Grain boundary structure in Al–Mg and Al–Mg–Sc alloys after equal-channel angular pressing

Samples of an Al-3% Mg alloy and an Al-3% Mg-0.2% Sc alloy were subjected to equal-channel angular pressing (ECAP) to reduce the grain size to approximately 0.2-0.3 μm. Some samples of each alloy were also annealed for 1 h at temperatures of either 423 or 673 K, respectively. High-resolution electron microscopy was used to examine the microstructure both before and after annealing. The grain boundaries after ECAP were wavy and faceted and in high-energy nonequilibrium configurations. These results were consistent with earlier observations of materials subjected to severe plastic deformation using high-pressure torsion. In addition, some grain boundaries in the Al-Mg-Sc alloy had a zigzag appearance after annealing at 673 K, where the straight portions of the boundary were identified as low-energy (111) planes. It is suggested these are mobile boundaries lying in a lowest energy configuration where mobility may be restricted by the presence of incoherent Al₃Sc particles.