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Annealing hardening in detwinning deformation of Mg–3Al–1Zn alloy
Materials Science & Engineering A 594 (2014) 287–291
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Annealing hardening in detwinning deformation of Mg–3Al–1Zn alloy Yunchang Xin a,n, Xiaojun Zhou a, Houwen Chen a, Jian-Feng Nie a,b, Hong Zhang a, Yuanyuan Zhang a, Qing Liu a a b
School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Department of Materials Engineering, Monash University, Victoria 3800, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2013 Received in revised form 24 November 2013 Accepted 25 November 2013 Available online 1 December 2013
The present work reports the effect of annealing treatment on detwinning deformation in Mg alloy AZ31 and pure Mg that have pre-strained twins. It shows that appropriate annealing enhances, rather than reduces, the yield strength of the pre-strained AZ31, but it does not cause any strengthening of the pre-strained pure Mg. STEM–EDS mapping shows that both Al and Zn segregate to twin boundaries in the pre-strained AZ31 after the annealing process. It is proposed that it is the pinning of twin boundary by segregated solute atoms that results in an increased activation stress for detwinning deformation and, hence, annealing hardening. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Detwinning Annealing Hardening
1. Introduction Basal slip and f1012g〈1011〉 twinning constitute the main deformation modes of Mg alloys at room temperature [1–4]. For hot rolled Mg alloys with a basal texture, f1012g〈1011〉 twinning dominates the initial plastic deformation when the alloys are compressed along the transverse direction (TD) (or the rolling direction (RD)), or are extended along the normal direction (ND) [1,3]. The compression in the extrusion direction of the extruded Mg alloys with a basal fiber texture is also governed by 〈1011〉f1012g twinning. In general, the f1012g twins generated by pre-loading can also detwin during reloading along some specific directions [5–7]. The detwinning of f1012g twins mainly takes place under two types of loading conditions: (1) loading, unloading, followed by reloading along the opposite direction of the initial loading; (2) compressing along TD (or RD) of hot rolled plates, unloading, followed by re-compressing along ND [5,6]. The twinning–detwinning process is very common in fatigue tests of textured Mg alloys that are subjected to cyclic loading of compression and tension [8–14]. As the detwinning proceeds by twin boundary migration that does not require the nucleation of new twins, the activation stress for detwinning is quite low. For example, in-situ neutron diffraction measurements demonstrate that the critical resolved shear stresses (CRSS) for twinning and detwinning in the polycrystalline Mg alloy ZK60 (Mg–6 wt%Zn– 0.6 wt%Zr) are about 15 MPa and 6 MPa, respectively [7].
n
Corresponding author. Tel./fax: þ 86 23 65106407. E-mail address: [email protected] (Y. Xin).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.11.080
Previously, the detwinning deformation has been extensively studied for plastically deformed Mg alloys without any annealing treatments [2,5,7,14]. The influence of annealing treatment on detwinning deformation of Mg alloys is therefore unclear. It is traditionally accepted that annealing treatment generally removes the lattice defects in metals and, thus, causes strength drop. However, in the present paper, we demonstrate that an annealing hardening occurs in the detwinning deformation of Mg alloy AZ31. We further report that such an annealing hardening phenomenon does not occur in detwinning deformation of pure Mg. The corresponding strengthening mechanism is discussed. 2. Experiments and methods Hot rolled plates of Mg alloy AZ31 and pure Mg were used in the present study. They both had a typical basal texture. The samples containing f1012g twins were prepared by compression along TD of the plate. The detwinning deformation was studied by a further re-compression along ND of the pre-compressed samples at a strain rate of 10 3 s 1. Inhomogeneous deformation regions are commonly known to appear after the compression of blockshaped samples, which greatly affects the deformation characteristic during reloading. In order to acquire pre-strained samples with a more homogeneous microstructure, large blocks of 40 mm 40 mm 40 mm were pre-compressed along TD by 1.2% (designated PR1.2%) and 3.5% (designated PR3.5%), and the blocks, 9 mm (ND) 7 mm (RD) 7 mm (TD), for compression along ND were cut from the center of the pre-strained large blocks. Two pre-strain levels were used to prepare samples with
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Fig. 1. (a and b) Strain–stress curves under compression along ND of pre-strained AZ31. For comparison, strain–stress curve under compression along TD of hot rolled AZ31 is also shown (TD-Com). YS represents yield strength.
