{111}/{111} Near Singular Boundaries in an Al-Zn-Mg-Cu Alloy Recrystallized After Rolling at Different Temperatures

Recent development in grain boundary design and control indicates that manipulating the {111}/{111} near singular boundaries will be a promising pertinent to improve the performance against intergranular corrosion attacks for the high stacking fault energy face-centered cubic metals such as aluminum and its alloys. In the current study, five samples of a home-made Al-Zn-Mg-Cu super-high-strength aluminum alloy were rolled at temperatures of 250, 300, 350, 400, and 450oC, followed by 30 min annealing at 520 degrees C. A method of grain boundary interconnection characterization based on electron backscatter diffraction and five parameter analysis was utilized to assess the {111}/{111} near singular boundaries in the samples as processed. The preceding rolling temperature was discovered to have a significant impact on the formation of {111}/{111} near singular boundaries during the subsequent annealing at 520 degrees C, that is, the fraction of {111}/{111} near singular boundaries out of the entire grain boundaries increases at first and then decreases as the preceding rolling temperature increases from 250 degrees C to 450 degrees C. In the five samples as processed, the one rolled at 300 degrees C followed by annealing at 520 degrees C has a peak content of {111}/{111} near singular boundaries and the fraction reaches 5.0%, which is 10 times higher compared to that of the singular boundaries or namely the coherent twin boundaries. Further investigations reveal that the sample rolled at 300 degrees C possesses a specific deformation substructure as well as suitable stored energy, resulting in continuous recrystallization during the successive annealing. This type of behavior aids in the formation of {111}/{111} near singular boundaries. The samples rolled at or above 350 degrees C, on the other hand, exhibit discontinuous dynamic recrystallization, which is detrimental to the development of {111}/ {111} near singular boundaries during subsequent annealing. Compared to the sample rolled at 300 degrees C, the sample rolled at 250 degrees C has higher stored energy and it improves discontinuous recrystallization during the subsequent annealing. This also harms the formation of {111}/{111} near singular boundaries. Offline in-situ surface etching test and high-resolution transmission electron microscope (HR-TEM) observation demonstrate that the {111}/{111} near singular boundaries have much higher resistance to intergranular corrosion in comparison to the random boundaries, they possess disclination structures of which the atomic ordering is much higher than that of the random boundaries. The results show that the {111}/{111} near singular boundary is regulable, and to further improving the fraction of such boundaries by manipulating the microstructure evolution will be effective in the practice of how reducing the intergranular corrosion in the aluminum and its alloys.