Sb变质对原位Mg₂Si/AZ91复合材料组织的影响

Large Mg₂Si particles in in-situ Mg₂Si particulate reinforced magnesium matrix composites seriously reduce its mechanical properties. The addition of tiny amounts of Sb can refine the Mg₂Si reinforcements, but the refinement mechanism of Sb element in Mg₂Si/AZ91 composites is not clarified yet. Effects of different Sb content on the microstructure of Mg₂Si/AZ91 composites prepared by in situ casting are studied. The size and morphology of Mg₂Si in Mg₂Si/AZ91 composites with different Sb contents are observed by optical microscope, scanning electron microscope and electron microprobe micro-analysis, and the mechanism of Mg₂Si refinement by Sb element is investigated. The average size and the shape factor of Mg₂Si particles are counted by Image-Pro Plus software. Experimental results show that with the addition of Sb, the Chinese characters-like eutectic Mg₂Si is no longer formed in the matrix and the size of primary Mg₂Si keeps decreasing. When the mass fraction of Sb is 2%, the particle size of primary Mg₂Si is refined to 13. 8 μm (a 80. 1% reduction), with a more uniform distribution. With the increase of Sb content, the shape factor of Mg₂Si tends to 1, and the morphology of primary Mg₂Si is changed from the coarse dendritic and long strip shape to small polygon. On the one hand, Sb is enriched at the front of Mg₂Si solid-liquid growth interface during the solidification, which reduces the actual precipitation temperature of primary Mg₂Si and the interfacial tension between Mg₂Si and the melt, improves the degree of undercooling and thus increases the nucleation rate of Mg₂Si. On the other hand, a large amount of Sb enriched on the surface of Mg₂Si poisons the growth step of Mg₂Si, leading to isotopic growth and better morphology.