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 ${\beta }_{\mathrm{o}}-\mathrm{TiAl}(+\mathrm{Mo})$ and disordered $\beta -\mathrm{TiAl}(+\mathrm{Mo})$ to ordered $\gamma -\mathrm{TiAl}(+\mathrm{Mo})$ and hypothetically assumed disordered ${\gamma }_{\mathrm{dis}}-\mathrm{TiAl}(+\mathrm{Mo})$ 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 ${\beta }_{\mathrm{o}}\to \gamma$ tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the ${\beta }_{\mathrm{o}}$ phase. Upon alloying of $\approx 7.4\mathrm{at}.\%$ Mo, a small barrier between ${\beta }_{\mathrm{o}}$ and $\gamma$-related local energy minima is formed. Yet a higher Mo content of $\approx 9\mathrm{at}.\%$ leads to an opposite-direction barrierless transformation $\gamma \to {\beta }_{\mathrm{o}}$, i.e., fully stabilizing the ${\beta }_{\mathrm{o}}$ phase. Considering the disordered phases, the $\beta -{\mathrm{Ti}}_{0.5}{\mathrm{Al}}_{0.5-x}{\mathrm{Mo}}_x$ and ${\gamma }_{\mathrm{dis}}-{\mathrm{Ti}}_{0.5}{\mathrm{Al}}_{0.5-x}{\mathrm{Mo}}_x$ are energetically very close. Importantly, for all here-considered compositions up to $11\mathrm{at}.\%$ of Mo, a small energy barrier separates $\beta -\mathrm{TiAl}(+\mathrm{Mo})$ and ${\gamma }_{\mathrm{dis}}-\mathrm{TiAl}(+\mathrm{Mo})$ energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered $\beta$ and ${\gamma }_{\mathrm{dis}}$-TiAl phases, but it turns out that it exhibits an energy barrier over $60\mathrm{meV}/\mathrm{at}.$ which, in comparison to the Bain's path with $9\mathrm{meV}/\mathrm{at}.$ barrier, effectively disqualifies the trigonal transformation for the TiAl system.

• Received 30 July 2020
• Accepted 14 September 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.103604

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