The crystal structure and transformations of the omicron phase Ο in the Ti-Al-Nb system and on the ambiguity of its subvariants Ο1 and Ο2

ABSTRACT

Omicron, Ο-Ti 2 NbAl alloys have been under continuous development for more than 40 years as potential materials for the gas turbine engine;however, the crystal structure and phase transformations are still unclear. In this work, we clarified the crystal structure, atom positions and phase transformations of the Ο phase by in-situ neutron diffraction and high-temperature X-ray diffraction methods. From room temperature to the vanishing temperature of the Ο phase, the latter exposes a C m c m structure, with Ti and Nb atoms tending to respectively occupy the 8g and 4c 2 positions, but with a certain disorder, introducing an order parameter. The phase transformation sequences of Ti-24.8Al-24.3Nb alloy are Ο(975 °C) → β o + Ο(997.5 °C) → β o (1165.5 °C) → β. Moreover, we have proven that a distinction of the omicron subvariants Ο1 and Ο2 is crystallographic nonsense as Ο1 must equal to α 2 of P 6 3 /m m c structure, and Ti 2 NbAl is a prototype structure by itself. [Display omitted] [ FROM AUTHOR] Copyright of Scripta Materialia is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full . (Copyright applies to all s.)