Homo or hetero? How vacancy motion brings discreteness into phase transformations

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