Microstructure And Properties Of SiC Particle Reinforced Al-20Si Composites Solidified Under High Pressure

SiC reinforced hypereutectic Al-Si matrix composite are widely used in automobile, aerospace and engineering applications due to the combination of excellent properties, such as low density, low coefficient of thermal expansion, high thermal conductivity, good wear resistance and corrosion resistance. Recently, high pressure solidification technique has gained favorable attention due to the unique features such as high relative density, refined microstructures, decreased experimental period. In our study, high pressure solidification technique was first used to fabricate Si Cp/Al-20 Si composite. The effects of different solidification pressures(0.0001 GPa, 1GPa, 2GPa, 3GPa) and Si C volume fractions(0, 35%, 40%, 45%) on the variation of the microstructures and phases were investigated systematically. Moreover, the precipitation behavior of the Si phase after aging treatment was analysed. Furthermore, the variation of the mechanical properties, the thermal expansion behaviour as well as thermal conductivity of the composites were studied.The effects of solidification pressures on the microstructure of Al-20 Si matrix were investigated. Primary Si phase exists after solidification at atmospheric pressure and at 1GPa, then it disappears at higher solidification pressures. The primary Si particles have polygonal and star-like shapes. The formation of primary silicon is hindered and a supersaturated dendritic α-Al phase is formed at the pressures of 2GPa and 3GPa. In addition, the morphology of the eutectic silicon changes from long acicular to short rod-like shape with increased solidification pressure. The size of eutectic Si decreases to 1.88μm when solidified under 3GPa. After the addition of the Si C particles, the morphology of primary Si transforms from coarse polygonal to fine blocky shape, the size of primary Si decreases to 11.2μm in 45%Si Cp/Al-20 Si composite. The short rod-like eutectic Si varies from lamellar to fine fibrous structure with increased Si C volume fraction.With increased solidification pressure, supersaturated Al(Si) solid solution is formed, and the concentration of Si in Al can reach 6.96 wt.% when solidified at 3 GPa, it is only 0.22 wt.% when solidified at atmospheric pressure. Spherical Si-rich zones precipitated in α-Al matrix after natural aging, the structure of the Si-rich zones was investigated. The results show that a diamond cubic crystal structure form in the supersaturated α-Al matrix. The Si-rich zones disappear and Si particles with nanoscale size are precipitated in the α-Al matrix after heating to 673 K. The DSC results show that the peak temperature and the activation energy for the Si particles precipitation decreases with increasing of the solidification pressure and the volume fraction of Si C.The relative density(RD) and mechanical properties of the matrix alloy and composites were studied. The RD increases with increased solidification pressure and decreases with increased Si C volume fraction. The RD of the Al-20 Si solidified at atmospheric pressure is 94.37%, and can reach 99.83% when solidified at 3GPa. The RD of the 35%Si Cp/Al-20 Si composite can be as high as 98.65%. As a result of solid solution strengthening, the hardness, strength of the matrix alloy increases gradually with increased solidification pressure. The hardness, the ultimate tensile strength(UTS) and the yield strength(YS)of Al-20 Si alloy solidified at 3GPa show an increase of 61.2%, 83.25%, 56.83%, respectively, compared with the alloy solidified at atmospheric pressure. The elongation(δ) increases by four times of Al-20 Si alloy solidified at 3GPa compared with the alloy solidified at atmospheric pressure. The alloy solidified at atmospheric pressure shows typical cleavage fracture, whereas, the alloy solidified at 3GPa exhibits quasi-cleavage fracture. When solidified at the same pressure, the compressive strength of the composite increases and the ductility decreases with increasing Si C contents.Effects of solidification pressure, the volume fraction of Si C particles and aging treatment on the coefficients of thermal expansion(CTEs) as well as thermal conductivity(TC) of the alloy and composite were investigated. The results show that a more obvious CTE peak in the Al-20 Si matrix alloy and composites at the temperature range from 408 K to 596 K can be obtained after high pressure solidification due to the precipitation of Si from supersaturated Al(Si) solid solution during the first heating, whereas the CTE decreases with increased solidification pressure after aging treatment at 433 K for 6h. Investigation on TC of Si Cp/Al-20 Si composite shows that lower values was obtained after high pressure solidification as a result of increased solid solubility, which increases lattice distortion and electron scattering as well as phonon scattering. A higher TC value is acquired in matrix alloy and composite solidified under 3GPa after heat treatment at 433 K for 6h because of decreased defects. The composites show lower CTE and higher TC solidified under 3GPa after aging treatment. The CTE and the TC of the composites decrease with increasing Si C volume fractions, 45%Si Cp/Al-20 Si composite solidified under 3GPa after aging treatment at 433 K for 6h shows comprehensive excellent thermal properties: The average CTE value between room temperature and 473 K is 7.8×10-6/K, the TC at room temperature is 166.8W/m·K, and is 125.7 W/m·K at 473 K.