Effects of Grain Growth on the {111}/{111} Near Singular Boundaries in High Purity Aluminum

The {111}/{111} near singular boundary is more resistant to intergranular corrosion than random boundary. At present, enhancing the fraction of such boundary to improve the performance against intergranular corrosion has been the latest issue in microstructure design and control for aluminum and its alloys. In the current work, high-purity aluminum was selected as an experimental material, and the effects of grain growth on {111}/{111} near singular boundary were investigated. First, the sample was given multi-directional forging at room temperature followed by recrystallization annealing at 370oC. The recrystallized samples were heated at 500oC for varied time to promote grain growth and to obtain microstructures with various grain sizes. Then, the {111}/{111} near singular boundary in the samples was measured by grain boundary inter-connection characterization, which was established on the basis of EBSD and five-parameter analysis. Results show that the length fraction of {111}/{111} near singular boundary increases with the increase of grain size. For example, the fraction of {111}/{111} near singular boundaries is 3.91% when the averaged grain size is 38 mu m, whereas it increases to 6.56% as the averaged grain size reaches 77 mu m. Off-line in situ EBSD coupled with grain boundary trace analysis indicates that the {111}/{111} near singular boundary is primarily formed via the encounter of two growing grains with <111>/. misorientation relationships (theta is the rotation angle). Meanwhile, the {111}/{111} near singular boundary is also formed via the re-orientation of grain boundaries with <111>/theta misorientation. HRTEM observation reveals that the {111}/{111} near singular boundary has disclination, and the degree of atomic ordering of such a boundary is higher than that of random boundaries. Therefore, such a boundary is more resistant to intergranular corrosion compared with random boundary.