Modification mechanism of Sm on the microstructure of Mg-8Al-0.3Zn alloy

A systematic investigation into the impact of samarium (Sm) addition (x = 0.2, 0.5, 1.0, 2.0, and 3.0 wt%) on the microstructure formation in a cast Mg-8Al-0.3Zn (wt.%, AZ80) alloy was performed and the modification mechanism was revealed. Adding 0.2 wt% Sm causes α-Mg grains coarsening (from ∼192 μm to ∼327 μm) due to phase transition from Al-Fe-C-O phase to Al-Fe-Sm-C-O phase, the latter showing weaker nucleation-promoting ability. The addition of 1.0 wt% Sm led to the concurrent refinement of both α-Mg grains and the β-Mg₁₇Al₁₂ phases. The α-Mg grain size decreased to ∼105 μm and coarse β-Mg₁₇Al₁₂ phases with continuous net-work structure were refined to fine discontinuous rod-like or spherical morphology. The reduced grain size is primarily ascribed to in situ formed Al₂Sm phases that could serve as active nucleation sites for α-Mg. The notable β-Mg₁₇Al₁₂ refinement is attributed to the formation of Al₂₀Sm₄ phases, which promote the heterogeneous nucleation of β-Mg₁₇Al₁₂. After adding 1.0 wt% Sm, the alloy demonstrated superior mechanical properties, achieving the ultimate tensile strength (UTS) of ∼220 MPa and elongation to failure (EF) of ∼11 %, which is due to the significant concurrent refinement of both primary and eutectic phases. However, when the amount of Sm reached 2.0 or 3.0 wt%, the ductility was declined owing to the coarsening of the Al₂Sm phase. This study could shed light on the design of strong and ductile Mg-Al alloys, especially with high Al content for lightweight structural material applications.