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Interactions between kinking and {101‾2} twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase
Materials Science & Engineering A 767 (2019) 138418
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Short communication
Interactions between kinking and {1012 ‾ } twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase
T
Tao Chen, Zhiyong Chen∗, Jianbo Shao, Renke Wang, Kai Li, Longhui Mao, Chuming Liu School of Materials Science and Engineering, Central South University, Changsha, 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium alloys Kinking Twinning LPSO phase
Two interaction modes between kinking and {101 ‾ 2} twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase are thoroughly investigated: (i) when kinking forms prior to twinning, twins concomitantly nuclear at kink boundaries (KBs) and twinning propagation is frequently restricted by two adjacent KBs. By contrast, (ii) when twinning forms prior to kinking, kink deformation locally breaks the twins’ theoretical misorientation to their adjacent matrixes, during which basal slip plays a vital role in accommodating the asynchronous deformation between kinking and twinning.
1. Introduction Recently, a series of Mg–Zn-RE (RE = rare earth) alloys containing long-period stacking ordered (LPSO) phase have attracted increasing attention [1–6]. Compared with conventional Mg alloys, those Mg/ LPSO alloys show superior mechanical properties and unique microstructure [7–12]. Due to the lamellar structure of LPSO phase, kinking was frequently detected in deformed Mg/LPSO alloys [13–16], especially in those grains align with their basal planes parallel to the compressive stress. The unique deformation geometry of kinking makes it an important mechanism for Mg/LPSO alloys to generate homogenous strain when basal slip is extremely inhibited. {101 ‾ 2} twinning, known as the most common twinning mode in Mg alloys, which is prevalently detected in the grains with basal planes parallel to the compressive stress or perpendicular to the tensive stress [17–19]. Although the twinning activity is more or less hindered by LPSO phase [20,21], {101 ‾ 2} twinning (hereinafter referred to as twinning) is still considered as an essential mechanism for Mg/LPSO alloys to generate homogenous strain when basal slip is inhibited. On top of this, both kinking and twinning play important roles in plastic deformation of Mg/LPSO alloys; and moreover, when compression stress parallels to the grain's basal plane, both of them are prone to develop. Therefore, it is quite reasonable to expect that the kinking and twinning may occur concurrently in a same grain and interact with each other under certain conditions. In fact, Wang et al. [22] have reported a unique microstructure in a deformed Mg–Y alloy that the formation of kinking was accompanied by nucleation of twins at kink boundaries (KBs). However, their primary concerns were on the
∗
phenomenological description via in-situ observation, the underlying interaction mechanism between kinking and twinning was less discussed. In this study, another new interaction mode between kinking and twinning was observed for the first time. Moreover, in-depth discussions were conducted on interaction modes between kinking and twinning, which can provide valuable references to lucubrate the basic deformation behaviors of Mg/LPSO alloys. 2. Materials and methods The investigated Mg–Zn-Gd alloy (Mg-0.8Zn-2.4Gd in at%) filled with LPSO phase was prepared by semi-continuous casting and homogenization (detailed material preparation method and initial microstructure of the homogenized alloy can be seen in our recent study [23]). The homogenized alloy (with a dimension of Φ10 mm × 15 mm) was compressed at ambient temperature on a standard universal testing machine (CSS-44100) with a constant loading speed of 1 mm/min. Compression test was halted and unloaded halfway, and the compressed sample without any cracks has eventually obtained 10% plastic deformation (typical strain-stress curve was shown in Fig. 1). Grain orientations of the selected area were acquired using a Helios Nanolab 600i scanning electron microscope (SEM) equipped with an Oxford instruments electron backscattered diffraction (EBSD) analysis system. High-angle annular dark-field (HAADF-STEM) images and related selected area electron diffraction (SAED) patterns were obtained using a FEI Titan G2 60–300 transmission electron microscope (TEM). Samples for EBSD/TEM observation were prepared by mechanical
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.msea.2019.138418 Received 27 July 2019; Received in revised form 12 September 2019; Accepted 12 September 2019 Available online 13 September 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 767 (2019) 138418
