Texture control by {10-12} twinning to improve the formability of Mg alloys: A review

1. Introduction

Wrought Mg alloys generally show remarkable plastic anisotropy due to the strong texture formed during plastic processing. Moreover, strong texture will also deteriorate the secondary formability of wrought Mg alloys [1,2]. A typical example is that Mg and its alloys usually develop strong basal texture during rolling, resulting in poor plastic formability (e.g. rollability and stretch formability) [1]. Thus, texture control is still a key issue for the development and application of rolled Mg alloy sheets/plates. In the past decades, some new plastic processing techniques (e.g. asymmetry rolling/extrusion) have been developed to overcome this issue [[1], [2], [3], [4]]. Moreover, addition of some alloying elements (e.g. rare-earth elements) can also weaken or change deformation texture [[5], [6], [7], [8]]. However, these methods require special processing equipment or expensive rare earth elements, which could be an obstacle to industrial applications.

{10-12} <10-11> twinning is an important deformation mechanism in most Mg alloys. It can cause the $\widetilde{8}$6.3° lattice rotation and generate a new texture component (i.e. twin-texture) [9]. Thus, twinning deformation can be a feasible way to regulate texture components of the wrought Mg alloys. Mg-Zn and Mg-Al series alloys are the most common commercial Mg alloys. In these alloys, {10-12} twinning can be easily activated and even become the dominant deformation mechanism under certain deformation conditions [[10], [11], [12], [13]]. It provides an opportunity to introduce profuse {10-12} twins into the wrought Mg alloys. Recently, the deformation characteristics and texture evolution during {10-12} twinning deformation have been studied extensively [11,[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. Pre-inducing {10-12} twinning deformation has also been utilized to enhance plastic formability of rolled Mg alloy sheets/plates [9,[24], [25], [26], [27], [28], [29]]. Thus, {10-12} twinning deformation has been considered as an effective and low-cost method to adjust the texture of rolled Mg alloys. In the present review, the latest research results of texture control via {10-12} twinning deformation, such as control in twin-orientation and processing techniques of pre-inducing twins, were summarized. The research progress in improving stretch formability of rolled AZ31 alloys by pre-twinning deformation was also reviewed. Finally, further research directions on this field were proposed.

2. Texture control via {10-12} twinning deformation

2.1. Control in twin-orientation

For Mg alloys, {10-12} extension twinning can arouse crystallographic lattice rotation of $\widetilde{8}$6.3° and generate a new orientation (i.e. twin-orientation). Generally, {10-12} twinning has six equivalent twin variants (see Table 1) [9]. Clearly, twin-orientation is related to selection of the twin variants during twinning deformation. It has been reported that selection of {10-12} twin variants is generally controlled by the Schmid factor (SF) law [11]. Thus, understanding the changes in SF under various loading conditions facilitates the twin-orientation design. The influence of uniaxial loading on selection of twin variants has been systematically studies by Hong et al. [11] and Song et al. [30], as shown in Fig. 1. The compression perpendicular to the c-axis and the tension parallel to the c-axis have the largest SF (0.5) for {10-12} twinning, but they exhibit different variants selection during twinning deformation. Tension parallel to the c-axis is favorable to activate all the six twin variants, while compression perpendicular to the c-axis can only activate one twin variant or a twin variant pair with the highest SF within one grain. Clearly, these two situations show distinctly different texture evolutions during {10-12} twinning, as shown in Fig. 2 [11]. With the change in the angle between loading axis and the c-axis of texture, SF of various twin variants exhibits different change, as shown in Fig. 1. For example, as the angle between the c-axis of texture and the tensile axis increases, the SF values of the six twin variants tend to decrease and exhibit different decelerations. In other tensile orientation, only one twin variant or a pair of twin variants could be favorable activated. When the angle between c-axis and the loading axis is fixed, the number of active twin variants can be also influenced by the distribution of the a-axis. For example, compression along <10-10> orientation can only advantageously activate one twin variant or a twin variant pair with a misorientation of $\widetilde{7}$.4°, while compression along <11-20> orientation can favorably activate multiple twin variants with a misorientation angle of $\widetilde{6}$0° [11,31].

