Study On Microstructure And Mechanical Properties Of SiC Particulate Reinforced Mg-Zn-Ca-Mn Magnesium Matrix Composites

SiCp/Mg-Zn-Ca-Mn magnesium matrix composites with average size of 10μm and three volume fractions of 5%, 10% and 15% were fabricated by compocasting method. Mg-Zn-Ca-Mn alloy was cast in the same conditions as the composites. The composites and the Mg alloy were extruded at different temperatures. The microstructures of the materials were observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The room temperature mechanical properties and high temperature tensile properties of the extruded materials were studied.The grain size of the as-cast alloy was very coarse and the precipitated phase in the matrix alloy mainly distributed along the grain boundaries, which caused the poor mechanical properties.The grain sizes of as-cast composites decreased with the increase of the volume fraction of particles. The grain size of 15vol.% SiCp/Mg-Zn-Ca-Mn composite was about 20μm, which was only 1/5 of the matrix alloy's. It was found by SEM observation that many particles at grain boundaries were coated by the large second phase, which weakened the bonding between matrix alloy and particles. As the volume fraction of particles increased, the content of the second phase decreased. TEM observation indicated that there was no chemical reaction between matrix and SiC particle. The Ca2Mg6Zn3 phase is observed at the interfaces. The yield strength, ultimate tensile strength and elastic modulus of the composites significantly increased with the increase of the volume fractions of particles while the elongations to fracture were less than alloy's.After hot extrusion, the second phase distributed as significant banded structure along the extrusion direction. Hot extrusion significantly refined the grain size of Mg-Zn-Ca-Mn magnesium alloy, and thus improved the mechanical properties of the alloy at room temperature. With the increase of extrusion temperatures, dynamic recrystallization (DRX) proceeded until it completed and the DRX grain size increased which caused that the yield strengths and ultimate tensile stresses of the extruded alloys decreased while their elongations gradually increased.Hot extrusion improved the distribution of particles and refined the grain sizes of the martrix. As extrusion temperatures increased, the particles distribution was improved and the DRX grains grew up. The content of the second phase decreased with the increase of particle in the extruded composites. Hot extrusion significantly improved the mechanical properties of the compistes. But the extrusion temperature differently influenced the mechanical properties in the composites with different volume fractions of particles. This was mainly because the mechanical properties of the extruded composites depended on the combined factors of texture, content of the second phase, grain size, and SiCp distribution and particle cracking induced by extrusion. In the extruded 5vol.%composite, the second phase were fine and distributed uniformly after extrusion at 300oC, which significantly improved the mechanical properties of the composite. In the 10vol.% composite extruded at 250 oC, unrecrystallized grains with high density of dislocations existed, which significantly increased the strengths. Both grain size and particle cracking had an important effect on mechanical properties of 15vol.% composite. The particle cracking was very significantly in 250 oC, and the grain size of the composite extruded at 350 oC was larger. Thus, the mechanical propertied of the composite extruded at 300 oC was the best in the 15vol.% composite. At the same extrusion temperature, the mechanical properties of the composite decreased with the increase of SiCp volume fractions because of the reduction of the second phase and particle cracking.With the increase of the test temperatures, the ultimate tensile strengths of the composites decreased, and their elongations increased. Their ultimate tensile strengths could still keep above 250MPa below 150 oC, so the composite materials were suitable for use below 150 oC.