Interplay between effect of Mo and chemical disorder on the stability of β/βo-TiAl phase

•We study chemical, mechanical and dynamical phase stability of beta-Ti-Al-Mo system.•Chemical disorder is modelled using special quasi-random structures.•Relationship between beta and gamma phases is addressed.•Chemical disorder is found to stabilise the beta phase even with zero Mo content.

The effect of Mo and chemical disorder on chemical, mechanical and dynamical phase stability of β(bcc)/βo (B2)-TiAl is studied using Density Functional Theory. The ordered βo structure has more negative energy of formation, hence it is chemically more stable than the disordered β   structure. Our calculations further suggest that the ordered structure is mechanically unstable for Mo concentrations <≈4at.%. Surprisingly, the disordered β phase is found to be mechanically stable regardless the Mo content. It is therefore concluded that the chemical disorder and the Mo alloying play a similar role in the stabilisation of β/βo-TiAl. This trend is further confirmed by studying phonon and electronic density of states. Finally, the ordered βo structure is shown to sit in a saddle point of a potential energy surface. A barrier-less transformation path βo→γ exists, allowing for a spontaneous transformation from the symmetry stabilised βo to the ground state γ-phase.