Indian Journal of Engineering & Materials Sciences Vol. 25, December 2018, pp. 480-486

Twinning and dynamic recrystallization in AZ31 magnesium alloy under medium-high strain rate

Xiao Liua,b*, Guangjie Huangc, Luoxing Lib, Chanping Tanga, Biwu Zhua & Wenhui Liua

aKey Laboratory of High Temperature Wear Resistant Materials Preparation Technology of Hunan Province, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China

bState Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China cCollege of Materials Science and Engineering, Chongqing University, Chongqing, Chongqing 400045, China

Received 24 June 2016; accepted 26 December 2017

The uniaxial compression tests were carried out on AZ31 magnesium alloy samples at 150°C and 300°C and strain rates of 0.3s-1, 3s-1 and 10s-1. The microstructures at various deformation conditions were detected by optical microscopy (OM) and the electron back scatter diffraction (EBSD). Schmid factor (SF) was employed to analyze the initiation of deformation modes. The secondary derivative method was used to identify the critical strain for twinning and DRX. Twinning is activated in the early stage of deformation and induces DRX, causing grain refinement. The microstructure and SF results show that the number of twins increases with strain rate and decreases with temperature increasing. Extension twinning is the main deformation mode (beside basal slip).

Keywords： Twinning; Dynamic recrystallization; Schmid factor; Magnesium alloy

Due to their stiffness-to-weight ratio, magnesium and its alloy are promising candidates for substitution of steel and aluminum components in the automotive industry. Hexagonal close-packed (hcp) magnesium has limited number of easy slip systems for low symmetry hcp crystal structures. This causes poor formability at relative low temperature and high strain rate1-3. Under these conditions, twinning can play a significant role by reorientation grains and permitting the activation of additional slip system4.

Many investigators1-13 focused on the effect of twinning on the deformation processing. A series of tensile and compression tests were carried out by Barnett5,6. They indicated that {10-12} tension twinning could increase the uniform elongation in tensile tests, while “contraction” double twinning would decrease the uniform elongation, and twin induced softening was important during compressive deformation. Yan et al.7 hot rolled specimens of Mg-2.0%Zn-0.8%Gd from 11.5 mm to 1.2 mm and pointed out that twins and shear bands were the key deformation mechanism during hot rolling. In our previous study8, we carried out compression at various deformation conditions and indicated that

flow softening was mainly caused by the grain orientation brought about by twinning. Recently, as-casting AZ31B samples were rolled at a high (1000 m/min) and a low (15 m/min) rolling speed by Su et al.3 They found that twinning played an important role on texture evolution. Hou et al.9 investigated AZ31 magnesium alloy during interrupted in situ compressive tests and indicated that {10-12} tensile twins refined grains and weaken the basal texture.

Dynamic recrystallization (DRX) can easily occur during various deformation processing in magnesium alloy, contributing to refined grains and improvement in mechanical properties11. There are several types of DRX, such as continuous dynamic recrystallization and discontinuous dynamic recrystallization14. Numerous researchers found a novel and interesting DRX, called twinning induced DRX11-13,15-17. Ma and co-workers2,12 investigated as-extruded AZ61 Mg alloy during extrusion and as-extruded AM30 Mg alloy during interrupted compression and found a novel DRX that was induced by twinning. Somewhat similar results were reported by Yu et al.13, in wrought AZ31B Mg alloy during compressing test under a wide range of strain rates and temperatures. They also indicated that the twinning induced DRX, resulting in more effective grain refinement.

—————— *Corresponding author (E-mail: liuxiao0105@163.com)

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Changizian et al.15 investigated the effects of mechanical twins and γ precipitates on DRX during compression tests and found that dynamic precipitation of γ phase and twins were corresponding to DRX and refined grains.

In the present study, the uniaxial compression tests were carried out at 150°C and 300°C and at strain rates of 0.3s-1, 3s-1 and 10s-1 in as-casting AZ31 magnesium alloy. The microstructures at various deformation conditions were detected by optical microscopy (OM). The electron back scatter diffraction (EBSD) was used to identify the type of twins and obtain the orientation of grains. Schmid factor was applied to analyze deformation modes during compression. The double minima were detected and the critical strain for twinning and DRX was identified in this way. The relationship between twinning and DRX was also discussed.

Experimental Procedure The alloy used in present study was the commercial

Mg-Al-Zn alloy, AZ31 (3.19%Al、0.81%Zn、0.33%Mn). The material was received as-casting bar in the form of Ф90 mm. The bars were solution heat treatment at 420°C for 5 h. Cylindrical specimens with diameters of 8 mm and heights of 12 mm were machined from the as-casting bars.

