Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations
Abstract
In titanium alloys, the ω (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 ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β -titanium alloyed with β -stabilizing elements, there are locally unstable regions having fewer β -stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation 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 of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111} β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω -phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β /ω 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 β /ω interface energy is the rate-determining process in the diffusionless ω transformations.
- Publication:
-
Physical Review Materials
- Pub Date:
- April 2019
- DOI:
- 10.1103/PhysRevMaterials.3.043604
- Bibcode:
- 2019PhRvM...3d3604T