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Formation and disintegration investigation of fivefold annealing twins in copper nanoparticles
Scripta Materialia 169 (2019) 42–45
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Formation and disintegration investigation of fivefold annealing twins in copper nanoparticles Ziliang Deng, Jun Luo, Wenjuan Yuan ⁎, Wei Xi Center for Electron Microscopy, TUT-FEI Joint Laboratory, Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 23 February 2018 Received in revised form 20 December 2018 Accepted 6 January 2019 Available online 16 May 2019 Keywords: Fivefold twin In-situ TEM Cu nanoparticle
a b s t r a c t Fivefold annealing twins (FAT) have been assumed to form by grain boundary migration according to the Ashby model. However, the mechanism of boundary migration lacks experimental support. Moreover, the FAT disintegration process has never been reported. In our experiment, the complete formation and disintegration process of FAT in copper nanoparticles was investigated by in-situ transmission electron microscopy with high temporal and spatial resolution. The results show that FAT formation and disintegration arises from the layer-by-layer migration of twin boundaries, induced by partial dislocation emission. This experimental observation is significant to the basic understanding of twinning mechanisms. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Twinning is an important deformation and strengthening mechanism [1–6] that can improve the physical and chemical properties of materials [7–13]. The special example of fivefold twins has attracted significant attention since they were first observed in 1957 [14], not only in crystal growth and crystallography research [15,16], but also in research on the plastic deformation of metal [17], materials surface science [18], films, and general materials [19,20]. Two kinds of fivefold twin have been observed in nanocrystals: fivefold deformation twins (FDT) and fivefold annealing twins (FAT) [21–24]. Both experimental observations and molecular dynamics (MD) simulations have indicated that FDT are formed mainly by a series of partial dislocations emitted from grain or twin boundaries (TBs) [21,22,25–27]. Theoretically, FATs are formed by grain-boundary migration according to the Ashby model [24,28]. However, these formation theories require support and improvement by in-situ experimental studies. Furthermore, Zhu et al. noted that the de-twinning process must be observed for comparison with the proposed mechanisms [29]. However, no example of fivefold twin de-twinning or disintegration has yet been reported. Thus, it is necessary to investigate the formation and disintegration process of FAT in-situ. In order to improve the understanding of the twinning mechanism, the complete formation and disintegration process of FATs in copper nanoparticles at 750 °C was observed by in-situ transmission electron microscopy (TEM) with high temporal and spatial resolution. The results indicated that the formation and disintegration of FAT arise from the layer-by-layer migration of TBs caused by the emission of partial
⁎ Corresponding author. E-mail address: [email protected] (W. Yuan).
https://doi.org/10.1016/j.scriptamat.2019.04.043 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
dislocations. This provides an important scientific basis for understanding the formation mechanism of fivefold twins. The copper nanoparticles used in this research were synthesized by ball-milling and high-temperature reduction [30]. Pure copper powder (5 g, 99.99%, 100 mesh) was ball-milled in a CuCl2 solution with a concentration of 0.75 × 102 mol L−1. The ball mill rotated at 400 rpm for 25 h. The weight ratio of the powder was 20:1. The milled product was then rinsed with distilled water and vacuum-dried at 40 °C for 2 h. Finally, the obtained sample was reduced in an NH3 atmosphere at 400 °C for 15 min to obtain the desired copper nanoparticles. In the in-situ observation experiment, the copper nanoparticles were dispersed on a heating chip (Wildfire S5, DENS solutions, Netherlands). The experimental heating temperatures were 23–950 °C with the heating rate of 30 °C/min, and the temperature error was b5% [31]. The TEM images were obtained by a Talos F200X (Thermo Fisher, USA) apparatus at an acceleration voltage of 200 kV. The experimental process was recorded by a Ceta2 high-speed camera (Thermo Fisher, USA). The copper nanoparticles have good crystallinity with the particle size range of approximately 3–15 nm, as shown in Fig. 1a and b. The (111) interplanar spacing is marked in Fig. 1(b). After heating to 950 °C, the particles are melted completely, as shown in Fig. 1(c). Under rapid temperature reduction at 200 °C/min to 750 °C, the molten copper nanoparticles recrystallize. Fig. 2(a) shows the highresolution TEM (HRTEM) image of a crystal particle after recrystallization with five TBs. Unlike the four other TBs, TB5 is not a straight line but comprises several steps (short black lines in Fig. 2(a)). Furthermore, the five TBs do not intersect at one point, and the lattice is distorted in the intersection region. As the heat treatment progresses, the five TBs gradually migrate, and finally hand over to one point during annealing
Z. Deng et al. / Scripta Materialia 169 (2019) 42–45
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Fig. 1. Characterizations of copper nanoparticles at different temperatures: (a) Low-magnification TEM image of nanoparticles at room temperature. (b) High-resolution TEM image of copper nanoparticles shown in (a). (c) Low-magnification TEM image of nanoparticles after melting at 950 °C. The insets in (a) and (c) show the corresponding diffraction patterns.