T. Chen, et al.
either in LABs or HABs. Those above detected deformation geometries of kinking are both in accordance with the previous studies on Zn single crystal [24] and LPSO-containing Mg–Zn–Y alloy [25]. As we turn our perspective to the twinned areas in Fig. 2, two kinds of twins with different morphologies can be detected. Firstly, as commonly shown in both Fig. 2a and Fig. 2b, most of the twins (the typical ones are highlighted by dashed rectangles) are embedded in a single kink band, and the twinned zones are frequently sandwiched by two adjacent KBs (hereinafter named as I type twin). Secondly, as highlighted by thicker boundaries in Fig. 2b, part of the twins stretches across more than one kink band, and their TBs do not terminate at KBs (hereinafter named as II type twin). Such two kinds of twins with different morphologies indicate two interaction modes between kinking and {101 ‾ 2} twinning, which would be further characterized and discussed in Fig. 3. Fig. 3a and Fig. 3b are Euler angle coloring maps of the selected rectangular areas in Fig. 2b (re-scanned with finer step size). In such maps, the TBs with misorientation of < 112 ‾0> /86.3° (with a tolerance of ± 5°) are highlighted by yellow lines, and some of the typical KBs are marked by dashed lines. By inspecting the eight selected areas contain typical II type twins (numbered 1–4 in Figs. 3a and 5–8 in Fig. 3b), the difference between the above-mentioned two kinds of twins can be more clearly distinguished: once stretch across a KB, the II type twins uniformly lose the theoretical misorientations to their near matrixes, which is entirely different from that of the I type twins with uninterrupted theoretical TBs. Such difference should relate with the formation order of kinking and twinning during deformation. Firstly, as for the I type twins, kinking should form earlier than that of twinning (hereinafter referred as “kinking + twinning” mode), otherwise the KB (s) should unavoidably intersect with the TB(s). As has been reported in Ref. [22], KBs are potential sites favor the nucleation of twinning. On the other hand, such earlier-formed KBs should also hinder the twinning propagation that attempt to across it, which eventually makes the I type twins are frequently sandwiched by two adjacent KBs. Secondly, as for the II type twins, twinning should form earlier than that of kinking (hereinafter referred as “twinning + kinking” mode), otherwise the constituent part of twins that stretch across KB(s) should also have the
Fig. 1. Typical stress-strain curve of the compressed sample with 10% plastic deformation.
polishing and electro polishing (twin-jet electro polishing for TEM sample), with a solution of 15 mL perchloric acid and 285 mL of ethyl alcohol, at 233 K and 10 mA.
3. Results and discussion Fig. 2 shows two inverse pole figure (IPF) coloring maps detected in the compressed sample where kinking and twinning occur concurrently in a same grain and interact with each other. The low angle boundaries (LABs, 3°~15°), high angle boundaries (HABs, > 15°) and twin boundaries (TBs, < 112 ‾0> /86.3°, with a tolerance of ± 5°) are respectively highlighted by white, black and yellow lines. By inspecting the misorientations of KBs numbered by 1–12 (seen in Fig. 2a) and A~L (seen in Fig. 2b), it can be seen that their rotation axes are uniformly perpendicular to the c-axis and show a scattered distribution on the basal plane, which can be concluded as < uvt0> and the indexes u , v , t are variables. Beyond that, their misorientation angles are also unfixed,
Fig. 2. (a) (b) IPF coloring maps showing the kinking and {101 ‾ 2} twinning occur concurrently in a same grain and interact with each other, where the rectangles “A” and “B” in (b) show further EBSD detection positions for Fig. 3a and b. 2
Materials Science & Engineering A 767 (2019) 138418
T. Chen, et al.
Fig. 3. Euler angle coloring maps showing detailed interactions between kinking and {101 ‾ 2} twinning, where the eight dashed squares showing further orientation analyses positions for Table 1.
theoretical misorientations to their near matrixes. In other words, in the “twinning + kinking” mode, the later-formed kinking has locally broken the theoretical misorientation of the earlier-formed twins to their adjacent matrixes. According to the above discussion, the “kinking + twinning” mode can be simply interpreted as twins concomitantly nuclear at KB(s) and their subsequent twinning propagation are restricted by two adjacent KBs. This can be verified by the fact that both the deformation geometry of kinking and twinning in this interaction mode are same to that of their independent occurrence under non-interaction situation [18,24]. By contrast, the “twinning + kinking” mode is more complicated, since kinking has locally changed the deformation geometry of the earlierformed twins. To further study this, orientation analyses of the eight selected areas in Fig. 3 are listed in Table 1. Based on the deformation geometry, each of the selected area is divided into four different regions: “twin” (marked as ○, as typically demonstrated in the No. 3 selected area, the same below), “kinked twin” (+), “matrix” (△) and “kinked matrix” (□), and their crystal orientations are schematically shown at the bottom of Fig. 3. From Table 1 and Fig. 3, it can be seen that the misorientation angles between “twin” and “kinked twin” (compare ○ with +) are rather small (ranges from 0.7° to 4.7°), which are much lower than of “matrix” and “kinked matrix” (compare △ with □, ranges from 11.8° to 23.7°). This indicates the orientation of the “twin” hasn't changed much when it stretches across a KB and becomes the “kinked twin”, but the orientation of the “matrix” has significantly changed by kink deformation when it becomes the “kinked matrix”. Such unsynchronized orientation change makes the “kinked twins” lose
