Reactive synthesis and characterization of titanium aluminides produced from elemental powder mixtures

The formation of titanium aluminides in Ti–Al elemental powder mixtures containing 25, 50 and 75 at.% Al, has been studied using differential scanning calorimetry (DSC). Phase evolution in the mixture was followed by heating the compacted samples up to 1273 K at 7.5 and 15 K min−1. The cooled samples were characterized using X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. The results showed that the primary combustion product in all the samples was TiAl3, and the combustion reaction occurred below the melting point of aluminum only in Ti-rich samples. In Al-rich samples (75 at.% Al), TiAl3 was obtained as a porous, single-phase product after combustion. In samples containing 25 and 50 at.% Al, the combustion reaction was incomplete and the unreacted titanium particles were covered by a layer of TiAl3. In these samples, other intermetallic compounds such as TiAl2, TiAl and Ti3Al were observed to form upon heating beyond the combustion peak and are attributed to the solid-state reaction between unreacted titanium and TiAl3. Heating the samples with 25 at.% Al to 1273 K for an hour led to the formation of a homogenous Ti3Al product, while a multiphase product with a dominant TiAl phase was observed in samples containing 50 at.% Al. Calculations based on DSC data show that the formation of TiAl3 through the reaction between solid titanium and molten aluminum is associated with an apparent activation energy of 195 ± 20 kJ mol−1 and an enthalpy of −114 ± 5 kJ mol−1.