Microstructure and compressive properties of chill-cast Mg-Al-Ca alloys

Mg₈₂Al₈Ca₁₀ was determined to be a pseudo-binary eutectic composition [liquid solidifying into α-Mg and (Mg,Al)₂Ca in the Mg-Al-Ca ternary system with a eutectic melting temperature of 789 K]. A series of Mg_x(Al_0.44Ca_0.56)_100-x alloys, where 75 ≤ x ≤ 95, were cast into φ4 mm rods using copper mold casting. The eutectic alloy exhibits the highest fracture strength, σ_f = 609 MPa. For 75 ≤ x ≤ 79, the alloys have hypereutectic microstructures with Mg₂Ca as the primary phase, and σ_f is reduced together with diminishing plasticity. For hypoeutectic alloys with 86 ≤ x ≤ 95, the volume fraction of the primary α-Mg dendrites dispersed in the eutectic matrix increases with increasing x, resulting in a gradual decrease of the yield and fracture strengths but improved plastic strain to as large as 9%. The refined microstructures created in bulk samples via chill casting can lead to a good combination of strength and plasticity, with specific strength superior to commercial Mg alloys.