On the athermal nature of the β to ω transformation

Abstract

One characteristic feature of the athermal α →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.