Stability of twins in Mg alloys – A short review

Tingting Liu, Qingshn Yng, Ning Guo, Yun Lu, Bo Song,∗

a School of Materials and Energy, Southwest University, Chongqing 400715, China

b School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China

c Graduate School of Science and Engineering, Chiba University, Chiba 263-8522, Japan

Abstract In this article, the stability of twins, especially {10–12} twins, in Mg alloys was critically reviewed. In the last decade, pre-twinning is considered to be an effective method for adjusting the microstructure and properties of wrought Mg alloys. Especially, formation of twin-texture can remarkably improve formability of rolled Mg alloys. However, the stability of {10–12} twins determines the effect of this method. Previous work revealed that initial {10–12} twins may either grow or shrink under further deformation, and may be removed by static recrystallization under thermal effect. It is considered that improving the stability of twin structure could enhance the contribution of twin-texture on subsequent plastic formability. Based on this background, stress stability and thermal stability of {10–12} twin structure were summarized and reviewed in this article. Finally, a few critical scientific problems in this research field were pointed out.

Keywords: Mg alloys; Twin structure; Stability; Texture; Recrystallization.

1. Introduction

Mg alloys have received great attention in the aviation, automobile and electronics industries because of their low density, high specific strength and ability to be recycled [1,2].However, the further wide applications of Mg alloys are limited due to their poor corrosion resistance and poor mechanical properties [2–4]. At present, the most widely used components of Mg alloys were fabricated by die casting. In order to further enhance the mechanical properties of Mg alloys,plastic processing is commonly used to produce wrought Mg alloys. However, traditional plastic processing usually generates relatively coarse grain and strong texture, resulting in low strength and poor plastic formability [5,6]. Thus, refining grain and regulating texture have been the key to the development of high-performance wrought Mg alloys.

In the past ten years, some new plastic processing technologies and new alloy systems have been developed to weaken or change the texture of wrought Mg alloy profiles[7–14].In addition,twinning deformation is a popular method for tailoring texture of wrought Mg alloys due to its lowcost [15]. Twinning deformation is an important deformation mechanism in Mg and its alloys [16]. On the one hand, it can provide additional deformation modes to coordinate deformation. Twinning activation can also remarkably influence mechanical behaviors (i.e., anelastic behavior, yield behavior and strain hardening behavior etc.) of as-cast and wrought Mg alloys [17–19]. On the other hand, twinning deformation can also cause the changes in microstructure [19–23]. The influence of twinning deformation on microstructure can be summarized as follows: (i) grain refinement via subdivision of twin boundaries [23]; and (ii) changes in crystal lattice orientation [24,25]. Thus, pre-twinning has been considered as a new strategy to regulate the microstructure of Mg alloys[19]. It has been reported that twin boundaries can enhance the yield strength of Mg alloys. However, the hardening effect is limited owing to that the spacing of twin boundaries is usually in micrometer scale [26, 27]. In contrast, the change in orientation caused by twinning can greatly affect the mechanical properties of Mg alloys. Especially for wrought Mg alloys with strong texture, it is expected that pre-twinning can effectively improve mechanical anisotropy and formability by regulating texture [28–34].

Mg alloys contain profuse twinning modes which correspond to various misorientation angles between twin and matrix [35–38]. Thus, texture control via twinning deformation is dependent on active twin types. For most Mg alloys, the most commonly observed twin modes in Mg alloys are {10–12} extension twins and {10–11} contraction twins which are rotated by ∼86° and ∼56° from the matrix, respectively [39–42]. Among them, {10–12} twinning has a very low critical resolved shear stress (CRSS). Thus, extensive {10–12} twins can be easy introduced into Mg matrix and generate a strong twin-texture[43].It has also been confirmed that formation of the twin-texture can remarkably enhance stretch formability of rolled AZ31 sheets [44–46]. Moreover, the effect of twintexture is closely related to stability of the twin structure.Stability of initial twins determines the contribution of twintexture to plastic forming[47].This indicates that the research on the stability of twins is very important for the application of this method. It has been reported that plastic deformation may induce the growth or shrinkage of initial twins [48–50].Heat treatment may cause static recovery/recrystallization, resulting in the destruction of twin structure and the change of twin-texture [51–53]. Thermal deformation can also initiate the twin assisted dynamic recrystallization to eliminate the initial twin structure [54]. Therefore, both stress and heat treatment will affect the twin structure/texture. Based on the recent research trends,the stress stability and thermal stability of {10–12} twin structure in Mg alloys are mainly summarized and discussed in this review. Finally,some key scientific issues in the field of research are proposed.

