Effect of in situ Mg.sub.2Si.sub.p contents on microstructure and mechanical properties of Mg.sub.2Si.sub.p/AZ91D composites

The in situ Mg.sub.2Si nanoparticles (Mg.sub.2Si.sub.p)-reinforced AZ91D composites were fabricated by powder thixoforming that involved introduction of Si nanoparticles (Si.sub.p) by ball milling followed by partial remelting and thixoforming. The results indicated that Si.sub.p can be homogeneously dispersed on the surface of AZ91D powders, while they will be clustered when their amount exceeded 0.91 vol%. The thixoformed microstructure of the composites consisted of spheroidal primary [alpha]-Mg particles, intergranular secondary solidified structures (SSS) and in situ Mg.sub.2Si.sub.p that dispersed in the SSS. The tensile strength and elongation of Mg.sub.2Si.sub.p/AZ91D composites increased with the increased content of Mg.sub.2Si.sub.p, but the improved degree in the mechanical properties was reduced when its content exceeded 3vol%. The 4.5 vol.%Mg.sub.2Si.sub.p/AZ91D composite has the excellent mechanical properties, achieving an ultimate tensile strength (UTS) of 288 MPa, yield strength (YS) of 205 MPa and elongation (É.sub.f) of 6.9%, which were increased by 50% and 48.6% and decreased by 9.2%, respectively, compared to those of the AZ91D alloy. The strong Mg.sub.2Si.sub.p/Mg interfacial bonding and homogeneously distributed Mg.sub.2Si.sub.p were beneficial to activating the combined strengthening effects of thermal mismatch and Orowan mechanisms, improving the mechanical properties of the Mg.sub.2Si.sub.p/AZ91D composites. However, the Mg.sub.2Si.sub.p clustering degraded the strength of the composites with higher Mg.sub.2Si.sub.p content.