The hot compression tests were performed on the Gleeble-3500 hot simulator. The specimens were heated to the test temperatures (150°C and 300°C), held for 5 min and then compressed to selected strains (0.1 and 0.2) at three strain rates (0.3 s-1, 3 s-1 and 10 s-1). During these tests, the true strain rate was maintained constant. In order to keep microstructure, the specimens were water quenched immediately after compression. To check the repeatability of the results, three experiments were conducted for each condition.

The deformed samples were sectioned along the compression direction and then subjected to metallographic preparation using SiC paper, followed by polishing. The polished samples were etched with 98 mL distilled water, 1 mL HNO and 1 g oxalic acid for times that varied from 18 to 22 s. The Axiovert 40 MAT (OM) was applied to detect the microstructure. To measure the twinning and orientation distribution, the deformation samples’ cross-section parallel to the compression axis were polished by AC2 solution. The EBSD was applied to determine the nature of the twin boundaries and orientation distribution and was performed on a Philips VP 3000N SEM equipped with the HKL data acquisition system.

Results and Discussion Microsturcure

The microstructure of as-casting AZ31 magnesium alloy under different hot compression conditions are displayed in Fig. 1. At 150°C and a strain rate of 0.3 s-

1 with a strain of 0.1 (in Fig. 1(a)), twins can be detected in initial grains and small amount of intersecting twins are also observed. The thickness of intersecting twins are relatively large. At 150°C and a strain rate of 0.3 s-1 with a strain of 0.2, it can be seen from Fig. 1(b), the number of twins increases in contrast to that at small strain. Large number of intersecting twins is detected. EBSD map under a temperature of 150°C and a strain rate of 0.3 s-1 with a strain of 0.2 is illustrated in Fig. 2. Here EBSD techniques were employed to identify recrystallized grains and twinning. Grey color represents deformed grains, while white color is corresponding to substructure, and green color is representing to recrystallized grains. Black lines represent grain boundary (θ>15°), while red lines are corresponding to {10-12} extension twins, and yellow lines are representing to {10-12}-{0-112} extension-extension twins, aqua lines correspond to {10-12}-{-1012} extension-extension twins, and blue lines are associated with {10-11} contraction twins. Finally, fuchsia lines are related to {10-11}-{10-12} double twins. Tiny recrystallized grains form inside the twins and serrations are observed, indicating that DRX has been initiated. Molodov et al.18 took place a plane strain compression along <11-20> direction at specially oriented magnesium single crystals at strain of -0.11 and at room temperature with a strain rate of 10-3 s-1 and found that recrystallized grains were detected in numerous bands associated with {10-11} contraction twinning within the primary extension twinned matrix. They indicated that the prismatic glide played a crucial role on initiation of DRX in the fragmentation of contraction twins. It can be concluded that the formation of DRX at low temperature may result from the prismatic glide inside twins. Increasing strain rate to 3 s-1 at 150°C and a strain of 0.2 (in Fig. 1(c)), tiny twins cross each other and initial grains are nearly occupied by intersecting twins. The density of twins dramatically increases with strain rate. The recrystallized grains are also observed at high magnification. At 300°C and a strain rate of 10s-1 with a strain of 0.2, twinning morphology can be detected. Large number of recrystallized grains forms inside twinning and segments twins into small

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pieces. The size of recrystallized grains is around 5 µm. This suggests that the density of twin increases with strain rate increasing and DRX is related to twin.

The critical resolved shear stress (CRSS) of slip systems are sensitive to temperature and strain rate, while CRSS of twinning is hardly change with temperature and strain rate19,20. As shown from Fig. 3,

at 150°C, the CRSSs for basal slip, prismatic slip, <c+a> pyramidal glide, contraction twinning and extension twinning are 0.5 MPa, 20 MPa, 30 MPa, 28 MPa and 3 MPa, separately. This indicates that prismatic slip and <c+a> pyramidal glide are hardly initiated at low temperature in contrast to basal slip and

Fig. 1 — Optical microstructure of as-casting AZ31 magnesium alloy under different hot compression conditions: (a) T=150°C, =0.3, ε=0.1; (b) T=150°C, =0.3, =0.2; (c) T=150°C, =3, ε=0.2; and (d) T=300°C, ε =10, ε=0.2

Fig. 2 — EBSD map of as-casting AZ31 magnesium alloy with astrain of 0.2 at 150°C and a strain rate of 0.3 s-1 at highmagnification

