Mg-3%Al alloy was refined by carbon inoculation combining with 0.2%Ca addition. High grain refining efficiency was obtained and the synergistic refining mechanism was deeply discussed in the present study. Al–C–O particles, actually Al4C3 particles, were formed in the carbon-inoculated Mg-3%Al alloy acting as nuclei for α-Mg grains. Ca addition had no obvious effect on size distribution of the nucleating particles. Ca segregation was proved on Al4C3 particles, which should reduce the interfacial energy of nuclei/Mg. The constitutional undercooling in front of nucleus/liquid was increased from 0.12 °C to 0.15 °C induced by 0.2%Ca addition. The synergistic grain refining efficiency can be attributed to the higher constitutional undercooling and lower the interface energy of nucleus/Mg induced by Ca addition. More nucleating particles with small size could be activated acting as potent nuclei of α-Mg grains. Consequently, Mg-3%Al alloy could be effectively refined due to the synergistic effect induced by carbon inoculation combining with Ca.
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