Stability of Ordered B2-β_o and Disordered bcc-β Phases in TiAl—A First Principles Study

Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β_o (B2). The relatively soft β phase can facilitate hot deformation, but β_o is detrimental for creep strength and ductility. Thus, knowledge about β_o→β phase transformation is desirable. Surprisingly, for the binary Ti-Al system it is under discussion whether the ordered β_o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.