Influence of Oxygen on the Kinetics of Omega and Alpha Phase Formation in Beta Ti-V

A detailed understanding of the kinetics of phase formation in fi-stabilised titanium is of decisive importance for the applicability of these materials. However, the complex nature and long timescales of the various transformations, calls for specialized measurement techniques. In this work high-stability isothermal laser dilatometry is used to study the temporal volume changes associated with the various phase formation processes. Distinctly different behaviours between samples of Ti-21 at. pct V with different solute oxygen content could be detected and quantified. Temperature regimes for both diffusionless and diffusion-assisted isothermal w-formation as well as for w-to-a-transformation were determined. Low oxygen contents promote the diffusionless w-formation mechanism, but retard the diffusion-assisted one as well as the w-to-a-transformation process. The results confirm recent findings of a clear distinction between the diffusionless and diffusion-assisted isothermal w formation modes. Modelling of the w-phase formation applying Austin-Rickett kinetics revealed the temperature-dependent formation rates, on the basis of which the isothermal TTT-diagrams were developed which reflect the strong influence of the oxygen content.

Automated summary

A detailed understanding of the kinetics of phase formation in fi-stabilised titanium is crucial for their applicability, but specialized measurement techniques are needed due to the complex nature and long timescales of the transformations. High-stability isothermal laser dilatometry was used to study the volume changes of Ti-21 at. pct V samples during phase formation. Different behaviors were observed between samples with different solute oxygen content, and the temperature ranges for diffusionless and diffusion-assisted isothermal w-formation as well as w-to-a transformation were determined. The results confirm recent findings on the distinction between diffusionless and diffusion-assisted isothermal w formation modes. Modelling of w-phase formation using Austin-Rickett kinetics revealed temperature-dependent formation rates, which were used to develop isothermal TTT-diagrams reflecting the strong influence of oxygen content.