Refinement mechanism of in-situ Al4C3 particles on AZ91 magnesium alloys

The effects of SrCO₃ on the microstructure of AZ91 magnesium alloy were investigated by OM, SEM, EDS, XRD and DTA analyses. The results show that AZ91 magnesium alloy with 1.0% SrCO₃ addition has the best refining effect at 730 ° for 20 min, the average size of the α-Mg grain in AZ91 matrix alloy is reduced from about 91 μm to 58 μm, with reduction of about 36%. Based on the analysis results of EDS, binding energy, and Gibbs free energy, it is evident that the grain is refined because of the generation of A1₄C₃ particles which can be used as the crystal nucleus of the magnesium alloy when SrCO₃ is added. Owing to the generation of A1₄C₃ in AZ91 magnesium alloy, the grain boundary is pined and the grain growth is inhibited. In the accession to the Al₄C₃, small subcooling temperature difference leads to the formation of fine grain microstructure in alloy melt. Fine grain microstructure in AZ91 alloy melt with the addition of SrCO₃ can be obtained on the condition of lower undercooling temperature according to the DTA analysis results.