.46Cu (wt.%), T6 state) were compared with those of a previous study (Al-5.16Zn-2.2Mg-1.46Cu (wt. %), solid solution state) [23]. The four yield strength reductions in this study are recorded as T–σJ–In, T–σew–In, T–σJ–G(d), and T–σ ew–G(d). The corresponding four yield strength reductions given in Ref. [23] are denoted as S–σ J–In, S–σ ew–In, S–σ J–G(d), and S–σew–G(d), respectively, as shown in Fig. 6. The four points marked A, B, C, and D in Fig. 6 are the intersections of T–σew–In and T–σ ew–G(d), T–σJ–In and T–σ J–G(d), S–σew–In and S–σew–G(d), S–σJ–In and S–σJ–G(d), respectively. The grain sizes corresponding to points A, B, C, and D are 65 μm, 58 μm, 128 μm, and 71 μm, respectively.
(2)
Fig. 5. Hall-Petch relationship for three experimental conditions: no pulse assistance (T), electric-pulse-assisted and air-cooled (T-EP-W), and electric-pulse-assisted with no air-cooling (T-EP). 5
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
The results show that there is no obvious tendency for Joule heating to soften either the intragranular structure or grain boundaries in the studied grain size range (18–103 μm). The drifting electrons have no obvious tendency to soften either the intragranular structures or the grain boundaries in the aging state. However, in the solid solution state, the drifting electrons have a greater effect on softening the grain boundaries than the intragranular structures. Overall, in grain size range in this study, the softening effect of an electric pulse with fixed parameters on a sample in the solid solution state is greater than that on a sample in the aging state, as shown in Equation (6): ðT σ J In Þ þ ðT σ ew In Þ þ ðT σJ G ðdÞÞ þ ðT σew G ðdÞÞ < ðS σJ In Þ þ ðS σew In Þ þ ðS σ J G ðdÞÞ þ ðS σew G ðdÞÞ
4.2. Grain boundary softening behavior caused by drifting electrons under different strengthening mechanisms Drifting electrons can interact with defects or structures in metals. Generally, the stronger the scattering effect of the defects is on the drifting electrons, the more marked the softening of the defects by the drifting electrons will be. The intensity of the scattering of drifting electrons in the metal can be characterized by the resistivity of the metal: the larger the resistivity of the metal, the stronger the scattering effect of the defects in the metal will be. As the sample transitions from a solid solution state to an aging state, solid solution atoms are precipitated from the matrix, and thus the grain boundary structure will change, as shown in Fig. 7. Fig. 7a and b shows the grain boundary morphology and elemental analysis perpendicular to the grain boundary in the solid solution state, respectively, indicating that the solid solution state has a good solid solution effect. There are effectively no large precipitate particles pre sent at the grain boundary, and the alloying elements are evenly distributed near the grain boundary. Fig. 7c and d shows the grain boundary morphology and elemental analysis perpendicular to the grain boundary in the aging state, respectively. Fig. 7c shows the occurrence of intermittent precipitates with a size of approximately 20 nm on the grain boundary. A precipitate free zone (PFZ) with a width of approxi mately 50 nm is formed near the grain boundary. Fig. 7d shows that the content of alloying elements in the region between the precipitates on the grain boundary is consistent with that in the PFZ, and the content of alloying elements these two regions is significantly lower than that in the solid solution state. Studies have shown that for Al-Zn-Mg-Cu alloys, the resistivity in the solid solution state is greater than that in the peak aging state [25,26], indicating that the solid solution sample has a greater scattering effect on drifting electrons compared with the aging sample. A region with a width of 25 nm on each side of the grain boundary is examined in greater detail. The mechanisms for the scattering of drifting electrons at the grain boundary and its vicinity under different strengthening states are shown in Fig. 8. From Fig. 4g and h, it can be concluded that the electric pulse under
(6)
Comparing the softening behavior caused by Joule heating under two strengthening mechanisms reveals that within the range of grain sizes studied, the degree of intragranular and grain boundary softening caused by Joule heating in the solution state is higher than that in the aging state. The reason for this phenomenon is that the precipitate has a higher thermal stability than the atoms in solid solution at the test temperature. The Al-Zn-Mg alloy has a tendency to precipitate in the range of 25–150 � C [24]. The test temperature is approximately 52 � C, which is within that precipitation range. Thus, it is concluded that the precipitate has a higher thermal stability than the atoms in solid solution at the test temperature. Within the range of grain sizes studied, the degree of intragranular softening caused by drifting electrons in the solid solution state is lower than that in the aging state; however, the degree of grain boundary softening caused by drifting electrons in the solid solution state is higher than that in the aging state. The reasons for these phenomena lie in the different scattering effects of the precipitate and solid solution atoms on electrons and the different grain boundary structures present in alloys in the solid solution and aging states. A more detailed analysis of this is given below.
