Quantitative data of the twins

Apart from dislocation glide, twinning is another important deformation mechanism. In Mg alloys with HCP structure, tensile twinning with a small twinning shear and atomic shuffling is the dominant twinning mode at room temperature [114]. The EBSD data of the quasi in-situ experiments show a good agreement that more than 90 percent of the observed twins are tension twins in both AN12 and ZN12 samples, Figure. 3.1.4.

Thus, it is reasonable to give a focus on the tensile twinning mode during deformation.

The microstructure of the ZN12 sample at ε = 0.02 shows some small twins, whereas it is almost twin free in the AN12 at same strain, Figure. 3.1.4 (a) and (b), respectively. With increasing the strain, twin growth

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as well as the formation of new twins occur simultaneously. The growing twin lamellae merge to a large twin when they are the same twin variant. The twins in the ZN12 sample have a thin lamellar shape, while the twins formed in the AN12 sample are much thicker. This can be derived from the grain size effect. Grain boundaries are the barrier for twin boundary migration. The twins in the ZN12 sample with a smaller grain size are more restricted compared to those in the AN12 sample. The statistical data in Table. 3.1.5 also show that 80 out of 378 in the AN12 and 74 out of 588 grains in the ZN12 are twinned at ε = 0.08. The number of twins observed in the AN12 and ZN12 samples are 255 and 147, respectively.

Table. 3.1.5 Number fraction of twinned grains at different strain.

Regarding the texture, tensile twinning mode is not preferred in both samples, since the a-axis of most grains undertake a tensile load while tensile twinning requires either c-axis tension or a-axis compression. This is supported by the calculated SF for tensile twinning in Figure. 3.1.9 (a). It should be mentioned that only the highest SF value for tensile twinning, from the six possible variants, is shown. The SF for tensile twinning of all grains in AN12 and ZN12 samples are concentrated in the range of -0.2 to 0.5. There is no significant difference in the max tensile twinning SF distribution of the AN12 and the ZN12 samples. The real active twin variants at ε = 0.08 were indexed by comparing to the ideal twin orientations, which are described in the experiment chapter 2.5. The SF calculated for the real active tension twins are shown in Figure. 3.1.9 (b). Obviously, a considerable number of the active twins have a negative SF, especially in the AN12 sample.

In addition, the ZN12 sample has more twins with the SF value for tensile twinning larger than 0.1, while in the AN12 sample more twins with a negative SF are activated.

An interesting result is found in terms of the twinning activity relating to the strain. The twins having a SF lower than -0.3 are formed at high strain in the AN12 sample, as shown in Figure 3.1.9 (c). There is no certain preference of SF range at different strain in the ZN12 sample in Figure. 3.1.9 (d). Table. 3.1.5 shows that 70 percent of the twins in the AN12 sample are formed at 0.08 strain. This strain is high enough to induce sufficient localized stress, deviating from the applied stress, that triggers the low SF twins.

However, some questions can arise relating the real twinning activities in Figure. 3.1.9 (b) with the theoretical SF values in Figure. 3.1.9 (a). First, the difference in the SF values between the AN12 and ZN12 sample is quite small, but the twin number fraction at ε = 0.02 in the ZN12 (f = 0.46) is much higher than

Sample Fraction of twinned grains

Fraction of newly formed twin lamellar at different strain 0.02 strain 0.04 strain 0.08 strain

AN12 0.22 0.05 0.25 0.70

ZN12 0.13 0.46 0.13 0.41

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that (f = 0.05) in the AN12. Interestingly, a surge of twin formation is observed in the AN12 sample at ε = 0.08, whereas the ZN12 sample shows a gradual increase of twin fraction with strain. Besides, the total number fraction of twinned grains and twin lamellae at ε = 0.08 in the ZN12 (f = 0.13) is smaller compared to that in the AN12 (f = 0.22).

Figure. 3.1.9 (a) Distribution of max SF for tensile twinning, (b) real active SF for tensile twinning, (c) and (d) active SF for tensile twinning at different strains in the AN12 and ZN12 sample, respectively.

The stress and strain accumulation at grain boundaries can be released by the formation of twins. Tensile twinning rotates the lattice of parent grain with 86° around <112�0> axis. With such a large rotation angle, the twinned volume will behave differently from its parent during further deformation. It has been shown from the quasi in-situ tensile tests that the basal slip is a dominant deformation mode. It is necessary to consider the basal slip activity inside the twins. The orientation of parents and twins at 0.08 strain are plotted in Figure. 3.1.10. The parent grains have a basal-type orientation and the scattered orientation in the AN12 and ZN12 sample, respectively. They are similar to those of the initial samples at fully recrystallized condition in Figure. 3.1.3 (c) and (d). Meanwhile, the twinned volumes have the basal poles reoriented to the TD in both samples, which is responsible for the formation of the TD component at ε = 0.08. The corresponding SF value for basal slip of the parents and twins in the AN12 and ZN12 samples are shown in

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Figure. 3.1.10 (c) and (d). The parent grains and twins in the AN12 sample have comparable SF values for basal slip because both basal oriented parent and TD oriented twins are not preferred for basal slip. In the ZN12 sample, high SF values for basal slip of parent grains are found while the twins have diverse SF value.

This means that the twinning in the ZN12 sample rotates the grains to a hard orientation for basal slip.

Figure. 3.1.10 (a) and (b) Discrete (0001) pole figure of parent and twins in the AN12 and ZN12 samples respectively, (c) and (d) corresponding distribution of SF value for basal slip.