Microstructural evolution in recrystallized and unrecrystallized Al–Mg–Sc alloys during superplastic deformation

The microstructural evolution of unrecrystallized (extruded) and recrystallized (friction stir processed, FSP) Al–Mg–Sc alloys during superplastic straining was investigated using electron backscatter diffraction (EBSD). The unrecrystallized structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. This evolution was divided into three stages based on true stress–strain curves and EBSD maps, i.e. subgrain rotation and coalescence in the early stage, dynamic recrystallization in the middle stage, and grain boundary sliding (GBS) and dynamic grain growth in the final stage. By comparison, the recrystallized grains in the FSP Al–Mg–Sc maintained a random distribution during the whole deformation process, however the grain size increased significantly with increasing strain, indicating that the main deformation mechanism was always GBS and dynamic grain growth. A deformation model was proposed to explain the microstructural evolution during superplastic deformation. The microstructure with the random boundary misorientations reaches a dynamic balance because the transformation between high-angle grain boundaries and low-angle grain boundaries is equivalent.