at 750 °C (Fig. 2).Similar phenomena was also observed in other copper nanoparticle, shown in Fig. S1. The migration of each TB is investigated by in-situ TEM, as shown in Fig. 2(b–f). At 0.0125 s after Fig. 2(a) is obtained, TB4 in the particle has migrated downward by one atomic layer (the original position of the boundary is marked as a short blue line), and the step number of TB5 is reduced, as shown in Fig. 2(b). At 0.025 s (Fig. 2(c)), both TB1 and TB4 in the particle have migrated down by one atomic layer, and the step number of TB5 is further reduced. At 0.0875 s (in Fig. 2(d)), TB1 has migrated down again by one atomic layer, and TB3 has migrated
to the left by one atomic layer. At this moment, TB5 becomes a straight boundary. In the subsequent migration of the TBs, the TBs in the particle constantly migrate back and forth with the consistent tendency to form a standard fivefold twin as shown in Fig. 2(d) to (f). At 0.1875 s, shown in Fig. 2(f), a standard fivefold twin is formed, with five TBs intersecting at one point. At this moment, the formation process of the fivefold twin is completed by the layer-by-layer migration of TBs. The migration of TBs during formation is very fast and the standard structure is maintained for approximately 3 s after formation, indicating
Fig. 2. Formation process of FAT in copper nanoparticle at 750 °C. (a–f) Layer-by-layer migration of FAT boundaries in copper nanoparticle over time. (a–c) Steps are present in TB5. As time progresses, the number of steps in TB5 decreases and TB5 becomes a straight boundary. (f) A standard fivefold twin is formed. Black dashed lines show the positions of the TBs; short blue lines indicate the TB positions from the previous step. In (a), b4 and b5 are Shockley partial dislocations with the Burgers vectors of b = 1/6〈11−2〉. (a)–(f) are sourced from Supplementary Video S1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. FAT disintegration in copper nanoparticle at 750 °C. (a–f) HRTEM images of copper nanoparticle at different times. (a) A standard fivefold twin. (b) Lattice distortion zone at the center of the particle. (c) Migration of TB3 and TB4. (d) New TB is formed near TB4. (e) Step present in TB1. (d–f) New twin is formed and thickened. (f) Complete disintegration of FAT. Black dashed lines show TB positions; short blue lines indicate TB positions from the previous step. In (b), b3 and b4 are Shockley partial dislocations with Burgers vectors of b = 1/6〈11–2〉. (a)–(f) are sourced from Supplementary Video S2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Schematic of FAT formation mechanism. (a, b) TB migration caused by emission of partial dislocations with Burgers vector b1. (c) Lattice distortion at the TB intersection as the dislocation source. (d–e) Schematic and HRTEM image of surface defects caused by geometric relations. (f) Schematic of standard FAT.