2. Stress stability of {10–12} twin structure

Stress stability of prefabricated twins directly affects mechanical properties and formability of the Mg alloys with initial twins. According to the Schmid law, {10–12} twins can be introduced by compression perpendicular to the c-axis or tension along the c-axis [43]. Reloading along pre-twinning deformation direction will cause coarsening of prefabricated twins [31]. However, reverse loading activates detwinning, resulting in shrinking and even disappearance of prefabricated twins [55]. Obviously, the coarsening and shrinking of prefabricated twins will strengthen and weaken the intensity of twin-texture, respectively [56]. Indeed, both the coarsening and shrinking of twins are associated with the migration of the twin boundaries under stress [55,57]. Therefore, the stress stability of twin structure depends on the mobility of twin boundary during plastic deformation[58].Recent studies have shown that internal stress, structure of twin boundary and microstructure on both sides of twin boundary are all important factors affecting the mobility of twin boundary.

2.1. Influence of internal stress

It is well known that the activation of twinning or detwinning is related to the loading direction, so it is also affected by the internal stress at the twins. Wu et al. [56] found that after twinning deformation,a significant residual tensile stress remained in {10–12} twins due to the redistribution of load between soft orientation and hard orientation. This residual tensile stress is beneficial to the activation of detwinning.Therefore, the active stress of detwinning is usually lower than that of twin-growth. Park et al. [47] investigated the effect of stress relief annealing on detwinning in AZ31 alloy.The results showed that annealing treatment can effectively delay the activation of detwinning at the initial stage of deformation (see Fig. 1). Moreover, local internal stress at twins can also be changed by subsequent deformation due to the different deformability between twins and matrix. It may also affect the stress stability of twin structure. At present, the regulation of internal stress in the twinned Mg alloys has not been systematically studied [59]. For this reason, the influences of internal stress on the stress stability of twin structure are unclear.

2.2. Influence of crystal defects

Crystal defects (e.g. dislocations, grain boundaries, solid solution atoms and precipitates etc.) are usually used to strengthen metals [60–63]. Similarly, they can also increase the active stress of twin-growth and detwinning [64–66]. Dislocations will generate dislocation hardening effect on twinning and detwinning. In fact, twinning deformation can also generate profuse dislocations [43]. Cui et al. [65] found that the dislocations via twinning deformation can also impede twin boundary motion. As reported by Lv et al. [67], the contribution of dislocation hardening to yield strength increased with the increase of pre-twinning strain(see Fig.2).They also found that recovery annealing (100°C/48h) can reduce the active stress of twin-growth in AZ31 alloy. It infers that the increase of dislocation density can increase the active stress of twin-growth, thus reducing the mobility of twin boundary.Moreover, the type of dislocations in twins and matrix is different during twinning deformation [63]. However, the effect of dislocation configuration on twin-growth and detwinning has not been studied. Besides, dislocations generally appear as low-angle grain boundaries. It is also reported that misorientation of grain boundary can generate a large influence on the hardening effect of {10–12} twinning [21]. With increasing misorientation between two neighboring grains, the hardening effect tends to increase.

Fig. 1. In situ EBSD measurement results of rolled AZ31 plate (inverse pole figure map and corresponding (0002) pole figure): (a) the 5% pre-compressed material and the same material tensioned to the strains of (b) 2% and (c) 5%; (d) the 5% pre-compressed material annealed at 300°C for 60min, and the same material tensioned to the strains of (e) 2% and (f) 5% (Vf = twin volume fraction). [47].