Table 1

Table 1   {10-12} twin variants in Mg alloys [9].

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Fig. 1.   SF of {10-12} twinning system as a function of the angle between loading axis and c-axis. (a) Tension [11] and (b) compression [30]. Information for six twin variants is given in the insert.

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Fig. 2.   Optical microstructures and (0002) pole figures with (a) compression strain along rolling direction (RD) (i.e. compression perpendicular to c-axis of texture) and (b) tension strain along the normal direction (ND) (i.e. tension along c-axis of texture). The twinned regions are indicated as a bright color in (a) and a dark color in (b). TD means transverse direction [11].

Rolled Mg alloys usually contain a single texture component (i.e. strong basal texture with c-axis//ND), as shown in Fig. 3(a) [32]. Such single texture offers a convenient condition to selectively activate twin variants by changing the strain path in rolled Mg alloys. Tension or compression along different directions of rolled plates can generate various twin-orientations, as shown in Fig. 1. However, in-plane loading could be a more realistic approach to induce {10-12} twins in rolled Mg alloy sheets/plates. For rolled Mg alloys with basal texture, {10-12} twinning is suppressed during in-plane tension, while in-plane compression strain can favorably activate {10-12} twinning. Generally, the c-axes of twins concentrate to the compression direction, according to Fig. 1, Fig. 2. It is reported that the compression along TD (or RD) can generate {10-12} twins and result in formation of a twin-texture with c-axis//TD (or RD) in rolled Mg alloy sheets [[32], [33], [34]], as shown in Fig. 3(a) and (b). Multi-axial compression or uniaxial multi-path compression can also be used to tailor activation of twin variants [21,[35], [36], [37]]. When rolled Mg alloy plate is subjected to repeated compression along RD and TD, the sample will generate twin-texture in both directions (c-axis//TD and c-axis//RD texture components), except for the initial c-axis//ND texture [32] (see Fig. 3(c)). Moreover, as shown in Fig. 1, Fig. 2, tension along c-axis can generate a more spread twin-orientation than compression. If the c-axis of texture can be adjusted to TD or RD direction, tension along TD or RD could generate more spread texture components. This idea can also be achieved by multi-step twinning deformation.

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Fig. 3.   (0001) pole figure of (a) as-rolled AZ31 plate, (b) after 3% compression along the TD, (c) after 3% compression along the TD and then 3% compression along the RD. The blue arrows and red arrows indicate the twin-orientations with c-axis//RD and c-axis//TD, respectively [32].

In addition, change in the initial orientation of the rolled Mg alloy sheets can also be employed to tailor active twin variants during in-plane loading according to the SF law. Song et al. [31] has tailored a-axis distribution of the basal texture by a tension strain of 5% along RD. It revealed that the control of <11-20> orientation concentrating towards TD increases activation of multiple twin variants during compression along TD. Moreover, asymmetry rolling/extrusion processing can generate tilt basal texture or non-basal texture [4,38,39] and addition of some alloying elements (e.g. rare-earth elements [5] and Li element [40,41] etc.) could change the rolling texture of Mg alloys. It will also modify twin-orientation evolution during in-plane tension or compression. Recently, He et al. [41] has obtained a c-axis//TD texture in the hot-extruded sheet of a Mg-3Al-Zn alloy with 3 wt.% Li addition. For this alloy, tension along TD is favorable to {10-12} twinning deformation, as shown in Fig. 4.

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Fig. 4.   (0002) pole figures and misorientation angle maps of the various pre-stretched samples: (a) 2.3% pre-stretched; (b) 5.4% pre-stretched and (c) 8% pre-stretched along the TD of the hot-extruded sheet (Mg-3Al-Zn alloy with 3 wt.% Li addition) [41].

In general, the SF law controls the variant selection of {10-12} twinning. Thus, it can be utilized to design twin-orientation, as reviewed above. However, in some situations, the selection of twin variants could also escape the SF law. For example, localized inhomogeneous straining or coordination of strain compatibility could activate non-Schmid twin variants [[42], [43], [44], [45], [46], [47]]. Volume fraction of non-Schmid twinning is quite small, but it usually generates an unexpected twin-orientation. How to use and even enlarge the role of non-Schmid twins on texture control and mechanical properties could be very interesting in the further work. Recently, Lee et al. [14] reported that a high volume fraction of non-Schmid twinning behavior. They found that initial twins can influence subsequent twinning process, as shown in Fig. 5. After pre-inducing {10-12} twins with c-axis//RD, tension along ND promotes growth of the pre-existing twins, rather than nucleation of new twins. The non-Schmid twinning behavior generates a distinct twin-texture evolution.

