The effect of Laves phases (C15-Al₂Ca, C36-(Mg, Al)₂Ca) on tensile strength was investigated for the Mg-5.0Al-1.5Ca-0.3Mn alloy produced by die-casting. The die-cast specimen was aged at 523 K for 10 h to precipitate the C15 phase within the α-Mg grains, and the aging treatment at 623 K for 100 h was carried out to divorce the interconnected skeleton of C36 phase. The tensile tests were conducted at temperatures between 298 and 473 K. The superior tensile strength is obtained for the as die-cast and 523 K/10 h-aged specimens relative to the 623 K/100 h-aged specimen at 473 K. The interconnected skeleton of C36 phase acts as a barrier of dislocations which glide in the α-Mg grains, and prevents the dislocation recovery at dendrite boundaries during high-temperature deformation. The precipitation of the C15 phase within the α-Mg grains increases the tensile strength at temperatures below 373 K, while it is not effective to enhance the tensile strength at 473 K. It is deduced that dislocations are easy to climb and overcome the C15 precipitates at 473 K, while the dislocation climb is negligible below 373 K.

This study systematically investigated the solidification microstructures of Al-Si-Fe and Al-Si-Mn ternary alloys solidified at various cooling rates ranging from 0.3 to 145°C・s^‒1. The thermodynamic calculations assessed three alloy compositions of Al-5%Si-1.5%Fe, Al-8%Si-1%Fe, and Al-5%Si-1.5%Mn exhibiting two-phase eutectic reactions of α-Al and intermetallics (β-Al₅FeSi, α_h-Al₇Fe₂Si or α_c-Al₁₅Mn₃Si₂) phases in solidification. The needle-shaped β phase was formed in the Al-Si-Fe ternary alloy. The microstructural characterizations revealed that the α_c phase (rather than α_h phase) and Si phase were dominantly formed in eutectic reactions. The α_c phase with Chinese-script morphology was formed in the Al-5%Si-1.5%Mn ternary alloy. The dendritic α-Al phase was dominantly formed as the primary solidified phase, and the intermetallic phases were subsequently solidified at lower temperatures, which was independent of the cooling rate in solidification. The observed microstructures can provide insights into the solidification sequences, which were consistent with exothermal reactions detected by differential scanning calorimetry measurements at controlled cooling rates in solidification. The solidification sequence may not follow the calculated liquidus projections and its associated Scheil solidification simulations. The difference between experiments and calculations would be associated with the thermodynamic stability of the constitute phases below the solidus temperatures of Al-Si-Fe and Al-Si-Mn ternary systems.