Modification mechanism of eutectic Si phases in Al-Si-Mg series alloys with Sc addition

The modification mechanism of Sc additions to the eutectic Si phase in Al-7.0Si-0.3Mg alloys was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The results indicated that the element Sc was able to modify the morphology of eutectic Si phase in the experimental alloys from a coarse plate-like shape to highly branched fibrous structures, inducing the formation of twinned branches at angles of 35.25° and 54.75°. In addition, a large number of twins, stacking faults, and Al-Si-Sc nanoclusters were observed in the modified Si phase. These nanoclusters are firstly preferentially segregated at the 70.5° twin intersections and twin fronts in the Si phase, demonstrating the applicability of the impurity-induced twinning (IIT) mechanism. And secondly, they are linearly aligned along the <112> growth direction of Si, supporting the poisoning of twin plane re-entrant edge (TPRE) mechanism was also applicable. Notably, this study found that the segregation of the nanoclusters in the Si phase was the key factor in inducing its modification effect, and these clusters only induced the formation of micro-twins or stacking faults of a few atomic layers thickness in the Si phase. This was different from the IIT mechanism and the poisoning of TPRE mechanism, which proposed that the modifier atomic themselves are adsorbed in the Si, inducing the formation of overall twins to achieve the modification effect.