Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations

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[111](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 {111}(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.