their theoretical misorientations of < 112 ‾0> /86.3° to the near matrix as they stretch across the KB. However, by inspecting of the eight selected areas in Fig. 3, most of the II type twins also show a bended morphology at KBs, and the bending angles are frequently much larger than the calculated trace angles of their {101 ‾ 2} twinning planes (seen in Table 2, it is the reason why we named such part as “kinked twin”). Therefore, this arises a quite counter-intuitive confusion that the orientation of the “twin” hasn't changed much (seen in Table 1) when it stretches across a KB but its body is obviously bended by kink deformation (seen in Fig. 3 and Table 2). To further clarify this counter-intuitive confusion, TEM works were carried out on such interested area and the corresponding results and analyses are shown in Fig. 4. Among the presented HAADF-STEM images (Fig. 4a, Fig. 4b and Fig. 4d), the white lines with higher Z contrast are LPSO building blocks [1–6]. Fig. 4b and Fig. 4d show enlarge views of the rectangles marked as “B” and “D” in Fig. 4a, which respectively aims at the above discussed “twin” and “kinked twin” regions (schematically depicted in Fig. 4c). From Fig. 4a, it can be clearly seen that the “twin” and “kinked twin” regions show an obvious bending angle at the KB. Meanwhile, the SAED pattern selected from the boundary between the “matrix” and “twin” (seen at the left bottom of Fig. 4c) shows a near theoretical TB with misorientation of < 112 ‾0> /88.9°, while the SAED pattern selected from the boundary between the “kinked matrix” and “kinked twin” (seen at the right bottom of Fig. 4c) shows a tilt boundary with misorientation of < 112 ‾0> /78.1°. This further confirms that there only exists a rather small misorientation
Table 1 Misorientation angles between “twin” (○) and “kinked twin” (+), “matrix” (△) and “kinked matrix” (□) of the eight selected areas in Fig. 3.
Table 2 Calculated trace angles of {101 ‾ 2} planes and measured bending angles between “twin” (○) and “kinked twin” (+) of the eight selected areas in Fig. 3.
Area No.
1
2
3
4
5
6
7
8
Area No.
1
2
3
4
5
6
7
8
twin/kinked twin (○/+) matrix/kinked matrix (△/□)
2.7°
2.9°
0.7°
1.5°
4.7°
1.1°
1.5°
3.1°
2.5°
0.6°
0.6°
1.1°
1.0°
1.0°
0.4°
2.5°
11.8°
23.7°
18.6°
14.6°
19.4°
22.9°
19.2°
21.9°
Calculated trace angle (○/+) Measured bending angle (○/+)
3.3°
30.9°
10.5°
10.4°
6.3°
12.9°
10.8°
13.1°
3
Materials Science & Engineering A 767 (2019) 138418
T. Chen, et al.
Fig. 4. TEM observations on the interaction between kinking and {101 ‾ 2} twinning, where (b) and (d) are enlarged views of rectangles “B” and “D” that aim at “twin” and “kinked twin” regions in (a). Schematic diagram of such interaction is shown in (c).
Fig. 5. Schematic diagram showing two interaction modes between kinking and {101 ‾ 2} twinning: (a) for “kinking + twinning” mode and (b) for “twinning + kinking” mode.
basal slip have occurred in such “kinked twin” region, and the accumulated slip step has eventually made the “kinked twin” region show an obvious bending angle to the “twin” region (schematically illustrated on the right top of Fig. 4c). According to the polycrystal plasticity theory, such occurrence of local basal slip would only bring small orientation change on the slipped area [26]. Thus, the TEM works well explain the above counter-intuitive confusion, and further reveals the orientation change of the “kinked twin” is caused by basal slip, rather than synchronous to the kink deformation. Moreover, from another perspective, those above discussion also revealed that kinking is such a macroscopic deformation mechanism accomplished by slip, which agrees with the viewpoints in Refs. [15,25,27]. This also explains the
between the “twin” and “kinked twin” (~6.3°), while a quite large misorientation between the “matrix” and “kinked matrix” (~17.1°), which is in accordance with the above EBSD results. Beyond that, a subtle but important morphology change of the LPSO building blocks was found under TEM. As shown in Fig. 4b, the LPSO building blocks in the “twin” region have a flat and straight morphology. However, as shown in Fig. 4d, the LPSO building blocks in the “kinked twin” region show a wavy morphology, and most importantly, the joint line of those wave nodes is found to parallel to its basal plane (the basal plane traces of the four concerned regions are marked by doted lines in Fig. 4c, and further highlighted by blue and red for the “twin” and “kinked twin” regions). This evidently indicates massive 4
Materials Science & Engineering A 767 (2019) 138418
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phenomenon that why the massive basal slip occurs in only in the “kinked twin”, rather than the “twin” region.
