| The mechanisms operating during continuous static and continuous dynamic recrystallization in the heat-treatable aluminum alloy Al-4Mg-0.3Sc have been determined. Bulk tensile specimens were fabricated from cold-worked material in the peak-aged and over-aged conditions. These specimens were superplastically tested at 733 K and a strain rate of 10−3 sec −1 to final elongations exceeding 300%. During the early stages of superplastic testing, every specimen dynamically recrystallized to a fine grain structure; A series of specimens from both the peak-aged and over-aged conditions were deformed to intermediate true strains of 0.1, 0.2, 0.4 and 0.8, and quenched under load with a liquid nitrogen spray. Post-mortem TEM examinations revealed microstructural changes with increasing strain in the specimen gauge sections such as subgrain growth, inhomogeneous dislocation distribution, and increasing complexity of dislocation interactions with particles, boundaries and other dislocations. These observations indicate the importance of dislocation dynamics during continuous dynamic recrystallization. Orientation imaging microscopy (OIM) revealed a variation in texture with distance from the surface of the specimen. A rolling texture dominated near the center of the specimen and randomized with increasing strain. A rotated cube texture dominated near the surface and sharpened with increasing strain.; In-situ TEM experiments were used to directly observe the mechanisms operating during continuous static and continuous dynamic recrystallization. The static process was observed to occur much more rapidly than the dynamic process. The dominant mechanism of continuous static recrystallization appears to be the rapid generation of dislocations at sources in grain boundaries and at grain boundary triple points. These dislocations slip across the subgrains and either interact with other dislocations or enter nearby grain boundaries. Low-angle grain boundaries were observed to disintegrate, but the mechanism by which this occurred was unclear. The primary mechanism of continuous dynamic recrystallization appears to be the process of subgrain rotation, manifested as the migration and disintegration of dislocation structures and the disintegration of subgrain boundary triple points. Subgrain rotation resulting in increased boundary misorientation was not observed; however, the restraints imposed by the thin foil geometry of the tensile TEM samples may be the reason for this. |