different volume fractions of f1012g twins. Some of the PR1.2% samples and PR3.5% samples were subjected to two different annealing schemes: (1) annealing at 170 1C for 12 h followed by 200 1C for 8 h; (2) annealing at 170 1C for 15 min. The PR1.2% samples and PR3.5% samples annealed at 170 1C for 12 h and 200 1C for 8 h were designated PRA1.2%-1 and PRA3.5%-1, respectively, while the PR1.2% samples and PR3.5% samples annealed at 170 1C for 15 min were designated PRA1.2%-2 and PRA3.5%-2, respectively. To reveal the microstructure evolution during plastic deformation, in-situ electron back-scattered diffraction (EBSD) analysis was conducted on a scanning electron microscope equipped with the HKL-EBSD system. In the in-situ EBSD analysis, EBSD mapping was carried out first, followed by reloading outside the SEM chamber, and subsequent EBSD data collection from the same area. The samples for EBSD mapping were mechanically ground and electrochemically polished in the AC2 electrolyte. A Cs-corrected FEI
Fig. 2. (a and b) Strain–stress curves under compression along ND of pre-strained pure Mg. YS represents yield strength.
G2 80-200 Titan was used for high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, and for energy dispersive X-ray (EDS) mapping in STEM, of twin boundaries, with a view to examine the distribution of Al and Zn solute atoms in the twin boundaries.
3. Results Fig. 1 shows the mechanical properties of pre-strained AZ31 samples under compression along ND. For comparison, the compression strain–stress curve along TD of the hot rolled AZ31 plate, which deforms predominantly by f1012g twinning, was also shown (designated TD-Com). The strain–stress curve of the TDCom sample has a plateau, which is the signature for the predominant occurrence of f1012g twinning. Such a plateau also appears in all curves of the pre-strained samples. It is generally accepted that hot rolled AZ31 samples that are similar to the
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Fig. 3. Crystallographic orientation maps of the 3.5% pre-compressed AZ31 alloy (a) and the 3.5% pre-compressed pure Mg (b).
PR1.2% and PR3.5% specimens in the present study mainly deform by detwinning when they are compressed along ND [2,7]. Consistent with previous observations [5,13], the yield strengths of the PR1.2% samples (38 MPa) and PR3.5% samples (54 MPa) are both lower than that of the TD-Com specimen (60 MPa). Interestingly, the yield strengths of all pre-strained samples subjected to annealing treatments are increased by about 20 MPa. This is in contrast with the traditional view that annealing treatment generally softens the plastically deformed Mg alloys [3,15]. It is important to note that the annealing treatment does not cause any strengthening in pre-strained pure Mg samples, as shown in the strain–stress curves in Fig. 2(a and b). Fig. 3 presents the crystallographic orientation maps of the 3.5% pre-compressed AZ31 Mg alloy and the 3.5% pre-compressed pure Mg. Many f1012g twin lamellae exist in both AZ31 and pure Mg. Fig. 4 shows in-situ EBSD maps of a pre-strained and annealed AZ31 sample and a pre-strained and annealed pure Mg sample before and after the compression along ND. Obviously, the twins in the pre-strained samples are well retained after annealing treatment. After the compression along ND, the majority of pre-existing f1012g twins have contracted or disappeared, as denoted by the arrows. Although it is well demonstrated in the literature [2,7] that the detwinning is one of the major deformation modes of Mg alloys that have already been plastically deformed by twinning, it was unclear whether annealing treatment had any influence on the detwinning mode. The present work demonstrates that annealing does not change the major deformation mode of samples with pre-existing f1012g twins.