Fig. 6. Decrease in stress caused by the electrical pulse in solid solution and aging sample states: S–σew–In and T–σ ew–In are the decrease in stress caused by drifting elec trons softening the intragranular structures; S–σJ–In and T–σ J–In are the decrease in stress caused by Joule heating softening the intra and granular structures; S–σew–G(d) T–σ ew–G(d) are the decrease in stress caused by drifting electrons softening the grain boundaries; S–σ J–G(d) and T–σJ–G(d) are the decrease in stress caused by Joule heating softening the grain boundaries.
6
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 7. Grain boundary morphology and elemental distribution obtained with TEM: a and b are the grain boundary morphology and elemental analysis (using energy-dispersive spectroscopy (EDS) attachment with the trajectory shown by the yellow arrow), respectively, in the solid solution sample; c and d are the grain boundary morphology and elemental analysis (using EDS with the trajectory shown by the yellow arrow), respectively, in the aging sample; in c, the trajectory of the elemental analysis crosses vertically between two adjacent precipitates on the boundary without passing through the precipitate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
the parameter in this study have no significant effect on the precipitation and dissolution behavior in the aging state. Previous studies have shown that the electric pulse under this parameter has no significant effect on the precipitation and dissolution behavior in the solid solution state [23]. So the physical model in Fig. 8 is reasonable. For solid solution samples, the drifting electrons are scattered by the solid solution atoms around the grain boundary, as shown in Fig. 8a. With aging and the precipitation of solid solution atoms, the content of solid solution atoms within the 50 nm width around the grain boundary decreases sharply. A “channel” with a low content of solid solution atoms (i.e., low resistivity) is formed between the precipitates. The drifting electrons thus detour from the precipitate to this “channel,” which reduces the scattering effect of the grain boundary structure, as shown in Fig. 8b. The effect of scattering by the grain boundary on drifting electrons in the aging state should thus be less than that in the solid solution state, which is consistent with the conclusions in Refs. [25, 26]. After aging, the scattering of drifting electrons by the grain boundary decreases, resulting in a degree of grain boundary softening caused by drifting electrons in the solid solution state that is greater than that in the aging state.
The direction of the electron wind force is the same as that of the drifting electrons [27]. The electron wind force can be calculated as shown in Equation (7): few ¼ Kew lJ
(7)
where Kew is electron wind force constant, l is the dislocation length, J is the current density, and few is the electron wind force on a dislocation. In this study, the sample matrix is aluminum in both the solid solu tion state and the aging state. According to the literature [28], the effective atomic valence for electromigration of aluminum atoms is ZAl* < 0, i.e., the direction of the electromigration of aluminum atoms is the same as that of the drifting electrons. In both the solid solution state and the aging state, the effect of the electron wind force on dislocation can be considered consistent. The drifting electrons strike the matrix atoms near the vacancies below the dislocations, causing the matrix atoms to migrate toward the vacancies along the direction of the drifting elec trons, eventually causing dislocations to occur and a reverse movement of the drifting electrons, as shown in Fig. 9 (gray arrow and gray rectangle). In the solid solution state, the drifting electrons interact with dislo cations as well as with the solute atoms pinning dislocations, resulting in the electromigration of solute atoms. Previous studies [28] have re ported that the effective atomic valences of zinc and copper atoms are ZZn* < 0 and ZCu* < 0, and the migration direction is the same as that of the drifting electrons, as shown in Fig. 9 (blue arrow and blue rectangle). The calculation of the electromigration force is given by Equation (8):
4.3. Intragranular softening behavior caused by drifting electrons under different strengthening mechanisms In the aging state, the intragranular softening caused by drifting electrons can be considered equivalent to the effect of the electron wind force (between drifting electrons and dislocations) on the dislocations. 7
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
Fig. 8. a Schematic diagram of the scattering of electrons by solid solution atoms in the 50 nm width around the grain boundary in the solid solution sample; b schematic diagram of the scattering of electrons by precipitates on the grain boundary in the aging sample; in b, the region between the precipitates and the PFZ together form a low-resistivity “channel,” which will induce the current to detour from the precipitate to this “channel".
