Strain induced grain boundary migration - Recrystallization mechanisms of Mg-Nd based alloys

4.2 Recrystallization mechanisms of Mg-Nd based alloys

4.2.3 Strain induced grain boundary migration

Twins play an important role in recrystallization behaviors because twins are severely deformed and have high stored energy. In addition, grain boundaries also hinder the dislocation movement, and they are the potential sites for recrystallization. However, there are very few recrystallized grains found at the grain boundary regions in the present quasi in-situ experiments. Nevertheless, the strain induced grain boundary migration is observed in the AN12 sample, as shown in Figure. 4.2.5. After the annealing at 400 °C for 1 min, the boundaries of the grains A and B move towards the twins of neighboring grain, forming a dislocation-free region marked with the white arrows in Figure. 4.2.5 (b). The misorientations between these regions and grain A or B are lower than 10° as no HAGBs are observed. Rather than regarding the marked

regions as newly formed recrystallized grains, it is more reasonable to relate them with grain boundary migration. These regions can continuously grow by consuming other twins with annealing time, as shown in Figure. 4.2.5 (c). TKD observation of the same sample also supports the mechanism, Figure. 4.2.5 (d).

The grain C stabs into the grain D. The grain boundary between C and D has a very high curvature that seems to be uncommon. The assumption makes sense that the grain boundary from its original place marked with the white dash line move towards grain D by consuming the twin band. The grain boundary migration is preferred at the junctions of twin and grain boundary. Compared with normal grain boundaries, the junctions of twin and grain boundary have a higher dislocation density and stored energy because of twinning. The dislocation and energy localization at the junctions can be released by grain boundary migration that forms the low dislocation region.

Figure. 4.2.5 EBSD maps of quasi in-situ AN12 sample: (a) as-rolled, (b) annealed at 270 °C for 1 min and (c) annealed at 270 °C for 3 min, (d) IPF map of the AN12 TKD sample annealed at 270 °C for 1 min.

Various mechanisms are considered to comprehend the recrystallization behaviors of the AN12 and ZN12 samples. The recrystallization processes of both samples, based on the present experimental results, are schematically illustrated in Figure. 4.2.6. Twins are observed in the as-rolled AN12 and ZN12 samples.

Secondary twins are dominant in the AN12 sample, Figure. 4.2.6 (a). Other types of twins, e.g., ternary twin, play an important role in the ZN12 sample. During recrystallization annealing, small nuclei are formed inside the secondary twin of the grain A. These nuclei inherit the orientation of the secondary twins having the RD split of basal poles, and they grow rapidly along the twins, as shown in Figure. 4.2.6 (c). Besides, the grain boundaries of grain C migrate towards the twin in grain B because of a high stored energy and dislocation density. It helps strengthen the deformation basal texture as some deformed grains are becoming larger. On the contrary, some small precipitates are formed at the grain and twin boundaries in the ZN12

sample. These precipitates pin the boundaries and retard the recrystallization process. Recovery accompanying the formation of dislocation cells is pronounced in the ZN12 sample. It is difficult for the dislocation cells to merge and form large recrystallized grains because the LAGBs can also be pinned by precipitates or segregation of Zn and RE. The preferential growth of RD split nucleus inside secondary twins is restricted by precipitation. As a result, the texture of recrystallized grains becomes more scattered in ZN12 sample than AN12 sample.

Figure. 4.2.6 Schematic of recrystallization mechanisms in: (a) and (c) the AN12 sample, (b) and (d) the ZN12 sample.

5 Summary

The understanding of the deformation and recrystallization mechanisms of Mg-RE alloys is of importance to develop high performance Mg alloys. To reach these complicated goals, quasi in-situ tensile and recrystallization annealing experiments were applied in this study. Distinct differences in the yielding, fracture behaviors and strain hardening are observed during the quasi in-situ tension. The slip traces and active twin variants reveal the contributions of different deformation modes to accommodate the strain during uniaxial tension.

1. The ZN12 sample shows a lower yield strength and higher elongation than the AN12 in conventional tensile tests. An obvious yielding is observed in the ZN12 sample. It is related to the formation of twins which releases the local stress. The calculated strain hardening rate exhibits a higher strain hardening exponent of the ZN12 than that of the AN12 sample.

