Fabrication And Mechanical Properties Of Particulate Reinforcing Magnesium Matrix Composites

The magnesium matrix composites, AZ91 magnesium alloy reinforced with SiC particles (average sizes are 2 μm, 10 μm and 20 μm, respectively) and nano SiO2 particles, have been fabricated using fluxless casting technique. Mechanical properties of SiCp/AZ91 and SiO2/AZ91 composites at different temperature and heat treatment conditions were determined. Microstructure was observed to investigate changes in matrix grain size, morphology of precipitates and distribution of reinforcing particles in matrix. By means of SEM and XRD, some analysis on fracture surface and phase constituent was carried out. And reinforcing mechanism in magnesium matrix composites was discussed.Experimental results showed that the addition of SiC particles could considerably improve mechanical properties of Mg alloy. Among the three kinds of composites reinforced with different sizes of SiC particles, SiC particles with the size of 2 μrn exhibited the best strengthening effect on AZ91 magnesium alloy. The SiCp/AZ91 composite had lower fracture elongation, and temperature had no significant effect on the elongation. Even in the temperature range of 200℃ to 300℃, with an increase in test temperature, the fracture elongation was a little enhanced. Mechanical properties of composites reinforced with different amount of SiO2 nano-particles were not stable. Analysis indicated that interface reaction between Mg and SiO2 should be the main reason for the undesirable strengthening effect.After adding SiC particles which homogeneously distributed along the grain boundaries, the matrix grains were obviously refined. There are two kinds of strengthening mechanism in particulate reinforcing magnesium composites. One is refinement of matrix grains caused by both heterogeneous nucleation of a-Mg on the surface of SiC particles and prohibited growth of a-Mg phase. The other is the increased dislocation density. Heat treatment, especially solution-aging treatment, could enhance mechanical properties of composites. The ultimate tensile strength of as solution-aging SiCp(2 μm)/AZ91 at room temperature was 264.3 MPa, andincreased by 27.5% compared with that of the matrix alloy. Increased dislocation in composites could promote the formation of precipitates, which could both fix dislocation and improve the interfacial structure between SiC particles and matrix alloy.Fracture surface analysis revealed that combination between matrix alloy and SiC particles was good. At low temperature, the cleavage surface and tearing ridge, which was taken as typical brittle fracture features, could be seen on fracture surfaces. With increasing test temperature, brittle features decreased, while ductile ones increased. At elevated temperatures, cleavage surface was replaced by large amount of homogeneously distributed dimples.