Initial and Cold Rolled Microstructure
Figure 2(a) shows an EBSD map (band contrast) of the starting material prior to cold rolling. The microstructure is largely recrystallized, and contains grains with a wide distribution of sizes, ranging from 5 to 50 μm. The average grain size (determined by mean linear intercept) is 12 μm. Boundaries with misorientations matching those of tensile, compression, and double twins are marked on the map. It can be seen there are very few twin boundaries identified in the initial material.
Figure 2(b) shows an EBSD band contrast map after straight cold rolling (higher magnification). A highly heterogenous microstructure is observed, characterized by bands of intense deformation. Indexing was not possible in the most heavily deformed regions within these bands; unindexed regions appear black. Again, special boundaries corresponding to twins are highlighted. Both compression (\(\{10{\bar{1}}1\}\)) and double (\(\{10{\bar{1}}1\}\)–\(\{10{\bar{1}}2\}\)) boundaries are seen to be dominant, with very few of the low CRSS \(\{10{\bar{1}}2\}\) twins. A high fraction of the boundary identified as double twins is in the very heavily deformed regions, where limited indexing and the very fine scale of the microstructure make it hard to unambiguously identify whether this is really twinned material. It is also important to note that many of the grains are not twinned, including the largest grains, and the overall twin volume fraction is very small. This point is consistent with the VPSC modeling, as shown later.
Bulk Textures After Cold Rolling and Recrystallization
The bulk textures of the deformed and fully recrystallized ZEK100 after each rolling process are presented in Figure 3. The RD and TD in each figure relate to the hot rolling and transverse directions of the original sheet, respectively. The texture of the as-received fully recrystallized sheet is given in Figure 3(a). This texture shows the characteristic split basal texture of ZEK100, forming two lobes split by ≈ 30 deg toward the TD.
The as-rolled texture of the material after each cold rolling schedule is presented in Figures 3 (b, d, and f). In each case, the material behaves typically for magnesium with basal poles rotating toward the ND. A texture split is still evident, but the deviation from the ideal basal orientation (≈ 10 deg) is much less than in the starting material. The basal pole figure of the SCR material is split toward the RDCR (parallel to RDHR) direction, Figure 3(b). The as-rolled TCR and ECR basal texture is split toward the TDHR (which corresponds to RDCR of the final rolling pass), Figures 3(d) and (f). In the TCR as-rolled material, although the basal poles are split toward the TDHR direction, the spread in this direction is reduced. In all cases, the prismatic pole figures show a weak texture; there is evidence in the prismatic pole figure of the SCR material for a weak \(\{11{\bar{2}}0\}\) ∥ RDHR texture.
Annealing for 60 minutes at 673 K (400 °C) fully recrystallizes each alloy. The recrystallized textures are shown in Figures 3(c, e, g). The texture for the SCR material (Figure 3(c)) is very similar to that of the parent hot rolled sheet (Figure 3(a)) as recrystallization has reproduced the TDHR spread in the basal pole figure. The recrystallized texture of the TCR material is shown in Figure 2(e). In this case, recrystallization has produced a similar split in the basal texture; however, it is rotated by 90 deg with respect to the hot rolling direction, Figure 3(a). Although rotated by 90 deg, both the SCR and TCR material have formed the Zn-RE recrystallization texture with a spread in the basal poles toward the cold rolling TD. It is notable that in the TCR material, the texture split observed in the original hot rolled plate has been completely replaced by a texture split rotated by 90 deg (i.e., basal poles rotated toward the cold rolled TD, which corresponds to the hot rolled RD).
The recrystallized texture of the ECR material is shown in Figure 3(g). In contrast to the SCR and TCR material, recrystallization after ECR does not produce a significant change in the texture components observed, but only in the texture intensity. The recrystallization texture in ECR material is best described as a weakened deformation texture, where the as-rolled split basal texture of Figure 2(f) has simply weakened. Notably the split in the recrystallized basal poles after ECR is ≈ 10 deg, which is significantly less than the ≈ 30 deg split observed in the other recrystallized materials.