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Fig. 5.   In-situ EBSD measurement results (inverse pole figure map, and corresponding (0001) and (10-10) pole figures): the rolled AZ31 plate (a) compressed to a strain of 2% along the RD and subsequently tensioned to strains of (b) 3% and (c) 5% along the ND (V_f: twin volume fraction) [14].

2.2. Control in proportion of twin-orientation

Generally, twinned Mg alloy plate contains a basal texture component (initial orientation) and a twin-texture component. The proportion of twin-orientation (i.e. volume fraction of {10-12} twins) increases with strain [11,48], as shown in Fig. 2. The strain accommodated by the {10-12} twinning can be estimated by the following formula [49]: ε_twin=f_twin×m×γ_twin, where f_twin is the twin volume fraction, m is the average Schmid factor for the {10-12} twinning, and γ_twin is the twinning characteristic shear ($\widetilde{0}$.13 for {10-12} twinning). Obviously, maximum plastic strain that can be coordinated by the {10-12} twinning is 6.5%. However, the {10-12} twinning can nucleate and grow over a wider range of strains, as the deformation also involves dislocation slipping [11], as shown in Fig. 6. The contribution of {10-12} twinning to plastic strain determines the amount of twin-orientation at a specific strain, and also influences evolution of dislocation density in the twinned microstructure through dislocation slip.

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Fig. 6.   Twinned volume fraction with strain along the RD and ND of a rolled AZ31 plate. The value at each condition is an average measured over five different micrographs [11].

Recently, a large number of works have quantitatively examined the plastic deformation mechanism of Mg alloys by experiment and simulation [[50], [51], [52], [53], [54], [55], [56]]. The results show that the contribution of {10-12} twinning on plastic strain can be influenced by initial microstructure (e.g. initial texture, grain size, solute atom, and precipitates etc.). Initial texture will influence the SFs of various deformation modes and further affect their active stress, as shown in Fig. 1, Fig. 7. That is, initial orientation not only affects the selection of twin variants, but also the relative activities of twinning and slipping, as shown in Fig. 6, Fig. 8 [52]. In addition, grain size, solute atom and precipitates can alter the critical resolved shear stresses (CRSS) of various deformation modes, and the changes in the CRSS of twinning and slipping are also different [12,[57], [58], [59], [60], [61], [62], [63], [64], [65]]. Thus, they can also change the contribution of {10-12} twinning on strain. Robson et al. [12] has predicted that precipitates can generate different strengthening responses on slipping and {10-12} twinning, which is dependent on shape and habit of the precipitates. The hardening effect of the c-axis rods on prismatic slip is higher than that on twin growth, whereas basal plates exhibit the opposite effect, as shown in Fig.9(a). Thus, the basal plate shaped precipitates can reduce the contribution of {10-12} twinning on plastic strain in Mg alloys. Barnett et al. [65] has revealed that with the decrease of grain size, both CRSS of slip and {10-12} twinning increase via the Hall-Petch hardening. However, the Hall-Petch slope (k value) for the {10-12} twinning is higher than for slip, as shown in Fig. 9(b). It suggests that the contribution of {10-12} twinning on strain tends to reduce with grain refinement. In addition, solute atoms can generate solid solute strengthening or softening effect. For Mg-Zn and Mg-Al alloys, the CRSS of {10-12} twinning is still far lower than non-basal slip. It has been reported that solute concentrations of rare-earth elements can largely harden twinning resistance [66]. As a result, the twin activity in Mg-Re alloys is lower compared to the conventional Mg-Zn and Mg-Al alloys [59,67].

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Fig. 7.   Stresses required for activating basal and prismatic slips, {10-12} twinning under tension (a) and compression (b) in an AZ31 alloy [50].

