Effects of Ca addition on the microstructures and mechanical properties of Mg-5Al-0.4Zn based casting alloy

In the sand-casting magnesium alloys Mg-5Al-0.4Zn-0.2Mn-Sr-Ti-xCa, existed α -Mg, (α + Al₂Ca) eutectic and Mg₁₇Al₁₂ when x = 0.37, 0.70, 0.93, 1.69, or α-Mg and (α +(Mg, Al)₂Ca) eutectic when x = 2.93. With increasing x, the room temperature mechanical properties (σ_b, δ) of the alloys decreased, meanwhile the strength at 200°C increased first and then decreased slightly, and the elongation decreased rapidly. The yield strength at 200°C (σ_2,200°c) of the alloys was greatly improved with the increase of x value. DSC analysis proved that the thermal stability of Mg₁₇Al₁₂ phase was improved by dissolution of Ca. The grain boundary strength at 200°C was improved through the formation of the Al₂Ca or (Mg, Al)₂Ca phases at the grain boundary. TEM analysis shows that the dislocations slip not only at the basal slip plane (0001), but also at the (101̄0) prismatic slip plane and (1̄1̄21) pyramidal slip plane at 200°C. The dislocation can be piled up by Al₂Ca phases at the grain boundary resulting in the higher yield strength of the alloy containing Ca than those without Ca at 200°C.