Z. Deng et al. / Scripta Materialia 169 (2019) 42–45
the stability of the fivefold twin at 750 °C. At 2.7375 s, the fivefold twin shown in Fig. 3(a) is still standard, with five TBs intersecting at one point. At 3.6 s (Fig. 3(b)), a lattice distortion zone appears around the intersection (as shown in the inset of Fig. 3(b)) and the ordered arrangement of the center zone is destroyed. A dislocation source is formed by the misalignment of a few atoms. After the dislocation source appears (Fig. 3(b)), the five TBs begin to migrate randomly again. TB3 and TB4 migrate away from each other during the disintegration process, shown in Fig. 3(a–f). The rest of the TBs also undergo migration by varying distances and directions. During disintegration, shown in Fig. 3(d), a new TB is formed near TB4. During the subsequent migration process, it gradually becomes a twin with several atomic layers (Fig. 3(e)). With the continuous TB migration, the fivefold twin is disintegrated at 35.4125 s (Fig. 3(f)). In the experiment, the complete TB migration process for fivefold twin formation and disintegration was observed. Therefore, the results present insight into the formation mechanism of fivefold twins. The observation results showed that FAT formation and disintegration is attributed to layer-by-layer TB migration. In experiments and MD simulations, TB migration has been attributed to the emission of partial dislocations [25,27,32–34]. The effect of partial dislocation emission on TBs is shown in Fig. 4(a) and (b). The gliding of partial dislocations b1 causes TBs to migrate up or down, inducing the formation or disappearance of steps [34]. According to this principle, TB5 in Fig. 2(a–d) changed from several steps to a straight TB, and TB1 of Fig. 3 (e) became a stepped TB. It has been reported that the emission of partial dislocations (grain-boundary migration) requires stress as a driving force [22,35]. During the formation process of a standard FAT, the internal stress induced by the lattice distortion (Fig. 4c) at the intersection of TBs or at surface defects caused by geometric relations (as shown by Fig. 4d and e) provided the driving force for dislocation emission. Therefore, without lattice distortion in the particle (Figs. 3(a) and 4f), the standard FAT is relatively stable. However, during the annealing process, lattice distortion generated by high-temperature effects (Fig. 3(b)) functions as a new driving force for dislocation emission. With partial dislocation emission, the TBs migrate again, thus disintegrating the fivefold twin (Fig. 3(f)). In summary, the formation and disintegration process of FAT was investigated by in situ TEM with high temporal and spatial resolution. The TEM observations indicated that lattice distortion functioned as a dislocation source, and that dislocation emission induced the layer-by-layer migration of TBs, finally yielding a standard fivefold twin. As annealing continued, a new dislocation source was formed under the applied high temperature and emitted partial dislocations. The emission of new partial dislocations caused further layer-by-layer migration of TBs, thus inducing disintegration of the fivefold twin. Our work provides experimental support for the current fivefold twin formation mechanism and may inspire new ideas for the study of twinning. Supplementary data to this article can be found online at https://doi. org/10.1016/j.dummy.2019.01.002.