force on the solute atoms. In the solid solution state, because part of the electron wind force on the dislocation is absorbed by the solid solution atoms pinning the dislocation, the dislocation de-pinning force caused by drifting electrons is the difference between the electron wind force and the electro migration force, as shown in Equation (9): fS ¼ few
fem ¼ ðKew l
Z * eρÞJ
(9)
where fS is the dislocation de-pinning force caused by drifting electrons in the solid solution state. Equations (7) and (9) show that under the same current parameters, the dislocation de-pinning force provided by drifting electrons in aging samples is greater than that in solid solution samples, which may be the reason that the degree of intracrystalline softening caused by drifting electrons is greater in aging samples than in solid solution samples. 5. Conclusions (1) Over the range of grain sizes (18–103 μm), both with or without air-cooling, the electric pulse has obvious softening effect under the solid solution state in our previously observed reference [23] and the aging state in this study. The softening effect caused by Joule heating is greater than that of drifting electrons. (2) Combined the results of this study with reference [23], softening effect of drift electrons on grain boundary in the solid solution state is greater than that in the aging state. However, softening effect of drift electrons on intragranular in the aging state is greater than that in the solid solution state. (3) A “channel” with low solid solution atom content is formed at the grain boundary during the aging process. The drifting electrons thus detour from the precipitate to this “channel,” which reduces the scattering of drifting electrons by the grain boundary in the original solid solution state. As a result, the degree of grain
Fig. 9. Mechanisms of the electron wind force (between drifting electrons and dislocations) and electromigration force (between drifting electrons and the solid solution atoms pinning dislocations); in solid solution samples, the elec tron wind force acts on dislocations by interacting with matrix atoms near the vacancies below dislocations; the electron wind force on the dislocations is in the same direction as that on the solid solution atoms pinning dislocations, and part of the electron wind force on the dislocations will be absorbed by the solid solution atoms pinning dislocations; in other words, the solid solution atoms pinning dislocations reduce the effect of drifting electrons on the dislocations.
fem ¼ Z * eρJ
(8)
where Z* is the effective atomic valence for the electromigration of solid solution atoms, e is the electronic charge, ρ is the resistivity caused by the solute atoms pinning dislocations, and fem is the electromigration 8
H. Zhang and X. Zhang
Materials Science & Engineering A 771 (2020) 138582
boundary softening caused by drifting electrons in the solid so lution state is greater than that in the aging state. (4) Drifting electrons provide a greater dislocation de-pinning force in aging samples than in solid solution samples. As a result, the degree of intracrystalline softening caused by drifting electrons in aging samples is greater than that in solid solution samples.
[8] G. Chen, J.T. Li, Z.K. Yin, G.M. Xu, Improvement of microstructure and properties in twin-roll casting 7075 sheet by lower casting speed and compound field, Mater. Char. 127 (2017) 325–332. [9] V.Y. Kravchenko, Effect of directed electron beam on moving dislocations, Sov. Phys. JETP 24 (1967) 1135–1142. [10] K. Okazaki, M. Kagawa, H. Conrad, An evaluation of the contributions of skin, pinch and heating effects to the electroplastic effect in titatnium, Mater. Sci. Eng. 45 (1980) 109–116. [11] G.V. Stepanov, A.I. Babutskii, Effect of electric current on stress relaxation in metal, Strength Mater. 28 (1996) 125–128. [12] X. Li, X. Li, Y. Ye, R. Zhang, S.Z. Kure-Chu, G. Tang, Deformation mechanisms and recrystallization behavior of Mg-3Al-1Zn and Mg-1Gd alloys deformed by electroplastic-asymmetric rolling, Mater. Sci. Eng. A 742 (2019) 722–733. [13] M. Li, D. Guo, J. Li, S. Zhu, C. Xu, K. Li, Y. Zhao, B. Wei, Q. Zhang, X. Zhang, Achieving heterogeneous structure in hcp Zr via electroplastic rolling, Mater. Sci. Eng. A 722 (2018) 93–98. [14] K. Li, M. Li, Y. Zhao, W. Shan, Y. Cao, D. Guo, Achieving ultralow elastic modulus in TiNi alloy by controlling nanoscale martensite phase, Mater. Lett. 233 (2018) 282–285. [15] X. Li, F. Wang, X. Li, J. Zhu, G. Tang, Mg-3Al-1Zn alloy strips processed by electroplastic differential speed rolling, Mater. Sci. Technol. 