2. Basal slip is the dominant deformation mode in quasi in-situ tensile tests. Most of the grains already show obvious basal slip traces after the yielding in the ZN12 sample, indicating a homogeneous deformation.

Meanwhile, some non-basal slip traces are also observed in the grain boundary region of the ZN12 sample.

The slip traces become wavy with increasing the strain. On the contrary, basal slip traces are rarely found in the AN12 sample at ε = 0.02. Even at ε = 0.08, the basal slip traces are localized in limited number of grains.

3. Apart from basal slip, tensile twinning plays a role in accommodating the strain. The number of the twins in the AN12 and ZN12 samples at ε = 0.08 are 255 and 147, respectively. It is of interest to find that half of the twins in the ZN12 sample are already found at ε = 0.02 while most of the twins in the AN12 sample are formed at ε = 0.08. Tensile twinning reorients the basal poles of the matrix grains from the ND to TD, i.e., the basal-type to TD spread orientation. With increasing strain, more twins are formed, and the twins formed at low strain gradually grow at the same time. As a result, the texture component corresponding to the TD spread in basal pole figure becomes sharper.

4. To minimize the effects of grain size and initial texture on the quasi in-situ tensile experiments, the EBSD data are carefully evaluated. The grains are categorized into several subsets based on the grain size and SF.

Thus, the grains in the same subsets can be compared between the AN12 and ZN12 samples without concerning the influence of grain size and orientation. There still remains an obvious trend that non-basal slip and tensile twinning are more preferred in ZN12 than AN12 sample, especially at low strain.

5. The chemical composition is one of the main factors that induced the distinct mechanical properties in the AN12 and ZN12. It results in a lower CRSS for basal slip and a lower CRSS ratio of non-basal to basal slip in the ZN12 compared with the AN12 sample.

Recrystallization annealing is used to modify the microstructure and texture after the thermomechanical processing. The addition of RE into Mg alloys was reported to alter the recrystallization mechanisms of Mg alloys and produce the so-called RE texture. Obvious differences in the recrystallization kinetics, nucleation and grain growth process are observed between the AN12 and ZN12 during quasi in-situ annealing.

1. The AN12 sample has higher recrystallization kinetics than the ZN12 sample. Fully recrystallization is achieved in the AN12 after 18 min annealing at 350 °C, while the recrystallization is negligible in the ZN12 sample with the same annealing parameter. Due to the significant difference in recrystallization kinetics, quasi in-situ annealing at 270 °C and 400 °C are applied to the AN12 and ZN12, respectively.

2. Secondary twinning is the dominant twinning mode in the as-rolled sample. Secondary twins are the preferred sites for nucleation in the AN12 sample. The c-axis of recrystallization nuclei of both alloys are tilted from ND to RD, similar to the secondary twin orientation. In addition, the secondary twins in the ZN12 sample have a wider spread towards TD compared with those in the AN12 sample.

3. Obvious precipitation at twin and grain boundaries is observed in the ZN12 sample. It is responsible for the retarded recrystallization. In the early stage of annealing, the dislocation cells as an indicator of recovery are formed in the ZN12 sample, which reduces the stored energy. On the contrary, the AN12 sample with clear boundaries and a high dislocation density promotes the twin induced nucleation mechanisms.

4. The growth preference is evaluated by tracing the diameter of recrystallized grains. In the AN12 sample, the growing grains (g ≥ 0.2) showed the RD split orientations which are responsible for the texture of recrystallized grains with a wide spread of the basal pole from the ND to RD. However, the newly formed grains (g = ∞) other than the growing ones are the majority of the recrystallized grains after 8 min annealing in the ZN12 sample. They showed a weak and scattered orientations in the (0001) pole figure.

To conclude, the ZN12 sample shows better formability than the AN12 sample, owing to the easier activation of different deformation modes. Apart from the effect of grain size and texture, the chemical composition by changing the CRSS value is important to explain the multiple dislocation slip and twinning activities in the ZN12 sample. During annealing, the recrystallization in the ZN12 sample is retarded by the segregation and precipitation at the twin boundaries. The complex twinning behaviors in the ZN12 sample result in a wide spread orientation of the recrystallized nuclei. The recrystallized grain originating from secondary twins in the AN12 sample can easily grow which inherit the RD spread texture of secondary twins.

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