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Fig. 8.   Calculated slip/twinning activities under uniaxial compression along various angle from ND to TD of a rolled AZ31 plate. (a) 0° (ND), (b) 30°, (c) 60° and (d) 90° (TD). The Y-axis is simulated relative activity of various deformation modes [52].

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Fig. 9.   (a) A calculation of the ratio of the increase in CRSS for {10-12} twin growth compared to the increase in CRSS for prismatic slip for plate, rod and spherical particles. Particle volume fraction = 5%, aspect ratio of plates = 0.1, aspect ratio of rods = 10. [12] (b) Hall-Petch plots showing the influence of grain size on σ_0.002 and σ_0.2 for 150 °C [65].

Moreover, deformation conditions (stress state, strain rate and deformation temperature, etc.) can also affect the contribution of {10-12} twinning to plastic strain. In fact, the effect of stress state is similar to the influence of initial orientation. Strain rate and deformation temperature also generate a large impact on the twinning process. Generally, a higher strain rate enhances twin activation [68]. Recently, Zhu et al. [45] has found that a high strain rate can also increase the trend of non-Schmid twining behavior. With increasing deformation temperature, the CRSS of basal slip and {10-12} twinning change little, while those of non-basal slip are remarkably reduced by thermal activity [69]. Moreover, hot deformation can arouse other microstructure changes due to dynamic recovery, recrystallization or precipitation [[70], [71], [72], [73], [74]]. Recently, Huo et al [75] also found that twinning appears to be inhibited at liquid nitrogen deformation temperature. These conclusions can also be used to design the relative proportion of {10-12} twins and dislocation features (density, type and distribution etc.) under specific strains. It is also necessary to investigate the influences of twin-precipitate interactions and twin-dislocation interactions on texture control. It should be stressed that the dislocations via pre-strain could also generate enormous influence on texture evolution during subsequent thermal treatment, as reviewed in Sect. 3.

3. Twin-orientation control during annealing

For twinning-dominated deformation (e.g. compression along RD/TD of rolled plate or along ED of extruded rod at a low strain), dislocation accumulation in the matrix and twins is very low due to limited active slip. Thus, the {10-12} twins have a good thermal stability in twinned Mg alloys [[76], [77], [78]]. Zhang et al. [79] found that {10-12} twin structure can be well remained at 250 °C in twinned AZ31 alloys. Li et al. [80] also pointed out that some {10-12} twins can even be retained after annealing at 350 °C for 11 h. It indicates that twin-texture is stable over a wide temperature range. Even so, thermally activated twin boundary migration may also take place extensively under certain conditions [81]. It is found that almost all the {10-12} twins can be removed when the annealing temperature is over 375 °C [79]. In addition, the recrystallization temperature of twinned Mg alloys decreases with increasing dislocation accumulation [50,76].

Once static recrystallization occurs, texture components could be altered owing to the formation of recrystallization texture [82]. It provides an opportunity to further design texture by combining twinning with subsequent annealing. Recently, Xin et al. [83] reported that initial twin size can influence recrytallization texture of twinned AZ31 rod (pre-compressed along ED of extruded rod), as shown in Fig. 10. It is found that twin-orientation can be enhanced or weakened by recrystallization annealing, which is dependent on the relative size between twin lamellae and neighboring matrix, as shown in Fig. 10(b) and (d). When matrix is thicker than twin lamellae, the matrix prefer to consume the twin, and vice versa. Moreover, when matrix and twins have a similar size, both twin-orientation and matrix-orientation can be remained, as shown in Fig. 10(c).

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Fig. 10.   Pole figures (measured by XRD) before and after annealing at 450 °C for 4 h for samples 0%, 2% 4% and 5% pre-compressed along the ED of an extruded AZ31 rod. RD-radial direction, and TD-tangential direction [83].