For precipitation-hardenable Mg alloys, the precipitates will hinder the migration of twin boundary, thus increasing the active stress of twin-growth and detwinning [30,68].In addition, the precipitation hardening effect depends on the shape and distribution of precipitates [69]. For example,the basal-plate shaped precipitates in Mg–Al alloys have a higher strengthening effect on twin-growth than the c-axis rod-shaped precipitates in Mg–Zn alloys[70].It has also been reported that twins and matrix show different precipitation behaviors [22,30,71,72]. In the AZ80 alloy, the twins are almost occupied by fine continuous precipitates,while the coarse discontinuous precipitates are usually precipitated in the matrix[22]. In the ZK60 and Mg–Zn–Y alloys, the number density of precipitates in twins is higher than that in matrix [30,73].The distribution of precipitates may have different hardening effects on twin-growth and detwinning. Moreover, the precipitates usually tend to be formed at the twin boundary [26].

He et al. [53] found that the precipitation behavior of the two twin boundaries of each twin in AZ61 alloy was also different. The precipitation characteristics at twin boundaries may also affect the stress stability of twins.

2.3. Effect of solute segregation

Fig. 2. The quantified contributions of dislocations associated mechanisms(△σ1) and other mechanisms (△σ2) to work hardening. △σ1 and △σ2 are calculated as the follows: △σ1=YSB−YSA and △σ2=YS−YSA. YS, YSB and YSA are yield stresses of ED compression of the extruded rod, the ED recompression without annealing and the ED recompression after annealing,respectively. [67].

{10–12} twins have a specific orientation relationship with the matrix (86.3° <1–210>), and the twin boundary is usually a fully coherent boundary with low energy [74]. The change of the twin boundary structure is bound to affect the mobility of the twin boundary.Nie et al.[74]reported that solute atoms can be stably segregated on the deformation twin boundaries. They found that heat treatment guided the preferred segregation of Gd and Zn atoms on the deformation twin boundaries of Mg–Gd, Mg–Zn and Mg–Gd–Zn alloys,as shown in Fig. 3. Xin et al. [75] also found that recovery annealing also caused the segregation of Al and Zn atoms on the {10–12} twin boundary of AZ31 alloy. Drozdenko et al.[76] found that annealing at 200°C/8h resulted in the segregation of Zn atoms at the twin boundary of extruded Mg–1Zn alloy, which generated a pinning effect on the migration of twin boundary. Nie et al. [74] reported that the segregation of Gd atoms at the {10–12} twin boundary in Mg–Gd alloy increases the active stress of twin-growth. Cui et al. [55] found that the segregation of Al and Zn atoms at the twin boundary can destroy the synchronous movement of atoms in the twin boundary during the twinning process, thus reducing the mobility of twin boundaries. Therefore, the ordered segregation of solute atoms at {10–12} twin boundary can effectively pin the twin boundary. Previous results have revealed that Al, Zn,and Re elements can generate the pinning effect [55,74–76].Whether other solute elements have similar effects requires more research to reveal. In addition, it is necessary to investigate the effect of various solute elements on the tendency of solute segregation at twin boundaries and their pinning effects on twin boundaries.

2.4. Influence of twin-dislocation interaction

The interaction between dislocation and {10–12} twin boundary can also change the structure of twin boundary, and even change the orientation relationship between matrix and twin [77,78]. The atomic simulation results revealed that the lattice dislocations caused by dislocation slip have a complex reaction at the twin boundary. Wang et al. [79] found that the interaction between basal 〈a〉dislocation and coherent twin boundary will generate twin dislocations and residual defects at the coherent twin boundary. Kadiri et al. [80] found the interaction between dislocation and twin boundary can generate a basal/prismatic(BP/PB)interface at{10–12}twin boundary.Recently, Zhang et al. [81] systematically investigated the effect of basal 〈a〉dislocation on {10–12} twin boundary. They found that the BP/PB interface was created by the interaction between the basal 〈a〉dislocation and {10–12} twin boundary, as shown in Fig. 4. This is the reason why the actual twin boundary deviates from the theoretical twin boundary.Wang et al. [82] pointed out that the BP/PB interface may have a dragging effect on movement of twin boundary. Xin et al. [83] found that the interaction between {10–12} twin boundary and dislocation can greatly enhance the active stress of detwinning, thus hindering the migration of twin boundary during reloading, as shown in Fig. 5. Moreover, the twin boundary structure may also be destroyed completely by large plastic strain or thermal deformation, thus avoiding the occurrence of twin-growth and detwinning [77,84].

