The effect of aluminum on the microstructure and phase composition of boron carbide infiltrated with silicon

Reaction-bonded boron carbide is prepared by pressureless infiltration of boron carbide preforms with molten silicon in a graphite furnace under vacuum. The presence of Al₂O₃ parts in the heated zone, even though not in contact with the boron carbide preform, causes aluminum to appear in the liquid silicon. The formation of aluminum sub-oxide (Al₂O) stands behind the transport of aluminum into the composite. The presence of aluminum in the boron carbide-silicon system accelerates the transformation of the initial boron carbide particles into B_x(C,Si,Al)_y and Al_1.36B₂₄C₄, newly formed carbide phases_. It also leads during cooling to the formation of some Si-Al solid solution particles. The effect of Al on the microstructural evolution is well accounted for by the calculated isothermal section of the quaternary Al-B-C-Si phase diagram, according to which the solubility of boron in liquid silicon increases with increasing aluminum content. This feature is a key factor in the evolution of the microstructure of the infiltrated composites.