- The present communication is concerned with the effect of the carbon source on the morphology of reaction bonded boron carbide (B 4 C). Molten silicon reacts strongly and rapidly with free carbon to form large, faceted, regular polygon-shaped SiC particles, usually embedded in residual silicon pools. In the absence of free carbon, the formation of SiC relies on carbon that originates from within the boron carbide particles. Examination of the reaction bonded boron carbide revealed a core–rim microstructure consisting of boron carbide particles surrounded by secondary boron carbide containing some dissolved silicon. This microstructure is generated as the outcome of a dissolution–precipitation process. In the course of the infiltration process molten Si dissolves some boron carbide until its saturation with B and C. Subsequently, precipitation of secondary boron carbide enriched with boron and silicon takes place. In parallel, elongated, strongly twinned, faceted SiC particles are generated by rapid growth along preferred crystallographic directions. This sequence of events is supported by X-ray diffraction and microcompositional analysis and well accounted for by the thermodynamic analysis of the ternary B–C–Si system.