| Carbon inoculation is considered as the best grain refining method for aluminum-bearing magnesium alloys. But untill now there are no commercially available carbon-containing inoculants. This is mainly because the grain refining mechanism of carbon inoculation has not yet been revealed.A number of hypothesises have been proposed to explain the grain refinement of carbon inoculation. Among them, the Al4C3 nuclei hypothesis, which considers that the AI4C3 formed by reaction of Al and C can act as the nucleis of primary Mg crystals, has been commonly accepted. But there are still many questions to be answered about the Al4C3 nuclei hypothesis. Furthermore, the effect of impurities such as Mn, Fe, et al. on grain refinement of carbon inoculation has been studied since the Al4C3 nuclei hypothesis was proposed. But untill now the role of impurities is still not identified mainly because the effect of impurities on grain size of Mg-Al alloys has not yet been clarified. On the other hand, native grain refinement occurs in high-purity Mg-Al alloys, which implies that there may be some intrinsic relationship among native grain refinement, the role of impurities, and grain refinement by carbon inoculation.In this study, grain refining efficiency of common carbon-containing inoculants like C2CI6, Al4C3, SiC was investigated, and the possible mechanism of carbon inoculation was discussed. The effect of Mn or Fe on grain size of Mg-Al alloys was also investigated based on native grain refinement of high-purity Mg-Al alloys through adding Mn or Fe into melts directly. The relationship among native grain refinement, the role of impurities, and grain refinement of carbon inoculation was analyzed. Based on these results, the thermodynamic parameters during solidification of Mg-Al alloys, such as the radius of nuclei, wetting angles, the nucleation efficiency, et al. were investigated, and the relationship between these parameters and grain size of Mg-Al alloys was analyzed. The "crystal separation" phenomenon (CSP) plays an important role during solidification of high-purity Mg-Al alloys. So the effect of CSP on grain size of Mg-Al alloys was also analyzed. The main conclusions are as follows:(1) Grain refining efficiency of C2C16, Al4C3, SiC is increased by increasing inoculating temperature, and when the inoculating temperature is above 760℃, the grain size of AZ91 alloy can be decreased to 100μm. The grain refining efficiency of C2Cl6 is not affected by inoculating time, but the grain refining efficiency of Al4C3 or SiC can be improved by increasing the inoculating time, and if the inoculating time is long enough, the grain size of AZ91 alloy can also be decreased to 100μm by Al4C3 or SiC.(2) When Al4C3 or SiC particles are directly added into melts, these particles can not directly act as nucleis of primary crystals because these particles are pushed to the solid/liquid interface. Satisfactory grain refinement can still be obtained by adding Al4C3 or SiC particles. The possible reason for the grain refinement stems from the release of carbon by these particles.(3) The possible mechanism of carbon inoculation is:carbon released from some carbon-containing substances firstly is dissolved and then precipitated to form carbon-containing nucleis in subsequent cooling and solidification process.(4) Grains of Mg-Al alloys can be coarsened by a small amount of Mn or Fe, but when the Mn or Fe content is high enough to precipitate Al-Fe or Al-Mn-Fe particles, the grain size can be markedly decreased because these particles can act as nucleis during solidification. The possible mechanism of Mn or Fe affecting grain size of Mg-Al alloys is:the interaction between Mn or Fe and carbon-containing nucleis decreases the potency of nucleis.(5) Mn or Fe has great effects on grain refining efficiency of carbon inoculation. When the Mn or Fe content is relatively low, the interaction between Mn or Fe and the nucleis is weak, and can be neglected. On the other hand, if the Mn or Fe content is relatively high, the grain refining efficiency of carbon inoculation can be seriously deteriorated because the nucleis can be poisoned by the Mn (or Fe), which can seriously degrade the potency of nucleis.(6) Thermodynamic parameters of solidification are analyzed. It is demonstrated that wetting angles are decreased and the nucleation efficiency during solidification is increased by carbon inoculation. For high-purity Mg-Al alloys, the wetting angles are increased and the nucleation efficiency is decreased by Mn or Fe addition, and subsequent carbon inoculation can decrease the wetting angles and increase the nucleation efficiency again. It is demonstrated that the radius of carbon-containing nucleis formed by carbon inoculation is only 20-40nm. Thus the carbon-containing nucleis can be easily dissipated during sample preparation. For this reason, it is extremely difficult to find them in microstructures.(7) CSP in binary magnesium alloys is studied. It is indicated that grain refinement by CSP is more notable in Mg-Al alloys than in Mg-Zn or Mg-RE alloys. So CSP may be an important factor in native grain refinement of high-purity Mg-Al alloys. CSP can be influenced by Mn or Fe, which may be the reason for Mn or Fe to affect grain size of Mg-Al alloys.The originalities of this study are as follows:(1) The common hypothesis "carbon-containing nucleis are formed by the reaction of carbon and aluminum" was renewed experimentally and thermodynamically, and a new hypothesis was further proposed:"carbon firstly is dissolved in the melt and then precipitated to form carbon-containing nucleis". (2) The role of impurities during solidification of Mg-Al alloys was investigated based on native grain refinement. The relationship among native grain refinement, the role of impurities, and grain refinement of carbon inoculation was established. (3) The role of "crystal separation" phenomenon during solidification of Mg-Al alloys was studied, and a semiquantitative relationship between CSP and grain size was established. It was suggested that CSP was an important factor to affect grain size of Mg-Al alloys.
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