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.