Recrystallization within the twins

As mentioned above, recrystallization in the AN12 sample is triggered within the twin bands. The twins are indexed as secondary twins with a misorientation of 38° around <112�0> axis to the parent grains, Figure.

3.2.3. In the AN12 sample, the recrystallized grains continuously grow with the annealing time. The development of recrystallized grains in the ZN12 sample is more difficult to trace compared to that in the AN12 sample. The large recrystallized grains suddenly appear after 8 min annealing, Figure. 3.2.3 (h), without indication from the EBSD data after 3 min annealing, Figure. 3.2.3 (g).

Secondary twins play an essential role in recrystallization. The question whether the recrystallization nuclei will inherit the twin orientation or not is important to investigate the texture evolution during recrystallization annealing. The orientations of secondary twins are extracted from the EBSD data of as-rolled condition, shown in Fig. 4.2.1 (a) and (b). The main feature of the secondary twins is that they have tilted c-axis from ND to RD in the (0001) pole figure in both alloys. The RD split texture is common in as-rolled Mg alloys, which is explained by the twin variants selection. The secondary twins with the misorientation of 38° around <112�0> axis to the matrix involve two twinning processes, first the primary compression twinning with 56° around <112�0> axis and secondary tensile twinning with 86° around <112�0>

axis within the compression twins. Contrary to the primary compression twinning in which the twin variants with the first or second highest SF are preferred, the secondary tensile twinning occurs in a non-Schmid factor manner. The SF for compression twinning of rolling process is calculated with a biaxial stress state, sigma:

sigma = �1 0 0 0 0 0 0 0 1��

which means a tension load on the RD and compression on the ND. The matrix grains are mostly oriented in the basal-type orientations after rolling. The calculated SF of six compression twin variants for two different orientation sets of the matrix grains, <101�0> // RD or <112�0>// RD, are shown in Figure. 4.2.1 (c) and (d), respectively. These two orientation sets represent the basal-type texture, in agreement with the as-rolled texture of this study. The blue and red arrows in Figure. 4.2.1 (c) and (d) exhibit how the secondary twins will rotate their basal poles in the (0001) basal pole figure corresponding to the rotation of the matrix grain. It is clear that compression twin variants locating at the RD (filled diamond symbols) are more favorable than the variants tilted to the TD (empty diamond symbols) due to the higher SF. It has been reported that the variants of the primary compression and secondary tension twins usually share the same

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<112�0> rotation axis because of the ease of growth and low associated accommodation strain [139]. Thus, secondary twin variants tilted towards the RD (blue triangle) are more favored than other secondary twin variants which exactly match the observed RD split texture of secondary twins. Besides, it is interesting that the secondary twins of the ZN12 sample have a wider TD spread in the (0001) pole figure than those of the AN12 sample. This can be understood by the strong basal type texture in the AN12 sample in which more matrix grains have the basal orientation. On the other hand, the ZN12 sample with a weak texture has a high potential to produce the secondary twins having the tilted basal poles to the TD. Recalling the texture of recrystallized grains in Figure. 3.2.4, small recrystallized grains also show the RD split texture component, similar to the texture of secondary twin in Figure.4.2.1. It is reasonable to assume the recrystallization nuclei triggered at the twin bands maintain the orientations of secondary twins.

From the statistic evaluation of quasi in-situ data, recrystallization is related to the secondary twins in both samples. However, it is not clear how the recrystallization nuclei are formed inside the twins. To reveal the underlying mechanisms, TKD characterization was applied on the shortly annealed samples. Figure. 4.2.2 shows the TKD maps of secondary twins in the AN12 sample annealed at 270 °C for 1 min. The IPF map in Figure. 4.2.2 (a) shows an orientation gradient inside the secondary twin as its color gradually changes from pink in the bottom to orange in the top part. The orientations of the matrix grain and the twinned volume are plot in Figure. 4.2.2 (b). In the (0001) pole figure, the c-axis of secondary twins spread towards the RD. It can be explained by the common <112�0> axis of the matrix and twin which is aligned parallel to TD in the {112�0} pole figure. It derives from the shear strain in RD by the rolling process. The misorientation dramatically increases from A to B, about 5 °/μm, as shown in Figure. 4.2.2 (c). The misorientation between neighboring points is less than 2 °. The transmission and band contrast images are shown in Figure. 4.2.2 (d) and (e), respectively. The yellow lines indicate the secondary twin boundaries which match well with the IPF map.

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Figure. 4.2.1 (a) and (b) basal pole figures of secondary twins in the AN12 and ZN12 in as-rolled condition, respectively, (c) and (d) calculated compression and secondary twin orientation and compression twinning SF in basal orientated matrix grains with <101�0> // RD and <112�0>// RD,

respectively.