4. Discussion AZ31 Mg alloy has a nominal concentration of 2.7 at% Al and 0.4 at% Zn. Al has an equilibrium solid solubility of 11.8 at% at the eutectic temperature and 3.2 at% at 200 1C in the Mg matrix [16]. The solubility of Zn in Mg is about 2.4 at% at the eutectic temperature of 340 1C and about 1.3 at% at 200 1C [16]. Generally, precipitates are absent in AZ31. It is generally accepted that solute atoms in alloys can segregate to high-energy grain boundaries, but not to low-energy twin boundaries. However, a very recent publication [17] reported that, after annealing, periodic segregation of solute atoms can take
place in twin boundaries in Mg alloys. In that report [17], the segregation of Zn or Gd to different types of twin boundaries, namely f1011g, f1012g and f1013g, is clearly shown by HAADF-STEM. As the atom number of Al is next to that of Mg, it is impossible to use HAADF-STEM to reveal whether Al atoms have segregated to f1012g twin boundaries or not. Therefore, the STEM–EDS mapping was employed in the present study to examine the distribution of Al and Zn solutes in the f1012g twin boundaries in the pre-strained and annealed AZ31 sample. Fig. 5(a–c) shows the segregation of both Al and Zn in the f1012g twin boundary. The segregated solutes in the twin boundary will pin the boundary, which makes the migration of the twin boundary more difficult. Since detwinning mainly involves the migration of the f1012g twin boundaries, any strong segregation of solute atoms to the f1012g twin boundaries will lead to an increased activation stress for detwinning, as demonstrated in Fig. 1. In addition, the annealing of pre-strained AZ31 sample at 170 1C for 15 min also results in a remarkable increase in the yield stress for detwinning, which implies that the segregation of solute atoms to f1012g twin boundaries is a relatively fast process. The segregation of solute atoms in twin boundaries is probably a common phenomenon in annealed Mg alloys containing twins. Similar to the solute pinning effects for twinning [17], the segregation of solute atoms in twin boundaries can also make the detwinning process more difficult. In fact, solute segregation may generate different pinning effects on twinning and detwinning in Mg alloys. It is well known that f1012g twinning proceeds by both nucleation and growth of twins, while detwinning occurs only by twin boundary migration. The segregated solute atoms can pin the twin growth (twin boundary migration process), but cannot influence the twin nucleation. Therefore, the annealing hardening phenomenon is expected to be more pronounced in the detwinning deformation of Mg alloys.
5. Conclusion In summary, we report an annealing hardening phenomenon that occurs during compression tests of pre-strained Mg alloy AZ31, in which detwinning is the major deformation mode. Our experiments further demonstrate that the annealing hardening phenomenon does not take place in the detwinning deformation
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Fig. 4. In-situ EBSD maps of pre-strained AZ31 alloy (3.5% pre-compressed along TD and annealed at 170 1C for 12 h and 200 1C for 8 h) before 2.2% compression along ND (a) and after 2.2% compression along ND (b). In-situ EBSD maps of pre-strained pure Mg (3.5% pre-compressed along TD and annealed at 170 1C for 12 h and 200 1C for 8 h) before 3.2% compression along ND (c) and after 3.2% compression along ND (d).
Fig. 5. (a) HAADF-STEM image showing a coherent f1012g twin boundary in Mg alloy AZ31 that was subjected to 3.5% pre-compression along TD and subsequent annealing at 170 1C for 12 h and 200 1C for 8 h. (b) and (c) STEM–EDS maps showing the segregation of both Zn and Al in the twin boundary shown in (a).
of pure Mg that is pre-strained and tested under a similar condition. In Mg alloy AZ31, the solute atoms segregate to f1012g twin boundaries after annealing, and pin the twin boundaries, which leads to an increased activation stress for detwinning.