[7] J.K. Kim, S. Sandlöbes, D. Raabe, On the room temperature deformation mechanisms of a Mg–Y–Zn alloy with long-period-stacking-ordered structures, Acta Mater. 82 (2015) 414–423. [8] W.W. Hu, Z.Q. Yang, H.Q. Ye, Cottrell atmospheres along dislocations in longperiod stacking ordered phases in a Mg–Zn–Y alloy, Scr. Mater. 117 (2016) 77–80. [9] X.H. Shao, Z.Z. Peng, Q.Q. Jin, X.L. Ma, Atomic-scale segregations at the deformation-induced symmetrical boundary in an Mg-Zn-Y alloy, Acta Mater. 118 (2016) 177–186. [10] S.M. Zhu, R. Lapovok, J.F. Nie, Y. Estrin, S.N. Mathaudhu, Microstructure and mechanical properties of LPSO phase dominant Mg85.8Y7.1Zn7.1 and Mg85.8Y7.1Ni7.1 alloys, Mater. Sci. Eng. A 692 (2017) 35–42. [11] X.J. Zhou, C.M. Liu, Y.H. Gao, S.N. Jiang, W.H. Liu, L.W. Lu, Microstructure and mechanical properties of extruded Mg-Gd-Y-Zn-Zr alloys filled with intragranular LPSO phases, Mater. Char. 135 (2018) 76–83. [12] R. Chen, S. Sandlöbes, C. Zehnder, X. Zeng, S. Korte-Kerzel, D. Raabe, Deformation mechanisms, activated slip systems and critical resolved shear stresses in an MgLPSO alloy studied by micro-pillar compression, Mater. Des. 154 (2018) 203–216. [13] H. Gao, K.I. Ikeda, T. Morikawa, K. Higashida, H. Nakashima, Analysis of kink boundaries in deformed synchronized long-period stacking ordered magnesium alloys, Mater. Lett. 146 (2015) 30–33. [14] K. Hagihara, T. Okamoto, M. Yamasaki, Y. Kawamura, T. Nakano, Electron backscatter diffraction pattern analysis of the deformation band formed in the Mg-based long-period stacking ordered phase, Scr. Mater. 117 (2016) 32–36. [15] T. Matsumoto, M. Yamasaki, K. Hagihara, Y. Kawamura, Configuration of dislocations in low-angle kink boundaries formed in a single crystalline long-period stacking ordered Mg-Zn-Y alloy, Acta Mater. 151 (2018) 112–124. [16] Z.Z. Peng, X.H. Shao, Q.Q. Jin, J.F. Liu, X.L. Ma, Dislocation configuration and solute redistribution of low angle kink boundaries in an extruded Mg–Zn–Y–Zr alloy, Mater. Sci. Eng. A 687 (2017) 211–220. [17] M.H. Yoo, Slip, twinning, and fracture in hexagonal close-packed metals, Metall. Trans. A 12 (1981) 409–418. [18] X.Z. Liao, J. Wang, J.F. Nie, Y.Y. Jiang, P.D. Wu, Deformation twinning in hexagonal materials, MRS Bull. 41 (2016) 314–319. [19] L.H. Mao, C.M. Liu, T. Chen, Y.H. Gao, S.N. Jiang, R.K. Wang, Twinning behavior in a rolled Mg-Al-Zn alloy under dynamic impact loading, Scr. Mater. 150 (2018) 87–91. [20] M. Matsuda, S. Ii, Y. Kawamura, Y. Ikuhara, M. Nishida, Interaction between long period stacking order phase and deformation twin in rapidly solidified Mg97Zn1Y2 alloy, Mater. Sci. Eng. A 386 (2004) 447–452. [21] X.H. Shao, Z.Q. Yang, X.L. Ma, Interplay between deformation twins and basal stacking faults enriched with Zn/Y in Mg97Zn1Y2 alloy, Philos. Mag. Lett. 94 (2014) 150–156. [22] L. Wang, J. Sabisch, E.T. Lilleodden, Kink formation and concomitant twin nucleation in Mg–Y, Scr. Mater. 111 (2016) 68–71. [23] K. Li, Z.Y. Chen, T. Chen, J.B. Shao, R.K. Wang, C.M. Liu, Hot deformation and dynamic recrystallization behaviors of Mg-Gd-Zn alloy with LPSO phases, J. Alloy. Comp. 792 (2019) 894–906. [24] K. Hagihara, T. Mayama, M. Honnami, M. Yamasaki, H. Izuno, T. Okamoto, T. Ohashi, T. Nakano, Y. Kawamura, Orientation dependence of the deformation kink band formation behavior in Zn single crystal, Int. J. Plast. 77 (2016) 174–191. [25] M. Yamasaki, K. Hagihara, S.I. Inoue, J.P. Hadorn, Y. Kawamura, Crystallographic classification of kink bands in an extruded Mg–Zn–Y alloy using intragranular misorientation axis analysis, Acta Mater. 61 (2013) 2065–2076. [26] C.N. Reid, Deformation Geometry for Materials Scientists: International Series on Materials Science and Technology vol 11, Elsevier, Amsterdam, 2016. [27] T. Inamura, Geometry of kink microstructure analysed by rank-1 connection, Acta Mater. 173 (2019) 270–280.
