Improvement of Strength-Elongation Balance of Al-Mg-Si Sheet Alloy by Utilising Mg-Si Cluster and Its Proposed Mechanism

The tensile properties of an Al-Mg-Si alloy with Mg-Si clusters were compared with those of an Al-Mg-Si alloy with beta '' precipitates of the same strength. The elongation of the alloy with Mg-Si clusters was found to be greater than that of the alloy with beta '' precipitates because of the high work hardening rate of the former alloy, particularly in the high-strain region. Decomposition of Mg-Si clusters into solute Mg and Si atoms during the tensile deformation was revealed by differential scanning calorimetry. Transmission electron microscopy revealed three types of dislocation characteristics in these alloys: homogeneous distribution of dislocations with beta '' precipitates, cell structures in the alloy with solute Mg and Si, and a combination of these two types in the alloy with Mg-Si clusters. In the case of the alloy with Mg-Si clusters, the yield strength increased significantly owing to the dislocation cutting mechanism; simultaneously, the elongation of this alloy improved greatly because of the presence of solute Mg and Si atoms formed by decomposition via plastic deformation, which were inferred to prevent dynamic recovery in the later stage of tensile deformation. Consequently, a comparison of conventional 6000 series and 7000 series Al alloys revealed that the alloy with clusters had advantages over the alloy with precipitates and the alloy with solutes in terms of the balance between strength and elongation.