Study on solid-state reactions of nanocrystalline TiAl synthesized by mechanical alloying

The nanocrystalline TiAl intermetallic compound was synthesized by mechanical alloying of the Ti–50 at.% Al powder mixture and subsequent heat treatment. The powder particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and microhardness measurements. It was found that a Ti(Al) solid solution was formed at the early stage of milling, followed by the formation of an amorphous phase at longer milling times. On further milling, the amorphous structure transformed to a supersaturated Ti(Al) solid solution. Annealing of amorphous as well as supersaturated Ti(Al) solid solution structure separately resulted in the formation of nanocrystalline TiAl intermetallic compounds with a grain size of about 50 nm and high microhardness value of about 1190 HV. Phase transformation during heating of amorphous structure could be represented as amorphous → metastable hcp → disorder fcc TiAl → TiAl(L10). The transition of the supersaturated Ti(Al) solid solution into the equilibrium TiAl intermetallic during annealing proceeds through an intermediate stage in which a metastable fcc phase forms.