Study on the microstructures and mechanical properties of Mg-xCa-5Zn-3Al-0.2Mn sand-casting alloy

As sand-cast microstructure of Mg-5Zn-3Al-0.2Mn magnesium alloy contains δ-Mg and τ (Mg₃₂ (Al, Zn)₄₉) phases. The τ phase is fully dissolved into δ-Mg matrix after annealed at 343°C for 17 h. Meanwhile the room temperature mechanical properties reach the maximum values, i.e., σ_b=245 MPa, δ=12%. By adding Ca into the experimental alloy, τ phase is gradually replaced by Mg-Zn-Al-Ca complex compound when Ca content is 0.43wt%, or Al₂Ca and Mg-Zn-Al-Ca compound and δ-Mg when Ca content was over 0.95wt%. With the increase of Ca content, the grain of δ-Mg is refined apparently but the room temperature mechanical properties (σ_b and δ) decrease gradually. The room temperature mechanical properties slightly increase when the Ca modified alloys are quenched after annealed at 350°C for 17 h, and meanwhile σ_b^250° and σ_0.2^250° increase with the increase of Ca content.