In titanium alloys, the $\omega$(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal $\omega$ transformation that can occur even above the $\omega$ transformation temperature. In body-centered cubic $\beta$-titanium alloyed with $\beta$-stabilizing elements, there are locally unstable regions having fewer $\beta$-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal $\omega$ transformation occurs even when the entire $\beta$ region is stable on average so that athermal $\omega$ transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of $2/3{\left[111\right]}_{\beta }$ longitudinal phonon, which cannot be suppressed by the stabilization of $\beta$ phase on average. In the diffusionless isothermal and athermal $\omega$ transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of ${\left\{111\right\}}_{\beta }$ pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the $\omega$-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent $\beta /\omega$ interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the $\beta /\omega$ interface energy is the rate-determining process in the diffusionless $\omega$ transformations.

• Received 22 January 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.043604