| The stir casting processing, hot compression and extrusion behaviors of SiCp/AZ91 magnesium matrix composites were studied in this paper. Optical microscope, scanning electronic microscope (SEM) and transmission electronic microscope (TEM) were employed to observe microstructure and interface structure of as-cast composites and to investigate the microstructure evolution of particle and matrix during hot compression and extrusion. The deformation mechanism for hot compression was analyzed and discussed. The fracture mechanism of as-cast composites was studied using in-situ tensile technique. The evolution of extrusion textures was investigated using neutron diffraction. And the mechanical properties of as-cast and as-extruded composites were studied.Stir casting in liquid condition is not suitable for fabrication of magnesium matrix composites, but compocasting method is very fit. Compocasting with vortex formation is able to fabricate fine particle reinforced magnesium matrix composites with uniform particle distribution and low porosity. Most SiCp are segregated at a microscopic scale near grain boundary regions, which is typical"necklace-type"particle distribution for metal matrix composite fabricated by stir casting. SiCp are very stable in the AZ91 melt, and chemical reactions don't take place at the interfaces. The yield strength (YS) of as-cast composites increases with decreasing particle size and increasing volume fraction. The elastic modulus of composites increases as volume fraction increases, but particle size doesn't have significant effect on elastic modulus. The investigations using in-situ SEM tensile technique reveal that"necklace-type"particle distribution results in that the dominant microcrack nucleation mode is interface decohesion in particle segregation regions, and that microcrack propagation tends to pass through particle segregation regions. The"necklace-type"particle distribution, which results in the weak interface between SiCp and matrix, is the main reason for the low ultimate tensile strength (UTS) of as-cast composite fabricated by stir casting, so it is necessary to employ hot deformation to improve particle distribution and interface bonding.According to the power law equation modified by threshold stress, the true activation energy for hot compression deformation of composite is calculated to be 95 kJ/mol, and stress exponent n=5, which indicates that the controlled deformation mechanism is dislocation climb controlled by grain boundary diffusion. Our compression experiment data agree with the slip band model established by W.D. Nix et. al, which is a model for creep based on the climb of dislocation at grain boundaries. Microstructure investigations have demonstrated that SiCp and their"necklace-type"distribution make the composite under study more suitable for the slip band model. A modified slip band model, which is based on the deformation condition of the composite under study, has been established. Compression temperature and strain rate have significant influence on dynamic recrystallization (DRX), dislocation and twins in the matrix. The results of room temperature compression experiments show that dislocations pile up in the matrix near particles, and particle deformation zones (PDZ) which are preferential site for DRX nucleation are formed. During hot compression, DRX takes place first in the regions near particle segregation at the original grain boundaries, and then extends to the interior of original grains. The"necklace-type"particle distribution leads to the"necklace"DRX mechanism of the composites.Hot extrusion can eliminate particle segregation in as-cast composites and improve particle distribution. Higher extrusion temperatures and larger extrusion ratios are favorable for improving particle distribution. SiCp can be cracked during extrusion. Particles are easy to be cracked when low extrusion temperature and large extrusion ratios are employed and the particle size is large. What's more, particle cracking is sensitive to the local particle content. SiCp can stimulate DRX nucleation and lower DRX temperature of matrix. SiCp influence the growth of DRX grains during extrusion in two ways: SiCp promote the growth of DRX grains, whereas SiCp hinder grain growth when DRX grains grow to the point to get in touch with the surfaces of SiCp.As-extruded alloys and composites exhibit (1010) fiber texture. When SiCp is introduced into a Mg alloy, the main component of texture is not modified, but the intensity of texture evolves with the SiCp content with two ways: in the composite with 5% SiCp the intensity of Mg is higher than in the non-reinforced alloy, whereas for an SiCp volume fraction higher than 10% becomes more and more isotropic. The intensity of Mg texture doesn't vary monotonically with particle size, and there is a peak of intensity when particle size is 10μm.Hot extrusion significantly improves the mechanical properties of composites. In temperature range from 250 to 350℃, the YS and UTS of composites increase with increasing extrusion temperature. The mechanical properties of 350R5 composites are lower than that of 350R12 composites. The variation of mechanical properties of AZ91 alloy with extrusion temperature and ratio is contrary to that of composites. The evolution of microstructure and texture in matrix during extrusion is not the leading factor of the mechanical properties of as-extruded composite, and the leading factor is the evolution of SiCp during extrusion. Both YS and UTS increase evidently with increasing SiCp volume fraction and decreasing particle size.
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