Effects of trace Be and Sc addition on the thermal stability of Al-7Si-0.6Mg alloys

In the present study, the effects of trace amounts of beryllium (Be, 0.05wt%) and scandium (Sc, 0.04wt%) addition on the microstructures and thermal stability of Al-7Si-0.6Mg alloys were investigated. The results show that traces of Be and Sc significantly reduce the amount of the iron-bearing phase and the interdendritic shrinkage. Be transformed the acicular iron-bearing phases into the nodular Al-Fe-Si iron-bearing phase, which is less harmful to ductility. Moreover, the addition of Be increased the Mg content of the solid solution within the matrix, prompting greater precipitation of the metastable Mg₂Si phase after T6 heat treatment and effectively enhancing the mechanical properties of the alloy. However, during the following thermal exposure at 250°C for 100h, the metastable Mg₂Si phase grew into the coarse β-Mg₂Si equilibrium phase, resulting in a decrease in the mechanical strength of the alloy. Meanwhile, the addition of Sc had insignificant effect on the amount of metastable Mg₂Si phase that precipitated. However, here, the iron-bearing phase was a nodular Al₁₂Si₆Fe₂(Mg,Sc)₅ phase, which significantly enhanced the density of the castings. After the same thermal exposure procedure, it was remarkably found that the precipitation of fine Al₃Sc particles effectively inhibited grain growth and hindered the movement of dislocations. These factors led to the Sc-containing alloy having better mechanical properties (strength and ductility) than the alloys without Sc or with Be during the following thermal exposure at 250°C.