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Acknowledgement This work was financially supported by the National Natural Science Foundation of China (21603161, 51671145, 51701143), the National Program for Thousand Young Talents of China, the Tianjin Municipal Education Commission, the Tianjin Municipal Science and Technology Commission (15JCYBJC52600), and the Fundamental Research Fund of Tianjin University of Technology. References [1] K. Lu, L. Lu, S. Suresh, Science 324 (2009) 349–352. [2] L. Lilensten, Y. Danard, C. Brozek, S. Mantri, P. Castany, T. Gloriant, P. Vermaut, F. Sun, R. Banerjee, F. Prima, Acta Mater. 162 (2019) 268–276. [3] J.F. Nie, Y.M. Zhu, J.Z. Liu, X.Y. Fang, Science 340 (2013) 957–960. [4] Y.T. Zhu, X.L. Wu, X.Z. Liao, J. Narayan, L.J. Kecskés, S.N. Mathaudhu, Acta Mater. 59 (2011) 812–821. [5] Q.S. Pan, Q.H. Lu, L. Lu, Acta Mater. 61 (2013) 1383–1393. [6] Z.S. You, X.Y. Li, L.J. Gui, Q.H. Lu, T. Zhu, H.J. Gao, L. Lu, Acta Mater. 61 (2013) 217–227. [7] O. Heczko, V. Drchal, S. Cichon, L. Fekete, J. Kudrnovský, I. Kratochvílová, J. Lancok, V. Cháb, Phys. Rev. B 98 (2018), 184407. [8] Y.G. Zhou, J.Y. Yang, L. Cheng, M. Hu, Phys. Rev. B 97 (2018), 085304. [9] V.O. Golub, V.A. Lvov, I. Aseguinolaza, O. Salyuk, D. Popadiuk, Y. Kharlan, G.N. Kakazei, J.P. Araujo, J.M. Barandiaran, V.A. Chernenko, Phys. Rev. B 95 (2017), 024422. [10] K. Aoyama, L. Savary, M. Sigrist, Phys. Rev. B 89 (2014), 174518. [11] L.Y. Zhang, Y.Y. Gong, D.B. Wu, G.L. Wu, B.H. Xu, L. Bi, W.Y. Yuan, Z.M. Cui, J. Colloid Interface Sci. 537 (2019) 366–374. [12] X. Wang, L. Figueroa-Cosme, X. Yang, M. Luo, J. Liu, Z. Xie, Y. Xia, Nano Lett. 16 (2016) 1467–1471. [13] S. Kameoka, M. Krajčí, A.P. Tsai, Appl. Catal. A Gen. 569 (2019) 101–109. [14] R.L. Segall, JOM 9 (1957) 50. [15] C.T. Chen, S.J. Nagao, J.T. Jiu, H. Zhang, T. Sugahara, K. Suganuma, Appl. Phys. Lett. 108 (2016) 263105. [16] M.F. Sun, R.G. Cao, F. Xiao, C. Deng, Comput. Mater. Sci. 79 (2013) 289–295. [17] L. Wang, W. Wang, B.L. Chen, X.Y. Zhou, Z.W. Li, B.X. Zhou, L.M. Wang, Mater. Charact. 95 (2014) 12–17. [18] S.L. Huang, Z.Y. Zhang, C. Xu, Z.W. Zhu, J.F. Cui, B. Wang, Mater. Lett. 229 (2018) 111–113. [19] O.R. Monteiro, J. Mater. Sci. 54 (2019) 2300–2306. [20] Y. Zhu, Q.Q. Qin, F. Xu, F.R. Fan, Y. Ding, Y. Zhang, B.J. Wiley, Z.L. Wang, Phys. Rev. B 85 (2012), 045443. [21] X.H. An, Q.Y. Lin, S.D. Wu, Z.F. Zhang, R.B. Figueiredo, N. Gao, T.G. Langdon, Scr. Mater. 64 (2011) 249–252. [22] Y.T. Zhu, X.Z. Liao, Appl. Phys. Lett. 86 (2005), 103112. [23] P. Huang, G.Q. Dai, F. Wang, K.W. Xu, Y.H. Li, Appl. Phys. Lett. 95 (2009) 203101. [24] Z.H. Cao, L.J. Xu, W. Sun, J. Shi, M.Z. Wei, G.J. Pan, X.B. Yang, J.W. Zhao, X.K. Meng, Acta Mater. 95 (2015) 312–323. [25] A.J. Cao, Y.G. Wei, Appl. Phys. Lett. 89 (2006), 041919. [26] Z.Y. Zhang, S.L. Huang, L.L. Chen, Z.W. Zhu, D.M. Guo, Sci. Rep. 7 (2017), 45405. [27] Y.G. Zheng, H.W. Zhang, Z. Chen, L. Wang, Z.Q. Zhang, J.B. Wang, Appl. Phys. Lett. 92 (2008), 041913. [28] E.M. Bringa, D. Farkas, A. Caro, Y.M. Wang, J. McNaney, R. Smith, Scr. Mater. 59 (2008) 1267–1270. [29] Y.T. Zhu, X.Z. Liao, X.L. Wu, Prog. Mater. Sci. 57 (2012) 1–62. [30] D. Chen, S. Ni, J.J. Fang, T. Xiao, J. Alloys Compd. 504 (2010) S345–S348. [31] F. Niekiel, S.M. Kraschewski, J. Müller, B. Butz, E. Spiecker, Ultramicroscopy 176 (2016) 161–169. [32] N. Lu, K. Du, L. Lu, H.Q. Ye, Nat. Commun. 