Abnormal Grain Growth of Off-Cube Grains in High Purity Aluminum Foils with Cube Texture

Abstract

In present study, abnormally coarsened grains in high purity aluminum foils with cube texture (f100gh001i) after a final annealing was investigated using SEM/EBSP method. Most of abnormally coarsened grains had Goss (f110gh001i )o r S( f123gh634i) orientations. The coarsened grains with Goss and S-orientations were surrounded by �5 and �7 coincidence boundaries, respectively. This result clarified that these coincidence boundaries would enhance the abnormal grain growth of Goss and S oriented grains. Grain boundary energy of sub-boundaries between cube-oriented grains provided the driving force for the abnormal grain growth. The distribution of Goss oriented grains in the partially annealed foils was characterized by SEM/EBSP method. This characterization revealed that many Goss-oriented grains were distributed in the transition band on the central layer of the sheets. aluminum and its alloys, which is closely related to forming or etching property. Several thermo-mechanical processes have been developed for the control of cube texture in commercially pure aluminum and its alloys. A thermo- mechanical process through a cold rolling, a partial annealing (the annealing temperature is 240-260 � C), an additional rolling (the reduction in thickness is approximately 20%) and a final annealing has been developed for the formation of sharp cube texture in a high purity aluminum and an Al-Mn- Mg alloy, 2) which are referred to as Pechiney process. 3) This process is the production process for aluminum foils for electrolytic capacitors. The mechanism of preferential growth of cube-oriented grains (cube grains) by the Pechiney process has been investigated using SEM/EBSP method. 4,5) The previous investigations have clarified that the difference of accumu- lated strains between cube grains and off-cube grains induced by an additional rolling could provide the driving force for the preferential growth of cube grains during the final annealing. This mechanism is expected to have come from the property that cube grains have less accumulated strains than off-cube grains. 6) Hence Controlling the formation of substructure accelerated the preferential growth of cube grains and inhibited growth of off-cube grains, leading to the production of higher volume fraction of cube texture more than 95%. 7) However, in some cases, off-cube grains abnormally grow to over 10 cm in grain size in high purity aluminum sheets through the Pechiney process, and bring about decline of a volume fraction of cube texture. Most of recent studies 8-14) are concerned to the preferential growth of cube grains, so that there are few studies on the character of residual off-cube grains in aluminum sheets with cube texture. Accordingly, establishing the mechanism of the formation and growth of off-cube grains in aluminum sheets with cube texture is important for not only the development of processes for control of cube texture but also under- standing the fundamental mechanism of cube texture devel- opment in aluminum and its alloy. In the present study, off-cube grains in high purity aluminum foils with cube texture were analyzed by SEM/ EBSP method. From the results, the mechanism of abnormal growth of off-cube grains is discussed focusing an attention to mobility and driving force of grain boundary migration between cube grains and off-cube grains. A distribution of orientation in aluminum foils before the final annealing was characterized by SEM/EBSP method. Comparing the dis- tribution of off-cube grains in the foils before and after the final annealing, we discuss the mechanism of the formation of off-cube grains during the thermo-mechanical process. 2. Experimental