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

Abstract

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. Unlike conventional displacive phase transitions, the diffusionless isothermal ω transformation occurs in locally unstable nanoscale β-phase regions formed by quenched-in statistical compositional fluctuations. In the present study, the effects of Al and oxygen additions in ternary β-phase Ti–V-based alloys on the diffusionless isothermal ω transformation and its attempt process, called dynamic atomic shuffling, were studied. Elastic constant and internal friction measurements for Ti–16.6V–2.1Al and Ti–17.5V–2.5O (at.%) alloy single crystals and analysis of quenched-in compositional fluctuations using the thermodynamic theory of fluctuations indicated that the oxygen addition increases the β-phase stability, 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 ω transformation that occurs during aging at 298 K. In contrast, low β-phase stability, which promotes ω-phase nucleation, is retained by Al addition. 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.