Effect Of Antimony On Microstructure Of Magnesium And Magnesium Alloy

Magnesium and magnesium alloys, as the lightest structural metallic materials, have been widely applied to military and civil fields. The mechanical property of magnesium alloy is poor because of the wide crystallizing range, the big shrinkage ratio and the large grain size. In this thesis, the effect factors of the content of antimony, smelting temperature and stillness time on the microstructure of pure magnesium and magnesium alloy were investigated. The antimony was added by antimony oxide in the process of smelting. The mechanism of microstructure refinement was also discussed based on the nucleation and the growth of metals. The main research work and conclusions are presented as follows:(1) Antimony existed in AZ91D magnesium alloy with two forms of solution phase and new phase according to the analysis results of SEM (scanning electron microscopy) and EDS (energy dispersive spectrometer). The result of TEM analysis indicated that the new phase was Mg₃Sb₂ particle, which has the close-packed hexagonal structure.(2) As the weight percent of antimony was 0.69%, the microstructure of AZ91D magnesium alloy was refined effectively, which the grain size was reduced from 128μm to 62μm, and the fragile β phase also decreased. When the smelting temperature reached 800℃, the microstructure of AZ91D magnesium alloy contained antimony was more uniform than that at 720℃ and 850℃. In addition, the experimental result showed that the antimony had the permanent effect on the microstructure of AZ91D magnesium alloy.(3) The reason for the refinement of the microstructure of AZ91D magnesium alloy by Sb was that Mg₃Sb₂ was not only the heterogeneous nucleus of the crystal but also the countercheck of the crystal growth. Mg₃Sb₂ had the same close-packed hexagonal structure as a-Mg, which conformed to the fundamental of heterogeneous nucleation, and was the favorable nucleating center. Mg₃Sb₂ precipitation, which had the higher melting point, was formed before any other phases and restrained the growth of the grain. Meanwhile, the solid solubility of aluminium in a-Mg was improved by the addition of antimony, which resulted in the reduction of |3 phase.(4) The microstructure of pure magnesium has not been refined by the addition of the same antimony as AZ91D magnesium alloy. When pure magnesiumcontained 0.21% Sb, antimony existed in the solution phase, which showed that the solid solubility of Sb in pure magnesium was bigger than in AZ91D.