The thermal stability of {10-12} twins at high temperature may be related to the accumulation of dislocations due to slip deformation [81]. Even if twinning dominates the deformation, there is also an initiation of dislocation slips [11]. Enhancement of dislocation slip could destroy the twin boundaries and increase the storage energy in matrix and twins [50,84]. These could change the static recrystallization behavior of the rolled plate with {10-12} twins. Xin et al. [76] reported that the thermal stability of {10-12} twins can be remarkably reduced by increasing storage energy, and the twin-orientation from the {10-12} twins with small size can be remained after recrystallization annealing in a pre-deformation AZ31 alloy (5.5% pre-compression along the ND and a subsequent 2.8% re-compression along the TD of rolled plate). Song et al. [85] has fabricated a twinned AZ31 sheet with a twin area fraction of $\widetilde{4}$9% by a continuous bending channel rolling, as shown in Fig. 11(a). It is found that recrystallization annealing eliminates the twin-texture (c-axis//RD texture) and generates a double-peak texture, as shown in Fig. 11(b). Cheng et al. [24] also observed a similar phenomenon. In these twinned samples, a common feature is that matrix or twins contain mass of dislocations [24,85]. It indicates that the dislocations can significantly affect recrystallization behavior of twinned AZ31 alloys. According to Sect. 2.2, both initial microstructure and deformation conditions can influence dislocation evolution during pre-inducing {10-12} twins. It is expected that the initial dislocations in the matrix and twins can also be regulated. In the further work, research on influence of dislocation features on static recrystallization of twinned Mg alloys could become an important topic. The abnormal recrystallization texture of twinned AZ31 alloys reported in Ref. [24,85] can also be interpreted by this study.

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Fig. 11.   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 [85].

4. Processing technologies of {10-12} twinning on rolled Mg plates/sheets

It has showed that the compression strain of rolled Mg plate in the thickness direction can be an effective method of inducing {10-12} twins in rolled Mg alloy. To achieve a continuous production of the twinned plate, twinning deformation can also be induced into the rolling processing. For example, vertical rolling (i.e. side-rolling or transverse rolling, as shown in Fig. 12(a)) along TD can induce twinning deformation [29,86,87]. For bulk Mg alloys with large size, cold forging is suitable to induce {10-12} twinning deformation, as shown in Fig.12(b) [[88], [89], [90]].

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Fig. 12.   The schematic diagram of deformation of (a) vertical rolling and (b) vertical compression [86,87].

Another important issue is that the compression in the thickness direction is not suitable for thin sheet due to bending buckling. Recently some methods have been designed to induce twinning deformation in thin Mg alloy sheets [[24], [25], [26],91], as shown in Fig. 13. Cheng et al. [24] and Kim et al. [26] have developed special dies for in-plane compression of thin sheets to avoid the instability, as shown in Fig. 13(a) and (b). Besides, an additional friction force between sheet and die may also affect the microstructure evolution during compression. To reduce such influence, the clamping force just ensures that the sheet does not bend during compression in their studies. He et al. [25] used the 0.4 mm Teflon films placed between Mg sheet and the die to accommodate the strain of Mg sheet along the ND and reduce the effect of friction, as shown Fig. 13(c). Kim et al. [91] developed a new method that {10-12} twins can be directly introduced into Mg sheet by conventional rolling, as shown in Fig. 13(d). In the method, rolled AZ31 sheet is pre-embedded in a steel block. During rolling, a compressive stress along TD will act on the embedded Mg alloy sheet due to the different strength between the Mg alloy and the steel block.

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Fig. 13.   Overview about the thin sheet compression die from (a) Ref. [24], (b) Ref. [26], (c) Ref [25]. and Ref. [91].

Although some deformation ways, e.g. in-plane shear deformation and bending deformation, are not dominated by {10-12} twinning, they can also be used to generate {10-12} twins in thin sheet. Fig. 14 shows deformation feature of the in-plane shear [92]. It is found that the shear stress can generate a compression stress component which is favorable to active {10-12} twinning for Mg alloy sheets with basal texture, as shown in Fig. 14(c). Kang et al. [93] has calculated the activity of deformation mode with applied strain in a rolled AZ31 sheet during the in-plane shear. The calculation results show that prismatic slip and basal slip are the major mode of strain accommodation, while {10-12} twinning is highly active. The c-axis of {10-12} twins via the in-plane shear locates to 45° away from RD (i.e. compression axial direction) in rolled AZ31 sheets (see Fig. 14(e)), which is also in line with the SF law [[92], [93], [94]].

