Microstructural evolution during region III deformation (T=723 K, ε̇=10⁻¹ s⁻¹) in a superplastic Al–Mg–Mn alloy (ALNOVI-U) was investigated. Surface makers drawn by focused ion beams (FIB) and internal observation by the electron back-scattered diffraction (EBSD) method were used for surface studies and internal studies, respectively. The surface markers clarified that both grain boundary sliding and dislocation activity significantly contributed for deformation. In the EBSD studies, formation of new low-angle boundaries (LABs) was observed at core/mantle boundaries and in core regions. The new LABs were formed by dynamic recovery of dislocation walls; it is the initial process of continuous dynamic recrystallization in region III superplasticity.

The hot tearing (crack) often occur on the surface of the Al–Mg system alloy ingot during casting process. To explain the mechanism of crack generation in the Al–Mg system alloy, the mechanical properties and deformation behaviors of semi solid state was investigated by the tensile test in the semi solid state. The tensile strength and the elongation of Al–Mg system alloy decrease with increase the test temperature in the solid state to semi solid state. Other system aluminum alloys, for example Al–Si–Fe system alloy and Al–Zn–Mg–Cu system alloy, have large elongation in the solid state near the solidus. Because of low ductility in the temperature range near the solidus, the hot tearing often occur on the surface of the Al–Mg system ingot, in which the casting mold contacts at the solidification.