On the athermal nature of the β to ω transformation

One characteristic feature of the athermal β → ω transformation is the short time scale of the transformation. So far, no clear understanding of this issue exists. Here we construct a model that includes contributions from a Landau sixth-order free energy density, kinetic energy due to displacement, and the Rayleigh dissipation function to account for the dissipation arising from the rapid movement of the parent–product interface during rapid nucleation. We also include the contribution from ω-like fluctuations to local stress. The model shows that the transformation is complete on a time scale comparable to the velocity of sound. The estimated nucleation rate is several orders higher than that for diffusion-controlled transformations. The model predicts that the athermal ω phase is limited to a certain range of alloying composition. The estimated nucleation rate and the size of “isothermal” particles beyond 17% Nb are also consistent with experimental results. The model provides an explanation for the reprecipitation process of the ω particles in the “cleared” channels formed during deformation of ω-forming alloys. The model also predicts that acoustic emission should be detectable during the formation of the athermal phase.