Formation of TiB₂-MgAl₂O₄ Composites by SHS Metallurgy

TiB₂-MgAl₂O₄ composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO₂ or B₂O₃ as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al₂O₃ or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B₂O₃/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO₂/Al/Mg thermite and Al₂O₃ as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB₂ and MgAl₂O₄. It is believed that MgAl₂O₄ was synthesized through a combination reaction between Al₂O₃ and MgO, both of which can be totally or partially produced from the metallothermic reduction of B₂O₃ or TiO₂. The microstructure of the TiB₂-MgAl₂O₄ composite exhibited fine TiB₂ crystals surrounded by large densified MgAl₂O₄ grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl₂O₄-containing composites.