6 (2015) 7648. [33] L. Wang, J. Teng, D. Kong, G. Yu, J. Zou, Z. Zhang, X. Han, Scr. Mater. 147 (2018) 103–107. [34] Y.B. Wang, M.L. Sui, E. Ma, Philos. Mag. Lett. 87 (2007) 935–942. [35] O.A. Ruano, O.D. Sherby, Mater. Sci. Eng. 56 (1982) 167–175.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Formation and disintegration investigation of fivefold annealing twins in copper nanoparticles Ziliang Deng, Jun Luo, Wenjuan Yuan ⁎, Wei Xi Center for Electron Microscopy, TUT-FEI Joint Laboratory, Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 23 February 2018 Received in revised form 20 December 2018 Accepted 6 January 2019 Available online 16 May 2019 Keywords: Fivefold twin In-situ TEM Cu nanoparticle
a b s t r a c t Fivefold annealing twins (FAT) have been assumed to form by grain boundary migration according to the Ashby model. However, the mechanism of boundary migration lacks experimental support. Moreover, the FAT disintegration process has never been reported. In our experiment, the complete formation and disintegration process of FAT in copper nanoparticles was investigated by in-situ transmission electron microscopy with high temporal and spatial resolution. The results show that FAT formation and disintegration arises from the layer-by-layer migration of twin boundaries, induced by partial dislocation emission. This experimental observation is significant to the basic understanding of twinning mechanisms. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Twinning is an important deformation and strengthening mechanism [1–6] that can improve the physical and chemical properties of materials [7–13]. The special example of fivefold twins has attracted significant attention since they were first observed in 1957 [14], not only in crystal growth and crystallography research [15,16], but also in research on the plastic deformation of metal [17], materials surface science [18], films, and general materials [19,20]. Two kinds of fivefold twin have been observed in nanocrystals: fivefold deformation twins (FDT) and fivefold annealing twins (FAT) [21–24]. Both experimental observations and molecular dynamics (MD) simulations have indicated that FDT are formed mainly by a series of partial dislocations emitted from grain or twin boundaries (TBs) [21,22,25–27]. Theoretically, FATs are formed by grain-boundary migration according to the Ashby model [24,28]. However, these formation theories require support and improvement by in-situ experimental studies. Furthermore, Zhu et al. noted that the de-twinning process must be observed for comparison with the proposed mechanisms [29]. However, no example of fivefold twin de-twinning or disintegration has yet been reported. Thus, it is necessary to investigate the formation and disintegration process of FAT in-situ. In order to improve the understanding of the twinning mechanism, the complete formation and disintegration process of FATs in copper nanoparticles at 750 °C was observed by in-situ transmission electron microscopy (TEM) with high temporal and spatial resolution. The results indicated that the formation and disintegration of FAT arise from the layer-by-layer migration of TBs caused by the emission of partial
⁎ Corresponding author. E-mail address: [email protected] (W. Yuan).
