Ab initio study of chemical disorder as an effective stabilizing mechanism of bcc-based TiAl( plus Mo)


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Year of publication 2020
Type Article in Periodical
Magazine / Source Physical Review Materials
MU Faculty or unit

Faculty of Science

Web https://doi.org/10.1103/PhysRevMaterials.4.103604
Doi http://dx.doi.org/10.1103/PhysRevMaterials.4.103604
Keywords First-principles calculations; Phase transitions by order
Description 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(o)-TiAl(+Mo) and disordered beta-TiAl(+Mo) to ordered gamma-TiAl(+Mo) and hypothetically assumed disordered gamma(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 beta(o )-> gamma tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the beta(o) phase. Upon alloying of approximate to 7.4 at.% Mo, a small barrier between beta(o) and gamma-related local energy minima is formed. Yet a higher Mo content of approximate to 9 at.% leads to an opposite-direction barrierless transformation gamma -> beta(o )i.e., fully stabilizing the beta(o) phase. Considering the disordered phases, the beta(o)-Ti0.5Al0.5-xMox and gamma(dis)-Ti0.5Al0.5-xMox, are energetically very close. Importantly, for all here-considered compositions up to 11 at.% of Mo, a small energy barrier separates beta-TiAl(+Mo) and gamma(dis)-TiAl(+Mo) energy minima Finally, a trigonal path was studied as an alternative transformation connecting disordered beta and gamma(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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