Impact of Mo on the ω0 phase in β-solidifying TiAl alloys: An experimental and computational approach

Intermetallic structural materials based on TiAl have gained industrial importance in recent years. Especially beta-solidifying alloys, such as the TNM alloy, have emerged, as they provide balanced mechanical properties attained via robust manufacturing routes. However, through the incorporation of the essential beta-stabilizing elements additional phases, such as the omega(0) phase, are introduced, which predominantly precipitate within the beta(0) phase. In this study the impact of the strong beta-stabilizing element Mo on the formation mechanism of the omega(0) phase with particular emphasis on the prevailing elemental redistribution is investigated using differently heat- treated material conditions of a TNM alloy. To this end, scanning and transmission electron microscopy as well as atom probe tomography are employed. These methods provide evidence that omega(0) phase particles nucleate initially within the beta(0) phase without significant diffusional alloying element redistribution. In contrast, the growth sequence is governed by diffusion, i.e. the alloying element Mo is rejected from the omega(0) particles and piles up in the beta(0) phase as visualized and quantified by atom probe tomography. In order to understand the physical origin of the Mo redistribution phenomena, ab initio calculations are conducted. Calculations of the energies of formation reveal a thermodynamic destabilization of the omega(0) phase in the presence of dissolved Mo, while the beta(0) phase is stabilized energetically. Supporting analysis of the valence charge density distribution shows an increase of Ti-Mo bond directionality for the incorporation of Mo into the omega(0) structure in comparison to a Mo-free omega(0) phase. (C) 2017 Elsevier Ltd. All rights reserved.