Effects of aluminum and oxygen additions on quenched-in compositional fluctuations, dynamic atomic shuffling, and their resultant diffusionless isothermal ω transformation in ternary Ti-V-based alloys with bcc structure

Diffusionless isothermal ω transformation is a recently discovered ω transformation that occurs isothermally during aging near room temperature in Ti alloys with a bcc (β-phase) structure. In the present work, we studied the effects of Al and oxygen additions on the diffusionless isothermal ω transformation in ternary β-phase Ti-V-based alloys. It was revealed that the oxygen addition increases the β-phase stability in a Ti-V-O alloy, and it selectively increases the activation energy of dynamic atomic shuffling in locally unstable Ti-rich regions, which is caused by the strong attractive interaction between Ti and oxygen elements. As a result, oxygen addition suppresses the diffusionless isothermal transformation. In contrast, low β-phase stability is retained by Al addition in a Ti-V-Al alloy. Furthermore, dynamic atomic shuffling with a low activation energy is retained even though the average activation energy is enhanced by the Al addition, originating from the relatively weak atomic interaction between Ti and Al. Therefore, the diffusionless isothermal ω transformation cannot be suppressed by Al addition.