https://doi.org/10.1016/j.scriptamat.2019.04.043 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
dislocations. This provides an important scientific basis for understanding the formation mechanism of fivefold twins. The copper nanoparticles used in this research were synthesized by ball-milling and high-temperature reduction [30]. Pure copper powder (5 g, 99.99%, 100 mesh) was ball-milled in a CuCl2 solution with a concentration of 0.75 × 102 mol L−1. The ball mill rotated at 400 rpm for 25 h. The weight ratio of the powder was 20:1. The milled product was then rinsed with distilled water and vacuum-dried at 40 °C for 2 h. Finally, the obtained sample was reduced in an NH3 atmosphere at 400 °C for 15 min to obtain the desired copper nanoparticles. In the in-situ observation experiment, the copper nanoparticles were dispersed on a heating chip (Wildfire S5, DENS solutions, Netherlands). The experimental heating temperatures were 23–950 °C with the heating rate of 30 °C/min, and the temperature error was b5% [31]. The TEM images were obtained by a Talos F200X (Thermo Fisher, USA) apparatus at an acceleration voltage of 200 kV. The experimental process was recorded by a Ceta2 high-speed camera (Thermo Fisher, USA). The copper nanoparticles have good crystallinity with the particle size range of approximately 3–15 nm, as shown in Fig. 1a and b. The (111) interplanar spacing is marked in Fig. 1(b). After heating to 950 °C, the particles are melted completely, as shown in Fig. 1(c). Under rapid temperature reduction at 200 °C/min to 750 °C, the molten copper nanoparticles recrystallize. Fig. 2(a) shows the highresolution TEM (HRTEM) image of a crystal particle after recrystallization with five TBs. Unlike the four other TBs, TB5 is not a straight line but comprises several steps (short black lines in Fig. 2(a)). Furthermore, the five TBs do not intersect at one point, and the lattice is distorted in the intersection region. As the heat treatment progresses, the five TBs gradually migrate, and finally hand over to one point during annealing
Z. Deng et al. / Scripta Materialia 169 (2019) 42–45
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Fig. 1. Characterizations of copper nanoparticles at different temperatures: (a) Low-magnification TEM image of nanoparticles at room temperature. (b) High-resolution TEM image of copper nanoparticles shown in (a). (c) Low-magnification TEM image of nanoparticles after melting at 950 °C. The insets in (a) and (c) show the corresponding diffraction patterns.
at 750 °C (Fig. 2).Similar phenomena was also observed in other copper nanoparticle, shown in Fig. S1. The migration of each TB is investigated by in-situ TEM, as shown in Fig. 2(b–f). At 0.0125 s after Fig. 2(a) is obtained, TB4 in the particle has migrated downward by one atomic layer (the original position of the boundary is marked as a short blue line), and the step number of TB5 is reduced, as shown in Fig. 2(b). At 0.025 s (Fig. 2(c)), both TB1 and TB4 in the particle have migrated down by one atomic layer, and the step number of TB5 is further reduced. At 0.0875 s (in Fig. 2(d)), TB1 has migrated down again by one atomic layer, and TB3 has migrated
to the left by one atomic layer. At this moment, TB5 becomes a straight boundary. In the subsequent migration of the TBs, the TBs in the particle constantly migrate back and forth with the consistent tendency to form a standard fivefold twin as shown in Fig. 2(d) to (f). At 0.1875 s, shown in Fig. 2(f), a standard fivefold twin is formed, with five TBs intersecting at one point. At this moment, the formation process of the fivefold twin is completed by the layer-by-layer migration of TBs. The migration of TBs during formation is very fast and the standard structure is maintained for approximately 3 s after formation, indicating
Fig. 2. Formation process of FAT in copper nanoparticle at 750 °C. (a–f) Layer-by-layer migration of FAT boundaries in copper nanoparticle over time. (a–c) Steps are present in TB5. As time progresses, the number of steps in TB5 decreases and TB5 becomes a straight boundary. (f) A standard fivefold twin is formed. Black dashed lines show the positions of the TBs; short blue lines indicate the TB positions from the previous step. In (a), b4 and b5 are Shockley partial dislocations with the Burgers vectors of b = 1/6〈11−2〉. (a)–(f) are sourced from Supplementary Video S1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. FAT disintegration in copper nanoparticle at 750 °C. (a–f) HRTEM images of copper nanoparticle at different times. (a) A standard fivefold twin. (b) Lattice distortion zone at the center of the particle. (c) Migration of TB3 and TB4. (d) New TB is formed near TB4. (e) Step present in TB1. (d–f) New twin is formed and thickened. (f) Complete disintegration of FAT. Black dashed lines show TB positions; short blue lines indicate TB positions from the previous step. In (b), b3 and b4 are Shockley partial dislocations with Burgers vectors of b = 1/6〈11–2〉. (a)–(f) are sourced from Supplementary Video S2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Schematic of FAT formation mechanism. (a, b) TB migration caused by emission of partial dislocations with Burgers vector b1. (c) Lattice distortion at the TB intersection as the dislocation source. (d–e) Schematic and HRTEM image of surface defects caused by geometric relations. (f) Schematic of standard FAT.
