Formation of TiAl–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites by combustion synthesis

Abstract

Preparation of TiAl–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS). Sample compacts containing Ti, Al, and TiO 2 powders were employed to produce the TiAl–matrix composite reinforced by Al 2O 3, which was yielded through the displacement reaction of Al with TiO 2. On the other hand, elemental powder compacts were used to synthesize the TiAl–matrix composite with Ti 5Si 3 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 Ti 5Si 3 formed in the composite increases the combustion temperature and combustion wave velocity. However, the opposite trends were found in the synthesis of TiAl–Al 2O 3 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 Al 2O 3 content. Moreover, it was found that the combustion front propagated in a pulsating mode when the mole fraction of Al 2O 3 higher than 12.5 mol% was produced, because of the lower exothermicity of the reaction between Al and TiO 2 than that of Ti with Al. The Al 2O 3 content of the TiAl–Al 2O 3 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–Ti 5Si 3 and TiAl–Al 2O 3 in situ composites. In addition to TiAl, another aluminide compound Ti 3Al known as a major secondary phase in the Ti–Al reaction was detected in both types of the composites.