Fabrication of MoSi2–MgAl2O4 in situ composites by combustion synthesis involving intermetallic and aluminothermic reactions

Abstract

Combustion synthesis involving aluminothermic reduction and intermetallic reaction was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate MoSi2–MgAl2O4 in situ composites with a broad composition range. Two thermite mixtures with stoichiometries of MoO3 + 2Al and 0.6MoO3 + 0.6SiO2 + 2Al were incorporated into MgO-added Mo–Si combustion systems. The MoO3/Al-based combustion reaction is more exothermic and has a lower activation energy (Ea) of 62.5 kJ/mol compared to Ea = 70.0 kJ/mol for the synthesis reaction involving the MoO3/SiO2/Al mixture. The final products with the molar ratio of MoSi2/MgAl2O4 between 2.5 and 6.0 were synthesized from powder compacts with the MoO3/Al thermite and those from 0.8 to 4.5 were produced from samples containing the MoO3/SiO2/Al thermite. Formation of MgAl2O4 from a combination reaction between thermite-produced Al2O3 and pre-added MgO was attained. Simultaneously, it was found β-MoSi2 formed as the dominant silicide when the reaction temperature exceeded about 1500 °C, but α-MoSi2 prevailing once the combustion temperature fell below 1250 °C. Complete phase conversion was achieved, except for a trivial amount of Mo5Si3 present in the products of the MoO3/SiO2/Al-based samples. The as-synthesized composites exhibited a dense morphology with connecting MgAl2O4 crystals and distributed MoSi2 small grains.