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Fig. 14.   (a) Physical diagram, (b) schematic diagram of simple shear device, (c) deformation feature of in-plane shear, (d) {0001} pole figure of rolled AZ31 sheet and (e) {0001} pole figure of rolled AZ31 sheet underwent a strain of 5% by in-plane shear deformation. The red arrows represent twin-orientation [92].

For bending deformation, stress state varies continuously from compression to tension, [95]. It is found that localized twin bands can be formed in the compression zone of the sheet and can be extended to the tension zone with an increase of bending angle [95,96]. Thus, bending deformation can also be a feasible method to introduce local {10-12} twins in thin sheets, as shown in Fig. 15. Traditional bending deformation can only generate a local plastic deformation. To achieve homogenous bending strain and continuous processing, some continuous bending processing techniques have been developed [85,[97], [98], [99], [100]], as shown in Fig. 16. Song and Huang et al. [98] developed a repeated unidirectional bending (see Fig. 16(a)). Recently, Song et al. [85] introduced a continuous bending channel into equal channel angular rolling (ECAR-CB rolling) (see Fig. 16(d)). These continuous bending processing techniques also introduce additional traction stress (tension or compression) and friction stress to achieve continuous bending. Additional traction stress could also affect final deformation microstructure. For example, a large number of {10-12} twins and dislocations can be formed over the entire thickness of the AZ31 sheet during ECAR-CB rolling, as shown in Fig. 11. [85]

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Fig. 15.   EBSD maps and (0002) pole figures of various positions in a bended AZ31 sheet: (a) outer surface zone; (b) middle zone; (c) inner surface zone; and (d) measurement positions for EBSD analysis in the RD-ND plane. Position (b) is near the convex side, (c) at the center and (d) near the concave side [95].

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Fig. 16.   (a) Repeated unidirectional bending [98], (b) continuous bending process [99], (c) repeated roll bending process [100] and (d) continuous bending channel rolling [85].

Some processing techniques have been applied to induce {10-12} twins. However, the development of continuous production processes of twinned Mg alloys (especially thin sheets) has always been an urgent issue. In the current research, ECAR-CB rolling provides a new perspective and shows great potential to solve this problem. Moreover, features of twins and dislocations determine the deformation/recrystallization texture, as reviewed in the Sect. 2 and Sect. 3. Therefore, understanding the relationship between microstructure evolutions and processing parameters during pre-inducing twins is very important for precise regulation of texture.

5. Improvement in stretch formability of Mg alloy sheets

Rolled Mg alloys have comparable ductility to rolled Al alloys. However, plastic formability (e.g. stretch formability and rollability) of Mg alloys is far lower than that of Al alloys. An important reason is the limited activatable slip system and strong basal texture (c-axis//ND texture) in rolled Mg alloys [1]. For rolled Mg alloy plates/sheets with a basal texture, the thickness strain only can be coordinated by contraction twinning and pyramidal <c+a> slip with very large CRSS during stretch forming. Thus, the strong basal texture in rolled plates/sheets generates a very poor deformation capability of sheet thinning, resulting in low stretch formability [53]. Twinning deformation can be used to tailor the texture components of wrought Mg alloys, as reviewed above. For the twin-orientation with c-axis//RD or c-axis//TD textures, basal slip and prismatic slip within twins and detwinning can accommodate thickness strain during stretch forming. Thus, pre-twinning can remarkably enhance deformation capability of sheet thinning in rolled AZ31 sheets, which is considered to be benefit for the improvement in stretch formability, as reported by Song et al [9]. Recently, pre-twinning deformation has been successfully applied to improve the stretch formability of rolled Mg alloys. Stretch formability can be simply assessed by Erichsen test. Table 2 summarized Erichsen value of rolled AZ31 sheet underwent various twinning deformation from Ref. [[24], [25], [26], [27], [28],85,92].

Table 2

Table 2   The Erichsen values (IE) and basal pole figures of rolled AZ31 alloys underwent pre-inducing {10-12} twins and subsequent annealing.

*RD and TD in the brackets represent the loading direction in rolled AZ31 sheet.*CA and RA represent the recovery annealing and recrystallization annealing, respectively.*Basal pole figure corresponds to the processing history highlighted by bold for each research.