Modeling Deformation
The VPSC model was used to simulate each rolling process and produce predicted deformation textures together with slip system activities in each case. The results are shown in Figure 4. The main qualitative features of the deformation textures are reasonably well reproduced: the simulated basal pole figure for SCR, Figure 4(a), contains two intense lobes split toward the RDCR (RDHR) and the diffuse spreading of the texture in the TDHR, which is observed experimentally in Figure 3(b). The model also predicts a relatively strong \(\{11{\bar{2}}0\}\) ∥ RDHR texture component in the prismatic pole figure, the beginnings of which can be seen in Figure 3(b). The simulated deformation textures of TCR and ECR are in similarly good agreement with experimental results: TCR simulations reproduce a split in the TD, but a reduced spread in this direction Figure 4(b), as observed experimentally in Figure 3(d), and ECR simulations reproduce a diffuse spread of orientations about the TD, Figure 4(c), which is observed experimentally in Figure 3(e).
The activities of each deformation mode are shown in Figures 4(d) through (f). As expected in magnesium, basal slip is the dominant deformation mode in all cases and contributes strongly to the strengthening of basal textures. The contribution of prismatic slip is strongly sensitive to the direction of cold rolling, with high prismatic slip activity predicted during SCR and low prismatic slip during TCR. The change in prismatic slip activity is also observed in ECR with high levels of prismatic slip predicted when the cold rolling direction is parallel to the hot rolling direction and low prismatic slip activity when perpendicular to it. The overall percentage activity, or percentage of strain, accommodated by each deformation mode during the three deformation processes is plotted in Figure 5. This shows that the ECR schedule is predicted to have basal, prismatic, pyramidal slip and tension twin activities which are between those predicted for the SCR and TCR schedules. This may be significant as increased non-basal slip is often cited as a critical factor in nucleating Zn-RE textures.
Texture Development in Transverse Cold Rolled Material
As demonstrated, the transverse cold rolled (TCR) material produced a split in basal pole alignment (texture) that was completely reorientated (rotated by 90 deg) compared with the texture split in the original hot rolled plate after recrystallization. It is therefore useful to study the texture evolution in this material in an attempt to isolate the source of the texture change and the role played by the deformation structure compared with the recrystallization behavior. Note that as Figure 3 shows, the split in basal pole alignment toward the transverse direction (of the final cold rolling pass) clearly occurred during recrystallization and was not detectable in the bulk pole figure for the deformed condition.
EBSD maps of the deformed and partially recrystallized TCR material are shown in Figure 6. In the as-cold rolled condition, Figure 6(a), the microstructure contains slip bands and the beginnings of shear bands where deformation has been concentrated. These regions appear as black lines on the map, where severe deformation has prevented indexing of the Kikuchi patterns. During annealing, recrystallization begins in these regions as a very fine distribution of grains, shown in Figure 6(b). As recrystallization progresses, the recrystallized grains grow larger and begin to consume the microstructure outside the severely deformed regions, Figure 6(d).
The recrystallized grains can be further broken down by grain size. Figure 7 shows grains separated into small grains, with sizes less than 1 μm, and large grains, with sizes greater than 1 μm. In the case of the as-rolled material, the large grain population also includes the deformed grains. In the case of the annealed material, only recrystallized grains (grains with internal misorientations < 1 deg are shown.
The textures are also plotted for each of these populations. In all cases, the small grains are aligned more closely to the ideal basal texture orientation, with a slight split basal texture toward the cold rolling direction. It is only the large grains that have orientations spread in the TD. With longer annealing times, these grains become increasingly dominant, leading to the texture seen in the fully recrystallized condition. This evolution in texture during recrystallization is gradual and progressive; it is clear that in the early stages of recrystallization, there are new grains formed with basal poles spread toward both the TD and RD (as is observed in non-RE containing Mg alloys). It is only as recrystallization proceeds that the grains with the characteristic spread in basal pole orientations toward the TD become dominant, which is (apparently) unique to Mg-Zn-RE and related alloys (e.g., Mg-Zn-Ca).