Prime novelty statement Detwinning is one of the most important deformation modes of Mg alloys. For pre-strained Mg alloys containing deformation
Y. Xin et al. / Materials Science & Engineering A 594 (2014) 287–291
twins, commonly seen in Mg products such as plates, sheets and extrudates, detwinning often dominates the deformation process under certain loading conditions, which greatly influences the deformation behavior and mechanical properties. In this paper, we report an interesting annealing hardening phenomenon in the detwinning process of pre-strained Mg alloy AZ31, which overturns the traditional view that annealing treatments cause softening of plastically deformed Mg alloys. Acknowledgment The current study is co-supported by National Natural Science Foundation of China (51371203, 51101175 and 51131009) and National Key Basic Research Program of China (2013CB632204 and 2013CB632205). We are also particularly grateful to Professor Ze Zhang of Zhejiang University for the access to the HAADF-STEM and STEM–EDS work in his lab.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Annealing hardening in detwinning deformation of Mg–3Al–1Zn alloy Yunchang Xin a,n, Xiaojun Zhou a, Houwen Chen a, Jian-Feng Nie a,b, Hong Zhang a, Yuanyuan Zhang a, Qing Liu a a b
School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Department of Materials Engineering, Monash University, Victoria 3800, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 19 September 2013 Received in revised form 24 November 2013 Accepted 25 November 2013 Available online 1 December 2013
The present work reports the effect of annealing treatment on detwinning deformation in Mg alloy AZ31 and pure Mg that have pre-strained twins. It shows that appropriate annealing enhances, rather than reduces, the yield strength of the pre-strained AZ31, but it does not cause any strengthening of the pre-strained pure Mg. STEM–EDS mapping shows that both Al and Zn segregate to twin boundaries in the pre-strained AZ31 after the annealing process. It is proposed that it is the pinning of twin boundary by segregated solute atoms that results in an increased activation stress for detwinning deformation and, hence, annealing hardening. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mg alloy Detwinning Annealing Hardening
1. Introduction Basal slip and f1012g〈1011〉 twinning constitute the main deformation modes of Mg alloys at room temperature [1–4]. For hot rolled Mg alloys with a basal texture, f1012g〈1011〉 twinning dominates the initial plastic deformation when the alloys are compressed along the transverse direction (TD) (or the rolling direction (RD)), or are extended along the normal direction (ND) [1,3]. The compression in the extrusion direction of the extruded Mg alloys with a basal fiber texture is also governed by 〈1011〉f1012g twinning. In general, the f1012g twins generated by pre-loading can also detwin during reloading along some specific directions [5–7]. The detwinning of f1012g twins mainly takes place under two types of loading conditions: (1) loading, unloading, followed by reloading along the opposite direction of the initial loading; (2) compressing along TD (or RD) of hot rolled plates, unloading, followed by re-compressing along ND [5,6]. The twinning–detwinning process is very common in fatigue tests of textured Mg alloys that are subjected to cyclic loading of compression and tension [8–14]. As the detwinning proceeds by twin boundary migration that does not require the nucleation of new twins, the activation stress for detwinning is quite low. For example, in-situ neutron diffraction measurements demonstrate that the critical resolved shear stresses (CRSS) for twinning and detwinning in the polycrystalline Mg alloy ZK60 (Mg–6 wt%Zn– 0.6 wt%Zr) are about 15 MPa and 6 MPa, respectively [7].
n
Corresponding author. Tel./fax: þ 86 23 65106407. E-mail address: [email protected] (Y. Xin).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.11.080
Previously, the detwinning deformation has been extensively studied for plastically deformed Mg alloys without any annealing treatments [2,5,7,14]. The influence of annealing treatment on detwinning deformation of Mg alloys is therefore unclear. It is traditionally accepted that annealing treatment generally removes the lattice defects in metals and, thus, causes strength drop. However, in the present paper, we demonstrate that an annealing hardening occurs in the detwinning deformation of Mg alloy AZ31. We further report that such an annealing hardening phenomenon does not occur in detwinning deformation of pure Mg. The corresponding strengthening mechanism is discussed. 2. Experiments and methods Hot rolled plates of Mg alloy AZ31 and pure Mg were used in the present study. They both had a typical basal texture. The samples containing f1012g twins were prepared by compression along TD of the plate. The detwinning deformation was studied by a further re-compression along ND of the pre-compressed samples at a strain rate of 10 3 s 1. Inhomogeneous deformation regions are commonly known to appear after the compression of blockshaped samples, which greatly affects the deformation characteristic during reloading. In order to acquire pre-strained samples with a more homogeneous microstructure, large blocks of 40 mm 40 mm 40 mm were pre-compressed along TD by 1.2% (designated PR1.2%) and 3.5% (designated PR3.5%), and the blocks, 9 mm (ND) 7 mm (RD) 7 mm (TD), for compression along ND were cut from the center of the pre-strained large blocks. Two pre-strain levels were used to prepare samples with
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Fig. 1. (a and b) Strain–stress curves under compression along ND of pre-strained AZ31. For comparison, strain–stress curve under compression along TD of hot rolled AZ31 is also shown (TD-Com). YS represents yield strength.
