Microstructure And Mechanical Properties Of N-SiCp/Mg-9Al Composites Fabricated By Ultrasonic Assisted Hot Pressing Of Powder Under A Semi-solid State

Magnesium matrix composites reinforced with SiC nanoparticles have become a competitive lightweight and high-performance structural materials due to their low density,high strength and rich resources.However,it is extremely challenging for conventional synthesis and processing methods to distribute and disperse high-content SiC nanoparticles uniformly in magnesium and its alloys.Hot pressing of powder is an attractive method of achieving a uniform distribution and dispersion of dense SiC nanoparticles in magnesium matrix.Accordingly,the experiments described in the present work were designed to synthesize n-SiCp/Mg-9Al magnesium matrix composite with the combination of excellent strength and ductility by hot pressing of powder assisted ultrasonic vibration under semi-solid state.Systematic attempt was made to evaluate the effect of process parameters on microstructure and mechanical properties of magnesium matrix composites reinforced with SiC nanoparticles.X-ray diffractometer(XRD),optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)wereemployed to observe the microstructures of n-SiCp/Mg-9Al magnesium matrix composite with different mass fractions fabricated at different hot-pressing temperatures.The microhardness of the nanocomposites were measured by microhardness tester.The tensile properties of n-SiCp/Mg-9Al composites prepared under different process parameters were measured by electronic universal testing machine at room temperature and 200?.The results showed that most of the n-SiCp were evenly dispersed in the n-SiCp/Mg-9Al composites fabricated by hot pressing of powder with ultrasonic vibration under a semi-solid state.However,the density of the composite was increased significantly and the agglomeration of SiC nanoparticles was evidently reduced with the increase of the hot-pressing temperature from 450?to 510 ?.Additionally the elevated hot-pressing temperature resulted in remarkable grain refinement.However,as the hot-pressing temperature increased to 530?,the density decreased and the average grain size increased.The study of the interface between the n-SiCp and the matrix in the nanocomposite suggested that n-SiCp bonded well with the matrix without interfacial activity.With the increase of hot-pressing temperature,the hardness,strength and elongation to fracture of the n-SiCp/Mg-9Al composites first increased and then decreased.The n-SiCp/Mg-9%Al composites fabricated at the hot-pressing temperature of 510 ? achieved the optimal mechanical properties and the composites exhibited good high temperature plasticity at200 ?.The addition of n-SiCp reduced the cross-section of the Al element to the interior of the magnesium particles and prolonged the diffusion path of the Al element,which resulted in hindering the diffusion of the Al element into the interior of the magnesium particles.When the mass fraction of n-SiCp gradually increases,the inhibition of n-SiCp on the diffusion of Al element is gradually enhanced.With the addition of SiC nanoparticles,grains of matrix in the n-SiCp/Mg-9Al composites exhibit obvious refinement compared with the Mg-9Al alloy and the density of the nanocomposites was found to decrease with the increase of SiC nanoparticles content.The mechanical properties of the n-SiCp/Mg-9Al composites first increased and then decreased with the increase of Si C nanoparticles content.7.5 wt% n-SiCp/Mg-9Al composites achieved the optimal mechanical properties.The hardness,ultimate tensile strength,yield strength and elongation to fracture of the nanocomposites were improved by141.8%,67.5%,45.9% and 112%,respectively,compared with that of the Mg-9%Al alloy.This improvement could be attributed to obvious grain refinement,the Orowan strengthening mechanism and load transfer effect.