4. Conclusion In this study, we have discussed two different interaction modes between kinking and twinning that distinct from their formation order, i.e. the “kinking + twinning” mode and “twinning + kinking” mode. Their deformation geometries are concluded and schematically shown in Fig. 5. (i) As shown in Fig. 5a, the “kinking + twinning” mode can be simply interpreted as the twins concomitantly nuclear near the earlierformed KB(s) and the twinning propagation is frequently restricted by two adjacent KBs. In this interaction mode, both the deformation geometry of kinking and twinning are same to that of their independent occurrence under non-interaction situation. (ii) While in Fig. 5b, the “twinning + kinking” mode can be mainly summarized that kinking has locally broken the theoretical misorientation of the earlier-formed twins to their adjacent matrixes. In this procedure, basal slip in “kinked twin” plays a vital role in accommodating the asynchronous deformation between kinking and twinning. Acknowledgement This work was supported by the National Natural Science Foundation of China (grant number 51874367 and 51574291). References [1] J. Gröbner, A. Kozlov, X.Y. Fang, J. Geng, J.F. Nie, R. Schmid-Fetzer, Phase equilibria and transformations in ternary Mg-rich Mg–Y–Zn alloys, Acta Mater. 60 (2012) 5948–5962. [2] Q. Yang, B.L. Xiao, Q. Zhang, M.Y. Zheng, Z.Y. Ma, Exceptional high-strain-rate superplasticity in Mg–Gd–Y–Zn–Zr alloy with long-period stacking ordered phase, Scr. Mater. 69 (2013) 801–804. [3] J.K. Kim, W.S. Ko, S. Sandlöbes, M. Heidelmann, B. Grabowski, D. Raabe, The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy, Acta Mater. 112 (2016) 171–183. [4] N. Fujita, M. Matsushita, R. Tsukamoto, M. Yamasaki, Y. Kawamura, T. Irifune, E. Abe, The structure of a novel long-period superlattice phase in Mg97Zn1Yb2 alloys, Scr. Mater. 150 (2018) 78–81. [5] T. Chen, Z.Y. Chen, J.B. Shao, R.K. Wang, L.H. Mao, C.M. Liu, Evolution of LPSO phases in a Mg-Zn-Y-Gd-Zr alloy during semi-continuous casting, homogenization and hot extrusion, Mater. Des. 152 (2018) 1–9. [6] J.B. Shao, Z.Y. Chen, T. Chen, R.K. Wang, Y.L. Liu, C.M. Liu, Texture evolution, deformation mechanism and mechanical properties of the hot rolled Mg-Gd-Y-Zn-Zr alloy containing LPSO phase, Mater. Sci. Eng. A 731 (2018) 479–486.
5
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Short communication
Interactions between kinking and {1012 ‾ } twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase
T
Tao Chen, Zhiyong Chen∗, Jianbo Shao, Renke Wang, Kai Li, Longhui Mao, Chuming Liu School of Materials Science and Engineering, Central South University, Changsha, 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnesium alloys Kinking Twinning LPSO phase
Two interaction modes between kinking and {101 ‾ 2} twinning in a Mg–Zn-Gd alloy containing long period stacking ordered (LPSO) phase are thoroughly investigated: (i) when kinking forms prior to twinning, twins concomitantly nuclear at kink boundaries (KBs) and twinning propagation is frequently restricted by two adjacent KBs. By contrast, (ii) when twinning forms prior to kinking, kink deformation locally breaks the twins’ theoretical misorientation to their adjacent matrixes, during which basal slip plays a vital role in accommodating the asynchronous deformation between kinking and twinning.
