Effect of Al addition on the grain refinement and mechanical properties of as-cast Mg–5Y–4Sm alloys

In recent years, the utilization of Al as a grain refiner in Mg–RE alloys has gained widespread attention because of its advantages such as low cost. In this study, Mg–5Y–4Sm–xAl (x = 0, 0.6, 1.2 and 1.8 wt.%) alloys were fabricated, and high grain refinement efficiency of Al addition was obtained. Subsequently, the effect of different Al addition on the grain refinement was investigated and the refinement mechanism was discussed deeply in two major aspects. Firstly, Al2RE particles were formed in the solidification process and acted as the nucleation sites of α-Mg. The numerical density and diameter of those particles were obviously increased by increasing Al addition. Secondly, the addition of Al reduced the solute elements content in the α-Mg melt, resulting in a decrease in the grain growth restriction factor (Q value). Furthermore, the synergistic effect of solute elements and nucleation particles on grain refinement was described by the interdependent theory. Finally, the Al addition is beneficial to improve the mechanical properties of alloy. When the Al addition was 1.8%, the room temperature tensile strength of the alloy reached a maximum of 221 MPa and the maximum elongation was 9.2%. When the Al addition was 1.2%, the yield strength of the alloy reached a maximum of 142 MPa.