| In this research, a novel ultrasonic cavitations based stir casting processing method was developed to produce high performance 1%nano-SiCp/AZ91 magnesium matrix nanocomposites (MMNCs). Optical microscope (OM), scanning electronic microscope (SEM) and transmission electronic microscope (TEM) were employed to observe microstructure and interface structure of as-cast and as-extruded composites. The microstructure evolution of particle and matrix during extrusion were also investigated. The mechanical properties of as-cast and as-extruded composites at elevated and room temperatures were studied.High performance micro-size SiCp/AZ91 MMC can be fabricated by semi-solid stir method while nano-size SiCp/AZ91 MMC can be fabricated by ultrasonic cavitation based semi-solid stir method. The nano-SiC particles show a uniform distribution within the grains of composite while some nano-SiC particles are still segregated at a microscopic scale near grain boundary region. Nano-SiC particles are very stable in the AZ91 melt, and there are no chemical reactions taking place in the interfaces between matrix and SiC particles.With the combine effect of ultrasonic cavitation and nano-SiC particles, the grain size of composite is decreased significantly, and the area of massive second phase which is favorable to improving the mechanical properties of the composites is increased. Both of these are the unique properties of MMNCs, different from the traditional MMCs.Using TEM, both matrix and massive second phase, Mg17Al12, show the orientation relationship with nano-SiC particles. Also, the distribution of Mg17Al12 is similar to that of nano-SiC. So nano-SiC will be the nucleus for the matrix and massive second phase in the process of composites'solidification, which means the number of crystal nucleus is increasing. Also, the grain growth will be prevented because that the grain boundary can be pinned by nano-SiC particles, resulting in the refined grains. Thus, the strength and ductility of 1%nano-SiCp/AZ91 MMNC are simultaneously improved. The varieties of the tensile strength of alloys and composites with ultrasonic cavitations are very similar to the change of the number of massive second phase in materials, indicating that the massive second phase is critical to the strength improvement of alloys and composites.With increasing the volume content of the SiC particles, the grain size of composites is decreased while the number of nano-SiC particles agglomeration is increased. Due to nano-SiC particles agglomerations the elasticity modulus and yield strength increase while the tensile strength and elongation decrease. Also, the number of the massive Mg17Al12 is increased with the increase of the number of nano-SiC particles agglomeration which indicates that this massive second phase must be nucleated with the single nano-SiC particle, and the agglomeration could restrict the precipitation of massive Mg17Al12.Hot extrusion can be used to further improve particle distribution and eliminate particle segregation in the as-cast composites. Higher extrusion temperatures and larger extrusion ratios are favorable for improving particle distribution. Nano-SiC particles can pin grain boundary effectively, and this prevents the growth of grain together with the dislocations released by nano-SiC particles pinning in room temperature, resulting in smaller grain size of the composites with a extrusion temperature of 350℃compared with that of 250℃and 300℃. With increasing the extrusion temperatures, the strength increases and ductility decreases in the AZ91 alloy while both higher strength and ductility increases in the composite, which means nano-SiC particles can significantly improve the mechanical properties of composites. With increasing the volume content of the SiC particles, elasticity modulus and yield strength of the composites increase while tensile strength and elongation decrease, which are consistent with the as-cast composites. |
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