1. Introduction Recently, a series of Mg–Zn-RE (RE = rare earth) alloys containing long-period stacking ordered (LPSO) phase have attracted increasing attention [1–6]. Compared with conventional Mg alloys, those Mg/ LPSO alloys show superior mechanical properties and unique microstructure [7–12]. Due to the lamellar structure of LPSO phase, kinking was frequently detected in deformed Mg/LPSO alloys [13–16], especially in those grains align with their basal planes parallel to the compressive stress. The unique deformation geometry of kinking makes it an important mechanism for Mg/LPSO alloys to generate homogenous strain when basal slip is extremely inhibited. {101 ‾ 2} twinning, known as the most common twinning mode in Mg alloys, which is prevalently detected in the grains with basal planes parallel to the compressive stress or perpendicular to the tensive stress [17–19]. Although the twinning activity is more or less hindered by LPSO phase [20,21], {101 ‾ 2} twinning (hereinafter referred to as twinning) is still considered as an essential mechanism for Mg/LPSO alloys to generate homogenous strain when basal slip is inhibited. On top of this, both kinking and twinning play important roles in plastic deformation of Mg/LPSO alloys; and moreover, when compression stress parallels to the grain's basal plane, both of them are prone to develop. Therefore, it is quite reasonable to expect that the kinking and twinning may occur concurrently in a same grain and interact with each other under certain conditions. In fact, Wang et al. [22] have reported a unique microstructure in a deformed Mg–Y alloy that the formation of kinking was accompanied by nucleation of twins at kink boundaries (KBs). However, their primary concerns were on the
∗
phenomenological description via in-situ observation, the underlying interaction mechanism between kinking and twinning was less discussed. In this study, another new interaction mode between kinking and twinning was observed for the first time. Moreover, in-depth discussions were conducted on interaction modes between kinking and twinning, which can provide valuable references to lucubrate the basic deformation behaviors of Mg/LPSO alloys. 2. Materials and methods The investigated Mg–Zn-Gd alloy (Mg-0.8Zn-2.4Gd in at%) filled with LPSO phase was prepared by semi-continuous casting and homogenization (detailed material preparation method and initial microstructure of the homogenized alloy can be seen in our recent study [23]). The homogenized alloy (with a dimension of Φ10 mm × 15 mm) was compressed at ambient temperature on a standard universal testing machine (CSS-44100) with a constant loading speed of 1 mm/min. Compression test was halted and unloaded halfway, and the compressed sample without any cracks has eventually obtained 10% plastic deformation (typical strain-stress curve was shown in Fig. 1). Grain orientations of the selected area were acquired using a Helios Nanolab 600i scanning electron microscope (SEM) equipped with an Oxford instruments electron backscattered diffraction (EBSD) analysis system. High-angle annular dark-field (HAADF-STEM) images and related selected area electron diffraction (SAED) patterns were obtained using a FEI Titan G2 60–300 transmission electron microscope (TEM). Samples for EBSD/TEM observation were prepared by mechanical
Corresponding author. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.msea.2019.138418 Received 27 July 2019; Received in revised form 12 September 2019; Accepted 12 September 2019 Available online 13 September 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 767 (2019) 138418
T. Chen, et al.
either in LABs or HABs. Those above detected deformation geometries of kinking are both in accordance with the previous studies on Zn single crystal [24] and LPSO-containing Mg–Zn–Y alloy [25]. As we turn our perspective to the twinned areas in Fig. 2, two kinds of twins with different morphologies can be detected. Firstly, as commonly shown in both Fig. 2a and Fig. 2b, most of the twins (the typical ones are highlighted by dashed rectangles) are embedded in a single kink band, and the twinned zones are frequently sandwiched by two adjacent KBs (hereinafter named as I type twin). Secondly, as highlighted by thicker boundaries in Fig. 2b, part of the twins stretches across more than one kink band, and their TBs do not terminate at KBs (hereinafter named as II type twin). Such two kinds of twins with different morphologies indicate two interaction modes between kinking and {101 ‾ 2} twinning, which would be further characterized and discussed in Fig. 3. Fig. 3a and Fig. 3b are Euler angle coloring maps of the selected rectangular areas in Fig. 2b (re-scanned with finer step size). In such maps, the TBs with misorientation of < 112 ‾0> /86.3° (with a tolerance of ± 5°) are highlighted by yellow lines, and some of the typical KBs are marked by dashed lines. By inspecting the eight selected areas contain typical II type twins (numbered 1–4 in Figs. 3a and 5–8 in Fig. 3b), the difference between the above-mentioned two kinds of twins can be more clearly distinguished: once stretch across a KB, the II type twins uniformly lose the theoretical misorientations to their near matrixes, which is entirely different from that of the I type twins with uninterrupted theoretical TBs. Such difference should relate with the formation order of kinking and twinning during deformation. Firstly, as for the I type twins, kinking should form earlier than that of twinning (hereinafter referred as “kinking + twinning” mode), otherwise the KB (s) should unavoidably intersect with the TB(s). As has been reported in Ref. [22], KBs are potential sites favor the nucleation of twinning. On the other hand, such earlier-formed KBs should also hinder the twinning propagation that attempt to across it, which eventually makes the I type twins are frequently sandwiched by two adjacent KBs. Secondly, as for the II type twins, twinning should form earlier than that of kinking (hereinafter referred as “twinning + kinking” mode), otherwise the constituent part of twins that stretch across KB(s) should also have the
Fig. 1. Typical stress-strain curve of the compressed sample with 10% plastic deformation.
polishing and electro polishing (twin-jet electro polishing for TEM sample), with a solution of 15 mL perchloric acid and 285 mL of ethyl alcohol, at 233 K and 10 mA.