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Earliest, Park et al. [27] proved that pre-induced {10-12} twins by pre-compressed along RD can enhance the stretch formability of rolled AZ31 sheets. And Erichsen value increases (to 6.3mm) with increasing volume fraction of {10-12} twins with c-axis//RD texture. Song et al. [9] pointed out that selection of twin variants during twinning deformation could also influence the plastic anisotropy and stretch formability. He et al. [25] found that the AZ31 sheet with multi-directional {10-12} twins (with c-axis//TD and c-axis//RD textures) exhibits better stretch formability than single-directional {10-12} twins (with c-axis//TD texture).

During twinning deformation, some dislocations can also be introduced into matrix, which may be harmful to formability. Therefore, in previous work, after pre-twinning deformation, recovery annealing treatment (200 °C/6 h in Xin et al.’s work [101] and 250 °C/1 h in Park et al.’s work [27]) is usually performed to remove dislocations and retain twin structure. Additionally, for the twinned AZ31 sheet, detwinning is very easy to be activated during stretch forming. Detwinning exhausts the twin-orientation at low strain during stretch forming and restores the twin-orientation to initial basal texture [[102], [103], [104], [105]]. Clearly, occurrence of detwinning degrades the effect of twin-orientation on formability. Thus, retarding detwinning activity can enhance the influence of texture control via {10-12} twinning [106]. In previous work, it is found that thermal treatment also can retard detwinning activity [54,103,[106], [107], [108], [109]]. However, its influence on the stretch formability could be limited. If recrystallization annealing can remain twin-orientation, it will be a good method to eliminate dislocations and avoid detwinning. According to the above, recrystallization annealing could remain twin-texture (as Kim et al.’s work [26] and Wang et al.’s work [28] in Table 2) or restore basal texture (as Zhang et al. [92] in Table 2) in pre-twinned sheet. It may depend on twin size and dislocation accumulation, according to the report in Ref. [50,76,83,85]. In addition, recrystallization annealing can also generate new texture components in Mg alloy sheets with twin structure, as reported by Chen et al. [24] and Song et al. [85] (see Table 2). Chen et al. [24] obtained a double peak texture along TD in AZ31 thin sheet by pre-inducing cross twin lamellae (with c-axis//RD and c-axis//TD textures) and subsequent static recrystallization. After TD5.38%-RD3.3% pre-compression followed by annealing at 450 °C for 2 h, the Erischen value can be enhanced from 2.83 mm to 6.01 mm. Song et al. [85] used a continuous bending channel during rolling to obtain AZ31 sheet with {10-12} twins and high density of dislocations. After recrystallization annealing at 350 °C for 1 h, a double peak texture is formed, resulting in an excellent high Erichsen value of 7.4 mm. Additionally, according to Sect. 3, the unusual recrystallization texture in twinned AZ31 sheets could be attributed to special dislocations structure in matrix or within twins [85].

It should be noted that the effect of initial texture on stretch formability is highly temperature dependent [53]. With increasing temperature, thermal activation of non-basal slip (including prismatic <a> slip and pyramidal <c+a> slip) may weaken influence of texture on stretch formability. According to reports by Zhang et al. [110] and Chino et al. [111], weakening basal texture may even be detrimental to elevated-temperature formability of AZ31 alloys. Moreover, initial {10-12} twin structure can also generate largely influence on dynamic recrystallization behavior of Mg alloys during thermal processing [112]. It could also influence elevated-temperature formability. Recently, Park et al. [27] reported that pre-induced {10-12} twins can enhance the stretch formability of rolled AZ31 sheets from room-temperature to 300 °C. However, the related mechanism is lacking. Thus, influence of twin-orientation/twin structure on stretch formability at different temperatures should be systematically evaluated in the further work.

