Formation of TiAl-Ti5Si3 and TiAl-Al2O3 in situ composites by combustion synthesis

Preparation of TiAl-Ti5Si3 and TiAl-Al2O3 in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS). Sample compacts containing Ti, Al, and TiO2 powders were employed to produce the TiAl-matrix composite reinforced by Al2O3, which was yielded through the displacement reaction of Al with TiO2. On the other hand, elemental powder compacts were used to synthesize the TiAl-matrix composite with Ti5Si3 as the reinforcement. Experimental observations show that the addition of Si in the Ti-Al powder mixture enhances the sustainability of the reaction and the increase of Ti5Si3 formed in the composite increases the combustion temperature and combustion wave velocity. However, the opposite trends were found in the synthesis of TiAl-Al2O3 composites. Not only is a preheating temperature of 300 °C required to achieve self-sustaining combustion, but also the reaction temperature and flame-front propagation rate decrease with Al2O3 content. Moreover, it was found that the combustion front propagated in a pulsating mode when the mole fraction of Al2O3 higher than 12.5 mol% was produced, because of the lower exothermicity of the reaction between Al and TiO2 than that of Ti with Al. The Al2O3 content of the TiAl-Al2O3 composite obtained by this study has an upper limit at 28.6 mol%, beyond which combustion ceased to propagate. The XRD analysis of the end products confirms formation of TiAl-Ti5Si3 and TiAl-Al2O3 in situ composites. In addition to TiAl, another aluminide compound Ti3Al known as a major secondary phase in the Ti-Al reaction was detected in both types of the composites.