3. Results and discussion Fig. 2 shows two inverse pole figure (IPF) coloring maps detected in the compressed sample where kinking and twinning occur concurrently in a same grain and interact with each other. The low angle boundaries (LABs, 3°~15°), high angle boundaries (HABs, > 15°) and twin boundaries (TBs, < 112 ‾0> /86.3°, with a tolerance of ± 5°) are respectively highlighted by white, black and yellow lines. By inspecting the misorientations of KBs numbered by 1–12 (seen in Fig. 2a) and A~L (seen in Fig. 2b), it can be seen that their rotation axes are uniformly perpendicular to the c-axis and show a scattered distribution on the basal plane, which can be concluded as < uvt0> and the indexes u , v , t are variables. Beyond that, their misorientation angles are also unfixed,
Fig. 2. (a) (b) IPF coloring maps showing the kinking and {101 ‾ 2} twinning occur concurrently in a same grain and interact with each other, where the rectangles “A” and “B” in (b) show further EBSD detection positions for Fig. 3a and b. 2
Materials Science & Engineering A 767 (2019) 138418
T. Chen, et al.
Fig. 3. Euler angle coloring maps showing detailed interactions between kinking and {101 ‾ 2} twinning, where the eight dashed squares showing further orientation analyses positions for Table 1.
theoretical misorientations to their near matrixes. In other words, in the “twinning + kinking” mode, the later-formed kinking has locally broken the theoretical misorientation of the earlier-formed twins to their adjacent matrixes. According to the above discussion, the “kinking + twinning” mode can be simply interpreted as twins concomitantly nuclear at KB(s) and their subsequent twinning propagation are restricted by two adjacent KBs. This can be verified by the fact that both the deformation geometry of kinking and twinning in this interaction mode are same to that of their independent occurrence under non-interaction situation [18,24]. By contrast, the “twinning + kinking” mode is more complicated, since kinking has locally changed the deformation geometry of the earlierformed twins. To further study this, orientation analyses of the eight selected areas in Fig. 3 are listed in Table 1. Based on the deformation geometry, each of the selected area is divided into four different regions: “twin” (marked as ○, as typically demonstrated in the No. 3 selected area, the same below), “kinked twin” (+), “matrix” (△) and “kinked matrix” (□), and their crystal orientations are schematically shown at the bottom of Fig. 3. From Table 1 and Fig. 3, it can be seen that the misorientation angles between “twin” and “kinked twin” (compare ○ with +) are rather small (ranges from 0.7° to 4.7°), which are much lower than of “matrix” and “kinked matrix” (compare △ with □, ranges from 11.8° to 23.7°). This indicates the orientation of the “twin” hasn't changed much when it stretches across a KB and becomes the “kinked twin”, but the orientation of the “matrix” has significantly changed by kink deformation when it becomes the “kinked matrix”. Such unsynchronized orientation change makes the “kinked twins” lose
their theoretical misorientations of < 112 ‾0> /86.3° to the near matrix as they stretch across the KB. However, by inspecting of the eight selected areas in Fig. 3, most of the II type twins also show a bended morphology at KBs, and the bending angles are frequently much larger than the calculated trace angles of their {101 ‾ 2} twinning planes (seen in Table 2, it is the reason why we named such part as “kinked twin”). Therefore, this arises a quite counter-intuitive confusion that the orientation of the “twin” hasn't changed much (seen in Table 1) when it stretches across a KB but its body is obviously bended by kink deformation (seen in Fig. 3 and Table 2). To further clarify this counter-intuitive confusion, TEM works were carried out on such interested area and the corresponding results and analyses are shown in Fig. 4. Among the presented HAADF-STEM images (Fig. 4a, Fig. 4b and Fig. 4d), the white lines with higher Z contrast are LPSO building blocks [1–6]. Fig. 4b and Fig. 4d show enlarge views of the rectangles marked as “B” and “D” in Fig. 4a, which respectively aims at the above discussed “twin” and “kinked twin” regions (schematically depicted in Fig. 4c). From Fig. 4a, it can be clearly seen that the “twin” and “kinked twin” regions show an obvious bending angle at the KB. Meanwhile, the SAED pattern selected from the boundary between the “matrix” and “twin” (seen at the left bottom of Fig. 4c) shows a near theoretical TB with misorientation of < 112 ‾0> /88.9°, while the SAED pattern selected from the boundary between the “kinked matrix” and “kinked twin” (seen at the right bottom of Fig. 4c) shows a tilt boundary with misorientation of < 112 ‾0> /78.1°. This further confirms that there only exists a rather small misorientation
Table 1 Misorientation angles between “twin” (○) and “kinked twin” (+), “matrix” (△) and “kinked matrix” (□) of the eight selected areas in Fig. 3.
Table 2 Calculated trace angles of {101 ‾ 2} planes and measured bending angles between “twin” (○) and “kinked twin” (+) of the eight selected areas in Fig. 3.
Area No.
1
2
3
4
5
6
7
8
Area No.
