| Mg-Al base alloys have been widely paid attention due to their high specific strength. However, wider applications in industrial productions are limited because of their low absolute strength and poor workability. Grain refinement is an effective route to improve strength and toughness of Mg-Al alloys. Up to now, general and effective refinement method and refining for Mg-Al alloy have not been developed. Relatively, carbon inoculation method is considered as the most promising one for its operation characteristics of low temperature and low cost. Among many carbonaceous inoculant, graphite carbon and MgCO3 were widely researched due to their better refining effect. Unfortunately, the similarities and differences of their refining mechanism were not deeply studied. Specially, the effect of decomposition product of MgO by MgCO3 on the grain refinement effect has not carried out in other research works. In the present paper, optimization of carbon refining process and the effects of Al and Mn elements on the grain refinement effect were firstly studied. Moreover, contrastive study on the effects of holding time on grain refinement effect and mechanical properties of AM30 inoculated by graphite carbon and MgCO3 were investigated. In order to analyze the refinging mechanism of MgCO3, refinement effect of AM30 alloy inoculated by MgO and pure magnesium inoculated by MgCO3 were studied. The refining mechanism was also discussed from the point of view of thermodynamics and crystallography. The main research results were obtained in the present work as follows:The optimal refinement process of graphite was 0.2%C, operating temperature of 760℃ and the holding time of 30min. Mn content had no obvious effect on the grain refinement. However, the effect of Al content on grain refinement was significant. Only the Al content was more than 2%, significant refinement effect of graphite carbon could be achieved. The optimal addition amount of MgCO3 was 1%.For AM30 alloy, the grain refinement effect of graphite carbon inoculation was better than that of MgCO3. The higher grain refining efficiency could be obtained when the holding time was 20-30min. As for MgCO3, higher grain refining efficiency could be quickly obtained for a shorter holding time of 2min. With the prolonging of holding time, the fading behavior of MgCO3 was more obvious than that of graphite carbon.Tensile strength and fracture strain of AM30 alloy were improved after being inoculated.MgO could effectively refine the AM30 alloy,and its refinement effect and inoculation fading behavior were better than those of MgCO3.MgCO3 couldd not effectively refine pure magnesium.Al is an indispensable element in to refine Mg alloys by MgO and MgCO3 inoculations.Al-C-rich and Al-Mn-rich particles as well as the particle with duplex phase structure of A14C3 coated on Al-Mn were observed in AM30 alloy inoculated by graphite carbon.Al-C particles and the particle with duplex phase structure should be the effective nucleating substrates of α-Mg.In addition to A14C3 particles and the particle with duplex phase structure,Mg-Al-O particles were also observed AM30 alloy inoculated by MgCO3.Only Mg-Al-O particles were observed in AM30 alloy inoculated by MgO.These Mg-Al-O particles should be effective nuclei of α-Mg.These Mg-Al-O particles should be MgAl2O4.The calculation results by edge-to-edge model showed that there were no orientation relationships（ORs）between MgO and Mg matrix.There were two ORs between MgAl2O4 and Mg matrix: （<11（?）0>Mg//<110>MgAl2O4,{10（?）1}Mg//{111}MgAl2O4） （<11（?）0>Mg//<110>MgAl2O4,{10（?）1}Mg//{111}MgAl2O4）Interatomic spacing misfit of<11（?）0>Mg//<110>MgAl2O4 and interplanar spacing mismatches of{10（?）1}Mg//{111}MgAl2O4)were 1.78％and 2.80%,respectively.In AM30 ally inoculated by MgCO3,the active C atoms decomposed from MgCO3 reacted with Al should be effective nucleating substrates.In addition,MgAl2O4 particles formed by decomposition products MgO with Al should also act as nuclei of α-Mg. |
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