Fig. 3. Periodic segregation of solutes in twin boundaries. HAADF-STEM images showing {10–12} twin boundaries in (A and B) Mg-1.9 atomic%Zn and(C and D) Mg–1.0 atomic%Gd-0.4 atomic%Zn-0.2 atomic%Zr alloys. (E) and (F) are schematic illustrations of (B) and (D). [74].

Fig. 4. High-resolution transmission electron microscopy images of samples with incident beam parallel to the 〈1210〉axis: (a) PS-1; (b) PS-2 and (c) PS-3.The {10–12} coherent twin boundaries and BP/PB boundaries are colored in red and yellow, respectively. In Fig. 4(d), the yellow rectangle stands for the average number of BP/PB boundaries in each sample. PS-1 is the sample compressed along the RD to a strain of 5.5%. Three PS-1 samples were further compressed along 45° of RD and ND to a strain of 8.5% and 12.5%; the resultant samples were designated as PS-2 and PS-3, respectively. [81].

3. Thermal stability of {10–12} twin structure

Generally, {10–12} twins can exist stably at higher temperature due to the low strain accumulation within them [85].Moreover, Nie et al. [74] reported that the solute atoms can be rapidly segregated at the grain boundary during heat treatment. The segregation of the solute atoms can also reduce the thermal mobility of twin boundary, thereby improving the thermal stability of twin structure. Recently, the annealing behavior of {10–12} twins in AZ31 alloys has been widely studied. Zhang et al. [86] found that {10–12} twins exhibit less thermal mobility at 250°C. Levinson et al. [87] found that about 67%of{10–12}twins can be retained after annealing at 275°C/16h. Li et al. [88] reported that some {10–12}twins could even be retained after annealing at 350°C/11h.Thus, the {10–12} twin structure can be retained in a larger temperature range. The thermal mobility of the {10–12} twin boundary greatly depends on the annealing temperature and time. Almost all {10–12} twins can be removed when the temperature reaches 375°C [86].

For the deformation dominated by {10–12} twinning (e.g.compression along the thickness direction of rolled plates and compression along the extrusion direction of the extruded rods, etc.), the contribution of slip to the plastic strain is very limited, resulting in low dislocation storage in the matrix.Therefore, the high thermal stability of {10–12} twin structure is related to the limited activation of slip during twinning deformation[85].Zhang et al.[86]also investigated the effect of recovery annealing on the thermal stability of prefabricated{10–12} twins. They found that annealing at 200°C/6h is ineffective to improve the thermal stability of {10–12} twins.It also indicates that the effect of dislocations via twinning deformation on the thermal stability of twins is very limited.Levinson et al. [87] thought that the thermal mobility of {10–12} twins at high temperature is related to the destruction of coherent twin boundary caused by slip. The generation of profuse dislocations can not only increase the storage energy,but could also destroy the twin boundary structure. So, preinducing dislocations can reduce the thermal stability of {10–12} twins. Xin et al. [85] found that for the twinned AZ31 alloy with high dislocation density, the twin structure of {10–12} disappears completely after annealing at 250°C/1h, as shown in Fig. 6.

Fig. 5. (a) A schematic diagram showing the pre-straining to generate interaction of {10–12} twin boundaries with 〈a〉dislocation and the preparation of specimen for ND reloading and (b) stress-strain curves under compression along the ND. Here, RD, TD and ND represent rolling direction, transverse direction and normal direction of the initial plate. [83].

Fig. 6. The inverse pole figure maps and boundaries misorientation maps of various samples: (a) PS-1 sample, (b) PS-1 sample annealed at 250 °C for 1h,(c) PS-2 sample and (d) PS-2 sample annealed at 250 °C for 1h. The PS-1 sample was subjected to a 2.8% pre-compression along the TD, while the PS-2 sample experienced a 5.5% pre-compression along the ND and a subsequent 2.8% re-compression along the TD. [85].

Fig. 7. Calculated slip/twinning activities under various loading conditions. (a) Uniaxial compression along TD of a rolled AZ31 plate [89], (b) in-plane shear of a rolled AZ31 sheet [90], (c) free-end torsion of a rolled AZ31 plate [92] and (d) uniaxial compression along RD of a rolled Mg–Y–Nd plate [105]. The Y-axis is simulated relative activity of various deformation modes.