Fig. 3 — CRSSs of magnesium for different deformation modes19

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twinning. Five independent slips are required to satisfy the uniform plasticity deformation according to Von’s Mise criteria21. Thus, twinning will be initiated to accommodate plasticity deformation (Fig. 1(a)-(c)). At 300°C, the CRSSs for prismatic glide and <c+a>pyramidal slip rapidly decrease (CRSSprismatic=2 MPa and CRSSpyramidal=4 MPa). The prismatic glide will be easily activated in contrast to contraction and extension twinning. Then, the number of twins decreases. As strain rate increasing, slip systems can not accommodate plasticity. Then, it will require more twins to realize the uniform deformation at high strain rate. Thus, the density of twins increases with strain rate. EBSD analysis

The EBSD microstructure corresponding to a strain of 0.2 at 300°C and a strain rate of 10 s-1 is displayed in Fig. 4. Here EBSD techniques were employed to identify twins and grain orientation. Black lines

represent grain boundary (θ>15°), while red lines are corresponding to {10-12} extension twins, and yellow lines are representing to {10-12}-{0-112} extension-extension twins, aqua lines correspond to {10-12}-{-1012} extension-extension twins, and blue lines are associated with {10-11} contraction twins. Finally, fuchsia lines are related to {10-11}-{10-12} double twins. It can be seen from Fig. 4 that various twins can be detected in initial grains, including {10-12}-{0-112} extension-extension twins, {10-11} contraction twins and {10-12} extension twins.

Substantial twins loss their morphology and only can be observed beside small recrystallized grains. Recrystallized grains whose size is around 5 µm form inside, the initial grains and exhibit twin morphology. In the meantime, the disorientation between these recrystallized fined-grain and matrix grain are 32-39°, 56°or 82°. The disorientation distribution and the corresponding inverse pole figure with rotation axes are displayed in Fig. 5. The characteristic disorientation angles and rotation axes of the most frequently observed twin types in Mg alloys are also given in Table 1. As shown in Fig. 5, the most frequently detected boundaries are those with disorientations of 30-40°about the <-12-10> axis, 50-60°about the <01-10> axis and 70-90°about the <-12-10> axis. According to Table 1, these are boundaries between: (i) double twins and matrix (double twin-matrix with a characteristic disorientation of 38°<-12-10>), (ii) two extension twins of different types (extension twin-extension

Fig. 4 — EBSD map determined at compressive strain of 0.2 at atemperature of 300°C and a strain rate of 10s-1 (CD is compressiondirection and RD is radial direction)

Table 1 – The most frequently observed twin types in Mg alloys are listed together with corresponding disorientations

with respect to the matrix given in minimum angle-axis pairs

{10-12} extension twin 86°<-12-10>

{10-11} contraction twin 56°<-12-10>

{10-11}-{10-12} double twin 38°<-12-10>

(10-12)-(0-112) extension-extension twin 60.4°<8-1-70>

(10-12)-(01-12) extension-extension twin 60°<10-10>

(10-12)-(-1012) extension-extension twin 7.4°<1-210>

Fig. 5 — The disorientation distribution and the corresponding inverse pole figure with rotation axes

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twin with a characteristic disorientation of 60°<10-10>), and (iii) extension twins and the matrix (extension twin-matrix with a characteristic disorientation 86°<-12-10>). Accordingly, these recrystallized grains, displaying twin morphology, are induced by twinning. It can be concluded that twinning is initiated in the earliest stages of deformation8. Then, twin boundaries provide potential nucleation positions for recrystallization, followed the nucleation of recrystallized grains on twin boundaries and inside twins. Thus, twins are segmented by new grains and lose their initial orientations. This indicates that twins induce dynamic recrystallization (DRX), resulting in grain refinement.

It is generally known that the activation of deformation modes is related to SFs and CRSSs of deformation modes. In order to analyze the deformation modes of grains 1-11, the SFs of basal glide, prismatic glide, <c+a> pyramidal slip, extension twin and contraction twin are calculated. The detailed calculation method can be found in Ref.8 The SFs of various deformation modes for grains 1-11 are given in Table 2. As shown in Fig. 3, CRSSs

for basal glide, prismatic glide, <c+a> pyramidal slip, extension twin and contraction twin at 300°C are 0.4 MPa, 2 MPa, 4 MPpa, 3 MPa and 28 MPa, respectively. The grains 1-11 of (CRSS/m) for various deformation modes were then obtained and are displayed in Table 3. The CRSS/m for basal slip is pretty low in grains 1-11, indicating that basal slip is the main deformation mode. In grains 1-7, extension twinning is the good secondary deformation modes (after basal glide). In grains 8-11, the CRSS/m for prismatic glide is relatively lower than that for extension twinning, but the difference between prismatic glide and extension twinning is little. It can be concluded that prismatic slip and extension twinning are the key deformation modes (after basal glide) in these grains. Because of internal stresses, contraction twinning can be activated in some case22. The critical for twinning and DRX