different volume fractions of f1012g twins. Some of the PR1.2% samples and PR3.5% samples were subjected to two different annealing schemes: (1) annealing at 170 1C for 12 h followed by 200 1C for 8 h; (2) annealing at 170 1C for 15 min. The PR1.2% samples and PR3.5% samples annealed at 170 1C for 12 h and 200 1C for 8 h were designated PRA1.2%-1 and PRA3.5%-1, respectively, while the PR1.2% samples and PR3.5% samples annealed at 170 1C for 15 min were designated PRA1.2%-2 and PRA3.5%-2, respectively. To reveal the microstructure evolution during plastic deformation, in-situ electron back-scattered diffraction (EBSD) analysis was conducted on a scanning electron microscope equipped with the HKL-EBSD system. In the in-situ EBSD analysis, EBSD mapping was carried out first, followed by reloading outside the SEM chamber, and subsequent EBSD data collection from the same area. The samples for EBSD mapping were mechanically ground and electrochemically polished in the AC2 electrolyte. A Cs-corrected FEI
Fig. 2. (a and b) Strain–stress curves under compression along ND of pre-strained pure Mg. YS represents yield strength.
G2 80-200 Titan was used for high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, and for energy dispersive X-ray (EDS) mapping in STEM, of twin boundaries, with a view to examine the distribution of Al and Zn solute atoms in the twin boundaries.
3. Results Fig. 1 shows the mechanical properties of pre-strained AZ31 samples under compression along ND. For comparison, the compression strain–stress curve along TD of the hot rolled AZ31 plate, which deforms predominantly by f1012g twinning, was also shown (designated TD-Com). The strain–stress curve of the TDCom sample has a plateau, which is the signature for the predominant occurrence of f1012g twinning. Such a plateau also appears in all curves of the pre-strained samples. It is generally accepted that hot rolled AZ31 samples that are similar to the
Y. Xin et al. / Materials Science & Engineering A 594 (2014) 287–291
289
Fig. 3. Crystallographic orientation maps of the 3.5% pre-compressed AZ31 alloy (a) and the 3.5% pre-compressed pure Mg (b).
PR1.2% and PR3.5% specimens in the present study mainly deform by detwinning when they are compressed along ND [2,7]. Consistent with previous observations [5,13], the yield strengths of the PR1.2% samples (38 MPa) and PR3.5% samples (54 MPa) are both lower than that of the TD-Com specimen (60 MPa). Interestingly, the yield strengths of all pre-strained samples subjected to annealing treatments are increased by about 20 MPa. This is in contrast with the traditional view that annealing treatment generally softens the plastically deformed Mg alloys [3,15]. It is important to note that the annealing treatment does not cause any strengthening in pre-strained pure Mg samples, as shown in the strain–stress curves in Fig. 2(a and b). Fig. 3 presents the crystallographic orientation maps of the 3.5% pre-compressed AZ31 Mg alloy and the 3.5% pre-compressed pure Mg. Many f1012g twin lamellae exist in both AZ31 and pure Mg. Fig. 4 shows in-situ EBSD maps of a pre-strained and annealed AZ31 sample and a pre-strained and annealed pure Mg sample before and after the compression along ND. Obviously, the twins in the pre-strained samples are well retained after annealing treatment. After the compression along ND, the majority of pre-existing f1012g twins have contracted or disappeared, as denoted by the arrows. Although it is well demonstrated in the literature [2,7] that the detwinning is one of the major deformation modes of Mg alloys that have already been plastically deformed by twinning, it was unclear whether annealing treatment had any influence on the detwinning mode. The present work demonstrates that annealing does not change the major deformation mode of samples with pre-existing f1012g twins.
