An ab initio study on stacking and stability of TiAl3 phases

Abstract

TiAl 3 persists in many Al alloys and plays a detrimental role in solidification of related melts. Knowledge about TiAl 3 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 TiAl 3 with aid from ab initio density functional theory (DFT) calculations. The study confirms that the ground state of TiAl 3 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-TiAl 3 units. At elevated temperature, the equilibrium configuration contains neither pure D0 23 nor pure D0 22 but will consist of a combination of the TiAl 3 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.