Effect of Sc on microstructure and mechanical properties of as-cast Al–Mg alloys

Highlights: • Providing a plausible explanation that the addition of Sc can cause a remarkable grain refinement effect in as-cast Al-Mg alloy via EBSD analyses. • Primary particles are identified as the eutectics consist of 'Al₃Sc + α-Al + Al₃Sc····' multilayer structure rather than pure Al₃Sc phase. • A three-dimensional schematic diagram was provided to illustrate this proposed structure of these eutectic particles. The effect of Sc addition on microstructure and mechanical properties of as-cast Al–Mg alloys was investigated based on optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD) and properties testing. The results showed that a remarkable grain refining effect was observed for Sc content greater than 0.4 wt.%, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formed primary particles, which hindered the undesirable diffusion and subsequent crystal growth and acted as potent heterogeneous nucleation sites for α-Al matrix during solidification. In addition, these particles could cause a significant refinement strengthening effect. Atomic force microscopy (AFM) and SEM-elemental mapping were employed to investigate the internal structure and elemental distribution of these particles. They could be identified as the eutectics consisted of Al₃Sc and α-Al phases rather than individual Al₃Sc phase, which exhibited an 'Al₃Sc + α-Al + Al₃Sc ⋯' multilayer structure.