Microwave activated combustion synthesis of titanium aluminides

TiAl and Ti3Al titanium aluminide intermetallic compounds were synthesized using a microwave activated combustion synthesis (MACS) method and compared to thermal activation (TACS) and conventional combustion (CS) methods. MACS represents an example of our “synergistic processing” philosophy, wherein the useful attributes of microwave processing and combustion synthesis are used synergistically. Thermodynamic calculations indicated activation was required for combustion synthesis to be successful in the Ti–Al system. MACS and TACS yielded Ti–Al products, while no reaction occurred with CS. Peak temperatures for TiAl during combustion were measured at 1500 °C for TiAl and 1450 °C for Ti3Al in MACS, compared to 1400 °C for TiAl and <1000 °C for Ti3Al in TACS. The change in peak temperature between the activation routes most likely corresponds to absorption of microwave energy at the reaction front. In MACS, product Ti3Al was produced in single-phase using an off-stoichiometric reacted mixture of 86:14 Ti:Al (w/w), while TiAl with a minimized Ti3Al secondary phase was obtained using a reactant mixture of 60:40 Ti:Al (w/w). The level of Ti3Al present in TiAl was minimized in MACS and could be controlled by changing the reactant mixture. Ti3Al content in TiAl was higher and consistent for different reactant mixtures in TACS samples. The present results obtained using the simple MACS process are comparable to those obtained using the TACS (at high heating rates) and the field activated combustion synthesis (FACS). An increase in volume and porosity was noted in all samples by all processing routes and was attributed to the intrinsic volume change associated with the reaction and the vaporization of persistent volatile impurities.