During pre-inducing {10–12} twins, the relative contribution of twinning/slip to plastic strain may also affect the thermal stability of twin structures. Fig. 7(a) shows that compression along the TD of the rolled AZ31 sheet is dominated by {10–12} twinning [89]. However, for some methods of pre-inducing twins, dislocation slip could have a large contribute to the plastic strain. These methods include in-plane shear in rolled AZ31 plate (Fig. 7(b)) [90], free-end torsion deformation in extrusion rod (Fig. 7(c)) [91–93], additional friction and traction force during pre-inducing twins [28,94]etc. These deformation methods will also introduce a lot of dislocations while generating profuse {10–12} twins, which will lead to the decrease in thermal stability of prefabricated twins. Moreover, the changes of microstructure can also affect the relative activity of twinning/slip during plastic deformation. For example, with the decrease of grain size, the contribution of slip to plastic strain will increase [95–97].Zhang et al. [86] found that refining the initial grain size from 32μm to 15μm did not significantly reduce the thermal stability of {10–12} twins. However, it is still expected that further refinement of the initial grain size will reduce the thermal stability of the prefabricated twins. Yin et al. [98] found that when the grain size of AZ31 Mg alloy was refined to 0.8μm, {10–12} twinning can be greatly suppressed during plastic deformation.Moreover,precipitates will strengthen the different deformation modes to different extents, which will affect the relative activity of twinning and slip during preinducing twins [70]. Solid solution elements may also affect the balance of deformation modes [99–101]. For example, in Mg–Al and Mg–Zn alloys, the active stress of {10–12} twinning is close to that of basal slip, and is far lower than that of non-basal slip [102–104]. However, in Mg–Y–Nd alloy,the active stress of {10–12} twinning is far higher than that of basal slip, which makes the contribution of twinning to plastic strain very low [105] (Fig. 7(d)). Thus, the control of initial microstructure can also affect the storage energy during pre-inducing twins, and then affect the thermal stability of prefabricated twins.

Recrystallization annealing can eliminate the{10–12}twin structure. If the twin orientation can be retained after recrystallization annealing, then it can be considered that twintexture have good thermal stability [106]. Moreover, the elimination of twin boundaries can also improve the stress stability of twin-texture because the occurrence of twin-growth and detwinning is avoided. Xin et al. [52] reported that the static recrystallization behavior of twinned AZ31 alloy has a strong twin-size effect (see Fig. 8). During recrystallization annealing (450°C/4h), the narrow twin lamellae will be consumed by large adjacent matrix, and vice versa. Thus, after recrystallizion annealing, the twin-texture may be enhanced or weakened. Moreover, in some cases, twin-texture also has good thermal stability.When the matrix and twins have a close size,both matrix-orientation and twin-orientation can be retained after annealing. Prefabricated dislocations may also enhance the thermal stability of twin-texture. Xin et al. [85] found that the orientation of small-size twins in AZ31 alloy with high dislocation density can be retained after recrystallization annealing. Moreover, high-density dislocations may also produce a new recrystallization texture in the twinned Mg alloy. Song et al. [94] fabricates an AZ31 Mg alloy sheet with high density of dislocations and profuse {10–12} twins(∼49% area fraction) by continuous bending rolling. The results showed that after annealing, the twin-texture was eliminated and a double-peak texture was formed, as shown in Fig. 9. Cheng et al. [44] also observed a similar phenomenon.This indicates that initial dislocations have a significant effect on the recrystallizing behavior and recrystallizing texture of twinned AZ31 alloys. In addition to initial dislocations, solute elements and precipitates may also influence the static recrystallization behavior and recrystallization texture [107–110]. However, for the Mg alloys with initial twins, there is still very little research in this area.

Fig. 8. Inverse pole figure maps of the extruded rods with different conditions (a) compressed along ED to 2%,(c) compressed along ED to 4%, (d) compressed along ED to 5%, (d) compressed along ED to 2% annealed,(e) compressed along ED to 4% and annealed, and (f) compressed along ED to 5% and annealed.[52].

Fig. 9. EBSD maps and {0001} pole figure of twinned AZ31 sheets by a continuous bending channel in a rolling device: (a) before annealing and (b) after annealing at 350°C. [94].

