TY - JOUR

T1 - Ab initio study of chemical disorder as an effective stabilizing mechanism of bcc-based TiAl(+Mo)

AU - Abdoshahi, Neda

AU - Spörk-Erdely, Petra

AU - Friák, Martin

AU - Mayer, Svea

AU - Šob, Mojmír

AU - Holec, David

N1 - Publisher Copyright: © 2020 American Physical Society.

PY - 2020/10/2

Y1 - 2020/10/2

N2 - 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 βo-TiAl(+Mo) and disordered β-TiAl(+Mo) to ordered γ-TiAl(+Mo) and hypothetically assumed disordered γdis-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 βo→γ tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the βo phase. Upon alloying of ≈7.4at.% Mo, a small barrier between βo and γ-related local energy minima is formed. Yet a higher Mo content of ≈9at.% leads to an opposite-direction barrierless transformation γ→βo, i.e., fully stabilizing the βo phase. Considering the disordered phases, the β-Ti0.5Al0.5-xMox and γdis-Ti0.5Al0.5-xMox are energetically very close. Importantly, for all here-considered compositions up to 11at.% of Mo, a small energy barrier separates β-TiAl(+Mo) and γdis-TiAl(+Mo) energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered β and γdis-TiAl phases, but it turns out that it exhibits an energy barrier over 60meV/at. which, in comparison to the Bain's path with 9meV/at. barrier, effectively disqualifies the trigonal transformation for the TiAl system.

AB - 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 βo-TiAl(+Mo) and disordered β-TiAl(+Mo) to ordered γ-TiAl(+Mo) and hypothetically assumed disordered γdis-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 βo→γ tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the βo phase. Upon alloying of ≈7.4at.% Mo, a small barrier between βo and γ-related local energy minima is formed. Yet a higher Mo content of ≈9at.% leads to an opposite-direction barrierless transformation γ→βo, i.e., fully stabilizing the βo phase. Considering the disordered phases, the β-Ti0.5Al0.5-xMox and γdis-Ti0.5Al0.5-xMox are energetically very close. Importantly, for all here-considered compositions up to 11at.% of Mo, a small energy barrier separates β-TiAl(+Mo) and γdis-TiAl(+Mo) energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered β and γdis-TiAl phases, but it turns out that it exhibits an energy barrier over 60meV/at. which, in comparison to the Bain's path with 9meV/at. barrier, effectively disqualifies the trigonal transformation for the TiAl system.

UR - http://www.scopus.com/inward/record.url?scp=85094142110&partnerID=8YFLogxK

U2 - 10.1103/PhysRevMaterials.4.103604

DO - 10.1103/PhysRevMaterials.4.103604

M3 - Article

VL - 24.020

JO - Physical review materials

JF - Physical review materials

SN - 2475-9953

IS - 10

M1 - 103604

ER -