The second derivative method23 was applied to the flow curve at 150°C with strain rates of 0.3 s-1 and 3 s-1 and at 300°C with a strain rate of 10 s-1 so as to determine the stress associated with the onset of

Table 2 — Schmid factors of different deformation modes in grains 1-11

Grains Basal slip Prismatic slip <c+a> pyramidal slip Extension twin Contraction twin

1 0.28 0.05 0.23 0.25 0.27 2 0.33 0.19 0.18 0.33 0.40 3 0.09 0.15 0.42 0.50 0.45 4 0.34 0.138 0.30 0.28 0.31 5 0.02 0.28 0.40 0.50 0.40 6 0.43 0.01 0.40 0.23 0.32 7 0.21 0.23 0.30 0.45 0.47 8 0.48 0.3 0.31 0.28 0.32 9 0.32 0.13 0.20 0.27 0.34

10 0.15 0.48 0.45 0.48 0.45 11 0.27 0.33 0.32 0.35 0.49

Table 3 — Grains 1-11 of (CRSS/m) for various deformation modes

Grain Basal slip Prismatic slip <c+a> pyramidal slip Extension twin Contraction twin

1 1.4 40 17 12 104 2 1.2 10 22 9.1 70 3 4.4 13 9.5 6 62 4 1.2 14 13 11 90 5 20 7 10 6 70 6 0.93 200 10 13 88 7 1.9 8.7 13 6.7 59 8 0.83 6.7 13 11 88 9 1.3 15 20 11 82

10 2.7 4.2 8.9 6.3 62 11 1.5 6.1 13 8.6 57

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twinning. For this purpose, polynomials of order 12 or 13 were employed. The results obtained are exhibited in Fig. 6 (a), suggesting that there are two minima. It can be concluded that the first minima is corresponding to critical stress for twinning, the second is related to the initiation of DRX. The critical stress was converted into critical strain by locating them on the relating flow curves. This is identified in Fig. 6(b). As can be seen from Fig. 6, it is clear that twinning is initiated in the earliest stages of deformation. Such twinning reorients the basal planes from a hard slip to a softer direction.

In the present study, twins are activated as good secondary deformation mode in the earliest stage for uniform deformation. The twin boundaries could act as barriers for dislocation motions, contributing to dislocation pile-ups both in the twins and the matrix along twin boundaries24. These dislocation entanglements provide the driving force for DRX. Ma et al.12 and Kaibyshev et al.25 proved that the twin boundaries could facilitate the thermally driven grain boundary diffusion, and evolve new grains at high temperature. In the meantime, substantial intersecting twins forms inside the initial grains and divide initial gains into several pieces at high strain rate (in Figs 1(c) and 1(d)). Thus, intersecting twinning provide more potential nucleation positions to DRX, contributing to grain refinement. Conclusions

As-casting AZ31 magnesium alloys were compressed at 150°C and 300°C with strain rates of 0.3 s-1, 3 s-1 and 10 s-1. Microstructures at various deformation conditions were observed and the

orientation of grains was obtained by EBSD technology at 300°C and 150°C. The critical strains for twinning and DRX were identified by double differentiation. The main results of the analysis can be summarized as follows: (i) Substantial twins are activated at high strain rate and low temperature. The density of twinning increases with strain rate and decreases with increasing temperature. Extension twinning is the main type of twin for relative low CRSS in contrast to contraction twinning. (ii) Extension twinning plays an important role during compression and is the good secondary deformation mode (after basal slip).

(iii) Twins are activated in the earliest stages of deformation and promote the initiation of DRX inside twinning and twin boundaries, followed by grain refinement. Acknowledgments

The authors gratefully acknowledge research support from the National Natural Science Foundation of China (Grant No.51601062 and 51605159) and the Hunan Provinical Natural Science Foundation of China (Grant No. 2015JJ6040) as well as from the Science Fund of the State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body (Grant No.31415004). References 1 Stidikov O & Kaibyshev R, Mater Trans, 42 (2001)

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Fig. 6 — The critical strain for twining and DRX at 150°C with strain rates of 0.3 s-1 and 3 s-1 and at 300°C with a strain rate of 10 s-1 (a) dependence of the stress second derivate on stress, and (b) the critical strains determined by double differentiation located on the experimental flow curves

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