- Abstract This chapter examines the microstructure, mechanical properties and environmental behavior of RBBC composites infiltrated with Al-Si alloys under Mg vapor at temperatures lower than 1000°C. For infiltration at 850-1000°C small Mg lumps were placed around the ceramic preform that upon heating generate Mg-rich vapor atmosphere. Infiltration with pure Al was conducted at 1200°C. The infiltrated composites were cooled to room temperature with a 10°C/min cooling rate. The peaks of the Al4C3 phase were not detected in samples fabricated at temperature lower than 1200°C. The fractography of the samples reviles a trans-granular fracture mode, which indicates a good adhesion between boron carbide particles and newly formed ceramic phases which is one of the benefits of reaction bonded method. The chapter explains how fabrication of the RBBC composites infiltrated with pure aluminum and Al-Si was successfully conducted at temperature lower 1000°C under Mg vapor atmosphere. The Mg vapor provides extremely low oxygen partial pressure and conditions for the decomposition of the alumina layer covered the surface of the aluminum containing melt.