1
2
3
4
5
6
7
8
twin/kinked twin (○/+) matrix/kinked matrix (△/□)
2.7°
2.9°
0.7°
1.5°
4.7°
1.1°
1.5°
3.1°
2.5°
0.6°
0.6°
1.1°
1.0°
1.0°
0.4°
2.5°
11.8°
23.7°
18.6°
14.6°
19.4°
22.9°
19.2°
21.9°
Calculated trace angle (○/+) Measured bending angle (○/+)
3.3°
30.9°
10.5°
10.4°
6.3°
12.9°
10.8°
13.1°
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Fig. 4. TEM observations on the interaction between kinking and {101 ‾ 2} twinning, where (b) and (d) are enlarged views of rectangles “B” and “D” that aim at “twin” and “kinked twin” regions in (a). Schematic diagram of such interaction is shown in (c).
Fig. 5. Schematic diagram showing two interaction modes between kinking and {101 ‾ 2} twinning: (a) for “kinking + twinning” mode and (b) for “twinning + kinking” mode.
basal slip have occurred in such “kinked twin” region, and the accumulated slip step has eventually made the “kinked twin” region show an obvious bending angle to the “twin” region (schematically illustrated on the right top of Fig. 4c). According to the polycrystal plasticity theory, such occurrence of local basal slip would only bring small orientation change on the slipped area [26]. Thus, the TEM works well explain the above counter-intuitive confusion, and further reveals the orientation change of the “kinked twin” is caused by basal slip, rather than synchronous to the kink deformation. Moreover, from another perspective, those above discussion also revealed that kinking is such a macroscopic deformation mechanism accomplished by slip, which agrees with the viewpoints in Refs. [15,25,27]. This also explains the
between the “twin” and “kinked twin” (~6.3°), while a quite large misorientation between the “matrix” and “kinked matrix” (~17.1°), which is in accordance with the above EBSD results. Beyond that, a subtle but important morphology change of the LPSO building blocks was found under TEM. As shown in Fig. 4b, the LPSO building blocks in the “twin” region have a flat and straight morphology. However, as shown in Fig. 4d, the LPSO building blocks in the “kinked twin” region show a wavy morphology, and most importantly, the joint line of those wave nodes is found to parallel to its basal plane (the basal plane traces of the four concerned regions are marked by doted lines in Fig. 4c, and further highlighted by blue and red for the “twin” and “kinked twin” regions). This evidently indicates massive 4
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phenomenon that why the massive basal slip occurs in only in the “kinked twin”, rather than the “twin” region.
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4. Conclusion In this study, we have discussed two different interaction modes between kinking and twinning that distinct from their formation order, i.e. the “kinking + twinning” mode and “twinning + kinking” mode. Their deformation geometries are concluded and schematically shown in Fig. 5. (i) As shown in Fig. 5a, the “kinking + twinning” mode can be simply interpreted as the twins concomitantly nuclear near the earlierformed KB(s) and the twinning propagation is frequently restricted by two adjacent KBs. In this interaction mode, both the deformation geometry of kinking and twinning are same to that of their independent occurrence under non-interaction situation. (ii) While in Fig. 5b, the “twinning + kinking” mode can be mainly summarized that kinking has locally broken the theoretical misorientation of the earlier-formed twins to their adjacent matrixes. In this procedure, basal slip in “kinked twin” plays a vital role in accommodating the asynchronous deformation between kinking and twinning. Acknowledgement This work was supported by the National Natural Science Foundation of China (grant number 51874367 and 51574291). References [1] J. Gröbner, A. Kozlov, X.Y. Fang, J. Geng, J.F. Nie, R. Schmid-Fetzer, Phase equilibria and transformations in ternary Mg-rich Mg–Y–Zn alloys, Acta Mater. 60 (2012) 5948–5962. [2] Q. Yang, B.L. Xiao, Q. Zhang, M.Y. Zheng, Z.Y. Ma, Exceptional high-strain-rate superplasticity in Mg–Gd–Y–Zn–Zr alloy with long-period stacking ordered phase, Scr. Mater. 69 (2013) 801–804. [3] J.K. Kim, W.S. Ko, S. Sandlöbes, M. Heidelmann, B. Grabowski, D. Raabe, The role of metastable LPSO building block clusters in phase transformations of an Mg-Y-Zn alloy, Acta Mater. 112 (2016) 171–183. [4] N. Fujita, M. Matsushita, R. Tsukamoto, M. Yamasaki, Y. Kawamura, T. Irifune, E. Abe, The structure of a novel long-period superlattice phase in Mg97Zn1Yb2 alloys, Scr. Mater. 150 (2018) 78–81. [5] T. Chen, Z.Y. Chen, J.B. Shao, R.K. Wang, L.H. Mao, C.M. Liu, Evolution of LPSO phases in a Mg-Zn-Y-Gd-Zr alloy during semi-continuous casting, homogenization and hot extrusion, Mater. Des. 152 (2018) 1–9. [6] J.B. Shao, Z.Y. Chen, T. Chen, R.K. Wang, Y.L. Liu, C.M. Liu, Texture evolution, deformation mechanism and mechanical properties of the hot rolled Mg-Gd-Y-Zn-Zr alloy containing LPSO phase, Mater. Sci. Eng. A 731 (2018) 479–486.
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