TiC-TiB2 particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al-Ti-B4C system with various B4C particle sizes. The formation mechanism of TiC and TiB2 in the locally reinforced regions was investigated. The results showed that TiC and TiB2 are formed and precipitated from Al-Ti-B-C melt resulting from the dissociation of B4C into Al-Ti melt when the concentrations of B and C atoms in the Al-Ti-B-C melt become saturated. However, in the case of coarse B4C powders (>= 40 mu m) used, the primary reaction in the Al-Ti-B-C melt is quite limited due to the poor dissociation of B4C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al-Fe-Ti-B-C melt, thanks to the favorable reaction of molten Fe with remnant B4C, and then TiC and TiB2 are further formed and precipitated from the saturated Al-Fe-Ti-B-C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB2 in the electric resistance furnace and during casting was proposed. (C) 2011 Elsevier B.V. All rights reserved.