In the study of phase transformations experimental results were often interpreted in terms of the concept of homogeneous transformation, that is, the phase change involved a continuous amplification of short wavelength (ordering) or long wavelength (clustering) fluctuations in an appropriate order parameter. This quasi-continuum approach suggested that the system was not partitioned into transformed and untransformed regions but the order parameter proceeds towards equilibrium uniformly in all micro-volumes of the system. However, defect-mediated diffusion (vacancy mechanism) involving local atomic jump processes during the early stages of transformation kinetics could produce discrete regions within which the order parameter has changed significantly embedded in an unperturbed matrix. An estimate of this `graininess' was calculated in terms of vacancy diffusion parameters which permitted one to distinguish between heterogeneous and homogeneous regimes of transformation processes in the alloy.

Homo or Hetero? How Vacancy Motion Brings Discreteness into Phase Transformations. W.A.Soffa, W.Püschl, W.Pfeiler: Intermetallics, 2003, 11[2], 161-7