Microstructur And Properties Of SiC Particulate Reinforced Magnesium Matrix Composites Processed By Ultrasonic Based Stir Casting

In this thesis, a novel ultrasonic method based on stir casting processing wasused to prepare magnesium matrix composites. The optimal processing parameterswere obtained for the composites with different particle contents （10vol.%micro-SiCp/AZ91and5%,15%,20%）. The composites were subject to fot extrusion andheat treatment. The microstructure and mechanical properties of as-cast and as-extruded composites before and after heat treatment were investigated using,optical microscope （OM） scanning electronic microscope （SEM） and transmissionelectronic microscope （TEM）. The processing-microstructure-propertiesrelationship of the composites were studied.Compared with traditional semi-solid stir technique，the novel method wasmore effective to distribute SiC particle. The novel method can fabricate themicro-SiCp/AZ91composites with high particle contents. It was also found thatultrasonic based semi-solid stir method was better than traditional semi-solid stirmethod through the effects of semi-solid stirring, ultrasonic treatment and stirringin liquid melting on microstructure of the compoisites. Stirring in liquid meltingafter ultrasonic treatment was necessary because it can improve particle distributionand mechanical properties. SEM investigation confirmed that SiCp distributeduniformly along the grain boundaries. TEM results show that there were noreactions in the interfaces of the AZ91matrix and SiC particlates With the increaseof volume fraction of SiC particles, the average grain sizes of the compositesdecreased, the strengths increased, and the elongations decreased.The distribution of SiC particles was evidently imporved after hot extrusion,The SiC particle distribution in the as-extruded composites was significant affectedby the particle distribution of as-cast composites. The as-extruded composites maysome defects in as-cast composites although extrusion reduced casting defects andparticle clusters. The investigation on microstructure evolution show that, duringthe hot extrusion, deformation strains significantly affected the particle distributionand dynamic recrystallization （DRX） of matrix. With the increase of deformationstrains, particle distribution was improved and the extent of DRX increased. SiCparticles evidently influenced DRX of Mg matrix. At lower strain, DRX was priorto start near SiCp. The grains were smaller near the SiC particles （DRX grains）,while the grains were larger away from the SiC particles （non-recrystallized grains）.As the deformation strain increased, DRX completed, and then the early DRXgrains grew up. The microstructure of composites was homogenization after hot extrusion. So the particle distribution of as-extrused composites became uniform,and the grains became finer. Therefore, the strength and elongation of thecompoistes were dramatically improved. The room tempreture strength ofcomposites was higher than alloy, but elongation was lower than alloy. However,strength and elongation of composites were both lower than alloy at highertempreture.After extrusion,15%and20%SiCp/AZ91composites were subject to T4, T5and T6treatment. In T5composites, two kinds of phases were observed. One wasbig and lamellate on the grain boundary, and the other was small and claviforminside the grains. After T5treatment, the strength of the composites did nto change,but their elongation decreased. After T4treatment, the precipitation was solutedinto matrix alloy. The strength and elongation of the composites were increasedslightly after T4.. A large quantity of precipitates were observed in the interior ofgrains after T6. Many nano-Mg₁₇Al₁₂were observed in T6composites. Aorientation relationship was found between Mg₁₇Al₁₂and matrix. The relationshipwas （200）p//（101）m,[012]p//[113]m. T6improved the yield strength and of theultimate tensile strength composites.