  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.
