An ab initio study on stacking and stability of TiAl3 phases

TiAl3 persists in many Al alloys and plays a detrimental role in solidification of related melts. Knowledge about TiAl3 phases and phase relations is of importance to get some insight into the solidification processes, the microstructures and the properties of Al alloys. In this manuscript, we present a systematic study of the basic structures of TiAl3 with aid from ab initio density functional theory (DFT) calculations. The study confirms that the ground state of TiAl3 has the D0(23) structure, whereas the observed D0(22) type is a metastable phase. The calculations have identified the stability of a series of stacking composed of both D0(22)- and D0(23)-TiAl3 units. At elevated temperature, the equilibrium configuration contains neither pure D0(23) nor pure D0(22) but will consist of a combination of the TiAl3 cubes arranged to minimise its Gibbs energy. Stacking default investigation reveals a large energy barrier for the D0(22) <--> D0(23) transition. The obtained information sheds some light on the rich variety of the experimental observations in Al alloys, and further to understand the complex titanium aluminides and their thermo-structural properties.