  The Fokker-Planck (FP) equation has been derived for describing the temporal
evolution of the particle size probability density function (PDF) for KJMA
(Kolmogorov-Johnson-Mehl-Avrami) transformations. The classical case of
transformations with constant rates of both nucleation and growth, in 3D space,
has been considered. Integration of the equation shows that the PDF is given by
the superposition of one-parameter Gamma distributions with time dependent mean
size given by the KJMA theory. The asymptotic behavior of the FP solution
offers a demonstration of the conjecture, previously proposed by Pineda et al
[E. Pineda, P. Bruna, D. Crespo, Phys. Rev. E 70 (2004) 066119], according to
which the set of nuclei formed at the same time are Gamma-distributed, with
parameter depending on nucleus birth time. Computer simulations of the
transformation with constant nucleation and growth rates, do show that the
temporal evolution of the PDF, in the volume domain, is in good agreement with
the Johnson-Mehl PDF. The approach based on the FP equations provides a
particle size PDF that exhibits such behavior.
