| Summary: | AM30 was inoculated by MgCO3 powder with different holding time. The influence of MgO decomposed by MgCO3 on the grain refinement effect was mainly discussed in the present study. Three sets of comparative samples were prepared. They were AM30 alloy inoculated by MgO and pure Mg inoculated by MgO and MgCO3. The possible nucleating particles were observed and analyzed by EPMA and SEM. AM30 alloy could be effectively refined by either MgCO3 or MgO inoculation. Grain refining efficiency and fading effect of MgO inoculation were better than those of MgCO3 inoculation. However, pure Mg could not be refined by these two inoculants. Al is an indispensable element to determine the grain refinement of Mg alloys inoculated by either MgCO3 or MgO. MgO should not be the effective substrates for α-Mg phase. A novel grain refining mechanism of MgCO3 inoculation on AM30 alloy was proposed by combining experimental results with theoretical calculation, i.e., MgAl2O4 should be the potent nuclei of α-Mg grain for the AM30 alloy in addition to Al4C3. Keywords: AM30 alloy, Grain refinement, Magnesium carbonate, Nucleating substrates
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