Phase equilibria and transformation in the Ti–Al–Ta system

TiAl- or Ti3Al-based alloys have gained industrial applications in recent years. The incorporation of β-stabilizing element Ta has demonstrated to better adjust their microstructures and properties by introduce β and O. However undesirable phases B2 and ωo are also triggered. Phase equilibrium information and the formation mechanisms of phases B2, ωo and O including solubility limit and stability range can provide a necessary basis for selecting alloy composition and heat treatment process of excellent Ti–Al–Ta alloys. In this work, several isothermal sections from 973 to 1373 K in Ti–Al–Ta system are investigated using highly efficient method integrated equilibrated alloys and diffusion triples. Based on the present results, solubility of Al changes from 35.7 at.% at 1456 K to 6.5 at.% at 973 K in β(Ti)/B2. The β(Ti)/B2 phase region shrinks with the decrease of temperature, while Al-rich B2 phase is separated from β(Ti)/B2 to form an island-like region at the temperature between 1373 and 1273 K. At about 1182.3 K, B2 transforms into ωo-Ti3Al2Ta, leading to the obvious change of microhardness. Meanwhile, O-Ti2AlTa phase occurs via reaction β(Ti)/B2 + Ti3Al + Ta2Al → O at 1185.5 ± 12.5 K. As temperature drops, the compositional ranges of ωo and O expand, extending along the direction parallel to the ligature of Ta and Ti3Al. The phase region of ωo covers 11.3–20.6 at.% Ta and 29.7–33.6 at.% Al at 973 K. According to the evolution of phase relations, both invariant reactions Ti3Al + Ta2Al →  ωo  + O and Ti3Al + Ta2Al →  ωo  + TiAl can be defined.