Different effects of SiC dimensions on the microstructure and mechanical properties of magnesium matrix composites

The effects of silicon carbide (SiC) dimensions on the microstructure and mechanical properties have been investigated by comparing SiC nanowires (SiCnw) reinforced Mg-2Zn-0.1Y(ZW20) matrix composite with the other two composites reinforced by nano-SiC (SiCnp) and submicron-SiC (SiCsp) particles. Three kinds of 0.3 wt% SiC reinforced ZW20 matrix composites (SiCnw/ZW20, SiCsp/ZW20, SiCnp/ZW20) were successfully fabricated by ultrasonic assisted with semi-solid stirring cast followed extrusion. The microstructures of the three composites were analyzed by Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscope (TEM), X-Ray diffraction (XRD) and optical microscope (OM). Then, the tensile test was performed to unravel the mechanical properties of composites. The results showed that relatively complete dynamic recrystallization (DRX) occurred in the three composites generating plentiful fine grains, the second phases proven be MgZn2 phases by EDS and XRD precipitated near grain boundaries and SiC. Among the three composites, the average grain size of SiCnw/ZW20 was the finest (only 0.37 mu m), which was mainly ascribed to the fact that one-dimensional SiCnw could hinder the growth of grains more effectively by twisting along grain boundaries. And the most complete recrystallization occurred in SiCnp/ZW20 due to the maximum number of SiCnp. Moreover, the volume fraction of precipitates in SiCnw/ZW20 was ~50% more than that of SiCsp/ZW20 and SiCnp/ZW20, relevant with more nucleation sites contributed by SiCnw. Accordingly, the microstructure given by different dimensions of SiC led to the improvements of yield strength (YS) and ultimate tensile strength (UTS). The synergy of fine-grain strengthening from DRXed grains, dislocation strengthening caused by hot deformation and Orowan strengthening originating from precipitations and SiC may primarily account for the high YS, over 440 MPa.