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

In titanium alloys, theω(hexagonal)-phase transformation has been categorized as either a diffusion-mediatedisothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mecha-nism. Here we report a diffusionless isothermalωtransformation that can occur even above theωtransformationtemperature. In body-centered cubic β-titanium alloyed withβ-stabilizing elements, there are locally unstableregions having fewerβ-stabilizing elements owing to quenched-in compositional fluctuations that are inevitablypresent in thermal equilibrium. In these locally unstable regions, diffusionless isothermalωtransformationoccurs even when the entireβregion is stable on average so that athermalωtransformation cannot occur.This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization ofβphase on average. In the diffusionlessisothermal and athermalωtransformations, the transformation rate is dominated by two activation processes: adynamical collapse of{111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionlessisothermal transformation, theω-phase nucleation, resulting from the localized phonon softening, requiresrelatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slowerrates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, thenucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.