Effect of Sm on Microstructures and Mechanical Properties of Mg-4Al-0.5Si-0.2Mn Alloy

The effect of Sm content (0, 0.2%, 0.5%, 0.9%, mass fraction) on microstructure evolution and mechanical properties of Mg-4Al-0.5Si-0.2Mn alloy at room temperature was investigated. Transmission electron microscopy (TEM) observation and thermodynamic calculation analysis by Pandat software were used to elucidate the Sm modification mechanism of the coarse Chinese script type Mg₂Si phase. The results show that Mg-4Al-0.5Si-0.2Mn alloy mainly consists of ɑ-Mg, Mg₂Si and Mg₁₇Al₁₂ phases, the Chinese script type Mg₂Si phase can be refined significantly by adding 0.2% or 0.5% Sm. When the content of Sm increases to 0.9%, the morphology of Mg₂Si transforms to rod-like from Chinese script, and its size is further reduced. Furthermore, the addition of Sm leads to the formation of Sm-rich phases in the alloy microstructure. The modification mechanism of Mg₂Si with Sm addition is elucidated as follows: Sm atoms are enriched at the solid-liquid interface of Mg₂Si crystal, forming a compositional undercooling, effectively inhibiting the growth of the Chinese script type Mg₂Si phase. Compared with the Mg-4Al-0.5Si-0.2Mn alloy with no Sm addition, the ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) of Mg-4Al-0.5Si-0.2Mn-0.9Sm alloy are increased from 173.8 MPa, 106.8 MPa and 3.5% to 238.2 MPa, 152.9 MPa and 5.1%, respectively, this is mainly attributed to the improvement of the morphology and refinement of the size of Mg₂Si phase.