Effects of Sm on the grain refinement, microstructures and mechanical properties of AZ31 magnesium alloy

The effects of samarium (Sm) on the grain refinement, microstructures and mechanical properties of as-cast AZ31 (Mg-3Al-1Zn-0.3Mn) magnesium (Mg) alloy have been investigated. Very serious grain coarsening happens when Sm content is between 0.16% and 1.18%. This is due to both the reactions between Al and Sm which reduce the constitutional undercooling effect and the lack of Al2Sm heterogeneous nuclei. However, excellent grain refinement effect is achieved at Sm content above 2.17%, which is because the in-situ formed Al2Sm particles significantly promote heterogeneous nucleation. The main phases in AZ31-xSm alloys include α-Mg, β-Mg17Al12, Al11Sm3 and Al2Sm. The Mg17Al12 phase is gradually suppressed by the increase in Sm content, and the Al2Sm is present at a higher Sm content. Because of grain refinement strengthening and secondary phase strengthening effects, the room temperature tensile property of AZ31-3.13Sm alloy has the optimal value of YS78.7MPa-UTS216.7MPa-EL13.6%.