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To shed a new light on the complex microstructural evolution in the Ti-Al-Mo system, we employ ab initio calculations to study bcc-fcc structural transformations of ordered β o-TiAl(+Mo) and disordered β-TiAl(+Mo) to ordered γ-TiAl(+Mo) and hypothetically assumed disordered γ dis-TiAl(+Mo) alloys, respectively. In particular, tetragonal (Bain's path) and trigonal transformations are combined with the concept of special quasirandom structures (SQS) and examined. Our calculations of the ordered phases show that the β o → γ tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the β o phase. Upon alloying of ≈7.4 at.% Mo, a small barrier between β o and γ-related local energy minima is formed. Yet a higher Mo content of ≈9 at.% leads to an opposite-direction barrierless transformation γ → β o , i.e., fully stabilizing the β o phase. Considering the disordered phases, the β-Ti 0.5 Al 0.5−x Mo x and γ dis-Ti 0.5 Al 0.5−x Mo x are energetically very close. Importantly, for all here-considered compositions up to 11 at.% of Mo, a small energy barrier separates β-TiAl(+Mo) and γ dis-TiAl(+Mo) energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered β and γ dis-TiAl phases, but it turns out that it exhibits an energy barrier over 60 meV/at. which, in comparison to the Bain's path with 9 meV/at. barrier, effectively disqualifies the trigonal transformation for the TiAl system.

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