- Composites with a high fraction of the ceramic phase were fabricated by infiltration of 80% dense preforms with liquid Mg or Mg alloys, under an Mg vapor atmosphere. The infiltration was performed at 1123 K in a semi-hermetically closed container, from which gas had been evacuated. The Mg vapor atmosphere was achieved by heating subsequently to the relatively high temperature of the Mg infiltration process. The specific weight of the composites is about 2.44 g/cm 3 . The microstructure of the composite consists of the newly formed MgB 2 and ternary carbide MgB 2 C 2 phases that connect the initial B 4 C particles and some residual metal. A thermodynamic analysis of the interaction between B 4 C and liquid Mg was conducted and its results are in good agreement with the experimental observations. The mechanical properties of the composites were investigated and discussed.