The TKD maps of the ZN12 sample annealed at 400 °C for 1 min are shown in Figure. 4.2.3. Similar to the AN12 sample, a significant orientation gradient and the basal pole spread in the RD are also observed in the (0001) pole figure of the ZN12 sample. Unlike the continuous distribution of the basal poles of the twins in Figure. 4.2.2 (b), the twin in the ZN12 sample has a discrete distribution of the basal pole showing some separate dots, as shown in Figure. 4.2.3 (b). The cumulative misorientation reaches 22 ° within about 2.5 μm, Figure. 4.2.3 (c). It is of interest that the point-to-point misorientation in the ZN12 sample can reach 10 °, much higher than that in the AN12 sample. The transmission image shows some dislocation cells along the black arrow, as shown in Figure. 4.2.3 (d). The diameters of these dislocation cells are approximately 400 nm. The formation of dislocation cells is a typical feature of recovery prior to recrystallization. The dislocation density contrast that the dislocation cells are brighter than the neighboring volume, indicating a lower dislocation density within the cells. In the band contrast map, it seems that the ZN12 sample is more likely to form LAGBs compared with the AN12 sample during annealing, as shown in Figure. 4.2.3 (e). In fact, the dislocation cells are not only observed in the twins but also in the matrix grains which are not shown here. However, this phenomenon is not found in the AN12 sample.

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Figure. 4.2.2 TKD maps of AN12 sample annealed at 270 °C for 1 min: (a) IPF map, (b) (0001) and {112�0} pole figures, (c) misorientation profile along the black arrow, (d) transmission image, (e) band

contrast map. The yellow lines in (d) correspond to the secondary twin boundaries in (a).

The stored deformation energy act as the driving force of recrystallization. Therefore, a high dislocation density will promote the recrystallization process. It is difficult to quantitatively estimate the dislocation density, especially in the severely deformed sample in which the dislocations are tangled. The misorientation gradient can be used as an indicator of dislocation density. Higher shear strain introduces a higher dislocation density, leading to a larger lattice rotation and misorientation gradient. The TKD characterizations show high misorientation gradients inside the twins in both annealed samples. In the AN12 sample, the misorientation generally increases with accumulating the low point-to-point misorientation. On the contrary, the ZN12 sample shows a sudden increase of the misorientation. The misorientation distributions indicate two different configurations of dislocations, i.e., dislocation pile-ups in the AN12 and dislocation cells in the ZN12 sample. It is reported that the dislocation pile-ups have a higher energy while dislocation cells are more energetically stable [76]. The texture evolution during the recrystallization can be explained by these two configurations.

Assuming that a recrystallization nucleus is formed inside a secondary twin of AN12 sample. The c-axis of this nucleus slightly tilted from the twin orientation. This nucleus can easily grow by absorbing the neighboring dislocations. Growth of the nucleus within the twins is more favored because of a high stored energy of the dislocation pile-ups, leading to the non-equiaxed shape of the recrystallized grains, as shown in Figure. 3.2.3 (b). These recrystallized grains, to some extent, inherit the orientation of secondary twins,

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in good agreement with Figure. 3.2.4 (f). In the ZN12 sample, the formation of dislocation cells is originated by the rearrangement and annihilation of the dislocations. The merging of dislocation cells is difficult because the dislocation density inside the cells is too low. Considering that the segregation and precipitation are observed at LAGBs in Figure. 3.2.8 (b), it is also reasonable to assume they can pin the LAGBs of dislocation cells. As a result, the contribution of recrystallized grain from dislocation cell with secondary twin orientation is minor in the ZN12 compared with the AN12 sample.

Figure. 4.2.3 TKD maps of ZN12 sample annealed at 400 °C for 1 min: (a) IPF map, (b) (0001) and {112�0} pole figure, (c) misorientation profile along the black arrow, (d) transmission image, (e) band contrast map. Black and yellow lines in (d) correspond to the HAGBs and secondary twin boundaries in

(a).

As shown in Figure. 4.2.4 (a), a small recrystallized nucleus was formed within the secondary twin in the ZN12 sample annealed at 400 °C for 1 min. This nucleus has a misorientation of 75° around <6�104�3> axis to the matrix, which doesn’t match any twin misorientation relationships. The boundary between the nucleus and the secondary twin is indexed as tension twin boundary with a misorientation of 86° around <112�0>

axis in the {112�0} pole figure, which means the nuclei has a ternary twin orientation, as shown in Figure.

4.2.4 (c). The ZN12 sample has more complex twinning activities, forming the recrystallization nuclei with diffuse orientations inside the twins. The complex twinning activities in the ZN12 sample, e.g., tension, secondary and ternary twins, are exemplarily shown in Figure. 4.2.4 and Figure. 3.2.2 (j). On the contrary,

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the dominant twinning mode in the AN12 sample is secondary twinning, shown in Figure. 3.2.2 (i). The recrystallized grains in the AN12 sample inherit the RD spread texture of the secondary twins. The explanation for the complex twinning activity in the ZN12 sample can be the texture effect. The AN12 sample with strong basal texture suppresses the formation of tension twins because the c-axis of grains were compressed. The weak texture of the ZN12 sample means a high fraction of grains with tilted c-axis from ND, and thus a high potential for various twinning modes.

Figure. 4.2.4 TKD map of the ZN12 sample annealed at 400 °C for 1 min: (a) transmission image, (b) and (c) the misorientation profile and corresponding (0001) and {112�0} pole figure along line A, B and C.