The purpose of this study is to investigate the influence of purity on the cube texture in aluminum foil sheets for the electrolytic capacitors. For the accomplishment of this purpose, aluminum foil sheets of 99.9 % (3NAl) and 99.99 % (4NAl) purity for electrolytic capacitors were analyzed by SEM/EBSP method after each production process.

Figure 1(a) and (b) show the OIM (Orientation Imaging Microscopy) images for the crystal orientation and the local orientation in partially annealed foil sheets of 3NAl and 4NAl. Each crystal orientation is represented by the unique color according to the unit triangle. Fine lines indicate that the misorientation angle between adjacent scanning points is 1 15 degrees, while bold lines indicate that the angle is over 15 degrees. The grain size of 4NAl was larger than that of 3NAl in these foil sheets. This result shows that the behavior of grain growth during partial annealing differed due to the purity of foil sheets. Furthermore, the area with dense fine lines corresponds to the deformed substructure and stored strains, which were developed during cold rolling.  Whereas the area with no or little amount of fine lines, which correspond to the low strained grains that were recovered and recrystallized during partial annealing. Figure 1(c) and (d) show the inverse pole figures presenting the orientation of the low strained grains in the partially annealed foil sheets of 3NAl and 4NAl, respectively. The red points indicate cube-orientation, while the blue points indicate the grains with other orientations which were recovered and recrystallized during partial annealing. The plots show that most of the strain-free grains in the 3NAl had the -fiber orientations which were detected in the cold rolled foil sheets. On the other hand, the 4NAl showed many strain-free grains which had the orientations other than -fiber components. Considering that the typical cold rolled texture consists of the -fiber orientation, it was expected that, the dislocation density in the grains in the 3NAl would have been reduced by the recovery. While, the strain-free grains in the 4NAl were formed not only by the recovery but also by the discontinuous recrystallization.

Figure 2 shows the relation between the final annealing time at 573K and the area fraction of cube-oriented grains in additionally rolled foil sheets of 3NAl and 4NAl. The values in Fig. 2 indicate the average size of cube-oriented grains. The area fraction of cube-oriented grains in 4NAl increased linearly to 38% with final annealing time from 0 to 120 seconds, and then increased abruptly at 180 seconds to the area fraction of 95%. While in the 3NAl, the area fraction was 28%. Despite the area fraction of cube-oriented grains continued to increase, the average grain size decreased at the annealing time of 180 seconds. It was caused by the formation of the small cube-oriented grains less than 10 m in diameter. Presumably, cube-oriented grains in the subsurface region grew during the final annealing and eventually emerged on the surface. After the annealing for 600 seconds, the area fraction of cube-oriented grains in 3NAl became 95% and was as large as that in 4NAl. It was clarified that the area fraction of the cube-oriented grains in both 3NAl and 4NAl increased abruptly after the incubation period, but the incubation period of 3NAl was longer than that of 4NAl. The result suggests that impurities in the aluminum suppress the growth of the cube-oriented grains.

Figure 3 shows the OIM images representing the difference of local orientation in the additionally rolled foil sheets of 3NAl and 4NAl. The fine lines and the bold lines indicate as same as Figs. 1(a) and (b). The fine lines in the same grain can correspond to the amount of stored strains induced by the additional rolling. From the maps, it was observed that all of the cube-oriented grains, which are indicated as ''W'', were surrounded by high angle boundaries. And there were less fine lines in the cube-oriented grains than the other grains. Therefore, it was considered that the additional rolling preferentially introduced strains in the grains with different orientation from the cube-orientation. From these results, it could be concluded that the strain would hardly accumulate in cube-oriented grains by the additional rolling and that the difference of purity would not influence the anisotropy of deformation by the additional rolling. Consequently, it was clarified that the quantities and local difference of residual strains in the additionally rolled foil sheets did not depend on the purity.

[Published in Materials Transactions, Vo1. 45, No. 5 (2004), pp. 1687-1692]