Microstructure of interfaces between a magnesium matrix and preoxidized silicon carbide particles

Abstract

An amorphous SiO2 layer, 100–150 nm thick, has been produced on the surface of α-SiC particles by controlled oxidation in air. The as-treated particles were immersed for 1.5h at 1,000 K in a large excess of liquid magnesium, which resulted in the formation by pressureless infiltration of a composite region with a particle volume fraction about 50%. Conventional transmission electron microscopy (CTEM) and high-resolution TEM (HRTEM) were used to characterize and localize the different species present in the transition zone between the Mg matrix and the α-SiC particles. The results are discussed in terms of the physico-chemical processes occurring during formation of the composite region or during cooling of the Mg ingot. A reaction mechanism based on local equilibria and liquid phase diffusion is proposed. Four steps are involved in this mechanism: (i) rapid conversion of SiO2 into MgO and Si; (ii) formation of a true Mg/SiC interface; (iii) partial decomposition of SiC into carbon and silicon; and (iiii) precipitation of β-SiC and Mg2Si on cooling.