Twin nucleation and growth

In addition to dislocation slip, twinning is an important deformation mode. Tension twins are observed in deformed Mg alloys because basal slip, in spite of its low CRSS, cannot accommodate strain along the c-axis. The CRSS for tensile twinning is higher than that for basal slip, but lower than other deformation modes at room temperature. Grain size is another factor that influences the twinning activity. It is reported that twinning is favored with a large grain size [31]. Grain refinement results in a high volume fraction of grain boundaries which are the preferred sites for twin nucleation. The finer grain size sample will contain more twins per volume but fewer twins per grain compared with the coarse one[37]. Moreover, the finer grain size restricts the twin growth because of the impingement of grain boundaries.

The different twinning behaviors between the AN12 and ZN12 samples during quasi in-situ tensile tests are clearly shown in Table. 3.1.5. About half of twins are already formed in the ZN12 sample at ε = 0.02, while in the AN12 sample the majority of the twins, almost 70 percent, are formed at ε = 0.08. Considering the larger grain size in the AN12 sample, this result is controversial to the effect of grain size on twinning activity.

The effect of SF on the twinning activity is plotted in Figure.3.1.9. The max SF for tensile twinning of six twin variants in the ZN12 sample is comparable to that in the AN12 sample, as shown in Figure. 3.1.9 (a).

Both samples have unfavorable textures for tensile twinning as most grains have their c-axis perpendicular to the loading direction, as shown in Figure. 3.1.3 (c) and (d). This alignment is not favorable for tensile twinning. In the twinned grains, the active SF values for tensile twinning were calculated based on the active twin orientation, Figure. 3.1.9 (b). The ZN12 sample generally has a much higher active SF for tensile twinning than the AN12 sample, even though the calculated max SF of the ZN12 and AN12 samples are quite comparable. That is, the grain orientation and the corresponding SF for twinning are also not the key factor for the enhanced activity of tension twins in the ZN12 compared to the AN12 sample.

The CRSS value should be taken into account to explain the twinning mechanisms in the examined alloys.

Normally, it is difficult to directly measure the CRSS for certain deformation modes in polycrystal. However, the CRSS for tensile twinning can be estimated in relation to the stress when the twins are formed. For example, half of the twins in the ZN12 sample are observed at ε = 0.02, corresponding to the stress of 121 MPa in Table.3.1.2 and Table. 3.1.5. At this strain, the local stress that triggers twin nucleation is still low

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in the ZN12 sample. On the contrary, the majority of the twins in the AN12 sample are formed at ε = 0.08 and 197 MPa. In general, the deformation is more homogeneous in finer grain size sample than larger one.

The local stress to initiate twins in the AN12 sample with a larger grain size is obviously much higher than that of the ZN12 sample. In another word, the CRSS for tensile twinning is lower in the ZN12 than that in the AN12 sample.

As can be seen in Figure. 3.1.9, many twins observed during the in-situ tensile tests correspond to the tensile twin variants with a lower or even negative SF rather than those with the max SF. This non-Schmid behavior of twinning is more obvious in the AN12 sample. Jonas [49] and Guan [50] proposed that the twins with low SF can be formed when prismatic and basal slip are highly activated in the neighboring grains. Guo [137] reported that the impingement of a neighboring twin at grain boundary triggers twinning with the low SF variants. The deformation inhomogeneity in the AN12 sample results in a higher localized stress which is responsible for the non-Schmid twinning, compared with the ZN12 sample.

The ZN12 sample has a high twin nucleation rate at relatively low strain. Many twins were observed at ε = 0.02 in the ZN12 sample, and the twin nuclei or fine twins do not obviously grow with further loading. In the AN12 sample, most of the twins were observed at ε = 0.08. The low CRSS value of tensile twinning in the ZN12 sample should be one of the preconditions for the profuse twins at low strain. Twin nucleation is a complicated process that involves the cross slip and slip-boundary interaction and dislocation dissociations.

At ε = 0.08, the twins are thicker and larger in the AN12 sample compared with that in the ZN12, indicating a higher twin growth rate in the former. It is generally accepted that the CRSS for twin nucleation is higher than that for twin growth [36, 138]. As mentioned above, the ZN12 sample has a lower twin nucleation CRSS than the AN12 sample. Thus, it is expected that twins can grow easily in ZN12 sample due to an even lower twin growth CRSS. Comparing the IPF maps at ε = 0.02 and 0.08 in Figure. 3.1.4, however, the twins do not obviously grow in the ZN12 sample. One explanation can be the small grain size of the ZN12 sample restricts the twin growth. In addition, the phenomenon that the strain was accommodated by basal slip in the twinned volume rather than by twin growth could also explain the restricted twin growth in the ZN12 sample.

As shown in Figure. 4.1.3 (a), the AN12 sample shows no basal slip trace at ε = 0.02. On the contrary, basal slip traces are observed in the matrix and twins in the ZN12 sample even though the twin has a low basal SF value. Statistically, about 25 percent of the twins in the ZN12 sample have obvious basal slip traces while the fraction in AN12 is below 5 percent. The basal slip traces inside the twins in the ZN12 sample are attributed to the low CRSS for basal slip rather than the effect of grain orientation. This is confirmed in Figure. 3.1.10 (d) that the parents have higher SF values for basal slip than the twins. With increasing the strain, the twin grows easily in the AN12 sample, Figure. 4.1.3 (b) and (c), while the twin in the ZN12

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sample is more stable, Figure. 4.3 (e) and (f). In the AN12 sample, twin growth plays an important role in the strain accommodation owing to the poor activities of basal and non-basal slip. In opposite, various deformation modes, e.g., basal, non-basal and the in-twin slip, can easily accommodate the strain in the ZN12 sample so that the driving force for twin growth is reduced. Nie [60] reported the co-segregation of Gd and Zn on twin boundaries can obviously increase the CRSS for twin growth. The EDS maps of the AN12 and ZN12 samples in a fully recrystallized condition are shown in Figure. 4.1.4. It is interesting to find a similar co-segregation of Zn and Nd at the grain boundaries of the ZN12 sample. This boundary segregation is not observed in the AN12 sample which has some large Al and Nd rich precipitates. As proposed in [60], the co-segregation of Zn and RE effectively pin the twin boundaries, resulting in an increase of the CRSS for twin growth.

Figure. 4.1.3 SEM image and corresponding IPF maps of: (a) and (b) the AN12 sample at ε = 0.02, (d) and (e) the ZN12 sample at ε = 0.02, (c) and (f) IPF maps of the AN12 and ZN12 samples at ε = 0.05.

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Figure. 4.1.4 Transmission images and EDS maps of the grain boundaries in fully recrystallized samples:

(a) and (c) the AN12, (b) and (d) ZN12.