4. Discussion AZ31 Mg alloy has a nominal concentration of 2.7 at% Al and 0.4 at% Zn. Al has an equilibrium solid solubility of 11.8 at% at the eutectic temperature and 3.2 at% at 200 1C in the Mg matrix [16]. The solubility of Zn in Mg is about 2.4 at% at the eutectic temperature of 340 1C and about 1.3 at% at 200 1C [16]. Generally, precipitates are absent in AZ31. It is generally accepted that solute atoms in alloys can segregate to high-energy grain boundaries, but not to low-energy twin boundaries. However, a very recent publication [17] reported that, after annealing, periodic segregation of solute atoms can take
place in twin boundaries in Mg alloys. In that report [17], the segregation of Zn or Gd to different types of twin boundaries, namely f1011g, f1012g and f1013g, is clearly shown by HAADF-STEM. As the atom number of Al is next to that of Mg, it is impossible to use HAADF-STEM to reveal whether Al atoms have segregated to f1012g twin boundaries or not. Therefore, the STEM–EDS mapping was employed in the present study to examine the distribution of Al and Zn solutes in the f1012g twin boundaries in the pre-strained and annealed AZ31 sample. Fig. 5(a–c) shows the segregation of both Al and Zn in the f1012g twin boundary. The segregated solutes in the twin boundary will pin the boundary, which makes the migration of the twin boundary more difficult. Since detwinning mainly involves the migration of the f1012g twin boundaries, any strong segregation of solute atoms to the f1012g twin boundaries will lead to an increased activation stress for detwinning, as demonstrated in Fig. 1. In addition, the annealing of pre-strained AZ31 sample at 170 1C for 15 min also results in a remarkable increase in the yield stress for detwinning, which implies that the segregation of solute atoms to f1012g twin boundaries is a relatively fast process. The segregation of solute atoms in twin boundaries is probably a common phenomenon in annealed Mg alloys containing twins. Similar to the solute pinning effects for twinning [17], the segregation of solute atoms in twin boundaries can also make the detwinning process more difficult. In fact, solute segregation may generate different pinning effects on twinning and detwinning in Mg alloys. It is well known that f1012g twinning proceeds by both nucleation and growth of twins, while detwinning occurs only by twin boundary migration. The segregated solute atoms can pin the twin growth (twin boundary migration process), but cannot influence the twin nucleation. Therefore, the annealing hardening phenomenon is expected to be more pronounced in the detwinning deformation of Mg alloys.
5. Conclusion In summary, we report an annealing hardening phenomenon that occurs during compression tests of pre-strained Mg alloy AZ31, in which detwinning is the major deformation mode. Our experiments further demonstrate that the annealing hardening phenomenon does not take place in the detwinning deformation
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Fig. 4. In-situ EBSD maps of pre-strained AZ31 alloy (3.5% pre-compressed along TD and annealed at 170 1C for 12 h and 200 1C for 8 h) before 2.2% compression along ND (a) and after 2.2% compression along ND (b). In-situ EBSD maps of pre-strained pure Mg (3.5% pre-compressed along TD and annealed at 170 1C for 12 h and 200 1C for 8 h) before 3.2% compression along ND (c) and after 3.2% compression along ND (d).
Fig. 5. (a) HAADF-STEM image showing a coherent f1012g twin boundary in Mg alloy AZ31 that was subjected to 3.5% pre-compression along TD and subsequent annealing at 170 1C for 12 h and 200 1C for 8 h. (b) and (c) STEM–EDS maps showing the segregation of both Zn and Al in the twin boundary shown in (a).
of pure Mg that is pre-strained and tested under a similar condition. In Mg alloy AZ31, the solute atoms segregate to f1012g twin boundaries after annealing, and pin the twin boundaries, which leads to an increased activation stress for detwinning.
Prime novelty statement Detwinning is one of the most important deformation modes of Mg alloys. For pre-strained Mg alloys containing deformation
Y. Xin et al. / Materials Science & Engineering A 594 (2014) 287–291
twins, commonly seen in Mg products such as plates, sheets and extrudates, detwinning often dominates the deformation process under certain loading conditions, which greatly influences the deformation behavior and mechanical properties. In this paper, we report an interesting annealing hardening phenomenon in the detwinning process of pre-strained Mg alloy AZ31, which overturns the traditional view that annealing treatments cause softening of plastically deformed Mg alloys. Acknowledgment The current study is co-supported by National Natural Science Foundation of China (51371203, 51101175 and 51131009) and National Key Basic Research Program of China (2013CB632204 and 2013CB632205). We are also particularly grateful to Professor Ze Zhang of Zhejiang University for the access to the HAADF-STEM and STEM–EDS work in his lab.
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