4. Stability of other types of twins

In addition to {10–12} twins, Mg alloys also contain other twin modes, e.g. {10–11} twins (56.2°<11–20>), {10–13}twins (64.0°<11–20>) and {11–21} twins (34.2°<10–10>)[35–38]. Among these, {10–11} twin is the most common twin mode. Moreover, {10–12} twin can also be formed inside primary {10–11} twin and primary {10–12} twin, resulting in formation of the {10–11}-{10–12} twin and the {10–12}-{10–12} twin, respectively [49,111–113]. In this section,the stability of these twins is also simply discussed.

4.1. Stability of {10–12}-{10–12} twins

Initial{10–12}twins may also be re-twinned during subsequent deformation.Under specific deformation conditions,the prefabricated {10–12} twins will produce secondary or highorder twins (e.g. {10–12}-{10–12} secondary twins and {10–12}-{10–12}-{10–12} tertiary twins, etc.) [114–116]. Retwinning of prefabricated twins will inevitably change the orientation distribution of primary twin-texture. Similar to primary {10–12} twins, secondary {10–12} twins in primary{10–12} twin can also grow or shrink under further deformation [116]. It is considered that the factors affecting the stress stability of secondary {10–12} twins are similar to those that affecting primary {10–12} twins. Moreover, it has also been found that {10–12}-{10–12} secondary twins can serve as a barrier to impede detwinning of this primary {10–12} twin and enhance the active stress of the detwinning [114].

As illustrated above, primary {10–12} twins usually have a good thermal stability. Primary {10–12} twins did not generate new recrystallized grains during the whole annealing process [117]. However, new grains can be nucleated preferentially at the intersections of{10–12}twin variants[88,117].Clearly, multiple {10–12} twin variants and high-order {10–12} twins increase the intersections among twins. It inferred that secondary {10–12} twins may reduce the thermal stability of twins. Li et al. [88] found that the new grains at the intersections of {10–12} twins have a close orientation with initial {10–12} twins. However, those at the intersections between {10–12} twins and {10–11} twins are relatively random. Thus, the thermal stability of {10–12}-{10–12} twintexture is very complex. More systematical work is required in the further work.

4.2. Stability of {10–11} twins

Unlike {10–12} twinning, {10–11} twinning requires a high active stress and accommodate compression strain along the c-axis [118]. Thus, high stress localization and high store energy usually exists in the matrix with{10–11}twins,which will make the Mg alloys brittle [119]. Moreover, a secondary{10–12} twin usually nucleates inside {10–11} twin and could grow to fill the entire{10–11}twin during pre-inducing{10–11} twins [120]. In the previous studies [24,121], it showed that more than 90% of {10–11} twins were completely twinned by {10–12} twinning in a cold rolled AZ31 sheet. Thus, texture control via {10–11} twins is essentially dependent on orientation of {10–11}-{10–12} twins. There are four types of {10–11}-{10–12} twins (D-twin), which are referred to as D-twin30, D-twin38, D-twin66, and D-twin69,respectively according to the misorientation angle between the D-twin and parent grain. Clearly, the occurrence of the various types of D-twins can rotate grain lattice by different angles. The D-twin38 and D-twin30 dominate the {10–11}-{10–12} twins [24,121]. However, the interface of {10–11}twin (or {10–11}-{10–12} twin) has a far higher stress stable than that of {10–12} twins [122,123]. Thus, {10–11} twin(or {10–11}-{10–12} twin) remains a smaller width and low volume fraction, and thus hardly influences the macro-texture[24]. Compared with {10–12} twins, they have greater potential for producing high density of twin boundaries. Recently,Fu et al. [122] fabricated densely hierarchical {10–11}-{10–11} double contraction nanotwins in a duplex Mg–Li alloy,which generated a high refinement hardening effect.

{10–11}-{10–12} twins have a very low thermal stability and are the favorite nucleation sites for static recrystallization gains [24,111,117]. A low temperature (100°C) can induce profuse static recrystallized grains inside twins [111]. It has been found that the recrystallized grains do not inherit the orientation of the {10–11}-{10–12} twins, but have a large scatter from the twin-orientation[24,111].With increasing annealing temperature, recrystallized grains grow. However, the growth of new grains from twins into the matrix is difficult[111]. Finally, the new grains in a twin lamella are consumed by each other. Guan et al. [117] indicates that new grains inside twins have little influence on recrystallized texture. They also found that twin–twin and twin–grain boundary intersections can induce the recrystallized grains which can readily grow into the deformed parent grains. The new grains from twin–twin and twin–grain boundary intersections exhibited a weak non-basal texture and have a large contribution for the weakening of texture of AZ31 sheet. According to a previous report [24], complete static recrystallization in the rolled AZ31 sheets with {10–11}-{10–12} twins only weakened basal texture, while did not produce new macro-texture component.

