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

Abstract

Intermetallic structural materials based on TiAl have gained industrial importance in recent years. Especially β-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 β-stabilizing elements additional phases, such as the ωo phase, are introduced, which predominantly precipitate within the βo phase.In this study the impact of the strong β-stabilizing element Mo on the formation mechanism of the ωo 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 ωo phase particles nucleate initially within the βo 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 ωo particles and piles up in the βo 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 ωo phase in the presence of dissolved Mo, while the βo phase is stabilized energetically. Supporting analysis of the valence charge density distribution shows an increase of TiMo bond directionality for the incorporation of Mo into the ωo structure in comparison to a Mo-free ωo phase.