On the variability of critical size for homogeneous nucleation in a solid-state diffusional transformation

Abstract

In solid-state diffusional phase transformations involving nucleation and growth, the size of the critical nucleus for a homogeneous process (rhomo*=r∗) has been assumed to be time invariant (constant). The strain associated with the process leads to an increase in the value of r*, with respect to that for nucleation from a liquid. The strain energy stored in the matrix increases with transformation and the nuclei forming at a later stage encounter a strained matrix. Using devitrification of a bulk metallic glass as a model system, we demonstrate that r* is not a cardinal time invariant constant for homogeneous nucleation and can increase or decrease depending on the strain energy penalty. We show that the assumption regarding the constancy of r* is true only in the early stages and establish that with progress of the transformation leads to an altered magnitude of r*, which is a function of the microstructural details, geometrical variables and physical parameters. Using, HRLFI and computations, we argue that, ‘liquid-like' homogeneous nucleation can occur and that the conclusions are applicable to a broad set of solid-state diffusional transformations. The above effect ‘opens up' a lower barrier transformation pathway arising purely from the internal variables of the system.