Z. Deng et al. / Scripta Materialia 169 (2019) 42–45
the stability of the fivefold twin at 750 °C. At 2.7375 s, the fivefold twin shown in Fig. 3(a) is still standard, with five TBs intersecting at one point. At 3.6 s (Fig. 3(b)), a lattice distortion zone appears around the intersection (as shown in the inset of Fig. 3(b)) and the ordered arrangement of the center zone is destroyed. A dislocation source is formed by the misalignment of a few atoms. After the dislocation source appears (Fig. 3(b)), the five TBs begin to migrate randomly again. TB3 and TB4 migrate away from each other during the disintegration process, shown in Fig. 3(a–f). The rest of the TBs also undergo migration by varying distances and directions. During disintegration, shown in Fig. 3(d), a new TB is formed near TB4. During the subsequent migration process, it gradually becomes a twin with several atomic layers (Fig. 3(e)). With the continuous TB migration, the fivefold twin is disintegrated at 35.4125 s (Fig. 3(f)). In the experiment, the complete TB migration process for fivefold twin formation and disintegration was observed. Therefore, the results present insight into the formation mechanism of fivefold twins. The observation results showed that FAT formation and disintegration is attributed to layer-by-layer TB migration. In experiments and MD simulations, TB migration has been attributed to the emission of partial dislocations [25,27,32–34]. The effect of partial dislocation emission on TBs is shown in Fig. 4(a) and (b). The gliding of partial dislocations b1 causes TBs to migrate up or down, inducing the formation or disappearance of steps [34]. According to this principle, TB5 in Fig. 2(a–d) changed from several steps to a straight TB, and TB1 of Fig. 3 (e) became a stepped TB. It has been reported that the emission of partial dislocations (grain-boundary migration) requires stress as a driving force [22,35]. During the formation process of a standard FAT, the internal stress induced by the lattice distortion (Fig. 4c) at the intersection of TBs or at surface defects caused by geometric relations (as shown by Fig. 4d and e) provided the driving force for dislocation emission. Therefore, without lattice distortion in the particle (Figs. 3(a) and 4f), the standard FAT is relatively stable. However, during the annealing process, lattice distortion generated by high-temperature effects (Fig. 3(b)) functions as a new driving force for dislocation emission. With partial dislocation emission, the TBs migrate again, thus disintegrating the fivefold twin (Fig. 3(f)). In summary, the formation and disintegration process of FAT was investigated by in situ TEM with high temporal and spatial resolution. The TEM observations indicated that lattice distortion functioned as a dislocation source, and that dislocation emission induced the layer-by-layer migration of TBs, finally yielding a standard fivefold twin. As annealing continued, a new dislocation source was formed under the applied high temperature and emitted partial dislocations. The emission of new partial dislocations caused further layer-by-layer migration of TBs, thus inducing disintegration of the fivefold twin. Our work provides experimental support for the current fivefold twin formation mechanism and may inspire new ideas for the study of twinning. Supplementary data to this article can be found online at https://doi. org/10.1016/j.dummy.2019.01.002.
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