On the Variability of Critical Size for Homogeneous Nucleation in a Solid-State Diffusional Transformation

TL;DR

This paper investigates how the critical nucleus size in solid-state diffusional transformations varies during the process, challenging the assumption that it remains constant, and shows that it depends on strain energy and microstructural factors.

Contribution

It demonstrates that the critical nucleus size is not constant during transformation and depends on strain energy, microstructure, and physical parameters, with implications for transformation pathways.

Findings

r* can increase or decrease during transformation

Homogeneous nucleation can occur in a liquid-like state

Transformation pathways are influenced by internal strain energy

Abstract

In a solid-state diffusional phase transformation involving nucleation and growth, the size of the critical nucleus for a homogeneous process (r*homo=r* ) has been assumed to be a time invariant constant of the transformation. The strain associated with the process has a positive energy contribution and leads to an increase in the value of r*, with respect to that for nucleation from a liquid. With the progress of such a transformation, the strain energy stored in the matrix increases and 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 of the transformation and establish that the progress of the transformation leads to an altered magnitude of r*, which is a function of the microstructural details, geometrical variables and physical parameters. With the aid of high-resolution lattice fringe imaging and computations of r*, we further 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.