For the contraction twins, there is less work focusing on their stability. {10–11} twinning and {10–12} twinning have very different characteristics, including active stress, accommodated strain and reorientation angle [35–38]. Thus,internal stress evolution during twinning and the influence of internal stress on mobility of twin boundaries may be very different between them. In addition, {10–11} twinning have a very high CRSS and therefore contribute little to plastic strain[24].Profuse dislocations have usually formed in the matrix before{10–11} twinning and occurs strong twin-dislocation interaction during growth of {10–11} twins [124]. Thus, {10–11}twins exhibit higher stress stability than{10–12}twins.Moreover,{10–11} twins and{10–11}-{10–12} twins cause lattice to rotate to a soft orientation for basal slip[125].This slip activity will release local stress, which is also not conducive to twin growth.For{10–12}twins,detwinning can occur widely and greatly affect elastic/plastic behavior [55,126]. However,detwinning behavior of {10–11} twins has not been observed.In addition, the influences of crystal defects on the stability of {10–11} twins have not been systematically examined.

5. Conclusion and outlooks

Recently, pre-inducing {10–12} twins plays an important role in the texture control of wrought Mg alloys. The stress stability and thermal stability of twin structure determine the application potential of twinned Mg alloy. In this paper, the stress stability and thermal stability of {10–12} twin structure in AZ31 Mg alloy are summarized and reviewed.

The stress stability of twin-texture mainly depends on the mobility of twin boundary. The residual tensile stress introduced during pre-twinning deformation within the twin is beneficial to the activation of the detwinning. Both solute segregation in twin boundary and precipitates caused by heat treatment can hinder the migration of the twin boundary, thus improving the stress stability of twin-texture. The dislocations and the dislocation-twin interaction can also hinder migration of twin boundary. Therefore, the improvement in stress stability of twin structure can be realized by regulating internal stress, structure of twin boundary and microstructure near twin boundaries.

{10–12} twin structure has good thermal stability in a large temperature range. However, the twin boundary migration caused by thermal activation at high temperature can lead to the enhancement or weakening of twin-texture. Moreover,the thermal stability of {10–12} twin-texture also depends on the static recrystallization texture. The static recrystallization texture of twinned Mg alloys is related to twin-size and dislocation storage. Increasing the dislocation storage can reduce the thermal stability of {10–12} twin structure, but could increase the thermal stability of twin-texture. Moreover, special recrystallization texture may also be produced in twinned Mg alloys with high density of dislocations.

According to above, the research on the stability of {10–12} twin structure/texture has obtained some results. However, the influencing factors and control methods of the stability have not been systematically investigated. Some basic scientific problems still need to be revealed by more research work, e.g. the regulation of internal stress in twinned Mg alloy and its effects on the mobility of twin boundary; the effect of dislocation configuration and precipitates on both sides of twin boundaries on twin-growth and detwinning; the effect of solute element type on solute segregation strengthening; the microscopic mechanism of static recrystallization behavior of twinned Mg alloys etc. Moreover, the effects of twin structure/texture stability on the mechanical properties and formability of Mg alloys needs to be systematically evaluated.

Declaration of Competing Interest

The authors declare no conflicts of interest.

CRediT authorship contribution statement

Tingting Liu:Writing - original draft, Writing - review &editing, Supervision, Funding acquisition.Qingshan Yang:Writing - review & editing.Ning Guo:Writing - review &editing.Yun Lu:Writing - review & editing.Bo Song:Conceptualization, Writing - original draft, Writing - review &editing, Supervision, Project administration, Funding acquisition.

Funding

This project was financially supported by the National Science Foundation of Chongqing (Project no.cstc2018jcyjAX0070), Fundamental Research Funds for the Central Universities (Project No. XDJK2019B003) and the National Natural Science Foundation of China (Project no.51601154). The authors are also very grateful to Yanan Chen and Zhiwen Du for checking the language.