6. Conclusions and outlooks

{10-12} twinning deformation is an effective method to achieve texture control in wrought Mg alloys. It has been successfully used to improve the stretch formability of rolled AZ31 sheets. For this research area, current research progress and some key scientific issues are summarized as follows:

(1) Twin-orientation depends on twin variant selection during twinning deformation. In general, SF law controls the variant selection of {10-12} twinning. Thus, both the loading method and initial texture can influence the design of twin-orientation. Moreover, multi-step twinning deformation can also make the design of texture richer. Recently, some non-Schmid behaviour of variant selection during {10-12} twinning has been observed and reported. Non-Schmid behaviour can generate unexpected twin-orientations. How to use or enlarge the influence of limited twin-orientation via non-Schmid variant selection on mechanical properties is considered to be important in the further work.

(2) The contribution of {10-12} twinning on plastic strain determines volume fraction of twin-orientation under specific strains. The relative activity of various deformation modes is related to their active stress (CRSS/SF) during plastic deformation. The influencing factors and controlling methods have been well understood. The relative activity of various deformation modes can also influence dislocation evolution. However, the influences of dislocation features (density, type and distribution etc.) in matrix and twins on texture control and stretch formability still lack sufficient attention. Moreover, both twin-twin interactions and twin-dislocation interactions could affect twinning process and twin structure [84,[113], [114], [115], [116], [117]]. Thus, their influences on texture control and improvement in stretch formability of Mg alloys should be concerned.

(3) Recovery annealing can maintain twin-orientation and eliminate partial dislocations, while recrystallization annealing can generate new texture components to rich textural design. It has been reported that twin-orientation (or initial texture) could be retained, enhanced or weakened during static recrystallization. It is dependent on the twin size. Moreover, recrystallization behaviour has been reported to be closely related to dislocations in matrix and twins. At latest, abnormal texture evolution in AZ31 alloys with {10-12} twins and high density of dislocations has been found during recrystallization annealing. However, up to now, the static recrystallization mechanism of twinned Mg alloys and its influence on texture evolution is still unclear.

(4) The processing methods of pre-inducing {10-12} twins in rolled AZ31 sheets with basal texture have been summarized. In-plane compression is considered as an effective method for inducing {10-12} twinning deformation, and related processing techniques for thin sheet and thick plate have been reported. Moreover, some deformation ways which are not dominated by {10-12} twinning (in-plane shear and bending etc.) can also be used to induce {10-12} twins in rolled AZ31 thin sheet. During the processing of twinned Mg alloys, additional stresses (e.g. friction stress and traction stress etc.) could also play a key role in microstructure evolution and mechanical properties, so it should be taken seriously. In addition, in order to meet industrial requirements, continuous production process of twinned Mg alloys (especially thin sheets) should receive more attention in the further work.

(5) Texture change via twinning deformation can effectively enhance the stretch formability of rolled AZ31 alloys at room temperature. The influences of volume fraction of twins and the selection of twin variants on Erichsen value have been widely reported. However, systematical research on the relationship between stretch formability and twin features is still lacking. Moreover, influence of twin-orientation on stretch formability under other deformation conditions should be also evaluated. In addition, the texture control via pre-inducing {10-12} twins and its effect on stretch formability are mainly concentrated on rolled AZ31 alloys. It is well known that alloy elements have a large influence on deformation mechanisms and recrystallization behavior. Thus, it is also necessary to investigate the influence of alloying elements on texture control by {10-12} twinning in Mg alloys.

(6) Residual dislocations and detwinning behavior could weaken the contribution of twin-orientation on stretch formability. Recovery or recrystallization annealing can be used to solve this problem. It is considered that the recrystallization texture control of twinned Mg alloys may be the key for development of high stretch formability Mg alloy sheets. Moreover, it also should be stressed that annealing treatment can affect other changes in microstructure (e.g. grain size, solid solution and precipitation etc.), in addition to the texture components. And twin structure also has a remarkable effect on solid solution (e.g. solute segregation) and precipitation during thermal treatment [72,107,[118], [119], [120], [121], [122], [123]]. These effects on stretch formability should also be taken into account when evaluating the influence of pre-{10-12} twinning on the stretch formability.

Acknowledgements

This project was financially supported by the National Natural Science Foundation of China (project No. 51601154), the Fundamental Research Funds for the Central Universities (project No. XDJK2019B003), the National Natural Science Foundation of China (project No. 51871036), the Chongqing Science and Technology Commission (cstc2017jcyjAX0012), and China Postdoctoral Science Foundation (2018T110948).