The In-situ Aluminum Matrix Composites Fabricated By The Al-SiO₂-C(Mg) Systems

In this paper, the Al-SiO2-C(Mg) systems of aluminum matrix composites have been successfully fabricated by means of the Exothermic Dispersion (XD). The reaction mechanism of the Al-SiO2-C(Mg) systems were analysed by method such as thermodynamic calculation, optical microscope, DSC curve, XRD (X-ray diffraction), SEM (scanning electron microscopy); the activation energy value of the Al-SiO2-C(Mg) systems was calculated by DSC curve with different heating rates and the Kissenger equation; the mechanical performance was tested and its fracture mechanism was analysed.The study of the reaction mechanism showed that:when C/SiO2 molar ratio of the Al-SiO2-C system was 0, the reinforcing phases wereα-Al2O3 grains and Si elemental which had two kinds of morphologies with thick primary silicon and dendritic eutectic silicon, when the temperature reached 1090K. When C/SiO2 molar ratio was 0.5 and 1, the reinforcing phases of Al-Sio2-C system wereα-Al2O3 grains, fine particle SiC, and dendritic eutectic Si. With the carbon added into the system, SiC was produced in the matrix, and the growth of the thick primary silicon was inhibited. When C/SiO2 molar ratio was 0,0.5 and 1, the activation energy value of Al-SiO2-C system was 235 kJ·mol-1,313.7kJ·mol-1 and 203.7 kJ·mol-Al-SiO2-Mg system had two step reactions. The first step, the intermediate products, which have been composited by MgO, Si, MgSiO3 and Mg2SiO4, were created at 850K; the second step, the final products, which contained large particles MgAl2O4, fine particle Mg2Si and MgO, and dendritic eutectic Si, were created at 1000K. With the magnesium added into the system, the growth of the thick primary silicon was also inhibited. When Mg/SiO2 molar ratio was 1 and 2, the first step of the activation energy value of Al-SiO2-Mg system was 229 kJ·mol-1 and 218 kJ·mol-1, and the second step of the activation energy value was 256.3 kJ·mol-1 and 125.3 kJ·mol-1.The research of performance testing showed that:the reinforcement volume fraction of the Al-SiO2-C(Mg) systems was 30%. As C/SiO2 molar ratio grew from 0 to 0.5 and 1, the tensile strength of the composites increased from 134.5MPa to 150.7MPa and 208.5MPa, while the elongation rate rose from 3.95% to 4.07% and 4.20%; as Mg/SiO2 molar ratio grew from 0 to 1 and 2, the tensile strength of the composites increased from 134.5MPa to 141.9MPa and 187.0MPa, while the elongation rate rose from 3.95% to 5.46% and 7.18%. Tensile fracture surface observed by means of the SEM, the fractures of Al-SiO2-C(Mg) systems showed a lot of dimples. The fracture of Al-SiO2 system was the matrix of ductile fracture and brittle rupture of primary silicon; the fracture of Al-SiO2-C system showed ductile fracture; the fracture of Al-SiO2-Mg system was the matrix of ductile fracture and brittle rupture of MgAl2O4.