33 (2017) 215–219. [16] H.J. Jeong, J.W. Park, K.J. Jeong, N.M. Hwang, S.T. Hong, H.N. Han, Effect of pulsed electric current on TRIP-aided steel, Int. J. Precis. Eng. Manuf-Green Technol. (2018) 1–13. [17] A. Ghiotti, S. Bruschi, E. Simonetto, C. Gennari, I. Calliari, P. Bariani, Electroplastic effect on AA1050 aluminium alloy formability, CIRP Annals 67 (2018) 289–292. [18] H. Krishnaswamy, M.J. Kim, S.T. Hong, D. Kim, J.H. Song, M.G. Lee, H.N. Han, Electroplastic behaviour in an aluminium alloy and dislocation density based modeling, Mater. Des. 124 (2017) 131–142. [19] E.O. Hall, The deformation and ageing of mild steel: III discussion of results, Proc. Phys. Soc. Sect. B 64 (1951) 747–753. [20] N.J. Petch, The cleavage strength of polycrystals, Journal of the Iron and Steel Institute 174 (1953) 25–28. [21] S. Takaki, D. Akama, N. Nakada, T. Tsuchiyama, Effect of grain boundary segregation of interstitial elements on Hall-Petch coefficient in steels, Mater. Trans. 55 (2014) 28–34. [22] S. Araki, K. Fujii, D. Akama, T. Tsuchiyama, S. Takaki, T. Ohmura, J. Takahashi, Effect of low temperature aging on Hall-Petch coefficient in ferritic steels containing a small amount of carbon and nitrogen, ISIJ Int. 58 (2018) 1920–1926. [23] H. Zhang and X. Zhang, Hall-Petch relationship in electrically pulsed Al-Zn-Mg alloys, Adv. Eng. Mater., https://doi.org/10.1002/adem.2019006381900638; 1 to 13. [24] P.K. Rout, M.M. Ghosh, K.S. Ghosh, Microstructural, mechanical and electrochemical behaviour of a 7017 Al-Zn-Mg alloy of different tempers, Mater. Char. 104 (2015) 49–60. [25] X. Li, Q. Cai, B. Zhao, B. Liu, W. Li, Precipitation behaviors and properties of solution-aging Al-Zn-Mg-Cu alloy refined with TiN nanoparticles, J. Alloy. Comp. 746 (2018) 462–470. [26] K. Wen, Y. Fan, G. Wang, L. Jin, X. Li, Z. Li, Y. Zhang, B. Xiong, Aging behavior and fatigue crack propagation of high Zn-containing Al-Zn-Mg-Cu alloys with zinc variation, Prog. Nat. Sci.: Materials International 27 (2017) 217–227. [27] S.W. Nam, H.S. Chung, Y.C. Lo, L. Qi, J. Li, Y. Lu, A.T. Charlie Johnson, Y. Jung, P. Nukala, R. Agarwal, Electrical wind force-driven and dislocation-templated amorphization in phase-change nanowires, Science 336 (2012) 1561–1566. [28] P.S. Ho, T. Kwok, Electromigration in metals, Rep. Prog. Phys. 52 (1989) 301–348.
Data availability The data that support the finding of this study are available from the corresponding author upon request. Declaration of competing interest There are no conflicts of interest for the publication of “Softening Behavior of Al-Zn-Mg Alloys with Different Strengthening Mechanisms in a Coupled Field”. Acknowledgements The work was financially supported by National Natural Science Foundation of China (51874023, 51601011, U1860206), Fundamental Research Funds for the Central Universities (FRF-TP-18-003B1), Recruitment Program of Global Experts. References [1] O.A. Troitskii, V.I. Likhtman, Anisotropy of action of gamma-electronic emission and gamma rays on deformation of single zinc crystals in the brittle state, Dokl. Akad. Nauk SSSR 148 (1963) 332–334. [2] H.D. Nguyen-Tran, H.S. Oh, S.T. Hong, H.N. Han, J. Cao, S.H. Ahn, D.M. Chun, A review of electrically-assisted manufacturing, Int. J. Precis. Eng. Manuf-Green Technol. 2 (2015) 365–376. [3] X. Zhang, H. Li, S. Yan, N. Zhang, Experimental study and analysis on the electrically-assisted tensile behaviors of Inconel 718 alloy, Procedia Eng. 207 (2017) 365–370. [4] C. Montilla Monta~ na, V. Kallewaard, H.A. Gonz� alez Rojas, Effect of electropulses on the machinability of a C45E steel, Dyna Ingeniería E Industria 94 (2019) 94–99. [5] B.J. Ruszkiewicz, E. Gendreau, F.A. Niaki, L. Mears, Electroplastic drilling of lowand high-strength steels, J. Manuf. Sci. Eng. 140 (2018), 061017. [6] B.J. Ruszkiewicz, T. Grimm, I. Ragai, L. Mears, J.T. Roth, A review of electricallyassisted manufacturing with emphasis on modeling and understanding of the electroplastic effect, J. Manuf. Sci. Eng. 139 (2017) 110801. [7] L.M. Yan, J. Shen, D. An, Z.X. Du, J.B. Zhang, Recrystallization characterizations of an Al-Zn-Mg-Cu-Zr alloy during multi-pass hot rolling simulation, Rare Metal Mater. Eng. 46 (2017) 3233–3238.
9